Uniformity and reproducibility studies of low-pressure-grown Cd<Subscript>0.90</Subscript>Zn<Subscript>0.10</Subscript>Te crystalline materials for radiation detectors

A large number of room-temperature detectors have been produced from CdZnTe crystals grown with 10% Zn and 1.5% excess tellurium by the low-pressure, vertical-Bridgman technique. Radiation spectra obtained by these crystals using a ²⁴¹Am source reveal the characteristic 59.5-keV line as well as the six low-energy peaks, which include the Cd and Te escape peaks. Similarly, ⁵⁷Co spectra obtained also show a very well-defined 122-keV peak with a 3:1 peak-to-valley ratio. Seven CdZnTe crystals have been grown for reproducibility studies. Four of these crystals have resistivities over 1E9 Ω-cm. Considering that the indiumdoping level is on the order of 2E15 cm⁻³, the reproducibility is excellent. The theoretical basis of the high-resistivity phenomenon in CdZnTe is discussed in reference to a previous paper. The uniformity of these 6-in.-long CdZnTe crystals is studied, and various measurements are carried out, both laterally and vertically, along the boule. It is determined that, in general, roughly a 3.5-in. section near the middle of the 6-in. boule has sufficient resistivity for producing radiation detectors. This nonuniformity along the vertical direction is caused mostly by the composition change of Cd, Zn, Te, and In-doping level in the growth melt caused by differences in the segregation coefficients of these elements. Although, variations in resistivity are seen across some of the wafer slices, most show very good uniformity with high breakdown voltage. Some of the variations are attributed to the different grains within the boule. Similar results are seen in the measured radiation spectra obtain on 4 mm × 4 mm × 2 mm samples from different locations across the wafer, where some samples show well-resolved secondary peaks, while others display only the primary spectral lines.     

Progress in CdZnTe substrate producibility and critical drivers of IRFPA yield originating with CdZnTe substrates

With the goal of maximizing the yield of infrared focal plane arrays (IRFPAs), Santa Barbara Research Center’s (SBRC) Infrared Materials Producibility Program (IRMP) has focused on assessing and improving the quality, yield, and throughput of CdZnTe substrates. A baseline detector lot was fabricated to identify the critical drivers of IRFPA yield coming from the substrates and to evaluate the quality and yield of the current vendor base for CdZnTe substrates. Substrate induced defects and impurities that can potentially affect device performance and operability were carefully mapped out in detail on 44 × 67 mm² size substrates, received from IRMP substrate vendors as well as SBRC. This paper will report on the correlations found between this substrate characterization data base and the IRFPA level defect distributions. Key results from these correlation studies are: (1) extended defects found on the substrates with the Nakagawa etch correlated well with responsivity reduction in the final IRFPA; (2) cross-hatch patterns that were evident in the responsivity map correlated well with similar features seen by x-ray topography on LPE double layers; and (3) a possible correlation of device performance (leakage current at 78K) with copper and lithium impurities in the substrate. Recent initiatives toward improving the quality and yield of the substrate growth process have focused on improving purity in the pre-growth charge preparation, modification of growth parameters to reduce defects and scaling up of the vertical Bridgman growth process from its current 67 mm diameter boule size to 92 mm diameter boules. Promising initial results from the large diameter boule growth process will be shown. The 92 mm diameter CdZnTe boule (6 kg charge) shows two predominant single crystal grains encompassing 75% of boule volume. Defect characterization of boules grown under baseline and modified conditions is discussed.     

