Sulfur forming an isoelectronic center in zinc telluride thin films

ZnTe1-xSx epitaxial layers grown on GaAs by molecular-beam epitaxy were studied by photoluminescence (PL) as a function of temperatures, excitation powers, and hydrostatic pressures. A sulfur-related emission peak, labeled as P-2, is identified as a deep-level emission by hydrostatic-pressure PL measurement. This indicates that sulfur atoms form isoelectronic centers in a ZnTe matrix. The results qualitatively agree with the theoretical prediction and show experimental evidence of isoelectronic S in ZnTe. A model is proposed to explain the emission mechanisms in the ZnTe1-xSx system with small x values.

Wet etching and infrared absorption of AlN bulk single crystals

The defects and the lattice perfection of an AlN (0001) single crystal grown by the physical vapor transport (PVT) method were investigated by wet etching, X-ray diffraction (XRD), and infrared absorption, respectively. A regular hexagonal etch pit density (EPD) of about 4000 cm~(-2) is observed on the (0001) A1 surface of an AlN single crystal. The EPD exhibits a line array along the slip direction of the wurtzite structure, indicating a quite large thermal stress born by the crystal in the growth process. The XRD full width at half maximum (FWHM) of the single crystal is 35 arcsec, suggesting a good lattice perfection. Pronounced infrared absorption peaks are observed at wave numbers of 1790, 1850, 2000, and 3000 cm~(-1), respectively. These absorptions might relate to impurities O, C, Si and their complexes in AlN single crystals.

Ag surface plasmon enhanced double-layer antireflection coatings for GaAs solar cells

Surface plasmon enhanced antireflection coatings for GaAs solar cells have been designed theoretically. The reflectance of double-layer antireflection coatings (ARCs) with different suspensions of Ag particles is calculated as a function of the wavelength according to the optical interference matrix and the Mie theory. The mean dielectric concept was adopted in the simulations. A significant reduction of reflectance in the spectral region from 300 to 400 nm was found to be beneficial for the design of ARCs. A new SiO_2/Ag-ZnS double-layer coating with better antireflection ability can be achieved if the particle volume fraction in ZnS is 1%-2%.

折射率色散效应对晶体硅太阳电池反射率的影响

为了分析色散效应对晶体硅太阳电池反射率的影响,在考虑材料折射率色散效应的情况下,运用光学干涉矩阵计算了具有SiO_2单层减反射膜和MgF_2/ZnS双层减反射膜晶体硅太阳电池的反射率与波长的函数关系,并与实验结果和未考虑色散效应的计算结果进行了对比分析.结果表明:考虑折射率色散效应的计算结果与实验测量数据完全相符,而未考虑折射率色散效应的计算结果与实验测量数据相差较大,最大差值分别为21.5%和16.9%.

色散效应对GaAs太阳电池双层减反射膜的影响

考虑双层减反射膜材料的折射率色散效应，采用光学干涉矩阵法计算了SiO2／ZnSe和SiO2／ZnS两种GaAs太阳电池双层减反射膜的反射率与波长的函数曲线，以及加权平均反射率随着顶层减反射膜SiO2厚度变化的函数曲线，并与未考虑色散效应的情况进行了对比．计算结果表明，色散效应对双层减反射膜的反射率有较大的影响，特别是对300-500nm波长范围的影响更大，且对不同材料的减反射膜的影响也是不同的．与未考虑色散效应的情况相比，考虑色散效应后，SiO2／ZnSe双层减反射膜的最小加权平均反射率从1．14％增加到1．55％，而SiO2／ZnS双层减反射膜的最小加权平均反射率却从1．49％减小到1.46％．

GaN／AlN组织平顶金字塔量子点的应变场分布

利用有限元法，研究了GaN／AlN自组织量子点材料的应变场分布。量子点模型采用了实验观察到的六角平顶金字塔形状，材料的弹性常数考虑了晶体材料的Wurtzite结构特性。在晶格失配处理上，采用了三维各项异性伪热膨胀模型。将基于有限元法的计算结果与简化的基于格林函数理论的解析计算结果进行了比较，验证了模型和计算结论的正确性。最后，讨论了各向异性效应对层间量子点垂直对准的影响，指出量子点材料的各向异性效应可以忽略。本文采用的模型没有采用类似解析计算的假设条件，因此在计算精度和可靠性上，要优于格林函数法。

半导体低维结构的压力光谱研究

研究了一些半导体低维结构的压力光谱.测得平均直径为26、52和62nm的In0.55Al0.45As/Al0.5Ga0.5As量子点发光峰的压力系数分别为82、94和98 meV/GPa.表明这些发光峰具有Γ谷的特性,这些量子点为Ⅰ型量子点.而平均直径为7nm的量子点发光峰的压力系数为-17 meV/GPa,具有X谷的特性.所以这种小量子点为Ⅱ型量子点.测得ZnS:Mn纳米粒子中Mn发光峰的压力系数为-34.6meV/GPa,与晶体场理论的预计一致.而DA对发光峰基本不随压力变化,表明它应该与ZnS基体中的表面缺陷有关.测得ZnS:Cu纳米粒子中Cu的发光峰的压力系数为63.2meV/GPa,与ZnS体材料的带隙压力系数相同.表明Cu引入的受主能级具有浅受主的某些特点.测得ZnS:Eu纳米粒子中Eu发光峰的压力系数为24.1meV/GPa,与晶体场理论的预计不同.可能和Eu的激发态与ZnS导带间的相互作用有关.

