Ultra wide-band-gap semiconductors for energy and electronics (UWBG2E)

Resume : Ultra-wide bandgap (UWBG) semiconductors are at the very frontier of electronics for energy management or energy electronics. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and solar-blind deeper ultraviolet optoelectronics. Gallium oxide—Ga2O3 (4.5–4.9 eV), has recently emerged pushing the limits set by more conventional WBG (~3 eV) materials, such as SiC and GaN, as well as for transparent conducting oxides (TCO), such as In2O3, ZnO and SnO2, to name a few. During the last decade, the Ga2O3 power diode and transistor progress has been impressive, with devices now approaching the frontier of the field. Indeed, Ga2O3 as the first oxide used as a semiconductor for power electronics, has sparked an interest in oxide semiconductors to be investigated (oxides represent perhaps the largest family of UWBG). Among these new power electronic materials, AlxGa1-xO3 may provide high-power heterostructure electronic and photonic devices at bandgaps far beyond all materials available today (~8 eV) or ZnGa2O4 (~5 eV), enabling spinel bipolar energy electronics for the first time ever. These material systems also opens new optoelectronics avenues and new electronics perspectives based on stabile interfaces and a natural integration with extremely high-k functional oxides. Here, we review the state-of-the-art and prospects of some ultra-wide bandgap oxide semiconductor arising technologies as promising innovative material solutions towards a sustainable zero emission society.

Resume : At the 2022 Spring meeting of E-MRS, we had an invited talk which title was “growth mechanism of mist CVD” at N.1.1. In this talk, I will report on the fabrication and the characterization of metal oxide thin films by mist CVD. Our lab has designed and built a system to prepare thin films using solution mist flow---the mist chemical vapor deposition (mist CVD), which is similar to the spray pyrolysis and AACVD techniques. Compared with the conventional processes used the mono-phase fluid to prepare thin films, the mist CVD needs to control more operating parameters due to the use of multi-phases mist flow. However, because there are many operating parameters can be controlled, the mist CVD may solve some problems which cannot be solved by the conventional techniques, such as the uncontrollable complex chemical reactions and the unavoidable external environmental disturbances. In recent studies, to making the simultaneous existence of multi-phases fluids become possible, we have developed the 3rd generation mist CVD system, which used the following physics: the existence of Leidenfrost state droplets (ELSD) and the low probability of collisions between mist droplets (LPCD). [1-3] Until now, the film fabrication process of 3rd generation mist CVD system can achieve the low environmental load, large area homogeneity, high quality and composition controllability simultaneously, that information we have briefly discussed at N.1.1 of “growth mechanism of mist CVD” at the 2022 Spring meeting E-MRS. In this repost, we will introduce more details about the fabrication of metal oxide thin films by mist CVD and the characterization of relative thin films. For the film formation process of spray method, one solute is usually dissolved in one solvent, and a precursor solution is used to prepare thin films. For example, in the case of preparing ZnO films, Zn(acac)2 can be used as Zn precursor, H2O or MeOH is used as solvent. Usually, the Zn concentration and growth temperature is usually selected as the experimental parameters, and the effects of them on the film properties will be studied. The relative reaction equation is showed as E.1.[4] (Zn(C5H7O2)2 + H2O ←→ ZnO + 2C5H8O2　 E.1 From the E.1, it can be seen that H2O as an oxygen source supported the ZnO films formation, but too much H2O will cause the reaction to proceed in reverse. The reaction movement also may affect the film growth rate and the crystal orientation. However, we all know that it is not easy to change the H2O concentration in precursor solution, and there are few reports about it. In this research, the ZnO films are prepared by the 3rd generation mist CVD, thanks to the special system configuration of this novel mist CVD, which has more than two separate solution chambers, one mixing chamber and one fine channel reactor, the Zn precursor solution and H2O (oxygen source) are separately added to different solution chambers, which makes it easy to change the amount of oxygen source during experiment. For the film preparation process of 3rd generation mist CVD, there is no complex or troublesome work for changing the supply ratio of oxygen source. The experimental results reveal that ZnO film growth rate and crystal orientation are influenced by the [H2O]/[Zn] ratios. Moreover, the characterization of the AlOx, ATO, [5] In2O3, Ga2O3 [6] and MoS2 thin films prepared by 3rd generation mist CVD system will also be reported. References: [1] T. Kawaharamura, Jpn. J. Appl. Phys. 53, 05FF08, 2014. [2] T. Kawaharamura, G. T. Dang, L. Liu, E. K. C. Pradeep, M. Tatsuta, M. Furuta, Y. Suwa, S. Sato, Y. Nakasone, S. Yamaoki, M. Nishi, Y. Kobayashi, M. Sakamoto, and P. Rutthongjan, 64th JSAP Spring Meeting, 2017, p. 17a-502-10. [3] T. Kawaharamura, P. Rutthongjan, L. Liu, M. Nishi, M. Sakamoto, Y. Kobayashi, E. K. C. Pradeep, G. T. Dang, S. Sato, S. Yamaoki, Y. Nakasone, and M. Ueda, 78th JSAP Autumn Meeting, 2017, p. 7a-C17-2. [4] P. Rutthongjan, et al, Growth mechanism of zinc oxide thin film by mist chemical vapor deposition via the modulation of [H2O]/[Zn] ratios, APEX, 12, 065505, 2019. [5] L Liu, et al, Study on Fabrication of Conductive Antimony Doped Tin Oxide Thin Films (SnOx:Sb) Fabricated by 3rd Generation Mist Chemical Vapor Deposition, Jpn. J. Appl. Phys., 58, 025502, 2019. [6] T. Yasuoka, et al, The effect of HCl on the α-Ga2O3 thin films fabricated by third generation mist chemical vapor deposition, AIP Adv., 11,045123, 2021.

Resume : Gallium oxide (Ga2O3) is of growing interest for high-power and radio-frequency (RF) electronic devices as well as deep-UV optoelectronic devices. Among the five polymorphs of Ga2O3, the most stable phase, β-Ga2O3, has been extensively investigated, and its MOSFET with high performance was demonstrated [1]. On the other hand, metastable α-Ga2O3 has also attracted much attention due to its remarkable device performance [2]. For α-Ga2O3 on a sapphire substrate, however, it is well known that there are dislocations with high density of 10^10-10^11 cm^-2 in its films due to the large lattice mismatch [3]. These days, the carrier transport in α-Ga2O3 on sapphire substrate was reported to be significantly limited by the dislocations [4]. Thus, further studies are needed to elucidate the relationship between dislocations and electrical properties in detail. In this study, we prepared Si-doped n-type α-Ga2O3 films on m-plane sapphire substrates with different thicknesses and dopant concentrations by using mist chemical vapor deposition method. These samples were investigated by Hall effect measurements at room temperature. From the measurements, it was found that both carrier mobilities and carrier concentrations are apt to increase as the film thicknesses increase. This suggests that threading dislocations in the film, which restrict the carrier mobility, fluctuate in their density depending on the distance from the film/substrate interface, that is, threading dislocation density decreases with an increase in the distance from the interface. The trend of carrier concentrations also shows that dangling bonds along threading dislocations act as acceptor centers, which trap electrons, like GaN [5]. In order to confirm the depth-dependence of threading dislocation density and electrical properties, we estimated the carrier concentration and carrier mobility in the surface region, where both of them were found to be higher than those in the interface region. In addition, the effects of screening of charged dislocations by carriers can be seen by comparison of the results for films with different dopant concentrations. This work was, in part, supported by MIC research and development (JPMI00316). [1] Higashiwaki et al., APL 103, 123511 (2013). [2] Oda et al., APEX 9, 021101 (2016). [3] Kaneko et al., JJAP 51, 020201 (2012). [4] Takane et al., 2022 MRS Spring Meeting, EQ01.15.02, Hawaii, (2022). [5] Look et al., PRL 82, 1237 (1999).

Resume : One of the main goals of semiconductor material science is controlling the electrical and optical properties of materials, which is fulfilled by means of controlling charge carrier concentration. Most of the materials valuable for electronic application supper from monopolar doping problems; oxide semiconductors are easily n-doped, however, p-doping is a big challenge still. The difficulties of p-doping are connected to the creation of compensating donors, low mobility of holes, low lying valence band, high ionization energies of acceptors. Modulated doping, in which the impurities are separated from the layer where current flows, gives one possibility to decrease scattering with ionized impurities and increase carrier mobility. In this case, the barrier layers are doped, while carriers reside in quantum well layers and do not experience scattering with ions. However, it is counted that modulation doping cannot reduce the compensation processes. The essence of compensation is that the free energy of the pair [negatively charge acceptor-hole] is more than the free energy of the pair [negatively charge acceptor – positively charged donor]; that is why for crystal it is energetically more favorable to create a positively charged donor defect (hole killer) rather than to maintain a hole in a valence band. Therefore, the efficiency of compensation depends on the band-gap, the ionization energy of compensating donors, their creation enthalpy. In the condition of space confinement all these characteristics change and become size dependent. In this work, we study the possibilities of suppressing the formation of compensating intrinsic defects in modulated p-doped AlGaO3/Ga2O3 quantum well heterostructures in order to obtain p-type gallium oxide layers. The basic idea is that because of very high conduction band offset aluminum and gallium oxides, it is possible to construct two-step quantum wells with double barriers with different aluminum concentrations – Al(1-x)Ga(x)O3/Al(1-y)Ga(y)O3/Ga2O3 with y less than x. If the modulated doped barrier Al(1-y)Ga(y)O3 that immediately borders Ga2O3 quantum well, is of a few nanometer sizes, the ionization energy of compensating donors will increase because of space confinement. The ionization energy of acceptors is expected to be much less affected by space confinement. This is mainly conditioned by a very low barrier for holes in the heterostructure, and also a big effective mass of holes. We calculated the band structure and ionization energies of donors, acceptors for different thicknesses for quantum well and barrier of the Al(1-x)Ga(x)O3/Al(1-y)Ga(y)O3/Ga2O3 heterostructure in the frame of single-band effective mass approximation and perturbation theory. The obtained results then were used for the Kroger method of quasi-chemical reactions to define the equilibrium concentrations of careers, acceptor and donor defects vs system geometrical parameters and to define their optimal values.

Resume : Ultraviolet (UV) photodetectors are emerging for applications such as flame/spark detection, chemical/biological-agent detection, military counter-measures, environmental monitoring, UV sterilization monitoring, non-line-of-sight communications and UV space astronomy. To minimize false alarms and background clutter, many of these devices operate in the solar-blind UVC portion of the spectrum (<290 nm). Over the past few years, attention has turned to the naturally solar blind semiconductor beta gallium oxide (Ga2O3) (Eg ~4.9 eV). Moreover it has been demonstrated that the bandgap can be engineered further into the UV range by alloying with Al2O3 (Eg ~9.0eV). In this study AlGa2O3 layers were grown on c-sapphire substrates by pulsed laser deposition. Optical transmission spectra were coherent with a bandgap engineering from 4.9 to 6.2 eV controlled via the growth conditions. X-ray diffraction revealed that the films were mainly monoclinic with strong (-201) orientation. Photodetectors with state-of-the-art solar rejection ratios were demonstrated. Dark signals were <50 pA and spectral responsivities showed both exceptionally narrow linewidths and strong photoconductive gain.

