Search Results (18)

Hybrid systems consisting of highly transparent channels of low-dimensional semiconductors between superconducting elements allow the formation of quantum electronic circuits. Therefore, they are among the novel material platforms that could pave the way for scalable quantum computation. To this aim, InAs two-dimensional electron gases are among the ideal semiconductor systems due to their vanishing Schottky barrier; however, their exploitation is limited by the unavailability of commercial lattice-matched substrates. We show that in situ growth of superconducting aluminum on two-dimensional electron gases forming in metamorphic near-surface InAs quantum wells can be performed by molecular beam epitaxy on GaAs substrates with state-of-the-art quality. Adaptation of the metamorphic growth protocol has allowed us to reach low-temperature electron mobilities up to 1.3 × 10⁵ cm²/Vs in Si-doped InAs/In_0.81Ga_0.19As two-dimensional electron gases placed 10 nm from the surface with charge density up to 1 × 10¹²/cm². Shubnikov-de Haas oscillations on Hall bar structures show well-developed quantum Hall plateaus, including the Zeeman split features. X-ray diffraction and cross-sectional transmission electron microscopy experiments demonstrate the coexistence of (011) and (111) crystal domains in the Al layers. The resistivity of 10-nm-thick Al films as a function of temperature was comparable to the best Al layers on GaAs, and a superconducting proximity effect was observed in a Josephson junction. Full article

The difference in the crystallization kinetics during growth from the melt between II–VIs (CdTe, Cd_1−xZn_xTe, ZnSe, and ZnTe) and III–Vs (GaAs and InP) is discussed. At the melt growth of II–VI crystals, the most important difference is the lack of controllability of seeding and achievement of a desired growth orientation. A pronounced tendency of self-orientation toward <111>, <110>, and sometimes <112> and <122>, but almost never toward <100> direction, has been observed regardless of whether a seed has been used or not. The main reason proves to be the tetrahedral coordination due to the high binding ratio of ionicity remaining in the II–VI melts but not occurring in III–Vs. As a result, the general effect of pre-ordering into density layers, forced by the solid surface, is in the II–VI liquids superimposed by a {111} self-orientation via tetrahedral in-plane alignment. Fitting growth kinetics seem to only be possible when this melt configuration conforms to the crystal structure, like the {111} but hardly the {100}. Otherwise, the liquid self-orientation determines the continuing crystal orientation. Additionally, an <100>-oriented seed abruptly changed into an <122> direction via a congruent twin plane. Although such considerations still need verifying atomistic simulations, they are helpful to optimize the growth methodology even for larger crystal diameters. Full article

This study investigates the growth of gallium arsenide nanowires, using lead as a catalyst. Typically, nanowires are grown through the vapor–solid–liquid mechanism, where a key factor is the reduction in the nucleation barrier beneath the catalyst droplet. Arsenic exhibits limited solubility in conventional catalysts; however, this research explores an alternative scenario in which lead serves as a solvent for arsenic, while gallium and lead are immiscible liquids. Liquid lead easily dissolves in Si as well as in GaAs. The preservation of the catalyst during the growth process is also addressed. GaAs nanowires have been grown by molecular beam epitaxy on silicon Si (111) substrates at varying temperatures. Observations indicate the spontaneous doping of the GaAs nanowires with both lead and silicon. These findings contribute to a deeper understanding of the VLS mechanism involved in nanowire growth. They are also an important step in the study of GaAs nanowire-doping processes. Full article

In this work, we report on a comparative study of the topology, luminescence, and wettability properties of ZnMgO films prepared by a cost-effective spray pyrolysis technology on GaAs substrates with (100), (001), and (111) crystallographic orientations, as well as on Si(100) substrates. Deposition on nanostructured GaAs substrates was also considered. It was found that film growth is not epitaxial or conformal, but rather, it is granular, depending on the nucleating sites for the crystallite growth. The distribution of nucleation sites ensured the preparation of nanostructured films with good uniformity of their topology. The observed difference in columnar growth on Si substrates and pyramidal growth on GaAs ones was explained in terms of the impact of chemical bonding in substrates. The films grown on GaAs substrates with a (001) orientation were found to be made of larger crystallites compared to those deposited on substrates with a (111) orientation. These effects resulted in a difference in roughness of a factor of 1.5, which correlates with the wetting properties of films, with the most hydrophobic surface being found on films deposited on GaAs substrates with a (111) orientation. The prospects for photocatalytic and gas sensor applications of films produced on flat substrates, as well as for plasmonic and other applications of films deposited on nanostructured substrates, are discussed, taking into account the results of the analysis of their photoluminescence properties. Full article

