This paper reviews the historical progress of HgCdTe material and device development at Raytheon Vision Systems starting with the initial work in 1965 at what was then the Santa Barbara Research Center, a subsidiary of the Hughes Aircraft Company and progressing up to the present time. Because of the long history, all the details cannot be presented in a single paper; instead, we focus only on a few major accomplishments. In HgCdTe material preparation these include: the early bulk single crystal growth methods; the advent of liquid phase epitaxial growth from Hg melts; and, the most recent molecular beam epitaxial methods. For IR photodetector devices, we started with just single element detectors operating either in photoconductive or photovoltaic mode, then progressed to multi-element linear arrays, then to 2-D arrays on Si read-out circuits and, finally to the very large focal plane (>2k × 2k), dual-band, and APD arrays of today. Some applications of these devices in IR systems will be presented. Technical issues will be discussed only to the extent necessary to support the historical narrative. Some interesting anecdotes will be included.

We review the rapid progress that has been made during the past three years in the heteroepitaxial growth of HgCdTe infrared detector device structures on Si substrates by molecular-beam epitaxy. The evolution of this technology has enabled the fabrication of high performance, large-area HgCdTe infrared focal-plane arrays on Si substrates. A key element of this heteroepitaxial approach has been development of high quality CdTe buffer layers deposited on Si(112) substrates. We review the solutions developed by several groups to address the difficulties associated with the CdTe/Si(112) heteroepitaxial system, including control of crystallographic orientation and minimization of defects such as twins and threading dislocations. The material quality of HgCdTe/Si and the performance of HgCdTe detector structures grown on CdTe/Si(112) composite substrates is reviewed. Finally, we discuss some of the challenges related to composition uniformity and defect generation encountered with scaling the MBE growth process for HgCdTe to large-area Si substrates.

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 105 cm−2 for 9 µm thick CdTe films. HgCdTe p+-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 µm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R0A products in the 106–107 Θ-cm2 range for 3.6 µm cutoff at 125K and R0A products in the 104–105 Θ-cm2 range for 4.7 µm cutoff at 125K.

Extensive material and device statistics of performance and reproducibility are presented to show the maturity of this technology. The demonstration vehicles to monitor yields during this demonstration were long-wavelength infrared (LWIR) HgCdTe multilayer wafers with 128 X 128 detector arrays. The heterostructure photodetectors were of the p-on-n planar configuration. Device data show that MBE LWIR diode test structures have performance that equals that of p-on-n double heterostructure photodiodes made by LPE. Due to the special attention given to understanding and reducing epilayer growth-induced defects, we have achieved improvements in FPA operability values from 92% to 98%. These improvements have resulted in the demonstration of a 128 X 128 FPA hybrid that had detectivity (D*) background limited performance when operating at 80 K in a tactical background environment. Mean D* was 1.28 X 1011 cmHz1/2/W. The corresponding mean NE(Delta) T was an excellent 5.9 mK.

In-situ doped p-on-n devices were grown by molecular beam epitaxy, and their structural, optical and electrical properties were evaluated. Significant progress has been made toward the growth of high performance HgCdTe devices by molecular beam epitaxy. Long wave infrared detectors operating at 9.9 µm at 78K exhibited a mean RoAo product of 1170 Ωcm2 at 0-fov. Very long wave infrared detectors operating at 14 µm at 78K exhibited a mean RoA product of 3.5 Ωcm2 at f/2 fov. These values represent the state-of-the-art for molecular beam epitaxially grown HgCdTe detectors.

