Microstructure evolution of GaN buffer layer on MgAl2O4 substrate

Microstructure of GaN buffer layer grown on (111)MgAl2O4 substrate by metalorganic vapor phase epitaxy (MOVPE) was studied by transmission electron microscopy (TEM). It has been observed that the early deposition of GaN buffer layer on the substrate at a relatively low temperature formed a continual island-sublayer (5 nm thick) with hexagonal crystallographic structure, and the subsequent GaN buffer deposition led to crystal columns which are composed of nano-crystal slices with mixed cubic and hexagonal phases. After high-temperature annealing, the crystallinity of nano-crystal slices and island-sublayer in the buffer layer have been improved. The formation of threading dislocations in the GaN him is attributed not only to the lattice mismatch of GaN/MgAl2O4 interface, but also to the stacking mismatches at the crystal column boundaries. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Spectroscopic ellipsometry study of In2O3 thin films

In2O3 films grown by helicon magnetron sputtering with different thicknesses were characterized by spectroscopic ellipsometry in the energy range from 1.5 to 5.0 eV. Aside from one amorphous sample prepared at room substrate temperature, polycrystalline In2O3 films with cubic crystal structure were confirmed for other four samples prepared at the substrate temperature of 450 A degrees C. Excellent SE fittings were realized by applying 1 and/or 2 terms F&B amorphous formulations, building double layered film configuration models, and further taking account of void into the surface layer based on Bruggeman effective medium approximation for thinner films. Spectral dependent refractive indices and extinction coefficients were obtained for five samples. The curve shapes were well interpreted according to the applied dispersion formulas. Almost similar optical band gap values from 3.76 to 3.84 eV were obtained for five samples by Tauc plot calculation using extinction coefficients under the assumption of direct allowed optical transition mode.

A New Criterion Number for the Boundary Conditions at the Solid/Liquid Interface in Nanoscale

A simple, but important three-atom model was proposed at the solid/liquid interface, leading to a new criterion number, lambda, governing the boundary conditions (BCs) in nanoscale. The solid wall is considered as the face-centered-cubic (fcc) structure. The fluid is the liquid argon with the well-known LJ potential. Based on the concept, the two micro-systems have the same BCs if they have The same criterion number. The degree of the locking BCs is enhanced when lambda equals to 0.757. Such critical criterion number results in the substantial epitaxial ordering and one, two, or even three liquid layers are locked by the solid wall, depending on the coupling energy scale ratio of the solid and liquid atoms. With deviation from the critical criterion number, the flow approaches the slip BCs and there are little ordering structures within the liquid. Always at the same criterion number, the degree of the slip is decreased or the locking is enhanced with increasing the coupling energy scale ratio of the solid and liquid atoms. The above analysis is well confirmed by the molecular dynamics (MD) simulation. The slip length is well correlated in terms of the new criterion number. The future work is suggested to extend the present theory for other microstructures of the solid wall atoms and quasi-LJ potentials.

喷灌均匀系数的三次样条两次插值计算方法

为提高喷灌水量分布均匀性评价的准确性,当雨量筒径向布置时,为考虑所有测点数据对插值点降水深的影响,采用径向和周向两次的三次样条插值计算出未知点的降水深,从而计算喷灌均匀系数。以美国雨鸟30PSH型喷头雨量筒间隔为1m和2m的喷洒试验数据,计算网格点取1m和0.25m,分别采用三次样条两次插值法和邻近四点距离线性插值法计算了克里斯琴森均匀系数。结果表明,均匀系数由高至低的顺序依次为采样间隔为2m的线性插值、采样间隔为2m的三次样条两次插值、采样间隔为1m的线性插值和采样间隔为1m的三次样条两次插值。采样间隔2m比1m计算出的均匀系数总体高3~4个百分点,三次样条两次插值法比邻近点距离线性插值法略低1个百分点,2种计算网格点间距下的均匀系数差值小于1个百分点。结果证明,采样间距、插值方法、计算网格间距对均匀系数的影响依次降低,三次样条两次插值法可以用来评价喷灌组合均匀系数。

Near-Infrared Femtosecond Laser for Studying the Strain in Si1-xGex Alloy Films via Second-Harmonic Generation

The second-harmonic generation (SHG) from Si1-xGex alloy films has been investigated by near-infrared femtosecond laser. Recognized by s-out polarized SHG intensity versus rotational angle of sample, the crystal symmetry of the fully strained Si0.83Ge0.17 alloy is found changed from the O-h to the C-2 point group due to the inhomogeneity of the strain. Calibrated by double crystal X-ray diffraction, the strain-induced chi((2)) is estimated at 5.7 x 10(-7) esu. According to the analysis on p-in/s-out SHG, the strain-relaxed Si0.10Ge0.90 alloy film is confirmed to be not fully relaxed, and the remaining strain is quantitatively determined to be around 0.1%.

CHANNELING ALIGNMENT FOR EPITAXIAL LIGHT-MASS FILM ON HEAVY-MASS SUBSTRATE

A simple procedure for obtaining a background-free backscattering spectrum of a light-mass film on a heavy-mass substrate by a normal incidence/grazing exit geometry has been described. Using this method such films can be aligned rapidly and accurately, and the impurity or defect information on the films can be obtained without need for realignment. Example is given from MeV Li-3+ analysis of a deposited film of Si on a single crystal substrate of yttria-stabilized, cubic zirconia.