Resume : Ga2O3 (bandgap Eg~4.8-5 eV) is a representative ultra-wide bandgap (UWBG) oxide semiconductor which attracts considerable interest for power electronic application particularly for high voltage for the electrification of transport systems for instance. This because of its unique combination of material properties: availability in large single crystals (at relatively low cost), heteroepitaxy developments onto cheap substrates (such as sapphire or silicon), tunable n-type conductivity, and ultra-high breakdown electrical field. Nevertheless, p-type doping remains of serious concern to be solve and finding an effective acceptor dopant is a key issue. Zinc (Zn) as a group 2 element should be a potential candidate as an acceptor in Ga2O3 [1]. This study is carried out in order to characterize and identify the energy levels introduced by Zn doping in Ga2O3 homoepitaxy in its beta phase realized by MOCVD. For this, a Zn-doped Ga2O3 layer 1 um thick was grown using a horizontal RF-heated MOCVD reactor on n-type Sn-doped ([Sn] = 1x1018cm-3 ) Ga2O3 (-201) substrates. Gallium and oxygen precursor flow rates were 6 µmol/min and 600 sccm respectively. The growth temperature and total pressure in the reactor were set at 775°C and 30 torr. The Zn doping is introduced under the conditions ensuring native conductivity of the holes resulting from the gallium vacancies as shown previously [2]. The deep defect energy levels were determined by DLTS technique on the p/n junction obtained between the p-type layer with an apparent concentration of free hole carriers of 3x1015 cm-3 measured at room temperature and the n-type substrate . Five hole traps (H1 to H5) and one electron trap E1 were determined using appropriate DLTS bias conditions (Vr=-1V; Vp= 3V). Considering their concentration and energy, H3 (Ev 056 ± 0.06 eV) and H4 (EV 07 ± 06 eV) could be attributed to Zn acceptor levels. Our DLTS values are in rather good agreement with the energy found by EPR from Zn ions at the tetrahedral ZnGa1 (0.65 eV) and octahedral ZnGa2 (0.78 eV) sites in the bulk material [3]. They are also in good agreement with recently reported Hall effect measurement showing a ionization energy Ev = 0.7 eV of ZnGa in Zn:Ga2O3 thin films [1]. The H5 level (Ev 1.05±0.06 eV) could be tentatively attributed to the deep acceptor level VGa(0/-) or its complexes already reported by several teams [4,5]. Finally, the origin of the E1 electron trap at Ec-0.9 ± 0.05 eV is still under investigation. Its signature is relatively close to the FeGa0/- acceptor level at Ec-0.8eV and to the TiGa0/ donor level around 0.95 eV [6]. However, so far neither Fe nor Ti have been detected in MOCVD layers [7]. So, E1 could be the same level of unknown origin detected at Ec-1eV in UID substrates and PAMBE layers [8]. [1] E.Chikoidze et al, Journal of Vacuum Science & Technology A 40, 043401 (2022) [2] E Chikoidze et al, Mat;ToDay Phys.3,118(2017) [3] T.D. Gustafson, et al, J. Appl. Phys. 129, 155701, (2021). [4] A.Y. Polyakov et al, Appl Phys Letters,112,032107(2018) [5] E. Chikoidze et al., J. of Mat Chem C, 2019, 7, 10231 [6] C. Zimmermann et al, Appl Phys Letters 116,072101(2020) [7] H. Ghadi et al, APL Materials 8,02111(2020) [8] E. Farzana et al, J Appl Phys 123,161410(2018)

Resume : Wide band semiconductor ß-Ga2O3, with an energy gap of 4.7 eV, is a promising material for application in high power electronics, solar blind sensors etc. The weak point hindering its large scale use is the difficulty of realization of p-type doping required for the fabrication of bi-polar devices. While to-date attempts to introduce acceptors by chemical substitution failed to generate p-type ß-Ga2O, we present an alternative method of doping by introduction of gallium vacancy (VGa) defects by swift electron irradiation. To follow the evolution of free carrier concentration we measured the Hall coefficient of intentionally n-type doped crystals exposed to irradiation by 2.5 MeV electrons at low temperature (20K). The observed depression, proportional to the irradiation dose reflects the downward shift of the Fermi energy (EF) due to introduction of acceptor levels associated with VGa. Electron irradiation was realized on the Pelltron type accelerator of SIRIUS facility at the Ecole Polytechnique. To prevent defect migration and agglomeration, the sample was immersed in liquid hydrogen at 20K during irradiation. At an electron beam energy of 2.5 MeV, Rutherford collisions represent the main channel of energy transfer to the lattice of amount sufficient to create vacancy – interstitial, Frenkel pair. The migration energy of the interstitial is in general much lower than that of vacancies. For this reason, the interstitial annihilate during the transfer of the samples from the irradiation chamber to measurement platform, leaving behind exclusively vacancy type defects. The remaining defects are stable at room temperature, so that we do not observe any evolution of Hall coefficient over several months. We calculated the electron beam energy (Eb) dependence of the effective cross sections for defect creation on the oxygen and gallium sublattices and found that at Eb=2.5 MeV VGa outnumbers at least by factor two that of VO. In contrast, at low electron beam energy, below 800 keV, the rate of creation of VO is higher than that of VGa. Since theoretically VGa and VO are predicted to be respectively acceptor and donor type, tuning of the electron beam energy opens the possibility to shift EF down and up. Here we focus on demonstrating the acceptor character of VGa. We used two types of ß-Ga2O3 crystal, commercially available from Novel Crystal Technology: n-type Sn doped bulk substrates and epitaxial 50nm thick films. In pristine state, both materials show high mobility in the 100 cm2/Vs range. We carried out irradiations step by step irradiations interspersed with the measurement of Hall coefficient and magnetoresistance in function of temperature in 80-300K range. At constant temperature, we observed a linear decrease of the free carrier concentration with irradiation dose. The decline rate is comparable to defect formation rate, taking into account two processes: the capture of free carrier on VGa acceptor levels and the excess carriers generation from VO donors. The emperature dependence of carrier concentration is thermally activated with energy increasing with irradiation dose from 17 to 37 meV. Above some threshold dose, samples become insulating and remain in this state after exposure to doses exceeding it by a factor of 10 times. A decrease in carrier mobility precedes the transition to an insulating state, suggesting that increase of the compensation rate, due to simultaneous creation of donors and acceptors las the origin of transition.

Resume : β-Ga2O3, with its wide bandgap of 4.8 eV, high breakdown voltage, and excellent thermal stability, is a promising semiconductor for high power and high temperature electronics. Similar to mature semiconductor technology, in particular, in the silicon industry, ion implantation could greatly enhance the diversity of possible device designs, allowing selective area doping and isolation, as well as a control of dopant and defect profiles. The successful activation of implanted donor ions and deep acceptors has already been reported in the literature; however, little is known about the processes leading to implantation damage build-up, its annealing and the interaction of dopants and irradiation defects. Thanks to the wide bandgap, β-Ga2O3 is also an interesting host for optically active ions such as rare earth and transition metal ions, opening up its integration in photonic devices. In this work, we show the importance of irradiation defects on the incorporation and optical activation of these ions. The effect of ion implantation on the structural and optical properties of Ga2O3 will be discussed based on 300 keV Europium implantation at different temperatures. The narrow emission lines of this optically active ion are strongly dependent on the ion’s lattice site and local environment. Moreover, the possibility to assume different charge states (Eu2+ and Eu3+) allows additional information on dopant-defect interactions. Regarding transition metal doping, in-situ characterisation using iono-luminescence during proton irradiation revealed a strong enhancement of the Cr3+ emission efficiency in the red and infra-red spectral region. In contrast to Eu-dopants, a stable 3+ charge state was observed in as-grown and irradiated samples. Finally, a novel ion-beam technology to produce flexible and transferable Ga2O3 microtubes and sub-micrometer membranes and their potential for devices will be briefly discussed.

Resume : α-Ga2O3, one of the ultra-wide bandgap semiconductors, has attracted considerable attention because of its potential application as power devices. The dielectric properties of the compound are of great importance for the evaluation of practical performance as a device. However, researches on the dielectric properties of α-Ga2O3 are not so many[1], partly because of difficulty in preparing a bulk crystal. Terahertz time domain spectroscopy (THz-TDS) is a non-contact technique for investigating electrical properties such as dielectric constant, carrier mobility, and carrier concentration. This technique has been effectively utilized to evaluate electrical properties of semiconductor materials including GaN[2] and β-Ga2O3[3]. In this work, we have characterized the dielectric properties of α-Ga2O3 thin film by using the THz-TDS. α-Ga2O3 thin film was grown on M-plane sapphire substrate by using mist-CVD method. The transmitted THz time-domain spectra through the reference (without any samples), sapphire substrate and α-Ga2O3 thin film on sapphire substrate were measured with THz-TDS system (pluse IRS-2003). The α-Ga2O3 and sapphire (α-Al2O3), both of which have the corundum structure (R-3c), are uniaxial crystals with the optic axis along the c-axis ([0001] direction). Hence, we measured the THz spectra with electric field directions being perpendicular (⊥) and parallel (//) to the c-axis. The time-domain pulses of E//c have a longer delay time than that of E⊥c. This indicates that the larger refractive index along the direction parallel than perpendicular to the c-axis in both sapphire and α-Ga2O3. The THz time-domain spectra involve pulses due to the multiple reflection inside the sapphire substrate. Thus, the time-domain spectra, after cutting off the multiple reflection pulse, were Fourier transformed to obtain the frequency dependent complex THz spectra. The experimental complex transmission coefficient is given by the frequency dependent spectra of the reference and the sapphire or α-Ga2O3. The theoretical complex transmission coefficient is expressed by the complex refractive index and the thickness of sapphire or α-Ga2O3. By comparing the experimental and theoretical forms with the numerical method, the frequency dependent complex refractive index and dielectric properties of sapphire and α-Ga2O3 were extracted in THz regime. The dielectric properties were evaluated by fitting the first-order approximated Lorentz model to the experimental data. The static dielectric constant can be determined by extrapolating the Lorentz model to the frequency of 0. The static dielectric constant of α-Ga2O3 was estimated to be 10.9 and 15.6 along the direction perpendicular and parallel to the c-axis, respectively. [1] M. Stokey et al., Phys. Rev. Materials 6, 014601 (2022) [2] W. Zhang et al., Appl. Phys. Lett. 82, 2841 (2003) [3] V. Agulto et al., Appl. Phys. Lett. 4, 042101 (2021)

