双向低触发电压横向晶闸管放电管研究

依据触发电压VS、触发电流IS、维持电流IH及触发电压、维持电流高低温变化率指标要求,利用Silvaco-TCAD半导体器件仿真软件完成了双向低触发电压横向晶闸管(SCR)放电管的设计.详细分析了对触发特性产生显著影响的结构参数(N-衬底区、寄生PNP晶体管P-集电区、寄生NPN晶体管P-基区、N+阴极区、N+触发区、...     

Thermal stability and crystallization kinetics of new As–Ge–Se–Sb glasses

Thermal stability and crystallization kinetics of As₁₄Ge₁₄Se_72−xSb_x (where x = 3, 6, 9, 12 and 15 at.%) glasses are studied by the differential scanning calorimetry. The values of the glass transition temperature (T_g) and the peak temperature of crystallization (T_p) are found to be dependent on heating rate and antimony content. From the heating rate dependence of T_g and T_p the values of the activation energy for glass transition (E_t) and the activation energy for crystallization (E_c) are evaluated and their composition dependence discussed. Crystallization studies have been made under non-isothermal conditions with the samples heated at several uniform rates. Using a recent analysis developed for non-isothermal crystallization studies, information on some aspects of the crystallization process has been obtained. The stability calculations emphasized that the thermal stability decreases with increasing the Sb content.     

A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5–25 K), C_p, and low-frequency Raman scattering (<150 cm⁻¹) has been performed in amorphous silica samples synthesized by sol–gel method (xerogels) and thermally densified in a range of densities, from ρ=1250 kgm⁻³ to ρ=2100 kgm⁻³, close to the density of the melt quenched vitreous silica (v-SiO₂). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO₂, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870°C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm⁻¹ and of a peak in C_p(T)/T³, very close to those observed in v-SiO₂; (ii) in silica xerogels treated at temperatures less than about 800°C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.     

Effect of grain size on the electrical properties of ultraviolet photodetector with ZnO/diamond film structure

Highly c-axis-oriented ZnO films were successfully deposited on the nucleation sides of freestanding diamond films by RF reactive magnetron sputtering. I–V characteristics of ultraviolet (UV) photodetectors with ZnO/diamond structure were studied and a significant photoresponse was observed under UV light illumination. The dark-current and the photocurrent of the ZnO photodetectors were relative to the grain size and the quality of ZnO films. For the photodetector with a bigger grain size, a weaker dark current and a stronger photocurrent were obtained under 10 V bias voltage. The photocurrent rise and decay process confirmed the carrier-trapping effect.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

A comparison of commercial sources of epitaxial material for GaN HFETs fabrication

This paper describes a comparison of material and device results obtained from AlGaN/GaN epitaxial HFET wafers from three commercial sources. Although all three sources supplied material to the same nominal specification, X-ray diffraction, Hall effect and C–V profiling revealed significant differences between them. Wafers from two of the suppliers showed poor inter-device isolation characteristics, indicative of a conducting buffer layer. Wafers from the third supplier showed excellent inter-device isolation, but C–V measurements showed that the AlGaN was about twice as thick as specified, resulting in devices with high pinch-off voltages (−16 V). For the wafers with poor buffer isolation, RF measurements on 1.2 μm gate length devices gave values of f_T 5.0 GHz and values of f_max from 8.0 to 11.7 GHz (exact values depending on DC bias conditions), while for the wafer with over-thick AlGaN the corresponding values were 8.0 and 20.0 GHz.     

Growth of semi-insulating InP by GSMBE

Fe doped semi-insulating InP layers have been grown by gas source MBE with a solid iron source. Structure as n-i-n, p-i-n and p-n-i-n were characterized by I(V) measurements and secondary ion mass spectroscopy profiling (SIMS). As shown by SIMS, uniform Fe doping and abrupt transitions are achieved for the different structures studied. Resistivities as high as 1.5×10⁹ Ω cm are determined from I(V) curves for Fe concentrations in the 10¹⁷ cm^-3 range. Lasers with semi-insulating layers have been realized for the first time by gas source MBE. Preliminary results show power emission of 43 mW, without antireflecting coating, comparable to state-of-the-art characteristics.     