Growth characteristics of CdZnTe layers grown by metalorganic vapor phase epitaxy using dimethylzinc, dimethylcadmium, diethyltelluride, and dimethyltelluride as precursors

Growth characteristics of (100)-oriented CdZnTe layers grown by atmospheric-pressure metalorganic vapor phase epitaxy have been studied using dimethylzinc (DMZn), dimethylcadmium (DMCd), diethyltelluride (DETe), and dimethyltelluride (DMTe) as precursors. Variations of Zn composition and layer growth rate were examined by changing the DMZn supply ratio, defined as DMZn/(DMCd+DMZn), where the precursors are expressed in appropriate units of flow rate, from 0 (no DMZn) to 1.0 (no DMCd), while keeping the total group II supply rate constant. The growth rate of CdZnTe layers was found to decrease monotonically with increase of the DMZn supply ratio. On the other hand, the Zn composition x of grown layers increased gradually up to x=0.04 with increase of the DMZn supply ratio from 0 to 0.8, beyond which the Zn composition increased abruptly to ZnTe. The abrupt transition of Zn composition was suppressed by increasing the VI/II ratio. The growth mechanism of CdZnTe layers was studied based on the observed growth characteristics of CdTe and ZnTe. A higher desorption rate from the growth surface for Zn species than for Cd species, and a higher rate of CdTe formation than ZnTe formation are believed to cause the observed growth characteristics. CdZnTe layers with high crystal quality were grown in a wide range of Zn compositions. The full-width at half-maximum values for x-ray double-crystal rocking-curve measurements were lower than 320 arc-sec for x<0.3 and x>0.75.     

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.     

碲锌镉晶体中夹杂问题的实验研究

采用红外透射显微镜检测和研究了热解氮化硼(PBN)坩埚生长碲锌镉晶体(CdZnTe)中的缺陷—夹杂,并对夹杂的影响因素进行了分析。研究发现:不同原料的化学配比、自由空间体积、饱和蒸汽压、晶体生长温场对晶体中夹杂的种类、形状、密度和尺寸都存在着影响,通过一系列工艺的改进可以获得夹杂合格的碲锌镉晶体。     

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.     

磁光—光致发光分析CdZnTe单晶带边浅杂质能级

通过对Bridgeman方法生长的CdZnTe单晶样品进行光致发光(Photoluminescence,PL)光谱测量,发现CdZnTe样品表面Te沉淀物的存在明显影响能量低于1.5 eV的深能级发光过程.进一步对CdZnTe晶锭的不同位置取样进行低温变磁场光致发光光谱测试,获得高分辨光谱信息.拟合分析结果表明:(1)在不含Te沉淀物的CdZnTe样品内部存在应力分布,并因此导致轻、重空穴带分裂;(2)1.57 eV发光特征源于浅施主杂质与价带间的复合过程.     

Growth and characterization of large Nd,Cr:GSGG crystals forhigh-average-power slab lasers

Nd,Cr:GSGG crystal boules up to 13 cm in diameter and 20.5 cm long have been grown by the Czochralski method. Several problems with Nd,Cr:GSGG growth were identified, and solved separately but not all at once; these problems included spiral boule growth, 1-μm absorption loss, iridium on the melt surface, iridium inclusions, boule cracking, dislocation, and fine scattering (smoke). In the grown crystals parameters relevant to their eventual use as gain elements for large slab lasers were measured including optical homogeneity, birefringence, absorption loss at 1 μm, scattering loss, and iridium inclusions. The optical homogeneity of the boules is good, except for a radial gradient in refractive index of about 10^-5 cm^-1. The birefringence is low (<3 nm/cm) in slabs cut from the boules. Scattering losses in the boules range from 0.01 to 0.08 cm^-1. This absorption has been reduced to <0.0025 cm^-1 in small samples using reducing heat treatments, but appropriate treatment conditions for full-scale slabs have yet to be determined     

高温热处理对碲锌镉衬底中Zn组分的影响

随着高性能碲镉汞红外探测器对碲锌镉衬底质量要求的不断提高,研制高质量的碲锌镉衬底材料势在必行。本文采用真空高温热处理温场与源控制的衬底高温热处理工艺,系统地研究了主要热处理参数对碲锌镉衬底中Zn组分的影响,给出了Zn组分控制的热处理经验公式,初步获得了碲锌镉衬底中Zn组分的成分控制和高组分均匀性分布的方法,提高了碲锌镉衬底质量。     

