Epitaxial Samples Research Articles

Study of the Photo-Response of Doped GaAs with Aging

The aging of semiconductor materials is a topic of current interest. We studied the photo-response of epitaxial samples of GaAs doped with Ge and Sn up to 1 × 1019 atoms cm−3. These samples were stored in a dry and dark environment for 26 years. We realized photoluminescence measurements at different temperatures and photoreflectance spectra at 300 K in three periods: 1995, 2001 and 2021. We found that environmental oxygen formed defects in GaAs, leaving lattice vacancies that provoked changes in the optical photo-response. In addition, we found that the vacancy concentrations could be as large as 5 × 1017 atoms cm−3 over the 26 years. In this work, we demonstrate that the aging of semiconductor materials occurs even when they are not used within a functioning circuit, with the changes being greater when the material is not doped. Knowing about the aging of materials is important for the industry, particularly for the semiconductor industry, because aging-induced deterioration influences prices and guarantees.

Optical Response of Aged Doped and Undoped GaAs Samples.

We studied epitaxial GaAs samples doped with Ge and Sn up to 1×1019 cm -3, which were stored in a dry and dark environment for 26 years. The optical response of the GaAs samples was determined through the photoluminescence and photoreflectance techniques, taken at different times: just after their fabrication in 1995, 2001 and 2021. The evolution of defects formed by the action of O 2 in the samples and their correlation with doping with Ge and Sn impurities were studied. We obtained the result that aging formed defects of type vacancies, mainly As, which produced energy levels of deep traps linked to the L band. The concentration of vacancies over the 26 years could be as large as 1017 cm -3, and these vacancies form complexes with doping impurities.

Formation of Point Defects Due to Aging under Natural Conditions of Doped GaAs.

The aging dynamics of materials used to build the active part of optoelectronic devices is a topic of current interest. We studied epitaxial samples of GaAs doped with Ge and Sn up to 1×1019 cm-3, which were stored in a dry and dark environment for 26 years. Photoluminescence spectra were taken in three periods: 1995, 2001 and 2021. In the last year, time-resolved photoluminescence, Raman, and X-ray measurements were also performed to study the evolution of defects formed by the action of O2 in the samples and its correlation with the doping with Ge and Sn impurities. We found that oxygen formed oxides that gave off Ga and As atoms, leaving vacancies mainly of As. These vacancies formed complexes with the dopant impurities. The concentration of vacancies over the 26 years could be as large as 1×1018 cm-3.

Epitaxial light actinide oxide thin films

Epitaxial thin films of ThO2 and (UxTh1−x)O2 mixed oxides (MOX) have been synthesised by DC magnetron sputtering. The samples were characterised using x-ray diffraction and spectroscopic ellipsometry. Three epitaxial ThO2 samples of different crystallographic orientations have been synthesised and confirmed by x-ray diffraction analysis. The samples are [1 1 1], [1 1 0] and [0 0 1] oriented, with lattice parameters determined to be 5.714 ± 0.001 Å, 5.707 ± 0.001 Å and 5.624 ± 0.001 Å respectively. Four [0 0 1] oriented epitaxial (UxTh1−x)O2 samples have been fabricated, across 0≤x≤1, with all samples found to have a unique specular direction. The mixed oxides were found to obey Vegard’s law, with lattice parameter varying linearly with ThO2 content. Values for the optical band gap and optical constants of the epitaxial ThO2 and MOX samples have also been determined by spectroscopic ellipsometry. The band gap determined for the three ThO2 samples were in the 3.9 - 4.6 eV range, which is a slight underestimate compared to other experimental values. The optical band gap was found to vary approximately linearly with ThO2 content, across the sample series. These films can be implemented in the research of thorium-based nuclear fuels, mixed actinide oxide nuclear fuel and the long term storage of nuclear waste forms.

