As钝化Si(211)表面上Cd、Te原子行为的第一性原理研究

文章采用基于密度泛函理论的CASTEP计算软件模拟了单个及多个As原子在Si(211)重构表面上的吸附、置换行为,通过系统地计算各种可能的吸附、置换构型,并进一步分 析能量、键长等性质,对As在Si (211)表面的钝化机理进行了初步探讨。同时,对Te、Cd在As钝化前后Si (211)表面上的吸附行为也进行了研究。计算结果表明,单个Cd或Te原子都可以稳定地吸附在清洁表面,但在As钝化表面,难以在钝化区域吸附生长。     

碲镉汞富汞热处理技术的研究

利用自行研制的垂直开管富汞热处理设备，研究了碲镉汞（HgCdTe）富汞热处理技术。经富汞条件下3000C热处理和后续的常规N型热处理，As掺杂的Hg1-xCd，Te分子束外延材料中的As原子已被激活成P型受主，As原子激活率同石英管封管热处理试验的结果基本一致。对包括3in Si基衬底在内的材料退火前后表面形貌进行的比较显示，样品表面形貌可得到很好的保护。研究结果表明，碲镉汞开管富汞热处理技术可用于第三代碲镉汞红外焦平面技术所需大面积多层掺杂异质外延材料的制备。     

碲镉汞阳极氧化层的化学结构分析

碲镉汞（MCT）由于其特殊的禁带宽度,成为一种重要的红外探测器半导体材料.其表面复合中心会严重影响探测器性能,为了减少复合中心及保持电学和化学稳定性,表面钝化成为必不可少的工艺步骤.根据碲、镉、汞3种元素在碱性溶液中的电极电势,计算得出阳极氧化的先后顺序为Cd＞Te＞Hg,并以其构建了一种氧化过程模型,模拟溶液中负电荷离子渗入材料和材料内部各种离子的位置及价态变化,推导出组成结构,与XPS化学组分分析结果十分吻合.在结构方面,该模型成功解释了缺Hg区、富Cd区和富Te区的存在及其位置,并通过俄歇电子能谱（AES）测试得到了验证,说明模型具有一定的合理性.     

p型碲镉汞液相外延材料Ag掺杂的研究

利用SIMS和变温霍尔测量手段对P型Hg0.77Gd0.23Te液相外延材料的Ag掺杂技术、机理及掺杂碲镉汞材料的性能进行了研究。结果表明采用AgNO3溶液直接浸泡方式对该材料进行掺Ag是成功的，掺杂浓度与被掺杂材料中的汞空位浓度是一致的，掺杂后，P型碲镉汞材料的受主能级比掺杂前有明显的减小，从实验结果可看到掺Ag碲镉汞材料的电学性能在室温下保持稳定。     

Si基短波碲镉汞材料分子束外延生长研究

报道了在中波工艺基础上,Si 基碲镉汞分子束外延短波工艺的最新研究进展,通过温度标定、使用反射式高能电子衍射、高温计的在线测量和现有的中波 Si 基碲镉汞温度控制曲线建立及优化了 Si 基碲镉汞短波材料的生长温度控制曲线;获得的 Si 基短波 HgCdTe 材料表面光亮、均匀,表面缺陷密度小于3000 cm －2;基于此技术成功制备出了 Si 基短／中波双色材料。     

Hg1—xCdxTe材料中的Te离子空位共振能级

利用拉曼显微镜在室温下对金属有机化合物气相外延（MOVPE）和液相外延（LPE）方法生长的Hg1-xCdxTe薄膜材料以及用加速坩埚旋转布里奇曼（ACRT－Bridgman)和Te溶剂方法生长的Hg1-xCdxTe体材料进行了系统研究，在上述4种方法生长的材料的显微拉曼光谱中，均发现在导带底上方且远高于材料导带底对应能级的显微荧光发光峰，通过详细比较可以判定，高于导带底约1．5eV的显微荧光起源于Hg1-xCdxTe材料中的Te离子空位与材料导带底的共振能级发光，从而确定在碲镉汞材料中存在一个稳定的Te离子空位共振能级。     