MOCVD growth of cubic GaN: Materials and devices

Many impressive progresses have been made recently on the growth of cubic-phase GaN by MBE and MOCVD. In this paper, some of our recent progress will be reviewed, including the growth of high quality cubic InGaN films, InGaN/GaN heterostructure blue and green LEDs. Cubic-phase GaN films were grown on GaAs (100) substrates by MOCVD. Growth conditions were optimized to obtain pure cubic phase GaN films up to a thickness of 4 mum. An anomalous compressive strain was found in the as-grown GaN films in spite of a smaller lattice constant for GaN compared with that of GaAs substrates. The photoluminescence FWHM of high quality InGaN epilayers was less than 100 meV The InGaN/GaN heterostructure blue LED has intense electroluminescence with a FWHM of 20 nm.

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.

Electronic structure and optical property of semiconductor nanocrystallites

Semiconductor nanostructures show many special physical properties associated with quantum confinement effects, and have many applications in the opto-electronic and microelectronic fields. However, it is difficult to calculate their electronic states by the ordinary plane wave or linear combination of atomic orbital methods. In this paper, we review some of our works in this field, including semiconductor clusters, self-assembled quantum dots, and diluted magnetic semiconductor quantum dots. In semiconductor clusters we introduce energy bands and effective-mass Hamiltonian of wurtzite structure semiconductors, electronic structures and optical properties of spherical clusters, ellipsoidal clusters, and nanowires. In self-assembled quantum dots we introduce electronic structures and transport properties of quantum rings and quantum dots, and resonant tunneling of 3-dimensional quantum dots. In diluted magnetic semiconductor quantum dots we introduce magnetic-optical properties, and magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot. (C) 2004 Elsevier B.V. All rights reserved.

Pressure behavior of Te isoelectronic centers in S-rich ZnS1-xTex alloy

The photoluminescence from ZnS1-xTex alloy with 0 < x < 0.3 was investigated under hydrostatic pressure up to 7 GPa. Two peaks were observed in the alloys with x < 0.01, which are related to excitons bound to isolated Te isoelectronic impurities (Te-1 centers) and Te pairs (Te-2 centers), respectively. Only the Te-2 related emissions were observed in the alloys with 0.01 < x < 0.03. The emissions in the alloys with 0.03 < x < 0.3 are attributed to the excitons bound to the Te-n (n greater than or equal to 3) cluster centers. The pressure coefficient of the Te-1 related peak is 89(4) meV/GPa, about 40% larger than that of the band gap of ZnS. On the other hand, the pressure coefficient of the Te-2 related emissions is only 52(4) meV/GPa, about 15% smaller than that of the ZnS band gap. A simple Koster-Slater model has been used to explain the different pressure behavior of the Te-1 and Te-2 centers. The pressure coefficient of the Te-3 centers is 62(2) meV/GPa. Then the pressure coefficients of the Te-n centers decrease rapidly with further increasing Te composition.

Anisotropic strain in (100) ZnSe epilayers grown on lattice mismatched substrates

We show that part of the reflectance difference resonance near the E-0 energy of ZnSe is due to the anisotropic in-plane strain in the ZnSe thin films, as films grown on three distinctly different substrates, GaAs, GaP, and ZnS, all show the resonance at the same energy. Such anisotropic strain induced resonance is predicted and also observed near the E-1/E-1+Delta(1) energies in ZnSe grown on GaAs. The theory also predicts that there should be no resonance due to strain at, the E-0+Delta(0) energy, which is consistent with experiments. The strain anisotropy is rather independent of the ZnSe layer thickness, or whether the film is strain relaxed. For ZnSe films with large lattice mismatch with substrates, the resonance at the E-1/E-1+Delta(1) energies is absent, very likely due to the poor crystalline quality of the 20 nm or so surface layer. (C) 2000 American Vacuum Society. [S0734-211X(00)05604-3].

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.

Pressure behavior of deep centers in ZnSxTe1-x alloys

We have measured photoluminescence of ZnSxTe1-x alloys (x > 0.7) at 300 K and under hydrostatic pressure up to 7 GPa. The spectra contain only a broad emission band under excitation of the 406.7 nm line. Its pressure coefficients are 47, 62 and 45 meV/GPa for x = 0.98, 0.92 and 0.79 samples, which are about 26%, 7% and 38% smaller than that of the band gap in the corresponding alloys. The Stokes shifts between emission and absorption of the bands were calculated by fitting the pressure dependence of the emission intensity, being 0.29, 0.48 and 0.13 eV for the three samples, respectively. The small pressure coefficient and large Stokes shift indicate that the emission band observed in our samples may correspond to the Te isoelectronic center in the ZnSxTe1-x alloy.

MOVPE growth of GaN and LED on (111) MgAl2O4

The growth of wurtzite GaN by low-pressure metalorganic vapor-phase epitaxy on (1 1 1) magnesium aluminate (MgAl2O4) substrates have been studied. The morphological, crystalline, electrical and optical properties are investigated. A p-n junction GaN LED was fabricated on the MgAl2O4 substrate. (C) 1998 Elsevier Science B.V. All rights reserved.