Resume : Gallium oxide (Ga2O3) showed great potential for development of electronic devices capable of handling high power or delivering very high blocking voltages, potentially extending the range currently covered by GaN and SiC-based technology and even allowing for new applications such as DC power grids, electric transport, and defence. This is due to the ultrawide bandgap energy ranging from ~4.9 – 5.2 eV depending on the chosen polytype and very high critical field of ~8 MV/cm. Monoclinic β-Ga2O3 also offers a competitive advantage over GaN and SiC – potentially cheap bulk single crystal synthesis using well-established melt-growth techniques, e.g. Czochralski (CZ), Vertical Bridgman (VB), floating zone (FZ) or edge-defined film-fed growth (EFG). The often-discussed significant disadvantage of Ga2O3 is relatively low thermal conductivity (TC) (11 – 27 W/mK for β-Ga2O3) and its anisotropy that will represent a bottleneck in the handling of device on-state heat generation. Failure to maintain a low on-state Ga2O3 device temperatures will result in their much faster degradation or will lead to lowering the dissipated power densities, clearly wasting the material potential of Ga2O3. Another thermal issue is related to the metastable Ga2O3 polytypes (e.g. rhombohedral α or hexagonal/orthorhombic ε/κ) which undergo phase transformation to the only stable monoclinic β phase at the elevated temperatures. Here, we discuss some of the possible approaches to improve the thermal performance of the Ga2O3 devices. We take advantage of the β-Ga2O3 TC anisotropy to improve the heat transport through the layer by growing of the (010)-oriented β-Ga2O3 using liquid-injection metal-organic chemical vapor deposition (LI-MOCVD) employing a 2-step growth method on the α-Ga2O3 templates prepared on the m-plane sapphire substrates. [010] crystal direction in β-Ga2O3 shows the highest TC value, i.e. ~27 W/mK which can be used for improved device cooling. We also demonstrate LI-MOCVD-enabled growth of β- and ε/κ-Ga2O3 on 4H-SiC substrates. High TC of SiC (~370 W/mK) offers greatly improved heat-spreading capabilities and has great potential for large-scale industrial application. Finally, we discuss thermal stability of epitaxial LI-MOCVD-grown layers of α-, β-, and ε/κ-Ga2O3 prepared on sapphire substrates. Thermal stability was investigated in-situ by means of X-ray diffraction and reasonable thermal processing window was found for all Ga2O3 polytypes.

Resume : Ga2O3 is a wide-bandgap semiconductor attractive for its potentialities in power electronics and sensing, its intrinsic UV-C spectral selectivity, and cost-effective growth methods. It has five known polymorphs: α, β, γ, δ, and κ, all of them characterized by an energy gap of about 4.5-5 eV. Most research has so far been focused on the thermodynamically stable β-Ga2O3 that allows for the growth of crystals from the melt as well as homo- and heteroepitaxial thin films [1]. However, its monoclinic low symmetry structure results in anisotropic physical properties, strong tendency to cleavage, which pose serious challenges for device manufacturing. Therefore, the interest on more symmetric Ga2O3 polymorphs is increasing, in particular on the orthorhombic κ-phase. The κ-polymorph can be epitaxially deposited on various substrates at lower growth temperature with respect to the β polymorph. Although intrinsically metastable, no variations in the crystallographic structure of κ-Ga2O3 are observed up to about 700°C, whereas a complete conversion to the β-phase occurs only above 900°C [2]. We have demonstrated that κ-Ga2O3 is a suitable material for solar-blind high-performance UV-C detectors [3]. The main reasons are: a high resistivity of nominally-undoped epitaxial layers, resulting in a very low dark current [4], spectral selectivity and reproducibility. Due to these properties, κ-Ga2O3 UV-C detectors may be applied e.g. to corona discharge monitoring, to prevent failure of ceramic insulators in high voltage electrical systems, and for detection of flames, for the early forest fire detection, or anti-fire surveillance in high-risk facilities. In this work the performances of a resistive UV-C photodetector based on κ-Ga2O3 epitaxial layers deposited by metal-organic vapor phase epitaxy on c-oriented Al2O3 will be discussed. A rejection ratio higher than 1E4, reproducible on-off switching times and a remarkable photo-gain (up to 1E3) were obtained. We propose a physical interpretation of such high photo-gain values, by critically considering two possible effects, both consistent with literature data [5,6]: (1) a strong trapping effect of free holes by deep levels inducing a majority carrier excess Δn with respect to the minority carriers Δp, (2) a mobility of holes much lower than that of electrons. A saturation of the photo-gain, dependent on detector size, was observed beyond a certain applied voltage, that is consistent with a minority carrier diffusion length of a few 1E-5 cm. The critical role of the contacts will also be discussed. [1] S. J. Pearton et al. Appl. Phys. Rev, 5, 011301 (2018) [2] I. Cora et al. Acta Mater. 183 216 (2020) [3] M. Pavesi et al. Mater. Chem. Phys. 205 502 (2018) [4] A. Parisini et al. Mater. Sci. Semicond. Process. 138, 106307 (2022) [5] F. Akyol, J. Appl. Phys. 127, 074501 (2020) [6] J. B. Varley et al. Phys. Rev. B 85, 081109 (2012)

Resume : The wide band-gap material gallium oxide (Ga2O3) has a multitude of potential applications, e.g., in gas sensors, transistors, and solar cells. To further improve such devices, it is crucial to know the energies of the charge-carrier transport levels. Such information can be derived by direct and inverse photoelectron spectroscopy (PES and IPES), but a detailed analysis is required for such studies on transparent conductive oxides (TCOs), to account for the unique transport and transparency properties simultaneously.1 In this work, we have investigated the electronic structures of sputtered Ga2O3, used as buffer layer in Cu(In,Ga)Se2 (CIGSe)-based thin-film solar cells2, and a β-Ga2O3 single crystal by means of laboratory-based x-ray and ultra-violet PES (XPS and UPS, respectively). The studies are combined with synchrotron-based hard x-ray PES (HAXPES) using the X-SPEC beamline3 at the KIT Light Source. We present PES spectra over a wide range of excitation energies (21 – 6,300 eV) to study the electronic surface structure with a wide range of electron inelastic mean free paths (and hence depth information). The experimental data is complemented by density-functional theory (DFT) calculations, which help us to determine the position of the valence band maximum of the Ga2O3 buffer layer and of the single crystal. Our results are also discussed in view of the device performance of Ga2O3/CIGSe-based thin-film solar cells. (1) Duncan, D. A.; Mendelsberg, R.; Mezher, M.; Horsley, K.; Rosenberg, S. G.; Blum, M.; Xiong, G.; Weinhardt, L.; Gloeckler, M.; Heske, C. A New Look at the Electronic Structure of Transparent Conductive Oxides—A Case Study of the Interface between Zinc Magnesium Oxide and Cadmium Telluride. Adv. Mater. Interfaces 2016, 3 (22), 1600418. https://doi.org/10.1002/admi.201600418. (2) Witte, W.; Paetel, S.; Menner, R.; Bauer, A.; Hariskos, D. The Application of Sputtered Gallium Oxide as Buffer for Cu(In,Ga)Se2 Solar Cells. Phys. Status Solidi RRL – Rapid Res. Lett. 2021, 15 (9), 2100180. https://doi.org/10.1002/pssr.202100180. (3) Weinhardt, L.; Steininger, R.; Kreikemeyer-Lorenzo, D.; Mangold, S.; Hauschild, D.; Batchelor, D.; Spangenberg, T.; Heske, C. X-SPEC: A 70 eV to 15 keV Undulator Beamline for X-Ray and Electron Spectroscopies. J. Synchrotron Radiat. 2021, 28 (2), 609–617. https://doi.org/10.1107/S1600577520016318.

Resume : We have developed a new approach to β-Ga2O3 epitaxy based on the hot-wall MOCVD reactor [1]. Compared to the conventional cold-wall MOCVD with heated substrate only, the hot-wall MOCVD employs a heated susceptor providing uniform temperature distribution in the growth zone vertically and laterally, which facilitates high cracking efficiency of the precursors. An upgraded custom-built horizontal hot-wall MOCVD reactor was used for β-Ga2O3 growth [1]. The material quality was elucidated through atomic force microscopy, spectroscopic ellipsometry, cathodoluminescence and scanning transmission electron microscopy. The interplay between growth regimes and properties of homoepitaxial β-Ga2O3 layers is discussed and a process for state-of-the-art material on sapphire substrates is established and successfully transferred to homoepitaxy. Instead of widely employed annealing in oxygen to make the surface epi-ready for homoepitaxial growth we have developed annealing processes in Ar atmosphere. The substrate surface preparation via in-situ and ex-situ annealing routes will be presented and discussed. Excellent quality single-crystalline β-Ga2O3 material has been attained on β-Ga2O3 substrates with (010), (001) and (-201) surface orientations. The hot-wall reactor employed provides industry-relevant growth rates (≥1 µm/h) due to the high efficiency of the chemical reactions as well as high sample uniformity. Therefore, this new approach to β-Ga2O3 epitaxy has a large potential for cost-effective industrial applications upon additional fine tuning of the growth regimes and reactor design optimization and up scaling. Furthermore, we introduce frequency-domain Terahertz Electron Paramagnetic Resonance (EPR) ellipsometry as a new tool to study defects in β-Ga2O3 and related materials at very high magnetic fields and very high frequencies. For first investigations, we use our previously developed optical Hall effect setup [2]. We recently demonstrated this new approach analyzing the polarized spin response for the nitrogen defect in SiC [3]. Here, we investigate defects in Fe-doped β-Ga2O3, whose large range of spin signatures strongly vary with crystal orientation, THz frequency, and magnetic field parameters. We obtain the anisotropic g-factor as well as the zero-field Hamiltonian up to fourth order which allows to discuss the relevance of the monoclinic character of the local site symmetry. We compare our results with present knowledge from theory computation approaches. We further discuss the influence of phonons, strain, and local crystal symmetry, and we predict THz EPR ellipsometry as a new tool with potential for characterization of defects in heteroepitaxial systems. References [1] D. Gogova et al., AIP Adv. 12, 055022 (2022). [2] P. Kühne et al., IEEE Trans. Terahertz Sci. Technol. 8(3), 257 (2018). [3] M. Schubert et al., Appl. Phys Lett. 120, 102101 (2022).

Resume : The technological evolution of high power components involves a strong increase in power densities. This leads to very high temperature fields and gradients in the components. It is necessary to know the thermal constraints because efficiency and reliability are strongly influenced by device self-heating on the micron scale. For example local temperature hotspots on the chip can indicate areas of high power density and possible failure locations. The temperature mapping can also be used for the dimensioning elements of the converters and is at the centre of the designers' concerns. CCD-based thermoreflectance microscopy technique has a very interesting potential to be used as a high-resolution non-invasive technique for quick failure analysis and temperature profiling of the devices under operating conditions. Thermoreflectance principle is based on measurement of the relative change in the reflectivity of a sample (device) surface as a function of change in temperature. As the temperature of the sample changes, the refractive index, and therefore the reflectivity, varies. The relation between the relative change in reflectivity and the temperature change is linear with a proportional coefficient named K thermoreflectance calibration coefficient. K is typically of the order of 10e-5 – 10e-2 (1/Kelvin) and depends on the sample material, the wavelength of the illuminating light, the angle of incidence (and thus, by extension, the surface roughness) and the composition of the sample. We have developed a thermoreflectance bench and implemented an experimental methodology and images post processing in order to optimize the measurements carried out on the power components We will highlight the problem of registration required by the use of thermoreflectance. Our results demonstrate that subpixel cross-correlation registration with a precise step gives the best results and that this step is directly correlated with the noise level of the images. In the second part, we will present a measurement methodology for power components. This methodology aims to determine several experimental parameters (the optimal wavelength, the minimum excitation frequency and the influence of image accumulation on the noise) and to choose the most suitable calibration method for this type of components. All this allows to perform a detailed thermal measurement on the metallization of Insulated Gate Bipolar Transistor (IGBT) emitter packaged in a power module. Finally, we will demonstrate that it is possible to perform thermal measurements of power chip through silicone protective gel and study the influence of aging on the thermal mapping. The thermal measurements performed have shown the ability of thermoreflectance to obtain a high spatial resolution temperature map. Dynamics thermal measurements will be also exposed.

