AlN and AlGaN materials and devices

Resume : Density-functional calculations concerning the structure and stability of wurtzite polar Al-terminated AlN(0001) surface are presented. Aluminum and nitrogen adatoms entail states in the bandgap, as well as Al dangling bonds. The Al-Al bonds related to Al adatoms are located in the lower part of the bandgap and are completely filled. The energy of Al broken bond-related band extends for about 2 eV. In the case of N adatom, four states emerge in the bandgap, two are Al-N bonding states, the other two are Al dangling bonds states located high in the bandgap. A quantum overlap repulsion between the conduction band states and aluminum dangling bond states increases the energies of the band states at the surface. The band bending at the surface indicates on an acceptor behavior for p-type and semi-insulating bulk and donor behavior for p-type of the Al broken bond state. Adsorption energy was determained for p- and n-type doping. Adsorption of N adatoms in its lowest energy sites i.e. H3 positions, transfers electrons from the Al broken bond states to the topmost N adatom states. For low nitrogen coverage, i.e. below 0.25 monolayer, the Fermi level is pinned by Al broken bond states located below the conduction band minimum. Adsorption of nitrogen depends on the Fermi level due to the involved charge transfer. For low N coverage the adsorption energy is very high and molecular nitrogen dissociates at the surface. Above 0.25 monolayer coverage the nitrogen adsorption energy gain is reduced as the Al broken bond is empty and energy gain from electron transfer is not possible. For such a coverage the adsorption of N2 is not dissociative and molecules are attached vertically on top of Al atoms. An equilibrium pressure of molecular nitrogen above an Al terminated AlN(0001) surface was determined. It was found that the pressure depends sharply on the Fermi level position, thus on the nitrogen coverage. Nitrogen adsorption mechanism leads to 2x2 reconstruction and the diffusion of N-adatoms is limited by relatively high barriers between lowest energy sites of order of 2 eV. The lowest Al adatom energy site is located in T4 position, and the diffusion barrier is no higher than 0.6 eV so more than three times lower than barrier for nitrogen diffusion. Additional chanel for N diffusion occurs at high Al coverage below the Al adatoms surface.

Resume : The majority of UV LEDs require AlGaN layers with compositions in the mid-range between AlN and GaN. There is a significant difference in the lattice parameters of GaN and AlN. Therefore AlGaN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlGaN bulk crystals by any standard bulk growth techniques has not been developed so far. There are very strong electric polarization fields inside the wurtzite group III-nitride structures. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende (Al)GaN bulk crystals by any standard bulk growth techniques were not successful. Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used the plasma-assisted molecular beam epitaxy (PA-MBE) technique for bulk crystal growth and have produced, for the first time, free-standing layers of zinc-blende and wurtzite AlGaN up to 100?m in thickness and up to 3-inch in diameter. Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

Resume : Despite the great progress made in AlGaN mid-UV emitters (λ<300nm) during several recent years, their output power and emission efficiency are still much poorer in comparison with those of InGaN-based photonics. The AlGaN structures suffer from low carrier localization efficiency, low injection efficiency due to the difficulty in p-type doping of high-Al-content AlGaN layers, and suppressed light extraction efficiency owing to the TM-polarized emission involvement and the UV light absorption in p-GaN injection layers. The structures grown on sapphire also possess rather high defect density which reduces the internal quantum efficiency. The paper considers possibilities provided by plasma-assisted molecular beam epitaxy (PA MBE) for fabrication of high quality AlGaN-based quantum well heterostructures on c-sapphire, demonstrating enhanced internal quantum efficiency (>50%), relatively low lasing threshold under optical pumping (<150kW/cm2) [1], and high light output at electron-beam pumping (160 mW) [2]. In particular, different advanced approaches to the AlN nucleation stage on c-sapphire, the AlN and AlGaN bulk layer PA MBE growth under metal-rich conditions, and the buffer structure design will be discussed, which enable one to lower density of the screw and edge threading dislocations down to ~10^8 and ~10^9 cm-2, respectively, and obtain atomically flat droplet-free surface (rms ~0.4nm). Main impact will be made on the effect of fractional monolayer design of the active region comprising quasi-2D Ga-rich AlGaN quantum wells coherently grown in the Al-rich AlGaN matrix by a sub-monolayer digital allying technique, which provide strong carrier localization both in vertical and lateral directions and are also beneficial for strain-induced dominant TE-polarized emission beyond 280 nm. [1] S.V. Ivanov et al., Semicond. Sci. Technol. 29, 084008 (2014). [2] X. Rong, S. Ivanov et al., Adv. Mater. (2016), to be published.

Resume : AlN is largely used as a buffer layer for III-nitride growth on silicon substrates. Actually, AlN layers on silicon are much more than simple buffer layers. AlN layers on silicon, which are typically 50-200 nanometer thick, have a strong impact on the material quality (defect density) of heterostructures. Also, the strain in epilayers grown on silicon is largely influenced by the quality of AlN buffer layers. For these reasons, epitaxy of AlN on silicon has been largely studied and issues related to growth and quality of AlN have been identified. In this paper, epitaxial growth of AlN on silicon, including the complex nucleation process, will be discussed. Then the influence of the quality of AlN layers on properties of 2DEG AlGaN/GaN heterostructures will be presented. Also, the demonstration of microdisk lasing in the deep UV using very thin GaN/AlN quantum well heterostructures will be discussed. Also, it will be shown that the patterning of silicon substrates provide a solution to grow crack-free and high-reflectivity AlN/AlGaN DBRs for various UV applications. Finally, thanks to the very good control of the epitaxy of AlN on silicon, it is demonstrated that AlN templates on Si offer an interesting platform for various applications.

Resume : GaN/AlGaN nanostructures are building blocks for UV GaN-based optoelectronics. The use of nonpolar crystallographic orientations allows enhancing the oscillator strength of the optical transitions, but at the price of increasing the lattice mismatch. In this work, we assess the effect of Al incorporation on nonpolar m-plane GaN/AlGaN multi-quantum-wells (MQWs) grown by plasma-assisted MBE on free-standing m-plane GaN. Three series of samples (A, B and C) are considered, with Al mole fractions in the ranges of 6-8%, 26-44% and 100%, respectively. Series A shows atomically flat surface morphology with no extended defects (TEM) or any indication of strain relaxation (XRD). In spite of the absence of structural defects, HAADF-STEM shows alloy fluctuations up to 30% of the average concentration. Al compositions in the range 26-44% lead to an anisotropic degradation of the surface morphology, with appearance of elongated features that increase the surface roughness. SEM images show cracks that developed during the cooling process as a result of the temperature-dependent GaN/AlN lattice mismatch, with increasing density when increasing the alloy composition. Furthermore, TEM characterizations show the onset of the formation of stacking faults and dislocations, and the appearance of nm-sized Al-rich clusters. The effect of all these structural features on the MQWs optical performance is discussed.