Macro- and microsegregation of Zn in bridgman-grown CdZnTe

One of the remaining problems in the use of CdZnTe material as substrates in liquid phase epitaxy (LPE) of Cd_xHg_1-xTe (CMT) layers is that of variation in lattice matching, i.e. Zn content, across substrates. This wil become increasingly important in the future as larger focal plane arrays of infrared detectors are required. The basic Bridgman growth process for CdTe/Cd_0.96Zn_0.04Te has been extended by applying the accelerated crucible rotation technique (ACRT). A marked reduction in axial Zn segregtion is seen in 50 mm diameter ACRT material, but this effect is smaller in the case of 75 mm diameter crystals. Radial variations in Zn content are small in both sizes of crystal, demonstrating the benefits obtained from ACRT stirring. Both macro- and microsegregation effects have been studied in these crystals in an attempt to understand the growth mechanism. Zinc distributions have been assessed by near-infrared transmission, X-ray lattice parameter measurements, atomic absorption spectrometry (AAS) and Auger electron spectrometry (AES). The last technique was used for the microsegregation studies, while AAS provides the absolute calibration for Zn content. Comparisons with segregation behaviour found in the literature will be given. It will be shown that the low temperature gradient and low growth rate lead to a degree of supercooling in the first-to-freeze region and this leads to significant Zn segregation in both radial and axial directions. As the crystals reach full diameter, the radial variation is decreased, presumably by the action of the ACRT, and axial segregation is also reduced.     

Spectroscopic evaluation of n-type CdZnTe gamma-ray spectrometers

This work focuses on the evaluation of the spectroscopic performance of n-type CdZnTe gamma-ray spectrometers, grown by a modified horizontal Bridgman Technique developed at IMARAD Imaging Systems Ltd. Two types of devices are studied: (i) detector arrays grown and produced by IMARAD and employing ohmic indium contacts and (ii) detectors and arrays fabricated at Technion in crystals provided by IMARAD, employing different types of contacts. Alpha particle spectroscopy as well as gamma-ray spectroscopy is used to evaluate and characterize the energy resolution of gamma-ray spectrometers fabricated on n-type CdZnTe grown by a modified horizontal Bridgman and doped with indium. The electron and hole mobility lifetime products of the n-type CdZnTe material grown by IMARAD are estimated by measuring the dependence of charge collection efficiency upon the bias voltage, using a calibrated multichannel analyzer. The measured results indicate that the average electron and hole mobility-lifetime products are, respectively, of the order of μ_nτ_n=(1–2)·10⁻³ cm²/V and μ_pτ_p=6·10⁻⁶ cm²/V. The measured energy resolution of 122 keV photons is −(5–6)% when the source is not collimated and is reduced to −4.5% when the source is collimated. These results are obtained with ohmic cathode as well as with a rectifying cathode. A statistical model for the calculation of the pulse height spectra as a function of photon energy, electron and hole mobility-lifetime products and applied electric field, which has been recently reported in Applied Physical Letters, is used to determine the role of incomplete charge collection in the spectral performance of the n-type CdZnTe spectrometers. The comparison between the measured and modeled results indicates that the dark noise, cross talk and non-uniformity are the main limiting factors of the spectral performance of the n-type spectrometers rather than incomplete charge collection. The good spectroscopic performance of the arrays under study is attributed to an adequate hole mobility lifetime for the geometry of the pixilated arrays. The study indicates that the n-type CdZnTe spectrometers are useful for a wide range of imaging applications.     

Advancements in THM-Grown CdZnTe for Use as Substrates for HgCdTe

The focus of this work is to evaluate the suitability and substrate potential of Cd_0.9Zn_0.1Te and Cd_0.96Zn_0.04Te crystals grown by the traveling heater method (THM). THM-grown Cd_0.9Zn_0.1Te crystals used for gamma spectroscopy have shown very good spectral performance owing partly to the very low concentration of Te inclusions and precipitates. Inspection in the infrared (IR) of annealed THM-grown CdZnTe wafers reveals no inclusions >3 μm, and Fourier-transform infrared measurements show IR transmission values in excess of 60%. Wafer etch pit density values are typically less than 4 × 10^?4 pits/cm², and double-crystal x-ray rocking-curve measurements show full-width at half-maximum values approaching 40 arcsec. 〈211〉 wafers have been produced with off orientation within 0.3°. (111)-Oriented, seeded THM growth runs have the ability to provide 10 60 mm × 60 mm × 2 mm wafers from a 75-mm-diameter boule or 20 90 mm × 90 mm × 2 mm wafers from a 100-mm-diameter boule.     

