Ultra-doped semiconductors by non-equilibrium processing for electronic, photonic and spintronic applications

Resume : Hyperdoping of silicon, that is doping at levels several orders of magnitude above equilibrium solid solubility, can be achieved through a combination of ion implantation and pulsed laser melting. For the past two decades, this approach has been used to dramatically enhance the sub-bandgap absorption of light by silicon to open up prospects for efficient near-infrared photodetectors. This presentation reviews attempts to hyperdope silicon with transition metals that have deep levels within the silicon bandgap but very low equilibrium solubilities. A range of physical characterization techniques, coupled with theory, have been used to assess crystalline quality, bandgap impurity and defect levels, carrier lifetime, impurity profiles and atom location of hyperdoped systems. In many cases, physical characterization results have been corelated with optical absorption and photodetector properties. The most extensive data exists for Au-hyperdoped silicon and this review will focus on this hyperdoped system. The correlation of atom location and absorption measurements have shown that Au can potentially hyperdope silicon beyond the so-called Mott limit. However, the photodetector properties achieved to date fall well short of expectations, and reasons for this behavior will be given. Results for two other systems will be presented, Ti-Si and Ag-Si, where somewhat encouraging photodetector properties have been achieved but the silicon is somewhat defective. New data on the extent of hyperdoping in these systems will be presented, and comparisons made with the Au-Si system. Finally, an assessment of prospects for efficient infrared photodetectors, based on hyperdoping of silicon, will be given.

Resume : Hyperdoping with deep level impurities is widely known as an effective method to introduce infrared absorption into silicon. We have previously reported that the photoelectric response of silicon in the near-infrared band is achieved by hyperdoping silver. However, it is reported that the light absorption of infrared light will reduce after high temperature annealing. This reduction of infrared absorption has been observed in chalcogen elements hyperdoped silicon, which can be explained by the long-range diffusion of chalcogen elements during annealing. For silver, this annealing reduction cannot be explained by long-range diffusion. It is found that only the annealing process at a temperature up to a certain level will cause the reduction of infrared absorption, while the diffusion length of silver atoms at lower temperatures has exceeded the thickness of the doped layer. We used high-resolution TEM to observe the Ag-hyperdoped silicon after annealing at different temperatures. The presence of silver precipitates was observed in all samples with annealing. The particle size of these silver precipitates is between 5-30 nm. As the heat treatment temperature increases, silver precipitates are gradually formed and grow into a larger size. The appearance of silver precipitates indicates that the silver atoms have left the original electrically active lattice position after heat treatment, resulting in a change in the electrical properties of the hyperdoped silicon. This change is the main reason for the annealing reduction phenomenon of Ag-hyperdoped silicon. This work was supported by the National Natural Science Foundation of China (Nos. 62025403, 61974129 and 61721005)

Resume : Intermediate band photovoltaics is proposed to improve the utilization of the solar spectrum and hyperdoping is an effective method to realize such a material [1]. We investigate various hyperdoping methods and dopant concentration profile tailoring, to increase the charge carrier lifetime in intermediate band semiconductor materials. Hyperdoping is a well-established technique to introduce large concentrations of deep level dopants such as heavy chalcogens and transition metals for intermediate band formation [1,2]. Various hyperdoping and annealing methods — namely, ion implantation followed by pulse laser melting, ion implantation followed by flash lamp annealing, and thin film deposition followed by pulse laser melting all can produce high-quality, single crystalline materials. Dopant segregation near the surface, however, is a common issue and it leads to short charge carrier lifetimes. To address this matter, we remove the segregated region using several etching methods and examine the results. We engineer the dopant profile by implementing different etching techniques and investigate charge carrier lifetime using terahertz (THz) spectroscopy. Optical pump THz probe is a non-contact photoconductivity measurement for probing carrier lifetime with picosecond time resolution [3]. We evaluate the recombination dynamics of gold-hyperdoped silicon by thin film deposition followed by pulse laser melting. We found that hyperdoped sample without any dopant profile modification has a fast decay lifetime of 16 ps and the reactive ion etching (RIE) improves the lifetime by a factor of 2. We also characterize the structural and absorption properties of the material. Additionally, we simulate the lifetime dependence on dopant concentration profile. We use these THz lifetime measurements to verify results of dopant profile engineering, to optimize materials for intermediate band photovoltaics. Our results show that, in addition to the surface segregated layer with high gold concentration, implantation and annealing induced defects could limit the carrier lifetime. Furthermore, we reveal that optimizing charge carrier lifetime require different dopant profile distribution for different wavelengths of excitation. Our experimental and simulation results identify optimal concentrations and distribution profiles for harvesting sub band-gap photons using hyperdoped silicon [1] Warrender J.M., “Laser hyperdoping silicon for enhanced infrared optoelectronic properties”, Applied Physics Review 3, 031104 (2016) [2] Mailoa J. P., et al., “Room-temperature sub-band gap optoelectronic response of hyperdoped silicon”, Nature Communication 5, 3011 (2014) [3] Sher R., et al., “Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon,” Applied Physics Letters 105, 053905 (2014)

Resume : The problems of poor open-circuit voltage (VOC) and low photogenerated carrier collection efficiency usually exist in the hyperdoped monocrystalline Si solar cells for sub-bandgap optoelectronic response. In contrast, the tandem solar cells using the hyperdoped Si films can not only preserve the VOC but also effectively combine the intrinsic photocurrent with the sub-bandgap one. In this study, a Ti-hyperdoped Si (Si:Ti) film tandem solar cell was prepared. The (Si/Ti)n multilayer films were deposited on the n-type front surface of monocrystalline Si solar cell substrate by vacuum evaporation technology. The substrate has been prepared a p/n junction, antireflection layer, front Ag contacts, rear passivation film, and rear Al contacts. Then, the (Si/Ti)n films were molten and crystallized by 1064 nm nanosecond-pulsed laser to form Si:Ti film (as a n+ layer) tandem solar cells. To optimize the hyperdoping process, the microstructural evolution, Ti impurity distribution and concentration variation, and sub-bandgap (Ti impurity band) formation in the Si:Ti films were carefully studied. The results show that the hyperdoping concentration and depth can be manipulated by controlling the Si and Ti deposition ratio, (Si/Ti)n film thickness together with a matched ns-laser fluence. Furthermore, the reduction of light reflection, the increase of sub-bandgap infrared absorption, and the p/n/n+ structure for carrier collection were used to increase both the sub-bandgap and intrinsic photocurrents of the solar cells. Thus, the VOC of the tandem solar cell reached 613 mV and short-circuit current density (JSC) showed 41.1 mA∙cm-2, respectively. Furthermore, its sub-bandgap infrared external quantum efficiency and JSC was increased by ~100% and 79% over those of the monocrystalline Si solar cell substrate, respectively. Finally, the tandem solar cell with a Si:Ti film thickness of 105 nm and an active area of 14.4 cm2 showed the highest photoconversion efficiency (18.6%), which was increased by 29% over that of the monocrystalline Si solar cell substrate (14.4%). From the perspective of industrial application, the structure of tandem solar cells has a flexible selectivity (the types of substrates and doping elements are unrestricted), a controllability (the concentration and thickness of hyperdoped films are controllable), and a process stability (the fabrication process is simple). This study experimentally demonstrates the principle of fabricating the Si impurity band solar cells, namely, simultaneous optimization of separation, transportation, and collection of photogenerated carriers. It also shows that the Si:Ti films can contribute to the photocurrent in this tandem solar cell structure and have a great potential in the field of high-efficiency solar cells.

