Effects of excess tellurium on the properties of CdZnTe radiation detectors

Room-temperature radiation detectors have been fabricated on high-resistivity, indium-doped Cd_0.90Zn_0.10Te crystals grown under different amounts of excess Te. The effects of the excess Te on the properties of the detectors are explained by a simple model using only three parameters: the density of Cd vacancies, the density of Te antisites (Te at Cd sites), and the deep level of doubly ionized Te antisites. The best detectors, which can resolve the low-energy Np-L and Te-K peaks as well as Cd and Te escape peaks of ²⁴¹Am, are produced from crystals grown with 1.5% excess Te. The detectors fabricated from crystals grown without excess Te are unable to resolve any characteristic-radiation peaks of ²⁴¹Am and ⁵⁷Co. This result is explained by a model of networked p-type domains in an n-type matrix or vice versa, which is caused by the lack of sufficient deep-level Te antisites. Such conduction-type inhomogeneity causes massive electron and hole trapping. As for the detectors fabricated from Cd_0.90Zn_0.10Te crystals grown with 2% and 3% excess Te, they are able to resolve the ²⁴¹Am 59.5-keV, ⁵⁷Co 122-keV, and ⁵⁷Co 136-keV radiation peaks. However, the full-width at half-maximum (FWHM) values of these peaks are broadened, especially the high-energy ⁵⁷Co peaks. These phenomena are attributed to the hole and, possibly, electron trapping by Cd vacancies and Te antisites, respectively. The result of the analysis indicates that sufficient Te antisites and a low density of carrier traps in Cd_0.90Zn_0.10Te are essential for producing high-quality radiation detectors. In the analysis, it was discovered that most of the excess Te, on the order of 1–2 × 10²⁰ cm⁻³, remain electrically inactive. A possible explanation for this phenomenon is that the excess Te atoms form neutral Te-antisite and Cd-vacancy complexes, such as Te_Cd·(V_Cd)², during the post-growth cooling process.     

Long wavelength infrared, molecular beam epitaxy, HgCdTe-on-Si diode performance

In the past several years, we have made significant progress in the growth of CdTe buffer layers on Si wafers using molecular beam epitaxy (MBE) as well as the growth of HgCdTe onto this substrate as an alternative to the growth of HgCdTe on bulk CdZnTe wafers. These developments have focused primarily on mid-wavelength infrared (MWIR) HgCdTe and have led to successful demonstrations of high-performance 1024×1024 focal plane arrays (FPAs) using Rockwell Scientific’s double-layer planar heterostructure (DLPH) architecture. We are currently attempting to extend the HgCdTe-on-Si technology to the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) regimes. This is made difficult because the large lattice-parameter mismatch between Si and CdTe/HgCdTe results in a high density of threading dislocations (typically, >5E6 cm⁻²), and these dislocations act as conductive pathways for tunneling currents that reduce the R_oA and increase the dark current of the diodes. To assess the current state of the LWIR art, we fabricated a set of test diodes from LWIR HgCdTe grown on Si. Silicon wafers with either CdTe or CdSeTe buffer layers were used. Test results at both 78 K and 40 K are presented and discussed in terms of threading dislocation density. Diode characteristics are compared with LWIR HgCdTe grown on bulk CdZnTe.     

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.     

Monolithically integrated HgCdTe focal plane arrays

The cost and performance of hybrid HgCdTe infrared (IR) focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an IR sensor technology based on monolithically integrated IR focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts; (2) the low-temperature cleaning of Si(001) wafers; (3) the growth of CdTe and HgCdTe layers on read-out integrated circuits; (4) diode creation, delineation, electrical, and interconnection; and (4) demonstration of high yield photovoltaic operation without limitation from earlier preprocessing such as substrate cleaning, molecular beam epitaxy (MBE) growth, and device fabrication. Crystallographic, optical, and electrical properties of the grown layers will be presented. Electrical properties for diodes fabricated on misoriented Si and readout integrated circuit (ROIC) substrates will be discussed. The fabrication of arrays with demonstrated I–V properties show that monolithic integration of HgCdTe-based IR focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the third generation of IR systems.     