低暗电流高温工作碲镉汞红外探测器制备技术

报道了与传统的n-on-p结构相比具有更低暗电流的p+-on-n型碲镉汞红外探测器的研究进展。通过水平滑舟富碲液相外延生长的方法在碲锌镉衬底上原位生长In掺杂碲镉汞n型吸收层材料,然后再分别采用As离子注入技术和富汞垂直液相外延技生长技术实现了P型As外掺杂的p+-on-n型平面型和双层异质结台面型两种结构的芯片制备。碲镉汞探测器的主要缺点是需要低温制冷,期望碲镉汞探测器在不降低性能的前提下具有更高的工作温度(High Operating Temperature,HOT),成为红外探测器技术发展的主要方向。本文对基于标准的n-on-p(Hg空位)掺杂工艺及新研制的p-on-n型As离子注入及异质结制备技术的探测器进行了高温性能测试,测试结果表明采用p-on-n型的碲镉汞探测器能够实现更高的工作温度。     

碲镉汞材料中Hg空位、Au、As掺杂的研究进展

碲镉汞材料是制造红外探测器的基础,高性能红外探测器对碲镉汞材料的要求越来越高。为了提升器件性能,必须提高碲镉汞材料的电学性能。而掺杂是一个很好的选择。碲镉汞材料掺杂可以分为n型和p型两种。对于n型掺杂来说,In是一种理想的掺杂剂,其掺杂研究目前已比较成熟。相对而言,p型掺杂研究还不是那么深入。Hg空位、Au、As掺杂均为碲镉汞材料中常见的p型掺杂手段。通过分析和总结近些年的部分相关文献,介绍了碲镉汞材料中Hg空位、Au、As掺杂的研究进展。     

Cd饱和气氛退火对碲锌镉晶体导电类型转变界面的影响

在富Te生长条件下,通过垂直布里奇曼法制备的部分碲锌镉晶体内存在导电类型转变界面。采用富Te液相外延技术在含有导电类型转变界面的碲锌镉衬底上生长碲镉汞薄膜,制成的红外焦平面探测器响应图上存在明显的响应不均匀分界面。碲锌镉晶体的导电类型转变由缺陷类型的不同引起,为消除碲锌镉衬底的导电类型转变界面,提升碲镉汞红外焦平面的成像质量,对含有导电类型转变界面的碲锌镉晶体进行了Cd饱和气氛退火实验,研究了时间和温度等退火条件对晶体导电类型转变界面的影响,探讨了Cd间隙和Cd空位缺陷的形成机制,为晶体生长过程中的Cd空位缺陷抑制提出了解决思路。     

碲镉汞薄膜表面钝化的研究进展

赝二元体系碲镉汞(Mercury Cadmium Telluride, Hg_xCd_1-xTe)材料具有优异的光电特性,是制备高灵敏度红外探测器的最重要材料之一。为了获得性能优异的Hg_xCd_1-xTe探测器及其组件,目前已经发展了各种Hg_xCd_1-xTe材料制备技术和器件制作工艺。但在各种材料制备及器件应用过程中,Hg_xCd_1-xTe表面均会受到环境和不良表面效应的影响,所以需要采用先进的钝化工艺对其表面电荷态进行处理,改善材料表面的电学物理特性,从而实现器件探测性能的提升。因此,Hg_xCd_1-xTe薄膜表面钝化工艺对Hg_xCd_1-xTe红外探测器的性能提升至关重要。总结和分析了近年来碲镉汞薄膜表面钝化层的生长方法。按照本源钝化和非本源钝化进行了分类总结和综述,分析了不同钝化方法的优缺点,并对未来碲镉汞薄膜钝化工艺进行了展望。     

CdZnTe heteroepitaxy on 3″ (112) Si: Interface, surface, and layer characteristics