Resume : Infrared detection on inexpensive devices that operate at room temperature, with fast pixel refresh rates and high resolution has been largely pursued by both academy and industry institutions. In particular, there is a high demand for Focal Plane Array (FPA) Short Wave Infrared (SWIR) photodetectors, with applications ranging from food industry, climate, and environmental disasters, among others. SWIR range covers from 1.0 to 3.0 µm of wavelength; it is fairly close to the bandgap of Si. However, silicon, with its bandgap of 1.12 eV, is not responsive in such range, which goes down to 0.4 eV. Supersaturated materials have gathered the attention of many research groups around the globe. The reason behind this interest lies in the fact that supersaturated semiconductors may exhibit sub-band gap optical response. In the particular case of silicon, it would be possible to extend its sensitivity towards the SWIR range by supersaturating it with transition metals (Ti, V, Cr, Au), or chalcogenides (S, Se), making it an ideal candidate to cover the abovementioned SWIR applications. The basic idea behind supersaturating a semiconductor, is to provide enough discrete levels inside the bandgap, so that they may eventually form a third band of allowed states, an Impurity Band (IB). The IB could act as an intermediate step for the carriers to promote from the Valence Band to the Conduction Band. Therefore, absorption of light whose energy is lower than the bandgap would be also possible through the IB, as it has been already reported in the literature. Thus, supersaturated Si devices may be the key to fabricate SWIR photodetectors at room temperature. Experimentally, we supersaturated Si with Ti atoms by means of ion implantation in high concentrations (>1015 cm-2)¬, followed by a Pulsed Laser Melting (PLM) process. The first technique allows the introduction of the soaring amount of Ti deep levels required to form the IB, but it produces a large density of crystal defects that may amorphize the surface. The nanosecond PLM process helps restore the crystal quality, if the energy absorbed by the surface is above a certain threshold limit. A good crystalline quality is desirable to produce high efficiency photodetectors. In previous works, this PLM energy threshold of supersaturated Si was identified by means of TEM, Raman spectroscopy or XRD inspections. In this work, we present a novel approach, where Transmittance-Reflectance (T-R) measurements are fed into a bilayer optical model, to extract key properties of the Ti supersaturated Si layer, as its complex refractive index, the absorption coefficient and the thickness of the implanted layer. Furthermore, we were able to identify the melting threshold limit above which we can expect monocrystalline supersaturated layers. Data are then compared to TEM micrographs and Raman spectroscopy measurements

Resume : n-type doping of Si by the deep chalcogen donor Te in excess of the solubility limit was recently demonstrated to lead to hyperdoped material [1]. Our investigation by hard x-ray photoelectron spectroscopy (hXPS) reveals at least two different Te species with different binding energy and systematically varying concentrations with increasing ion implantation dose. At the highest doping we study the photoelectron scattering patterns using hard x-ray photoelectron diffraction (hXPD) [2]. Substitutional site occupation of both Te monomers as well as dimers is identified with increasing binding energy leading to the main features in the XPS spectra. The sharp hXPD patterns allow the detailed analysis of the local surrounding of the dopant atoms [3]. At the lowest binding energy, an additional species is found and the distinct, rather diffuse hXPD pattern at this binding energy suggests the assignment of this component to a small fraction of Te in clusters. References [1] M. Wang et al. Phys. Rev. Appl. 11 054039 (2019) and references therein. [2] O. Fedchenko et al NJP 21, 113031 (2019); [3] O. Fedchenko et al NJP 22, 103002 (2020).

Resume : Ultra-doped Silicon (i.e., silicon doped beyond the solid solubility limit) with chalcogen impurities is attracting increasing interest from electronic industry and material science community since it is a promising material for infrared absorber, intermediate band photovoltaics, and nanoelectronics. As it has been recently shown [1], among dopants, chalcogen impurities – i.e., impurities of VI column of the periodic table (S,Se,Te) -- provide superior electronic properties (high carrier concentration, and no saturation) than traditional column V dopants. Due to their high predictive capability, ab initio simulations are therefore desirable to provide new information on the microscopic mechanisms governing the electronic properties of the ultra-doped regime in silicon [2]. In the framework of density functional theory, by plane-wave pseudopotential and super-cell techniques, we systematically investigated Se ultra-doped silicon by computing, for different types of Se complexes, the formation energy as a function of dopant concentration. Our ab initio simulations enlighten the microscopic mechanisms responsible of the drastic reduction of electrical deactivation defects as the number of dopants approaches the critical concentration at which the insulator-to-metal transition occurs. We discuss the electrical properties of the Se point defects and Se complexes, studying the formation, the nature, and the evolution as a function of the Se concentration of the electronic band formed by the Se impurities in the gap. We establish the role played by the presence of different types of Se complexes to determine the insulator-to-metal transition, which, according to our simulation, is mainly driven by complexes involving substitutional Se. We identify the optimal doping range, in which is possible to engineer the width and the shallowness of the impurity band by tuning the dopant concentration, thus allowing the design of ultra-doped material that can be exploited for nano-electronic applications, ranging from long-wavelength infrared detectors to intermediate impurity band solar cell. Our study is completed by the calculation of the structural properties of the different types of complexes investigated and by the simulation of the dependence of lattice parameter as a function of dopant concentration. These findings can be extended to silicon ultra-doped with other chalcogen impurities, thus paving the way toward the intermediate impurity band nano-electronics. [1] M. Wang et al. Phys. Rev. B: Condens. Matter Mater. Phys., 2020, 102, 085204. [2] A.Debernardi, Phys. Chem. Chem. Phys., 2021, 23, 24699

Resume : Tellurium-hyperdoped silicon shows significant promise as a candidate as an intermediate band (IB) material for highly efficient solar cells and photodetectors. Previous work by Wang et al. shows ion implantation of Te in silicon followed by pulsed-laser melting results in stable silicon supersaturated with Te compared to the other dopant candidates [1] [2]. Heavy chalcogen dopants can be incorporated at very high concentrations without exhibiting cellular breakdown, and Te is more thermally stable than lighter chalcogen such as S or Se. In addition, Te-hyperdoped Si exhibit strong sub-band gap optical absorption. Wang et al, demonstrated room-temperature photodetection for infrared wavelength as long as 2.5 µm, and the photodetection range is extended to 5.0 µm at 20K. Photo responsivity and detectivity range increase with declining temperature and their values are comparable with commercially available photodetectors [1]. Literature results show that Te-hyperdoped Si as a promising IB material. In this work, using ps-time resolved terahertz photoconductivity measurements, we study charge carrier transport dynamics of Te-hyperdoped silicon as a function of temperature. Low charge carrier lifetime in hyperdoped semiconductors often limits device responsivities. Previously we showed that for S- and Se-hyperdoped silicon, balancing sub-bandgap infrared absorption and carrier lifetime and optimizing figure-of-merit revealed optimal dopant concentrations [3]. We study Te-hyperdoped silicon on both p-type and n-type substrates with varying laser processing conditions. Optical absorption and secondary ion-mass spectrometry are employed to investigate the sub-band gap absorption and concentration profiles, respectively. The results show that sub-band gap absorption increases with increasing dopant concentration. Time resolved terahertz spectroscopy is used to study the charge carrier lifetime. We find that lifetimes are much longer, consisting of a rapid decay in the order of ps followed by a longer, ns-lifetime component. To explain the observed results, we study two different optical pump wavelengths at 400 nm and 266 nm to investigate surface recombination, carrier mobility and diffusion properties. We plan to investigate temperature-dependent charge carrier dynamics in order to develop a better understanding of enhanced and extended infrared photodetection at low temperatures. Our goal is to understand the effect of temperature on transition time, diffusion length as well as conductivity of the material. References [1] M. Wang et al, "Silicon‐Based Intermediate‐Band Infrared Photodetector Realized by Te Hyperdoping," Advanced Optical Materials, 9, 2001546, 2021. [2] M. Wang et al, "Thermal stability of Te-hyperdoped Si: Atomic-scale correlation of the structural, electrical, and optical properties," Physical Review Materials, 3, 044606., 2019. [3] M.-J. Sher, "Picosecond carrier recombination dynamics in chalcogen-hyperdoped silicon.," Applied Physics Letters, 105, 053905, 2014.

