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。     

碲锌镉小角晶界的反射式X射线衍射形貌

碲锌镉晶体中存在着各种晶体缺陷,其中小角晶界是制约碲锌镉晶体质量的主要晶体缺陷之一。X射线衍射形貌术是一种非破坏性地全面研究小角晶界缺陷的有效方法。采用反射式X射线衍射形貌术对碲锌镉衬底的小角晶界缺陷进行了研究,讨论了小角晶界类型、样品扫描方向以及入射线发散度等对小角晶界缺陷的X射线衍射形貌的影响。为全面获得小角晶界缺陷的衍射形貌,应尽量选择宽的入射线狭缝,对于对称倾侧晶界和扭转晶界,应分别平行和垂直于小角晶界方向扫描。     

基于X射线衍射形貌术的碲锌镉晶体样品制备方法研究

基于传统X射线衍射形貌术(X-Ray diffraction Topography, XRT)的碲锌镉晶体样品制备方法是通过对测试晶片进行磨抛加工,获得满足测试条件的表面。随着单晶晶片尺寸的增大,晶片磨抛加工的难度变大、耗时变长,而且还容易导致晶片损坏。针对上述问题,通过对切割研磨后的晶片腐蚀方法进行研究,获得了一种新的XRT样品制备方法。该方法能够快速去除晶片的表面损伤,获得满足XRT测试要求的晶片表面,大幅减小制样难度并缩短制样时间。使用该方法制备的样品X射线衍射形貌图像衬度均匀、信噪比较好,各类型晶体缺陷均可被检测出来。此技术能够很好地应用于大尺寸碲锌镉晶片的后续筛选和加工。     

碲锌镉晶片的X射线貌相研究

X射线貌相测试是一种针对样品晶体结构完整性的非破坏性检测手段。通过高分辨率数字式X射线形貌仪对碲锌镉(111)晶片表层进行貌相测试,通过缺陷形式及密度来评价碲锌镉晶体质量。经过大量数据分析统计出5种常见于碲锌镉晶体中的缺陷：划痕、空洞、小角晶界、孪晶和杂晶。结合具体工艺阐述和分析了5类缺陷的形成原因,并针对碲锌镉晶体生长和加工工艺提出了建设性意见。这有利于获取高质量衬底材料,进而提升外延碲镉汞膜的质量。     

碲镉汞小晶片中缺陷的X射线形貌相检测

用X射线劳厄透射形貌术拍摄了用于制备多元线列探测器的碲镉汞小晶片的形貌相,观察到在碲镉汞小晶片中存在的各种晶体缺陷,如晶格扭曲、亚晶块、滑移线和其它缺陷。结合晶片扭曲和滑移的模型,分析了小晶片的受力状况。实验表明,在普通的X射线光源上,用劳厄透射形貌术能以20μm的分辨率无损检测用于制备多元线列探测器的碲镉汞小晶片中的晶体缺陷;在不良的器件工艺中,小晶片的晶格会受到明显的损伤,严重地影响了小晶片的电学参数和多元线列探测器的性能。     

用X射线双晶衍射测量碲镉汞晶片表面的加工损伤

用X射线双晶衍射法测量了x=0.273、晶向为[110]的碲镉汞晶体在切、磨过程中引入的加工损伤,测出了在切割、M5金刚砂研磨过程中产生的损伤层厚度分别为4μm～7#m、12μm～20μm,并测出在碲锅汞晶体中存在大量取向差为90"～810"之间的亚晶块.     

碲锌镉用于探测γ射线

ＣｄＺｎＴｅ晶体生长所取得的进展,使得人们有可能制备出供γ射线光谱技术使用在室温下工作并具有良好能量分辨率的探测器。     

碲锌镉晶体Zn组分的光致发光实用化研究

利用激光显微光致发光光谱仪测试了碲锌镉晶片的室温显微光致发光谱,对测得的光致发光谱进行拟合得到碲锌镉材料带隙的Eg值,根据实验总结出的Eg与Zn组分的室温计算公式,结合自主开发的Zn组分计算程序得到碲锌镉晶片上的Zn组分。所得的Zn组分结果用X射线双晶衍射进行验证,结果显示,室温下显微光致发光测得的Zn组分是相对准确可信的,可作为大量常规工艺测定Zn组分的有效工具,并且获得的Zn组分可成为外延碲镉汞薄膜时筛选匹配衬底的重要依据,同时还为研究和优化碲锌镉晶体生长工艺提供重要帮助。     