Tellurium adsorption studies were made on clean and arsenic passivated (112) silicon surfaces. Quantitative surface coverage values for tellurium were determined by Auger electron spectroscopy. Saturation coverage of up to 1.2 monolayers of tellurium could be obtained on a clean (112) silicon surface. On an arsenic passivated (112) Si surface however, the tellurium saturation coverage was limited to only ∼0.3 monolayer. Analysis of the adsorption behavior suggested that tellurium and arsenic chemisorption occurs preferentially at step edges and on terraces, respectively. The study revealed that arsenic passivation led to a significant decrease in the sticking coefficient of tellurium and an increase in it’s surface mobility. A model describing zinc telluride nucleation on a (112) Si surface is proposed. Thin templates of ZnTe followed by Cd_1−xZn_xTe layers were deposited on (112) Si by molecular beam epitaxy (MBE). The characteristics of the MBE Cd_1−xZn_xTe layers were found to be sensitive to the initial ZnTe nucleation and Si surface preparation.     

Arsenic deposition as a precursor layer on silicon (211) and (311) surfaces

We investigate the properties of arsenic (As) covered Si(211) and Si(311) surfaces by analyzing data from x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) images. We then create a model using total surface energy calculations. It was found that both Si(211) and Si(311) had 0.68±0.08 surface As coverage. Si(211) had 0.28±0.04 Te coverage and Si(311) had 0.24±0.04 Te coverage. The Si(211) surface replaces the terrace and trench Si atoms with As for a lower surface energy, while the Si edge atoms form dimers. The Si(311) surface replaces all terrace atoms and adsorbs an As dimer every other edge site. These configurations imply an improvement in the mean migration path from the bare silicon surface by allowing the impinging atoms for the next epitaxial layer, tellurium (Te), to bind at every other pair of edge atoms, and not the step terrace sites. This would ensure a nonpolar, B-face growth.     

Selective Growth of CdTe on Si(211): First-Principle Calculations

The adsorption of CdTe layers on clean and As-passivated Si(211) substrates has been simulated by first-principle calculations in this study. Based on the simulation results, we theoretically show the important roles of the As₄ passivation during the epitaxial growth. Arsenic can saturate part of the dangling bonds and weaken the surface states. The partial passivation finally induces the B-face polarity selection automatically. This conclusion can provide further explanations for the successful growth of large area high-quality CdTe(211)B layers on the Si(211) substrates.     

Comparison of HgTe materials grown in (100), (110), (111), and (211) Orientations

We calculated energies required to remove atoms from various configurations on (111), (110), (100), and (211) HgTe surfaces. The excess pair energies for various species are then calculated and are used in a thermodynamic model to study the growth. All energies are obtained using a Green’s function method. The pair distributions are calculated from these energies in a generalized quasi-chemical approximation. The calculated critical temperatures for surface roughness transition are found to be considerably higher than the usual growth temperature of 185°C, so the growth on these surfaces is expected to be layer-by-layer with formation of two-dimensional islands. However, among the surfaces studied, only the (211) surfaces have an attractive binding energy for Hg, making those surfaces suited for better growth. The critical temperature for growth on (21 l)Hg is slightly higher than that for (211)Te, but we also find that Hg sticking coefficient on (21 l)Hg surface is considerably lower than that on (21 l)Te surface. These calculations are consistent with the observed higher growth rate of the (211)Te surface. Our calculations suggest that there will be fewer grown-in vacancies and Te antisites, at the expense of growth rate and sticking coefficient, for crystals grown on (211)Hg surface. We further calculated the Hg and Te vacancy formation energies as functions of surface orientations and layer depth. The cation vacancy formation energies from completed surface regions (islands) are higher than bulk values near anion terminated surfaces and smaller than bulk values near cation terminated surfaces.     