Resume : Nitrogen-hyperdoped silicon is a promising material with many desirable properties, from high-efficiency infrared absorption for optoelectronics, photovoltaics, and detectors, to suppressing defects during wafer manufacturing, and potential high temperature operating spintronic devices. The fabrication of N-hyperdoped Si has proven difficult due to the di-interstitial pair being the most stable N defect in silicon. Ion implantation yields high concentrations of interstitials, implantation damage, and off-centre displacements of N atoms. Ion-implanted samples, as well as those treated by furnace or rapid thermal annealing, do not show substitutional N (NSi)-related signals. Only after samples are laser processed can EPR and IR vibrational mode signals related to the electrically active NSi defect be detected. Therefore, the development of a low-damage, controllable method for hyperdoping Si with NSi is vital. This work demonstrates how non-equilibrium pulsed excimer laser processing in high-pressure N-rich gaseous environments (gas immersion laser doping – GILD) can be used to fabricate N-hyperdoped Si, displaying concentrations several orders of magnitude beyond the solid solubility, while suppressing surface damage. X-band continuous-wave EPR measurements at temperatures ranging 10-300 K confirm the presence of NSi, and I-V, C-V, DLTS, and IR measurements provide complementary information on defects. In addition to NSi, signals attributed to other N complexes have been detected by EPR. Their nature will be discussed. Samples processed in Ar display, at room temperature, a single EPR line attributed to the laser-induced damage, and at low temperature, a line attributed to conduction electrons. The latter result reveals that shallow donors are produced during laser processing. Interestingly, the N doped samples do not show the conduction electron signal, a result consistent with I-V measurements, which leads to the conclusion that N introduced during GILD is compensating the donors. These results indicate the successful N-hyperdoping of Si. At high fluences, rapid localised melting of Si is expected in atmospheric pressure. However, we observe no evidence to suggest a melting and re-solidification process (to be studied further). At high pressure, the gaseous environment is possibly suppressing melting. Our proposed doping mechanism follows; laser light-matter interaction generates high energy plasma at the Si surface causing thermal expansion stresses, laser-induced shock waves, and vibrational excitations. Nitrogen subsequently diffuses through these, leading to greatly enhanced doping and low damage compared to other methods.

Resume : Highly mismatched alloys (HMAs) are semiconductors with strongly modified band structures due to the alloying of elements with strongly different electronegativities. Their band structures have been well described using the band anticrossing (BAC) model, according to which the alloy interaction splits the conduction or valence bands. In the case of a conduction band (CB) anticrossing, the CB splits into an upper (E+) and lower (E-) band, with a rapid change in the band gap as the alloy concentration changes. This large tunability has been used in LEDs and multijunction solar cells. We discuss the properties of heavily doped HMAs with appreciable population in the E- band, either from doping or photoexcitation, and present these materials as mid-IR plasmonic resources. We use a disorder-averaged Green’s function method, which goes beyond the BAC and approximately accounts for the alloy disorder, to determine the bulk plasmon frequency and the E- to E+ absorption spectrum. We show the distinctive signatures of the direct and indirect optical absorption processes, which are different from those in standard semiconductors. These signatures make optical absorption an effective way to determine the BAC and disorder parameters. We further describe a nonstandard scaling of the plasmon frequency, which does not reduce to the free-electron-gas form even when doping is low and the band is well approximated as parabolic. The proven tunability of HMA band structures permits a wide variety of plasmon frequencies to be realized, with the most commonly considered HMAs having resonances in the mid-IR. This exploration helps open up this new material class for plasmonic development.

Resume : Losses in plasmonics materials are one of the main challenges, limiting the promise of many proposed plasmonic materials and applications. To solve the loss problem, extensive efforts have been made in searching for alternative plasmonic materials, especially in the last decade. Theoretical analyses can provide insightful guidance for exploring the vast landscape of possible alternative plasmonic systems. Khurgin and Sun [APL 96, 181102 (2010)] proposed necessary conditions for the electronic structure of a material to dissipate a photon’s energy with electron-photon interactions. Such dissipative mechanisms need available empty electronic states, to which electrons can be excited by absorbing a photon and subsequently dissipating the absorbed energy into phonons. A material can have an electronic structure without such empty states for a range of photon frequencies. If there are also free electrons and the plasma frequency falls in the lossless window, then the system is an essentially lossless plasmonic medium. We observe that doped highly mismatched alloys (HMAs) can satisfy these conditions. HMAs are a class of semiconductor alloys where the alloying element has a very different electronegativity than that of the host. Localized states form around the mismatching atoms, which hybridize with a set of propagating states of the host to generate two split bands. Considering such a two-band model, we show that there is always a lossless window for some range of doping. We find the conditions for which the plasma frequency falls in the lossless window. We find a universal closed contour in the parameter space of all two-band HMAs, where the system can become a lossless plasmonic material for a specific range of doping. Our results suggest that such a remarkable promise is likely most experimentally achievable in HMAs with large effective masses.

Resume : Inorganic perovskites like CsPbI3 are emerging as a family of very promising materials to be used in the photovoltaic field as absorber materials. Nevertheless, besides their remarkable optical absorption properties, these materials have encountered two important bumps in their road towards commercialization, their poor stability and toxicity. In this work, we explore the possibility of lessening both stability and toxicity related problems, as well as obtaining improved photovoltaic efficiencies, through the propitious fine?tuning of the chemical composition. For that purpose, we have performed a systematic DFT study of RbaCs1?aSnbPb1?bI2Br (a = 0 ? 0.125, b = 0 ?1) perovskites, studying in detail the effects of composition changes and related solid structure distortions on the optoelectronic properties of these materials. Our results provide a complete description on the connections between the chemical composition, crystal structure, intrinsic stability, electronic properties, together with absorption features, pointing out that all?inorganic RbaCs1?aSnbPb1?bI2Br (1 > b > 0.5) perovskites would be adequate candidates for photovoltaic applications thanks to their improved stability and reduced toxicity, because of reduced content of Pb.

Resume : In decades, plasmonics has attracted considerable attentions among researchers in the field of ultrafast nonlinearity and integrated photonics. Under the electromagnetic field, the surface plasmons will oscillate at the matched frequency (known as surface plasmon resonance, i.e., SPR), which can lead to some intriguing optical properties, such as giant enhancement of linear and nonlinear optical responses. Previous research has established that the optical response band of SPR effect is closely associated to the free carrier concentration and the nanostructures of plasmonic materials and their dielectric environment. Therefore, it is of significant importance to tailor these properties for performance optimization of plasmonic devices with broad operating range. In general, noble metals with high carrier concentration (~ 10^22 cm^-3) are commonly used for plasmonics at visible and near infrared (NIR) bands. However, their large real and imaginary parts of the permittivity in the mid-infrared result in high losses and weak confinement to the surface, which limit the development of plasmonic devices at long-wavelength band. Recently, hyper-doped semiconductors (HDSCs) have emerged as new platforms for plasmonics in the terahertz band. Since the carrier concentration of HDSCs can be tailored by doping, HDSCs can be expected as the substitute of traditional plasmonic materials in the IR band. Based on Mie theory, the plasmonic wavelength decreases as the carrier concentration increases, and the realization of plasmonics at mid-infrared wavelengths requires ultra-high carrier concentration up to 10^21 cm^-3. However, one of the main obstacles faced by many researchers is the intrinsic doping limit, which is determined by the Fermi-level pinning and defect complexes at high doping concentration. More recently, our group has achieved the world-record-level doping concentration approaching ~ 10^21 cm-3 in Si and Ge by ion implantation and sub-second annealing, which shows significant potential in plasmonics from mid-infrared to far-infrared. In addition, combined with electron-beam lithography (EBL) technology, compact HDSC nanostructures can be designed with a lot of freedom to cover a wide spectral range. In this work, P doped Si thin film and nanostructures are designed and fabricated by ion implantation and EBL. Superior SPR response has been obtained by FTIR in the band of NIR and MIR. The third-order nonlinear optical response are measured within the plasmonic band. It is found that these samples can be served as excellent saturable absorbers for Q-switched lasing in tailored waveband. Further work will be focused on the optimization of nanostructures for improvement of laser performance.