P–T–X phase equilibrium and vapor pressure scanning of non-stoichiometry in the Cd–Zn–Te system

Technical applications of artificial crystals strongly depend on tailoring the defect structure. In compound semiconductors, native defect concentration is closely related to non-stoichiometry. Vapor pressure scanning (VPS) is a direct high precision method of in situ investigation of the composition of non-stoichiometric crystals at high temperatures. It is based on experimental measurements of the vapor pressure, from which three-dimensional P–T–X (pressure–temperature–composition) range of existence of the crystalline phase is outlined. In this communication VPS data on non-stoichiometry in the Cd–Zn–Te system are presented. Geometrical analysis of the phase equilibrium is performed, and composition of the crystal, melt and vapor is determined in the technologically most important melting region. It will be shown how to apply experimental P–T–X phase equilibrium data for preparation of the material with pre-determined composition, either stoichiometric or with a certain deviation from stoichiometry. Different technologies are analyzed: vapor-phase growth, vertical, horizontal and high-pressure Bridgman. VPS has proved to be a powerful analytical tool. For CdZnTe the accuracy of the VPS determination of non-stoichiometry was shown to be as high as 10⁻⁴ at.% for temperatures up to the melting point.     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Silicon nanowire growth by electron beam evaporation: Kinetic and energetic contributions to the growth morphology

The vertical and epitaxial growth of long (up to a few microns) silicon nanowires on Si(1 1 1) substrates by electron beam evaporation (EBE) (10⁻⁶–10⁻⁷ mbar) is demonstrated at temperatures between 600 and 700 °C following the vapour–liquid–solid (VLS) growth mechanism from gold nanoparticles. The silicon atoms are provided by evaporating silicon at varying evaporation currents (I_E) between 35 and 80 mA, which results in growth rates between 1 and 100 nm/min. The growth peculiarities in the interaction triangle, evaporation current (I_E), growth temperature (T_S) and gold layer thickness (d_Au) will be reported. Kinetic and energetic contributions to the morphology of silicon nanowires will be discussed.     

Dendrite spacing in unidirectionally solidified Al-Cu alloy

Directional solidification experiments have been carried out to study the variation in primary and secondary arm spacings with solidification parameters in the Al-Cu system. It is found that the primary arm spacing Z₁ obeys the common correlation Z₁ = KG^-aV^-b in the high velocity regime or low temperature gradient regime, where a and b are constants, the value of K included the composition dependence, G is the temperature gradient in the liquid and V the growth rate; however, it does not obey this correlation for the low velocity regime or high temperature gradient regime but goes through a maximum or a catastrophe as a function of V or G at V = V_cs/k or G = kG_cs, where k is the equilibrium distribution coefficient, and V_cs and G_cs are the critical velocity and temperature gradient at the limit of constitutional undercooling respectively. The initial secondary arm spacings Z₂₀ are nearly independent of G and mainly depended on V, Z₂₀ = 0.016 V^-0.54 (mm). The secondary arm spacing Z₂ tends to c oarsen with time and thus is a function of coarsening time t_f, Z₂ = 0.016t^0.34_f (mm). Theoretical analyses of the primary arm spac ing and the initial secondary arm spacing have been proposed, and the derived relationships agree reasonably well with the above experimental results.     

Glass-forming ability versus stability of silicate glasses. II. Theoretical demonstration

Possible relationships between measures of glass stability (GS) against devitrification on heating (evaluated by the Hrubÿ parameter K_H=(T_c^h−T_g)/(T_m−T_c^h), and the parameter K_w=(T_c^h−T_g)/T_m) and a criterion of glass-forming ability (GFA) – the critical cooling rate – were investigated by computing non-isothermal crystallization for typical values of the main quantities that control crystal nucleation and growth in silicate glasses. We limit these quantities to one thermodynamic parameter – the melting entropy (ΔS_m) and two kinetic parameters that control the viscosity (B and T₀ in the Vogel–Fulcher–Tamman equation or T_g and in Avramov’s equation). The effect of heterogeneous nucleation and, in particular, the possible role of the surface as active substrate is tested. The results presented herein demonstrate that GS and GFA are indeed related concepts.     

Precise electrical evaluation of active oxides thickness and comparison with TEM measurements

In this paper we propose a new model to describe the quantum effects at the SiO₂/Si interface of metal-oxide–semiconductor (MOS) devices. Using this model we developed a method to extract the thickness of thin oxides (in the range of 3–20 nm) from capacitance (C) as a function of voltage (V) measurements, C(V). The results of our extraction are in good agreement with transmission electron microscopy (TEM) measurements, within the accuracy of both techniques, while classical electrical methods are inadequate for a precise evaluation of the oxide thickness. Moreover this new method is suitable for in-line monitoring of oxide thickness in advanced MOS processes.     

A geometric and analytic approach to the process of melt vitrification. I. Thermodynamic treatment

By using simple geometric concepts, general thermodynamic dependences governing the process of vitrification are derived. The thermodynamic driving forces of crystallization, ΔG_f,c(T), and stabilization, ΔG_st(T), melts and glasses are considered in the entire temperature interval from temperatures below the glass transition temperature, T_g, to temperatures high above the melting point, T_m. It is show that at large deviations from T_g the temperature dependence of the driving force for stabilization, ΔG_st(T), is non-symmetric and expressions derived by a Taylor expansion become a poor approximation. By using an appropriate thermodynamic model, a more general expression for ΔG_st(T) is obtained and the thermodynamic basis for the existence of superheated glasses is established. The thermodynamic instability of glasses below and above T_g, i.e., of supercooled and superheated glasses is discussed.     

