Heavily doped polycrystalline 3C-SiC growth on SiO2/Si(100) substrates for resonator applications

3C-SiC is a promising material for the development of microelectromechanical systems (MEMS) applications in harsh environments. This paper presents the LPCVD growth of heavily nitrogen doped polycrystalline 3C-SiC films on Si wafers with 2.0 mu m-thick silicon dioxide (SiO2) films for resonator applications. The growth has been performed via chemical vapor deposition using SiH4 and C2H4 precursor gases with carrier gas of H-2 in a newly developed vertical CVD chamber. NH3 was used as n-type dopant. 3C-SiC films were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and room temperature Hall Effect measurements. It was shown that there is no voids at the interface between 3C-SiC and SiO2. Undoped 3C-SiC films show n-type conduction with resisitivity, Hall mobility, and carrier concentration at room temperature of about 0.56 Omega center dot cm, 54 cm(2)/Vs, and 2.0x 10(17) cm(-3), respectively. The heavily nitrogen doped polycrystalline 3C-SiC with the resisitivity of less than 10(-3) Omega center dot cm was obtained by in-situ doping. Polycrystalline SiC resonators have been fabricated preliminarily on these heavily doped SiC films with thickness of about 2 mu m. Resonant frequency of 49.1 KHz was obtained under atmospheric pressure.

Micro-raman investigation of defects in a 4H-SiC homoepilayer

Three types of defects, namely defect I, defect 11, defect 111, in the 4H-SiC homoepilayer were investigated by micro-raman scattering measurement. These defects all originate from a certain core and are composed of (1) a wavy tail region, (11) two long tails, the so called comet and (111) three plaits. It was found that there are 3C-SiC inclusions in the cores of defect 11 and defect III and the shape of inclusion determines the type of defect II or defect III. If the core contains a triangle-shaped inclusion, the defect III would be formed; otherwise, the defect 11 was formed. No inclusion was observed in the core of the defect I. The mechanisms of these defects are discussed.

Optical and structural properties of ZnO films grown on Si(100) substrates by MOCVD - art. no. 60290G

High quality ZnO films have been successfully grown on Si(100) substrates by Metal-organic chemical vapor deposition (MOCVD) technique. The optimization of growth conditions (II-VI ratio, growth temperature, etc) and the effects of film thickness and thermal treatment on ZnO films' crystal quality, surface morphology and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectrum, respectively. The XRD patterns of the films grown at the optimized temperature (300 degrees C) show only a sharp peak at about 34.4 degrees corresponding to the (0002) peak of hexagonal ZnO, and the FWHM was lower than 0.4 degrees. We find that under the optimized growth conditions, the increase of the ZnO films' thickness cannot improve their structural and optical properties. We suggest that if the film's thickness exceeds an optimum value, the crystal quality will be degraded due to the large differences of lattice constant and thermal expansion coefficient between Si and ZnO. In PL analysis, samples all displayed only ultraviolet emission peaks and no observable deep-level emission, which indicated high-quality ZnO films obtained. Thermal treatments were performed in oxygen and nitrogen atmosphere, respectively. Through the analysis of PL spectra, we found that ZnO films annealing in oxygen have the strongest intensity and the low FWHM of 10.44 nm(106 meV) which is smaller than other reported values on ZnO films grown by MOCVD.

Substrate temperature dependence of ZnTe epilayers grown on GaAs(001) by molecular beam epitaxy

ZnTe thin films have been grown on GaAs(0 0 1) substrates at different temperatures with constant Zn and Te beam equivalent pressures (BEPs) by molecular beam epitaxy (MBE). In situ reflection high-energy electron diffraction (RHEED) observation indicates that two-dimensional (2D) growth mode can be established after around one-minute three-dimensional (3D) nucleation by increasing the substrate temperature to 340 degrees C. We found that Zn desorption from the ZnTe surface is much greater than that of Te at higher temperatures, and estimated the Zn sticking coefficient by the evolution of growth rate. The Zn sticking coefficient decreases from 0.93 to 0.58 as the temperature is elevated from 320 to 400 degrees C. The ZnTe epilayer grown at 360 degrees C displays the narrowest full-width at half-maximum (FWHM) of 660 arcsec from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) measurements. The surface morphology of ZnTe epilayers is strongly dependent on the substrate temperature, and the root-mean-square (RMS) roughness diminishes drastically with the increase in temperature.