We report on the growth of high-quality GaAs semiconductor materials on an AlAs/sapphire substrate by molecular beam epitaxy. The growth of GaAs on sapphire centers on a new single-step growth technique that produces higher-quality material than a previously reported multi-step growth method. Omega-2theta scans confirmed the GaAs (111) orientation. Samples grown at 700 °C displayed the highest crystal quality with minimal defects and strain, evidenced by narrow FWHM values of the rocking curve. By varying the As/Ga flux ratio and the growth temperature, we significantly improved the quality of the GaAs layer on sapphire, as compared to that obtained in multi-step studies. Photoluminescence measurements at room temperature and 77 K further support these findings. This study underscores the critical role of the As/Ga flux ratio and growth temperature in optimizing GaAs epitaxial growth on sapphire. Full article

GaAsSb semiconductor material, a ternary alloy, has long been recognized as a crucial semiconductor in the near infrared range due to its ability to finely adjust the wavelength through controlling the Sb component. In this work, we report on the pattern of orientation variation in self-catalyzed grown GaAsSb nanowires (NWs). Utilizing solid-source molecular-beam epitaxy (MBE), self-catalyzed GaAs and GaAsSb nanowires (NWs) were grown on Si (111) substrates. The influence of various Sb components on the growth direction of the nanowires in the ternary GaAsSb alloy was examined using scanning electron microscopy (SEM). The inclusion of Sb components was discovered to alter the growth direction of the nanowires, transitioning them from a vertical and inclined orientation to a configuration that encompassed vertical, inclined, and parallel orientations with respect to the Si (111) substrate. As the Sb component in GaAsSb increased, there was an increased likelihood of the nanowires growing parallel to the surface of the Si (111) substrate. A combination of X-ray diffraction (XRD) and Raman spectroscopy validated the presence of Sb components and indicated a high crystalline quality. Additionally, XRD confirmed that the Sb components aligned with the intended structure. These findings establish a solid material foundation for the development of high-performance GaAsSb-based devices. Full article

Heteroepitaxial Zn-doped p-GaP was grown on (001) GaAs, (001) Si and (111) Si substrates by hydride vapor phase epitaxy for solar-driven photoelectrochemical applications of hydrogen generation by water splitting and CO₂ reduction. Growth of GaP on Si was realized through the implementation of a low-temperature buffer layer, and the morphology and crystalline quality were enhanced by optimizing the precursor flows and pre-heating ambient substrate. The p-GaP/GaAs and p-GaP/Si samples were processed to photoelectrodes with an amorphous TiO₂ coating for CO₂ reduction and a combination of TiO₂ layer and mesoporous tungsten phosphide catalyst for water splitting. P-GaP/GaAs with suitable Zn-doping concentration exhibited photoelectrochemical performance comparable to homoepitaxial p-GaP/GaP for water splitting and CO₂ reduction. Degradation of photocurrent in p-GaP/Si photoelectrodes is observed in PEC water splitting due to the high density of defects arising from heteroepitaxial growth. Full article

We report on the characterization of the thermoelectric properties of Bi₂Se₃ epifilms. MBE-grown Bi₂Se₃ films on GaAs (111) A are nanomachined with integrated Pt elements serving as local joule heaters, thermometers, and voltage probes. We suspended a 4 µm × 120 µm Bi₂Se₃ by nanomachining techniques. Specifically, we selectively etched GaAs buffer/substrate layers by citric acid solution followed by a critical point drying method. We found that the self-heating 3ω method is an appropriate technique for the accurate measurement of the thermal conductivity of suspended Bi₂Se₃. The measured thermoelectric properties of 200 nm thick Bi₂Se₃ at room temperature were $\kappa =1.95 \mathrm{W}/\mathrm{m} \mathrm{K}$, $S=-102.8 \mathsf{\mu }\mathrm{V}/\mathrm{K}$, σ = 75,581 S/m and the figure of merit was $ZT=0.12$. The study introduces a method to measure thermal conductivity accurately by suspending thin films. Full article