Resume : Hydride vapor phase epitaxy (HVPE) is widely used to fabricate GaN substrates because of its high growth rate and high crystal quality. To extend this approach to substrates for UVB LEDs a horizontal HVPE reactor was fitted with an additional aluminum source. Sapphire is used as substrate material because of its UVB transparency, but has a large lattice mismatch to AlGaN in the whole composition range. To cope with interfacial stress, epitaxial lateral overgrowth on isotropically patterned sapphire substrates (PSS) is employed. AlGaN growth on honeycomb-shaped PSS led to non-c-planar oriented domains originating from the sidewalls of the pattern. C-plane AlGaN growth on n-plane sapphire micro facets leads to crystallites protruding from the honeycomb openings and disturbing the coalescence process. To largely avoid these sapphire n-plane micro facets, triangular shaped mesa- and hole-like patterned sapphire was fabricated. Coalesced Al0.8Ga0.2N layers with a total thickness of 10 μm were achieved on such triangular PSS with FWHM of XRD rocking curves lower than 1000” in the symmetric 002-reflection and 1500” in the skew-symmetric 302-reflection. This opens the path to growth of thick AlGaN layers with good crystalline properties.

Resume : Hydride vapor-phase epitaxy (HVPE) plays an important role in group III-nitrides because it enables the production of thick films with either high purity or intentional doping. Thus it represents an independent or supplementary method for obtaining buffer layers or substrates. Compared to HVPE of GaN the high reactivity of the aluminum-containing reactants in the Al(Ga)N system results in special demands on reactor design (avoiding quartz) and process parameters. The state of development in the HVPE growth of AlN-containing layers will be discussed highlighting the special challenges and potential solutions. Own studies are presented that include simulation, design and optimization of reactor, patterned substrates and growth processes. A special situation is obtained for AlN, where AlN substrates made by sublimation (PVT) arise as a carrier for subsequent pseudomorphic growth of layers of high Al-content. By contrast, AlxGa1-xN layers in the medium region of composition cannot be grown without lattice relaxation. An overview of the state-of-art Al(Ga)N HVPE, achievable layer thicknesses and material quality will be given and discussed.

Resume : Achieving control of electrical conductivity in n- and p-AlGaN over all doping levels of interest has proven to be challenging. AlGaN exhibits a decrease in free carrier concentration as the dopant concentration increases above a critical doping concentration. This behavior is attributed to the formation of compensating defects due to lowering of their formation energy during doping. These compensators also contribute to impurity scattering, limiting the carrier mobility. A novel scheme to control point defects in wide bandgap semiconductors is presented. The scheme uses above bandgap UV-illumination to change the position of the quasi Fermi levels in a semiconductor, and, thus, increase the formation energy of compensating defects leading to a decrease in incorporation. Using AlGaN as a model system, we demonstrated that over an order of magnitude improvement in the electrical properties could be achieved in these materials by using this scheme for controlling the incorporation of point defects. In n- and p-doped AlGaN films grown by MOCVD, point defects such as hydrogen, carbon, nitrogen or metal vacancies and their corresponding complexes lead to dopant compensation, resulting in a high resistivity and a low mobility in these films. Generally, the energy of formation of a point defect is a function of the process conditions, as described by the chemical potentials of the species involved. In addition, the energy of formation of charged point defects is also a function of the Fermi energy, or the electrochemical potential. We have developed a non-equilibrium process scheme in which the Fermi level is controlled by external excitation in a steady-state condition. We introduce above-bandgap illumination as the excitation source for enhancing the doping capabilities, by controlling the energetics of point defects via Fermi level control during growth. This presentation will focus on the details of the theoretical background and related calculations. In addition, the discussion will also focus on the used experimental setup and influence of UV-light on the growth condition and possible gas-phase interaction. The proposed point defect control scheme is of great interest for all semiconductors in which compensation is the main limiting factor in obtaining the desired free carrier concentrations and conductivity. Samples were characterized using SIMS, photoluminescence, Hall measurements, XRD and TEM. The passivation and self-compensation of Mg acceptors by H and VN was significantly reduced by above-bandgap illumination. The H concentration was reduced by an order of magnitude and no post growth annealing was needed to activate the samples when grown with UV-light. In Si doped (n-type) AlGaN, UV-illumination during the growth, reduced the Si donor compensation by acceptor defects (e.g. C or VGa/Al). Here, UV-growth led to an increase in mobility and free carrier concentrations with a maximum increase of an order of magnitude in AlGaN films grown on sapphire for doping within the self-compensation regime. A significant reduction in PL intensity related to compensating defects was also observed in these n-type films.

Resume : Nitride semiconductors are the key materials for solid-state lighting and power electronics. I will discuss two issues of crucial importance for these applications: point defects and polarization fields. Point defects may act as compensating centers, charge traps, or recombination centers; in bulk crystals, defects may also reduce transparency [1]. Point defects can also be used beneficially, for instance in quantum information science as analogs of the NV center in diamond [2]. I will discuss the theoretical advances that are enabling us to calculate the energetics as well as electronic and optical properties of point defects with unprecedented accuracy [3]. Accurate knowledge of polarization constants is also critical for analysis and device design. We have identified shortcomings in the current implementation of polarization within simulation tools, and present a consistent approach for modeling spontaneous and piezoelectric polarization [4]. Work performed in collaboration with C. E. Dreyer, A. Janotti, J. Lyons, J. Varley, D. Vanderbilt, and Q. Yan, and supported by DOE, LEAST, and NSF. [1] Q. Yan, A. Janotti, M. Scheffler, and C. G. Van de Walle, Appl. Phys. Lett. 105, 111104 (2014). [2] J. B. Varley, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 93, 161201 (2016). [3] C. Freysoldt, B. Grabowski, T. Hickel, J. Neugebauer, G. Kresse, A. Janotti, and C. G. Van de Walle, Rev. Mod. Phys. 86, 253 (2014). [4] C. E. Dreyer, A. Janotti, and C. G. Van de Walle, Phys. Rev. X (in press).

Resume : Semiconductors and insulators obtain new functionality through the incorporation of dilute concentrations of impurities. Determining the properties that arise from each defect is a challenge, especially for wide band gap materials. As a result, there has been slow progress in identifying new material combinations (bulk material dilute dopants) that enable novel functionality. To overcome this obstacle, we have integrated results of first principles simulation together with synthesis and characterization to identify and eliminate unwanted defects while at the same time promoting the properties from desirable dopants. In this talk, results of density functional theory (DFT) calculations that utilize hybrid exchange correlation functionals will be presented. These methods are used to determine properties of native and impurity defects as well as defect complexes in both AlN and high Al content AlGaN. The results from DFT are compared to photoluminescence and optical absorption measurements of AlN samples grown by physical vapor transport (PVT), metal-organic chemical vapor deposition (MOCVD), and hydride vapor-phase epitaxy (HVPE) and high Al content AlGaN grown by MOCVD. In addition, routes to mitigate the impact of undesirable compensating defects during growth will be discussed.