Resume : We present a module for calculating the mobility and conductivity of semiconducting materials using Rode's iterative method [1]. In this module [2], the Density Functional Theory (DFT) is used to construct a variety of electronic structure inputs. The performance of the code for binary and ternary electron doped semiconductors is illustrated [3,4,5]. We also compare our results to those obtained using relaxation time approximation (RTA), and show that ours are clearly superior. Ionized impurity scattering, polar optical phonon scattering due to longitudinal phonons, acoustic deformation scattering, piezoelectric scattering, dislocation scattering, alloy scattering, intra-valley scattering, and neutral impurity scattering are among the eight types of scattering mechanisms included in this module.The current version of the module is interfaced to the Vienna ab initio simulation package (VASP). References: [1] D. L. Rode, Semiconductors and Semimetals (Academic Press, New York, 1975), Chapter 1. [2] Anup Kumar Mandia, Bhaskaran Muralidharan, Jung-Hae Choi, Seung-Cheol Lee, Satadeep Bhattacharjee, AMMCR: Ab initio model for mobility and conductivity calculation by using Rode Algorithm, Computer Physics Communications, Volume 259, 2021, 107697, [3] Mandia, A. K., Patnaik, R., Muralidharan, B., Lee, S. C., Bhattacharjee, S. (2019). Ab initio semi-classical electronic transport in ZnSe: the role of inelastic scattering mechanisms. Journal of Physics: Condensed Matter, 31(34), 345901 [4] Chakrabarty, S., Mandia, A. K., Muralidharan, B., Lee, S. C., Bhattacharjee, S. (2019). Semi-classical electronic transport properties of ternary compound AlGaAs2: role of different scattering mechanisms. Journal of Physics: Condensed Matter, 32(13), 135704 [5] Kumar, U., Nayak, S., Chakrabarty, S., Bhattacharjee, S., Lee, S. C. (2020). Gallium–Boron–Phosphide (GaBP2): a new III–V semiconductor for photovoltaics. Journal of Materials Science, 55(22), 9448-9460

Resume : There is a great interest on hyperdoping semiconductor materials because their unusual properties and the control capability of these properties through material engineering. Regarding photovoltaic industry, it is possible to increase the solar cells photoresponse by absorbing low energy photons. In this work we present a proof-of-concept device with sub bandgap absorption. We have fabricated a silicon hyperdoped solar cell based on a HIT structure (Heterojunction with intrinsic thin layer) and we have measured the J-V characteristics at different temperatures, and the external quantum efficiency (EQE). Three different solar cells have been fabricated based on silicon hyperdoped with different maerials/impurities: Ti, V and Cr. To introduce this high concentration we used ion implantation followed by a nanosecond pulsed laser melting process in order to recover the silicon crystallinity. The solar cell structure was completed by the deposition of the amorphous silicon layers by PECVD followed by the deposition of an ITO contact by magnetron sputtering. Into the rear contact a laser firing process were done previous to Al deposition by electron beam evaporation. The front electrode is finished by thermally evaporating a 2 µm thick silver grid. Preliminary results show an increase in the EQE of the silicon hyperdoped solar cells for energies lower than the silicon bandgap. This is a promising result to improve the efficiency of this type of solar cell. We fit the J-V characteristic measured at different temperatures to different models. Curves measured at high temperatures (from 360 K to 280 K) were fitted to a classic model and only one conduction processes is observed. However the behavior observed for low temperature changes drastically: a parallel transport mechanism is observed, which can be modelled by a second diode with a current limitation process in series. In order to explain these results, we propose a conduction model that fits with these measurements: The high concentration of the implanted transition metals produces a high concentration of allowed states within the silicon bandgap. This high concentration causes a decrease in carrier time life in silicon bulk. However, following the intermediated band theory, if this concentration exceeds the Luque’s limit, a band of allowed states could be formed and the silicon life time is recovered. As this band would be accessible for carriers, its presence affects to conduction processes in the material. In this work we explain the J-V and EQE measurements base on a impurity band formation within silicon bandgap. At high temperatures, a conduction path exist between the intermediate band (in hyperdoped material) and n-type amorphous silicon conduction band. As temperature decrease, the distance between those bands increase and therefore this process becomes more resistive. This effect is been observed in I-V curve as limiting mechanisms in series with the diode observed at high temperatures.

Resume : Research on amorphous oxide semiconductors (AOS) has gained momentum since the discovery of indium gallium zinc oxide (IGZO). AOS are made of at least two metal cations (Zn2+,In3+,Sn4+,Ga3+...) and oxygen. Oxide semiconductors with only one metal cation, like In2O3, SnO2, ZnO are usually polycrystalline. Applied in thin-film transistors (TFTs), they can be used in large area electronics like Active Matrix Light Emitting Diodes (AMOLEDs). Here, we demonstrate a strategy to fabricate single cation based oxide semiconductor: amorphous tin oxide. The amorphous phase is reachable by solution process, in a specific curing and annealing window. We then implemented them in TFTs, and compared their performances with polycrystalline SnO2 TFTs. TFTs with mobility in the ~100cm2Vs range are reachable, and simple circuits (inverters and ring oscillators) are also demonstrated.

Resume : Silicon doped with Tellurium (Te), a deep level impurity, at concentrations higher than the solid solubility limit (hyperdoping), was prepared by ion-implantation and nanosecond pulsed laser melting. The resulting materials exhibit strong sub-bandgap optical absorption showing potential for room-temperature broadband infrared photodetectors. As a thermodynamically metastable system, an impairment of the optoelectronic properties in hyperdoped Si materials occurs upon subsequent high-temperature thermal treatment. The substitutional Te atoms that cause the sub-bandgap absorption are removed from the substitutional sites to form Te-related complexes. In this work, we explore the phase evolution and the electrical deactivation of Te-hyperdoped Si layers upon furnace annealing through the analysis of optical and microstructural properties as well as positron annihilation lifetime spectroscopy. Particularly, Te-rich clusters are observed in samples thermally annealed at temperatures reaching 950 °C and above. Combining the analysis of polarized Raman spectra and transmission electron microscopy, the observed crystalline clusters are suggested to consist of Si2Te3.

Resume : N-type doping beyond the traditional limit has been achieved in Si:Te, where an extremely high free-electron density approaching 1021 cm-3 is obtained. In this work, the aspects of electrical doping in Si by Te ion implantation for ultralow metal/semiconductor ohmic contacts resistivity are further investigated. The Te doping concentration is in the range of 1.0 × 1021 cm-3 to 2.5 × 1021 cm-3, resulting in a free carrier density approaching 2 × 1021 cm-3. The multi-ring circular transmission line method (MR-CTLM) with simple process and high accuracy for rigorous contact resistivity study is applied to extract the contact resistivity of n-type Si:Te doped layers. The Ti/Au layers are deposited by metal evaporation and annealed by millisecond flash lamp annealing to form effective silicidation. In this way, ultralow contact resistivity on n-Si is achieved.

