Characteristics of InGaN multiple quantum well blue-violet laser diodes

Studies on InGaN multiple quantum well blue-violet laser diodes have been reported. Laser structures with long-period multiple quantum wells were grown by metal-organic chemical vapor deposition. Triple-axis X-ray diffraction (TAXRD) measurements show that the multiple quantum wells were high quality. Ridge waveguide laser diodes were fabricated with cleaved facet mirrors. The laser diodes lase at room temperature under a pulsed current. A threshold current density of 3.3 kA/cm(2) and a characteristic temperature To of 145 K were observed for the laser diode.

Self-consistent analysis of double-delta-doped InAlAs/InGaAs/InP HEMTs

We have carried out a theoretical study of double-delta-doped InAlAs/InGaAs/InP high electron mobility transistor (HEMT) by means of the finite differential method. The electronic states in the quantum well of the HEMT are calculated self-consistently. Instead of boundary conditions, initial conditions are used to solve the Poisson equation. The concentration of two-dimensional electron gas (2DEG) and its distribution in the HEMT have been obtained. By changing the doping density of upper and lower impurity layers we find that the 2DEG concentration confined in the channel is greatly affected by these two doping layers. But the electrons depleted by the Schottky contact are hardly affected by the lower impurity layer. It is only related to the doping density of upper impurity layer. This means that we can deal with the doping concentrations of the two impurity layers and optimize them separately. Considering the sheet concentration and the mobility of the electrons in the channel, the optimized doping densities are found to be 5 x 10(12) and 3 x 10(12) cm(-2) for the upper and lower impurity layers, respectively, in the double-delta-doped InAlAs/InGaAs/InP HEMTs.

Tunable spin transport in magnetic-electric superlattice

We study the spin-dependent electron transport in a special magnetic-electric superlattice periodically modulated by parallel ferromagnetic metal stripes and Schottky normal-metal stripes. The results show that, the spin-polarized current can be well controllable by modulating the magnetic strength of the ferromagnetic stripes or the voltage applied to the Schottky normal-metal stripes. It is obvious that, to the system of the magnetic superlattice, the polarized current can be enhanced by the magnetic strength of ferromagnetic stripes. Nevertheless, it is found that, for the magnetic-electric superlattice, the polarized current can also be remarkably advanced by the voltage applied to the Schottky normal-metal stripes. These results may indicate a useable approach for tunable spintronic devices. (c) 2006 Elsevier B.V. All rights reserved.

Comparison of valence band x-ray photoelectron spectrum between Al-N-codoped and N-doped ZnO films

The valence band structures of Al-N-codoped [ZnO:(Al, N)] and N-doped (ZnO:N) ZnO films were studied by normal and soft x-ray photoelectron spectroscopy. The valence-band maximum of ZnO:(Al, N) shifts up to Fermi energy level by about 300 meV compared with that of ZnO:N. Such a shift can be attributed to the existence of a kind of Al-N in ZnO:(Al, N), as supported by core level XPS spectra and comparison of modified Auger parameters. Al-N increased the relative quantity of Zn-N in ZnO:(Al, N), while N-N decreased that of Zn-N in ZnO:N. (c) 2006 American Institute of Physics.

Hydrogen passivation of multi-crystalline silicon solar cells

The effects of hydrogen passivation on multi-crystalline silicon (mc-Si) solar cells are reported in this paper. Hydrogen plasma was generated by means of ac glow discharge in a hydrogen atmosphere. Hydrogen passivation was carried out with three different groups of mc-Si solar cells after finishing contacts. The experimental results demonstrated that the photovoltaic performances of the solar cell samples have been improved after hydrogen plasma treatment, with a relative increase in conversion efficiency up to 10.6%. A calculation modelling has been performed to interpret the experimental results using the model for analysis of microelectronic and photonic structures developed at Pennsylvania State University.