Resume : Bulk AlN crystals typically contain high concentrations of oxygen, silicon, and carbon – as also still state-of-the art epitaxial layers do, depending on the specific growth conditions. In optical spectroscopy, such crystals show broad bands in the region from 2 – 5 eV in absorption and emission. We investigated in detail the emission bands centered at 2.0 eV and 2.4 eV, which under below-bandgap excitation with a HeCd laser (325 nm) dominate the photoluminescence (PL) spectra both at low and at room temperature. Using time-resolved PL on different time scales ranging from µs to msec at variable temperature and PL excitation spectroscopy, we find clear indications that these transitions occur between different states of the shallow donors Si or O, and a deep acceptor. The donors undergo large lattice relaxation and form DX centers, and similarly the acceptors are subject to Franck-Condon relaxation. The acceptor involved in these PL bands is likely linked to carbon. Depending on temperature, the initial state is either a long-lived (S=1) DX--center state of the donors, a shallow EMT-like conventional donor state, or a free electron – what changes drastically the relaxation time for the optical transitions under below-bandgap excitation. We determined the actual characteristic electron capture times for the DX- states. Based on our data, we develop configuration coordinate diagrams, which also give hints to the source of adjacent optical transitions in the range from 1.8 eV to 2.4 eV.

Resume : In the active layer of blue LEDs, InGaN QWs are exclusively used, since InGaN alloys of low InN mole fractions exhibit bright emissions with internal quantum efficiency higher than 80% in spite of high threading dislocation density (10^8 to 10^9 cm-2). This property is markedly different from the conventional (Al,In,Ga)(As,P) semiconductors or AlGaN alloys. For realizing compact, low power-consumption light sources for high CRI LED-lighting and for sterilization and disinfection purposes, near- to deep-ultraviolet LEDs are indispensable, and are under development using AlGaN. Whereas, as the growth of high-quality crystal is difficult, AlInN alloys have rarely been considered as light-emitting media, although their bandgap energies cover from UV-C to near-infrared wavelengths. In this presentation, we demonstrate planar vacuum-fluorescent-display devices emitting polarized UV-C, blue, and green lights using m-plane AlInN bulk epilayers. The materials are revealed by positron annihilation and time-resolved photoluminescence measurements to contain a large number of nonradiative recombination centers consisting of Al vacancies higher than 1019 cm-3. Such defective materials may not emit any light owing to one-sided nonradiative recombination of excited carriers. However, the films emit the light with the photoluminescence lifetime of 22-32 ps at 300 K. Such defect-resistant radiative performance suggests supernormal localized characteristics of electron-hole pairs in AlInN.

Resume : The anisotropy of the optical response of III-nitrides is a direct consequence of their wurtzite crystal structure leading to lifted degeneracy of the valence band. Selection rules and oscillator strengths of inter band transitions involving different valence bands can be described by the material parameters crystal field splitting and spin-orbit interaction energy. Interestingly, the crystal field splitting undergoes a change of sign from GaN (positive) to AlN (negative) when alloying AlGaN. This change of sign can be observed as polarization switching of the low energy absorption onset. Such observations are possible on non- or semipolar AlGaN films allowing to assess ordinary and extraordinary dielectric functions simultaneously. However, such films are usually influenced by epitaxial strain due to the lack of lattice matched substrates for ternary AlGaN. This strain introduces further symmetry breaking to the valence band structure altering the optical answer of the system considerably. This talk will present the current status of ongoing experimental studies on non- and semipolar AlGaN. Experimentally, spectroscopic ellipsometry extending from the far infrared to the vacuum ultraviolet allows access to the elements of the dielectric tensor while perturbation theoretical calculations are used to understand the influence of strain and composition.

Resume : High-pressure and time-resolved studies of the optical emission from polar GaN/AlN multi-quantum-wells (MQWs) with various well widths are analysed in comparison with ab initio calculations of the electronic and optical properties. The observed emission properties of MQWs were strongly affected by quantum confinement and polarization induced electric fields. The ambient pressure photoluminescence (PL) peak energies decreased by over 1 eV with QWs widths increasing from 1 up to 6 nm, and the respective PL decay times increased from 1 ns up to 10 µs, due to the strong internal electric field. The pressure coefficients of PL energy were significantly reduced as compared to bulk GaN, and they depended on the QWs widths. The transition energies, their oscillator strength and pressure dependence were modelled for tetragonally strained structures of the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs were also simulated using density functional theory calculations. A good agreement between these two approaches verified by experimental results indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy. Acknowledgements: The authors acknowledge support of the Polish National Science Centre by grant No. DEC-2012/05/B/ST3/03113.

Resume : Wide-bandgap semiconductors require precise defect control when used in modern electronic and optoelectronic devices. Compensation mechanisms make it especially challenging to reach either very high or very low free carrier concentrations. This is due to the formation energy dependence of charged point defects on the Fermi level. Control of the Fermi level position during growth offers an elegant way of designing defect profiles in epitaxial films independently of the chemical potential, which is commonly done by changing reactor conditions such as temperature, pressure, or flow of precursors. In this study, the method of defect quasi-Fermi level control was experimentally investigated through illumination of MOCVD-grown epitaxial films with above-bandgap light during growth. The results include the first intensity-dependent study of the effect, matching the data from photoluminescence and cathodoluminescence measurements with a comprehensive, quantitative model developed in our group. Furthermore, the applicability of three different illumination systems have been investigated and the feasibility of lateral defect control could be demonstrated by laser illumination of GaN and AlGaN epitaxial films. The Varshni bandgap shift due to the high temperatures during MOCVD allowed us to use commercially available visible laser diodes, in very good agreement with optical transmission measurements at high temperatures of the epitaxial samples.

Resume : Ab initio simulations were used to determine electron affinity of group III nitride AlN, GaN and InN polar surfaces both metallic and nitrogen sides. It was found that electron affinity at metal and nitrogen faces are drastically different. These values for AlN(0001) and AlN(000-1) are 1.66 eV and 3.14 eV, respectively. Similarly for GaN(0001) and GaN(000-1) 2.34 eV and 5.54 eV while for InN(0001) and InN(000-1) are 2.92 eV and 6.52 eV, respectively. These values are in reasonable agreement with the existing experimental data for AlN and GaN. For InN these values are different. Influence of adsorption of Cs at both metal and nitrogen surfaces was determined. It was shown that Cs drastically decreases affinity. The change of affinity was determined. It was shown that gradual change of the affinity, due to modification of surface dipole layer is followed by finite jump for the coverage corresponding to electronic passivation of the surface, i.e. depinning of the Fermi level.

