LWIR HgCdTe Detectors Grown on Ge Substrates

Long-wavelength infrared (LWIR) HgCdTe p-on-n double-layer heterojunctions (DLHJs) for infrared detector applications have been grown on 100 mm Ge (112) substrates by molecular beam epitaxy (MBE). The objective of this current work was to grow our baseline p-on-n DLHJ detector structure (used earlier on Si substrates) on 100 mm Ge substrates in the 10 μm to 11 μm LWIR spectral region, evaluate the material properties, and obtain some preliminary detector performance data. Material characterization techniques included are X-ray rocking curves, etch pit density (EPD) measurements, compositional uniformity determined from Fourier-transform infrared (FTIR) transmission, and doping concentrations determined from secondary-ion mass spectroscopy (SIMS). Detector properties include resistance-area product (RoA), spectral response, and quantum efficiency. Results of LWIR HgCdTe detectors and test structure arrays (TSA) fabricated on both Ge and silicon (Si) substrates are presented and compared. Material properties demonstrated include X-ray full-width of half-maximum (FWHM) as low as 77 arcsec, typical etch pit densities in mid 10⁶ cm⁻² and wavelength cutoff maximum/minimum variation <2% across the full wafer. Detector characteristics were found to be nearly identical for HgCdTe grown on either Ge or Si substrates.     

Investigation of 1/<Emphasis Type="Italic">f</Emphasis> Noise Mechanisms in Midwave Infrared HgCdTe Gated Photodiodes

In this work, gated midwave infrared (MWIR) Hg_1–x Cd _x Te photodiodes are used to investigate the physical origin of 1/f noise generation. Gated photodiodes were fabricated on liquid-phase epitaxy p-type HgCdTe MWIR material with a vacancy-doped concentration of 1.6 × 10¹⁶ cm⁻³ and x = 0.31. CdTe was thermally deposited and used as both a passivant and a mask for the plasma-based type conversion, and ZnS was used as an insulator. Fabricated devices show a R ₀ A of 1–5 × 10⁴ Ωcm² with zero gate bias. Application of 2 V to the gate improves the R ₀ A by more than two orders of magnitude to 6.0 × 10⁶ Ωcm², which corresponds to the p-type surface being at transition between depletion and weak inversion. Trap-assisted tunneling (TAT) current was observed at negative gate biases and reverse junction biases. For gate biases greater than 3 V, a field-induced junction breakdown was observed. An I _n = α I ^β f ^−0.5 trend was observed above 200 pA reverse bias dark current, with α = 3.5 × 10⁻⁵ and β = 0.82, which corresponds to the TAT dominated region. Below 200 pA, junction generation-recombination (GR) current starts to dominate and this previously mentioned trend is no longer observed. Junction GR current was not seen to be correlated with 1/f noise in these photodiodes.     

Comparison of characterization techniques in p-on-n HgCdTe LWIR photodiodes technology

In this paper standard techniques for characterization of HgCdTe liquid phase epitaxial layers (LPE) were presented. The performance of long wavelength p-on-n HgCdTe photodiodes fabricated by arsenic diffusion was described. The correlation between LPE HgCdTe material parameters and properties of the infrared photodiodes was demonstrated.     

Design and development of multicolor MWIR/LWIR and LWIR/VLWIR detector arrays

Multicolor infrared (IR) focal planes are required for high-performance sensor applications. These sensors will require multicolor focal plane arrays (FPAs) that will cover various wavelengths of interest in mid wavelength infrared/long wavelength infrared (MWIR/LWIR) and long wavelength infrared/very long wavelength infrared (LWIR/VLWIR) bands. There has been significant progress in HgCdTe detector technology for multicolor MWIR/LWIR and LWIR/VLWIR FPAs.^1–3 Two-color IR FPAs eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in simpler, reliable, and affordable systems. The complexity of a multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally, material characteristics are only relatively controllable and depend on the process repeatability. In this context, the ability of performing “simulation experiments” where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications.     

碲镉汞光伏探测器的变面积表面钝化研究

利用同一片碲镉汞材料制备了由单层ZnS和双层CdTe/ZnS作钝化膜的变面积光伏探测器,对两种钝化膜结构的变面积器件进行了对比研究.通过分析两种器件的电流-电压(I-V)特性曲线以及零偏电阻-面积乘积(RoA)与周长-面积比(p/A)的关系曲线,发现ZnS钝化的器件具有较大的表面漏电流;通过分析两种器件的电流噪声与暗电流的关系,发现ZnS钝化的器件的噪声特性较接近散粒噪声,CdTe/ZnS双层钝化的器件则表现出较好的基本1/f噪声特性,使得器件噪声要小于单层ZnS钝化的器件.     