Studies of 6H-SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H-SiC were systematically studied. The main works are summarized as follows:The homoepitaxial growth on the commercially available single-crystal 6H-SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p(+)n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K.Using thermal evaporation, Ti/6H-SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V.n-Type 6H-SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of polysilicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to gamma rays. (C) 2002 Published by Elsevier Science B.V.

High responsivity ultraviolet photodetector based on crack-free GaN on Si (111)

A Schottky-based metal-semiconductor-metal photodetector is fabricated on 1 mu m-thick, crack-free GaN on Si (I 11) substrate using an optimized AlxGal-xN/AlN complex buffer layer. It exhibits a high responsivity of 4600A/W at 366nm which may be due to both a crack-free sample and high internal gain. The relationship between responsivity and bias voltage is also investigated. The experiment results indicate that the responsivity increases with the bias voltage and shows a tendency to saturate. (c) 2007 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim.

Homoepitaxial growth and characterization of 4H-SiC epilayers by low-pressure hot-wall chemical vapor deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates purchased from Cree is performed at a typical temperature of 1500 degrees C with a pressure of 40 Torr by using SiH4+C2H4+H-2 gas system. The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope, atomic force microscopy (AFM), x-ray diffraction, Raman scattering, and low temperature photoluminescence (LTPL). The background doping of 32 pm-thick sample has been reduced to 2-5 x 10(15) cm(-3). The FWHM of the rocking curve is 9-16 arcsec. Intentional N-doped and B-doped 4H-SiC epilayers are obtained by in-situ doping of NH3 and B2H6, respectively. Schottky barrier diodes with reverse blocking voltage of over 1000 V are achieved preliminarily.

A novel low-power input-independent MOS AC/DC charge pump

This paper presents a novel fully integrated MOS AC to DC charge pump with low power dissipation and stable output for RFID applications. To improve the input sensitivity, we replaced Schottky-diodes in conventional charge pumps with MOS diodes with zero threshold, which has less process defects and is thus more compatible with other circuits. The charge pump in a RFID transponder is implemented in a 0.35um CMOS technology with 0.24 sq mm die size. The analytical model of the charge pump and the simulation results are presented.

Quantum well infrared photodetector simultaneously working in two atmospheric windows

We have demonstrated a two-contact quantum well infrared photodetector (QWIP) exhibiting simultaneous photoresponse in both the mid- and the long-wavelength atmospheric windows of 3-5 mu m and of 8-12 mu m. The structure of the device was achieved by sequentially growing a mid-wavelength QWIP part followed by a long-wavelength QWIP part separated by an n-doped layer. Compared with the conventional dual-band QWIP device utilizing three ohmic contacts, our QWIP is promising to greatly facilitate two-color focal plane array (FPA) fabrication by reducing the number of the indium bumps per pixel from three to one just like a monochromatic FPA fabrication and to increase the FPA fill factor by reducing one contact per pixel; another advantage may be that this QWIP FPA boasts broadband detection capability in the two atmospheric windows while using only a monochromatic readout integrated circuit. We attributed this simultaneous broadband detection to the different distributions of the total bias voltage between the mid- and long-wavelength QWIP parts.