Resume : Many research groups have been working in the last years to develop the Si-based IR detectors technology. This technology can be used in a great number of applications, in both the military and civil ambits. Obtaining room temperature focal plane arrays based on Si with detectivity in the SWIR (or the MWIR) range would be a worldwide revolution, due to its low-cost compatibility with CMOS read-out circuitry of today electronics. One way to achieve this could be the creation of a deep impurity band in the Si bandgap that would help the promotion of electrons from the valence band to the conduction band using low energy photons (<1.12 eV). The Thin Films and Microelectronics Group have been trying to obtain Si-based SWIR materials by supersaturating Si with deep level impurities (mainly transition metals) to create an impurity band. Since the solid solubility limit of these impurities is usually very low, Pulsed-Laser Melting (PLM), as an out-of-thermodynamical equilibrium technique, has been the main technology chosen up to now. For these reasons, the traditional Rapid Thermal Annealing (RTA) was discarded. PLM-based samples and devices showed very interesting properties (IR absorption and quantum efficiency, impurity band electronic transport properties, good crystal quality), which have been widely published. On the other hand, there are very few reports on transition metals supersaturation of Si by means of RTA. Until recently, the electronic transport properties of Si implanted with high Ti doses and processed by RTA were not understood. We implanted Si (n=2.2×1013 cm−3) at 32 keV with Ti at high doses (1015 cm-2 - 2×1016 cm-2) and process them at 850 °C during 10 s to recover the crystal quality damaged during the implantation. Also, for comparative purposes, after implantation some samples were PLM processed. We measured the sheet resistance and Hall effect with the van der Pauw set-up at variable temperature (15 – 410 K) and analyzed the results. Results indicate that the high Ti concentration surface layer has a negligible conductivity due to the high quantity of defects. On the contrary, the region of the tails of the implantation has a very high electron mobility. This region presents the activation of a very shallow donor (with an energy below 30 meV and a sheet carrier concentration in the 1010 – 1011 cm-2 range) and of a deep level (~0.21 eV). While the deep level has been previously reported for Ti in Si, such a shallow level has never been measured, and we suggest that it is originated from Ti-Si complexes. Finally, a decoupling effect between the implanted layer and the substrate seems to be present, and a bilayer model is applied to fit the measured properties. The resulting parameters follow the Meyer-Neldel rule. The role of the tails of the implantation in Si supersaturated with Ti is unraveled in this work, and the model developed can be applied also for PLM fabricated samples.

Resume : Many research groups have been working in the last years to develop the Si-based SWIR detectors technology. This technology can be used in a great number of applications, in both the military and civil ambits. Obtaining Room Temperature (RT) focal plane arrays based on Si with detectivity in the SWIR (or the MWIR) range would be a worldwide revolution, due to its low-cost compatibility with CMOS read-out circuitry of today electronics. One way to achieve this could be the creation of a deep impurity band in the Si bandgap that would help the promotion of electrons from the valence band to the conduction band using low energy photons (<1.12 eV). One method to obtain Si-based SWIR materials would be supersaturating Si with deep level impurities (for instance transition metals) to create an impurity band. Since the solid solubility limit of these impurities is usually very low, Pulsed-Laser Melting (PLM), as an out-of-thermodynamical equilibrium technique, has been the main technology chosen up to now. However, PLM is only capable of creating a very thin surface layer of supersaturated material, and therefore the potential efficiency is very low. On the other hand, Microwave Annealing (MWA) is a bulk process that can recrystallize thick implanted or deposited layers, with very low impurity diffusion, due to the non-thermal effects inherent to the microwave absorption process. We suggest that MWA could be an alternative to improve the efficiency of Si-based SWIR devices by increasing the thickness of the sensitive supersaturated material. As far as we know, this is the first attempt to use MWA for supersaturating semiconductors. We implanted Si samples (150-300 Ωcm) with Ti at 35 keV (1015 cm-2) and processed them by MWA at 1.8 – 3 kW during 100 – 600 s to recover the crystal quality damaged during the implantation. We measured the Ti depth profile by SIMS and the crystal quality by TEM. Also, we measured the electronic transport properties (15 – 300 K) and the photoconductivity (50 – 300 K). Results indicate that samples processed at 3 kW during 300s or more present a monocrystalline surface layer with a high density of defects. The crystal quality can be enhanced with increasing MWA time. Also, once the implanted layer is recrystallized, the Ti profile is unchanged, pointing to a very low Ti diffusion for longer MWA processes. This is very interesting, since a high impurity concentration is needed to form the impurity band. The electronic transport properties show a decoupling effect with the substrate, high mobility, and a very low impurity activation. Regarding photoconductivity, some samples showed IR response up to 1800 nm at RT. At lower temperature the IR response increases rapidly (4 orders of magnitude at 100 K). This work is a first step to obtain thick Si:Ti layers with SWIR response at RT. Next steps would be to reduce the defect density and to fabricate thicker Si:Ti layers.

Resume : There is a vast number of possible point defects that can exist in one material. Identifying which defect that best explains experimental observations is challenging and requires a lot of theoretical data. To produce this data, we performed high-throughput ab-initio calculations with ADAQ—a collection of automatic workflows that generates defects, screens for relevant properties such as formation energy and zero-phonon lines, fully calculates additional properties such as zero-field splitting and hyperfine coupling parameters for many different charge and spin states. We have created and screened 8355 single and double intrinsic defects in 4H-SiC. The results for these point defects show which defect type and configurations are the most stable, i.e., on the defect hull. In this presentation, we explore this database to highlight some interesting systems and explain experimental observations.

Resume : Silicon photonics is demonstrated to be instrumental for a broad range of applications such as long-haul telecommunications, sensing, signal processors, light-field displays and artificial neural networks [1]. Unlike silicon microelectronics, silicon photonics makes use of photons to detect, process and transmit information more efficiently than electrical signals, and yet have low manufacturing costs as a result of using processes of commercial silicon integrated circuits. In turn, photonic integrated circuits (PICs) offer important advantages such as scalability, low-latency control electronics and the inherent low noise and high speed of photons. A PIC is composed of a set of elementary building blocks such as photon sources, reconfigurable photonic elements and detectors. Although the advantages of silicon photonics are enormous, there are also crucial challenges imposed by the 1.12 eV-indirect Si band gap. This makes silicon an inefficient light emitting material, along with the difficulty to detect photons in the main optical telecom bands (1.3 and 1.5 μm). The hybrid integration in silicon of InP lasers and Ge- and III-V-based photodetectors has been adopted. Yet, these heterogeneous approaches, leading to silicon hybrid PICs are expensive and strongly limited in complexity. Therefore, the monolithic integration of all these building blocks on a silicon chip would be more suitable for practical and broader applications [2]. In this work, we show an approach for integrating all the three elementary building blocks in a commercial silicon-on-insulator wafer following standard CMOS processes. Our approach monolithically integrates, on a single SOI chip, an array of optically driven Si-based telecom photon sources coupled to telecom Si-based photodetectors through strip waveguides. We developed a lateral silicon telecom photodetector based on hyperdoped Si, which exhibits a strong absorption of 104 cm-1 in the strategic telecom O-band (1.3 µm) [3]. In turn, the telecom light source is based on one of the carbon-related damage centers in silicon, the so-called G center, which has a zero-phonon photoluminescence line at 1.28 µm that falls into the telecom O-band [4]. This work holds great promise for reducing the large coupling losses between the traditionally used off-chip light sources and off-chip detectors and the Si chip. References [1] Margalit, N., Xiang, C., Bowers, S. M., Bjorlin, A., Blum, R., & Bowers, J. E. (2021). Perspective on the future of silicon photonics and electronics. Applied Physics Letters, 118(22), 220501. [2] Pavesi, L. (2003). Will silicon be the photonic material of the third millenium?. Journal of Physics: Condensed Matter, 15(26), R1169. [3] Wang, M., García‐Hemme, E., Berencén, Y., Hübner, R., Xie, Y., Rebohle, L., ... & Zhou, S. (2021). Silicon‐Based Intermediate‐Band Infrared Photodetector Realized by Te Hyperdoping. Advanced Optical Materials, 9(4), 2001546. [4] Hollenbach, M., Berencén, Y., Kentsch, U., Helm, M., & Astakhov, G. V. (2020). Engineering telecom single-photon emitters in silicon for scalable quantum photonics. Optics Express, 28(18), 26111-26121.

