Dislocation density, infrared absorption and cathodoluminescence mapping of microstructure associated with dislocation cells in semi-insulating LEC GaAs

We have investigated the microstructure that occurs both inside and closely associated with dislocation cells in undoped, semi-insulating LEC GaAs. Using A/B etching, we have identified regions containing large volumes of this microstructure for examination by transmission electron microscopy. A precipitate with a possible associated dislocation loop has been observed in such a region. Cathodoluminescence and reverse contrast images show excellent agreement and demonstrate correlation between regions of high precipitate (and dislocation) density and low concentrations of particular unidentified point defects. In-situ, optical scattering imaging by A/B etched surfaces and reverse contrast absorption imaging demonstrate directly the excellent correlation between these techniques for the first time.     

二维原子晶体的低电压扫描透射电子显微学研究

二维原子晶体材料,如石墨烯和过渡金属硫族化合物等,具有不同于其块体的独特性能,有望在二维半导体器件中得到广泛应用.晶体中的结构缺陷对材料的物理化学性能有直接的影响,因此研究结构缺陷和局域物性之间的关联是当前二维原子晶体研究中的重要内容,需要高空间分辨率的结构研究手段.由于绝大部分二维原子晶体在高能量高剂量的电子束辐照下容易发生结构损伤,利用电子显微方法对二维原子晶体缺陷的研究面临诸多挑战.低电压球差校正扫描透射电子显微(STEM)技术的发展,一个主要目标就是希望在不损伤结构的前提下对二维原子晶体的本征结构缺陷进行研究.在STEM下,多种不同的信号能够被同步采集,包括原子序数衬度高分辨像和电子能量损失谱等,是表征二维原子晶体缺陷的有力工具,不但能对材料的本征结构进行单原子尺度的成像和能谱分析,还能记录材料结构的动态变化.通过调节电子束加速电压和电子辐照剂量,扫描透射电子显微镜也可以作为电子刻蚀二维原子晶体材料的平台,用于加工新型纳米结构以及探索新型二维原子晶体的原位制备.本综述主要以本课题组在石墨烯和二维过渡金属硫族化合物体系的研究为例,介绍低电压扫描透射电子显微学在二维原子晶体材料研究中的实际应用.     

Dislocation-related luminescence in Er-implanted silicon

The behavior of luminescence spectra and structural defects in single crystal Czochralski silicon after erbium implantation at 1–1.8 MeV energies and 1×10¹³ cm⁻² dose with subsequent annealing at 1000–1200°C for 0.25–3 h in argon and a chlorine-containing ambience (CCA) was studied by photoluminescence (PL), transmission electron microscopy and chemical etching/Nomarski microscopy. We have found that annealing in CCA gives rise to dislocation loops and pure edge dislocations with dominant dislocation-related lines in the PL spectrum. Pure edge dislocations are responsible for the appearance of the lines.     

Issues in ZnO homoepitaxy

Zinc oxide (ZnO) bulk single crystals, which are of high purity and transparency with a large size of 2 in., are successfully grown by the hydrothermal method. The sliced substrates are chemomechanically polished to form an epi-ready surface. The impurities existing on the as-polished substrate surface are characterized before and after annealing by SIMS (secondary-ion mass spectroscopy), and a damaged surface layer due to chemomechanical polishing is evaluated by an optical method. We attempt to remove the layer damaged due to chemomechanical polishing with two approaches, chemical etching and thermal annealing in N₂, O₂ or high vacuum. The improvement of the surface morphology and crystallinity is evaluated by means of high resolution X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM). In the PL measurements, the relative intensity of the first-order longitudinal optical phonon replica of the free exciton (FX-1LO) is compared against varying etching depth. The relative intensity becomes weak with increasing etch depth and finally saturates at the etch depth of 5 μm. After the annealing process, we grow ZnO thin films on these ZnO(0001) substrates by plasma-assisted molecular beam epitaxy. Films grown directly on the substrate show a 3D growth mode in the initial stage of growth with various surface treatments. To overcome this problem, we employ a low temperature grown ZnO buffer layer (LT-ZnO), and a two-dimensionally grown high quality ZnO film is attained.     

