Assessment of the structural properties of GaAs/Si epilayers using X-ray (004) and (220) reflections

We improved the method previously used to determine the lattice constants and misorientation of GaAs/Si by recording the patterns of X-ray (004) and (220) reflections. The (220) reflection was measured from the (110) cross section of a GaAs/Si epilayer. The structural properties of the GaAs/Si epilayers grown by metal-organic chemical-vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were investigated. The rotation angle of GaAs/Si epilayers grown by MOCVD using an a-Si buffer layer is very small and the lattice constants of these GaAs/Si epilayers agree quite well with elastic theory.

Characterization and improvement of GaAs layers grown on Si using an ultrathin a-Si film as a buffer layer

GaAs epilayers grown on Si by metalorganic chemical vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were characterized by deep-level transient spectroscopy (DLTS). Six electron traps with activation energies of 0.79, 0.67, 0.61, 0.55, 0.53 and 0.32 eV below the conduction band were determined by fitting the experimental spectra. Two of the levels, C (0.61 eV) and F (0.32 eV), were first detected in GaAs epilayers on Si and identified as the metastable defects M3 and M4, respectively. In order to improve the quality of GaAs/Si epilayers, another GaAs layer was grown on the GaAs/Si epilayers grown using MOCVD. The deep levels in this regrown GaAs epilayer were also studied using DLTS. Only the EL2 level was found in the regrown GaAs epilayers. These results show that the quality of the GaAs epilayer was greatly improved by applying this growth process.

Growth of GaAs on Si by using a thin Si film as buffer layer

We report a novel technique for growing high-quality GaAs on Si substrate. The process involves deposition of a thin amorphous Si film prior to the conventional two-step growth. The GaAs layers grown on Si by this technique using metalorganic chemical vapor deposition exhibit a better surface morphology and higher crystallinity as compared to the samples gown by conventional two-step method. The full width at half maximum (FWHM) of the x-ray (004) rocking curve for 2.2 mu m thick GaAs/Si epilayer grown by using this new method is 160arcsec. The FWHM of the photoluminescence spectrum main peak for this sample is 2.1 meV. These are among the best results reported so far. In addition, the mechanism of this new growth method was studied using high-resolution transmission electron microscopy.

PHOTOLUMINESCENCE SPECTRUM STUDY OF THE GAAS/SI EPILAYER GROWN BY USING A THIN AMORPHOUS SI FILM AS BUFFER LAYER

Recently, we reported successful growth of high-quality GaAs/Si epilayers by using a very thin amorphous Si film as buffer layer. In this paper, the impurity properties of this kind of GaAs/Si epilayers have been studied by using PL spectrum, SIMS and Hall measurement. Compared to a typical PL spectrum of the GaAs/Si epilayers grown by conventional two-step method, a new peak was observed in our PL spectrum at the energy of 1.462 eV, which is assigned to the band-to-silicon acceptor recombination. The SIMS analysis indicates that the silicon concentration in this kind of GaAs/Si epilayers is about 10(18) cm(-3). But its carrier concentration (about 4 x 10(17) cm(-3)) is lower than the silicon concentration. The lower carrier concentration in this kind of GaAs/Si epilayer can be interpreted both as the result of higher compensation and as the result of the formation of the donor-defect complex. We also found that the high-quality and low-Si-concentration GaAs/Si epilayers can be regrown by using this kind of GaAs/Si epilayer as substrate. The FWHM of the X-ray (004) rocking curve from this regrowth GaAs epilayer is 118 '', it is much less than that of the first growth GaAs epilayer (160 '') and other reports for the GaAs/Si epilayer grown by using conventional two-step method (similar to 200 '').

Influence of zinc phthalocyanines on photoelectrical properties of hydrogenated amorphous silicon

Composites consisting of hydrogenated amorphous silicon (a-Si: H, inorganic) and zinc phthalocyanine (ZnPc, organic) were prepared by vacuum evaporation of ZnPc and sequential deposition amorphous silicon via plasma enhanced chemical vapor deposition (PECVD). The optical and electrical properties of the composite film have been investigated. The results demonstrate that ZnPc can endure the temperature and bombardment of the PECVD plasma and photoconductivity of the composite film was improved by 89.9% compared to pure a-Si: H film. Electron mobility-lifetime products μτ of the composite film were increased by nearly one order of magnitude from 6.96 × 10~(-7) to 5.08 × 10~(-6) cm~2/V. Combined with photoconductivity spectra of the composites and pure a-Si: H, we tentatively elucidate the improvement in photoconductivity of the composite film.

