We report the development of high performance low cost SWIR infrared detectors from MBEgrown HgCdTe on 3-inch CdTe-buffered silicon substrates. The experimental findings demonstrate that despite the large lattice mismatch between HgCdTe and Si substrate, the materials and detector performances are sufficiently better than those reported for III-V mixed crystals. High minority carrier lifetime of the order 3 μs at room temperature was measured on the as grown material. Photodetectors fabricated from this material produced low dark current densities on the order of 10-6 A/cm2 and 10-3 A/cm2 at 200K and 300K. Quantum efficiency exceeding 70% at 2.0 μm, without antireflective coating, was measured on single element detectors. Further, 320 X 256, 30 μm pitch FPA’s have been fabricated with this HgCdTe on Si material and dark current operability of ~ 99.5% (mean dark current of 30 pA/Pixel) at 200K has been demonstrated.

CID imaging arrays were fabricated on Hg0.7Cd0.3Te produced by the solid state recrystallization technique. It was found that the most serious source of dark current was sub-grain boundaries. SEM studies of the microstructure revealed by etching showed that boundaries with a high denisty of dislocations were detectable sources of dark current, while those boundaries with a low density of dislocations, as well as individual dislocations were not. TEM showed that all dislocations were free of precipitates, and most were not dissociated. The sub-grain boundaries were found to arise from misorientation between dendrites which form during the solidification from the melt.

A new method for the nucleation of oxygen precipitates in Czochralski silicon is described. The temperature is ramped at approximately 100°C/hr from a very low value, near 400°C, to the highest temperature used for subsequent process steps. The technique generates a larger precipitate number density and a greater volume fraction of precipitated oxygen than standard isothermal nucleation anneals. The morphology of the precipitates changes from 0.1um sized (100) platelets to small particles unresovable by TEM. The new temperature ramping technique can reduce the time needed for precipitate nucleation by at least a factor of three. The details of oxygen precipitation can be totally controlled by adjusting the temperature ramp rate as a function of time.

A 1/f noise model is developed for reverse biased diodes based on McWhorter’s concept of charge tunneling into semiconductor states at passivation layer interfaces [A. L. McWhorter, in Semiconductor Surface Physics, edited by R. H. Kingston (University of Pennsylvania Press, Philadelphia, 1957), pp. 207–228]. The charge modulates the width of semiconductor surface charge layers on either side of the junction, resulting in fluctuations in dark current from these volumes due to the net difference in depletion and diffusion current generation rates per unit volume in the semiconductor. The 1/f spectrum associated with the fluctuating surface charge translates into a 1/f spectrum in thermally generated diode dark current. The model is applied to midwavelength infrared HgCdTe N+/P diode architectures.

The utilization of the non-equilibrium photodiode concept for high operating temperature (HOT) FPAs is discussed, both generically, and with regard to the specific example of MWIR HgCdTe. The issues of dark current, surface passivation, and 1/f noise are considered for three different architectures, namely N+/N-/P+, N+/P-/P+, and nBn. These architectures are examined with regard to possible FPA performance limitations, and potential difficulty in reduction to practice. Performance data obtained at DRS for the N+/N-/P+ and N+/P-/P+ HgCdTe architectures will be presented.

Anomalous secondary-ion mass spectroscopy (SIMS) profiles of copper in thin pieces of HgCdTe are explained using the model used for diode formation by ion milling and ion implantation. In this model, the SIMS ion beam injects mercury interstitials into the HgCdTe as it etches the HgCdTe. The interstitials fill metal vacancies and kick copper off the metal lattice sites. The copper interstitials then diffuse either to the surface being etched, where it is removed and detected by the SIMS instrument, or deeper into the HgCdTe, where it annihilates vacancies. Good agreement between model predictions and experimental SIMS profiles are obtained.

Ion implantation into HgCdTe results in the production of Hg interstitials, which can be subsequently driven into the HgCdTe by an annealing process. This diffusive drive-in of the Hg interstitials fills vacancies and kicks out group I impurities and results in the formation of an n–p junction. In this work we report on the production of interstitials during baking subsequent to the ion implantation process. Various concentrations of metal vacancies were first introduced into mid-wavelength infrared (MWIR, 3 μm to 5 μm) HgCdTe by annealing under tellurium-saturated conditions at various temperatures. Baking subsequent to planar implantation of boron produced n–p junctions whose depths were measured by defect etching. The results were modeled using a simple diffusion limited model from a fixed surface concentration. The surface concentration was allowed to decrease exponentially to zero after a time, found to be of the order of ∼80 h to 150 h. Exhaustion of the interstitials sources produced by the implantation was nearly complete after ∼400 h. The total number of mercury interstitials produced was approximately 50% of the implant dosage.

Hg1−x Cd x Te samples of x ~ 0.3 (in the midwave infrared, or MWIR, spectral band) were prepared by molecular beam epitaxy (MBE) for fabrication into 30-μm-pitch, 256 × 256, front-side-illuminated, high-density vertically-integrated photodiode (HDVIP) focal plane arrays (FPAs). These MBE Hg1−x Cd x Te samples were grown on CdZnTe(211) substrates prepared in this laboratory; they were ~10-μm thick and were doped with indium to ~5 × 1014 cm−3. Standard HDVIP process flow was employed for array fabrication. Excellent array performance data were obtained from these MWIR arrays with MBE HgCdTe material. The noise-equivalent differential flux (NEΔΦ) operability of the best array is 99.76%, comparable to the best array obtained from liquid-phase epitaxy (LPE) material prepared in this laboratory.

