Evaluation of Zn uniformity in CdZnTe substrates

The radial uniformity of Zn concentration in 4 in. CdZnTe crystals was improved by keeping uniform temperature distribution in the VGF furnace. The maximum temperature difference at the outside of crucible in radial direction was reduced to less than 1. The size of voids observed outside of the crystals became small and the distribution has become well-uniformed. To evaluate the Zn uniformity, a new NIR (Near Infrared) instrument was developed using the diode array type spectrometer. The NIR spectra were analyzed by C.D. Maxey et al.’s method. The Zn concentration in 4 in. CdZnTe substrates grown by modified furnace was more uniform than that of the conventional substrates.     

Vertical bridgman techniques to homogenize zinc composition of CdZnTe substrates

In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.     

Lattice mismatch induced morphological features and strain in HgCdTe epilayers on CdZnTe substrates

HgCdTe epilayers on CdZnTe substrates can exhibit a cross-hatch pattern of periodically varying strain and surface undulations as revealed by x-ray topography, and in some cases by Nomarski optical microscopy. On { 111 } oriented material, the pattern appears as three sets of parallel lines in the 〈 110 〉 slip directions (60° apart). To investigate this phenomenon and its impact on photovoltaic device performance, we have characterized several liquid phase epitaxy (LPE)-grown HgCdTe epilayer samples by means of Lang x-ray reflection topography, synchrotron white beam x-ray topography (SWBXT), etch pit density, and other techniques. The cross hatching generally shows a correlation with the ZnTe mole fraction of the substrate. In particular, the pattern is likely to appear when the natural lattice parameter of the layer at room temperature is slightly larger or smaller than that of the substrate in the same region. We also find the corresponding pattern in { 211 } oriented layers grown by MBE. Although substantial compositional interdiffusion occurs at the layer/substrate interface during LPE growth at around 500°C, this is not a necessary condition for the cross-hatch pattern, as demonstrated by the occurrence of the pattern in MBE material grown at less than 200°C. In terms of device performance, the pattern is manifested as lines of diodes in an array having greater leakage than their neighbors. In addition to these results, we have investigated other anomalies, by means of SWBXT applied to large-area diodes that have been electrically tested. A novel technique called absorption edge contour mapping, using synchrotron white beam x-rays with a molybdenum filter, was applied to reveal the longer range lattice strain.     

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.     

数值模拟在VB法生长碲锌镉晶体中的应用

数值模拟是晶体生长研究中一种非常有效的辅助分析技术,借助该技术可以模拟晶体生长过程,分析获得一定的规律或参数,从而指导工艺研究。本文介绍了晶体生长数值模拟的概念、基本方法,并结合VB法生长碲锌镉晶体难点,对数值模拟技术在碲锌镉晶体技术研究中的应用情况进行了归纳总结。     

Chemical polishing of CdZnTe substrates fabricated from crystals grown by the vertical-gradient freezing method

Chemical polishing is a process of crucial importance in the manufacture of epiready substrates for molecular beam epitaxy (MBE) of high-quality HgCdTe layers. With the aim of fabrication of (211) CdZnTe substrates, we focused on the fundamental research of polishing processes with respect to reducing subsurface damage. Wafers of the orientation (211) were prepared from the as-grown crystals by a process flow including oriented slicing, several steps of mechanical polishing, and finally chemical polishing. In the prechemical polishing process, several free and bound abrasives were applied to reach the surface roughness close to 1 nm. The surface polishing treatment included testing of the surface quality after each polishing step. We used an interferometer profiler, which yields detailed surface maps. Within chemical polishing processing, we have looked for an optimum composition of etchant based on the bromine-methanol/ethylene glycol solution and adequate polishing pad. We studied the substrate surface quality dependence on the rotation speed of the plate, sample loading weights, and duration of polishing. Correlation between the final surface roughness and layer thickness removed was established. The chemical polishing with a very low concentration of Br-methanol/ethylene glycol solution was found to yield very good CdZnTe surfaces with a perfect flatness.     