HgCdTe组分异质结的生长与表征

采用富碲水平推舟液相外延生长方式,在(111)晶向的碲锌镉衬底上生长了双层组分异质HgCdTe外延薄膜,并对生长后的薄膜质量进行了评价.使用染色法和红外透射光谱数值拟合的方法,对两层薄膜的厚度进行了表征,并建立了一个双层薄膜纵向组分分布模型;对材料的电学参数进行测量的结果显示,双层异质液相外延样品中长波层的载流子迁移率较之单层液相外延样品略高,原因可能是中波覆盖层对长波外延层起到了钝化作用.     

Study of contacts to CdZnTe radiation detectors

This study characterizes, for the first time, contacts to CdZnTe radiation detectors by measuring the dark noise spectra as a function of the applied bias. The noise currents are correlated with the dc dark current-voltage characteristics of CdZnTe x-ray and gamma-ray detectors. In order to identify and separate the role of the contacts in the overall performance, the measured noise phenomena is correlated with detector configuration and contact design as well as the growth method of the CdZnTe crystals, contact technology, and passivation. Several contact technologies (electroless gold, and a number of evaporated metallic contacts including gold, indium, zinc, titanium, aluminum, and platinum contacts) are compared. Contacts to CdZnTe crystals grown by high pressure Bridgman are compared with contacts to CdZnTe crystals grown by modified Bridgman. Contacts of resistive detectors as well as of Schottky detectors are reported. Large area symmetric contacts are compared with small area pixelized contacts. The role of the metallization used for contacts, the role of surface effects and passivation, and the role of contact design are discussed.     

Effect of constrained growth on defect structures in microgravity grown CdZnTe boules

In a microgravity environment obtainable in an orbiting space shuttle, it is possible to virtually eliminate gravity related effects such as buoyancy driven convection and hydrostatic forces thus providing an ideal environment for diffusion-controlled, containerless crystal growth processes. Under such conditions, it is possible to investigate the effects of gravity independent growth parameters on crystal growth. Studies of CdZnTe boules grown on space shuttle mission USML-1 revealed that regions of the boules grown with wall contact were associated with a higher defect density than regions grown with partial or no wall contact. Defect densities in certain regions grown without wall contact were as low as 5 × 10²/cm² to 1.2 × 10³/cm². More detailed studies on the effects of wall contact were sought in the USML-2 mission. Two CdZnTe boules (GCRC-1 and GCRC-2) were grown by the seeded Bridgman-Stockbarger method. Boule GCRC-1 was grown under constrained conditions to force full wall contact while boule GCRC-2 had a tapered geometry designed to minimize wall contact. Defect distributions in the boules were investigated by synchrotron white beam x-ray topography. The sample GCRC-1 was characterized by the presence of large inhomogeneous strains, numerous grains and twins, all of which are caused by effects related to wall contact. On the other hand, a part of the boule GCRC-2 that grew free from wall contact revealed minimum surface strains, the absence of twins and a very high structural uniformity. Results clearly verify that ampoule wall contact plays an important role in determining the incidence of crystal imperfections.     

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%.     

温度对CdZnTe载流子输运特性和能谱特性的影响

采用垂直布里奇曼法生长的CdZnTe(CZT)单晶,制备出单平面探测器。在210～300K温度范围内,测试了不同外加偏压作用下探测器的漏电流,并计算了低压下CZT的体电阻率。同时,在不同电场作用下,测试了CZT探测器对未经准直的241 Am@5.48 MeVα粒子脉冲响应信号的上升时间,计算出电子迁移率为1 360cm2/V-1s-1,并进一步推算出电子寿命随温度的变化规律。在220～300K温度范围内,对比了CZT探测器对241 Am@59.5keVγ射线的能谱响应结果,分析了载流子传输特性及器件性能随温度的变化规律。结果表明,在298～253K温度范围内,降低温度可以提高晶体的体电阻率,减少探测器工作时的漏电流,进而提高探测器的能量分辨率;但当温度低于253K时,电子寿命τe加剧减小,此时由上升时间起伏而引起的全能峰的展宽不能被忽略,导致探测器性能恶化。     