Effects of a-Si:H resist vacuum-lithography processing on HgCdTe

A vacuum-compatible process for carrying out lithography on Hg_1−xCd_xTe and CdTe films was previously demonstrated. It was shown that hydrogenated amorphous silicon (a-Si:H) could be used as a dry resist by projecting a pattern onto its surface using excimer laser irradiation and then developing that pattern by hydrogen plasma etching. Pattern transfer to an underlying Hg_1−xCd_xTe film was then carried out via Ar/H₂ plasma etching in an electron cyclotron resonance (ECR) reactor. Despite the successful demonstration of pattern transfer, the possibility of inducing harmful effects in the Hg_1−xCd_xTe film due to this vacuum lithography procedure had not been explored. Here we present structural and surface compositional analyses of Hg_1−xCd_xTe films at key stages of the a-Si:H vacuum lithography procedure. X-ray diffraction double crystal rocking curves taken before and after a-Si:H deposition and after development etching were identical, indicating that bulk structural changes in the Hg_1−xCd_xTe film are not induced by these processes. Cross-section transmission electron microscopy studies show that laser-induced heating in the 350 nm thick a-Si:H overlayer is not sufficient to cause structural damage in the underlying Hg_1−xCd_xTe surface. In vacuo surface analysis via Auger electron spectroscopy and ion scattering spectroscopy suggest that the hydrogen plasma development process produces Hg-deficient surfaces but does not introduce C contamination. However, after ECR plasma etching into the Hg_1−xCd_xTe film, the measured x value is much closer to that of the bulk.     

Diffusion of Selenium in Liquid-Phase Epitaxy–Grown Hg<Subscript>0.78</Subscript>Cd<Subscript>0.22</Subscript>Te

We present preliminary results on Se diffusion in liquid-phase epitaxy (LPE)–grown HgCdTe epilayers. The LPE Hg_0.78Cd_0.22Te samples were implanted with Se of 2.0 × 10¹⁴/cm² at 100 keV and annealed at 350–450°C in mercury saturated vapor. Secondary ion mass spectrometry (SIMS) profiles were obtained for each sample. From a Gaussian fit, we find that the Se diffusion coefficient D _Se is about 1–2 orders of magnitude smaller than that of arsenic. The as-implanted Se distribution is taken into account in case of small diffusion length in Gaussian fitting. The D _Se was found to satisfy the Arrhenius relationship .     

API SPEC 5L《管线钢管规范》第45版主要内容变化分析

API SPEC 5L《管线钢管规范》是广泛应用于世界石油天然气输送钢管生产、检验和使用的重要基础标准。依据美国石油学会(API)2012年12月发布的第45版API SPEC 5L英文版,对照2007年第44版API SPEC 5L和API在2009—2011年期间发出的3个补遗,按照标准章节顺序,从总体结构、规范性引用文件、钢管钢级及交货状态、购方提供信息、制造、验收极限、检验试验方法和附录入手,分析了新标准的主要变化情况,便于标准使用者总体了解新版标准的主要变化情况,利于新版标准2013年7月1日后的正式实施。     

A novel simultaneous unipolar multispectral integrated technology approach for HgCdTe IR detectors and focal plane arrays

In the last few years Rockwell has developed a novel simultaneous unipolar multispectral integrated HgCdTe detector and focal plane array technology that is a natural and relatively straightforward derivative of our baseline double layer planar heterostructure (DLPH) molecular beam epitaxial (MBE) technology. Recently this technology was awarded a U.S. patent. This simultaneous unipolar multispectral integrated technology (SUMIT) shares the high performance characteristics of its DLPH antecedent. Two color focal plane arrays with low-10¹³ cm⁻²s⁻¹ background limited detectivity performance (BLIP D^*) have been obtained for mid-wave infrared (MWIR, 3–5 m) devices at T>130 K and for long-wave infrared (LWIR, 8–10 m) devices at T∼80 K.     

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.     

An efficient media access protocol for packet-switched wavelength-division multiplexed photonic networks

A reservation-based protocol based on a Pipelining Cyclic Scheduling Algorithm (PCSA) is proposed for packet-switched single-hop photonic networks. This protocol contains a mechanism to avoid contention at the receivers. Packets arrive in order and the transmission delay and its variations are optimized. The effects of propagation delay and processing time are almost compensated for by a pipelining technique. Analytical models and analysis are developed. The transmission delay is calculated as a function of the offered load, the number of nodes, propagation delay and processing time. Finally, the results of the analysis and simulations are compared.