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HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm 2 in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048u 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6-in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 u 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations. Keywords: HgCdTe, HgCdTe/Si, infrared detectors, FPA, heteroepitaxy, MBE, Si, MWIR, LWIR

HgCdTe APDs and APD arrays offer unique advantages for high-performance eyesafe LADAR sensors. These include: operation at room temperature, low-excess noise, high gain, high-quantum efficiency at eyesafe wavelengths, GHz bandwidth, and high-packing density. The utility of these benefits for systems are being demonstrated for both linear and area array sensors. Raytheon has fabricated 32 element linear APD arrays utilizing liquid phase epitaxy (LPE), and packaged and integrating these arrays with low-noise amplifiers. Typical better APDs configured as 50-micron square pixels and fabricated utilizing RIE, have demonstrated high fill factors, low crosstalk, excellent uniformity, low dark currents, and noise equivalent power (NEP) from 1-2 nW. Two units have been delivered to NVESD, assembled with range extraction electronics, and integrated into the CELRAP laser radar system. Tests on these sensors in July and October 2000 have demonstrated excellent functionality, detection of 1-cm wires, and range imaging. Work is presently underway under DARPA's 3-D imaging Sensor Program to extend this excellent performance to area arrays. High-density arrays have been fabricated using LPE and molecular beam epitaxy (MBE). HgCdTe APD arrays have been made in 5 X 5, 10 X 10 and larger formats. Initial data shows excellent typical better APD performance with unmultiplied dark current < 10 nA; and NEP < 2.0 nW at a gain of 10.

Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.

Separate absorption and multiplication avalanche photodiode (SAM-APD) device structures, operating in the 1.1 - 1.6 micrometer spectral range, have been fabricated in the HgCdTe material system by molecular-beam epitaxy. These HgCdTe device structures, which offer an alternative technology to existing III-V APD detectors, were grown on CdZnTe(211)B substrates using CdTe, Te, and Hg sources with in situ In and As doping. The alloy composition of the HgCdTe layers was adjusted to achieve both efficient absorption of IR radiation in the 1.1 - 1.6 micrometer spectral range and low excess-noise avalanche multiplication. To achieve resonant enhancement of hole impact ionization from the split-off valence band, the Hg1-xCdxTe alloy composition in the gain region of the device, x equals 0.73, was chosen to achieve equality between the bandgap energy and spin-orbit splitting. The appropriate value of this alloy composition was determined from analysis of the 300 K bandgap and spin-orbit splitting energies of a set of calibration layers, using a combination of IR transmission and spectroscopic ellipsometry measurements. MBE-grown APD epitaxial wafers were processed into passivated mesa-type discrete device structures and diode mini-arrays using conventional HgCdTe process technology. Device spectral response, dark current density, and avalanche gain measurements were performed on discrete diodes and diode mini- arrays on the processed wafers. Avalanche gains in the range of 30 - 40 at reverse bias of 85 - 90 V and array-median dark current density below 2 X 10-4 A/cm2 at 40 V reverse bias have been demonstrated.

Significant progress has been made in the technology for MBE growth of HgCdTe infrared focal-plane arrays on Si substrates since the initial demonstration of MBE HgCdTe-on- Si heteroepitaxy in 1989. In 1995, the first all-MBE-grown detector arrays on Si were produced through direct MBE growth of (112)B-oriented II-VI films on Si without III-V initiation layers, culminating in detector performance comparable to LPE-grown detectors on bulk CdZnTe substrates. This achievement was enabled by the development of two key contributing technologies: CdTe on Si buffer layer growth and HgCdTe p-on-n double-layer heterojunction growth using p-type chemical doping with As. The MBE process for deposition of high crystalline quality CdTe buffer layers has been developed so that x-ray rocking curve FWHM less than 75 arc-sec and near-surface etch pit densities (EPD) of 2 multiplied by 106 cm-2 are routinely achievable for 9-micrometer-thick CdTe buffer layers. The dependence of CdTe EPD on ZnTe initiation layer thickness, insertion of CdTe/CdZnTe strained layer superlattices, and thermal cycling to cryogenic temperatures has been investigated and is reviewed. HgCdTe baselayers deposited by MBE on these CdTe/Si composite substrates exhibit x-ray FWHM as low as 72 arc-sec and EPD of 3 - 20 multiplied by 106 cm-2. To demonstrate the potential for MBE growth of large-area HgCdTe FPAs on Si, detectors with 78 K cutoff wavelength of 7.8 micrometer have been fabricated in this HgCdTe/Si epitaxial material with array-average R0A product of 1.64 multiplied by 104 (Omega) -cm2 (0 FOV).© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal epitaxial films of CdTe(112)B and HgCdTe(112)B directly onto Si(112) substrates without the use of GaAs interfacial layers. The films have been characterized with x-ray diffraction and wet chemical defect etching, and IR detectors have been fabricated and tested. CdTe(112)B films are twin- free and have x-ray rocking curves as narrow as 72 arc-seconds and near-surface etch pit density (EPD) of 2 X 106 cm-2 for 8 micrometers -thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 92 arc-seconds and EPD of 8 - 30 X 106 cm-2. HgCdTe/Si infrared detectors have been fabricated with R0A equals 4.3 X 103 (Omega) -cm2 (f/2 FOV) and 7.8 micrometers cutoff wavelength at 78 K to demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal films of ZnTe and CdZnTe/ZnTe onto Si(100) and Si(112) substrates. Parallel epitaxy of ZnTe(100) and CdZnTe(100)/ZnTe(100) has been observed for growth on Si(100) substrates misoriented from 0-8 degrees towards the [011] direction. With ZnTe initiation layers, high quality CdZnTe(100) films have been demonstrated on both 4° and 8° misoriented Si(100) with x-ray rocking curve FWHM as narrow as 158 arc-seconds, which is comparable to that obtained with GaAs/Si composite substrates. The observed surface morphologies are superior to those obtained on GaAs/Si composite substrates. HgCdTe(100) films with x-ray FWHM as low as 55 arcseconds and average etch pit densities of 5 x 106 cm2 have been deposited by liquid phase epitaxy on these MBE CdZnTe/ZnTe/Si(100) substrates. On vicinal Si(1 12) substrates, ZnTe films are observed to nucleate in either the (1 12) or its twin (552) orientation depending on the misorientation of the Si substrate away from (1 12). For Si(1 12) misorientations of 5° or 10° towards from the [1 1-1] direction, ZnTe nucleates in a parallel (1 12) orientation, while for misorientations of 0° or 5° away from the [1 1-1] direction, ZnTe is observed to nucleate in a (552) orientation. CdTe deposited on ZnTe/Si(112) is observed to nucleate in the same orientation as the ZnTe. CdTe(552) epilayers are of substantially higher quality than (1 12)oriented films. X-ray rocking curves as narrow as 1 10 arc-seconds have been observed for the CdTe(331) reflection in the case of (552)-oriented epitaxy.