Resume : The AlGaN alloy system has been the material system of choice for development of solid-state UV emitters and is gaining interest for high-voltage power electronics. Since the critical electric field scales as the bandgap to the 5/2 power, Al-rich AlGaN alloys with their much larger bandgap compared to SiC and GaN are appealing for high voltage PN diodes and transistors. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer device design options not possible for SiC-based devices. Here we demonstrate the potential of AlGaN alloys of high voltage power devices with both vertical and lateral architectures. Al0.3Ga0.7N PN diodes were grown on sapphire substrates by metal organic vapor phase epitaxy. The PN diode structure consists of an AlN template layer followed by an N+ Al0.3Ga0.7N contact layer similar to that reported for AlGaN-based laser diode structures. Rather than quantum wells, a 4.3 um-thick, lightly n-type doped Al0.3Ga0.7N drift layer is grown followed by a p-type Al0.3Ga0.7N layer graded to a p-GaN contact layer to complete the structure. Employing 1.3 mm-thick sapphire substrates allowed crack-free growth of the 9.1 um epi structure. Quasi-vertical diodes exhibited breakdown voltages in excess of 1600 V with less than 3 nA of reverse leakage current out to 1200 V. To our knowledge, this is the first vertical AlGaN-based PN diode exhibiting a breakdown voltage in excess of 1 kilovolt. An effective critical electric field of 5.9 MV/cm was determined from the epilayer properties and the reverse current-voltage characteristics. We note that a Baliga figure of merit (Vbr2/ Rspec,on) of 150 MW/cm2 found in this study is the highest reported for an AlGaN PN diode. We also report electrical characteristics of 2-dimensional electron gases (2DEGs) formed by Al0.85Ga0.15N/Al0.7Ga0.3N and AlN/Al0.85Ga0.15N heterostructures grown on sapphire substrates. These are the highest Al compositions where a 2DEG has been observed. Unlike previous reports, we show that the sheet resistance of Al-rich, AlGaN heterostructures is independent of the density of threading dislocations when employing growth conditions that minimize the density of electron compensating defects. Sheet resistances as low as 1650 Ohms/sqr. were measured using a Lehighton contactless sheet resistance system in Al0.85Ga0.15N/Al0.7Ga0.3N heterostructures. From capacitance-voltage measurements, using a Hg probe we find that changing the thickness of the Al0.85Ga0.15N barrier layer from 10 nm to 50 nm will shift the pinch-off voltage from ~0 V to -15 V. The addition of Si doping to the Al0.85Ga0.15N barrier results in a negative shift in pinch-off voltage indicating an increase in the sheet carrier concentration in the 2DEG. Based on the pinch-off voltage and sheet resistance we calculate the electron mobility to range from 200 – 250 cm2/Vs and the sheet charge density to range from ~2 x 10^12 to 2 x 10^13 cm-2. The higher electron mobility in these Al-rich heterostructures is consistent with a reduction in alloy scattering predicted by Bajaj[1] as the channel composition approaches AlN. Simple HEMT devices employing Schottky contacts and an etch / N+ GaN regrowth process were fabricated from AlN/Al0.85Ga0.15N heterostructures. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. This exceeds the 370 V breakdown reported for similar simple HEMT devices based on B-Ga2O3[2] but is less than the 1700 V breakdown reported for AlGaN HEMTs with field plates and a lower Al composition[3]. Source-drain current was low and limited by the resistance of the grown contact. Improvements in the contact regrowth process and implementation of field plate architecture are expected to increase transistor performance. 1. S. Bajaj, T.-H. Hung, F. Akyol, D. Nath, and S. Rajan, Applied Physics Letters 105, 263503 (2014). 2. M. Higashiwaki, K. Sasaki, T. Kamimura, M. Hoi Wong, D. Krishnamurthy, A. Kuramata, T. Masui and S. Yamakoshi, Appl. Phys. Lett. 103, 123511 (2013). 3. T. Nanjo, A. Imai, Y. Suzuki, Y. Abe, T. Oishi, M. Suita, E. Yagyu and Y. Tokuda, IEEE T. Electron. Dev. 60, 1046 (2013). This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Resume : As the wide-bandgap materials SiC and GaN mature and become commercially available, “Ultra” Wide-Bandgap (UWBG) semiconductors with bandgaps exceeding 3.4 eV are gaining interest as the next-generation materials that will enable dramatic leaps in power electronics performance. This is because the Figure-of-Merit (FOM) comparing breakdown voltage (VB) and area-normalized “specific” on-resistance (Ron,sp), defined as VB^2/Ron,sp, scales as the cube of the critical electric field for vertical devices and the square of the critical electric field for lateral devices. The critical electric field for avalanche breakdown in turn scales approximately as the 5/2 power of the bandgap, implying a very strong dependence of FOM on bandgap. Representative UWBG materials include diamond, Ga2O3, and AlxGa1-xN. The AlGaN system is particularly attractive due to the availability of heterostructures as well as the ability to utilize polarization doping; both of these properties mitigate the issues of energetically deep impurity dopants and consequent incomplete ionization which universally affect UWBG semiconductors. Thus, our group’s focus has been on the development of AlGaN for the fabrication of next-generation power devices with both vertical and lateral architectures. Quasi-vertical AlGaN PiN diodes were grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on sapphire substrates. The key epitaxial layer is the thick, low-doped n- drift layer, since it supports the high voltage in the reverse-bias blocking mode, and it also sets a lower limit for the on-state resistance in the forward-bias conducting mode. Growth of the p-layers follows, and ion implantation into these layers is utilized to form a Junction Termination Extension (JTE) structure to mitigate high lateral electric fields and prevent premature breakdown. The JTE is based on a design developed for vertical GaN PiN diodes, which we have studied extensively using Scanning Photocurrent Microscopy (SPCM). SPCM provides experimental measurements of the internal electric field as a function of position within the device as well as applied voltage. The experimental data are compared to numerical simulations, which not only reveals very rich behavior of the internal electric fields, but also permits informed optimization of the device design. The device fabrication is completed by a mesa-etch down to an n-contact layer and deposition of Ohmic contacts (TiAlMoAu) onto this layer; this is necessary because the substrate is insulating and, due to the lateral current transport in this layer, the device configuration is termed “quasi-vertical” (the conduction through the pn junction and drift layer is in the vertical direction). The top Ohmic contact is PdAu, the surface is passivated with silicon nitride, and a shallow trench isolates the JTE from the mesa sidewalls. For a device utilizing Al0.3Ga0.7N for the active layers and with a 4.3 um thick drift region doped in the mid-10^16 cm^-3 range, VB exceeded 1.6 kV; combined with the measured Ron,sp of 16 mOhm·cm^2, a FOM of approximately 150 MW/cm^2 is obtained. To our knowledge this is the first time a vertical kV-class AlGaN PiN diode has been demonstrated, and the FOM is the highest reported for an AlGaN diode. Further, the current density under forward bias exceeds 3.5 kA/cm^2, and the critical electric field for the Al0.3Ga0.7N is estimated to be 5.9 MV/cm, consistent with the expected EG^5/2 dependence of critical field on bandgap. Lateral High Electron Mobility Transistors (HEMTs) were also grown and fabricated in Al-rich AlGaN alloys. The epitaxial layers are likewise grown on sapphire substrates by MOVPE. Following the growth of an AlN nucleation layer, a 400 nm thick Al0.85Ga0.15N buffer/channel layer is grown, which is capped by a 48 nm thick AlN barrier layer. All layers are unintentionally doped. A combination of mercury-probe capacitance-voltage and contactless resistance measurements indicates a sheet resistance of approximately 4200 Ohm/square and a channel mobility of 250 cm^2/V·s, and the pinch-off voltage is approximately -4.0 V. Circular HEMTs were fabricated using a six-mask photolithographic process. The key step in the process is the fabrication of the source and drain Ohmic contacts, for which n+ GaN:Si is re-grown in trenches etched through the AlN barrier, and a TiAlNiAu metal stack is afterwards deposited. The Schottky gate is NiAu and the surface is passivated with silicon nitride; no field plates are utilized. Drain current vs. drain voltage curves exhibit clear gate control. However, the current density is much lower than expected based on the measured sheet resistance and known device geometry, indicating that further optimization of the Ohmic contacts is necessary (the contact resistivity is estimated to be 0.1 Ohm·cm^2), possibly through the use of compositional grading. Despite this, the devices show excellent performance with Ion/Ioff > 10^7 and VB > 800 V, and to our knowledge are the highest-bandgap transistors ever reported. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Resume : With our growing dependence on technologies driven by electricity, the role of power conversion is evolving to address higher conversion efficiency, higher temperatures of operation, higher power density and smarter applications. Switches, an integral part of every power converter, directly impact the efficacy of the converter, and indirectly its cost. Widebandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) have shown promising performance owing to their superior material quality (compared to Silicon (Si)) that make them the leading materials for next generation power electronics. While SiC and GaN are making impressive progress wider bandgap semiconductors like Aluminum Nitride and Diamond are attracting research interest. This presentation will focus on a developing technology, the vertical GaN technology and its current challenges, and elucidate some recent developments on an emergent one - diamond. Lateral GaN based HEMTs [1] have been designed into power converters successfully that allowed us to study the outcomes at all levels, all the way up to the system. The two important factors that benefited power converters due to the inclusion of a GaN-switch are 1) higher frequency of operation 2) higher temperature of operation, compared to what Si can offer as it reaches its material’s limit. High frequency operation leads to the reduction in electrical form-factor due to reduced size of passive components (capacitors and inductors). Higher temperature of operation allows reduction of cooling accessories hence reducing the mechanical form factor. Higher power (>15KW, although the limits are yet to be determined) applications benefits from vertical device design to maximize the power density. Vertical topology allows higher On-state currents and off-state voltages over a smaller chip area, where the lateral design become unattractive in both cost and performance due to large chip area. Our recent understanding of vertical GaN devices along with the cost of producing bulk GaN substrates portrays a sustainable solution exists for the high power market. CAVETs have demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design). Our recent research has identified vertical device designs that warranties single chip normally off devices, which will further improve switching performance by eliminating bond wire and PCB trace related inductances. GaN substrates with defect densities lower that 104cm-2 and allowing electron mobility >1100cm2V-1s-1 [2] and recent success on implantation activation [3] techniques create a very encouraging picture for bulk GaN based power devices. Energy bandgap above 3.4eV is very attractive to continue the roadmap of power electronics for higher power application (3kV and up). With a bandgap of 5.5eV, a critical electric field over 10 MV/cm, and thermal conductivity superior to Si, SiC and GaN, diamond offers the most appropriate combination to serve very high voltage power electronics. One of our recent research achievement demonstrated excellent contacts to n-type diamond, which has been challenging due to very low work function of diamond. Phosphorous doped diamond achieved by Koeck et al. [4] on homoepitaxial diamond substrates led to excellent ohmic contacts to n-type diamond. Using the same contact technology on a homoepitaxialy grown p (bottom)-i-n(top) structure, diode operation was achieved blocking over 400V. These early results obtained with diamond clearly show a path beyond GaN and SiC. Reference: [1] S.Chowdhury and U.K Mishra, IEEE Transaction on Electron Devices, 60 3060(2013) [2] P Kruszewski et al. The International Workshop on Nitride Semiconductor (2014) [3] T. J. Anderson et al, Electronics Letts. , 50, 197(2014) [4] F.A.M. Koeck et al, Diamond Relat. Mater. 18, 789(2009)