Resume : One of the promising ternary alloys is ZnCdO, which modulates the band gap of a pure ZnO from ~3.3 eV to ~1.9 eV as well as expands the radiation range from UV to the visible range, and as a result, significantly expands the functionality of devices and elements made on ZnO-basis[1]. The growth of high-quality ZnCdO films is complicated by the different crystalline phase of ZnO and CdO (wurtzite and rock salt, respectively). However, the deposition of quasi-ternary alloys based on ZnO/CdO superlattices (SLs) can significantly reduce the number of defects in the samples, thereby increase the efficiency of ZnCdO-based devices[2]. In this work we present the results of investigation of {ZnO/CdO}30 SLs deposited on (10-10) m-plane sapphire substrate (Al2O3) at different growth temperature within 360 – 550 ◦C by PA-MBE. The effect of the growth temperature (Tgr) on the structural, morphology and optical properties of SLs were examined by transmission electron microscopy (TEM), X-ray Diffraction techniques (XRD), as well as by temperature dependent photoluminescence measurements (PL). In the obtained samples, the main peaks at 10.0 and 20.0 correspond to SLs with the wurtzite phase, but the low intensity peak (220) originating from the CdO-cubic phase is also observed. The high quality of the obtained SLs was confirmed by the presence of satellite peaks from (10.0) on the XRD scan. The number of satellite peaks decreases with increasing Tgr, which indicates a deterioration in the crystallographic quality of the {ZnO/CdO}30 SLs. Satellite structures are also clearly visible on symmetrical (10.0) reciprocal space maps (RSM). The X'Pert Epitaxy software was used to simulate the XRD data and determine both the SLs period and the thickness of the individual ZnO and CdO layers. The obtained SLs period varies in the range 24-32 nm depending on the growth temperature and corresponds well to the SLs period determined from the TEM images. In the as grown SLs samples dominate emission is observed in the UV region at about 3.36 eV. In contrast, in all annealed samples emission peaks at about 3.31-3.33 eV are observed and also broad band of different intensity are detected. The luminescence spectra of annealed films strongly depends on the growth temperature applied. The experimental data demonstrated that the growth temperature is of great importance for the quality of the obtaining {ZnO/CdO}30 SLs, as well as their properties. This work was supported in part by the Polish National Science Center, Grants No. 2019/35/B/ST8/01937, and 2021/41/B/ST5/00216. [1]. J. Zúñiga-Pérez, Materials Science in Semiconductor Processing, 69, 36-43 (2017). [2]. E. Przeździecka et al., Crystal Growth & Design, (2022)

Resume : Recently, ultra-wide bandgap (UWBG) semiconductor β-Ga2O3 has been investigated as a novel platform for extending the current limits of power electronics and deep ultraviolet optoelectronics. However, the p-type conductivity is well-known to be a challenge in UWBG materials, finding effective acceptor dopants for Ga2O3 is thus a central issue. Zn was demonstrated to be a deep acceptor in β-Ga2O3 [1–3] in previous works, but the direct evidence of hole conductivity is not reported. However, due to its smaller atomic radius (137 pm) in comparison with that of Ga (153 pm), Zn can also occupy an interstitial position, thus acting as a donor. This amphoteric nature of Zn could be an origin of the possible so-called « impurity auto-compensation effect » [4]. The objective of this work is to determine the growth parameters when Zn related impurity conduction can take place and determine the ionization energy of Zn related acceptor. Zn doped Ga2O3 thin films were grown by the Metal-Organic Chemical Vapor deposition (MOCVD) technique on c-sapphire substrates. Estimated [Zn] concentration by secondary-ion mass spectrometry (SIMS) is 1016 at/cm-3. The optical absorption onset starts at ~4.6 eV, and the obtained bandgap energies by using the Tauc’s relationship were estimated to be E_g = 5.0 eV for the samples [5]. We performed electrical transport measurements in Van Der Pauw configuration in the temperature range of 500 – 830 K temperature range, the resistivity ρ = 5.4 ×106 – 3.6 ×103 Ω·cm for undoped Ga2O3 sample, while ρ = 1.3 ×105 – 3.4 ×102 Ω·cm for Zn doped Ga2O3 thin film. The Hall Effect measurements at T=830 K showed a hole concentration p = 4.7 ×1013 cm-3 for undoped Ga2O3 sample, which increases almost by 2 orders of magnitude up to p =2.3 ×1015 cm-3 with Zn incorporation. The acceptor ionization energy was estimated as 0.77 eV. We believe that Zn substitutes gallium atoms (ZnGa) thus creating an acceptor level at E_i = 0.77 eV which is completely consistent with the thermal activation energy of 0.65 eV and 0.78 eV determined by EPR spectroscopy for the two ZnGa sites [1]. Reference [1] T.D. Gustafson, J. Jesenovec, C.A. Lenyk, N.C. Giles, J.S. McCloy, M.D. McCluskey, L.E. Halliburton, Zn acceptors in β-Ga2O3 crystals, J. Appl. Phys. 129, 155701, (2021). [2] J. Jesenovec, J. Varley, S.E. Karcher, J.S. McCloy, Electronic and optical properties of Zn-doped β-Ga2O3 Czochralski single crystals, J. Appl. Phys. 129, 225702, (2021). [3] D. Skachkov, W.R.L. Lambrecht, Computational study of Electron Paramagnetic Resonance parameters for Mg and Zn impurities in β-Ga2O3, Appl. Phys. Lett. 114, 202102, (2019). [4] E. Chikoidze, T. Tchelidze, C. Sartel, Z. Chi, R. Kabouche, I. Madaci, C. Rubio, H. Mohamed, V. Sallet, F. Medjdoub, A. Perez-Tomas, Y. Dumont, Ultra-high critical electric field of 13.2 MV/cm for Zn-doped p-type β-Ga2O3, 15, 100263, (2020). [5] E. Chikoidze, C. Sartel, H. Yamano, Z. Chi, G. Bouchez, F. Jomard, V. Sallet, G. Guillot, K. Boukheddaden, Amador Pérez-Tomás, Electrical properties of p-type Zn:Ga2O3 thin films. Journal of Vacuum Science & Technology A, In press. hal-03651802, (2022).

Resume : Gallium oxide (Ga2O3) has received great research interest due to its ultrawide bandgap (Eg ~ 4.5 ÷ 5.3 eV) and high theoretical critical electric field (~8 MV/cm), holding promise for processing of high-voltage and high-power electronic devices exceeding the capabilities of current semiconductor technologies. Ultrawide bandgap of Ga2O3 makes it also very attractive for optoelectronic devices including solar-blind photodetectors. Ga2O3 crystalizes in several crystal phases differing in crystal structure, bandgap, and other material properties. Apart from well-studied monoclinic (β) phase, a metastable hexagonal (ε) Ga2O3 has been predicted to show piezoelectric properties, which can give rise to future polarisation-engineered heterostrucutres similar to III-N based electronic devices. Recently, it has been shown that standard X-ray diffraction (XRD) is unable to distinguish between hexagonal and orthorhombic (κ) Ga2O3 phase unless detail analysis of high-resolution transmission electron microscopy (TEM) is employed (CrystEngComm, 2017, 19, 1509). In this work, we analysed crystal structure of ε/κ-Ga2O3 thin epitaxial films grown by liquid-injection metalorganic chemical vapor deposition (LI-MOCVD) on sapphire substrates using XRD and TEM methods. In particular, we performed detail analysis of the XRD φ scans, where an appropriate hkl diffraction inclined by an angle χ from the sample normal is examined. The hexagonal Ga2O3 has higher symmetry and the structure can be described by smaller unit cell, while lower symmetry and larger unit cell of the orthorhombic Ga2O3 results in larger number of accessible diffractions. However, for all measurable diffractions of the ε phase, one can found at least one diffraction of the κ phase with almost identical 2θ and χ angles. In contrast, identification of κ phase in Ga2O3 thin films can be more conclusive. We identified several diffractions in orthorhombic phase suitable for φ scans, which do not have a counterpart among the hexagonal phase. As an example, the φ scan of the 122 diffraction can serve as an indicator of the κ-Ga2O3 phase. Yet, the existence of the maxima in orthorhombic 122 φ scan can also imply the appearance of maxima in hexagonal 10-15 φ scan, due to other orthorhombic diffractions, regardless of the hexagonal phase presence. Therefore, the only way to detect the hexagonal phase is the disappearance of the orthorhombic phase, i.e. the Ga2O3 layer has to be single-phased. In this case, the maxima are detected only in hexagonal 10-15 φ scan, while no maxima should be detected in orthorhombic 122 φ scan. We will also discuss electron diffraction patterns obtained from plan-view and cross-section TEM observations.

Resume : Gallium oxide Ga2O3 has gained much attention because of its unique properties such as ultra-wide bandgap (~ 5eV), high breakdown field, which makes it very promising for application opto- and power electronics. However, in gallium oxide, like other semiconductor oxides, obtaining low ohmic hole conductivity is still challenging. The difficulty is connected to low hole mobility caused by the big effective mass of holes, high ionization energies of native and impurity acceptors, compensation processes (appearance of hole killer defects) stipulated by high band-gap, and low lying valence band. Nitrogen and magnesium substituting oxygen and gallium respectively are expected to lead to p-type conductivity. We fulfilled the thermodynamic analysis of the concentration equilibrium for nitrogen and magnesium doped gallium oxide using the Kroger method of quasi-chemical reactions, which gives one possibility to find the dependence of concentrations of defects and free carriers on growth/processing parameters and impurity concentration, and correspondingly to find their optimal values. The analysis was carried out for the system Ga2O3 doped samples – vapor of components. In Mg-doped samples, the main defect which can compensate hole conductivity must be oxygen vacancy or interstitial gallium. The concentration of interstitial Mg atoms, which also can be compensating donors is expected to be very low because of the big atomic radius. High enough oxygen partial pressure is needed on the one hand to suppress the formation of compensating donors and on the other hand, the provides the room for substitutional Mg atoms. The analysis shows that the hole concentration increases with increasing oxygen pressure and at some critical value holes and substitutional magnesium become dominant spices (for T=800 K, Pcr=0.7 atm). However further increase of oxygen pressure does not affect significantly hole concentration. Pcr decreases with temperature, but it does not depend on the total concentration of impurity atoms. For nitrogen hole doping we took into account the formation of the nitrogen molecule, which is expected to reduce the effect of doping. The formation energy of nitrogen molecules is quite high – 9.756 eV. In Gallium oxide taking into account dielectric screening with local field correction, dissociation energy turned out to be approximately 2 eV. Thermodynamic analysis shows that, for nitrogen doping, the dependence on oxygen pressure still exists, but it is less pronounced. The critical pressure above which negatively charged acceptors and holes are dominant spices is again around 1 atm. Based on these results one can conclude, that for Mg doping of gallium oxide, as well as for N doping the native donors - oxygen vacancy or Gallium interstitial are responsible for p-conductivity compensation. Remarkably, in both cases, the expected hole concentration exceeds the one observed in undoped samples [E. Chikoidze, et all., Mater. Today Phys. 3, 118–126 (2017)].

