Papers by Keyword: Molecular Beam Epitaxy

Abstract: The electronic structure of ultrathin Na/GaN interfaces was studied using photoelectron spectroscopy with synchrotron radiation in the photon energy range of 75–770 eV. The experiments were carried out in situ in ultrahigh vacuum of 5·10^–10 Torr with submonolayer sodium coverages on the gallium nitride surface. The photoemission spectra of the Ga 3d and N 1s core levels were studied at different excitation energies. It was found that Na adsorption causes a decrease in the intensity and a shift in the spectra of the Ga 3d and N 1s core levels towards higher binding energies. It was found that the sodium adsorption leads to some changes in the spectra due to charge transfer between the Na adlayer and the surface Ga or N atoms.

Abstract: Photoconductive antennas based on low-temperature gallium arsenide (LT-GaAs) with a gold plasmonic-gold grating made of nanorods and nanoislands have been developed and fabricated. The antennas were produced using molecular beam epitaxy and electron-beam nanolithography. LT-GaAs samples with a high annealing temperature of 943 K were employed in the fabrication process. It has been demonstrated that plasmonic nanostructures significantly enhance the efficiency of light-to-terahertz radiation conversion by photoconductive antennas.

Abstract: Epitaxial GaAs-on-Si is an ideal material system for studying the physics of mismatched heteroepitaxy of a polar semiconductor layer grown on a non-polar substrate like silicon. Understanding and optimization of the initial nucleation of GaAs on silicon is the most crucial step in the success of GaAs/Si heteroepitaxy. Molecular beam epitaxy (MBE) technique has been used to deposit hetero-epitaxial GaAs thin-film on off-angle Si (100) substrate using the three-step growth method. After optimizing the growth parameters of low temperature (LT) buffer layer and high temperature (HT) epilayer, XRD analyses were performed. Characterization results of the GaAs (004) films which were not subjected to annealing, show a full-width half maximum (FWHM) of 760.1 arc sec and a root mean square (RMS) surface roughness of lower than 1 nm for a scanning area of 10 μm × 10 μm.

Abstract: We present here a study on the electrical and structural properties of p-type PbTe films doped with CaF₂. The layers were grown by molecular beam epitaxy on freshly cleaved (111) BaF₂ substrates. The doping level was monitored by the CaF₂ solid source cell temperature (T_CaF2), which varied from 500 to 1150 °C. The films with low doping level, T_CaF2 ≤ 1010 °C, exhibited flat surfaces with crystalline quality close to the undoped PbTe sample. In contrast, samples with high levels of doping (T_CaF2 > 1010 °C) presented CaF₂ agglomerates on the surface and a worse crystal quality. The hole density at 77 K versus T_CaF2 oscillated between 1.3 × 10¹⁷ and 3.6 × 10¹⁷ cm^-3 and did not exhibit a systematic behavior as the fluoride supply is raised. The results indicate that CaF₂ is not an effective p-type dopant for PbTe, due to the abscence of a resonant level close to the valence band or to compensation of extrinsic dopant levels.

Abstract: The design and technological conditions for the manufacture of photoconductive antennas based on low-temperature gallium arsenide (LT-GaAs) have been developed. The optimized photoconductive THz antenna is made based on LT-GaAs with the flag geometry of the contacts and with the interdigitated structure including metal closing through the dielectric of each second period. LT-GaAs samples were obtained by molecular beam epitaxy at temperatures of 210 °C, 230 °C, 240 °C on GaAs substrates (100). Dark and photocurrent were measured depending on the bias voltage of the LT-GaAs heterostructure at the EP6 probe station. Full wave finite element method solver has been used to investigate the proposed plasmon PCA electrical and optical behavior by combining the Maxwell's wave equation with the drift-diffusion/Poisson equations.

Abstract: We report our experiments based on the interfaces of a 5-period superlattice, containing GaAsP(3Å)/GaAs (190Å) heterostructures grown by molecular beam epitaxy (MBE). The atomic arrangement at the interfaces of GaAsP/GaAs is investigated using high resolution transmission electron microscopy (HRTEM). Our results indicate that the superlattice was grown coherently with strained layers. We propose that the atomic arrangement at the interface is GaP, assuming that phosphorus incorporation occurs primarily via substitution due to desorption of arsenic at the surface for substrate temperatures above 500°C. The incorporation of phosphorus has been investigated using fast Fourier transform (FFT) patterns and shows a form of strain distribution near the heterointerface. The FFT patterns of the superlattice reveal that strain distributes mostly near the interface and gradually decreases along the direction of growth. Phosphorus diffused into a GaAs layer changes the lattice constant in the growth direction, which reduces strain in the superlattice.

Abstract: Ternary (Si_1-xC_y)Ge_x+y solid solutions were grown on Si-face 4H-SiC applying atomic layer molecular beam epitaxy at low temperatures. The grown layers consist of twinned 3C-SiC revealed by cross section electron microscopy. The germanium was incorporated on silicon lattice sites as revealed by atomic location by channeling enhanced microanalysis transmission electron microscopy studies. The Ge concentration of the grown 3C-(Si_1-xC_y)Ge_x+y heteroepitaxial layers decreases with increasing growth temperatures, but exceeds the solid solubility limit.

Abstract: The article overviews the results of the research into the peculiarities of GaN and AlGaN epitaxy from metal organic compounds, the influence of its peculiarities on the formation of GaN epitaxial layers and gives recommendations on the development of new methods for GaN epitaxial growth as well as on the research how they impact the characteristics of grown layers. The use of such semi-conductors allows obtaining full-color RGB light sources, increasing record density of a digital data storage device, getting high-capacity and efficient sources of white light. The electronic properties of such semi-conductors allow using them as a basis for high-power and high-frequency transistors and other electronic devices the specifications of which are competitive with those of SiC-based devices.

Abstract: We report on improvement in the deep ultraviolet (DUV) photoresponse of SiC based detectors through the development of n- Al_xGa_1-xN / i-p SiC heterojunction photodiodes. Fabricated photodiodes have high external quantum efficiency (EQE), greater than 60%, over a wide spectral range from 215-255 nm that is ~10x enhancement in performance over comparable homogenous SiC photodiodes at the shortest wavelength. This is attributed to photogeneration of carriers within the SiC depletion region by DUV illumination of the diode through the n- Al_xGa_1-xN “window”, as compared to a typical homogenous SiC n-i-p structure where the carriers are photogenerated in the n-type neutral region, resulting in more efficient collection of holes through drift

Abstract: Conductivity and capacitance in structures with Ge nanoclusters grown on oxidized Si (001) with different morphology have been investigated for the temperature range 120-290 K and frequencies 1 kHz-1MHz in co-planar geometry. It was found that structures exhibited T^-1/3 conductivity dependence. The Mott’s variable range hopping through quasi-band of localized states at the Fermi level of Ge nanoclusters and their interfaces was found to be the dominant transport mechanism in the surface conductivity channel. The quasi-band width depends of surface morphology varying in the range 110-130 meV, while the middle of the band is located at E_v+140 meV. The peak of reduced conductivity and capacitance were observed under conditions when Fermi level is in the middle of this band.