Fabrication and Characterization of MOVPE-Grown CdTe-on-Si Heterojunction Diode-Type Gamma-Ray Detectors

We report on the growth of very thick (>260 μm) high-crystalline-quality single-crystal CdTe epitaxial films on (211) Si substrates in a metalorganic vapor-phase epitaxy reactor, and the development of gamma ray detectors and their radiation detection properties. Films were grown with a high growth rate varying from 40 μm/h to 70 μm/h. A heterojunction diode was fabricated by growing a 90-μm-thick CdTe layer on an n ⁺-Si substrate, which exhibited good rectifying behavior and had a low reverse bias leakage current of 0.18 μA/cm² at 100 V bias. The diode clearly demonstrated its gamma radiation detection capability by resolving energy peaks from the ²⁴¹Am radioisotope during room-temperature measurements. By cooling the diode detector to −30°C, the leakage current could be reduced by three orders of magnitude from the room-temperature value. At this operating condition dramatic improvements in the pulse height spectrum were observed.     

Thermoelectric Effect Spectroscopy and Photoluminescence of High-Resistivity CdTe:In

Thermoelectric effect spectroscopy and photoluminescence techniques were used to study the defect levels in samples from three crystals of CdTe:In grown by the vertical gradient freeze method. The main goal of the investigation was to study defects, which strongly trap charge carriers or act as recombination centers in order to eliminate them from the technological process. The main difference among detecting and non-detecting samples was the absence of electron traps with a very high capture cross-section and energy 0.6 eV to 0.7 eV, which act as lifetime killers even at low concentrations. Recently published ab initio calculations show a complex of Te antisite and Cd vacancy within this energy range.     

Synchrotron X-ray Based Characterization of CdZnTe Crystals

Synthetic CdZnTe (CZT) crystals can be used for the room temperature-based detection of gamma radiation. Structural/morphological heterogeneities within CZT, such as secondary phases (namely, precipitates and inclusions), can negatively affect detector performance. We used a synchrotron-based x-ray technique, specifically extended x-ray absorption fine-structure (EXAFS) spectroscopy, to determine whether there are differences on a local structural level between intact CZT of high and low radiation detector performance. These studies were complemented by data on radiation detector performance and transmission infrared (IR) imaging. The EXAFS studies revealed no detectable local structural differences between the two types of CZT materials.     

Screening Effects in High Resistivity CdTe for X-ray and Gamma Ray Detectors

The possibility to estimate an upper limit of concentrations of defects forming deep levels in high-resistivity CdTe is discussed based on evaluation of room-temperature photocurrent spectra. A model explaining the shift of the maximum of photocurrent with the applied electric field as a consequence of screening effects caused by space charge accumulated on deep levels is presented. Theoretical calculations show that a maximum concentration of deep levels leading to the observed shift of the photocurrent maximum is less than ∼10¹³ cm⁻³ for typical capture cross sections of electrons and holes. This result supports the models assuming formation of a high-resistivity state with a minimum deep level doping.     

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.     

Theory of te precipitation and related effects in CdTe Crystals

A thermodynamic calculation is presented which explains the origin of often reported large stress fields in and around Te precipitates and associated punching of dislocation loops in star like patterns. The calculation is based on the consideration that the stoichiometric deviation in Te saturated crystals are accommodated by Te interstitials, Cd multivacancy complexes and Te antisites; the Te interstitials are most mobile of them, and dominate the precipitation process; and the precipitates when they appear first are in droplet form. The droplet state is modelled on the basis of the Lennard-Jones 6-12 interaction potential. The droplet growth is envisaged to occur via the following two processes operating in tandem: first, capture of excess Te interstitials to cause droplet overpressurization, and then, punching of interstitial dislocation loops whenever the pressure exceeds a threshold value. The flow of Te interstitials into the droplets is driven by the difference between their formation energies in the lattice and the free energy change of the droplets per Te atom added. It is shown that the achievable droplet pressures far exceed that required for the loop punching and sustaining the growth cycle. The occurrence of varying precipitate morphologies are explained, and the possibility of nucleating high presssure Te phases is examined. Some experimental evidence is also presented which corroborate the theoretical arguments involved.     