Growth of CuS platelet single crystals by the high-temperature solution growth technique

Millimeter size CuS single crystals with a dark indigo blue color and a plate hexagonal shape have been successfully grown by the high-temperature solution growth technique using the KCl–LiCl eutectic as solvent. Surface microtopographic studies of the crystals indicated that the growth is made by the lateral spreading of the layers. Electrical resistivity measurements clearly show an anomaly at T55 K, related with the low-temperature structural transition, a high residual resistivity ratio of 400 and a sharp superconducting transition at T≈1.7 K confirming the very good quality of the crystals.     

基于神经网络和遗传算法的铸锭晶体硅质量控制及工艺优化

铸锭晶体硅是太阳能级晶硅材料的重要来源之一,为了进一步降低硅片成本,需要在保证晶体质量的同时发展大尺寸铸锭晶硅。影响铸造晶体硅质量的热场控制核心参数包括晶体生长速度与生长界面温度梯度之比V/G、壁面热流q、生长界面高度差Δh和硅熔体内部温差ΔT等。针对铸锭晶体硅生长过程中的质量控制问题,本研究基于人工神经网络(ANN)模型对晶体生长过程建立了工艺控制优化方法,利用实验测量数据和数值仿真模拟结果构建铸锭晶体硅生长过程的工艺控制数据集,以底部隔热笼开口和侧、顶加热器功率比作为主要工艺控制参数,V/G、|q|、|Δh|和ΔT为优化目标,建立用于研究晶体生长工艺控制参数和热场参数之间映射关系的神经网络模型。使用训练完成的模型分析底部隔热笼开口及侧、顶加热器功率比对晶体生长过程热场的影响规律,并采用遗传算法(GA)对铸锭晶体硅生长过程的工艺控制参数以提高晶体质量为目标进行优化,最后结合实际生产中的检测图像讨论了V/G对晶体质量的影响。研究表明晶体生长中期的V/G沿横向变化较平缓,对应缺陷较少且分布均匀,因此增大V/G在横向上的均匀度也是提高晶体质量的一个重要因素。     

Deep level defect luminescence in cadmium selenide nano-crystals films

Undoped CdSe monocrystals and CdSe nano-crystals films have been studied at various temperatures by continuous wave (cw) photoluminescence. We report on a characteristic deep level emission, which is consistently observed in the wurtzite bulk- and nanocrystalline forms of CdSe. Two broad luminescence bands, which are separated from the excitonic emission by 0.5 and 0.7 eV occur in CdSe, prepared by quite different techniques. These bands experience, similar to the excitonic emission, a spectral shift to high energy enforced by the quantum confinement in nano-CdSe. The defects responsible for this luminescence are probably two different V_Cd–V_Se divacancies: one is oriented along the hexagonal c-axis, the other is oriented along the basal Cd–Se bond directions.     

Relationship between T0, Tg and their pressure dependence for supercooled liquids

We show that simple expressions can be derived from the Vogel–Fulcher–Tammann (VFT) law relating the glass transition temperature T_g, the VFT temperature T₀, their pressure derivatives, the steepness index of the ‘Angell plot’ and the strength parameter D of the VFT equation, in good agreement with experimental data. In the same way one can describe the dependence of the dT_g/dP on the relaxation time τ_g chosen to define the temperature T_g. Thus, this procedure allows a consistent rescaling and comparison of pressure dependent parameters obtained from different experiments and simulations.     

Profiling band structure in GaN devices by electron holography

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V₀ across GaN/x nm In_0.1Ga_0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V₀ of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.     

Characteristics of ZnO–B2O3–CaO–Na2O–P2O5 glass powders prepared by spray pyrolysis

Fine-sized ZnO–B₂O₃–CaO–Na₂O–P₂O₅ glass powders with spherical shape were directly prepared by high temperature spray pyrolysis. The ZnO–B₂O₃–CaO–Na₂O–P₂O₅ powders prepared by spray pyrolysis at temperatures above 1200 °C had broad peaks at around 30° in the XRD patterns. The glass transition temperatures (T_g) of the glass powders obtained by spray pyrolysis at preparation temperatures between 900 °C and 1400 °C were near 480 °C regardless of the preparation temperatures. The dielectric layers formed from the glass powders prepared by spray pyrolysis at preparation temperatures above 1300 °C had clean surface and dense inner structure at the firing temperature of 580 °C. The transmittance of the dielectric layer formed from the glass powders obtained by spray pyrolysis at preparation temperature of 1400 °C was 90% at the firing temperature of 580 °C, in which the thickness of the dielectric layer was 13 μm. The UV cutoff edges gradually shift towards longer wavelength with increasing the preparation temperature of glass powders and the firing temperature of dielectric layers.