Optical responses of ZnSe quantum dots in silica gel glasses

The characteristic features of the absorption and photoluminescence spectra of ZnSe quantum dots (QDs) inside a silica matrix derived from a sol-gel method were studied at room temperature. Compared with the bulk materials, the absorption edges of ZnSe QDs in silica gel glass were shifted to higher energies and the spectra exhibited the discrete excitonic features due to the quantum confinement effects. Besides the band-edge emission, photoluminescence at ultraviolet excitation also showed the emissions related to the higher excitonic states. (C) 2004 Elsevier B.V. All rights reserved.

Optical and structural properties of self-assembled ZnO QD chains by L-MBE

ZnO complex 3D nano-structures have been self-organized on Al2O3 (0 0 0 1) substrate by laser molecular beam epitaxy (L-MBE). It is shown by AFM morphology that the structure is composed of ID quantum dot chains (QDCs) and larger nano-islands at the nodes of QDCs. The formation mechanism of the nano-structure is also investigated. XRD results indicate that the nano-structure is highly c-axis oriented, with the aligned in-plane oriented domains. Time-integrated photoluminescence (TIPL) of the sample shows obvious blue-shift and broadening of the near band-edge (NBE) emission at room temperature, which are related to the quantum confinement effects. Time-resolved PL (TRPL) result shows bi-exponential decay behavior of ZnO QDCs, with a fast decay time of 38.21 ps and a low decay time of 138.19ps, respectively, which is considered to be originated from the interdot coupling made by coherent emission and reabsorption of the photons in QDCs. (C) 2007 Elsevier B.V. All rights reserved.

复杂无机离子晶体晶格能的研究

对于简单二元晶体（I-VII，II-VI，III-V族晶体），我们发现了它们的晶格能、德拜温度、等离子体能量、体变模量间的一些简单关系，分析发现了德拜模型的局限性，得到离子品体晶格能最小可能值为546kjmol-1，也发现碱金属卤化物的晶格能与格波的纵、横光学支的波数都呈线性关系，相同阳离子及相同阴离子的碱金属卤化物的LEELS、HEELS，ORS的谱峰能量值也与晶格能呈线性关系，说明这些量都与离子间的｛妾合强度有一定的关系。基于复杂靛．体化学键介电理论，并考虑到晶格能和键性的关系，我们发展了一种新的经验地计算晶．格能的方法，晶格能被视为是由各类键贡献的，对于每类键，其对晶格能的贡献被分为离子性和共价性两部分，复杂离子晶体被分解后得到的各类键可做相似处理。我们又基于Born理论模型，考虑到离子晶体中离子间主要作用是库仑作用及电子壳层的排斥作用，发展了另一种简单的计算晶格能的方法。对于很多静电键为主的功能材料（如巨磁阻材料、超导体等），用这两种方法计算的晶格能值与实验值及他人的计算结果相当接近，它们可以作为复杂离子晶体晶格能可信I均估算方法。我们还计算了YBCO体系的晶格能，并得到其与氧浓度的函数关系。

半导体材料研究的新进展

首先对作为现代信息社会的核心和基础的半导体材料在国民经济建设、社会可持续发展以及国家安全中的战略地位和作用进行了分析，进而介绍几种重要半导体材料如，硅材料、GaAs和InP单晶材料、半导体超晶格和量子阱材料、一维量子线、零维量子点半导体微材料、宽带隙半导体材料、光学微腔和光子晶体材料、量子比特构造和量子计算机用材料等目前达到的水平和器件应用概况及其发展趋势作了概述。最后，提出了发展我国半导体材料的建议。本文未涉及II－VI族宽禁带 与II－VI族窄禁带红外半导体材料、高效太阳电池材料Cu(In,Ga)Se_2、CuIn(Se,S)等以及发展迅速的有机半导体材料等。

Controllable synthesis of highly luminescent and monodisperse CdS nanocrystals by a two-phase approach under mild conditions

Highly luminescent and monodisperse CdS nanocrystals (see Figure) have been synthesized using a two-phase approach. The synthesis of CdS nanocrystals at the liquid-liquid interface was easy, safe, and highly reproducible, and the reaction conditions were mild and controllable.