Resume : Ultrawide bandg-ap semiconductors such as aluminum gallium nitride AlxGa1xN have recently attracted attention as promising for quantum electronics due to Al-composition induced properties. The band-gap of AlxGa1xN can be varied from from 3 to more than 6 eV by tuninging the Al-composition. Eight-band wurtzite k.p Hamiltonian with efective mass method have been applied to calculate the energy subband structure and material gain for GaN/AlxGa1xN heterostructures. The efects of Al-composition are systematically analyzed and compared with the available results.

Resume : Nitride thin films provide a solid material platform for various applications in modern microtechnology owing to their outstanding physical properties and mechanical robustness, outperforming many oxides. AlN and especially its derivatives such as AlScN are widely employed as the piezoelectric layer in diverse piezo-micro-electro-mechanical systems (piezoMEMS) [1]. Nevertheless, Sc doping is sought to be substituted with more cost-efficient materials such as Yttrium for a manifold production scale-up [2]. In this work, we have investigated high-quality large-scale thin AlYN films with varied Y content deposited on 8’’ Si wafers by means of reactive magnetron co-sputtering. The co-sputtering technique requiring the synchronous sputtering of two, Al and Y, targets via a working gas mixture of Ar and N2 was optimized in a broad parameter space (power, pressure, substrate temperature, target to substrate distance). The films were characterized via SEM, XRD, AFM, EDS, and optical spectroscopy methods. The intricate, non-linear relations between the deposition parameters and the film properties revealed a dominant role of the sputtering power relation in the atomic composition of the films allowing us to obtain a wurtzite lattice with Y concentrations above 25%. Moreover, our results underline a critical role of a buffer interlayer (material) which prevents wurtzite-rock salt phase separations [3] within the nitride film. We show that residual film stress must be considered in large-scale growth as it promotes the formation of unwanted, non-wurtzite phases, deteriorating compositional film homogeneity. Piezoelectric properties of AlYN were characterized using double-beam laser interferometry, the results of which show a direct proportionality between the value of the longitudinal piezoelectric coefficient, d33, and the microstructure of the AlYN films deviating from the theoretical predictions [2]. Understanding the means and performance of Y doping in AlN helps establish Yttrium as a viable substitute for Sc in high-performance piezoMEMS with enhanced electro-mechanical coupling. [1] P. Muralt, Recent Progress in Materials Issues for Piezoelectric MEMS, J American Ceramic Society 91, 1385 (2008). [2] P. M. Mayrhofer, H. Riedl, H. Euchner, M. Stöger-Pollach, P. H. Mayrhofer, A. Bittner, and U. Schmid, Microstructure and Piezoelectric Response of YxAl1−xN Thin Films, Acta Materialia 100, 81 (2015). [3] C. Höglund, J. Birch, B. Alling, J. Bareño, Z. Czigány, P. O. Å. Persson, G. Wingqvist, A. Zukauskaite, and L. Hultman, Wurtzite Structure Sc1−xAlxN Solid Solution Films Grown by Reactive Magnetron Sputter Epitaxy: Structural Characterization and First-Principles Calculations, Journal of Applied Physics 107, 123515 (2010).

Resume : Metal oxide thin films are widely used in electronics and other fields. There are many methods for depositing metal oxide thin films using inexpensive solution processes, such as the sol-gel method, but depositing high-quality thin films at low temperatures is very difficult. Photochemical conversion of metal-organic precursors to metal oxide thin films using Vacuum Ultraviolet (VUV) irradiation is attracting attention as a new solution process method for metal oxide film formation at low temperatures. Employing polydimethylsiloxane (PDMS) and perhydropolysilazane (PHPS) as one-dimensional polymer precursors, void free and dense gas barrier films have been successfully fabricated by VUV irradiation [1]. Void free and dense thin films derived from the entanglement of precursor chains. With titanium oxide polymer precursor commonly known as polytitanoxane, we attempted to convert polytitanoxane precursor layers to void free and dense titanium oxide thin films by VUV irradiation in this study. Hydrazine monohydrochloride could control the rate of titanium alkoxide’s hydrolysis and condensation. Promoting one-dimensional growth, highly transparent polytitanoxane solution was fabricated [2]. The polytitanoxane precursor layers fabricated by spin-coating had a thickness of 198 nm and a refractive index of 1.65, whereas the thin films after 30 minutes of VUV irradiation had a thickness of 128 nm and a refractive index of 1.85. The thin films annealed at 500°C in air as a reference had a thickness of 74 nm and a refractive index of 2.04. The decrease in thickness and increase in refractive index after both VUV irradiation and annealing were attributed to the formation of titanium oxide as the organic ligands were removed. VUV irradiation could make thin films dense only on the surface [3], while annealing could make the entire thin films dense. Therefore, the thin films after annealing exhibited a higher refractive index than the VUV-irradiated films. VUV irradiation formed different thin film structure from annealing. Employing the monomer titanium alkoxides was used as a precursor, the VUV-irradiated films had a thickness of 156 nm and a refractive index of 1.74, the annealed thin films had a thickness of 82 nm and a refractive index of 1.91. After VUV irradiation and after annealing, the refractive index was lower than that of the polymer precursor, polytitanoxane. The polymer precursor formed higher refractive index films than the monomer precursor in titanium oxide was indicated. From SEM image of VUV-irradiated films, no voids were observed in the thin films. In conclusion, the polytitanoxane precursor layers were converted to void free and dense titanium oxide thin films by VUV irradiation at low temperatures. [1] Lina Sun et al., ACS Appl. Mater. Interfaces, 2019, 11,43425−43432. [2] Wataru Shimizu et al., Langmuir, 2012, 28, 12245−12255. [3] Tatsuki Sasaki et al., ACS Appl. Nano Mater. 2021, 4, 10344−10353

Resume : Zinc oxide (ZnO) is the subject of intense research due to its applications in optoelectronics, mainly in devices emitting light in blue and UV spectral range. A wide bandgap (3.37 eV at room temperature) of ZnO allows to adjust its emission properties through alloying it with selected elements. Among various elements, cadmium incorporation into ZnO serves the purpose of bandgap narrowing efficiently, because of lower bandgap of CdO (2.1 eV) as compared to ZnO. Therefore, ZnCdO/ZnO multiple quantum wells (MQWs) nanowires on silicon substrates, might find applications as laser sources and other oxide-based quantum devices. In our work we report on the structural and luminescence properties of the aforementioned ZnCdO/ZnO MQWs nanowires grown on Si substrate. The nanowires were prepared by molecular beam epitaxy (MBE) with a 250 nm thick ZnO buffer layer deposited at 380°C. The 30 periods of ZnO/ZnCdO MQWs located at ZnO nanowires were grown at 170°C without employing a catalyst such as Au or Ag at low temperatures in oxide rich conditions. The thickness of barriers and quantum wells was kept constant at 2 nm. The structure of nanowires has been characterized using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Optical properties of the structures were studied by photoluminescence technique. At low temperatures, the luminescence peaks near band edge and in the blue region were observed. The structures are hexagonal and polar. A cubic phase has not been registered. The analysis of the XRD data yielded the SL mean period equals to (52.5 ± 0.1) Å. From 2θ/ω scans the averaged c lattice parameter of SL is cSL = 5.2115 Å. The mean period calculated from XRD measurements equals to (40 ± 10) Å. Moreover, the XRD experiments show a broadening of XRD signal on RSMs. It can be due to micro-strain generated in nanowires structure caused by coalescence of nanowires. Coalescence of nanowires is visible on the SEM image [1]. The micro-Raman spectra obtained with 514.5 nm excitation wavelength show phonon modes originating from the Si substrate as well as the ZnO layer of ZnCdO/ZnO heterostructures. The positions of particular phonon modes are shifted towards lower frequencies at about 2-3 cm-1 with respect to the positions of the same phonon modes in Raman spectrum of bulk ZnO [2]. A red-shift of the ZnO-like phonon modes may indicate a tensile type of strain in the ZnCdO/ZnO sample, which can be caused by the presence of Cd in ZnCdO alloy of ZnCdO/ZnO MQWs or it may be due to coalescence of nanowires as well. [1] M.A. Pietrzyk, et al., Sensors Actuat. A – Phys. 315 (2020) 112305 [2] E. Zielony et al., Appl. Surf. Sci. 538 (2021) 148061 This work was supported in part by the Polish National Science Center, Grants No. 2019/35/B/ST8/01937, and 2021/41/B/ST5/00216.

Resume : Ion channelling is a powerful experimental technique used to analyze structural defects in single crystals, especially those created by ion implantation. However, measured energy spectra contain combined signals coming from different kinds of defects. Their separation requires the use of advanced computational methods, e.g., Monte Carlo simulations. One of the codes successfully used for that purpose is called McChasy-1.0. The software is capable to simulate trajectories of light ions (He, H) in small structures (up to hundreds of atoms) to fit the experimental channelling spectra. The presence of the lattice distortion is modelled in situ, i.e., during the ongoing simulations. The code deforms the structures based on implemented defect models and given depth distributions of defects. The most recent and unique achievements of the McChasy-1.0 code are the models of extended defects, namely dislocations and dislocation loops. The model of dislocations assumes that atomic planes adjacent to an extra half-plane of dislocation are bent following the arctan function. Hence, geometrical parameters of dislocations must be determined for a given structure. To date, coefficients of the arctan function have been determined using high-resolution Transmission Electron Microscopy (HRTEM) for several structures only (AlGaN, SrTiO3, and ZnO). It was also shown that the parameters decrease with the distance from a dislocation leading to the smoothing out of the bent planes [1]. Here we report the current state of the McChasy-1.0 code and describe the principle of the simulation procedure in the presence of point and extended defects. Moreover, having in mind that the HRTEM analysis is expensive and destructive for samples, we discuss another way to determine the dislocation parameters based on Molecular Dynamics (the LAMMPS code). We model edge dislocations and dislocation loops in hexagonal structures of zinc oxide (ZnO) and gallium nitride (GaN), which are wind band-gap semiconductors promising for a new generation of electronic and optical applications. The coefficients of the arctan function found this way are then used in the McChasy-1.0 code to fit channelling spectra recorded for ion-bombarded ZnO and GaN crystals. Moreover, both Ga-polar and N-polar orientations of GaN are studied to assess the role of the intrinsic electric field in defect creation and transformation. Defect profiles are presented and discussed as a result. [1] P. Jozwik, L. Nowicki, R. Ratajczak, A. Stonert, C. Mieszczynski, A. Turos, K. Morawiec, K. Lorenz, and E. Alves, Monte Carlo Simulations of Ion Channeling in Crystals Containing Dislocations and Randomly Displaced Atoms, Journal of Applied Physics 126, 195107 (2019).