Optical and Structural Properties of Aluminum Nitride Epi-Films at Room and High Temperature

The high-quality aluminum nitride (AlN) epilayer is the key factor that directly affects the performance of semiconductor deep-ultraviolet (DUV) photoelectronic devices. In this work, to investigate the influence of thickness on the quality of the AlN epilayer, two AlN-thick epi-film samples were grown on c-plane sapphire substrates. The optical and structural characteristics of AlN films are meticulously examined by using high-resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM), a dual-beam ultraviolet-visible spectrophotometer, and spectroscopic ellipsometry (SE). It has been found that the quality of AlN can be controlled by adjusting the AlN film thickness. The phenomenon, in which the thicker AlNn film exhibits lower dislocations than the thinner one, demonstrates that thick AlN epitaxial samples can work as a strain relief layer and, in the meantime, help significantly bend the dislocations and decrease total dislocation density with the thicker epi-film. The Urbach’s binding energy and optical bandgap (Eg) derived by optical transmission (OT) and SE depend on crystallite size, crystalline alignment, and film thickness, which are in good agreement with XRD and SEM results. It is concluded that under the treatment of thickening film, the essence of crystal quality is improved. The bandgap energies of AlN samples obtained from SE possess larger values and higher accuracy than those extracted from OT. The Bose–Einstein relation is used to demonstrate the bandgap variation with temperature, and it is indicated that the thermal stability of bandgap energy can be improved with an increase in film thickness. It is revealed that when the thickness increases to micrometer order, the thickness has little effect on the change of Eg with temperature.

Nonradiative Dynamics Induced by Vacancies in Wide-Gap III-Nitrides: Ab Initio Time-Domain Analysis.

Insightful understanding of defect properties and prevention of defect damage are among the biggest issues in the development of photoelectronic devices based on wide-gap III-nitride semiconductors. Here, we have investigated the vacancy-induced carrier nonradiative dynamics in wide-gap III-nitrides (GaN, AlN, and AlxGa1-xN) by ab initio molecular dynamics and nonadiabatic (NA) quantum dynamics simulations since the considerable defect density in epitaxy samples. E-h recombination is hardly affected by Vcation, which created shallow states near the VBM. Our findings demonstrate that VN in AlN creates defect-assisted nonradiative recombination centers and shortens the recombination time (τ) as in the Shockley-Read-Hall (SRH) model. In GaN, VN improves the NA coupling between the CBM and the VBM. Additionally, increasing x in the AlxGa1-xN alloys accelerates nonradiative recombination, which may be an important issue in further improving the IQE of high Al-content AlxGa1-xN alloys. These findings have significant implications for the improvement of wide-gap III-nitrides-based photoelectronic devices.

Supercurrent diode effect in thin film Nb tracks

We demonstrate nonreciprocal critical current in 65 nm thick polycrystalline and epitaxial Nb thin films patterned into tracks. The nonreciprocal behavior gives a supercurrent diode effect, where the current passed in one direction is a supercurrent and the other direction is a normal state (resistive) current. We attribute fabrication artifacts to creating the supercurrent diode effect in our tracks. We study the variation of the diode effect with temperature and the magnetic field and find a dependence with the width of the Nb tracks from 2 to 10 μm. For both polycrystalline and epitaxial samples, we find that tracks of width 4 μm provide the largest supercurrent diode efficiency of up to ≈30%, with the effect reducing or disappearing in the widest tracks of 10 μm. We propose a model based on the limiting contributions to the critical current density to explain the track width dependence of the induced supercurrent diode effect. It is anticipated that the supercurrent diode will become a ubiquitous component of the superconducting computer.

Surface Morphology and Photoluminescence Spectra of Pseudomorphic {InGaAs/GaAs} Superlattices on GaAs (100), (110), and (111)A Substrates

The production of superlattices with pseudomorphically strained quantum wells (QWs) {InxGa1–xAs/GaAs} grown by molecular beam epitaxy on GaAs substrates with (100), (110), and (111)A crys-tallographic surface orientations is reported. The quality of the crystal structure of epitaxial samples is assessed using the atomic force microscopy of their surface. The manifestation of a piezoelectric field in the photoluminescence spectra is reported.

Experimental and theoretical study on the complete phase separation of ceria-zirconia solid solution into two end members, ceria and zirconia

Solid solution CeO2–ZrO2 has long been used as a non-noble metal oxide promoter for three-way catalysts owing to its high oxygen storage capacity. However, the stability issue of the CeO2–ZrO2 has been controversial for a long time. In particular, the phenomena observed by phase instability are so diverse and inconsistent that the related causal analysis is still a matter of debate. In this study, for the first time, it was demonstrated theoretically and experimentally that a Ce0.75Zr0.25O2 (CZO) solid solution must be completely separated into CeO2 and ZrO2 phases owing to its inherent thermodynamic instability. According to an extensive evaluation via defect chemical calculations and well-controlled model experiments with grain-boundary-free epitaxial thin film samples, CZO materials undergo phase separation until they are completely separated, and the separation rate is particularly high in a reducing atmosphere. The underlying inherent stability problem and enhanced phase separation kinetics of the CZO material are attributed to the enhanced cation diffusion in a reducing atmosphere, where more mobile cationic defects (interstitial cations) are generated and an easier pathway with a lower migration energy is available.