Resume : One of the advantages of Si-Ge-Sn alloying is the enhanced optical absorption and tunable energy band gap of the alloy. In this talk, a short review of structural changes and segregation in SiGe(Sn) alloy layers during laser-induced melting and fast crystallization will be done. The layers of epitaxial or polycrystalline SiGe(Sn) alloy are deposited by MBE or CVD on Si substrate and are treated by a pulsed laser beam (25-100 ns, 0.2-3.5 J/cm2). Structural changes and optical properties are studied by using electron microscopy, time-resolved reflectivity, RBS/Channelling, atomic force microscopy and Raman spectroscopy. A special attention is devoted to fast crystallization of Si-based alloy layers and formation of cellular structure. We concentrate on segregation of dopant to a nanometer-scale cellular network; time-resolved reflectivity measurements of melting, crystallization and segregation. Optical properties of segregated cellular SiGe/Si structures are considered as well. The results of pulsed laser modification of Ge and GeSn nanodots will also be presented assuming that fast segregation is a powerful tool for production of non-equilibrium compounds, e.g. metastable Ge1-xSnx dots with a direct tuneable energy band gap. A new approach for nano-profiling of the surface is proposed and implemented. The approach consists of pulsed laser melting of SiGe(Sn) layers, self-organized formation of a cellular nanostructure followed by selective etching of Ge out of walls of the cells. The regimes of laser melting and anodic or thermal etching are achieved, in which the formation of epitaxial Si columns (200-300 nm in diameter and 500 nm in height), separated by grooves of 3-30 nm wide takes place. The nano-profiled SiGe/Si surface is finally applied for high-temperature growth of epitaxial SiC layers.

Resume : The epitaxial growth of group IV semiconductor materials is one of the backbones of modern Integrated Circuit (IC) production flows. It is used for the Source/Drain (S/D) engineering of both n- and p-MOS high performance transistors, allowing the deposition of Si:P and Si1-xGex:B layers with active carrier concentrations as high as ~ 1x1021 cm-3, ensuring low contact resistivities [1]. Still, with reducing device dimensions, the growing importance of contact resistance in devices has become a major concern [2]. This sets a need to further increase the active doping concentration in the epitaxial S/D layers, which can be achieved by using higher order precursors at very low growth temperatures [3,4]. A significantly lower parasitic S/D resistance could also be obtained thanks to the use of epitaxial and metal wrapped-around contacts [5]. In addition, new device concepts which are considered for the upcoming technological nodes of 3 nm and below, require process temperatures below 500oC. Examples are 3D transistor stacking and gate-all-around (GAA) nanosheet device concepts which include complementary FET (CFET) and forksheet devices. In this contribution, the different Si:P and Si1-xGex:B epitaxial growth processes considered for these Si-channel devices will be reviewed. Metal-to-semiconductor contact resistivities as low as 2x10-9 Ohmcm2 (n-type) and 1.5x10-9 Ohmcm2 (p-type) will be demonstrated. References [1] H. Wu et al., 2018 IEEE International Electron Devices Meeting (IEDM), 35.4.1. [2] P. Raghavan et al., 2015 IEEE Custom Integrated Circuits Conference (CICC), p. 1. [3] E. Rosseel et al., ECS Trans. 98 (5), 37 (2020). [4] A. Hikavyy et al., ECS Trans. 104 (4), 139 (2021) [5] S. Chew et al., 2017 IEEE International Interconnect Technology Conference (IITC) 1.

Resume : Although boron doped silicon (Si:B_N) is the best-known semiconductor material, superconducting (Si:B_S) is not yet a common material among the superconductivity community. A superconducting phase is observable in silicon provided that the boron concentration reaches a threshold value, which is larger than the solubility limit (n_(B-thresh)≳10^20 cm^(-3)∼n_solubility). These concentrations are achievable using nanosecond laser doping: a precursor gas, BCl_3, is chemisorbed on a Si wafer, which is then melted by a laser pulse (λ=308 nm,t_pulse=25ns,E_S≥0.5 mJ/cm^2). The boron diffuses in the liquid phase before being trapped as the melted phase recrystallizes. The doped depth (d) and the incorporated concentration of dopants (C_B) are controlled by respectively modulating the incoming energy (E_S) and the number of process repetitions (N). Fabrication of Si:B_S nanodevices generates a lot of interest as it could easily be implemented in more complex Si based micro-electronics systems. However, the structural and electrical properties of the doped layers are strongly dependent on the melting process and need to be understood in order to conceive and model such devices. The present work is dedicated to the analysis of the structural and electrical properties of several layers at both room and cryogenic temperature with respect to the doping parameters (d,C_B, N). Secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) analysis reveal sharp interfaces and homogeneous monolayers for small concentrations (n_B∼2.10^21). Superconductivity appears and rises with doping at higher concentrations where we also observe the presence of non-activated boron. It then is gradually suppressed as the interstitial boron content is increased. For such level of doping, TEM images reveal partial strain relaxation, dislocations and doping inhomogeneities. We have shown that T_c can be tuned by the concentration of dopants and studied its decline at large disorder. Moreover, X-ray diffraction measurements (XRD) were realized with the aim of better understanding the strong interaction mechanisms at play in such covalent superconductors. We have highlighted a correlation between the strain relaxation of the ultra-doped layer and the onset of superconductivity.

Resume : Silicon is one of the most well-known materials, and the main actor in today electronics. Despite this, silicon superconductivity was only discovered in 2006 [1] in laser doped Si:B samples. Laser annealing is instrumental to cross the superconductivity threshold, as the required doping is above the solubility limit, and cannot be reached using conventional micro-electronic techniques. Laser doping allows the realisation of epitaxial, homogeneous, thin silicon layers (5-300 nm) with extreme active doping values as high as 11 at. %, and without the formation of B aggregates. Silicon is a disordered superconductor, with a lower carrier density (1e20 – 1e21 cm-3) than metallic superconductors, a critical temperature modulable with doping from 0 to 0.7 K, and a resistivity easily matched to the void impedance. After demonstrating all-silicon SQUIDs and Josephson junctions [2,3], we have realised microwave silicon resonators [4,5], working in the 1-10 GHz range and with quality factors of the order of 4000. We have shown a strong non-linear response with power, observing a Kerr coefficient of the order of 300 Hz/photon where less than 1 Hz/photon was expected. Additionally, we have investigated the quasiparticle dynamics when submitting the resonators to either a light or a microwave pulse and following the resonators relaxation to equilibrium. We compared the results of these out-of-equilibrium measurements with the characteristic equilibrium generation-recombination time measured through the noise spectral density. Both techniques showed a long quasiparticle lifetime of the order of 0.6 ms, among the largest measured in similar systems. References [1] E. Bustarret et al., Nature 444, 465 (2006) [2] J.-E. Duvauchelle et al., Applied Physics Letters 107, 072601 (2015) [3] F. Chiodi et al., Phys. Rev. B 96, 024503 (2017) [4] P. Bonnet, F. Chiodi, et al., arXiv:2101.11125 [5] F. Chiodi, R. Daubriac and S. Kerdilès, Laser ultra-doped silicon: Superconductivity and applications (ch.9) Laser Annealing Processes in Semiconductor Technology, Elsevier (2021)