碲镉汞光伏探测器的变面积表面钝化研究

利用同一片碲镉汞材料制备了由单层ZnS和双层CdTe/ZnS作钝化膜的变面积光伏探测器,对两种钝化膜结构的变面积器件进行了对比研究.通过分析两种器件的电流-电压(I-V)特性曲线以及零偏电阻-面积乘积(RoA)与周长-面积比(p/A)的关系曲线,发现ZnS钝化的器件具有较大的表面漏电流;通过分析两种器件的电流噪声与暗电流的关系,发现ZnS钝化的器件的噪声特性较接近散粒噪声,CdTe/ZnS双层钝化的器件则表现出较好的基本1/f噪声特性,使得器件噪声要小于单层ZnS钝化的器件.     

Analysis of 1/f noise in LWIR HgCdTe photodiodes

We study the 1/f noise currents and dark currents in LWIR HgCdTe photodiodes. The measured dark currents of the diodes processed by post implantation annealing with different annealing times are analyzed using current model fitting methods. The different dark current components, such as diffusion current, generation-recombination current, band-to-band tunneling current, and trap assisted tunneling current, at various bias voltages can be separated from the measured dark currents. By the fitting analysis, some physical parameters are extracted and different annealing effects can be explained by the parameters. The improvements in diode characteristics by post implantation annealing can be explained by the changes of trap density, donor concentration, minority carrier lifetime, and generation lifetime. The 1/f noise currents are measured over a wide range of reverse bias voltages, and correlated with the extracted dark currents by superposition of the noise generated by the different dark current mechanisms. It turns out that the band-to-band tunneling has a smaller correlation with the 1/f noise than other current components, and the trap center seems to be responsible for the 1/f noise characteristics of the LWIR HgCdTe photodiodes.     

High-Performance LWIR MBE-Grown HgCdTe/Si Focal Plane Arrays

We have been actively pursuing the development of long-wavelength infrared (LWIR) HgCdTe grown by molecular beam epitaxy (MBE) on large-area silicon substrates. The current effort is focused on extending HgCdTe/Si technology to longer wavelengths and lower temperatures. The use of Si versus bulk CdZnTe substrates is being pursued due to the inherent advantages of Si, which include available wafer sizes (as large as 300 mm), lower cost (both for the substrates and number of die per wafer), compatibility with semiconductor processing equipment, and the match of the coefficient of thermal expansion with silicon read-out integrated circuit (ROIC). Raytheon has already demonstrated low-defect, high-quality MBE-grown HgCdTe/Si as large as 150 mm in diameter. The focal plane arrays (FPAs) presented in this paper were grown on 100 mm diameter (211)Si substrates in a Riber Epineat system. The basic device structure is an MBE-grown p-on-n heterojunction device. Growth begins with a CdTe/ZnTe buffer layer followed by the HgCdTe active device layers; the entire growth process is performed in␣situ to maintain clean interfaces between the various layers. In this experiment the cutoff wavelengths were varied from 10.0 μm to 10.7 μm at 78 K. Detectors with >50% quantum efficiency and R ₀ A ∼1000 Ohms cm² were obtained, with 256 × 256, 30 μm focal plane arrays from these detectors demonstrating response operabilities >99%. Work supported by the Missile Defense Agency (MDA) through CACI Technologies, Inc. subcontract no. 601-05-0088, NVESD technical task order no. TTO-01, prime contract no. DAAB07-03-D-C214, (delivery order no. 0016)     

1/f noise in HgCdTe photodiodes

1/f noise in HgCdTe photodiodes has been attributed to a variety of sources, most of which are associated with some form of excess current. At DRS, we have measured the 1/f noise in vertically integrated (VIP) and high-density vertically integrated photodiodes (HDVIP), over a wide range of compositions and temperature, for strictly well-behaved diffusion current limited operation. It is found that (1) the 1/f noise current is directly dependent on dark current density; (2) material composition and temperature are irrelevant, except in as much as they determine the magnitude of the current density; (3) in high-quality diodes, the 1/f noise is independent of background flux; and (4) surface passivation is relevant. These observations have been compared to the 1/f noise theory of Schiebel, which uses McWhorter’s fluctuation of the surface charge tunneling model to modulate diode diffusion current. Agreement is obtained with Schiebel’s theory for realistic surface trap densities in the 10¹²/cm² range, which will obviously be characteristic of the passivation used. The relevance of this work relative to high operating temperature phtodiodes is discussed.     

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.     

Status of LWIR HgCdTe-on-Silicon FPA Technology

The use of silicon as an alternative substrate to bulk CdZnTe for epitaxial growth of HgCdTe for infrared detector applications is attractive because of potential cost savings as a result of the large available sizes and the relatively low cost of silicon substrates. However, the potential benefits of silicon as a substrate have been difficult to realize because of the technical challenges of growing low-defect-density HgCdTe on silicon where the lattice mismatch is ∼19%. This is especially true for long-wavelength infrared (LWIR) HgCdTe detectors where the performance can be limited by the high (∼5 × 10⁶ cm⁻²) dislocation density typically found in HgCdTe grown on silicon. The current status of LWIR (9 μm to 11 μm at 78 K) HgCdTe on silicon focal-plane arrays (FPAs) is reviewed. Recent progress is covered including improvements in noise equivalent differential temperature (NEDT) and array operability. NEDT of <25 mK and NEDT operability >99% are highlighted for 640 × 480 pixel, 20-μm-pitch FPAs.     