Direct growth of high-quality thick CdTe epilayers on Si (211) substrates by metalorganic vapor phase epitaxy for nuclear radiation detection and imaging

Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 μm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 μm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n⁺-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.     

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.     

Assessment of the purity of cadmium and tellurium as components of the CdTe-based substrates

In order to assess the purity of their Cd and Te components, CdTe crystals have been grown by Bridgman using commercial batches of elements of purity ranging from 5N to 6N⁺. The assessment has been achieved through the classical optical and electrical techniques that solid state physics offers, on as-grown and annealed crystals. Some contamination by residual acceptors like Cu, Li, and P is shown to occur during the high temperature growth, mainly if uncoated silica tubes are used, as is emphasized within a comparison between the electrical and optical properties of crystals grown by melt-growth or at lower temperature by solution-growth.     

Development of nuclear radiation detectors based on epitaxially grown thick CdTe layers on n<Superscript>+</Superscript>-GaAs substrates

A CdTe/n⁺-GaAs heterojunction diode for a room-temperature nuclear radiation detector has been developed and demonstrated. The heterojunction diode was fabricated by growing a 2–5-μm-thick iodine-doped n-CdTe buffer layer on the n⁺-GaAs substrates, followed by about 100-μm-thick undoped p-like single crystalline CdTe layer using metalorganic vapor-phase epitaxy. The n-type buffer layer was found to be essential to improve the junction property of the diode detector. The diode detectors exhibited good rectification property and had the reverse leakage currents typically from 1 μA/cm² to 5 μA/cm² at 40 V bias. The detector clearly demonstrated its energy resolution capability by resolving the 59.54-keV gamma peak from the ²⁴¹Am radioisotope during the radiation detection test.     

固体可调谐氟化物激光晶体的生长与性能

文章总结了LiCaAlF6(LiCAF)、LiSrAlF6(LiSAF)以及YLiF4(YLF)等具有重要应用价值的固体可调谐氟化物激光晶体的生长、结构和性能,讨论了Nd^3 、Cr^3 等离子掺杂氟化物激光晶体的发光特性及其在激光器上的应用,分析了这些激光晶体的市场及应用前景。     

PINT铁电晶体的助熔剂法制备工艺与介电性能

用助熔剂法制备了准同型相界附近的弛豫铁电体基钛铌铟酸铅（Pb（In1／2Nb12）0.63Ti0.37O3，简称PINT）铁电单晶体，研究了四氧化三铅（Pb3O4）与二氟化铅（PbF2）助熔剂对钙钛矿结构相稳定性的不同作用。用四氧化三铅（Pb3O4）和三氧化二硼（B2O3）作为助熔剂获得尺寸达5mm的纯钙钛矿相结构PINT单晶体，利用显微分析方法、X射线衍射技术研究了单晶体的微观形貌和相结构，测量了〈100〉取向单晶体样品的介电温度谱。     

Multiple-Scale Analysis of Charge Transport in Semiconductor Radiation Detectors: Application to Semi-Insulating CdZnTe

The transport, trapping, and subsequent detrapping of charge in single crystals of semi-insulating cadmium zinc telluride (CdZnTe) has been analyzed using multiple-scale perturbation techniques. This method has the advantage of not only treating impulse charge generation typical in spectroscopic analysis, but also a large class of continuous generation sources more relevant to high-flux x-ray imaging applications. We first demonstrate that the multiple-scale solutions obtained for small-current transients induced by an impulse generation of charge are consistent with well-known exact solutions. Further, we use the multiple-scale solutions to derive an analytic generalization of the Hecht equation that incorporates detrapping over times much longer than the carrier transit time (i.e., delayed signal components). The method is then applied to a continuous charge generation source that approximates that of an x-ray source. The space–time solutions obtained are relevant to detector design in high-flux x-ray imaging applications. Throughout this work the multiple-scale solutions are compared with exact solutions as well as full numerical solutions of the fundamental charge conservation equations.     

