Relaxed GexSi1-x layer formation with reduced dislocation density grown by ultrahigh vacuum chemical vapor deposition

A new alternative method to grow the relaxed Ge0.24Si0.76 layer with a reduced dislocation density by ultrahigh vacuum chemical vapor deposition is reported in this paper. A 1000-Angstrom Ge0.24Si0.76 layer was first grown on a Si(100) substrate. Then a 500-Angstrom Si layer and a subsequent 5000-Angstrom Ge0.24Si0.76 overlayer followed. All these three layers were grown at 600 degrees C. After being removed from the growth system to air, the sample was first annealed at 850 degrees C for 30 min, and then was investigated by cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy. It is shown that the 5000-Angstrom Ge0.24Si0.76 thick over layer is perfect, and most of the threading dislocations are located in the embedded thin Si layer and the lower 1000-Angstrom Ge0.24Si0.76 layer. The relaxation ratio of the over layer is deduced to be 0.8 from Raman spectroscopy.

Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (mu c-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the mu c-Si:H films with different H-2/SiH4 ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAX). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of mu c-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of mu c-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si--H bonds in mu c-Si:H and in polycrystalline Si thin films are located at the grain boundaries. (C) 2000 Elsevier Science S.A. All rights reserved.

Low-temperature growth of cubic GaN by metalorganic chemical-vapor deposition

Low-temperature growth of cubic GaN at 520 degrees C was achieved using CCl4 as an additive by metalorganic chemical-vapor deposition (MOCVD) on GaAs substrate. X-Ray measurement confirmed that the films are single-phase cubic GaN. Scanning electron microscopy (SEM) and reflection high-energy electron diffraction (RHEED) were also used to analyze the surface morphology and the quality of films. The evolution of surface morphology suggests that CCl4 can reduce the hopping barrier and thus Ga adatoms are able to diffuse easily on the GaN surface. (C) 1998 Elsevier Science S.A. All rights reserved.

The dependence of GexSi1-x epitaxial growth on GeH4 flow using chemical vapour deposition

GexSi1-x epilayers were grown at 700-900 degrees C by atmospheric pressure chemical vapour deposition. GexSi1-x, Si and Ge growth rates as functions of GeH4 flow are considered separately to investigate how the growth of the epilayers is enhanced. Arrhenius plots of Si and Ge incorporation in the GexSi1-x growth show the activation energies associated with the growth rates are about 1.2 eV for silicon and 0.4 eV for germanium, indicating that Si growth is limited by surface kinetics and Ge growth is limited by mass transport. A model based on this idea is proposed and used to simulate the growth of GexSi1-x. The calculation and experiment are in good agreement. Growth rate and film composition increase monotonically with growth pressure; both observations are explained by the model.

Homoepitaxial growth and characterization of 4H-SiC epilayers by low-pressure hot-wall chemical vapor deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates purchased from Cree is performed at a typical temperature of 1500 degrees C with a pressure of 40 Torr by using SiH4+C2H4+H-2 gas system. The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope, atomic force microscopy (AFM), x-ray diffraction, Raman scattering, and low temperature photoluminescence (LTPL). The background doping of 32 pm-thick sample has been reduced to 2-5 x 10(15) cm(-3). The FWHM of the rocking curve is 9-16 arcsec. Intentional N-doped and B-doped 4H-SiC epilayers are obtained by in-situ doping of NH3 and B2H6, respectively. Schottky barrier diodes with reverse blocking voltage of over 1000 V are achieved preliminarily.

Effects of annealing treatment on the formation of CO2 in ZnO thin films grown by metal-organic chemical vapor deposition

Post-growth annealing was carried out on ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD). The grain size of ZnO thin film increases monotonically with annealing temperature. The ZnO thin films were preferential to c-axis oriented after annealing as confirmed by Xray diffraction (XRD) measurements. Fourier transformation infrared transmission measurements showed that ZnO films grown at low temperature contains CO2 molecules after post-growth annealing. A two-step reaction process has been proposed to explain the formation mechanism of CO2, which indicates the possible chemical reaction processes during the metal-organic chemical vapor deposition of ZnO films.

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H-2 selectivity at above 800 degrees C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H-2/CO molar ratio close to 2:1 at a temperature of 800-900 degrees C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 degrees C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H-2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION GROWTH OF GAN

Single-crystal GaN films have been deposited on (01 (1) over bar 2) sapphire substrates using trimethylgallium (TMGa) and NH3 as sources. The morphological, crystalline, electrical and optical characterizations of GaN film are investigated. The carrier concentration ofundoped GaN increases with decreasing input NH3-to-TMGa molar flow ratio.

The content calculation of hexagonal phase inclusions in cubic GaN films on GaAs(001) substrates grown by metalorganic chemical vapor deposition

Cubic GaN(c-GaN) films are grown on GaAs(001) substrates by metalorganic chemical vapor deposition (MOCVD). Two GaN samples were grown with different buffer layer, the deposition time of each was 1 and 3 min, respectively. 4-circle X-ray double crystal diffraction (XRDCD) was used to study the secondary crystallographic phases presented in the c-GaN films. The phase composition of the epilayers was determined by X-ray reciprocal space mapping. The intensities of the c-GaN(002) and h-GaN(10 (1) over bar 1) planes detected in the mapping were investigated by omega scans. The content of the hexagonal phase inclusions in the c-GaN films was calculated to about 1.6 and 7.9%, respectively. The thicker buffer layer is not preferable for growing high quality pure c-GaN films. (C) 2000 Elsevier Science S.A. All rights reserved.

Influences of initial buffer layer deposition on electrical and optical properties in cubic GaN grown on GaAs(100) by metalorganic chemical vapor deposition

We present some results on the effect of initial buffer layer on the crystalline quality of Cubic GaN epitaxial layers grown on GaAs(100) substrates by metalorganic chemical vapor deposition. Photoluminescence and Hall measurements were performed to characterize the electrical and optical properties of cubic GaN. The crystalline quality subsequently grown high-temperature (HT) cubic GaN layers strongly depended on thermal effects during the temperature ramping process after low temperature (LT) growth of the buffer layers. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to investigate this temperature ramping process. Furthermore, the role of thermal treatment during the temperature ramping process was identified. Using the optimum buffer layer, the full width at half maxim (FWHM) at room temperature photoluminescence 5.6 nm was achieved. To our knowledge, this is the best FWHM value for cubic GaN to date. The background carrier concentration was as low as 3 x 10(13) cm(-3). (C) 2000 Published by Elsevier Science S.A. All rights reserved.

The role of zinc dopant and the temperature effect on the controlled growth of InNnanorods in metal-organic chemical vapor deposition system

This work was supported by the National Science Foundation of China (60976008 and 60776015), the Special Funds for Major State Basic Research Project (973 program) of China (2006CB604907), and the 863 High Technology R&D Program of China (2007AA03Z402 and 2007AA03Z451). The authors express their appreciations to Prof. Yongliang Li (Analytical and Testing Center, Beijing Normal University) for FE-SEM measurements, to DrTieying Yang and Prof. Huanhua Wang (Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences) for XRD measurements and helpful discussions.