X射线衍射形貌术在碲锌镉晶体中的应用

碲锌镉晶体中存在着各种典型晶体缺陷,X射线衍射形貌术是一种非破坏性地整体研究晶体材料结构完整性、均匀性的有效方法。本文将反射式X射线衍射形貌术应用于碲锌镉晶体质量的评价,研究了入射线狭缝宽度、积分时间、扫描步长等测试参数以及样品表面加工状态对X射线衍射形貌的影响。结果表明入射线狭缝宽度对碲锌镉晶体的X射线衍射成像及晶体质量筛选应用影响很大,积分时间、样品扫描步长等测试参数的选择与入射线狭缝宽度密切相关。     

碲锌镉晶体的X射线衍射形貌与腐蚀形貌

碲锌镉晶体中存在着各种典型晶体缺陷,其缺陷研究一直倍受关注,X射线衍射形貌术是一种非破坏性地研究晶体材料结构完整性、均匀性的有效方法.采用反射式X射线衍射形貌术对碲锌镉衬底的质量进行了研究,并将衬底的X射线衍射形貌与Everson腐蚀形貌进行了对比分析,碲锌镉衬底的X射线衍射形貌主要有六种特征类型,分别对应不同的晶体结构或缺陷,包括均匀结构、镶嵌结构、孪晶、小角晶界、夹杂、表面划伤,对上述特征类型进行了详细的分析.目前,衬底的X射线衍射形貌主要以均匀结构类型为主,划伤和镶嵌结构缺陷基本已消除,存在的晶体缺陷主要以小角晶界为主.通过对比分析碲锌镉衬底和液相外延碲镉汞薄膜的X射线衍射形貌,发现小角晶界等晶体结构缺陷会延伸到外延层上,碲锌镉衬底质量会直接影响碲镉汞外延层的质量,晶体结构完整的衬底是制备高质量碲镉汞外延材料的基础.     

Improved morphology and crystalline quality of MBE CdZnTe/Si

We report on continuing efforts to develop a reproducible process for molecular beam epitaxy of CdZnTe on three-inch, (211) Si wafers. Through a systematic study of growth parameters, we have significantly improved the crystalline quality and have reduced the density of typical surface defects. Lower substrate growth temperatures (∼250–280°C) and higher CdZnTe growth rates improved the surface morphology of the epilayers by reducing the density of triangular surface defects. Cyclic thermal annealing was found to reduce the dislocation density. Epilayers were characterized using Nomarski microscopy, scanning electron microscopy, x-ray diffraction, defect-decoration etching, and by their use as substrates for HgCdTe epitaxy.     

Impact of CdZnTe Substrates on MBE HgCdTe Deposition

The highest sensitivity, lowest dark current infrared focal plane arrays (IRFPAs) are produced using HgCdTe on CdZnTe substrates. As-received state-of-the-art CdZnTe 6 × 6 and 7 × 7.5 cm substrates were analyzed using Nomarski phase contrast microscopy, Auger electron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and scanning profilometry. On all CdZnTe substrates tested, we observed as-received large area macro-defect contamination. Using a defect specification limit of 50 contiguous defective pixels, we identified non-compliant 1280 × 720 12-μm pitch focal plane arrays due to as-received substrate macro-defect contamination. Using the above specification, up to 20% IRFPA wafer yield loss is due to state-of-the-art as-received CdZnTe substrate macro-contamination.     

Synthesis and microstructure of Cd4SiS6/Si composite nanowires

Cd4SiS6/Si composite nanowires are produced through co-thermal evaporation of CdS and Si powders with a small amount of tin sulfide as an additive. A vapor-liquid-solid growth mechanism is proposed for the anisotropic growth of the composite nanowires based on the presence of metallic tin particles at their tip-ends. Both side-by-side and core-shell composite nanowires are obtained. The product is characterized using X-ray powder diffraction and scanning electron microscopy. Detailed structural and chemical analyses of individual composite nanowires are carried out using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffraction (ED) and energy-dispersive X-ray spectroscopy. Planar defects, including microtwins and stacking faults, are abundant in a Si core, as revealed by HRTEM and selected-area ED. The formation of composite nanowires is discussed in the light of thermodynamic and kinetic aspects.     