Resume : Gallium oxide Ga2O3 has recently attracted a lot of scientific attention as a prospective ultra-wide bandgap semiconductor. It has five different polytypes α, β, δ, γ and ε among which the most studied and thermodynamically stable phase is β-Ga2O3. However, corundum-structured α-Ga2O3 with 5.2 eV bandgap is a better alternative for power electronics and ultraviolet optoelectronics applications. α-Ga2O3 is a metastable phase and it cannot be obtained as bulk crystals used for homoepitaxial growth. On the other hand, sapphire (α-Al2O3) is a convenient substrate for heteroepitaxy since some of its crystalline planes have a small lattice mismatch with α-Ga2O3, but there are only few and recent reports on the use of other orientation sapphire substrates than c-plane. In this work, we demonstrate growth of α-Ga2O3 thin films on a-, r- and m-plane sapphire wafers at various substrate temperatures via two different methods – reactive magnetron sputtering and pulsed laser deposition. Crystalline structure, elemental composition, surface morphology and optical properties were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, atomic force microscopy and UV-VIS measurements. α-phase stability dependence on film thickness was also investigated. Such epitaxial stabilization of high-quality thin films with commonly used deposition methods is a perspective way how to integrate α-Ga2O3 on available substrates. The financial support of Latvian Council of Science FLPP project LZP-2020/1-0345 is greatly acknowledged.

Resume : The precise control of concentration and depth distribution of dopant provided during the introduction of rare-earth (RE) into the ZnO matrix by ion implantation is a very promising technique for the production of monochromatic light sources, displays, and other optoelectronic devices with emission in the visible region. However, an important limitation of this technique is the build-up of lattice disorder due to the ballistic nature of the process. Usually, structural defects are unwanted because they quench the luminescence and adversely affect the lifetime of devices based on defected materials. Additionally, in the as-implanted stage, most dopants remain optically inactive and thus post-implantation annealing is necessary. The annealing issue turns out to be non-trivial because of the diffusion effect of RE ion and their precipitation on the surface, due to the low solubility limit, where RE ions become optical inactive [1-2]. In this paper, a comprehensive approach aiming to obtain the most efficient luminescence of RE3+ ions into the ZnO matrix is presented. We compare the deep and shallow implantation, implantations performed at room and high temperature, and finally, the post-implantation thermal annealing performed at O2, N2, and Ar atmospheres. The damage build-up in the ZnO lattice after RE ion bombardment, the structure recovery, and the RE lattice site location before and after annealing were monitored by the Channelling Rutherford Backscattering Spectrometry (RBS/c). The optical response was studied by photoluminescence (PL) spectroscopy. In the studies, the Yb ions were chosen because the optical response from Yb3+ is in the infrared region where the band-gap and defect-related PL emission are not present. Our studies show that only keeping the balance between the perfect recovery of the crystal and a stable impurity depth profile is a reasonable solution. The Yb-implantation of ZnO performed at RT and subsequent thermal annealing at O2 at 800 C for 10 min result in the most efficient optical system based on ZnO doped with RE ions, and the emission is so bright that can be observed by the naked eye. Reference: [1] R. Ratajczak et al. J. App. Phys, 121, 075101 (2017). [2] R. Ratajczak, Thin Solid Films, 643, 24 (2017). Acknowledgments This research was carried out under the co-financed international project supported by the Polish Ministry of Education and Science (5134/HZDR/2020/0) and Helmholtz-Zentrum Dresden-Rossendorf (20001987-ST, 21002630-ST, 21002632-ST)

Resume : The interest to zinc orthotitanate, Zn2TiO4, a wide band gap semiconductor of inverse spinel structure has been motivated by its application in microwave dielectrics, photocatalysts, pigments, gas sensors, etc. The doping with Mn often used to improve quality factor of Zn2TiO4 also produces its coloration and causes intense photoluminescence (PL) in the red spectral range. The PL originates from optical transitions of Mn4+ ion substituted Ti4+ cites. The PL band is peaked at 680 nm and can be easy excited by UV or blue light making this material attractive as the low cost and environmental safety red phosphor. In this work, the peculiarities of Mn defect formation in Zn2TiO4 synthesized by conventional solid state reaction have been investigated by optical and electron paramagnetic resonance (EPR) methods. In particular, the influence of rutile and anatase TiO2 on Mn charge state in Zn2TiO4 has been studied. The Mn-doped Zn2TiO4 ceramics was obtained by sintering in the air at temperatures of 900-1200ºC of ZnO and TiO2 powders where TiO2 was either rutile or anatase. The MnSO4 aqueous solution was added to provide Mn content of about 0.1 mol.%. All ceramics showed wide absorption band in 400-600 nm spectral range and Mn4+-related red PL. The intensity of absorption increased with the increase of annealing temperature being larger for the ceramics produced using anatase. At the same time, the ceramics made of rutile showed about twice larger intensity of the Mn4+-related PL. It is decreased in several times as the annealing temperature increased up to 1100 ºC for the samples made of anatase and 1200 ºC for those made of rutile. The PL showed decay time of about 150 μs which decreased slightly when the annealing temperature increased. No other PL bands were recorded under UV or blue light excitation. In the EPR spectra, the signal ascribed to Mn2+ ion in Zn2TiO4 was identified and the spin-Hamiltonian parameters of the center were determined. No EPR signal that could be ascribed to Mn4+ ion in Zn2TiO4 was detected. The intensity of Mn2+-related EPR signal was larger for the ceramics made of anatase. It increased sharply tenfold as the annealing temperature increased up to 1100ºC for ceramics made of anatase and 1200ºC for those made of rutile. It is proposed that the Mn2+ and Mn4+ centers are formed in Zn2TiO4 simultaneously, and Mn tends to incorporate as Mn4+ ion at lower sintering temperatures and as Mn2+ at higher. Both these defects contribute to optical absorption and are responsible for pronounced yellow-brownish coloration of the ceramics. The rutile is supposed to be more preferred to generate Mn4+ centers in Zn2TiO4, while anatase - to create Mn2+ defects.

Resume : Aiming for optimizing the performance of ZnO-related material devices, the crucial point is to grow high-quality ZnO-based thin films. The development of growth techniques for high-quality films is highly required. Among the various kinds of deposition methods, the physical vapor deposition techniques generally demanded high vacuum situation and caused very large energy consumption. On the other hand, the mist chemical vapor deposition (mist-CVD) technique, which is one of the spray pyrolysis related systems under atmospheric ambiance, has successfully achieved the high-quality thin films deposition for its various advantages. For example, mist-CVD system could be capable of easily manipulating the different component ratios of the solution source by setting different carrier gas/dilution gas (c.g. / d.g.), and easily depositing the multilayers and devices for its separated solution chambers, etc. In our presentation at the international conference [1], we succeed in the fabrication of the heterojunctions of ZnMgO/AgxO, and was found that the quality of the ZnMgO layer profoundly influences the device performance [2]. Meanwhile, the studies based on 3rd generation of mist-CVD system that can supply the precursors separately for ZnMgO deposition was found and verified that setting different Mg carrier gas/dilution gas can precisely control the Mg component ratios, and the increasing of Mg carrier gas flow rate led to higher incorporation of Mg atom in the grown films at 400℃, and the morphology and crystallinity of ZnMgO films were extensively impacted by the Mg content [3]. There are still lots of issues that need to be investigated for the improvement in characteristics of ZnMgO. Currently, we are investigating the dependence of [H2O]/{[Zn] [Mg]} supply ratio on the characteristics of ZnMgO. Even though we had already reported on the changes of ZnO characteristics with the supply ratio of [H2O]/[Zn] [4], after introducing Mg the properties of ZnMgO have changed very much. The results indicate that the H2O supply amount was influenced by Mg c.g./d.g., at the same content of Mg, the optical band gaps of ZnMgO films were widened from 3.22 to 3.8 eV by increasing the H2O concentration; the supply ratio of [H2O]/{[Zn] [Mg]} has strongly impacted on the growth rate and crystal orientations; and at the same supply ratio of [H2O]/{[Zn] [Mg]}, after adding the support oxidant O3, the resistivity increased from 3.91 × 107 Ωcm (Mg c.g./d.g.= 2.0/3.0) under without O3, and decreased to 2.4 × 102 Ωcm (Mg c.g./d.g.= 2.0/3.0) under with O3. The details about the fabrication of ZnMgO films by mist-CVD, the dependence of [H2O]/{[Zn] [Mg]} supply ratio on ZnMgO properties, the influence of support O3 gas on ZnMgO properties, and mechanism of growth of ZnMgO films will be specifically presented in the EMRS conference. References [1] Xiaojiao Liu, Giang T. Dang, Li Liu, Tatsuya Yasuoka, Yoshiro Kawanishi, Toshiyuki Kawaharamura, Solid State Devices and Materials (SSDM 2021), J-6-06 [2] Xiaojiao Liu, Giang T. Dang, Li Liu, Toshiyuki Kawaharamura, Applied Surface Science 596 (2022) 153465. [3] P. Rutthongjan, L. Liu, M. Nishi, M. Sakamoto, S. Sato, G. T Dang, and T. Kawaharamura, Jpn. J. Appl. Phys. 58 (2019) 035503. [4] Phimolphan Rutthongjan, Misaki Nishi, Li Liu, Shota Sato, Yuya Okada, Giang T. Dang, Toshiyuki KAWAHARAMURA, Appl. Phys. Express 12 (2019) 065505.

Resume : Over the last few decades, the optical properties of nanoscale objects have been intensively investigated. The reason for this is the size dependence of electronic structure and connected to this the possibility of engineering optical parameters in a wide range. The excitons are main intrinsic emitters in short wavelength region in semiconductors and therefore, optimization of excitonic emission is very important for emitting device fabrication. Gallium oxide is very promising for optical-electronics material due to its ultra-high band gap. We present a theoretical approach to calculate the electronic state of excitons and biexciton in gallium oxide nanowires (NW) and nanotubes in the framework of the effective-mass model using the Born-Oppenheimer approximation. We consider the formation of excitons and biexcitons under the action of both the lateral confinement and the Coulomb potential. The analytical expressions for the binding energy and eigenfunctions of the excitons and biexciton are obtained in dependence on system geometry. The approach is based on the fact that the confinement effect is stronger than the Coulomb term. The Coulomb term is significant only along nanowire/nanotube axes (z-axes). That is why to calculate exciton states in the presence of space confinement we averaged the Coulomb potential over the lateral (perpendicular to nanowire/nanotube axes) wave functions of electrons and holes. This procedure reduces the dimensionality of Coulomb potential to one, and the 1D Schrödinger equation is solved in the frame of variational technique. Having known single exciton states biexciton states are calculated using Born-Oppenheimer approximation. The calculations reveal strong dependence of binding energy of excitons and biexcitons on geometrical size for nanowires as well as for nanotubes.