π Band Folding and Interlayer Band Filling of Graphene upon Interface Potassium Intercalation

AbstractThe structural and electronic properties of epitaxial monolayer graphene on SiC(0001) are examined upon potassium intercalation. Notably, the first real‐space observation via scanning tunneling microscopy of the (2 × 2) superstructure in graphene‐based thin films formed by the potassium atoms below the uppermost graphene layer is presented. Therein, additional signatures stemming from quasiparticle interference effects are found allowing investigations of the electronic bands of K‐intercalated epitaxial monolayer graphene on a local scale. Those data are compared to area‐averaged results obtained from photoelectron spectroscopy. In particular, backfolding of the graphene π bands are found as a consequence of the (2 × 2) superstructure of the K atoms with respect to the graphene lattice. This is accompanied by a strong n‐type doping effect that causes a rigid shift of the Dirac bands to higher binding energies and filling of two parabolic interlayer bands at the Γ point of the surface Brillouin zone of the graphene lattice as well. This electronic configuration is promoted by additional penetration of potassium atoms into the interspace between the SiC substrate and the buffer layer that is located between the substrate and the quasi‐freestanding graphene sheet. Consequently, the epitaxial monolayer graphene sample transforms to n‐doped epitaxial bilayer graphene upon K intercalation.

(Invited) Vertically Stacked Junction Devices Fabricated Using Single-Crystal Graphene on SiC Substrate

Graphene is a potential post-silicon nanomaterial because of its tremendous properties. Single-crystal graphene epitaxially grown on a SiC substrate is the most promising candidate for future electron devices. Single-crystal graphene with a 100 mm2 size has been successfully fabricated using a high-temperature rapid thermal annealing system [1]. In this study, two types of stacked graphene devices using graphene on SiC are demonstrated. The first device is a bi-crystal system consisting of two graphene samples as shown in Fig. 1 (a). These samples are directly bonded in a face-to-face manner. Contact pins are attached to the graphene surface for ohmic contact. The electrical characteristics of the stacked graphene junction were ohmic at a low bias voltage below 20 V. The linear characteristic at a low bias voltage is attributed to the linear dispersion relation in the band structure of graphene, Dirac cone. At a high bias voltage, the conductance of the graphene junction drastically increased. This nonlinear behavior originates from a semi-metallic property far from the Dirac point. Thermal imaging results indicated that electrical power was directly converted to infrared radiation. The radiation efficiency was estimated to be approximately 10%, which was high compared to that of a previously reported graphene blackbody emitter. The infrared emission spectrum measured using FT-IR was blackbody-like [2]. The peak wavelength of the measured spectrum was almost constant at 10.2 µm with changing applied power. The estimated junction voltage was approximately 5 V at a bias voltage of 80 V. Hot electrons were injected to the graphene layer. The results suggested that infrared emissions from the graphene–graphene stacked junction could be attributed to a nonthermal emission mechanism, such as phonon–plasmon coupling. The other vertically stacked graphene junction is a tunneling diode as shown in Fig. 1 (b). An insulative layer is sandwiched between two graphene samples. A structured water layer [3] formed by deionized water treatment on the epitaxial graphene sample acts as a tunneling barrier. Nonlinear I–V characteristics were observed. The dynamic range of the current change reached the sixth order of magnitude. The estimated thickness of the structured water layer was approximately 0.5 nm. As the tunneling barrier was sufficiently thin, a direct tunneling phenomenon could be observed in a low bias regime. In a high bias regime, the Fowler–Nordheim tunneling phenomenon was observed. A sandwich structure of a vertically stacked graphene junction with a sub-nm-thick structured water layer was constructed through simple water treatment and a direct bonding technique [4]. [1] T. Aritsuki, et al., Jpn. J. Appl. Phys. 55 (2016) 06GF03. [2] N. Murakami, et al., Jpn. J. Appl. Phys. 60 (2021) SCCD01. [3] M. Kitaoka, et al., Jpn. J. Appl. Phys. 56 (2017) 085102. [4] J. Du, et al., Jpn. J. Appl. Phys. 58 (2019) SDDE01. Figure 1