关于半导体电极上极化成份的分解和电极反应动力学参数的确定

电极动力学在金属电极上已经发展得比较成熟，对半导体来说，由于电极反应的复杂性，还有不少问题比较模糊。其中之一是怎样根据表观的极化测量求得反映界面电荷转移的动力学参数。目前有关这方面的工作不多，还没有见到具体对极化成份进行分解以求解半导体电极反应的动力学系统的报导。本工作从一定的电极物理模型出发，在电化学测量的基础上结合电子计算技术，对这方面问题的研究提出了测量计算方法。本文提出的半导体电极的物理模型如图A所示。I，I_J, I_S, I_D, I_H均取阴极性电流方向为正，η则按电极电势的方向取阳极过电热为正，阴极过电势为负。模型中的J反映空间电荷层的Schottky结特性，其数学描述可概括（对n-型半导体）为：I_J = I_0[exp(-n_s/a)-1] I_0： 结的反向饱和电流。（a:k_T）/q或（RT）/F D反映界面电荷转移反应的特性，其数学描述适用Butler-Volmer方程：I_D = i~0[exp(-(1-β)n η_H)/a) - exp(βη-η_H/a)] i~0:交换电流。β：阳极传递系数 C_s，C_H分别表示空间电荷层和Helmholtz层的电容。R_B和R_l分别为半导体体内和溶液电阻。由于J，D二者特性不同，可以通过极化测量利用电子计算机加以鉴别。实验上针对电路主要是串联结构的特点，采用恒电流极化，利用恒电流恒电位仪实现一系列的电流阶跃（I_(K-1) I_K K = 1, 2, 3 ……）记录相应的电位随时间变化的响应曲线如图B所示。根据曲线各段的特点，利用电子计算机曲线拟合，分别求解有关参数。（1）在t = 0时，找出一系列不同I下的φ（I_K, t = 0）值，根据φ（I_K, t = 0) = φ(I = 0)-I_KR拟合求解φ（I = 0）， R。（2）找出一系列不同I下的稳态极化数据φ（I_K,t →∞），推导出电位随电流变化关系式，拟合求解I_0, j~0, β。（3）利用暂态过程的φ（I_K, t = 0) ～ t曲线，拟合求解每阶电位变化区间的C_s，C_H。由于在我们的电极模型中，D采取的是完整的Butler-Volmer表达式，没有作任何简化或近似，因而在数据处理和计算时，涉及隐式超越代数方程和隐式超越微分方程，无法通过一般解析法求解。为此我们把牛顿迭代法和Runge-Kutta法引入相应的曲线拟合计算程序中。这样做虽然计算上比较复杂困难，但方法的通用性更广泛。无论Helmholtz层处于线性极化，弱极化，或强极化区部同样适用。我们用BASIC语言编写了梯度法，线性化法联合使用的曲线拟合源程序及牛顿迭代法和Runge-Kutta法于程序。利用上述研究方法，对不同掺杂浓度的n-型GaAs电极在S~(2-)/S_x~(2-)体系中的电化学行为进行了研究。求解的电荷转移反应的动力学参数I_0, i~0, β分别在7.27 * 10~(-8) - 4.66 * 10~(-1) A/cm~2, 2.08 * 10~(-6)-4.62 * 10~(-6)A/cm~2, 0.70 - 0.78的范围之内。并于Pt电极连同一体系中的i~0，β进行了比较。i_（半导体）~0 < i_（导体）~0。但β在二种材料上差别不大。将半导体电极极化分解为空间电荷层极化和Helmholtz层极化两部分。测量了空间电荷层电学及Helmholtz层电容与电极电位的关系。从实际测量中证明，本文提出的电极模型比较恰当的反映了半导体电极的特性，本工作的测量和拟合计算方法对研究半导体电极行为是一种可行的方法。

Fabrication of Silicon-Based Template-Assisted Nanoelectrode Arrays and Ohmic Contact Properties Investigation

A convenient fabrication technology for large-area, highly-ordered nanoelectrode arrays on silicon substrate has been described here, using porous anodic alumina (PAA) as a template. The ultrathin PAA membranes were anodic oxidized utilizing a two-step anodization method, from Al film evaporated on substrate. The purposes for the use of two-step anodization were, first, improving the regularity of the porous structures, and second reducing the thickness of the membranes to 100 similar to 200 nm we desired. Then the nanoelectrode arrays were obtained by electroless depositing Ni-W alloy into the through pores of PAA membranes, making the alloy isolated by the insulating pore walls and contacting with the silicon substrates at the bottoms of pores. The Ni-W alloy was also electroless deposited at the back surface of silicon to form back electrode. Then ohmic contact properties between silicon and Ni-W alloy were investigated after rapid thermal annealing. Scanning electron microscopy (SEM) observations showed the structure characteristics, and the influence factors of fabrication effect were discussed. The current voltage (I-V) curves revealed the contact properties. After annealing in N-2 at 700 degrees C, good linear property was shown with contact resistance of 33 Omega, which confirmed ohmic contacts between silicon and electrodes. These results presented significant application potential of this technology in nanosize current-injection devices in optoelectronics, microelectronics and bio-medical fields.