Super deformability and thermoelectricity of bulk γ-InSe single crystals

Indium selenide, a III–V group semiconductor with layered structure, attracts intense attention in various photoelectric applications, due to its outstanding properties. Here, we report super deformability and thermoelectricity of γ-In Se single crystals grown by modified Bridgeman method. The crystal structure of In Se is studied systematically by transmission electron microscopy methods combined with x-ray diffraction and Raman spectroscopy. The predominate phase of γ-In Se with dense stacking faults and local multiphases is directly demonstrated at atomic scale. The bulk γ-In Se crystals demonstrate surprisingly high intrinsic super deformative ability which is highly pliable with bending strains exceeding12.5% and 264% extension by rolling. At the meantime, In Se also possesses graphite-like features which is printable,writable, and erasable. Finally, the thermoelectric properties of γ-In Se bulk single crystals are preliminary studied and thermal conductivity can be further reduced via bending-induced defects. These findings will enrich the knowledge of structural and mechanical properties' flexibility of In Se and shed lights on the intrinsic and unique mechanical properties of In Se polytypes.     

Evaluation of Charged Defect Energy in Two-Dimensional Semiconductors for Nanoelectronics: The WLZ Extrapolation Method

Defects play a central role in controlling the electronic properties of two-dimensional (2D) materials and realizing the industrialization of 2D electronics. However, the evaluation of charged defects in 2D materials within first-principles calculation is very challenging and has triggered a recent development of the WLZ (Wang, Li, Zhang) extrapolation method. This method lays the foundation of the theoretical evaluation of energies of charged defects in 2D materials within the first-principles framework. Herein, the vital role of defects for advancing 2D electronics is discussed, followed by an introduction of the fundamentals of the WLZ extrapolation method. The ionization energies (IEs) obtained by this method for defects in various 2D semiconductors are then reviewed and summarized. Finally, the unique defect physics in 2D dimensions including the dielectric environment effects, defect ionization process, and carrier transport mechanism captured with the WLZ extrapolation method are presented. As an efficient and reasonable evaluation of charged defects in 2D materials for nanoelectronics and other emerging applications, this work can be of benefit to the community.     

Novel silicon nanowire film on copper foil as high performance anode for lithium-ion batteries

A novel silicon nanowire film anode was successfully prepared by a combination of magnetron sputtering deposition and metal-catalyzed electroless etching technology. Scanning electron microscopy revealed the formation of a Si film composed of nanowires with a diameter of ~70 nm and lengths of ~3.5 μm. As-prepared Si nanowire film is directly grown on current collectors without binders and carbon additives, which provides a good contact and adhesion of them to current collector. Furthermore, the defined spacing of nanoscale Si nanowire allows Si to undergo large volume change during the alloying/dealloying process without loss of its integrity. These structural features of the resulting Si nanowire make it a promising anode for lithium-ion batteries with remarkably improved electrochemical performance compared with the Si film-based electrode prepared without metal-catalyzed electroless etching process.     

Statistical model for the formation of the alloy

The electronic structures of most semiconductor alloys are smooth functions of their composition. Binary alloys of group IV semiconductors are usually easy to prepare at any concentration, but this is not the case for the Ge_1-xSn_x alloy. Homogeneous alloys as required for nano- and optoelectronics device applications have proved difficult to form for x above a temperature-dependent critical concentration, above which Sn exhibits the tendency to segregate in the metallic cubic β phase, spoiling the semiconducting properties. The underlying mechanism for this segregation and critical concentration was not known.Through previous accurate ab initio local defect calculations we estimated the scale of energies involved in the immediate environment around a large number of Sn defects in Ge, the relaxed configurations of the defects, and the pressure directly related to the elastic field caused by the defects. This detailed information allowed us to build a simple statistical model including the defects most relevant at low x, namely substitutional α-Sn and non-substitutional β-Sn (in which a single atom occupies the centre of a Ge divacancy). Our model enables us to determine at which concentration β defects, which exhibit a tendency to segregate, can be formed in thermal equilibrium. These results coincide remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases.     

Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

用于红外探测的极度非简并非线性光学

半导体中的双光子吸收(2PA)与能隙的三次方的倒数(Eg-3)成正比,这一关系限制了在宽带隙半导体中获得双光子吸收系数。但是已知在输入波长迥异的高度非简并情况下,双光子吸收率较简并状态会有大幅提升。在飞秒和皮秒脉冲下的泵浦探测传输实验中证实了几种直接能隙半导体具有这种性质。发现在许多直接能隙半导体中,非简并双光子吸收率较简并情况下可增强约100倍。在GaAs中,观察到双光子吸收系数达到1cm/MW,这么大的系数以前只在窄带半导体如InSb中观察到过。基于这种作用可以利用传统的半导体光电二极管获得敏感的选通探测。采用标准的GaN和GaAs光电二极管和高达14∶1的能量比的极度非简并光子证实了此技术。此外,还采用强紫外选通脉冲探测弱红外辐射,如采用GaN光电二极管,最小可探测红外脉冲能量可低至20pJ,而现有的标准制冷型MCT探测器的最小可探测能量为200pJ。我们现在还证实这种方法也可在连续波探测中应用。值得注意的是,这个过程没有用到类似χ(2)上转换的红外晶体或相位匹配。在本报告中将介绍怎样将这种探测技术用于其他半导体,以及如何优化器件的几何结构以满足实用化探测要求。     

Study of laser-deposited metallic thin films by a combination of high-resolution ex situ and time-resolved in situ experiments

Laser-deposited metallic alloys and multilayers were studied in detail by a combination of high-resolution ex situ and time-resolved in situ experiments. The purpose of these experiments is to better understand the special properties of laser-deposited metallic films in comparison with conventionally prepared thin films. During deposition, thickness, resistance, and electron diffraction (THEED) experiments show that the film surface is resputtered, local mixing at the interfaces of multilayers on a nanometre scale occurs, and metastable phases up to large film thicknesses are formed. After deposition, a compressive stress of 1-2 GPa was measured using four-circle diffractometry, and growth defects were observed on an atomic scale by electron microscopy (HRTEM) and field ion microscopy (FIM). The obtained structural details of the metallic films can be explained by an implantation model for the laser deposition process.     

Reflectometric studies of the etching of a silica fiber with a germanium silicate core in sub- and supercritical water

Fiber optic reflectometry (FOR) and scanning electron microscopy (SEM) were used to study the regularities of the etching of a single-mode optical silica fiber with a germanium silicate core in subcritical and supercritical water. It was demonstrated that the rate of etching of the germanium silicate core, being higher than that of etching of the silica cladding, was responsible for the formation of a well at the fiber end face, the depth of which increased with the time of etching. The temporal behavior of the FOR signal was of oscillatory character, an observation that accounted for the interference effects that accompany the reflection of radiation (from the photodiode used in the FOR) from the fiber end face during its etching (well deepening). The interference-controlled character of the FOR signal made it possible to directly measure the rate of etching of the fiber end face in water in its different phase states (gaseous, liquid, and supercritical) at various temperatures and pressures. The lowest measured rate of etching of the germanium silicate core (at 200°C and 54 atm) was 10⁻³ nm/s, whereas the highest measured rate was 30 nm/s (at 400°C and 246 atm). The temperature dependence of the etching rate was demonstrated to obey the Arrhenius law, with an activation energy of 58 ± 3 kJ/mol. At later stages of etching, the FOR signal changed from regular oscillatory to irregular noisy due to the formation defects of various sizes, as could be clearly seen in SEM images.     