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a “top-down” inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Position-controlled quantum emitters with reproducible emission wavelength in hexagonal boron nitride

Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological steps towards scalable quantum photonic devices. Here, we evidence SPEs in high purity synthetic hexagonal boron nitride (hBN) that can be activated by an electron beam at chosen locations. SPE ensembles are generated with a spatial accuracy better than the cubed emission wavelength, thus opening the way to integration in optical microstructures. Stable and bright single photon emission is subsequently observed in the visible range up to room temperature upon non-resonant laser excitation. Moreover, the low-temperature emission wavelength is reproducible, with an ensemble distribution of width 3 meV, a statistical dispersion that is more than one order of magnitude lower than the narrowest wavelength spreads obtained in epitaxial hBN samples. Our findings constitute an essential step towards the realization of top-down integrated devices based on identical quantum emitters in 2D materials.

Xenon Plasma Focused Ion Beam Milling for Obtaining Soft X-ray Transparent Samples

We employ xenon (Xe) plasma focused ion beam (PFIB) milling to obtain soft X-ray transparent windows out of bulk samples. The use of a Xe PFIB allows for the milling of thin windows (several 100 nm thick) with areas of the order of 100 µm × 100 µm into bulk substrates. In addition, we present an approach to empirically determine the transmission level of such windows during fabrication by correlating their electron and soft X-ray transparencies. We perform scanning transmission X-ray microscopy (STXM) imaging on a sample obtained by Xe PFIB milling to demonstrate the conceptual feasibility of the technique. Our thinning approach provides a fast and simplified method for facilitating soft X-ray transmission measurements of epitaxial samples and it can be applied to a variety of different sample systems and substrates that are otherwise not accessible.

Towards high-performance linear piezoelectrics: Enhancing the piezoelectric response of zinc oxide thin films through epitaxial growth on flexible substrates

Ferroelectrics are popular for sensors, actuators and transducers. However, the energy barrier for reversing the polarization leads to extra energy losses, while the hysteresis causes phase-lags and non-linearities between input and output signals. The high piezoelectric constants of ferroelectrics dictate the figure-of-merit of piezoelectric devices. Enhancing the piezoelectric constant of linear-piezoelectrics potentially leads to next-generation piezoelectric devices free from issues associated with ferroelectrics. By epitaxial growth of undoped ZnO films on flexible Hastelloy substrates, we demonstrated an ~85% improvement in the piezoelectric constant, and more importantly maintained the linearity of the piezoelectric response. Strong out-of-plane and in-plane crystallographic alignments are detected in the epitaxial sample, yet modeling of the effective thin-film piezoelectric constant shows only a limited contribution of the optimized textures. The improvement in the piezoelectric response is ascribed mostly to the in-plane epitaxial strain resulted from the lattice mismatch of ZnO to the substrate. The epitaxial ZnO film on Hastelloy promisingly offer high-precision positioners with nano-/pico-meter resolutions. Enhancing the piezoelectric constant by strain engineering potentially removes the barrier for applications of linear-piezoelectrics and enriches the piezoelectric research community.

КВАНТОВЫЙ РАЗМЕРНЫЙ ЭФФЕКТ В СВЕРХПРОВОДНИКАХ

In the middle of the last century, it was demonstrated that with a decrease in the size of superconducting structures, for example, the thickness of a thin film, its critical temperature TC shifts by a certain amount. It increases in aluminum, tin, and indium, and decreases in mercury, niobium, and lead. However, there is still no generally accepted theory explaining this effect. In the 1970s, during the largest volume of research on this topic, V.L. Ginzburg assumed that the transition temperature of a sufficiently pure, monocrystalline superconductor film will be exactly the same as in a bulk body. However, this assumption has not yet been verified, and the question about the nature of this effect still remains open. For the study, we chose aluminum, due to the fact that the dependence of TC of the film on its thickness is very predictable and increases with decreasing size. Despite a number of works on studying this dependence in aluminum, it is not always possible to accurately establish a correspondence with a theory. This is because characteristics vary from sample to sample, even made in the same batch. In our case, polycrystalline films were prepared, the crystallite sizes in which are comparable to the film thickness, and epitaxial samples with an atomically smooth surface. The films were fabricated using electron-beam deposition and molecular-beam epitaxy on various substrates. Within the BCS model, the critical temperature of the superconducting transition exponentially depends on the density of electronic states at the Fermi level N ( EF ) and the electron-phonon interaction constant V : TC ~ exp(-1/[ N ( EF )* V ]). It is shown in this work that, due to QSE in thin superconducting films, both parameters N ( EF ) and V change nonmonotonically with the sample thickness. This behaviour is a consequence of the form resonance theory. Presumably, the effect caused by disordering of crystallites, as well as by the surface or substrate, does not play a dominant role specifically in our case, since the aluminum films are of high quality, and their thicknesses go far beyond the limits of ultrathin objects, in which surface phenomena become of decisive importance. As a result of this study, the experimental and theoretical dependences of TC on the thickness of films prepared by different methods on different substrates were obtained.