Abnormal photoabsorption in high resistance GaN epilayer

Unintentionally doped GaN epilayers are grown by the metalorganic chemical vapor deposition (MOCVD). Photovoltaic (PV) spectroscopy shows that there appears an abnormal photoabsorption in some undoped GaN films with high resistance. The peak energy of the absorption spectrum is smaller than the intrinsic energy band gap of GaN. This phenomenon may be related to exciton absorption. Then metal-semiconductor-metal (MSM) Schottky photodetectors are fabricated on these high resistance epilayers. The photo spectrum responses are different when the light individually irradiates each of the two electrodes with the photodetector which are differently biased. When the excitation light irradiates around the reverse biased Schottky junction, the responsivity is almost one order of magnitude larger than that around the forward biased junction. Furthermore, when the excitation light irradiates the reverse biased Schottky junction, the peak energy of the spectrum has a prominent red-shift compared with the peak energy of the spectrum measured with the excitation light irradiating the forward biased Schottky junction. The shift value is about 28 meV, and it is found to be insensitive to temperature. According to the analyses of the distribution of the electric field within the MSM device and the different dependences of the response on the electric field intensity between the free carriers and excitons, a reliable explanation for the different response among various areas is proposed.

STUDIES OF THE PROMISING MATERIAL COSI2 ON GAAS SUBSTRATES

The thermal stability of CoSi2 thin films on GaAs substrates has been studied using a variety of techniques. The CoSi2 thin films were formed by depositing Co(500 angstrom) and Si(1800 angstrom) layers on GaAs substrates by electron-beam evaporation followed by annealing processes, where the Si inter-layer was used as a diffusion/reaction barrier at the interface. The resistivity of CoSi2 thin films formed is about 30 muOMEGA cm. The Schottky barrier height of CoSi2/n-GaAs is 0.76 eV and the ideality factor is 1.14 after annealing at 750-degrees-C for 30 min. The CoSi2/GaAs interface is determined to be thermally stable and the thin film morphologically uniform on GaAs after 900-degrees-C/30 s anneal. The CoSi2 thin films fulfill the requirements in GaAs self-aligned gate technology.

SELF-CONSISTENT TREATMENT OF 3-DIMENSIONAL - 2-DIMENSIONAL AND 2-DIMENSIONAL - 2-DIMENSIONAL RESONANT-TUNNELING IN DOUBLE-BARRIER STRUCTURES

By using a transfer-matrix method on the basis of two-dimensional (2D) Bloch sums in accordance with a tight-binding scheme, a self-consistent calculation on the resonant tunneling in asymmetric double-barrier structures is presented, in which contributions to resonant tunneling from both three-dimensional (3D) electrons in the contacts and 2D electrons in the spacer or accumulation layers are considered simultaneously. The charge buildup effect on the current versus voltage (I-V) curves is evaluated systematically, showing quantitatively how it results in the I-V bistability and enhanced differences between I-V curves for positive and negative bias in an asymmetric double-barrier structure. Special attention is focused on the interaction between 3D-2D and 2D-2D resonant-tunneling processes, including the suppression of 2D-2D resonant tunneling by the charge buildup in the well accompanying the 3D-2D resonant tunneling. The effects of the emitter doping condition (doping concentration, spacer thickness) on the presence of two types of quasi-2D levels in the emitter accumulation layers, and on the formation of a potential bulge in the emitter region, are discussed in detail in relation to the tunneling process.