原位退火对碲锌镉晶体第二相夹杂缺陷的影响

采用传统布里奇曼法生长碲锌镉晶体,在配料过程中添加适当过量的Cd,并在晶体生长结束阶段的降温过程中加入晶锭原位退火工艺,晶体的第二相夹杂缺陷得到了有效抑制.根据晶体第二相夹杂缺陷的形成机理,结合热扩散理论和碲锌镉晶体的P-T相图,研究了退火温度对晶体第二相夹杂缺陷密度和粒度(尺寸)的影响,获得了抑制碲锌镉晶体第二相夹杂...     

Study of Morphological Defects on Dual-Band HgCdTe on CdZnTe

HgCdTe dual-band epitaxial layers on lattice-matched CdZnTe substrates often have morphological defects. These defects, unlike normal void and microvoid defects, do not contain a polycrystalline core and, therefore, do not offer a good contrast for observation using optical and electron microscopes. This paper reports a way of identifying these defects by using a Nomarski optical microscopy image overlay on focused ion beam microscopy images for preparation of thin cross-sectional foils of these defects. Transmission electron microscopy was used to study the defect cross-sections to identify the origin and evolution of the morphological defects and their effect on the epitaxial layer. This paper reports cross-sectional analysis of four morphological defects of different shape and size.     

Investigation of HgCdTe p-n device structures grown by liquid-phase epitaxy

The microstructure of p-n device structures grown by liquid-phase epitaxy (LPE) on CdZnTe substrates has been evaluated using transmission electron microscopy (TEM). The devices consisted of thick (∼21-μm) n-type layers and thin (∼1.6-μm) p-type layers, with final CdTe (∼0.5 μm) passivation layers. Initial observations revealed small defects, both within the n-type layer (doped with 8×10¹⁴/cm³ of In) and also within the p-type layer but at a much reduced level. These defects were not visible, however, in cross-sectional samples prepared by ion milling with the sample held at liquid nitrogen temperature. Only isolated growth defects were observed in samples having low indium doping levels (2×10¹⁴/cm³). The CdTe passivation layers were generally columnar and polycrystalline, and interfaces with the p-type HgCdTe layers were uneven. No obvious structural changes were apparent in the region of the CdTe/HgCdTe interfaces as a result of annealing at 250°C.     

Impurity ‘Hot Spots’ in MBE HgCdTe/CdZnTe

In this work, impurity ‘hot spot’ macro-defects—high impurity level macro-defect contaminates were examined. ‘Hot spots’ have very high localized concentrations of: K, Mg, Ni, Cr, Mn, Ca, Al, Na, Fe, and Cu. For example, these ‘hot spot’ macro-defects can have Cu concentrations >?1?×?10¹⁸ cm^?3. Focused ion beam scanning transmission electron microscopy analysis of four ‘hot spots’ was performed. The origin of ‘hot spot’ defects is unresolved—however, our analysis has shown ‘hot spots’ can arise due to molecular beam epitaxy spit defects and CdZnTe substrate defects. The estimated ‘hot spot’ density is ～?30 cm^?2. The presence of impurity ‘hot spot’ macro-defects in HgCdTe/CdZnTe is confirming evidence for the occurrence of L. Bubulac’s impurity ‘pipe’ mechanism.     

Structural analyses by TEM of iridium deposited on TiO2 powder and rutile single crystal

It was found by transmission electron microscopy (TEM) that deposition precipitation of Ir onto TiO2 powder (Degussa P-25) produced Ir particles preferentially on the rutile phase. Iridium oxide particles supported on rutile TiO2 single crystal by deposition precipitation were also observed by TEM and scanning electron microscopy, and were found to exist as thin films of a few atoms. A structural change caused by hydrogen reduction was observed using high-resolution TEM and this was confirmed to be the iridium oxide film being reduced to metallic iridium particles.     

Electron microscopy of surface-crater defects on HgCdTe/CdZnTe(211)B epilayers grown by molecular-beam epitaxy

Surface-void defects observed in Hg_1−xCd_xTe (x ∼ 0.2–0.4) alloys grown by molecular-beam epitaxy (MBE) have been investigated using scanning and high-resolution transmission-electron microscopy (HRTEM) as well as atomic force microscopy (AFM). These surface craters, which have been attributed to Hg-deficient growth conditions, were found to originate primarily within the HgCdTe epilayer, rather than at the CdZnTe substrate, and they were associated with the local development of polycrystalline morphology. High-resolution observations established the occurrence of finely spaced HgCdTe/Te intergrowths with semicoherent and incoherent grain boundaries, as well as small HgCdTe inclusions embedded within the Te grains. This study is the first time that high-resolution electron microscopy has been used to investigate this type of defect.     

