Chunlin Shao

Anomalous compositional pulling effect in InGaN/GaN multiple quantum wells

A series of InGaN/GaN multiple quantum wells (MQWs) was grown by metalorganic chemical vapor deposition with different well thickness. High-resolution x-ray diffraction studies revealed that the In composition is increasing along the growth direction from the bottom to the top of each well layer in these MQWs. While the In composition at the bottom of each well layer almost keeps constant, the increasing rate of In composition becomes obviously larger when the growth temperature is decreased. The important conclusion of this study is that the InGaN/GaN MQWs is shaped like a triangle due to the increasing of In composition from the bottom to the top of the well layer. The emission mechanism of the InGaN/GaN MQWs has to be discussed based on the triangular band gap structure.

Porous GaAs formed by a two-step anodization process

Abstract In this letter, we proposed a method in which a n-type GaAs wafer was first anodized in HF for a short time to form a high density of etch pits on its surface and then anodized in KOH solution to form a high density of pores with uniform distribution in it. Scanning electron microscopy revealed feature sizes of the porous structure in the nanometer range. The porous GaAs has been investigated using double crystal X-ray diffraction technique. Crystal quality and crystallographic orientation of the porous layer were similar to those of the substrate. The lattice constant of porous layer is slightly larger than that of the substrate in the growth direction.

Visible-Blind Metal-Semiconductor-Metal Photodetectors Based on Undoped AlGaN/GaN High Electron Mobility Transistor Structure

Visible-blind Schottky metal-semiconductor-metal photodetectors (MSM-PDs) fabricated on an undoped AlGaN/GaN high electron mobility transistor (HEMT) structure were reported. Dark-current density as low as 1.8 ×10-8 A/cm2 at 10 V bias was obtained. A peak responsivity of 114 mA/W at 350 nm was measured under top illumination with a constant irradiation power density of 10 µW/cm2, which corresponds to an external quantum efficiency of 40%. The MSM-PDs based on this structure are of advantage for monolithic integration with HEMT circuits in one epitaxial step.

CHARACTERIZATION OF EXCIMER LASER ANNEALED POLYCRYSTALLINE SI1-XGEX ALLOY THIN FILMS BY X-RAY DIFFRACTION AND SPECTROSCOPIC ELLIPSOMETRY

Thin films of Si1−xGex alloys of different compositions x have been deposited, on single-crystal Si (100) surface and glass substrates, by simple ion beam sputtering, at room temperature. Crystallization of these films has been done using excimer laser annealing. Structural and optical properties of as-deposited and annealed Si1−xGex alloy films are characterized by x-ray diffraction (XRD), uv-visible spectrophotometry, spectroscopic ellipsometry (SE), and Auger electron spectroscopy (AES). The as-deposited films, both on Si and glass, have been found to be amorphous by XRD. Polycrystalline nature of laser-annealed samples has been evidenced by both x-ray and SE measurements. The results of x-ray, uv-visible, AES, and SE are compared and discussed. The poly-Si1−xGex films were oriented predominantly to (111) and the grain sizes were determined from half-width of x-ray peaks. The compositions x of Si1−xGex films have been evaluated from the SE dielectric function e(ω) data, using the second-derivative tech...

A theoretical model for the tilt of the GaAs/Si epilayers

The crystallographic tilt of the epilayers with respect to their substrates has been observed in many heteroepitaxial systems. Many models have been proposed to explain this phenomenon, but none of them is suitable for the large mismatched system, such as GaAs/Si. Here a new model is proposed for GaAs/Si epilayers, which can also be used in other large mismatched systems. The magnitude of the tilt calculated from this model coincide well with the experimental results. Especially, this model can correctly predict the tilt direction of the GaAs/Si epilayers.

Formation Mechanism for High-Surface-Area Anatase Titania Nanoparticles Prepared by Metalorganic Chemical Vapor Deposition

High-surface-area anatase titania nanoparticles are prepared by metalorganic chemical vapor deposition (MOCVD) using thermophoretic collection. Based on the dependence of X-ray linewidth (FWHM) on collection distance, a formation mechanism for the nanoparticles is proposed in which it is assumed that the relative changes in the grain size of and vacancy concentration in the nanoparticles are proportional to the velocity of the gas flow in the reactor. Three nondimensional equations that describe the dependence of the properties of the nanoparticles on collection distance are derived. This model explains well the effect of collection distance on X-ray linewidth, and it is not only in good agreement with XPS results but also consistent with TEM observations.

Optical properties of excimer laser annealed polycrystalline Si by spectroscopic ellipsometry

Abstract Using spectroscopic ellipsometry (SE), optical properties of poly-Si obtained by excimer laser annealing of amorphous silicon (α-Si) are evaluated. Both, amorphous silicon and SiO 2 are deposited on Si substrate (α-Si/SiO 2 /Si) by low temperature chemical vapor deposition (LPCVD). SE measurement shows that the crystallization starts with laser annealing of energy 144–280 mJ/cm 2 and above at which it remains amorphous. A linear regression analysis (LRA) and a Bruggeman effective-medium approximation (EMA) indicate that the laser annealed layer consists of 91.3% crystalline Si and 8.7% amorphous Si for samples irradiated in two steps, i.e., 344 and 276 mJ/cm 2 of each pulse. The thickness of each layer determined by SE is in good agreement with that of cross-section transmission electron microscopy (TEM).

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.

Emission Properties of Electrodeless Argon Gas Discharge in VUV Region

The dependences of the VUV light intensity from an electrodeless Argon gas discharge on various parameters such as the gas pressure, gas flow rate, input voltage, input current and discharge current were investigated. These data can be utilized in designing this type of VUV light source. The peak intensity of the light in the 50 to 110 nm wavelength region was estimated to be about 1.8×10-4 W/cm2 at the output aperture of the discharge tube.

Barrier-height-enhanced n-GaN Schottky photodiodes using a thin p-GaN surface layer

A p+-GaN surface layer of 15 nm was incorporated in n-GaN Schottky photodiode to enhance the effective Schottky barrier height. A barrier height of 1.09 eV for the normal n-GaN Schottky photodiode was increased to the effective barrier height of 1.16 eV. The resulting photodiodes show a reverse dark current density of as low as 4.8×10-10 A/cm2 at -2 V bias, which is about three orders of magnitude lower than that of the normal n-GaN Schottky photodiode. The lower dark current leads to a significant improvement in the visible rejection ratio. A peak responsivity of 107 mA/W was obtained at -2 V bias under the incident power density of 10 µW/cm2, corresponding to an external quantum efficiency of 38%.