Measurement of threading dislocation densities in GaN by wet chemical etching

We demonstrate a technique based on wet chemical etching that enables quick and accurate evaluation of edge- and screw/mixed-type threading dislocations (TDs) in GaN. Large and small etch pits are formed by phosphoric acid on the etched surfaces. The large etch pits are attributed to screw/mixed TDs and the small ones to edge TDs, according to their locations on the surface and Burgers vectors of TDs. Additionally, the origin of small etch pits is confirmed by a transmission electron microscopy. The difference in the size of etch pits is discussed in view of their origin and merging. Overetching at elevated temperatures or for a long time may result in merging of individual etch pits and underestimating of the density of TDs. Wet chemical etching has also been proved efficient in revealing the distribution of TDs in epitaxial lateral overgrowth GaN.

Evolution of mosaic structure in InN grown by metalorganic chemical vapor deposition

Mosaic structure in InN layers grown by metalorganic chemical vapor deposition at various temperatures has been investigated by X-ray diffraction (XRD). With a combination of Williamson-Hall measurement and fitting of twist angles, it was found that variation of growth temperature from 450 to 550 degrees C leads to the variation of the lateral coherence length, vertical coherence length, tilt and twist of mosaic blocks in InN films in a, respectively, monotonic way. In particular, mosaic tilt increases whereas mosaic twist decreases with elevating temperature. Atomic force microscopy shows the morphological difference of the InN nucleation layers grown at 450 and 550 degrees C. Different coalescence thickness and temperature-dependent in-plane rotation of InN nuclei are considered to account for the XRD results. (c) 2006 Elsevier B.V. All rights reserved.

InN nanoflowers grown by metal organic chemical vapor deposition

Hexangular indium nitride nanoflower pattern is observed from scanning electron microscopy and atomic force microscopy. The sample is grown on c-plane (0001) sapphire by metal organic chemical vapor deposition with intentional introduction of hydrogen gas. With the aid of hydrogen, a stable existence of metallic indium is achieved. This will induce the growth of InN nanoflowers via self-catalysis vapor-liquid-solid (VLS) process. It is found that the VLS process is modulated by the interface kinetics and thermodynamics among the sapphire substrate, indium, and InN, which leads to the special morphology of the authors' InN nanoflower pattern. (c) 2006 American Institute of Physics.

Crack control in GaN grown on silicon (111) using In doped low-temperature AlGaN interlayer by metalorganic chemical vapor deposition

The effects of In doped low-temperature (LT) AlGaN interlayer on the properties of GaN/Si(111) by MOCVD have been investigated. Using In doping LT-interlayer can decrease the stress sufficiently for avoiding crack formation in a thick (2.0 mu m) GaN layer. Significant improvement in the crystal and optical properties of GaN layer is also achieved. In doping is observed to reduce the stress in AlGaN interlayer measured by high-resolution X-ray diffraction (HRXRD). It can provide more compressive stress to counteract tensile stress and reduce crack density in subsequent GaN layer. Moreover, as a surfactant, indium is observed to cause an enhanced PL intensity and the narrowed linewidths of PL and XRD spectra for the LT-interlayer. Additionally, the crystal quality of GaN layer is found to be dependent on the growth parameters of underneath In-doped LT-AlGaN interlayer. The optimal parameters, such as TMIn flow rate, TMAl flow rates and thickness, are achieved to obtain nearly 2.0 mu m thick crack free GaN film with advanced optical and crystal properties. (c) 2005 Elsevier B.V. All rights reserved.

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Growth of ZnO single crystal by chemical vapor transport method

ZnO crystals were grown by CVT method in closed quartz tube under seeded condition. Carbon was used as a transport agent to enhance the chemical transport of ZnO in the growth process. ZnO single crystals were grown by using GaN/sapphire and GaN/Si wafer as seeds. The property and crystal quality of the ZnO single crystals was studied by photoluminescence spectroscopy and X-ray diffraction technique.

Magnetophotoluminescence of Zn0.88Mn0.12Se grown by metal-organic chemical vapor deposition on GaAs substrates

Magnetophotoluminescence properties of Zn0.88Mn0.12Se thin films grown by metal-organic chemical vapor deposition on GaAs substrates are investigated in fields up to 10 T. The linewidth of the excitonic luminescence peaks decreases with the increasing magnetic field (< 1 T), but the peak energy is almost unchanged. There is a crossover of the photoluminescence intensities between interband and bound excitonic transitions as the magnetic field is increased to about 1 T. These behaviors are interpreted by the strong tuning of the local alloy disorder potential by the applied magnetic field. In addition, the magnetic field-induced suppression of the energy transfers from excitons to Mn2+ ions is also observed.

Growth of InAs quantum dots on vicinal GaAs (100) substrates by metalorganic chemical vapor deposition and their optical properties

The growth of InAs quantum dots on vicinal GaAs (100) Substrates was systematically studied using low-pressure metalorganic chemical vapor deposition (MOCVD). The dots showed a clear bimodal size distribution on vicinal substrates. The way of evolution of this bimodal size distribution was studied as a function of growth temperature, InAs layer thickness and InAs deposition rate. The optical properties of dots grown on vicinal substrates were also studied by photoluminescence (PL). It was found that, compared with dots on exact substrates, dots on vicinal substrates had better optical properties such as a narrower PL line width, a longer emission wavelength, and a larger PL intensity. (c) 2006 Elsevier B.V. All rights reserved.