Resume : Conventionally III-Nitride devices had been based on GaN, grown on foreign substrates. However, recently GaN and AlN substrates of high quality have become available and thus of increasing interest, and heterostructure devices start to cover the entire spectrum of AlGaN and InAlN compositions with electronic properties now reaching from those of classical semiconductors with shallow doping to ultra-wide bandgap compounds with deep doping levels typical for insulators. Their high chemical/thermal stability should allow the realization of devices with extreme properties and applications in harsh environments. Thus, device technologies need also to cover the related spectra like extreme modes of operation. This means that many of the traditional technological building blocks and their materials and processing tools used should be re-evaluated and re-adjusted. This concerns first of all fundamental building blocks like ohmic and blocking contacts and surface passivation with dielectrics. This will be the focus of this talk. However, the field of device processing technologies is rather heterogeneous with electronic, optical, physical and chemical aspects producing often conflicting requirements for processing routines. Thus a selection of cases needs to be made, which may highlight the difficulties and challenges encountered in respect to the conventional procedures used up to now. Thus, the focus will be on the following three topics: (1) Ohmic contacts to P-GaN (with some reference to N-AlGaN), where often non-linear behavior is seen, (2) Ni-based Schottky barriers to n-GaN, which is the standard barrier contact material in HEMTs, but where still often differences in barrier height and low thermal stability are observed and (3) Surface passivation by SiN, which is amorphous (and thus highly defective) with a variable stoichiometry, often resulting in distinctly different interface properties, but which is nevertheless the common passivation material for III-Nitrides presently. This is then contrasted with an InAlN/GaN heterostructure technology of extreme thermal stability, tested up to 1000 °C in HEMT devices. In fact, the intrinsic carrier concentration at 1000 °C of the wide bandgap branch of the III-Nitride materials matrix is below 1014 cm-3 and will thus not impose a temperature limit on most electronic (and perhaps even optoelectronic) device applications, (posing of course also a challenge to the related materials development).