Structural investigations on YbRh2Si2: from the atomic to the macroscopic length scale

YbRh2Si2 has advanced to a prototype material for investigating physics related to the Kondo effect. An optimization of the synthesis resulted in single crystals of extraordinary crystalline quality. At the atomic scale, we utilize scanning tunneling microscopy to study the topography of cleaved single crystals. A structural and chemical analysis was conducted by highly accurate x-ray diffraction and wavelength dispersive x-ray spectroscopy measurements. The latter indicate a homogeneity range of the YbRh2Si2 phase between approximately 40.0–40.2 at.% Rh. For our high-quality samples the number of defects found on the atomic scale (of the order of 0.3% of the visible lattice sites) is in quantitative agreement with a very small off-stoichiometry within this homogeneity range. Comparing our results for these samples allows an assignment of the structural defects observed at the cleaved surfaces to Rh occupying Si sites and, even less numerous Si in Rh sites. Such an analysis is hampered for samples of lesser quality, but there seem to be numerous empty Si-sites. Based on these observations the results of scanning tunneling spectroscopy can be analyzed in further detail and provide insight into the Kondo physics.     

Improvement of spatial homogeneity in GdBCO/IBAD-MgO coated conductor

Recent process optimization allows improving homogeneity and a significant increase of J_c in GdBa₂Cu₃O_7-δ deposited on ion-beam assisted deposited MgO template (GdBCO/IBAD-MgO). We applied low-temperature laser scanning microscopy and laser scanning thermo-electric microscopy (LSTEM) to investigate local dissipation and defects simultaneously in recent GdBCO/IBAD-MgO coated conductor. By using high-resolution LSTEM, we could detect current blocking obstacles which are responsible for the large scale local dissipation. Data on the present GdBCO/IBAD-MgO coated conductors point out a significant reduction of current blocking obstacles compared to the previous process. We have shown the improvement of spatial homogeneity in the recent GdBCO/IBAD-MgO coated conductors. Current blocking obstacles are much less densely distributed as compared to previous YBCO/IBAD-GZO coated conductor. Obstacles larger than several micrometer significantly increase local dissipation whereas smaller defects do not influence it noticeably. 2D map of the phase delay component of the thermoelectric voltage lock-in signal is effective to analyze current blocking obstacles having action upon local dissipation.     

Defect detection in solar cells via electroluminescence,LBIC, and EBIC methods

Structural defects affecting the efficiency of multicrystalline silicon solar cells are investigated by the electron-beam-induced current (EBIC) mode of a scanning electron microscope and the laser-beam-induced current (LBIC) method. It is experimentally demonstrated that the LBIC technique is more sensitive to electrically active 2D defects than the EBIC approach at large values of the diffusion length and excitation-beam penetration depth. The comparison of LBIC (or EBIC) and electroluminescence images enables us to reveal the correlation between solar-cell short circuits and electrically active structural defects.     

基于压缩表示的离子刻蚀仿真三维表面演化方法

为了研究表面演化过程的机理, 提出了一种基于压缩表示的三维表面演化方法来模拟等离子体刻蚀工艺,并着重探讨了对离子刻蚀的仿真. 为了解决三维元胞自动机内存需求量大的问题, 该方法将二维数组和动态存储方式相结合, 既实现元胞信息的无损压缩存储, 又保持三维元胞间的空间相关性. 实验结果也表明该方法不仅节省了大量内存, 而且在高分辨率条件下查找离子初始碰撞的表面元胞效率较高, 满足高分辨率仿真的要求. 将该方法应用于实现刻蚀工艺三维表面仿真中, 模拟结果与实验结果对比验证了该方法的有效性. 关键词： 等离子体刻蚀 元胞自动机 表面演化方法 高分辨率仿真     

Structures of planar defects in ZnO nanobelts and nanowires

Quasi-one-dimensional (1D) nanostructures, such as nanowires, nanobelts and nanorods, are the forefront materials for nanotechnology. To date, such nanostructures have been synthesized for a wide range of semiconductors and oxides, and they are potential building blocks for fabricating numerous nano-scale devices. 1D ZnO nanostructures, due to its unique semiconducting, piezoelectric, and bio-safe properties, have received wide attention. From structure point of view, a common characteristic of ZnO nanostructures is that they are mostly dislocation-free. However, planar and point defects do frequently exist in such nanostructures. The objective of this paper is to present detailed electron microscopy study about the structures of planar defects, such as stacking faults, twins, inversion domain walls that existed in 1D ZnO nanostructures. These planar defects are important for understanding the growth mechanism and relevant physical and possibly chemical properties of 1D ZnO nanostructures.     