Parasitic reaction and its effect on the growth rate of AlN by metalorganic chemical vapor deposition

The Al composition of metalorganic chemical vapor deposition (MOCVD)-grown AlGaN alloy layers is found to be greatly influenced by the parasitic reaction between ammonia (NH3) and trimethylaluminum (TMAI). The growth process of AlN is carefully investigated by monitoring the in situ optical reflection. The abnormal dependencies of growth rate on growth temperature, reactor pressure, and flux of NH3 are observed and can be well explained by the effect of parasitic reaction. The increase of growth rate with increasing flux of TMAI is found to depend on the growth temperature and reactor pressure due to the presence of parasitic effect. A relatively low growth temperature and a reduced reactor pressure are suggested for the effective decrease of parasitic reaction during the MOCVD growth of AlN and probably lead to a more effective incorporation of Al into the AlGaN layers. (c) 2005 Elsevier B.V. All rights reserved.

Electrical properties of B-doped polycrystalline silicon thin films prepared by rapid thermal chemical vapour deposition

In this paper, about 30 mu m thick B-doped polycrystalline silicon (poly-Si) thin films were deposited on quartz substrates, n-type single crystalline silicon wafers and p(++)-type poly-Si ribbons by a rapid thermal chemical vapour deposition system in a temperature range from 1000 to 1150 degrees C. Activation energy measurement and room temperature/temperature dependent Hall effect measurement were performed on the poly-Si thin films prepared on the former two kinds of substrates, respectively. It seems that the electrical properties of as-prepared poly-Si thin films could be qualitatively explained by Seto's grain boundary (GB) trapping theory although there is a big difference between our samples and Seto's in gain size and film thickness etc. The experimental results reconfirm that GB itself is a kind of most effective recombination center with trapping level near the midgap and trapping state density in the order of 1012 cm(-2) magnitude. Electron beam induced current measurements on the poly-Si thin films prepared on the poly-Si ribbons also show that severe recombination occurs at the positions of GBs. (c) 2005 Elsevier B.V All rights reserved.

Structual and optoelectronic properties of polycrystalline silicon thin films prepared by hot-wire chemical vapor deposition at low temperatures

Polycrystalline silicon thin films were prepared by hot-wire chemical vapor deposition ( HWCVD) on glass at 250 degreesC with W or Ta wire as the catalyzers. The structual and optoelectronic properties as functions of the filament temperature, deposition pressure and the filament-substrate distance were studied, and the optimized polycrystalline silicon thin films were obtained with X-c > 90 % ( X-c denotes the crystalline ratio of the film), crystal grain size about 30-40nm, R-d approximate to 0.8nm/s, sigma(d) about 10(-7) - 10(-6) Omega(-1) cm(-1), Ea(a) approximate to 0.5eV and E-opt less than or equal to 1.3eV.

A passively mode-locked diode-end-pumped Nd : YAG laser with a semiconductor saturable absorber mirror grown by metal organic chemical vapour deposition

We report the experimental results of a mode-locked diode-end-pumped Nd:YAG laser with a semiconductor saturable absorber mirror (SESAM) from which we achieved a 10 ps pulse duration at 150 MHz repetition rate. The SESAM was grown by metal organic chemical vapour deposition at low temperature. The recovery time was measured to be 0.5 ps, indicating the potential pulse compression to sub-picoseconds.

Defects in GaN films grown on Si(111) substrates by metal-organic chemical vapour deposition

We report the transmission-electron microscopy study of the defects in wurtzitic GaN films grown on Si(111) substrates with AIN buffer layers by the metal-organic chemical vapour deposition method. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations passing through the MQW. No evident reduction of the edge dislocations density by the MQW was observed. It was found that dislocations with screw component can be located at the boundaries of sub-grains slightly in-plane misoriented.

Microstructure of GaN films grown on Si(111) substrates by metalorganic chemical vapor deposition

We report the transmission electron microscopy (TEM) study of the microstructure of wurtzitic GaN films grown on Si(I I I) substrates with AlN buffer layers by metalorganic chemical vapor deposition (MOCVD) method. An amorphous layer was formed at the interface between Si and AlN when thick GaN film was grown. We propose the amorphous layer was induced by the large stress at the interface when thick GaN was grown. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations from passing through the MQW. But no evident reduction of the edge dislocations by the MQW was observed. It was found that dislocations located at the boundaries of grains slightly in-plane misoriented have screw component. Inversion domain is also observed. (C) 2003 Elsevier B.V. All rights reserved.

Photoluminescence of Mg-doped GaN grown by metalorganic chemical vapor deposition

Two Mg-doped GaN films with different doping concentrations were grown by a metalorganic chemical vapor deposition technique. Photoluminescence (PL) experiments were carried out to investigate the optical properties of these films. For highly Mg-doped GaN, the PL spectra at 10 K are composed of a blue luminescence (BL) band at 2.857 eV and two excitonic luminescence lines at 3.342 eV and 3.282 eV, in addition to a L2 phonon replica at 3.212 eV. The intensity of the L1 line decreases monotonously with an increase,in temperature. However, the intensity of the L2 line first slowly increases at first, and then decreases quickly with an increase in temperature. The two lines are attributed to bound excitonic emissions at extended defects. The BL band is most likely due to the transition from deep donor Mg-V-N complex to Mg shallow acceptor. From the temperature dependence of the luminescence peak intensity of the BL band, the activation energy of acceptor Mg was found to be 290 meV. (C) 2003 American Vacuum Society.