Resume : Amorphous materials have seen substantial interest in recent times due to the significant benefits they can bring to electronic devices. One such material of promise is ZnxSn1-xOy (a-ZTO). a-ZTO, which is only composed of common and abundant materials, has been shown to possess high mobilities even in an amorphous state [1]. In addition, the mobility and charge carrier concentration in the sample can be manipulated by the intercation ratio in the material. Already a-ZTO has been implemented in a wide range of applications such as organic light emitting diodes (O-LEDs) [2], photovoltaic cells [3], and thin film transistors (TFTs) [4]. Considering the on-going effort to shrink these devices to increase their power density, the knowledge of how thickness influences the electronic properties of a-ZTO is critical, and to date missing from literature. In this work a combination of ellipsometry, in-situ scanning tunnelling microscopy/spectroscopy (STM/STS) and UV-vis spectrophotometry were employed to investigate the bandgap variation of a-ZTO as thickness was reduced. In vacuum (1e-8 mbar) transfer from Magnetron sputtering growth chamber to STM was employed to ensure uncontaminated measuring of the surface. A variation of 0.3 eV was observed as the thickness was reduced from 50 to 5 nm regardless of whether the bandgap was modelled as a direct or indirect transition. STS measurements support the hypothesis that the bandgap in the thin films depended upon the sample thickness, and also aligned significantly better with the assumption of an indirect rather than direct bandgap. X-Ray Photoelectron Spectroscopy measurements confirmed a consistent Zn/Sn ratio across the films, ruling this out as an explanation for the bandgap shift. References 1. Kim, Y. H., Han, J. I. & Park, S. K. Effect of Zinc/Tin composition ratio on the operational stability of solution-processed Zinc-Tin-Oxide Thin-Film transistors. IEEE Electron Device Lett. 33, 50–52 (2012). 2. Fioretti, A. N. & Morales-Masis, M. Bridging the p-type transparent conductive materials gap: synthesis approaches for disperse valence band materials. J. Photonics Energy 10, 1 (2020). 3. Pandey, R. et al. Fluorine doped zinc tin oxide multilayer transparent conducting Oxides for organic photovoltaic’s Cells. Sol. Energy Mater. Sol. Cells 134, 5–14 (2015). 4. Fakhri, M., Theisen, M., Behrendt, A., Gorrn, P. & Riedl, T. Top-gate zinc tin oxide thin-film transistors with high bias and environmental stress stability. Appl. Phys. Lett. 104, 251603-251603.5 (2014).

Resume : Providing Wi-Fi at speeds of 520 miles/h at 30,000 feet has proven to be difficult, yet it is estimated that the market for in-flight connectivity is likely to be more than USD 130 Billion in 2035. Demand is high, we all want higher data transfer and more stable connections not only at work and home but also when on the move! To achieve this, there are three key technology areas to focus on: 1) the satellite network quality and accessibility, 2) the antenna on top of a plane collecting and redistributing the satellite network signal, and 3) the protection of the antenna through a radome consisting of dedicated composite materials which allow constant service even under challenging conditions and different environments during a journey. Our work concerns the development of next-generation of radome materials which will be to achieve higher data throughput. Some systems which work at higher radio frequencies (RF) (Ka-band) exist, however, just using higher RF creates its own problems given that higher RF do not automatically travel through all materials. To maximise the signal, the radome material must be as transparent as possible to higher RF allowing uninterrupted transfer of radio waves. It is important to remember that, especially when working at higher frequencies, the signal will be impacted by the presence of water or ice on the radome. We have been focusing on finding a solution for this challenge by developing a suitable lightweight composite building block and material with outstanding dielectric properties. Such material should not be affected by electrical fields and should prevent any loss in frequency intensity, whilst simultaneously exhibiting mechanical strength against external environments and good thermal properties in a range of temperatures. Developing routes to apply heat throughout the radome will be essential to stop ice formation on the radome. Whilst there are material solutions out there, the question is whether they can be employed to build radomes. The material hexagonal boron nitride (h-BN) presents all of these outstanding properties. This poster presents the work on a novel synthesis route of a new h-BN foam structure - foam because it is a light material with open porosity that can easily be incorporated to form a composite. The foams can help to improve the mechanical and thermal properties of the composite. Moreover, foams are an interconnected web that can redistribute mechanical stress and heat throughout the entire structure. To make such foam, we developed a sacrificial templating method using multi-walled carbon nanotubes and graphene oxide nanomaterials as a template solution free of any additives. This method allowed the generation of a dedicated foam precursor without any chemical additives. Achievements hitherto are the successful construction of a 3D interconnected foam of desired chemistry which is crucial for the radome to be transparent to relevant frequencies. The next big step is scaling up the production of this material and we are in the process of solving it. Through the development of this process, we might be one step closer to Wi-Fi at 30,000 feet and (even faster?) than 520miles/h.

Resume : Heterojunction solar cells based on ZnMgO and Si employed as an emitter and absorber, respectively, have a potential theoretical efficiency of ~ 24% if the conduction band gap misalignment between ZnO and Si is eliminated by Mg alloying [1]. This approach has been also experimentally tested and it has been shown that an increase in efficiency from ~3.7% to ~7.1% can be achieved following this route [2]. However, as discussed in detail in Ref.2, further improvements in efficiency are hindered by the increase in resistivity of the ZnO based layer when the Mg content overcomes ~2-3 at.%. In the work presented the possibility of increasing the Mg content above ~2-3 at.%, while maintaining a low resistivity by Al doping, has been investigated. It has been found that, as expected, by keeping the Al content ~2 at.% up to ~12 at.% Mg can be incorporated into the ZnO layers with the films still maintaining excellent electrical properties: carrier concentration and mobility equal to ~2x10^20 cm^-3 and ~2 cm^2/V s. This permits to increase the open circuit voltage, VOC, of the test solar cells from ~0.33 V to ~0.43 V as consequence of the reduction in the conduction band misalignment. The test solar cells with the highest Mg content exhibits a VOC, short circuit current density, efficiency and fill factor equal to ~0.43 V, ~29 mA/cm^2, ~7.4 and ~60%, respectively. The low fill factor related to relatively high series resistance, >10 Ω as well as the low shunt resistance ~10 kΩ appears to be, at the moment, among the main limiting factors to achieve higher performances in the present structures. Acknowledgement The work has been performed within the National Science Centre project UMO-2016/22/E/ST3/00553. [1] K. E. Knutsen , R. Schifano et al. Phys. Status Solidi A 210, No. 3, 585–588 (2013) [2] R. Pietruszka, R. Schifano et al. Solar Energy Materials & Solar Cells 147, 164–170 (2016).

Resume : The fast rise of computational power was linked with the 4th Industrial revolution, which aims to insert automatization via AI of the production steps. When relating these advancements towards Pulsed Laser Deposition (PLD) we realize that the technique is at crossroads in terms of usability and implementation. Available experimental solutions are near impossible to replicate as the synthesis of thin films is strongly linked to the particularities of the laser beam, deposition geometry and working atmosphere. This work presents research performed towards developing high-quality oxide semiconducting materials by PLD technique (e.g., CuO2, MnO, NiO, AgO, SnO) as potential candidates for p-type semiconductors. Various deposition conditions were used in order to develop the deposition recipe and highlight optimum deposition conditions. Given the particularities of the PLD technique the angular distribution of the ablated particles kinetic energy plays an important role in the quality of the deposited films. On and off axis deposition geometries were implemented and the PLD process was monitored by optical emission spectroscopy and Langmuir Probe to investigate the formation of new oxide phases inside the plasma. The deposited films were investigated with Atomic Force Microscopy, Scanning Electron Microscopy, X-ray Diffraction, X-ray reflectivity, X-ray Photoelectron Spectroscopy and electrical measurements.

Resume : For enabling critical next generation technologies, significant interests have been directed toward next-generation ultra-wideband gap materials. These materials have various applications in photoinduced hydrogen production [1], energy harvester Micro- and Nano- electromechanical systems (MEMS / NEMS) [2], high-capacity charge storage [3], low loss transistors, power devices [4], optoelectronic devices [5], two-dimensional (2D) devices [6], neuromorphic [7], hybrid devices, nanophotonic and quantum technology applications. In order to move toward Ga2O3 wafer scale devices nanofabrication, there is a need for both n-type and p-type Ga2O3. The objective of the present work is to compare the effect of two acceptor dopants (cation and anion substitution) on optoelectronic properties of Gallium oxide thin films. For this purpose, we have grown Ga2O3 thin films with nitrogen and zinc both with incorporation as high as at 1021cm-3 concertation. Theoretical results [8] indicate that Nitrogen acts as a deep acceptor with an acceptor level at 1.33 eV above the valence band maximum (VBM). Ion implantation of N in Ga2O3 has been reported [9], showing that nitrogen acts as an efficient compensator for residual donors, with an additional photosensitive Mid-bandgap at ~2.47eV below the conduction band. It was recently reported [10], the nitrogen doped p-type Ga2O3 thin layers with Ea=0.165eV, ionization energy [11]. While for Zn It is known the it has preference for Ga tetrahedral site incorporation and showing 0.7eV ionization energy However, highly doped samples have not yet been investigated. N and Zn: Ga2O3 thin films were grown by Metal-Organic Chemical Vapor deposition (MOCVD) technique on sapphire substrates. Epilayer deposition was done in the conditions (pressure, temperature) which is already well known for us to obtain native hole conductivity in Ga2O3 [12]. In such a growth condition, ultra-high incorporation of acceptor dopants might be more efficient than direct implantation which damage the crystal matrix. Morphological and surface roughness characterization were carried out by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. We aimed to determine the solubility limit of nitrogen and zinc in β-Ga2O3 thin films by incorporating maximum concertation of dopant while keeping crystalline structure and the quality of epilayers. The secondary-ion mass spectrometry (SIMS) analysis and X-ray diffraction showed that solubility limit is 1021 at/cm-3. Such high level of incorporated dopants leads to the creation of an acceptor related impurity band for nitrogen doped sample. The optical spectroscopies were carried out in the 20K-700K temperature range. Furthermore, low temperature photoluminescence properties for highly doped samples were studied. At room temperature, the central peak emission for N:Ga2O3 and Zn:Ga2O3, were 1.81 eV and 2.58eV, respectively. This exhibits a significant shift for the nitrogen doped samples. Furthermore, optical bandgap absorption edge of nitrogen doped p-type Ga2O3 was estimated as E_g= 4.1 eV, which shows its absorption edge shift by about 0.8 eV if compared with undoped Ga2O3 thin films (E_g = 4.9 eV) grown in the same conditions, while no significant shift of band gap edge was observed for highly doped Zn:Ga2O3. Electrical transport properties were studied in detail with a Van Der Pauw configuration in 300 – 850 K temperature range and 0-1.6T magnetic field ranges. Resistivity versus temperature shows semiconducting behavior at room temperature. The Hall voltage measurements versus applied magnetic field, confirms p-type conductivity for both Zn and N doped samples with different resistivity and carrier concentrations. These results point toward highly doped p-type for bandgap tailoring and p-type conduction at room temperature in Ga2O3 for better contacts and versatile optically control gate for photonics and optoelectronics devices. References: [1] Yamakata, A. et al. ACS Catal. 11, 1911–1919 (2021). [2] Aleksandrova, M. et al Coatings 10, 650 (2020). [3] Li, D. et al. 46, 1025–1033 (2022). [4] Wong, M. H. et al. IEEE Transactions on Electron Devices 67, 3925–3937 (2020). [5] Chikoidze, E. et al. Mater. Today Phys. 3, 118–126 (2017). [6] Pérez-Tomás, A. et al, Oxide-based Materials and Devices XII vol. 11687 135–162 (SPIE, 2021). [7] Zhu, R. et al. Advanced Electronic Materials 8, 2100741 (2022). [8] Dong, L. et al. Journal of Alloys and Compounds 712, 379–385 (2017). [9] Nakanishi, M. et al. AIP Advances 11, 035237 (2021). [10] Mishra, A. et al. Appl. Phys. Lett. 117, 243505 (2020). [11] Wu, Z. Y. et al. Materials Today Physics 17, 100356 (2021). [12] Chikoidze, E. et al. C 7, 10231–10239 (2019).