Resume : Epitaxial MgO-CaO (Mg0.52Ca0.48O) alloys are promising gate oxide candidates for next generation GaN power electronics, owing to its wide bandgap, high dielectric constant and ability to lattice-match to GaN. However, the cubic oxide| hexagonal nitride interface is challenging due to the highly dissimilar structure and symmetry across the interface. Adding to the complexity, the 111-growth direction of epitaxial rocksalt oxides compatible with (0001) GaN is polar, necessitating the formation of rough 100-oriented facets. We have reported the utility of surfactant- MBE and PLD growth, utilizing water vapor, of Mg0.52Ca0.48O alloys on GaN. Using this technique, the 111-surface is hydroxylated, changing the equilibrium habit from cubic to octahedral, enabling 2D growth of (111) MCO on GaN. In this presentation, we will utilize this technique to produce and characterize smooth, commensurate MCO | (Al)GaN interfaces. Band offsets for the highest Al-content samples- Al0.67Ga0.23N- are 1eV ± 0.2 for the valence band and 1.6 eV ± 0.2 for the conduction band. We will further characterize these interfaces through electrical analysis- IV, CV, and Dit curves- that highlight interface properties for each growth ambient. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Resume : Owing to the presence of the 2DEG and to the high critical electric field, AlGaN/GaN heterostructures are attractive materials for high power and high frequency devices. For this technologies, a good understanding of the behaviour of Schottky contacts to AlGaN/GaN heterostructures is the key to obtain the desired performances, in terms of leakage current, temperature dependent characteristics, etc. However, the behaviour of Schottky contacts to AlGaN/GaN heterostructures can be affected by the high defects density (such as dislocations) of the material, especially if grown on Si substrates. While many works have already discussed the Schottky contacts on GaN layers, Schottky barriers on AlGaN/GaN are still widely debated. In this context, monitoring the temperature dependence of the electrical characteristics can provide useful insights for a better understanding of the current transport mechanisms in these systems. In this work, the forward and reversed I-V characteristics of Ni/Au Schottky contacts on AlGaN/GaN heterostructures grown on Si substrates have been investigated at different temperatures (25-175°C). In particular, the Schottky barrier height has been determined from the forward I-V curves, assuming a ?two diode model? to take into account of the potential well at the AlGaN/GaN heterojunction. The behaviour of the electrical parameters (ideality factor and barrier height) indicated the presence of an inhomogeneous barrier. In addition, a post-metal-deposition at 400°C revealed a modification of the forward I-V curves, which has been discussed considering the possible contribution of dislocations in the diode current.

Resume : UV-photodetectors based on the semiconductor material aluminum gallium nitride (AlGaN) are particular suited for monitoring of ultraviolet radiation sources. Typically, AlGaN-detector structures are fabricated on sapphire substrates. Due to the large lattice mismatch, a high density of threading dislocations (TDDs) of ~109 cm−2 is present in AlGaN-layers grown on sapphire substrates. Therefore, freestanding AlN-substrates with low defect density and high UV-transparency are the substrate material of choice for the realization of AlGaN-based detectors with superior properties. Recently, AlN-substrates have become available by various manufacturers. The AlN-substrates differ considerably in the structural perfection, homogeneity and deep UV-transparency. In this work we present a detailed analysis of the structural properties of AlGaN- heterostructures for UV-detectors deposited on AlN/sapphire (0001) templates and freestanding AlN (0001) substrates. The structural perfection of the AlN-substrates is investigated by X-ray topography and HRXRD. The UV-transparency of the AlN-substrates is analyzed by optical transmission measurements. All AlGaN-layers are grown by MOVPE. After growth HRXRD is used for measurement of the Al-concentration and mosaicity of the device structures. We will show that the AlN-substrates contain a heterogeneous distribution of TDDs, basal plane dislocations and grain boundaries. The comparison of AlGaN-layers grown on AlN/sapphire-templates with layers grown on AlN-substrates clearly shows that a significantly improved material quality can be achieved on AlN-substrates.

Resume : We recently reported the formation of high-quality AlN films grown on sapphire substrates by MOVPE by thermal annealing at a high temperature in a carbon-saturated N2-CO gas mixture [1,2]. The TD density in the AlN films was reduced to 4.7×10^8 cm^-2, and the films can be used to fabricate conventional LEDs. However, there have been few reports on the high-temperature thermal annealing of sputtered AlN films grown on sapphire substrates, which has great potential for preparing highly uniform and large-scale AlN films for commercial use. In this work, we studied the effects of thermally annealing sputtered AlN films grown on sapphire substrates on the crystallinity and surface morphology, where the thermal annealing was performed in nitrogen ambient. The effects of annealing were investigated as a function of the sputtered AlN film thickness and the thermal annealing temperature. AlN films were grown on c-plane sapphire substrates by sputtering. The misorientation angle of the 2-inch substrates was -0.2^o toward the [1-100] direction. AlN powder was used as a target, and the ambient was an argon (Ar) and nitrogen (N2) gas mixture with the ratio [Ar]/[N2] = 0.33. The radio frequency (RF) power and growth temperature were 700 W and 650 oC, respectively. The thicknesses of the sputtered AlN films were 170 and 340 nm. Subsequently, the sputtered AlN films were thermally annealed in N2 at 1600 - 1700 oC for 1 h. The (0002) and (10-12) XRCs for the 170-nm-thick sputtered AlN films without and with thermal annealing at 1700 oC were examined. Both the (0002) and (10-12) XRCs of the AlN films had a single sharp peak after thermal annealing owing to the elimination of the tilt and twist components from the sputtered AlN films by high-temperature thermal annealing. The FWHMs of the (0002) and (10-12) XRCs were markedly reduced from 532 to 49 and 6031 to 287 arcsec, respectively. The improved crystallinity is related to the solid-phase reactions that occur at high annealing temperatures, which annihilated domain boundaries and concurrently increased the compressive stress in the films. REFERENCES: [1] H. Miyake, G. Nishio, S. Suzuki, K. Hiramatsu, H. Fukuyama, J. Kaur, N. Kuwano, Appl. Phys. Exp. 9 (2016) 025501. [2] H. Fukuyama, H. Miyake, G. Nishio, S. Suzuki, K. Hiramatsu, Jpn. J. Appl. Phys, 55 (2016) 05FL02.