Stress and resistivity controls on in situ boron doped LPCVD polysilicon films for high-Q MEMS applications

The simultaneous control of residual stress and resistivity of polysilicon thin films by adjusting the deposition parameters and annealing conditions is studied. In situ boron doped polysilicon thin films deposited at 520 ℃ by low pressure chemical vapor deposition (LPCVD) are amorphous with relatively large compressive residual stress and high resistivity. Annealing the amorphous films in a temperature range of 600-800 ℃ gives polysilicon films nearly zero-stress and relatively low resistivity. The low residual stress and low resistivity make the polysilicon films attractive for potential applications in micro-electro-mechanical-systems (MEMS) devices, especially in high resonance frequency (high-f) and high quality factor (high-Q MEMS resonators. In addition, polysilicon thin films deposited at 570 ℃ and those without the post annealing process have low resistivities of 2-5 mΩ·cm. These reported approaches avoid the high temperature annealing process (> 1000℃), and the promising properties of these films make them suitable for high-Q and high-f MEMS devices.

Boron-doped silicon film as a recombination layer in the tunnel junction of a tandem solar cell

Boron-doped hydrogenated silicon films with different gaseous doping ratios (B_2H_6/SiH_4) were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The microstructure of the films was investigated by atomic force microscopy (AFM) and Raman scattering spectroscopy. The electrical properties of the films were characterized by their room temperature electrical conductivity (σ) and the activation energy (E_a). The results show that with an increasing gaseous doping ratio, the silicon films transfer from a microcrystalline to an amorphous phase, and corresponding changes in the electrical properties were observed. The thin boron-doped silicon layers were fabricated as recombination layers in tunnel junctions. The measurements of the Ⅰ-Ⅴ characteristics and the transparency spectra of the junctions indicate that the best gaseous doping ratio of the recombination layer is 0.04, and the film deposited under that condition is amorphous silicon with a small amount of crystallites embedded in it. The junction with such a recombination layer has a small resistance, a nearly ohmic contact, and a negligible optical absorption.

Effect of Carrier Gas Flux on ZnO Nanorod Arrays Grown by MOCVD

ZnO nanorod arrays with different morphologies were grown by metalorganic chemical vapor deposition （MOCVD）. The diameters of nanorods range from 150 nm to 20 nm through changing the carrier gas flux during the growth process. Measurements such as scanning electron microscope （SEM）, X-ray diffraction （XRD）, Raman scattering and photoluminescence （pL） spectrum were employed to analyze the differences of these nanorods. It was found that when both carrier gas flux of Zn and O reactant are 1 SLM, we can obtain the best vertically aligned and uniform nanorods. Furthermore, the PL spectrum reveals a blueshift of UV emission peak, which may be assigned to the increase of surface effect.

Heteroepitaxial Growth of 3C-SiC Films on Maskless Patterned Silicon Substrates

Heteroepitaxial growth of 3C-SiC on patterned Si substrates by low pressure chemical vapor deposition (LPCVD) has been investigated to improve the crystal quality of 3C-SiC films. Si substrates were patterned with parallel lines, 1 to 10μm wide and spaced 1 to 10μm apart, which was carried out by photolithography and reactive ion etching. Growth behavior on the patterned substrates was systematically studied by scanning electron microscopy (SEM). An air gap structure and a spherical shape were formed on the patterned Si substrates with different dimensions. The air gap formed after coalescence reduced the stress in the 3C-SiC films, solving the wafer warp and making it possible to grow thicker films. XRD patterns indicated that the films grown on the maskless patterned Si substrates were mainly composed of crystal planes with (111) orientation.