Distribution of the high resistivity region in CdZnTe and its effects on gamma-ray detector performance

The effect of the location of the high resistivity region on gamma-ray detector performance within the crystal boule is investigated for 10% zinc with 1.5% excess Te. By varying the indium-doping concentration in several CdZnTe boules, the region of high resistivity is seen to move along the vertical length of the crystal. The variation of the zinc concentration within the crystal boule is compared with the location of the high resistivity region along the length of the crystals. The concentration of zinc is extracted from Fourier transform infrared (FTIR) measurements, and the segregation coefficient is calculated using data obtained from the CdZnTe crystals. The zinc distribution is plotted in terms of the location along the crystal length in order to correlate the concentration with detector performance. Radiation spectra obtained from the 122-KeV gamma rays using a ⁵⁷Co source reveal a strong dependence between detector performance and the relative location of the high resistivity region within the crystal. Initial results suggest that there are three semi-distinct regions along the length of the boule that give very different characteristics, where it can be said that the best detector performance is in the middle region with a 6% resolution of the 122-KeV peak, which is quite good for test detectors without a guard ring such as these. It is determined that this middle region has a zinc concentration of ∼9–11%, which varies slightly from the original concentration of 10%. The differences in the performance characteristics are discussed, and defect distribution within the crystal as the main source of the variation is suggested. Also, based on the results, it is believed that the role of indium is essentially to compensate for the vacancies in the crystal and, therefore, is secondary to the crystalline properties and impurities within the boule. Overall, it is believed that crystalline defects and inclusions play a greater role in determining the performance characteristics of CdZnTe radiation detectors.     

Application of eddy current technique to vertical bridgman growth of CdZnTe

The eddy current technique was used to reveal the interface shape during vertical Bridgman growth of CdZnTe and to follow changes in the properties of the solidified ingot as it was cooled to room temperature after growth. Experiments were performed where partially solidified charges were decanted to show the interface shape. Eddy current analysis of the partially solidified charge indicated a concave interface shape in qualitative agreement with the shape of the decanted ingot. However, due to noise, interference, and possibly the inhomogeneous nature of the melt, only some of the eddy current signals could be analyzed empirically for interface shape; absolute values of conductivity could not be calculated from the eddy current data. Eddy current measurements made to follow changes in conductivity during post-growth cooling showed a minimum in the data during an 800°C annealing step indicative of a transition in the electrical properties of the ingot. On further cooling, a dramatic increase in the bulk conductivity of the ingot was noted. Such a transition can probably be described as a Mott transition.     

Parameter Optimization of CdZnTe Crystal Growth Simulated by Finite Element Method

During the crystal grown by VBM, the solid/liquid interface configurations greatly influence the quality of as-grown crystals. In this paper, finite element method (FEM) was used to simulate the growth process of CdZnTe crystal. The effects of different crucible moving rates and temperature gradient of adiabatic zone on crystal growth rate and solid-liquid interface configuration were studied as well. Simulation results show that when crucible moves at the rate of about 1 mm/h, which is nearly equal to crystal growth rate, nearly flat solid/liquid interface and little variation of axial temperature gradient near it can be attained, which are well consistent with the results of experiments. CdZnTe crystal with low dislocation density can be obtained by employing appropriate crucible moving rate during the crystal growth process.     

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.     

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.     

CdZnTe晶片中的Zn组分的研究

用X 射线双晶衍射、光致发光谱和红外透射光谱研究了CdZnTe 晶体中的Zn 的组分.研究表明透射光谱的Syllaios经验公式结果与X 射线双晶衍射和光致发光谱精确测量结果对比,偏差小于4% .透射光谱可以做为测量Zn 组分的常规方法     

碲锌镉缓冲层液相外延技术的研究

文章报道了采用液相外延方法，在碲锌镉衬底上进行碲锌镉薄膜缓冲层生长的情况，并且采用X光双晶衍射仪、X光形貌仪、红外傅里叶光谱仪、二次离子质谱仪等手段对碲锌镉薄膜进行了表征，碲锌镉薄膜具有较好地组分及均匀性，晶体结构质量也较好。采用碲锌镉缓冲结构生长了碲镉汞液相外延片，其碲锌镉与碲镉汞薄膜界面附近的杂质得到了有效的控制。     