Raman and AFM studies of epitaxial graphene intended for manufacturing of transistors

Graphene is considered as a promising material for electronic applications. In this work, micro-Raman spectroscopy, atomic force microscopy and Kelvin-probe force microscopy are used to study three epitaxial graphene samples: two commercially-available ones and one grown using a home-made setup. The obtained data indicates significant differences in homogeneity of strain and doping level, concentration of bilayer inclusions, surface topography and surface potential. Analysis of the measurements data allows us to establish connections between the topography of the samples and their structural properties. The results of these studies will help to understand the characteristics of transistors that are to be made on the basis of the studied samples.

Photo-Enhanced Grain Boundary Ionic Conductivity in Gadolinium Doped Ceria

Oxide-based ionic and mixed ionic-electronic conductors (MIEC) play strategic roles in applications ranging from energy conversion and storage to thin film nano-ionic devices such as fuel cells, batteries, permeation membranes and nano-electronic memory and sensing devices. Most cost-effective production methods for fabricating both bulk and thin films samples generally result in the formation of polycrystalline microstructures with grain boundaries, where the crystallographic periodicity is interrupted.Grain boundaries possess significantly different properties compared to the bulk and often block ion migration. Due to the high resistance and large activation energies, they represent a ubiquitous problem to overcome for a variety of electrochemical energy storage and conversion devices especially for low temperature applications. Space-charge potential barriers, featuring depletion zones and band bending, forming at the interface between adjacent grains, have been used to explain their blocking nature in polycrystalline semiconductors.In the field of photoelectrochemistry, above band-gap light is well-known to contribute to reduce band bending at interfaces by providing additional charge carriers to screen potential barriers. Here, we show that the same principle can be applied to ionic conductors as well, and that we can significantly decrease the grain boundary impedance contribution in such systems.We illustrate photo-enhanced ionic conduction in a thin film of 3% Gd-doped ceria upon illumination by UV light, whose photon energy is slightly above ceria’s band-gap. In fact, we demonstrate the ability to decrease the grain boundary resistance by 72% at 250°C and reduce its activation energy nearly in half from 1.12 eV in the dark to 0.68 eV under UV light illumination. We further demonstrate that this effect is not caused by heat, electronic conductivity, surface catalysis or other effects. Our findings are based on electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) measurements, performed on polycrystalline and epitaxial samples, and backed by theoretical considerations of grain boundary potential height and space charge regions.This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks: “Solid Oxide Interfaces for Faster Ion Transport”, Department of Energy, Basic Energy Sciences and the Swiss National Science Foundation. Figure 1

The relationship between the doping concentration and d ferromagnetism in n-type 4H-SiC

To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the NCVSi complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals.

Structure of quasi-free-standing graphene on the SiC (0001) surface prepared by the rapid cooling method

A systematic structural study of epitaxial graphene samples on the SiC (0001) surface has been performed by the surface x-ray diffraction method, which is a non-contact technique. For samples with only a buffer layer, one layer graphene, and multilayer graphene, the distances between the buffer layer and the surface Si atoms were found to be 2.3 Å. This value is the same as reported values. For quasi-free-standing graphene samples prepared by the rapid cooling method [Bao et al., Phys. Rev. Lett. 117, 205501 (2016)], there was no buffer layer and the distance between the quasi-free-standing graphene and the surface Si atoms was 3.5 Å, which is significantly shorter than the value in hydrogen-intercalated graphene and slightly longer than the interplane distance in graphite. The Si occupancy deviated from unity within 1 nm of the SiC surface. The depth profile of the Si occupancy showed little sample dependence, and it was reproduced by a simple atomistic model based on random hopping of Si atoms.