THE STUDY OF MICROSTRUCTURE IN IRON-DOPED AND SULFUR-DOPED INP CRYSTALS BY MEANS OF HREM AND COMPUTER-SIMULATION

The microstructures in iron- and sulphur-doped InP crystals were studied using both electron microscopy and electron diffraction. A modulated structure has been found in S-doped InP crystal, where the commensurate modulations corresponded to periodicities of 0.68 nm and 0.7 nm in real space and were related to the reflections of the cubic lattice in [111] and [113BAR] directions; they were indexed as q111* = 1/2(a* + b* + c*) and q113BAR* = 1/4(-a* - b* + 3c*), respectively. Single atomic layers of iron precipitate were observed, with preferred orientations along which precipitates are formed. Simulated calculations by means of the dynamical theory of electron diffraction using models for the precipitate structure were in good agreement with our experimental results. The relation between the modulated structure and the precipitates is also discussed.

PHYSICAL VAPOR TRANSPORT AND CRYSTAL-GROWTH OF GESEXTE1-X SOLID-SOLUTIONS

Physical vapor transport studies of GeSe(x)Te1 - x (x = 0.1, 0.2, 0.3, and 0.4) solid solutions demonstrated, that individual, large single crystals of these materials can be grown in closed ampoules. A compositional analysis of the grown crystals revealed, that the mass transport (crystal growth) process under steady-state conditions is pseudo-congruent and controlled by diffusion processes in the source material. From these experiments, the degree of non-stoichiometry (Ge-vacancy concentrations) of GeSe(x)Te1 - x single crystals could be estimated. The effects of the cubic to rhombohedral phase transformation during cooling on the microstructure and morphology of the grown mixed crystals are observed. This work provides the basis for subsequent defect studies and electrical measurements on these crystals.

INVESTIGATION OF C-60 SINGLE-CRYSTAL BY X-RAY-METHODS

C-60 Single crystals grown by a single-temperature-gradient technique were characterized by synchrotron radiation white beam x-ray topography and x-ray double crystal diffraction with Cu K-alpha 1 radiation on conventional x-ray source. The results show that the crystal is rather well crystallized, The x-ray topographies give an evidence of dendritic growth mechanism of C-60 Single crystal, and x-ray double crystal diffraction rocking curve shows that there are mosaic structural defects in the sample. A phase transition st 249+/-1.5% K from a simple cubic to a face centered cubic structure is confirmed by in situ observation of synchrotron radiation white beam x-ray topography with the temperature varing from 230 to 295 K.

Etching Behavior of GaN/GaAs(001) Epilayers Grown by MOVPE

Wet etching characteristics of cubic GAN (c-GaN) thin films grown on GaAs(001) by metalorganic vapor phase epitaxy (MOVPE) are investigated. The samples are etched in HCl, H_3PO_4, KOH aqueous solutions, and molten KOH at temperatures in the range of 90～300 ℃. It is found that different solution produces different etch figure on the surfaces of a sample. KOH-based solutions produce rectangular pits rather than square pits. The etch pits elongate in [1(1-bar)0] direction, indicating asymmetric etching behavior in the two orthogonal <110> directions. An explanation based on relative reactivity of the various crystallographic planes is employed to interpret qualitatively the asymmetric etching behavior. In addition, it is found that KOH aqueous solution would be more suitable than molten KOH and the two acids for the evaluation of stacking faults in c-GaN epilayers.

The growth and characterization of GaN grown on a gamma-Al2O3/(001) Si substrate by metalorganic vapor phase epitaxy

Wurtzite single crystal GaN films have been grown onto a gamma-Al2O3/Si(001) substrate in a horizontal-type low pressure MOVPE system. A thin gamma-Al2O3 layer is an intermediate layer for the growth of single crystal GaN on Si although it is only an oriented polycrystal film as shown by reflection high electron diffraction. Moreover, the oxide is not yet converted to a fully single crystal film, even at the stage of high temperature for the GaN layer as studied by transmission electron microscopy. Double crystal x-ray linewidth of (0002) peak of the 1.3 mu m sample is 54 arcmin and the films have heavy mosaic structures. A near band edge peaking at 3.4 eV at room temperature is observed by photoluminescence spectroscopy. Raman scattering does not detect any cubic phase coexistence.

The effect of low temperature GaAs nucleation on the growth of GaN on Silicon (001) during MOVPE process

High quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10-20nm GaAs nucleation layer grown by ALE.

Epitaxial growth of SiC on complex substrates

Epitaxial growth of SiC on complex substrates was carried out at substrate temperature from 1200 degreesC to 1400 degreesC. Three kinds of new complex substrates, c-plane sapphire, AlN/sapphire, and GaN/AlN/sapphire, were used in this study. We obtained a growth rate in the range of 1-6 mum/h. Thick (6 mum) SIC epitaxial layers with no cracks were successfully obtained on AlN/sapphire and GaN/AlN/sapphire substrates. X-ray diffraction patterns have confirmed that single-crystal SiC was obtained on these complex substrates. Analysis of optical transmission spectra of the SIC grown on sapphire substrates shows the lowest-energy gap near 2.2 eV, which is the value for cubic SiC. The undoped SIC showed n-type electrical conductivity. (C) 2001 Elsevier Science B.V. All rights reserved.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Optimized sinusoidal phase mask to extend the depth of field of an incoherent imaging system

Wavefront coding is a powerful technique that can be used to extend the depth of field of an incoherent imaging system. By adding a suitable phase mask to the aperture plane, the optical transfer function of a conventional imaging system can be made defocus invariant. Since 1995, when a cubic phase mask was first suggested, many kinds of phase masks have been proposed to achieve the goal of depth extension. In this Letter, a phase mask based on sinusoidal function is designed to enrich the family of phase masks. Numerical evaluation demonstrates that the proposed mask is not only less sensitive to focus errors than cubic, exponential, and modified logarithmic masks are, but it also has a smaller point-spread-function shifting effect. (C) 2010 Optical Society of America