Resume : Surface acoustic waves (SAW) devices are key components in communication systems and are more and more used as wireless and passive sensors including in harsh conditions. AlN/Sapphire structure has been widely investigated for both applications as it allows high-frequency operations. However, this structure shows a relatively low electromechanical coefficient K², close to 0.3%. ScxAl1-xN, which shows stronger piezoelectric properties, is a serious candidate to advantageously replace AlN in SAW applications. The aim of this work is to achieve highly-textured c-oriented ScxAl1-xN thin films with a FWHM rocking-curve below 1°, to maximize the SAW properties of the film. The optimal film composition, with the best compromise between K2 and SAW velocity, will be obtained by testing Sc rates between 0 and 50%. The films are deposited by magnetron sputtering, with a composite target. The microstructure is determined by X-ray diffraction (XRD) measurements (θ-2θ, rocking-curve and pole figure) and transmission electron microscopy. Energy dispersive X-ray as well as Raman spectroscopy methods are used to get the film composition. K² and SAW velocity are determined by RF characterization. Firstly, c-oriented AlN thin films were deposited to get a reference sample without Sc. XRD θ 2θ diagram shows strong (002) orientation with a FWHM rocking-curve below 0.5° for the optimized films. Pole figures show that the films are also textured in the plane. RF measurements confirm the piezoelectricity of the film. Ongoing experiments are dedicated to the optimization of the deposition parameters throughout the whole investigated Sc rate range, to obtain ScxAl1-xN films that are highly textured in and out of the plane such as AlN reference ones.

Resume : Crystalline aluminum nitride (AlN) is a semiconductor with excellent properties such as wide bandgap of 6.2 eV, high chemical stability, high thermal conductivity and high dielectric breakdown voltage, which allow this material to be applied in opto-electronics [1]. Due to its optimal low reflection as well as to its fixed negative charges, AlN has also attracted attention as anti-reflective coating and passivation layer for silicon surfaces [2,3]. In this work, the dependence of the structural and optical properties of AlN on different hydrogen content is demonstrated. The layers are prepared by RF-magnetron sputtering in an Ar/H2 atmosphere. Elemental composition was determined by Energy Dispersive Spectroscopy (EDS). The structural properties were investigated by Fourier Transform Infrared spectroscopy, X-ray diffraction at grazing incidence (XRD) and Scanning Electron Microcopy (SEM). The optical characterization was performed through transmittance measurements using a modified envelope method. Hydrogen induces the increase of the grain size and the optical bandgap, showing a good correlation between the optical and structural properties. [1] B.N. Pantha et al, Appl. Phys. Lett. 90 (2007) 3-5. [2] P.M. Kaminski et al, Phys. Status Solidi C 8, N0 4 (2011) 1311-1314 [3] G. Krugel et al, Energy Procedia. 55 (2014) 797-804.

Resume : Photoconductors are the most simple semiconductor detectors; their operation is related to change in device conductivity by photogenerated carriers, with an absorption cutoff wavelength related to the semiconductor band gap. AlInN presents a direct band gap that can be tuned with the In composition from the mid-UV to the near IR, while keeping the intrinsic properties of III-nitrides. In addition, the growth of AlInN by RF sputtering presents some advantages due to the low cost of the system and the low temperature of the deposition process, which enables the integration with other materials. In this work, we have developed photoconductor devices based on AlInN layers grown on sapphire by RF sputtering. We have studied the effect of the thickness and composition of the AlInN layer on the photoresponse characteristics. Developed devices show a sublinear dependence of the photocurrent as a function of the incident optical power (I_pc∝P_opt^(-γ) ). In particular, for 20 nm-thick Al0.38In0.62N-based devices a value of γ ~ 0.7 has been estimated for incident light of 405 nm. For these devices, above-the-band-gap (405 nm wavelength) responsivity higher than 0.1 A/W can be obtained for an irradiance of 1 W/m2, and the response drops by more than two orders of magnitude for excitation below the band gap (633 nm). The devices present persistent photoconductivity effects associated to the grain boundaries.

Resume : Sputtered AlN is a piezoelectric material with applications in electroacoustic devices such as high frequency filters or biological sensors. For most of them, films are deposited with the c-axis of their wurtzite structure perpendicular to the surface. The most commonly-used resonator structure consists in an electrode/AlN/electrode piezoelectric capacitor acoustically isolated from the substrate by an air gap (suspended structures) or by an acoustic reflector (solidly mounted resonators). In these devices the propagating acoustic wave mode is longitudinal, that is, the movement of the material is parallel to the propagating direction, which is parallel to the c-axis. For biosensing applications, which require in-liquid operation, this mode is not suitable due to its high energy radiation to the medium, which gives rise to a reduction of the quality factor. For proper operation, shear mode resonators, where the movement of the material is perpendicular to the propagation direction and parallel to the surface, are need. To generate these modes, it is essential to create a component of the electric field perpendicular to the c-axis. The most straightforward way to do so is to deposit AlN films with grains whose c-axis is tilted with respect to the normal to the surface. To deposit such films, well-controlled directional deposition and rough substrates are needed. In this communication, we present the deposition process and material characterization of tilted grain AlN films. Bulk acoustic wave resonators, made with this material, are also presented together with their application to biosensors operating in-liquid medium. Some details about device design and operation in the detection of proteins are also included.

Resume : One motivation for studying the growth and properties of AlGaN nanowires (NWs) is the growing interest in UV-C emission for sanitization applications. Indeed, the versatility of NW growth on a large variety of substrates, the easier doping of NWs with respect to layers, the absence of extended defects, make them particularly attractive and a potential candidate to overcome the limitations of bulk-like layers. Accordingly, we report here on the molecular beam epitaxy growth of AlGaN NW sections grown on top of GaN NWs on Si (111). We show that structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states [1,2]. These carrier localization features are present at very small scale, as they are also observed for AlGaN NW insertions in AlN, less than 2-3 nm thick. Moreover, related micro-photoluminescence (PL) and time-resolved PL data do not show any dependence of decay times with growth temperature. Photon correlation experiments have been performed on the micro-PL sharp lines associated to localized electronic states. Antibunching has been put in evidence, typical of a quantum dot-like behaviour. Interestingly, such a behaviour has been observed for a wide range of AlGaN NW chemical composition and growth temperature. Preliminary results on the p and n-type doping of AlGaN p-n junction (35 % AlN molar fraction) studied by a combination of Raman spectroscopy and Kelvin probe microscopy will be additionally reported, paving the way to the realization of UV NW-based UV emitting devices. References [1] A.Pierret, et al., Nanotechnology 24 115704 (2013). [2] A.Pierret, et al., Nanotechnology 24 305703 (2013).