Resume : The discovery of superconductivity in Si:B [1] opens the door to powerful new applications since silicon technology is the most mature one for nanoelectronics. Obtained by laser doping, Si:B layers can be epitaxied over a silicon substrate forming a cristalline interfaces without Schottky barriers and ensuring ohmic contact with metal. The thickness and dopant, B, concentration are well controled. This control allows, in turn, to tune the superconconding critical temperature , Tc, in the 0.1 K – 0.7 K range [2,3]. Having successfully realized SQUID [4] superconductor/normal-metal junctions [5] and superconducting resonators [6] based on Si : B, we are working now on the conception of all-Si superconducting JoFET and have demonsrated that the interface resistance between the superconducting and the metallic doped Si layer (i) depends on the dopant, B, concentration and (ii) is lower enough to enable a long-range proximity effect and thus Josephson effect. [1] E. Bustarret, C. Marcenat, P. Achatz, J. Kacmarcik, F. Levy, A. Huxley, L. Ortega, E. Bourgeois, X. Blase, D. Débarre, and J. Boulmer, Nature (London) 444, 465 (2006). [2] A. Bhaduri, T. Kociniewski, F. Fossard, J. Boulmer, and D. Débarre, Applied Surface Science 258, 9228 (2012). [3] A. Grockowiak, T. Klein, E. Bustarret, J. Kacmarcík, C. Dubois, G. Prudon, K. Hoummada, D. Mangelinck, T. Kociniewski, D. Débarre, J. Boulmer, and C. Marcenat, Superconductor Science and Technology 26, 045009 (2013). [4] J. E. Duvauchelle, A. Francheteau, C. Marcenat, F. Chiodi, D Débarre, K. Hasselbach, J. R. Kirtley, and F. Lefloch. Applied Physics Letters, 107(7), 2015. [5] F. Chiodi, J. E. Duvauchelle, C. Marcenat, D Débarre, and F. Lefloch. Physical Review B, 96(2) :1–7, 2017. [6] P. Bonnet, F. Chiodi, D. Flanigan, R. Delagrange, N. Brochu, D. Débarre and H. le Sueur, arXiv:2101.11125

Resume : Diamond is one of the prospective materials for realization of optical integrated circuits, including waveguides, single-photon sources and single-photon detectors on a single chip [1,2]. The complete realization of the so-called 'diamond platform' requires an efficient single-photon detector, which is technologically compatible with the platform. Single-photon detectors that are used in such hybrid circuits are currently based on disordered superconducting NbN films [1,2]. These detectors’ performance depends strongly on the structural uniformity, which is determined by the smoothness of the diamond platform. Development of a superconducting single-photon detector based on a boron-doped diamond (C:B) enables fabrication of a monolithic diamond platform hence studying thermal transport in this material is required. We expand the methods of our studies of nonequilibrium processes occurring in superconducting boron-doped single-crystal diamond films [3] to understand better energy relaxation of electrons (holes) in this material. We used noise thermometry for measure the rate of energy transport between the electrons and lattice at low temperatures [4]. The measurements of 130-nm thick C:B epilayer on IIa-(100)-oriented diamond substrate were made. The epilayer was also characterized with a critical temperature Tc =1.7 K and a carrier density of 1.46·10^21 cm^-3 measured at 20 K. The studied samples were structured into wires with a length varied from 2.5 to 330 μm and a width from 150 nm to 30 μm respectively. We measured noise spectral power in the samples at low heating (joule) powers and at low bath temperature (0.5 K) and observed that the heat flow rate is associated either with electrons diffusion into reservoirs or with electron-phonon interaction. The latter is characterized by the power-law dependence Р=Σ·Ω(TN^5 – Tb^5) [5] where Ω is the sample volume, TN is the noise temperature, Σ = 0.028 nW/(K^5µm^3) is the thermal coupling constant. The obtained value for Σ is lower than conventional metals have (Au-2.4, Al-0.3 nW/(K^5µm^3)), but close to values for P-doped Si – 0.04 nW/(K^5µm^3) [6]. At high heating powers and at bath temperature (4.2 K), we observe a deviation from the T^5 dependence of the heat flow rate towards the T^3-dependence and also a strong decrease of the thermal coupling in large samples. To describe the observed effects, we propose the phenomenological model taking into account ballistic and diffusive phonon transport regimes. Our findings are important for understanding operation of devices embedded in crystalline substrates. This work was supported by the RSF project No.19-72-10101. 1. P.Rath et.al, Light:Science&Applic., 4, 2015 2. Lenzini, et al, Adv. Quantum Technol. 2018 3. A. Kardakova et. al, Phys.Rev B 93, 2016 4. M. Roukes, et al, Phys.Rev.Lett.55, 1985 5. B. Huard, et al, Phys. Rev. B 76, 2007 6. Giazotto et.al, Rev. Mod. Phys. 78, 2006

Resume : As an important silicon material in the nanometer-sized regime, silicon nanocrystals (Si NCs) have various potential applications [1]. Among all kinds of approaches for the synthesis of silicon nanocrystals nonthermal plasma is very attractive [2], especially given its capability of hyperdoping Si NCs in a non-equilibrium environment. In the past few years we have hyperdoped Si NCs by using nonthermal plasma. These hyperdoped Si NCs exhibit novel electronic and optical properties. We have explored the use of hyperdoped Si NCs in photodetectors and synaptic devices [3-5]. [1] Z. Y. Ni, et al., Materials Science & Engineering R 138, 85-117 (2019). [2] L. Mangolini, et al., Nano Lett. 5, 655–659 (2005). [3] Z. Y. Ni, et al., ACS Nano 11, 9854-9862 (2017). [4] W. Huang, et al., Nano Energy 73, 104790 (2020). [5] Y. Wang, et al., Advanced Functional Materials 32, 2107973 (2022).

Resume : Due to their potential use in a wide range of applications such as nanophotonics, light harvesting, sensing, photocatalysis, photothermal therapies, nanocrystals (NCs) exhibiting surface plasmon resonance have been the subject of an intense research activity. Highly doped semiconductors NCs have recently gained a significant attention mainly because the frequency of the LSPR is expected to be tuned by controlling the dopant concentration and thus the free carrier concentration. This is not achievable with noble metals in which the free carrier concentration is fixed [1]. Moreover the energy range of the LSPR in semiconductors NCs is redshifted as compared to metals. As a low cost, non-toxic and CMOS compatible material, silicon (Si) has emerged as a promising material for plasmonic application. LSPR were obtained in highly phosphorus-doped and boron-doped Si NCs produced via nonthermal plasma synthesis [2,3]. However, when being exposed to air, the plasmonic properties of doped Si-NCs can be modified by oxidation and surface modification [4]. In this work, highly phosphorus-doped Si-NCs were obtained by elaboration of phosphorus-doped SiO/SiO2 multilayers, subsequently annealed by a rapid thermal annealing process at temperatures up to 1100 °C. The sample structure and the localization of P atoms were both studied at the nanoscale by scanning transmission electron microscopy and atom probe tomography. It is found that P incorporation promotes the phase separation process in the SiO layers leading to the growth of doped Si-NCs that reach sizes ranging from 10 to 30 nm in diameter with P concentration up to several at%. Infrared absorption spectroscopy experiments demonstrate that depending on the P content, a tunable plasmon-related absorption is obtained. Using a core-shell structure to model doped Si-NCs in a SiO2 environment, the plasmonic response is well described by the Drude theory. The model allows us to extract free carriers concentrations values up to 2.5x1020 cm-3. Moreover, with increasing P concentrations and for the smallest Si-NCs, the photoluminescence properties disappear as expected, as a consequence of the Auger effect induced by the free carriers. [1] J.M. Luther, P.K. Jain, T. Ewers, et al. Nature materials, 2011, 10(5): 361-366; [2] D.J. Rowe, J.S. Jeong, K.A. Mkhoyan, U.R. Kortshagen, Nano Lett. 2013, 13, 1317−1322; [3] S. Zhou, X. Pi, Z. Ni, Y. Ding, Y. Jiang, C. Jin, C. Delerue, D. Yang, T. Nozaki, ACS Nano, 2015, 9(1):378-386 ; [4] N.J. Kramer, K.S. Schramke, U.R. Kortshagen, Nano letters, 2015, 15(8): 5597-5603.