New developments in CdTe and CdZnTe detectors for X and γ-ray applications

There has been considerable recent progress in II-VI semiconductor material and in methods for improving performance of the associated radiation detectors. New high resistivity CdZnTe material, new contact technologies, new detector structures, new electronic correction methods have opened the field of nuclear and x-ray imaging for industrial and medical applications. The purpose of this paper is to review new developments in several of these fields. In addition, we will present some recent results at LETI concerning first the CdTe 2-D imaging system (20 × 30 mm² with 400 × 600 pixels) for dental radiology and second the CdZnTe fast pulse correction method applied to a 5 × 5 × 5 mm³ CdZnTe detector (energy resolution = 5% for detection efficiency of 85% at 122 keV) for medical imaging.     

Growth of large size AgGaGeS4 crystal for infrared conversion

Single crystals of AgGaGeS4 (AGGS) were grown in a modified Bridgman furnace with 25 mm in diameter and 70 mm in length. The transmission spectra of as-grown AGGS slices were measured on a Hitachi 270-30 spectrophotometer, the fabricated device crystal was 5 mm×5 min×3.5 mm in dimension and its absorption was 0.04-0. 15 cm-1. Frequency doubling of 2.79 and 8 μm laser radiation were investigated using fabricated device crystals with thicknesses of 3.5 and 2.7 mm respectively.     

两种新型声光晶体的生长及特性

论述了两种新型声光晶体ＰｂＢｒ２和Ｈｇ２Ｃｌ２的原料提纯、生长方法及晶体的光学与声学特性。     

Excimer Laser Etching Process of CdTe Crystals for Formation of Deep Vertical Trenches

Deep isolation trenches with high aspect ratio were formed on CdTe crystals using the excimer laser etching technique. Vertical trenches 80-μm deep and 55-μm wide were formed in 5 min by irradiating a laser on the CdTe crystal through a contact-type metal mask in vacuum. Surface chemistry of the laser-irradiated CdTe crystal was examined by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and photoluminescence (PL) to determine the extent of laser-induced damage. It was found that the damaged layer remains confined to the thin surface layer in the submicron range, which could be easily removed by a light Br-methanol etch.     

Comparison of normal and inverted band structure HgTe/CdTe superlattices for very long wavelength infrared detectors

The type III band alignment of HgTe/CdTe superlattices leads to the interesting possibility of achieving very long wavelength infrared (VLWIR) (15 μm and longer) cutoff wavelengths with either normal (HgTe layer thickness less than about 70 ? for CdTe layer thickness of 50 ?) or inverted (HgTe thickness greater than about 70 ?) band structures. The inverted band structure superlattices promise even greater cutoff wavelength control than the normal band structure ones. However, the electronic band gaps of inverted band structure superlattices are substantially less than their optical band gaps, leading to large thermal carrier concentrations even at temperature as low as 40 K. These high carrier concentrations in turn give rise to more rapid Auger recombination than normal band structure superlattices with the same cutoff wavelengths. We conclude that the highest performance is expected from VLWIR HgTe/CdTe superlattice-based detectors with normal band structure absorber layers.     

Characterization of CdTe for HgCdTe surface passivation

The objectives of this work are to study the physical and chemical structure of CdTe films using secondary ion mass spectrometry (SIMS) and atomic force miroscopy (AFM) and to demonstrate the usefulness of these analytical techniques in determining the characteristics of CdTe-passivation films deposited by different techniques on HgCdTe material. Three key aspects of CdTe passivation of HgCdTe are addressed by different analytical tools: a) morphological microstructure of CdTe films examined by atomic force microscopy; b) compositional profile across the interface determined by Matrix (Te)—SIMS technique; c) concentration of various impurities across the CdTe/HgCdTe structure profiled by secondary ion-mass spectrometry.     

掺杂硅酸铋晶体的生长及吸收光谱的研究

用提拉法生长了质量较高的Ａｌ∶Ｂｉ１２ＳｉＯ２０和Ｃｒ∶Ｂｉ１２ＳｉＯ２０（简称Ａｌ∶ＢＳＯ和Ｃｒ∶ＢＳＯ）晶体，测定了掺杂ＢＳＯ晶体的吸收光谱。首次发现：Ａｌ∶ＢＳＯ（摩尔比）为０．１％的晶体的透过率（８０％）远高于纯ＢＳＯ晶体的透过率（７０％）；Ｃｒ∶ＢＳＯ晶体中除Ｃｒ３＋外，还存在少量Ｃｒ４＋