Resume : For many years, diamond has been trying to find its place in the world of materials for power electronics. It is a wide band-gap material that competes with many other materials and, despite its remarkable intrinsic properties, competition is strong. Its theoretical voltage withstand, high carrier mobility and high thermal conductivity are to its advantage, while the difficulty of obtaining large-area substrates with low defects control is a limiting factor in its use. This paper will present the different steps leading to the fabrication of Schottky diodes by detailing the current limitations but also the successes obtained in recent years. It will discuss and attempt to be objective about the results in comparison with other materials.

Resume : Development of nitride based optoelectronic devices is supported by extensive computational efforts in several areas: simulations of molecular processes at semiconductor surfaces; incorporation of atoms into the solid phase, electric transport in the device structures, strain and spontaneous polarization and related electric fields in optically active structures, light emission and other recombination types. These subject require different analysis tools which will be discussed. Molecular processes require use of ab initio simulations in density functional theory (DFT) formulation. These basic techniques were recently supplemented by new aspects: electric field at surfaces and charge transfer contribution to adsorption energy. The subject was supplemented by incorporation of enthalpy and entropy contributions that allow to obtain pressure/temperature dependence of the surface state. Incorporation of atoms, including dopants was also investigated by ab initio methods. The calculations incorporating above mentioned field and charge effects provide deep insight into the semiconductor doping in the growth stage. That allows to determine the doping problems, especially acute in AlN-rich wide bandgap devices. The use of polarization doping allows to alleviate the doping problems these AlN-rich device structures.

Resume : This study reports on the use of thin films of polycrystalline diamond grown on doped silicon and titanium substrates for in vitro electrophysiological sensing devices. The electrical properties of the polycrystalline diamond surfaces were evaluated using electrochemical impedance spectroscopy, and electrical noise measurements. The polycrystalline diamond surface when immersed in the cell culture medium, establishes an electrical double-layer which in series with the bulk solution (cell culture medium) behaves as a classical Maxwell-Wagner relaxation with a relaxation frequency located at approximately 2 kHz. The low frequency (100 Hz) interfacial capacitance has a value of 10 µF/cm2. Furthermore, the interfacial resistance is low which minimizes the 1/f noise as well as the thermal noise, which is as low as 0.2 µV rms for a sensing electrode with an active area of 0.16 cm2. The high interfacial capacitance associated with the low thermal and 1/f noise makes these polycrystalline diamond coatings particularly suited to measure weak bioelectrical signals generated by non-electrogenic cells. Unlike neurons, non-electrogenic cells generated weak signals (micro-volts) in the mHz frequency range. The diamond coatings with different surface roughness were evaluated to record cell signals generated by the population of glial cells (C6 immortal cell line). Electrophysiological recordings were complemented with a detailed surface morphology analysis to gain insight into the best diamond morphology to minimize the low-frequency electrical noise and achieve bioelectrical recordings with a signal-to-noise ratio above 20 at frequencies as low as 1 Hz.

Resume : The epitaxial growth of high electron mobility transistors (HEMT) on bulk AlN substrates is very attractive to fabricate high-frequency and high-power switching devices. This is due to the high resistivity and good thermal conductivity of AlN as well as the possibility to manage at least in a given thickness or composition range the lattice parameter mismatch between AlGaN, GaN and the substrate. However, few studies have been dedicated to the effect of GaN channel thickness downscaling in Al(Ga)N/GaN HEMTs, especially on bulk AlN substrate. In a previous work, we investigated the effect of GaN channel thickness and AlGaN barrier composition on lateral breakdown voltage of HEMT structures grown by ammonia source molecular beam epitaxy on AlN-on-Sapphire. A similar trend on the three terminal breakdown voltage of transistors was noticed while reducing from 500 nm to 50 nm the thickness of the GaN channel grown on bulk AlN substrate. To go further in the understanding of this trend, HEMTs were grown on bulk AlN substrate with GaN channel thickness varying from 500 nm to 8 nm. Most of these structures contain a barrier layer consisting of a 1 nm AlN spacer layer plus a 19 nm AlGaN layer with a nominal Al content of 30% capped with a thin GaN layer. X-ray diffraction (XRD) shows that reducing the GaN channel down to 20 nm induces a broadening of the diffraction peaks. This indicates a degradation of the crystal quality which is consistent with a drop of the electron mobility. The change of the in-plane lattice parameter with channel thickness shows that the 20-50 nm region is the range corresponding to the most rapid change in strain relaxation rate. The cross-section transmission electron microscopy (TEM) analysis performed on 50 nm and 500 nm GaN channel HEMTs reveals the presence of a 20-30 nm thick contrasted region close to the GaN on AlN interface, with a large number of defects. The present observations indicate that downscaling the channel thickness increases the residual compressive strain in GaN at the expense of the rapprochement to a highly defective region. Nevertheless, the nucleation of defects at this interface is not likely occurring for thicknesses below a critical value estimated around 10 nm. Thus, HEMT structures with 8-9 nm GaN channels using 86% Al content AlGaN barriers are demonstrated with sheet resistances between 980 and 1300 Ohm/sq, which is promising for power switching applications. This work was supported by French technology facility network RENATECH and the French National Research Agency (ANR) through the projects BREAkuP (ANR-17-CE05-0013) and the “Investissements d’Avenir” program GaNeX (ANR-11-LABX-0014).

Resume : Gallium oxide (Ga2O3) is an ultra-wide bandgap semiconductor exhibiting a number of unique properties and promising applications for power and optoelectronics. Importantly, Ga2O3 can be crystallized in different polymorphs having different structure and, consequently, different physical properties that can be potentially exploited by gaining control over the phase transitions. In the present contribution we review our recent activity on radiation phenomena in Ga2O3. Specifically, we have used a combination of experimental methods with DFT calculations to investigate the response of monoclinic beta-Ga2O3 single crystals to ion bombardment. Exploring a wide range of experimental parameters, such as ion species, accumulated dose, ion energy, irradiation temperature and beam flux we demonstrate that ion-beam-induced disorder and associated strain affect the stability of beta-phase leading to the polymorph transformations. As an example, we demonstrated a controlable formation of highly-oriented single-phase orthorhombic κappa-Ga2O3 layer with sharp b/κ interface on the top of the beta-Ga2O3 wafer [1]. Our findings pave the way for a new synthesis technology of the metastable polymorph heterostructures and regularly shaped interfaces in the device components not atchivable by the conventional deposition methods. References: [1] A. Azarov, C. Bazioti, V. Venkatachalapathy, P. Vajeeston, E. Monakhov, and A. Kuznetsov, Phys. Rev. Lett. 128 (2022) 015704

Resume : Beta gallium oxide is an emerging wide bandgap semiconductor with bandgap (4.8 eV), good saturation velocity and a large predicted breakdown field up to 8 MV.cm-1. As shown in the Baliga’s Figure of Merit, the on-resistance of ß-Ga2O3 devices is expected to be lower than that of SiC or GaN at the same breakdown voltage. These properties make Ga2O3 a very attractive semiconductor for high voltage and radio-frequency applications. However, the technological advances of Ga2O3 are limited by the lack of control over defects, impurities and doping level which determine the electronic properties of devices. In the literature, electrical investigations of ß-Ga2O3 crystals grown by Cz and EFG (edge defined film fed growth) method were carried out using deep level transient spectroscopy (DLTS). Several deep levels have been observed, both intrinsic and extrinsic origins have been suggested. But precise identification is still expected. This work aims to describe and report on the electrical properties of ß-Ga2O3 crystals, doped by Si, grown by the Floating Zone method which is crucible free, in theory generates fewer impurities. Electrical characterizations were performed on Schottky diodes doped with Si. DLTS measurements were carried out by varying the parameters to observe the surface and bulk of the substrate distinctly. We observed at least six deep traps in the 77-550K range. The thermal activation energy and the apparent capture cross-section of the traps were extracted from Arrhenius plots: ES (EC – 0.31e eV), E1 (EC – 0.54 eV), E2 (EC – 0.77 eV), E3 (EC – 0.97e eV), E4 (EC – 1.1eV) and E5 (EC – 1.31 eV). Of these levels, ES occurs only near the surface with a high concentration. There have been several previous reports of E1, E2 and E3 and they were observed in substrates grown by EFG and Cz techniques. It is possible that the origin of these deep electron traps is the same since we find similar signatures in materials grown by different crystal growth methods. We also compared two diodes of the same sample. Static analysis and DLTS spectra showed that the higher the concentration of trap ES, the higher the saturation current and the lower the barrier height. Topological analysis of the substrate was carried out and AFM images revealed the presence of scratches on the surface which are expected to be produced by the mechanical polishing of the substrate. They are not evenly distributed on the surface and have different depths. We also carried out DLTS measurement on a diode which have undergone a different polishing cycle (chemical) and trap ES disappeared. We assumed the presence of these scratches could contribute to the defect Es.

Resume : Ga2O3 is a semiconductor with a large bandgap (~4.9 eV at room temperature), a high breakdown field (> 8 MV/cm), and a tunable conductivity from almost semi-insulating to highly conductive. Combining its electrical and optical properties, Ga2O3 has aroused interest in different electronic and optoelectronic applications, such as high-power electronics, UV photodetectors, solar cells, and sensors. Furthermore, and thanks to its monoclinic structure, Ga2O3 presents two easy cleavage planes, (100) and (001), that have been exploited to produce thin, flexible, and transferable nanomembranes with high crystalline quality by mechanical exfoliation. More recent works have been reported about the production of thin membranes based on ion implantation, that contrary to mechanical exfoliation using conventional scotch tape, the thickness of the obtained membranes can be tuned via the implantation energy. In this work, (100)-oriented membranes with different thicknesses in the range from 200 nm to 500 nm were produced using a process based on ion implantation. In more detail, these membranes result from the unrolling of Ga2O3 microtubes formed during implantation upon annealing at temperatures higher than 500 °C. In this work, the microtubes were produced by Cr implantation with different energies from 200 keV to 300 keV, with a fluence of 5x1014 ions/cm2. Using nanoindentation it was found that these thin membranes, produced by this innovative process, present a hardness and a Young Modulus comparable with the values measured by other authors for commercial single crystals produced by Novel Crystal Technology, Inc.. This study of the local physical properties by nanoindentation was complemented with a structural characterization by x-ray diffraction and Raman spectroscopy. Furthermore, a systematic study of the effects induced by argon irradiation on the surface potential, mechanical and morphological properties of these thin membranes will be presented. A correlation of the mechanical properties as a function of the thickness and defect profile will be discussed.