High-energy X-ray diffraction and topography investigation of CdZnTe

High-energy transmission x-ray diffraction techniques have been applied to investigate the crystal quality of CdZnTe (CZT). CdZnTe has shown excellent performance in hard x-ray and gamma detection; unfortunately, bulk nonuniformities still limit spectroscopic properties of CZT detectors. Collimated high-energy x-rays, produced by a superconducting wiggler at the National Synchrotron Light Source’s X17B1 beamline, allow for a nondestructive characterization of thick CZT samples (2–3 mm). In order to have complete information about the defect distribution and strains in the crystals, two series of experiments have been performed. First, a monochromatic 67 keV x-ray beam with the size of 300×300 μm² was used to measure the rocking curves of CZT crystals supplied by different material growers. A raster scan of a few square centimeter area allowed us to measure the full-width at half-maximum (FWHM) and shift in the peak position across the crystal. The rocking curve peak position and its FWHM can be correlated with local stoichiometry variations and other local defects. Typically, the FWHM values ranging from 8.3 arcsec to 14.7 arcsec were measured with the best crystal used in these measurements. Second, transmission white beam x-ray topography (WBXT) was performed by using a 22 mm×200 μm beam in the energy range of 50 keV to 200 keV. These types of measurements allowed for large area, high-resolution (50 μm) scans of the samples. Usually, this technique is used to visualize growth and process-induced defects, such as dislocations, twins, domains, inclusions, etc. the difference in contrast shows different parts of the crystal that could not be shown otherwise. In topography, good contrast is indicative of a high quality of the sample, while blurred gray shows the presence of defects. Correlation with other techniques (e.g., infrared (IR) mapping and gamma mapping) was also attempted. Our characterization techniques, which use highly penetrating x-rays, are valid for in-situ measurements, even after electrical contacts have been formed on the crystal in a working device. Thus, these studies may lead to understanding the effects of the defects on the device performance and ultimately to improving the quality of CZT material required for device fabrication. It is important to study crystals from different ingot positions (bottom, center, and top); consequently, more systematic studies involving scans from center to border are planned.     

HgCdTe晶体缺陷的快速X射线形貌检测法

本文讨论了HgCdTe晶体缺陷的两种快速X射线形貌检测方法:使用X射线像眼的反射形貌法和反射Laue形貌法。在Laue形貌相机上用X射线像眼,经数分钟的曝光时间可拍摄分辨率约50μm的HgCdTe样品的反射扫描形貌相,而用特制的Laue相机可用20min的曝光时间拍得分辨率约60μm的反射形貌相。     

Correlation of CdZnTe(211)B substrate surface morphology and HgCdTe(211)B epilayer defects

We present results on the surface morphology and recombination lifetimes of molecular-beam epitaxy (MBE)-grown HgCdTe (211)B epilayers and correlate them with the roughness of the CdZnTe substrate surfaces. The substrate surface quality was monitored by in-situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The SE roughness of the substrate was measured after oxide desorption in the growth chamber. The RHEED patterns collected show a strong correlation with the SE roughness. This proves that SE is a valuable CdZnTe prescreening tool. We also found a correlation between the substrate roughness and the epilayer morphologies. They are characterized by a high density of thin elongated defects, “needle defects,” which appear on most samples regardless of growth conditions. The HgCdTe epilayers grown on these substrates were characterized by temperature-dependent, photoconductive decay-lifetime data. Fits to the data indicate the presence of mid-gap recombination centers, which were not removed by 250°C/24-h annealing under a Hg-rich atmosphere. These centers are believed to originate from bulk defects rather than Hg vacancies. We show that Te annealing and CdTe growth on the CdZnTe substrates smooth the surface and lower substantially the density of needle defects. Additionally, a variety of interfacial layers were also introduced to reduce the defect density and improve the overall quality of the epilayer, even in the presence of less than perfect substrates. Both the perfection of the substrate surface and that of its crystalline structure are essential for the growth of high-quality material. Thus, CdZnTe surface polishing procedures and growth techniques are crucial issues.     