A robust process has been developed for the reproducible growth of in-situ doped Hg1−xCdxTe:As alloys by molecular beam epitaxy. Net hole concentrations in excess of 5 x 1017 cm−3, with peak mobilities >200 cm2/Vs were measured in Hg0.74Cd0.26Te:As films. The p-type layers were twin-free and consistently exhibit narrow x-ray rocking curves (<40 arc sec). The reproducible growth of small lots of p-on-n LWIR detector structures has been established. For a typical lot consisting of 13 layers, the average x-value of the n-type base layer was 0.226 with a standard deviation of 0.003. The lateral compositional uniformity across a 2.5 cm 2.5 cm wafer was = +- 0.0006. High performance p-on-n LWIR diodes were fabricated that exhibited RoAo values (0-fov at 78K) as large as 350 Q cm2 at 10.4 m.

HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.

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).

HgCdTe has become the detector material of choice for many development and production electro-optical systems whose applications cover the IR spectrum from 2 to 16 micrometers , with operating temperatures ranging from 300 K to 40 K and background flux levels from 1018 to 1012 photons/cm2-sec. At the base of this success is the ongoing development and perfection of the HgCdTe material from which these detectors are fabricated. This paper examines the expressions that describe leakage currents, signal current, and capacitance for HgCdTe p+-on-n diodes to identify the critical material properties and their influence on the performance of the resulting detectors. In addition to lifetime, doping density, mobility, and composition, the compositional grading within the absorbing layer must be managed to achieve the desired performance. Equally important are parameters such as size and uniformity which must be emphasized to meet IR FPA cost targets. Caution must also be taken not to attempt to reduce IR FPA cost at the expense of material quality, because doing so would most likely have the opposite effect.

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 106 cm−2 for 8 µm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 106 cm−2. These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R0A = 1.64 × 104 Ωm2 (0 FOV) for devices with 7.8 µm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.

Molecular beam epitaxy (MBE) offers benefits such as the capability for growth of compositionally-tailored heterostructures and in-situ doping of HgCdTe alloys. These capabilities were applied to the growth of long wave infrared unispectral focal plane arrays (FPA) with 480×4 elements. The detectivity (D*) map of the FPA demonstrated performance that was higher than the specification value, with no defective channels. Two-color detectors with the n-p-n architecture, for the simultaneous detection of two closely spaced bands in the midwave infrared spectrum were also grown by MBE. These devices exhibited sharp turn-off and turn-on in both bands. The quantum efficiency was greater than 70% and average R°A values exceeded 1×106 Ω-cm2 in both bands. These result demonstrate that high performance HgCdTe devices can be grown by MBE.

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.

High crystalline quality epitaxial CdTe(112)B/ZnTe films were deposited by molecular-beam epitaxy directly onto vicinal Si(112) substrates, without use of GaAs interfacial layers. The films were characterized with x-ray diffraction, optical microscopy, and wet chemical defect etching. Single crystal, twin-free CdTe(112)B films exhibit structural quality exceeding that previously reported for CdTe(112)B heteroepitaxy on GaAs/Si(112) or GaAs(112)B substrates. X-ray rocking curve full width at half-maximum of 72 arcsec for CdTe(224) reflection and near-surface etch pit densities (EPD) of 2×106 cm−2 have been observed for 8-μm-thick CdTe films. EPD depth profiles indicate that the threading dislocation density decreases with increasing II–VI epilayer thickness up to approximately 5 μm thickness and saturates at 2×106 cm−2 for thickness exceeding 5 μm. The CdTe epilayer orientation was observed to tilt 2° away from the Si(112) substrate orientation toward the [001] direction.

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.

HgCdTe MBE technology is becoming a mature growth technology for flexible manufacturing of short-wave, medium-wave, long-wave, and very long-wave infrared focal plane arrays. The main reason that this technology is getting more mature for device applications is the progress made in controlling the dopants (both n-type and p-type in-situ) and the success in lowering the defect density to less than 2 x 105/cm2 in the base layer. In this paper, we will discuss the unique approach that we have developed for growing As-doped HgCdTe alloys with cadmium arsenide compound. Material properties including composition, crystallinity, dopant activation, minority carrier lifetime, and morphology are also discussed. In addition, we have fabricated several infrared focal plane arrays using device quality double layers and the device results are approaching that of the state-of-the-art liquid phase epitaxy technology.