Optical and Electrical Properties of GaN.Mg Grown by MOCVD

Mg-doped GaN layers prepared by metalorganic chemical vapor deposition were annealed at temperatures between 550 and 950℃. Room temperature （RT） Hall and photoluminescence （PL） spectroscopy measurements were performed on the as-grown and annealed samples. After annealing at 850℃, a high hole concentration of 8 × 10~(17) cm~(-3) and a resistivity of 0. 8lΩ·cm are obtained. Two dominant defect-related PL emission bands in GaN.. Mg are investigated; the blue band is centered at 2. 8eV （BL） and the ultraviolet emission band is around 3.27eV （UVL）. The relative intensity of BL to UVL increases after annealing at 550℃, but decreases when theannealing temperature is raised from 650 to 850℃, and finally increases sharply when the annealing temperature is raised to 950C. The hole concentration increases with increased Mg doping, and decreases for higher Mg doping concentrations. These results indicate that the difficulties in achieving high hole concentration of 10~(18)cm~(-3) appear to be related not only to hydrogen passivation, but also to self-compensation.

High power red-light GaInP/AlGaInP laser diodes with nonabsorbing windows based on Zn diffusion-induced quantum well intermixing

The layer structure of GaInP/AlGaInP quantum well laser diodes (LDs) was grown on GaAs substrate using low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. In order to improve the catastrophic optical damage (COD) level of devices, a nonabsorbing window (NAW), which was based on Zn diffusion-induced quantum well intermixing, was fabricated near the both ends of the cavities. Zn diffusions were respectively carried out at 480, 500, 520, 540, and 580 Celsius degree for 20 minutes. The largest energy blue shift of 189.1 meV was observed in the window regions at 580 Celsius degree. When the blue shift was 24.7 meV at 480 Celsius degree, the COD power for the window LD was 86.7% higher than the conventional LD.

Persistent Photoconductivity in n-type GaN

The persistent photoconductivity（PPC） phenomena in n-type GaN Films grown by metalorganic chemical vapor deposition（MOCVD） have been studied. After using some testing and analysis methods, such as the double crystal X-ray diffraction（DCXRD）, the photolumineseence（PL） spectra, etc, it is found that the issue which influences PPC in n-type GaN is not relative to the dislocations and yellow band （YB）, and is caused by the doping level of Si most likely.

渐变掩蔽图形超低压选择区域生长法制备高质量InGaAsP多量子阱材料

采用超低压（22mbar）选择区域生长（Selective Area Growth，SAG）金属有机化学汽相沉积（Metal-organic Chemical Vapor Deposition，MOCVD）技术成功制备了高质量InGaAsP／InGaAsP多量子阱（Multiple Quantum Well，MQW）材料．在较小的掩蔽宽度变化范围内（15—30μm），得到了46nm的光荧光（Photoluminescence，PL）波长偏移量，PL半高宽（Full-Width-at-Half-Maximum，FWHM）小于30meV．为了保证选择区域内的MQWs材料的均匀性，我们采用了新型的渐变掩蔽图形，并且运用这种新型渐变掩蔽图形，研究了渐变区域的过渡效应对材料生长的影响．我们还观察到，渐变区域的能量偏调量随着掩蔽图形宽度与渐变区域长度比值的增大而出现饱和现象．

MOCVD-Grown AlGaN/AlN/GaN HEMT Structure with High Mobility GaN Thin Layer as Channel on SiC

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with a high-mobility GaN thin layer as a channel are grown on high resistive 6H-SiC substrates by metalorganic chemical vapor deposition. The HEMT structure exhibits a typical two-dimensional electron gas (2DEG) mobility of 1944cm2/(V · s) at room temperature and 11588cm2/(V· s) at 80K with almost equal 2DEG concentrations of about 1.03 × 1013 cm-2 High crystal quality of the HEMT structures is confirmed by triple-crystal X-ray diffraction analysis. Atomic force microscopy measurements reveal a smooth AlGaN surface with a root-mean-square roughness of 0. 27nm for a scan area of 10μm × 10μm. HEMT devices with 0.8μm gate length and 1.2mm gate width are fabricated using the structures. A maximum drain current density of 957mA/mm and an extrinsic transconductance of 267mS/mm are obtained.

Dislocation Reduction in GaN on Sapphire by Epitaxial Lateral Overgrowth

High quality GaN is grown on GaN substrate with stripe pattern by metalorganic chemical vapor deposition by means of epitaxial lateral overgrowth. AFM,wet chemical etching, and TEM experiments show that with a two-step ELOG procedure, the propagation of defects under the mask is blocked, and the coherently grown GaN above the window also experiences a drastic reduction in defect density. In addition, a grain boundary is formed at the coalescence boundary of neighboring growth fronts. The extremely low density of threading dislocations within wing regions makes ELOG GaN a potential template for the fabrication of nitride-based lasers with improved performance.