Development of inclusion-free CdZnTe substrates from crystals grown by the vertical-gradient freeze method

With the aim of fabrication of (111) and (211) CdZnTe inclusion-free substrates for molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE) growth of mercury cadmium telluride (MCT), we focused on fundamental research of the process of crystallization and cooling to room temperature. Based on the study of inclusion formation in dependence of Cd overpressure above the melt, an optimized process of crystal solidification was established. A key result of this study is the position of the Cd pressure, where inclusion-free crystals were fabricated without post-growth annealing. The crystals with a diameter of 100 mm and a height 40–50 mm were fabricated by the vertical-gradient freeze method (VGFM). The resulting ingots exhibit very good crystallographic quality, with a single-crystalline part filling 60–80% of the crystal volume. Substrates with orientation (111) and (211) and dimensions up to 4 × 4 cm were fabricated.     

CdZnTe材料的表面钝化新工艺

研究了一种新的钝化CdZnTe(CZT)器件表面的工艺，即先采用KOHKCl溶液对CZT表面进行处理，再用NH4F/H2O2溶液对其进行表面氧化的二步法钝化工艺．并借助俄歇电子能谱（AES）、微电流测试仪等手段对其表面钝化层的质量进行了鉴别，同时与KOH KCl和NH4F/H2O2两种工艺进行了比较．AES能谱分析表明，采用二步法工艺钝化，既可获得化学计量比较好的CZT表面，又可在表面形成一层起保护作用的氧化层．I-V特性曲线显示，两步法钝化后CZT器件的漏电流与KOH KCl和NH4F/H2O2钝化相比都有一定程度的下降．说明文中提出的新工艺在CZT器件制备方面具有良好的应用前景．     

Material quality characterization of CdZnTe substrates for HgCdTe epitaxy

Cd_1−xZn_xTe (CZT) substrates were studied to investigate their bulk and surface properties. Imperfections in CZT substrates affect the quality of Hg_1−xCd_xTe (MCT) epilayers deposited on them and play a role in limiting the performance of infrared (IR) focal plane arrays. CZT wafers were studied to investigate their bulk and surface properties. Transmission and surface x-ray diffraction techniques, utilizing both a conventional closed-tube x-ray source as well as a synchrotron radiation source, and IR transmission micro-spectroscopy, were used for bulk and surface investigation. Synchrotron radiation offers the capability to combine good spatial resolution and shorter exposure times than conventional x-ray sources, which allows for high-resolution mapping of relatively large areas in an acceptable amount of time. Information on the location of grain boundaries and precipitates was also obtained. The ultimate goal of this work is to understand the defects in CZT substrates and their effects on the performance and uniformity of MCT epilayers and then to apply this understanding to produce better infrared detectors.     

Misfit relaxation behavior in CdHgTe layers grown by molecular beam epitaxy on CdZnTe substrates

Uniform layers of cadmium mercury telluride have been grown on inhomogeneous cadmium zinc telluride substrates by molecular beam epitaxy so that a single epitaxial layer experiences a laterally varying lattice mismatch. The lateral variations of layer and substrate lattice parameters, layer lattice tilt, diffraction peak width, etch pit density (EPD) and surface crosshatch have been characterized, and all measured quantities are reported as functions of the substrate lattice parameter. At small mismatch, the layer appears to be elastically deformed. Beyond a certain critical mismatch, the onset of relaxation is clearly observed in the layer lattice parameter. Relaxation leads to appearance of surface crosshatch and an increased diffraction peak width, but a reduction in EPD, suggesting a reduction in the density of threading dislocations within the layer.     

Thermomigration of tellurium precipitates in CdZnTe crystals grown by vertical bridgman method

Te precipitates in CdZnTe have been characterized by x-ray diffraction at room and higher temperatures. From the x-ray results at room temperature, it has been confirmed that Te precipitates in CdZnTe have the same structural phase as observed in elemental Te under high pressure. The x-ray results at higher temperature indicate that Te precipitates melt around 440°C. CdZnTe samples containing Te precipitates have been annealed at temperatures below and above 440°C with thermal gradient of ∼70°C/cm. Results of the observation with infrared microscope before and after the annealings indicate distinct occurrence of thermomigration of Te precipitates in samples annealed at temperature above 440°C compared with ones annealed at temperature below 440°C. Thermomigration velocity obtained from these results is ∼50 μm/h. The average value for the effective diffusion coefficient of the metallic atoms in Te precipitates calculated by using the thermomigration velocity is ∼3 x 10⁻⁵ cm²/s.