Resume : Materials consisting of silicon nanocrystals (Si-ncs) embedded in SiO2 are the subject of an intense research activity due to their potential applications in optoelectronic. Providing charged carriers by introducing n- or p-type impurities in the core of Si-ncs can drastically modify their properties. A perfect control of the doping level should allow to tune the material properties for specific applications. In fact, it has been shown that highly P or B doped Si-ncs should exhibit strong plasmonics properties. However, due to the low solubility of impurities in silicon and side effects like self purification (i.e. impurity are expelled toward Si-ncs), reaching high doping level could be difficult. To understand and to improve the quality of these systems, an accurate control of the dopant location is necessary. In this work, phosphorous doping of silicon nanocrystals carried out by several elaboration strategy and annealing process have been investigated using Atom Probe Tomography. By computing 3D mapping of chemical species, we studied the structure of these films at the atomic scale to investigate the location of dopants and the Si clustering characteristics. We will discussed the influence of the dopant on the Si-ncs growth and showed that high level of phosphorous impurities can be introduced in the core of every Si-ncs. Behavior of phosphorous as function of the elaboration strategy will be discussed.

Resume : The n-type doping of Ge is a self-limiting process due to the formation of vacancy-donor complexes (DnV with n ≤ 4) that deactivates the donors. Based on data density functional theory calculations, at temperature higher than 850 K, the concentration of D4V clusters progressively decreases liberating unbounded vacancies and donor atoms. Similar problems apply to wide-band gap semiconductors where the p-type doping is challenging, mainly due to the high activation energy for acceptors, low equilibrium solid solubility and deactivation of acceptors by the formation of acceptor-vacancy clusters. Here, we report on experiments and theoretical calculations solving the basic problem of donors and acceptors deactivation in heavily doped semiconductors. The dissolution of donor/acceptor-vacancy clusters in heavily doped semiconductors is achieved by ms-range FLA with a peak temperature close to the melting point of the semiconductor. Positron annihilation lifetime spectroscopy (PALS) reveals that dopant-vacancy clusters are the main defect centers deactivating both acceptors and donors. Millisecond-range high-temperature treatment dissociates the dopant-V clusters and, as shown by SIMS, fully suppresses the dopant diffusion in both group IV semiconductors and III-V compound semiconductors. For the first time, using structural characterization (PALS, SIMS) and electrochemical capacitance-voltage profiling combined with DFT calculations, we were able to address, understand, and solve the fundamental problem hindering the doping of semiconductors above the solid solubility limit.

Resume : The search for room-temperature ferromagnetism (RTFM) in wide band gap semiconductors has emerged as an important area of research from fundamental physics and for possible applications in future spintronic devices. Due to the coexistence offerromagnetism (FM) and semiconducting properties in one material, the dynamics of electron spin can be controlled and manipulated, which is essential for spin-based devices to be operated in ambient condition. Both III–V and II–VI semiconductors in pure form and doped with transition metals (TMs have been investigated in detail by various researchers. It has been observed that FM in these materials is originated due to the following reasons: (i) doping of TM, which causes creation of quasi-free electrons and interaction with localized magnetic moments, leading to FM (intrinsic FM), (ii) segregation of TM clusters leading to FM (extrinsic FM), and (iii) FM without incorporation of any TM can also be triggered due the presence of some type of defects in these materials (defect-mediated FM). In case of zinc oxide and Gallium nitride FM due to all the three aforementioned reasons have been observed and various reports can be found in the literature. There are still quite a few fundamental issues, which need to be resolved before integrating this material in spintronic devices.This work describes investigations on intrinsic, ion-implanted/irradiated ZnO and GaN and the role of defects and correlation with electronic structure are discussed in three sections: i. Microstructure controlled RTFM in implanted ZnO films, ii. Tuning FM strength in ZnO and GaN by controlled variation of defects and iii. Correlation of RTFM with electronic structure based on synchrotron based x-ray absorption results. Correlation of the experimental results with first principle based calculation.

Resume : In the present work, on the basis of outstanding crystalline AlN template whose full width at half maximum (FWHMs) of XRD rocking curves (002) and (102) are as low as 50 and 180 arcsec, respectively, the highly activated p-type AlGaN was achieved by MOCVD. Due to the compressive-strain induced splitting of valence band in p-AlGaN, an ultra-high hole concentration at around 1018 /cm3 level was realized in spite that the Al concentration is as high as 60% in p-AlGaN. Moreover, such a high-activated material on outstanding AlN template acts as an ideal p-type source for high performance UVC-LED, and accordingly we achieved the high performance 4-inch UVC-LED for the first time, which sweeps the main obstacle to match the UVC-LED preparation with current GaN based blue-LED process.

Resume : Solid solutions of spinel nitrides with cations from the 14 Group of the periodic table are thermally stable materials very suitable for optoelectronic applications, thanks to outstanding properties like their great hardness and oxidation resistance, plus a wide bandgap range when modifying partial concentration of the cations. Specifically, tin-germanium nitride solid solutions have been previously reported as promising materials for intermediate-band solar cells (IBSC), used as host materials where an intermediate band (IB) could be achieved through transition metal hyperdoping of the optimal host. Discovery of an operative IBSC material could contribute to a groundbreaking change in the field of photovoltaics, thanks to ideal efficiency limits up to 63% with a single layer configuration. To be able to explore the vast compositional and configurational space of these materials in an efficient way to find an optimal host, we have performed a thorough study combining density functional theory (DFT) and machine learning (ML) techniques, aiming at a full and accurate description of the electronic properties for all the possible configurations within the space. The implementation of ML and other statistical analysis techniques allows us to reduce the number of DFT calculations required to fulfill that task, accelerating the process by several orders of magnitude. Mixing energies, to give a broad perspective of thermodynamics and intrinsic stability of the different configurations, as well as accurate bandgaps, obtained through correlation of some HSE results and more basic DFT calculations, are successfully reproduced by the ML models. We test several statistical models with different complexity levels and a variety of atomic-level descriptors based on structural information in the process. Our results demonstrate the usefulness of ML for predictions in the configurational space of alloys.

Resume : Material development holds promise as the basis of topological quantum computing with Majorana fermions. These quasiparticles have been predicted to be formed in semiconductor (SE) nanowires coupled to conventional superconductors (SUs) [1-2]. This prediction has been followed by a series of experiments providing strong evidence [3-4]. In the system, the Zeeman energy, that is required for hybrid nanowires to enter the topological phase, is provided by a strong external magnetic field. However, such a magnetic field imposes additional restrictions on the device fabrication layout and components. Is it possible to further develop hybrid materials and thus minimize the need for the external field? It is well-known that materials combining ferromagnetism and semi-conductivity, that is ferromagnetic insulators (FMIs), have been developed for spin-based electronics, and intrinsic magnetism can be induced by them. The ferromagnetic hybrid nanowires, which integrates a FMI into SE–SU NWs, i.e. SE–SU–FMI NWs, is derived from this idea. In this work, we grow epitaxial SE–SU–FMI InAs-Al-EuS hybrid nanowires in-situ in the molecular beam epitaxy system. The results show the superconducting hard gap, the transport hysteresis and the shape-defined magnetic single domain structures based on well-controlled epitaxy, which suggests that this highly ordered material system is a promising platform for scalable topological quantum computing. References [1] R. M. Lutchyn et al, Phys. Rev. Lett. 105, 077001 (2010). [2] Y. Oreg et al, Phys. Rev. Lett. 105, 177002 (2010). [3] V. Mourik et al, Science 336, 1003 (2012). [4] A. Das et al, Nat. Phys. 8, 887 (2012).