Observation on Defects in Poly-Si Films Prepared by RTCVD Under Nonideal Conditions

Polycrystalline silicon (poly-Si) thin films were deposited on quartz substrates by rapid thermal chemical vapor deposition (RTCVD) under nonideal conditions. Then, crystallographic defects in the poly-Si films were investigated by using transmission electron microscopy (TEM) and optical microscopy combined with defect etching. We found that as-deposited poly-Si films contain a lot of twin crystals, including first-order, second-order, third-order, and higher-order twinned crystals. Besides twinned crystals, stacking faults, dislocations, dislocation nets, dislocation loops, extended dislocations, and dislocation line arrays were also found. Finally, the origins of the defects were analyzed, being attributed to the peculiarities of the RTCVD-quartz growth system, stress caused by lattice and thermal mismatch, a huge temperature ramp, and nonideal deposition conditions. Although our experimental results cannot represent the crystallographic quality of poly-Si films prepared by RTCVD, they at least indicate what kinds and how many defects exist in poly-Si films when deposition conditions severely deviate from the optimum.     

Surface Morphology and Defect Formation Mechanisms for HgCdTe (211)B Grown by Molecular Beam Epitaxy

The surface morphology and crystallinity of HgCdTe films grown by molecular beam epitaxy (MBE) on both CdZnTe and CdTe/Si (211)B substrates were characterized using atomic force microscopy (AFM), as well as scanning (SEM) and transmission (TEM) electron microscopy. Crosshatch patterns and sandy-beach-like morphologies were commonly found on MBE (211) HgCdTe epilayers grown on both CdZnTe and CdTe/Si substrates. The patterns were oriented along the , , and directions, which were associated with the intersection between the (211) growth plane and each of the eight equivalent HgCdTe slip planes. This was caused by strain-driven operation of slip in these systems with relative large Schmid factor, and was accompanied by dislocation formation as well as surface strain relief. Surface crater defects were associated with relatively high growth temperature and/or low Hg flux, whereas microtwins were associated with relatively low growth temperature and/or high Hg flux. AFM and electron microscopy were used to reveal the formation mechanisms of these defects. HgCdTe/HgCdTe superlattices with layer composition differences of less than 2% were grown by MBE on CdZnTe substrates in order to clarify the formation mechanisms of void defects. The micrographs directly revealed the spiral nature of growth, hence demonstrating that the formation of void defects could be associated with the Burton, Cabrera, and Frank (BCF) growth mode. Void defects, including microvoids and craters, were caused by screw defect clusters, which could be triggered by Te precipitates, impurities, dust, other contamination or flakes. Needle defects originated from screw defect clusters linearly aligned along the directions with opposite Burgers vector directions. They were visible in HgCdTe epilayers grown on interfacial superlattices. Hillocks were generated owing to twin growth of void or needle defects on (111) planes due to low growth temperature and the corresponding insufficient Hg movement on the growth surface. Therefore, in addition to nucleation and growth of HgCdTe in the normal two-dimensional layer growth mode, the BCF growth mode played an important role and should be taken into account during investigation of HgCdTe MBE growth mechanisms.     

空间微重力汽相生长CdZnTe的研究进展

微重力条件下汽相生长CdZnTe晶体可以克服浮力对流,实现“无接触”生长,获得厚度均匀、结构完整、纯度高的材料.本文综述了国内外空间汽相生长CdZnTe晶体的研究进展.     

Correlative microscopy analyses of thin-film solar cells at multiple scales

In the present work, a brief overview is given on how to apply transmission (TEM) as well as scanning electron microscopy (SEM) and their related techniques (electron diffraction, energy-dispersive X-ray spectrometry, electron energy-loss spectroscopy, electron holography; electron backscatter diffraction, electron-beam-induced current, cathodoluminescence) for the analysis of interfaces between individual layers or extended structural defects in a thin-film stack. All examples given in the present work were recorded on Cu(In, Ga)Se₂ thin film solar cells, however, the shown experimental approaches may be used on any similar thin-film semiconductor device. A particular aspect is the application of various techniques on the same identical specimen area, in order to enhance the insight into structural, compositional, and electrical properties. For (aberration-corrected) TEM, the spatial resolutions of such measurements can be as low as on the subnanometer scale. However, when dealing with semiconductor devices, it is often necessary to characterize electrical and optoelectronic properties at larger scales, of few 10 nm up to even mm, for which SEM is more appropriate. At the same time, these larger scales provide also enhanced statistics of the analysis. In the present review, it is also outlined how to apply SEM techniques in combination with scanning-probe and optical microscopy, on the same identical positions. Altogether, a multiscale toolbox is provided for the thorough analysis of structure-property relationships in thin-film solar cells using correlative microscopy approaches.     