Investigation of etch characteristics of non-polar GaN by wet chemical etching

We characterized the surface defects in a-plane GaN, grown onto r-plane sapphire using a defect-selective etching (DSE) method. The surface morphology of etching pits in a-plane GaN was investigated by using different combination ratios of H₃PO₄ and H₂SO₄ etching media. Different local etching rates between smooth and defect-related surfaces caused variation of the etch pits made by a 1:3 ratio of H₃PO₄/H₂SO₄ etching solution. Analysis results of surface morphology and composition after etching by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) demonstrated that wet chemical etching conditions could show the differences in surface morphology and chemical bonding on the a-plane GaN surface. The etch pits density (EPD) was determined as 3.1 × 10⁸ cm⁻² by atom force microscopy (AFM).     

Characterization of wide-band-gap semiconductors (GaN, SiC) by defect-selective etching and complementary methods

Two defect-selective etching approaches used for revealing and analysis of defects in wide-band-gap semiconductors (GaN, SiC) are described in detail: (i) orthodox etching in molten salts (KOH, NaOH) and hot acids (H₂SO₄,H₃PO₄) and (ii) electroless photo-etching (photoelectrochemical or PEC) in aqueous solutions of KOH. Characteristic features of these two techniques, their reliability and limitation in revealing different types of defects (dislocations, stacking faults, micro-defects and electrically active chemical non-homogeneities) will be discussed. Examples of the use of both etching approaches to reveal defects in bulk and epitaxial layers of different crystallographic orientation are given. Numerous references to previous work on calibration of the etch features by means of TEM, X-ray diffraction, Raman and PL methods are cited.     

多元半导体光伏材料中晶格缺陷的计算预测

半导体光伏材料的发展在过去60多年中表现出了清晰的多元化趋势. 从20世纪50年代的一元Si太阳能电池, 到20世纪60年代的GaAs和CdTe电池、70年代的CuInSe₂电池、80年代的Cu(In, Ga) Se₂、90年代的Cu₂ZnSnS₄电池, 再到最近的Cu₂ZnSn(S, Se)₄和CH₃NH₃PbI₃电池, 组成光伏半导体的元素种类从一元逐渐增多到五元. 元素种类的增多使得半导体物性调控的自由度增多, 物性更加丰富, 因而能满足光伏等器件应用的需要. 但是, 组分元素种类的增多也导致半导体中晶格点缺陷的种类大幅增加, 可能对其光学、电学性质和光伏性能产生显著影响. 近20年来, 第一性原理计算被广泛应用于半导体中晶格点缺陷的理论预测, 相对于间接的实验手段, 第一性原理计算具有更加直接的、明确的优势, 并且能对各种点缺陷进行快速的研究. 对于缺陷种类众多的多元半导体体系, 第一性原理计算能预测各种点缺陷的微观构型、浓度和跃迁(离化)能级位置, 从而揭示其对光电性质的影响, 发现影响器件性能的关键缺陷. 因而, 相关的计算结果对于实验研究有直接、重要的指导意义. 本文将首先介绍半导体点缺陷研究的第一性原理计算模型和计算流程; 然后, 总结近5年来两类新型光伏半导体材料, 类似闪锌矿结构的Cu₂ZnSn(S, Se)₄半导体和有机-无机杂化的钙钛矿结构CH₃NH₃PbI₃半导体的点缺陷性质; 以这两类体系为例, 介绍多元半导体缺陷性质的独特特征及其对太阳能电池器件性能的影响.