Effect of As Passivation on Vapor-Phase Epitaxial Growth of Ge on (211)Si as a Buffer Layer for CdTe Epitaxy

We report an investigation of epitaxial germanium grown by chemical vapor deposition (CVD) on arsenic-terminated (211)Si, which is the preferred substrate in the USA for fabrication of night-vision devices based on mercury cadmium telluride (MCT) grown by molecular-beam epitaxy (MBE). The films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), cross-sectional transmission electron microscopy (XTEM), and x-ray diffraction (XRD). Arsenic passivation was found to be effective in preventing cross-contamination of unwanted residual species present inside the reactor chamber and also in prolonging the evolution of layer-by-layer growth of Ge for significantly more monolayers than on nonpassivated Si. The two-dimensional (2D) to three-dimensional (3D) transition resulted in Ge islands, the density and morphology of which showed a clear distinction between passivated and nonpassivated (211)Si. Finally, thick Ge layers (∼250 nm) were grown at 525°C and 675°C with and without As passivation, where the layers grown with As passivation resulted in higher crystal quality and smooth surface morphology.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Defect modeling studies in HgCdTe and CdTe

We have used a quasichemical formalism to calculate the native point defect densities in x = 0.22 Hg_1−xCd_xTe and CdTe. The linearized muffin-tin orbital method, based on the local density approximation and including gradient corrections, has been used to calculate the electronic contribution to the defect reaction free energies, and a valence force field model has been used to calculate the changes to the vibration free energy when a defect is created. We find the double acceptor mercury vacancy is the dominant defect, in agreement with previous interpretations of experiments. The tellurium antisite, which is a donor, is also found to be an important defect in this material. The mercury vacancy tellurium antisite pair is predicted to be well bound and is expected to be important for tellurium antisite diffusion. We consider the possibilities that the tellurium antisite is the residual donor and a Shockley-Read recombination center in HgCdTe and suggestions for further experimental work are made. We predict that the cadmium vacancy, a double acceptor, is the dominant defect for low cadmium pressures, while the cadmium interstitial, a double donor, dominates at high cadmium pressures.     

含空位缺陷碳化硅纳米管的电子结构和光学性质

Based on first-principle calculations,the electronic structures and optical properties of a single-walled (7,0) SiC nanotube(SiCNT) with a carbon vacancy defect or a silicon vacancy defect are investigated.In the three silicon atoms around the carbon vacancy,two atoms form a stable bond and the other is a dangling bond.A similar structure is found in the nanotube with a silicon vacancy.A carbon vacancy results in a defect level near the top of the valence band,while a silicon vacancy leads to the formation of three defect levels in the band gap of the nanotube.Transitions between defect levels and energy levels near the bottom of the conduction band have a close relationship with the formation of the novel dielectric peaks in the lower energy range of the dielectric function.     

Passivation of HgCdTe p-n diode junction by compositionally graded HgCdTe formed by annealing in a Cd/Hg atmosphere

Cadmium telluride (CdTe) is being widely used for passivating the HgCdTe p-n diode junction. Instead of CdTe, we tried a compositionally graded HgCdTe as a passivation layer that was formed by annealing an HgCdTe p-n junction in a Cd/Hg atmosphere. During annealing, Cd diffuses into HgCdTe from the Cd vapor, while Hg diffuses out from HgCdTe, forming compositionally graded HgCdTe at the surface. The Cd mole fraction at the surface was constant regardless of the annealing temperature in the range of 250–350°C. Capacitance versus voltage (C-V) curves for p-type HgCdTe that were passivated with compositionally graded HgCdTe formed by Cd/Hg annealing at 260°C showed a smaller flat-band voltage than the one passivated by thermally deposited CdTe, indicative of the better quality of the passivation. A long-wave infrared (LWIR) HgCdTe p-n junction diode passivated by compositionally graded HgCdTe showed about a one order of magnitude smaller R_dA value than the one passivated by thermally deposited CdTe, confirming the effectiveness of the compositionally graded HgCdTe as a passivant.