Ni thin films with different thicknesses were grown on a GaAs substrate using the magnetron sputtering technique followed by in situ X-ray diffraction (XRD) annealing in order to study the solid-state reaction between Ni and GaAs substrate. The thickness dependence on the formation of the intermetallic phases was investigated using in situ and ex situ XRD, pole figures, and atom probe tomography (APT). The results indicate that the 20 nm-thick Ni film exhibits an epitaxial relation with the GaAs substrate, which is (001) Ni//(001) GaAs and [111] Ni//[110] GaAs after deposition. Increasing the film’s thickness results in a change of the Ni film’s texture. This difference has an impact on the formation temperature of Ni₃GaAs. This temperature decreases simultaneously with the thickness increase. This is due to the coherent/incoherent nature of the initial Ni/GaAs interface. The Ni₃GaAs phase decomposes into the binary and ternary compounds xNiAs and Ni_3−xGaAs_1−x at about 400 °C. Similarly to Ni₃GaAs, the decomposition temperature of the second phase also depends on the initial thickness of the Ni layer. Full article

InGaAs quantum well (QW) lasers have attracted significant attention owing to their considerable potential for applications in optical communications; however, the relationship between the misorientation of the substrates used to grow InGaAs QWs and the structural and optical properties of QWs is still ambiguous. In this study, In-rich InGaAs/GaAsP single QWs were grown in the same run via metal organic chemical vapor deposition on GaAs (001) substrates misoriented by 0°, 2°, and 15° toward (111). The effects of substrate misorientation on the crystal quality and structural properties of InGaAs/GaAsP were investigated by X-ray diffraction and Raman spectroscopy. The 0° substrate exhibited the least lattice relaxation, and with increasing misorientation, the degree of lattice relaxation increased. The optical properties of the InGaAs/GaAsP QWs were investigated using temperature-dependent photoluminescence. An abnormal S-shaped variation of the peak energy and inverse evolution of the spectral bandwidth were observed at low temperatures for the 2° substrate, caused by the localization potentials due to the In-rich clusters. Surface morphology observations revealed that the growth mode varied with different miscuts. Based on the experimental results obtained in this study, a mechanism elucidating the effect of substrate miscuts on the structural and optical properties of QWs was proposed and verified. Full article

This paper presents the current status of medium-wave infrared (MWIR) detectors at the Military University of Technology’s Institute of Applied Physics and VIGO System S.A. The metal–organic chemical vapor deposition (MOCVD) technique is a very convenient tool for the deposition of HgCdTe epilayers, with a wide range of compositions, used for uncooled infrared detectors. Good compositional and thickness uniformity was achieved on epilayers grown on 2-in-diameter, low-cost (100) GaAs wafers. Most growth was performed on substrates, which were misoriented from (100) by between 2° and 4° in order to minimize growth defects. The large lattice mismatch between GaAs and HgCdTe required the usage of a CdTe buffer layer. The CdTe (111) B buffer layer growth was enforced by suitable nucleation procedure, based on (100) GaAs substrate annealing in a Te-rich atmosphere prior to the buffer deposition. Secondary-ion mass spectrometry (SIMS) showed that ethyl iodide (EI) and tris(dimethylamino)arsenic (TDMAAs) were stable donor and acceptor dopants, respectively. Fully doped (111) HgCdTe heterostructures were grown in order to investigate the devices’ performance in the 3–5 µm infrared band. The uniqueness of the presented technology manifests in a lack of the necessity of time-consuming and troublesome ex situ annealing. Full article