On the metallic bonding of GaN-based vertical light-emitting diode

In this work, metallic bonding in GaN-based vertical light-emitting diode (VLED) is systematically characterized by using combined methodology of transmission Kikuchi diffraction (TKD) and energy dispersive X-ray spectroscopy (EDS) in a scanning electron microscope (SEM). SEM-based TKD with EDS identifies chemical composition, grain morphology, orientation, and phases at metallic bonding, while transmission electron microscopy (TEM) provides nanoscale characteristics of metallic diffusion bonding, and its interface-related defects and nano-twinned boundaries. Our results from SEM-TKD and TEM techniques provide unparalleled insight into the metallic bonding, and its future optimization.     

Extended defects and polarity of hydride vapor phase epitaxy GaN

Hydride vapor phase epitaxy (HVPE) GaN layers on sapphire substrates and so-called free-standing platelets (layers removed from the sapphire) were studied by different transmission electron microscopy (TEM) techniques. Polarity, determined by convergent beam electron diffraction (CBED), and distribution of structural defects, determined by conventional TEM, are discussed. The HVPE layers were found to grow primarily with Ga-polarity. A few inversion domains (areas with N-polarity) were observed on the substrate side of one of the free-standing layers. The dominant structural defects in HVPE GaN layers are threading dislocations. A systematic reduction of their density with an increase in layer thickness was observed for all of the samples. The experimental results indicate that the density of dislocations is inversely proportional to the distance from the substrate, which agrees with the theoretical model.     

Cross-Sectional Study of Macrodefects in MBE Dual-Band HgCdTe on CdZnTe

HgCdTe dual-band mid-wave infrared/long-wave infrared focal-plane arrays on CdZnTe are a key component in advanced electrooptic sensor applications. Molecular beam epitaxy (MBE) has been used successfully for growth of dual-band layers on larger CdZnTe substrates. However, the macrodefect density, which is known to reduce the pixel operability and its run-to-run variation, is larger when compared with layers grown on Si substrate. This paper reports the macrodefect density versus size signature of a well-optimized MBE dual-band growth and a cross-sectional study of a macrodefect that represents the most prevalent class using focused ion beam, scanning transmission electron microscopy, and energy-dispersive x-ray spectroscopy. The results show that the macrodefect originates from a void, which in turn is associated with a pit on the CdZnTe substrate.     

A study of the damage on FIB-prepared TEM samples of AlxGa1-xAs

The damage produced by focused ion beam (FIB) milling on a TEM sample of AlGaAs crystals has been studied. The damage observed on the sidewall of an AlGaAs transmission electron microscopy (TEM) sample was an amorphous layer. The thickness of the amorphous layer linearly increased with an increase in FIB accelerating voltage from 5 to 30 kV. The thickness of the amorphous layer of Al(x)Ga(1-x)As was constant at 3 nm and was independent of the Al concentration x when the accelerating voltage was below 5 kV. The thickness of the amorphous layer of Al(x)Ga(1-x)As decreased with an increase in Al concentration x when the accelerating voltage was above 5 kV. FIB milling at 5 kV effectively minimizes the thickness of the amorphous layer and also provides flat sidewalls on multilayer samples of Al(x)Ga(1-x)As that are prepared for TEM and scanning electron microscopy (SEM).     

Vanadium-Doped Cadmium Manganese Telluride (Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te) Crystals as X- and Gamma-Ray Detectors

CdMnTe offers several potential advantages over CdZnTe as a room- temperature gamma-ray detector, but many drawbacks in its growth process impede the production of large, defect-free single crystals with high electrical resistivity and high electron lifetimes. Here, we report our findings of the defects in several vanadium-doped as-grown as well as annealed Cd_1−x Mn _x Te crystals, using etch pit techniques. We carefully selected single crystals from the raw wafer to fabricate and test as a gamma-ray detector. We describe the quality of the processed Cd_1−x Mn _x Te surfaces, and compare them with similarly treated CdZnTe crystals. We discuss the characterization experiments aimed at clarifying the electrical properties of fabricated detectors, and evaluate their performance as gamma-ray spectrometers.