Investigation of Artificial Forced Cooling in the Bridgman Crystal Growth of Cadmium Zinc Telluride

The effects of artificial forced cooling on the solid–liquid interface and on solute segregation were investigated by modeling the vertical Bridgman method for the single-crystal growth of CdZnTe, taking into consideration effects such as increasing the axial outward heat flux from the crucible bottom, the radial outward heat flux from the crucible wall, and the carbon film thickness on the crucible inner wall. Axial artificially forced cooling noticeably increases convection and the temperature gradient in the melt next to the solid–liquid interface, and substantially reduces interface concavity at the initial solidification stage. Interface concavity increases a little when the solidification proceeds further, however. Axial artificially forced cooling reduces radial solute segregation of the initial segment of the grown crystal and slightly increases the solute iso-concentration segment. Radial artificially forced cooling enhances melt convection substantially, affects solid–liquid interface concavity only slightly, and hardly affects solute segregation in the grown crystal. Doubling the carbon film thickness weakens convection of the melt in front of the interface, substantially increases interface concavity, and hardly affects solute segregation in the grown crystal.     

CdZnTe晶体缺陷的交流阻抗谱研究

宽禁带II-VI族半导体化合物碲锌镉(CdZnTe)晶体是制备室温X和γ射线探测器的理想半导体材料,但其晶体缺陷特性对探测器性能有重要的影响,一直是人们研究的热点与难点。文中采用垂直布里奇曼法生长了CdZnTe晶锭,XRD测试表明晶片呈现(111)取向。通过测试样品不同温度下的交流阻抗谱,研究了晶体缺陷的阻抗特性。结果表明,制备的CdZnTe单晶具有负温度系数效应,化学法制备的Au电极与晶片之间形成了欧姆接触,没有出现电极界面和晶界对阻抗谱曲线影响,晶粒导电机制占主导。利用Arrhenius方程拟合曲线获得晶体缺陷的激活能为0.48 eV,表明晶体缺陷以Cd空位为主。     

Direct growth of CdZnTe/Si substrates for large-area HgCdTe infrared focal plane arrays

Direct epitaxial growth of high-quality 100lCdZnTe on 3 inch diameter vicinal {100}Si substrates has been achieved using molecular beam epitaxy (MBE); a ZnTe initial layer was used to maintain the {100} Si substrate orientation. The properties of these substrates and associated HgCdTe layers grown by liquid phase epitaxy (LPE) and subsequently processed long wavelength infrared (LWIR) detectors were compared directly with our related efforts using CdZnTe/ GaAs/Si substrates grown by metalorganic chemical vapor deposition (MOCVD). The MBE-grown CdZnTe layers are highly specular and have both excellent thickness and compositional uniformity. The x-ray full-width at half-maximum (FWHM) of the MBE-grown CdZnTe/Si increases with composition, which is a characteristic of CdZnTe grown by vapor phase epitaxy, and is essentially equivalent to our results obtained on CdZnTe/GaAs/Si. As we have previously observed, the x-ray FWHM of LPE-grown HgCdTe decreases, particularly for CdZnTe compositions near the lattice matching condition to HgCdTe; so far the best value we have achieved is 54 arc-s. Using these MBE-grown substrates, we have fabricated the first high-performance LWIR HgCdTe detectors and 256 x 256 arrays using substrates consisting of CdZnTe grown directly on Si without the use of an intermediate GaAs buffer layer. We find first that there is no significant difference between arrays fabricated on either CdZnTe/Si or CdZnTe/GaAs/Si and second that the results on these Si-based substrates are comparable with results on bulk CdZnTe substrates at 78K. Further improvements in detector performance on Si-based substrates require a decrease in the dislocation density.