Resume : High-power optical sources that emit in the UV spectral range have applications that include bio-chem identification, decontamination, medical diagnostic and treatment, communications, and materials curing. Attempts to realize semiconductor UV sources by the conventional approach with a current-injection p-/n-junction diode have encountered numerous difficulties. This presentation will discuss the challenges, illustrate the current status of materials/device development, and describe the necessary building blocks, which include highly conductive p-type cladding layers, sophisticated electron-blocking layers, and high performance active zones. High current densities of more than 20kA/cm2 of a full LD heterostructure have been achieved. Optically pumped lasers have been demonstrated with low pump threshold down to a wavelength of 237nm. However, issues related to effective carrier injection and low absorption losses remain challenging when using high band gap p-type materials. Thus, a laser configuration that omits the use of p-doped materials is an attractive alternative. We will present recent results towards realizing deep-UV lasers in the AlGaN materials system using electron beam excitation as the laser pumping strategy. A custom-built e-beam-pumping system has been developed that enables excitation of AlGaN gain chips with high-energy electrons (10-30keV) and high-power densities exceeding 1MW/cm2. AlGaN gain chip heterostructures for vertical laser emission will be described. Toward development of a compact deep-UV laser source, we report record high pulsed peak optical output power of more than 200mW of spontaneous emission at λ=246nm.

Resume : The field of ultraviolet (UV) photonics at wavelengths <400nm is an area of increasing practical interest and coherent light sources and fast, sensitive photodetectors are needed for many important applications. In this study, AlGaN p-i-n APDs are demonstrated on “free-standing” (FS) n-type (0001) GaN substrates having low dislocation densities. The AlGaN PIN UV-APDs and UV lasers were epitaxially grown on a c-axis n-type free-standing GaN and sapphire substrates using metalorganic chemical vapor deposition (MOCVD). For top-illuminated APD structures, step-graded layers from n-GaN:Si to Al0.02Ga0.98N:Si were introduced instead of a single n-Al0.05Ga0.95N:Si layer as a strain management scheme for crack-free growth. The APD epitaxial layer structure consists of a 0.45-μm thick GaN:Si layer followed by a 0.15-μm thick n-Al0.02Ga0.98N:Si layer for the step grading, a 0.3-μm thick unintentionally-doped Al0.05Ga0.95N layer, a 0.1-μm thick Al0.05Ga0.95N:Mg layer, and a 0.02-μm thick Al0.05Ga0.95N:Mg++ grown on FS-GaN. The growth and doping conditions for epitaxial layers were optimized to achieve improved crystalline and structural quality by modifying the MOCVD growth parameters. The APD wafers were fabricated into circular mesas with various diameters using standard photolithography and inductively coupled plasma etching, followed by SiO2 passivation deposited using plasma-enhanced chemical vapor deposition. The APDs exhibit a maximum avalanche gain >105 at a reverse bias of -102V. We have also fabricated UV optically pumped InGaN/AlGaN MQW lasers at 375nm employing an AlGaAs-GaAs 40 pair n-type bottom DBR operating pulsed at 300K, and will describe the growth, processing, and output characteristics of these lasers.

Resume : In order to manage and decrease the lattice mismatch induced strain in AlGaN heterostructures for UV-C light emitters, we have started investigations about the growth and basic characterisation of AlBGaN layers. A major problem of this material system is the very poor solubility of boron (B) in Al(Ga)N, which may be increased by MOVPE growth at elevated temperatures of up to 1400 °C. At first, we have carefully optimised the quality of MOVPE-grown AlN templates on sapphire wafers. A multi-step process was applied by alternating slow and fast growth leading to a reduction of the threading dislocation density as determined by TEM. This improvement is also confirmed by XRD showing narrow peaks with FWHM below 80 and 600 arcsec for the (0002) and (10-12) ω-scan, respectively, and also supported by strongly improved photoluminescence (PL) signals. On such templates, first 500 nm thick AlBN layers were grown with various B and Al flow ratios, still using conventional growth temperatures around 1100°C. For layers grown with a TEB/(TEB+TMAl) ratio of 1.7% in the gas phase, XRD scans show a weak peak besides the main AlN peak corresponding to approximately 0.4% boron in AlN assuming a fully relaxed layer, while larger B ratios resulted in polycrystalline layers. First PL investigations show pronounced defects in both AlN and AlBN layers. However, the main defect-related contribution shifts in energy from ~4.4 eV towards 3 eV with increasing boron content, indicating a change in defect generation in presence of TEB in the gas phase.

Resume : With a “honeycomb” lattice structure similar to graphene, hexagonal boron nitride (h-BN) has attracted great interest for its promising properties such as atomic flat surface free of dangling bonds, high thermal conductivity and electrical resistivity, high neutron capture cross section and wide bandgap around 6 eV, which make it suitable material for graphene electronics, van der Waals heterostructures, neutron detectors and deep UV devices. Since BN atomic layers are hold together by weak van der Waals forces, h-BN can also serve as a buffer layer for the growth of III-nitrides on top and as a release layer at the same time allowing mechanical transfer of device structures to foreign substrates. The quality and the scalability of h-BN are the main issues for its applications. In this work, we have successfully grown 2D layered hexagonal BN in 2 inch wafer scale by MOVPE on sapphire substrates[1]. The h-BN layer shows (0002) and (0004) X-ray diffraction peak with small mosaic spread. The scanning transmission electron microscopy images present highly oriented lattice with hexagonal stacking sequence. The whole wafer exhibits continuous and uniform surface, with growth steps for 3 nm BN and organized wrinkles for 30 nm BN. The growth of layered h-BN on 2 inch sapphire substrates has enabled us to investigate the large-scale mechanical lift-off and transfer of GaN-based device structures[2]. In the present work, high quality InGaN/GaN multi-quantum well (MQW) structures were grown on 3 nm h-BN on sapphire substrate. The nucleation and epitaxial growth process have been studied. Wafer-scale mechanical exfoliation of the device structures using commercially available adhesive tapes was demonstrated. Structural and optical characterization performed before and after the lift-off confirms conservation of structural properties and optical functionalities by this technology. The ability to grow wafer-scale 2D layered h-BN epi-layer is promising for future electronics and optoelectronics. The high quality III-nitride device structures grown on top of BN and subsequent mechanical lift-off from the entire substrate present an exceptional opportunity to realize high-efficiency and low-cost next generation device structures on any platform. [1] X. Li, S. Sundaram, Y. El Gmili, T. Ayari, R. Puybaret, G. Patriarche, P.L. Voss, J.P. Salvestrini, and A. Ougazzaden, Cryst. Growth Des. 16, 3409 (2016). [2] T. Ayari, S. Sundaram, X. Li, Y. El Gmili, P.L. Voss, and J.P. Salvestrini, Appl. Phys. Lett. 108, 171106 (2016).