Improvement of stoichiometry in semi-insulating gallium arsenide grown under microgravity

A semi-insulating (SI) GaAs single crystal was recently grown in a retrievable satellite. The average etch pit density (EPD) of dislocations in the crystal revealed by molten KOH is 2.0 x 10(4) cm(-2), and the highest EPD is 3.1 x 10(4) cm(-2) This result indicates a quite good homogenity of the EPD which is much better than the ground-grown crystals. A similar better homogenity of the stoichiometry i.e., the [As]/([As] + [Ga]) ratio has been found in the space-grown SI-GaAs single crystal studied nondestructively using a new mapping method based upon X-ray Bond diffraction. The average stoichiometry in the space-grown crystal is 0.50007 with mean-square deviation of 6x10(-6), while the average stoichiometry in ground-grown SI-GaAs crystal is more than 0.50010. (C) 1998 Elsevier Science B.V. All rights reserved.

Influence of rapid thermal annealing on the optical properties of gallium nitride grown by gas-source molecular-beam epitaxy

Raman scattering, photoluminescence (PL), and nuclear reaction analysis (MA) have been employed to investigate the effects of rapid thermal annealing (RTA) on GaN films grown on sapphire (0001) substrates by gas-source molecular-beam epitaxy, The Raman spectra showed the presence of the E-2 (high) mode of GaN and shift of this mode from 572 to 568 cm(-1) caused by annealing. The results showed that RTA has a significant effect on the strain relaxation caused by the lattice and thermal expansion misfit between the GaN epilayer and the substrate. The PL peak exhibited a blueshift in its energy position and a decrease in the full width at half maximum after annealing, indicating an improvement in the optical quality of the film. Furthermore, a green luminescence appeared after annealing and increased in intensity with increasing annealing time. This effect was attributed to H concentration variation in the GaN film, which was measured by NRA. A high H concentration exists in as-grown GaN, which can neutralize the deep level, and the H-bonded complex dissociates during RTA, This leads to the appearance of a luminescent peak in the PL spectrum. (C) 1998 American Institute of Physics.

Influence of fluorine on radiation-induced charge trapping in the SIMOX buried oxides

Charge trapping in the fluorinated SIMOX buried oxides before and after ionizing radiation has been investigated by means of C-V characteristics. Radiation-induced positive charge trapping which results in negative shift of C-V curves can be restrained by implanting fluorine ions into the SIMOX buried oxides. Pre-radiation charge trapping is suppressed in the fluorinated buried oxides. The fluorine dose and post-implantation anneal time play a very important role in the control of charge trapping.

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.

GROWTH OF HIGH-QUALITY GALLIUM-ARSENIDE ON HF-ETCHED SILICON (001) BY CHEMICAL BEAM EPITAXY

HF etching followed by relatively low temperature (almost-equal-to 600-degrees-C) pretreatment is shown to provide a suitable substrate for the heteroepitaxial growth of GaAs on Si(100) by CBE using TEGa and AsH3 as sources. Rutherford backscattering (RBS), photoluminescence (PL), transmission electron microscopy (TEM), and Raman measurements show the low-defect nature of the GaAs epilayer.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.

Nondestructive measurements of stoichiometry in undoped semi-insulating gallium arsenide by x-ray bond method

The influences of microdefects and dislocations on the lattice parameters of undoped semi-insulating GaAs single crystals were analyzed, and a novel nondestructive method for measuring stoichiometry in undoped semi-insulating GaAs was established in this letter. The comparison of this method with coulometric titration indicates that the method of nondestructive measurements is indeed convenient and reliable. (C) 1996 American Institute of Physics.

Dislocations and precipitates in semi-insulating gallium arsenide revealed by ultrasonic Abrahams-Buiocchi etching

The dislocations and precipitates in SI-GaAs single crystals are revealed by ultrasonic-aided Abrahams-Buiocchi etching (USAB), and the etch pits are observed and measured by metalloscope and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), respectively. The size of etch pit revealed by USAB etching is about 1 order of magnitude smaller than that revealed by molten KOH. The amount of arsenic atoms in the dislocation-dense zone is about 1% larger than that in an adjacent dislocation-free zone measured by EDS attached to SEM, which indicates that the excess arsenic atoms adjacent to the dislocation-dense zone are attracted to the dislocations and precipitate there due to the deformation energy.

CHARACTERISTICS OF OXIDES FORMED FROM A SI0.5GE0.5

X-ray photoelectron spectroscopy (XPS) combined with Auger electron spectroscopy (AES) have been used to study the oxides from a Si0.5Ge0.5 alloy grown by molecular beam epitaxy (MBE). The oxidation was performed at 1000 degrees C wet atmosphere. The oxide consists of two layers: a mixed (Si,Ge)O-x layer near the surface and a pure SiOx layer underneath. Ge is rejected from the pure SiOx and piles up at the SiOx/SiGe interface. XPS analysis demonstrates that the chemical shifts of Si 2p and Ge 3d in the oxidized Si0.5Ge0.5 are significantly larger than those in SiO2 and GeO2 formed from pure Si and Ge crystals.

Sensitivity of Total-Dose Radiation Hardness of SIMOX Buried Oxides to Doses of Nitrogen Implantation into Buried Oxides

In order to improve the total-dose radiation hardness of the buried oxides(BOX) in the structure of separa tion-by-implanted-oxygen(SIMOX) silicon-on-insulator(SOI), nitrogen ions are implanted into the buried oxides with two different doses,2 × 1015 and 3 × 1015 cm-2 , respectively. The experimental results show that the radiation hardness of the buried oxides is very sensitive to the doses of nitrogen implantation for a lower dose of irradiation with a Co-60 source. Despite the small difference between the doses of nitrogen implantation, the nitrogen-implanted 2 × 1015 cm-2 BOX has a much higher hardness than the control sample (i. e. the buried oxide without receiving nitrogen implantation) for a total-dose irradiation of 5 × 104rad(Si), whereas the nitrogen-implanted 3 × 1015 cm-2 BOX has a lower hardness than the control sample. However,this sensitivity of radiation hardness to the doses of nitrogen implantation reduces with the increasing total-dose of irradiation (from 5 × 104 to 5 × 105 rad (Si)). The radiation hardness of BOX is characterized by MOS high-frequency (HF) capacitance-voltage (C-V) technique after the top silicon layers are removed. In addition, the abnormal HF C-V curve of the metal-silicon-BOX-silicon(MSOS) structure is observed and explained.

Space grown semi-insulating gallium arsenide single crystal and its application

Low noise field effect transistors and analogue switch integrated circuits (ICs) have been fabricated in semi-insulating gallium arsenide (SI-GaAs) wafers grown in space by direct ion-implantation. The electrical behaviors of the devices and the ICs have surpassed those fabricated in the terrestrially grown SI-GaAs wafers. The highest gain and the lowest noise of the transistors made from space-grown SI-GaAs wafers are 22.8 dB and 0.78 dB, respectively. The threshold back-gating voltage of the ICs made from space-grown SI-GaAs wafers is better than 8.5 V The con-elation between the characterizations of materials and devices is studied systematically. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.