碲锌镉缓冲层液相外延技术的研究

文章报道了采用液相外延方法，在碲锌镉衬底上进行碲锌镉薄膜缓冲层生长的情况，并且采用X光双晶衍射仪、X光形貌仪、红外傅里叶光谱仪、二次离子质谱仪等手段对碲锌镉薄膜进行了表征，碲锌镉薄膜具有较好地组分及均匀性，晶体结构质量也较好。采用碲锌镉缓冲结构生长了碲镉汞液相外延片，其碲锌镉与碲镉汞薄膜界面附近的杂质得到了有效的控制。     

未掺杂CdZnTe与掺铟CdZnTe晶体的热处理

对于未掺杂Cd0.9Zn0.1Te晶片,采用在Cd/Zn气氛下,以In作为气相掺杂源进行热处理;而对于低阻In-Cd0.9Zn0.1Te晶片,则采用在Te气氛下进行热处理.分别研究了不同的热处理条件,包括温度、时间、pIn或pTe等对晶片电学性能、红外透过率以及Te夹杂/沉淀相的影响.结果表明,在Cd/Zn气氛下适当的掺In热处理和在Te气氛下适当的热处理均有效地提高了晶片的电阻率,分别达到2.3×1010和5.7×109Ω·cm,同时晶片的其他性能也得到明显改善.     

Molecular beam epitaxy growth of high-quality HgCdTe LWIR layers on polished and repolished CdZnTe substrates

We report here molecular beam epitaxy (MBE) mercury cadmium telluride (HgCdTe) layers grown on polished and repolished substrates that showed state-of-the-art optical, structural, and electrical characteristics. Many polishing machines currently available do not take into account the soft semiconductor materials, CdZnTe (CZT) being one. Therefore, a polishing jig was custom designed and engineered to take in account certain physical parameters (pressure, substrate rotational frequency, drip rate of solution onto the polishing pad, and polishing pad rotational velocity). The control over these parameters increased the quality, uniformity, and the reproducibility of each polish. EPIR also investigated several bromine containing solutions used for polishing CZT. The concentration of bromine, as well as the mechanical parameters, was varied in order to determine the optimal conditions for polishing CZT.     

Linear x-ray detector array made on bulk CdZnTe for 30∼100 keV energy

A CdZnTe detector grown by the high pressure Bridgman (HPB) growth technique was tested using high energy x-rays (30∼100 keV), and the performance was compared with a commercially available Nal scintillating detector of 5 cm thickness. The charge collection efficiency of a CdZnTe detector is as high as 90% at relatively low electric field, 600 V/cm. At high x-ray photon energies, the detection efficiency is reduced due to the thickness of the CdZnTe. A 32 channel linear array was fabricated on 1.2∼1.7 mm thick CdZnTe, of which the detector area was 175 × 800 μm² and the pitch size 250 μm. The measured dark current for the 16 element detector was as low as 0.1 pA at 800 V/cm with an excellent uniformity. Energy spectra were measured using a Co⁵⁷ radiation source. A small pixel effect and charge sharing were observed. The energy resolution was improved and compared with the large area detector. The array detector gave an average 5.8% full-width-half-maximum (FWHM) at 122 keV photopeak. The large area detector of the same material before fabrication exhibited a low energy tail at the photopeak, which limits the photopeak FWHM to 8%.     

碲锌镉晶体定向研究

碲锌镉材料是制备高性能碲镉汞红外焦平面探测器不可或缺的衬底材料.液相外延工艺和分子束外延工艺分别需要使用(111)晶面和(211)晶面碲锌镉衬底制备碲镉汞薄膜材料.低偏角、高精度衬底的选取有利于高质量碲镉汞外延层的获得.介绍了孪晶线快速定向法、使用X射线衍射仪(X-Ray Diffractome-ter,XRD)定向法...     

Recent progress in CdZnTe crystals

CdZnTe crystals were grown by vertical gradient freezing (VGF) method with a Cd reservoir for controlling the Cd pressure under conditions such that the crystals are in equilibrium with a Cd vapor corresponding to the minimum deviation from stoichiometry. The precipitate size became smaller by a post growth annealing method in the VGF furnace after the crystal growth without using wafer annealing. The size became less than 2 μm. Precipitate-free crystals were also grown by controlling the cooling method. In addition, the carrier concentration of p-type CdZnTe crystals was reduced using polycrystals grown in pBN boats. We have found that the carrier concentration of p-type ingots is dependent on Na and Li impurity concentrations.     

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.