采用叠盖电极的高级长波60元光导探测器

介绍一种采用叠盖电极制备高级长波60元探测器的方法,并进行了较深入的实验,证明此方法是一种行之有效的提高光导探测器的技术。并在此基础上,确定了器件的最佳工作条件,而且证明应用有叠盖电极的器件是有方向性的。     

HgCdTe长波红外n-on-p结线列的串音研究

Crosstalk of HgCdTe long-wavelength infrared （LWIR） n-on-p diode arrays was measured using scanning laser microscopy. During the measurement, HgCdTe diode arrays with different diode pitches were frontside illuminated by a He-Ne laser at liquid nitrogen temperature and room temperature. The experimental results show that crosstalk between the nearest neighboring diodes decreases exponentially as the diode pitch increases, and the factors that affect the obtained crosstalk are presented and analyzed. Crosstalk out of the nominal diode area （optically sensitive area） is also measured and discussed.     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

Crosstalk of HgCdTe LWIR n-on-p diode arrays

stalk out of the nominal diode area (opti-cally sensitive area) is also measured and discussed.     

HgCdTe光导探测器的优化设计

利用牛顿迭代法,在一维理论的基础上,计算机模拟优化设计了工作在8～14μm波段HgCdTe光导探测器的各项参数。结果表明,器件厚度取6μm,长度取100～150μm,环氧树脂胶粘层<3μm,净掺杂浓度取1.4×10~(15)cm~(-3),表面复合速度取500cm/s,电场强度取10V/cm为佳。该法亦可使用于其他光导探测器的优化设计中。     

CdTe/ZnS双层钝化碲镉汞长波探测器制备的研究

开展了CdTe/ZnS双层钝化碲镉汞长波探测器制备的研究。CdTe钝化膜经退火热处理后,可实现CdTe/MCT界面的互扩散,并改善CdTe钝化膜的质量。通过全湿法腐蚀方法完成了金属化开口,制备了长波碲镉汞600×18@15 μm规格线列和640×512@15 μm规格面阵。线列I-V测试表明:CdTe/ZnS双层钝化膜能有效地减少长波碲镉汞器件的表面漏电流,器件的反向结特性良好。面阵在77K测试:NETD 26.7 mK,有效像元率95.4%,并对室温目标进行了凝视成像。测试过程出现了4%左右由噪声引起的零散盲元,是由芯片面阵局部钝化失效引起的,表明钝化膜沉积工艺及芯片加工工艺尚有改进的空间。     

Passivation effect on optical and electrical properties of molecular beam epitaxy-grown HgCdTe/CdTe/Si layers

The effects of passivation with two different passivants, ZnS and CdTe, and two different passivation techniques, physical vapor deposition (PVD) and molecular beam epitaxy (MBE), were quantified in terms of the minority carrier lifetime and extracted surface recombination velocity on both MBE-grown medium-wavelength ir (MWIR) and long-wavelength ir HgCdTe samples. A gradual increment of the minority carrier lifetime was reported as the passivation technique was changed from PVD ZnS to PVD CdTe, and finally to MBE CdTe, especially at low temperatures. A corresponding reduction in the extracted surface recombination velocity in the same order was also reported for the first time. Initial data on the 1/f noise values of as-grown MWIR samples showed a reduction of two orders of noise power after 1200-Å ZnS deposition.     

Equilibrium properties of indium and iodine in LWIR HgCdTe

The equilibrium properties of indium and iodine HgCdTe are calculated from ab initio methods and compared with experimental results. Indium, a group III element, is found to substitute exclusively on the group II sublattice, behaving as a singly ionizable donor throughout the existence region. Indium forms a neutral bound complex with a cation vacancy, and although this complex accounts for less than 1% of the indium incorporation at the temperatures considered >250°C, it will be important in the diffusion of the indium. Iodine, a group VII element, is found to substitute on the group VI sublattice, also behaving as a singly ionizable donor throughout the existence region. Iodine is found to bind strongly to the cation vacancy, and this neutral complex dominates the incorporation under mercury-deficient conditions. Even under near mercury-saturated conditions, at low temperature the complex is still present in significant concentrations. Although iodine incorporation on the cation sublattice is predicted, it never represents a significant source of inactive incorporation.     

1/f noise and material defects in HgCdTe diodes

1/f noise is measured on long wavelength diodes as a function of device geometry, band gap, temperature, diode bias, and anneal temperature for a Te-rich CdTe passivation layer. The results show that for these diodes the 1/f noise is a bulk phenomena due to the modulation of generation recombination current associated with defects formed by the interdiffusion of Te-rich CdTe, and that these defects are located in the junction region. No 1/f noise is observed for the lowest interdiffusion anneal temperature.