To facilitate the production of HgCdTe IR detectors on Si substrates, epitaxial films of ZnTe and CdZnTe/ZnTe have been deposited by molecular-beam epitaxy (MBE) onto both Si(001) and Si(112) substrates. On Si(001) substrates misoriented from 0 to 8-degree toward [110], parallel epitaxy of ZnTe(001) and CdZnTe(001)/ZnTe(001) has bene observed. Using ZnTe initiation layers, high quality CdZnTe(001) films have been demonstrated with (004) reflection x-ray rocking curves as narrow as 158 arc-secs for Cd0.96Zn0.04Te and 78 arc-secs for CdTe. HgCdTe(001) films grown by liquid-phase epitaxy (LPE) on these MBE CdZnTe/ZnTe/Si(001) substrates have x-ray rocking curves as low as 55 arc-secs and average etch pit densities of 5 x 10(6) cm-2. IR detectors, fabricated from LPE-grown p-on-n heterojunctions on CdZnTe/Si, are comparable in performance to detectors on bulk CdZnTe substrates with R0A > 2 x 10(3) OMEGA-cm2 at 78 K for a 9.4 mum cutoff wavelength. On vicinal Si(112) substrates, ZnTe nucleates in either the (112) or twin (552) orientation, depending on the Si misorientation. CdTe deposited on ZnTe/Si(112) nucleates in the same orientation as the ZnTe. X-ray rocking curves as narrow as 110 arc-secs have been obtained for CdTe(552) epitaxy.

Epitaxial structures of ZnTe(100) and CdZnTe(100)/ZnTe(100) have been deposited by molecular-beam epitaxy onto Si(100) substrates misoriented from 0° to 8° towards the [011] direction. The films were characterized with x-ray diffraction, photoluminescence spectroscopy, optical microscopy, and stylus profilometry. Single-crystal CdZnTe(100) films comparable in structural quality to those obtained with growth on GaAs/Si composite substrates have been demonstrated on both 4° and 8° misoriented Si with the use of ZnTe buffer layers. X-ray rocking curves with FWHM less than 300 arcsec for ZnTe (400) and less than 160 arcsec for CdZnTe(400) have been obtained for as-grown films. Specular surface morphologies, superior to those obtained on GaAs/Si composite substrates, are also observed.

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.

Large-area HgCdTe 480×640 thermal-expansion-matched hybrid focal plane arrays were achieved by substituting metalorganic chemical vapor deposition (MOCVD)-grown CdZnTe/GaAs/Si alternative substrate in place of bulk CdZnTe substrates for the growth of HgCdTe p-on-n double-layer heterojunctions by controllably-doped mercury-melt liquid phase epitaxy (LPE). (100) CdZnTe was grown by MOCVD on GaAs/Si using a vertical-flow high-speed rotating disk reactor which incorporates up to three two-inch diameter substrates. Layers having specular surface morphology, good crystalline structure, and surface macro defect densities <50 cm−2 are routinely achieved and both the composition uniformity and run-to-run reproducibility were very good. As the composition of the CdZnTe layers increases, the x-ray full width at half maximum (FWHM) increases; this is a characteristic of CdZnTe grown by VPE techniques and is apparently associated with phase separation. Despite a broader x-ray FWHM for the fernary CdZnTe, the FWHM of HgCdTe grown by LPE on these substrates decreases, particularly for [ZnTe] compositions near the lattice matching condition to HgCdTe. An additional benefit of the ternary CdZnTe is an improved surface morphology of the HgCdTe layers. Using these silicon-based substrates, we have demonstrated 78K high-performance LWIR HgCdTe 480×640 arrays and find that their performance is comparable to similar arrays fabricated on bulk CdZnTe substrates for temperatures exceeding approximately 78K. The performance at lower temperatures is apparently limited by the dislocation density which is typically in the low-mid 106 cm−2 range for these heteroepitaxial materials.