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(001) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios (R-VI/II) in a wide range of 0.96-11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R-VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R-VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R-VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R-VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R-VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.

Suppression of ballistic electron transmission through a semiconductor II-structure by an external transverse electric field

We have conducted numerical studies of ballistic electron transport in a semiconductor II-structure when an external transverse electric field is applied. The device conductance as a function of electron energy and the strength of the transverse electric field is calculated on the basis of tight-binding Green's function formalism. The calculations show that a relatively weak electric field can induce very large decrease in the electron transmission across the structure. When the transverse electric field is sufficiently strong, electrons can hardly be transported through the device. Thus the performance of the device can be greatly improved for it is much easier to control electron transport through the device with an external transverse electric field.

Combining chemical reduction with an electrochemical technique for the simultaneous detection of Cr(VI), Pb(II) and Cd(II)

This work herein reports the approach for the simultaneous determination of heavy metal ions including cadmium (Cd(II)), lead (Pb(II)), and chromium (Cr(VI)) using a bismuth film electrode (BFE) by anodic stripping voltammertry (ASV). The BFE used was plated in situ. Due to the reduction of Cr(VI) with H2O2 in the acid medium, on one hand, the Cr(III) was produced and Cr(VI) was indirectly detected by monitoring the content of Cr(III) using square-wave ASV. On the other hand, Pb(II) was also released from the complex between Pb(II) and Cr(VI). Furthermore, the coexistence of the Cd(II) was also simultaneously detected with Pb(II) and Cr(VI) in this system as a result of the formation of an alloy with Bi. The detection limits of this method were 1.39 ppb for Cd(II), 2.47 ppb for Pb(II) and 5.27 ppb for Cr(VI) with a preconcentration time of 120 s under optimal conditions (S/N = 3), respectively. Furthermore, the sensitivity of this method can be improved by controlling the deposition time or by using a cation-exchange polymer (such as Nafion) modified electrode.

Bulk modulus of ternary chalcopyrite A(I)B(III)C(2)(VI) and A(II)B(IV)C(2)(V) semiconductors

Based on the complex crystal chemical bond theory, the formula of Liu and Cohen's, which is only suitable for one type of bond, has been extended to calculate the bulk modulus of ternary chalcopyrite A(I)B(III)C(2)(VI) and A(II)B(IV)C(2)(V) which contains two types of bonds. The calculated results are in fair agreement with the previous theoretical values reported and experimental values. (C) 1998 Elsevier Science Ltd. All rights reserved.

Photoluminescence of low-dimensional semiconductor structures under pressure

Photoluminescence of some low-dimensional semiconductor structures has been investigated under pressure. The measured pressure coefficients of In0.55Al0.45 As/Al0.5Ga0.5As quantum dots with average diameter of 26, 52 and 62 nm are 82, 94 and 98 meV/GPa, respectively. It indicates that these quantum dots are type-I dots. On the other hand, the measured pressure coefficient for quantum dots with 7 nm in size is -17meV/GPa, indicating the type-II character. The measured pressure coefficient for Mn emission in ZnS:Mn nanoparticles is -34.6meV/GPa, in agreement with the predication of the crystal field theory. However, the DA emission is nearly independent on pressure, indicating that this emission is related to the surface defects in ZnS host. The measured pressure coefficient of Cu emission in ZnS: Cu nanoparticles is 63.2 meV/GPa. It implies that the acceptor level introduced by Cu ions has some character of shallow level. The measured pressure coefficient of Eu emission in ZnS:Eu nanoparticles is 24.1 mev/GPa, in contrast to the predication of the crystal field theory. It may be due to the strong interaction between the excited state of Eu ions and the conduction band of ZnS host.