Papers by Keyword: MBE

Abstract: The temperature dependence Raman scattering from m-plane GaN thin films grown on m-plane sapphire substrate by Molecular Beam Epitaxy (MBE) has been investigated. Three pieces of m-plane GaN films grown with different Ⅲ/Ⅴ ratios were studied by confocal micro-Raman spectrometer from -180 °C to 240 °C. Raman shift and the full width at half maximum (FWHM) were fitted by lorentzian line shape, which reveal the quality and compressive stress of sample. It’s obvious that the Raman shift and FWHM exhibit a quadratic dependence on temperature, and that the redshift of Raman peak position with increasing temperature should be due to anharmonic coupling to phonons of other branches, volume expansion or lattice dilation. Comparing the experiment data and calculated results, the three-phonons processes are dominant in the redshift of E₁(LO) and E₂(high).

Abstract: Anisotropies in misfit dislocations (MDs) at the interface of InGaAs/GaAs (001) are investigated by monochromatic X-ray topography (XRT) technique. Single MD line or several MD lines (MD bunching) are observed as white lines in XRT. Distribution, density, and number of MDs in one MD bunching are evaluated. The density of α-MDs is larger than that of β-MDs. Number of MDs in one MD bunching distribute with two peaks in both in-plane directions. In macroscopic view, α-MD bunching lay more orderly than the other. While in microscopic view, α-MDs gather for the range narrower than β-MDs.

Abstract: In this paper, we studied growth of AlN/GaN/AlN on Si (111) by using plasma assisted molecular beam epitaxy (PA-MBE) system. The structural and optical characteristics of the sample have been investigated by using high resolution X-ray diffraction (HR-XRD), Raman spectroscopy and photoluminescence (PL). PL spectrum of the sample has shown sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the sample. The silver (Ag) metal contact was then deposited on the sample followed by thermal treatment at 500°C and 700°C, respectively. Treated sample at 700°C showed good spherical Ag islands on sample compared to the treated sample at 500°C. The effect of Ag islands on the electrical characteristics of sample was also examined by using I-V measurement. The results showed that the treated sample at 700°C has decreased the photo-current of Schottky diode.

Abstract: A high-quality crack-free AlN cap layer on GaN layer has been achieved using an AlN buffer layer directly grown on a silicon substrate at high temperature by radio frequency (RF) plasma-assisted molecular beam epitaxy. A two dimensional (2D) growth process guide to AlN cap layer of high grade crystal quality. The nucleation and the growth dynamics have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ by high resolution transmission electron microscopy (HR-TEM). The microstructure was investigated by energy-dispersive X-ray spectroscopy (EDX). It was disclosed that AlN is single crystalline with low defect. High densities of V-shaped pits were not detected at the interface between AlN and GaN layers. Contradictory the earlier reported V-shaped defects in nitride-based alloys; these V-shaped pits were condensed on top of the AlN layer because of H2 etching of the surface when a high temperature growth discontinuity between AlN and GaN layers.

Abstract: The doping of Cu in the BaSi₂ films grown by molecular beam epitaxy (MBE) with various Cu concentrations for the suitability of the solar cells was studied in this paper. The main objective of the present work is to investigate and compare the carrier concentration of Cu-doped BaSi₂ films grown with different Cu Knudsen cell temperatures and qualify as a potential candidate for more efficient solar cells. The reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements and secondary ion mass spectroscopy (SIMS), were used to determine the structure, depth profile and composition of the grown samples. The electrical properties like resistivity as well as carrier concentration were measured by using a four point probe method and Van der Pauw technique, respectively. During the MBE growth, different temperatures for Cu Knudsen cell ranging from 800 to 1200 °C were chosen and the optimum growth condition for both heavily doped n-type as well as p-type in the MBE was investigated. In our previous work, the Al, Sb doped BaSi₂ were used as a potential candidate for the formation of pn-junction for solar cells, but the result was not encouraging one due to diffusion and segregation problems in the surface and BaSi₂/Si interface regions. In the present work n-type BaSi₂ layers with their dopant atoms uniformly distributed in the grown layers for the formation of high-quality of BaSi₂ pn-junction with single crystal nature were successfully developed. The realizations to develop cost effective and more efficient solar cells are inevitable for both terrestrial as well as space applications.

Abstract: Several methods have been introduced to study and simulate homoepitaxial growth of III-V materials. GaAs (001) surface has widely been used in the last three decades due both to its importance as substrate and for characterization of epitaxial growth. In this paper, we firstly study the initial stage of homoepitaxial growth on a GaAs (001) β2(2x4) reconstructed surface using As2 . The simulation was carried out with Kinetic Monte Carlo simulations including the zinc blend structure β2 (2x4) reconstruction of GaAs surface. Then we discus results of the homoepitaxy GaAs on GaAs particularly morphological evolution of the two dimensional islands and observations were made in real-time at the growth temperature using reflection high energy electron diffraction (RHEED) and roughness morphology.

Abstract: GaN/SiC Heterojunction Bipolar Transistors (HBTs) with ultra-thin AlN insertion layers at the n-GaN/p-SiC emitter junction are proposed to improve carrier injection efficiency. The current-voltage characteristics of n-GaN/AlN/p-SiC heterojunctions have exhibited very small reverse leakage and good rectification. The capacitance-voltage measurement have revealed that the conduction band offset between n-GaN and p-SiC has been reduced from -0.74 eV to -0.54 eV by insertion of AlN, indicating that the GaN/AlN/SiC heterojunction may show better electron-injection efficiency. A significantly improved common-base current gain (α~0.2) is obtained for GaN/AlN/SiC HBTs with initial N* pre-irradiation, while it was very low (α~0.001) for GaN/SiC HBTs without AlN layers.

Abstract: An overview of various cleaning procedures for silicon surfaces is presented. Because in-situ cleaning becomes more and more important for nanotechnology the paper concentrates on physical and dry chemical techniques. As standard ex-situ wet chemical cleaning has a significant impact on surface quality und thus device properties, its influence on further processes is also considered. Oxygen and carbon are unavoidable contaminations after wet chemical treatment and therefore we discuss their in-situ removal as one of the main goals of modern silicon substrate cleaning. As surface roughness strongly influences the electrical quality of interfaces for epitaxy and dielectric growth, we concentrate on techniques, which meet this requirement. It will be shown that multi-step thermal sequences in combination with simultaneous passivation of the clean surface are necessary in order to avoid recontamination. This can be achieved not only for ultra hich vacuum but also for inert gas atmosphere. In this case the process gases have to be extremely purified and the residual partial pressure of contaminats such as oxygen and carbon has to be negligible. It will be demonstrated that 800°C is an upper limit for thermal treatment of silicon surfaces in the presence of carbon because at this temperature SiC formation in combination with a high mobility of silicon monomers leads to surface roughness. In addition mechanical stress causes dislocations and crystal defects.

Abstract: Vertically aligned GaN nanocolumn arrays were grown by molecular beam epitaxy on Gallium coated silicon substrate. The dense packing of the NCs gives them the appearance of a continuous film in surface view, but cross-sectional analysis shows them to be isolated nanostructures. The GaN nanocolumns have uniform diameters of 85 nm, lengths up to 720 nm and possess a pyramid like tip. Photoluminescence measurements of NCs show excitonic emission with a dominant, narrow peak centered at 363 nm and FWHM of 68 meV. From the Raman spectrum, peaks at 566.9 and 730 cm-1 are assigned to the E2 and A1(LO) GaN phonons modes which clearly indicates that the grown nanocolumns are highly crystalline. The grown nanocolumns are highly oriented and perpendicular to the growth surface.