Based on first-principle calculations, the mechanical anisotropy and the electronic and optical properties of seven kinds of carbon materials are investigated in this work. These seven materials have similar structures: they all have X-type structures, with carbon atoms or carbon clusters at the center and stacking towards the space. A calculation of anisotropy shows that the order of elastic anisotropy in terms of the shear modulus, Young’s modulus and Poisson’s ratio of these seven carbon materials with similar structure is diamond < supercubane < T carbon < Y carbon < TY carbon < cubane-diyne < cubane-yne. As these seven carbon materials exhibit cubic symmetry, Young’s modulus has the same anisotropy in some major planes, so the order of elastic anisotropy in the Young’s modulus of these seven main planes is (111) plane < (001) plane = (010) plane = (100) plane < (011) plane = (110) plane = (101) plane. It is also due to the fact that their crystal structure has cubic symmetry that the elastic anisotropy in the shear modulus and the Poisson’s ratio of these seven carbon materials on the seven major planes are the same. Among the three propagation directions of [100], [110], and [111], the [110] propagation direction’s anisotropic ratio of the sound velocity of TY carbon is the largest, while the anisotropic ratio of the sound velocity of cubane-diyne on the [100] propagation direction is the smallest. In addition, not surprisingly, the diamond has the largest Debye temperature, while the TY carbon has the smallest Debye temperature. Finally, TY carbon, T carbon and cubane-diyne are also potential semiconductor materials for photoelectric applications owing to their higher or similar absorption coefficients to GaAs in the visible region. Full article

We demonstrate the feasibility of growing GaAs nanomembranes on a plastically-relaxed Ge layer deposited on Si (111) by exploiting selective area epitaxy in MBE. Our results are compared to the case of the GaAs homoepitaxy to highlight the criticalities arising by switching to heteroepitaxy. We found that the nanomembranes evolution strongly depends on the chosen growth parameters as well as mask pattern. The selectivity of III-V material with respect to the SiO₂ mask can be obtained when the lifetime of Ga adatoms on SiO₂ is reduced, so that the diffusion length of adsorbed Ga is high enough to drive the Ga adatoms towards the etched slits. The best condition for a heteroepitaxial selective area epitaxy is obtained using a growth rate equal to 0.3 ML/s of GaAs, with a As BEP pressure of about 2.5 × 10⁻⁶ torr and a temperature of 600 °C. Full article

The effect of the gaseous atmosphere in the growth of gallium arsenide (GaAs) films was studied. The films have been grown by close-spaced vapor transport (CSVT) technique in a home-made hot filament chemical vapor deposition (HFCVD) reactor using molecular hydrogen and molecular nitrogen as the transport agent. An important point about the gaseous atmosphere is the ease in creating volatile compounds when it makes contact with the GaAs source, this favors the transport of material in a CSVT system. Chemical reactions are proposed in order to understand the significant difference produced from the gaseous atmosphere. The films grown with hydrogen are (almost) continuous and have homogeneous layers with preferential orientation (111). The films grown with nitrogen are granular and rough layers with the coexistence of the orientations (111), (220) and (311) in the crystals. The incorporation of impurities in the films was corroborated by energy dispersive spectroscopy (EDS) showing traces of oxygen and nitrogen for the case of the samples obtained with nitrogen. Films grown in a hydrogen atmosphere show a higher band gap than those grown in a nitrogen atmosphere. With the results of XRD and micro-Raman we observe a displacement and broadening of the peaks, characteristic of a structural disorder. The calculations of the FWHM allow us to observe the crystallinity degree and determine an approximate crystallite size using the Scherrer’s equation. Full article

We report observations of field emission from self-catalyzed GaAs nanowires grown on Si (111). The measurements were taken inside a scanning electron microscope chamber with a nano-controlled tungsten tip functioning as anode. Experimental data were analyzed in the framework of the Fowler-Nordheim theory. We demonstrate stable current up to 10⁻⁷ A emitted from the tip of single nanowire, with a field enhancement factor β of up to 112 at anode-cathode distance d = 350 nm. A linear dependence of β on the anode-cathode distance was found. We also show that the presence of a Ga catalyst droplet suppresses the emission of current from the nanowire tip. This allowed for the detection of field emission from the nanowire sidewalls, which occurred with a reduced field enhancement factor and stability. This study further extends GaAs technology to vacuum electronics applications. Full article