Exposure to specific damage introduced by either ion implantation or ion milling converts p-type short wavelength infrared (SWIR) HgCdTe to n-type in a manner similar to the conversions in medium wavelength infrared (MWIR) or long wavelength infrared (LWIR) mercury cadmium telluride. However, the depth of conversion for SWIR Hg1−xCdxTe, with x=0.48, is approximately 300% smaller when compared to the depth of conversion for MWIR HgCdTe for an identical degree of ion milling. The depth of conversion, or the n/p junction depth, tracks linearly the extent of surface removals by ion milling when the metal vacancy concentration is held invariant. These results can be correlated to the interaction between metal vacancies and a product of the lattice damage process resulting from ion milling. The observation of a linear dependence of this depth on the degree or time of ion milling rules out the existence of a diffusive barrier in the transfer of this product for both MWIR and SWIR HgCdTe.

Transmission spectra of liquid-phase epitaxy (LPE) Hg1-xCdxTe with Cd mole fractions in the range of 0.23<x<0.30 have been obtained as a function of temperature and thickness. The results are described using a model consisting of exponential (Urbach) absorption in the band tail region and band-to-band absorption, predicted by a Kane k p model, for the above band gap region. Modifications to the Urbach and transmission expressions are found to be necessary to properly describe the shape, temperature, and thickness dependence of the spectra. A known composition gradient was found to be sufficient to describe the spectra obtained for thin (<20-µm) samples.

Arsenic incorporated in HgCdTe epifilms at levels ranging from 6×1014 to 4×1016 cm−3, and activated completely as an acceptor, converts into a donor upon introduction of specific damage introduced by ion milling, ion implantation, or ECR. The results could be correlated to the formation of a complex between the arsenic and a product of the lattice damage process. If this product is assumed to be an atom, which could be an Hg interstitial, then a direct mass balance relationship dictates the formation of one Hg interstitial for each 49,000 atoms of Hg removed by the damage process. A very similar relationship is observed in the annihilation of metal vacancies pre-existing in the solid by damage introduced by ion milling. The deactivated arsenic, present as a donor, can be restored back to complete activation as an acceptor upon annealing for relatively short durations at 120°C.

Arsenic is known to be an amphoteric impurity that may occupy either sublattice in HgCdTe depending upon sample annealing. As an acceptor in low concentrations, it offers several features that are attractive for the fabrication of certain n + -on-p detector diode structures. The epitaxial growth of arsenic-doped HgCdTe from a Te-rich melt can fulfill the requirements for application in a variety of devices where low vacancy concentrations and low defect densities are critical requirements in minimizing dark currents. These devices may include the high operating temperature (HOT) detectors operated in a strong nonequilibrium and reverse bias mode to suppress the Auger-generated dark currents. For the materials’ growth process to be effective, the segregation coefficient determining the incorporation of arsenic from the Te-rich melt needs to be established. This coefficient was measured during these investigations and was observed to vary with arsenic concentration. Within the range of interest, this parameter varied between 8×10 −6 and 1×10 −4 . These extremely small values limit the doping that can be achieved to <5×10 16 cm −3 in the grown epifilm. Furthermore, the large addition of arsenic to the melt, necessitated by the extremely small segregation coefficients, leads to a condition where the concentration of arsenic in the liquid-phase epitaxy (LPE) nutrient melt exceeds that of cadmium. The melt chemistry, phase diagram, and epigrowth process fundamentally change as a result. This new epigrowth process was developed and tuned during these investigations. For acceptor levels at 1×10 15 cm −3 and lower, the growth of arsenic-doped HgCdTe from a Te-rich LPE melt has been determined to be an extremely reproducible, powerful, and controllable technique.

Hg1−xCdxTe(x∼0.22) samples grown by LPE on CdZnTe(111B)-oriented substrates were exposed to various doses of thermal neutrons (∼1.0×1016−1.7×1016n/cm2) and subsequently annealed for ∼24h in Hg overpressure to remove damage and reduce the presence of Hg vacancies. Extensive magnetotransport measurements were performed on these samples as part of an investigation into the use of elemental transmutation for efficient p-type doping of this material. The data were analyzed using a multi-carrier approach which incorporates various scattering mechanisms and the presence of two conduction channels of differing alloy content to describe the changes in the transport properties due to neutron irradiation.

An overview of the DRS HDVIP architecture for realization of large-area infrared focal plane arrays (IRFPAs) is given. Improvements needed to meet more stringent application requirements are discussed and modeled. Both theoretical and experimental data are presented.

: An overview on DRS' approach towards realization of HgCdTe photonic infrared detectors based on DRS's proven HDVIP(registered) technology is given A summary of recent progress is described.

The recently developed Te-rich, liquid-phase-epitaxy growth technology for low arsenic-doped mid-wavelength infrared (MWIR) HgCdTe with p-type doping concentrations <10 15 cm −3 has enabled the fabrication of n + /p photodiodes using the damage associated with a boron ion implantation. The diode properties are presented and compared to similar diodes fabricated in p-HgCdTe doped with Group IBs. The attraction of the arsenic-doped diode technology is associated with the fact that the arsenic resides on the Te sublattice and is immune to the Hg interstitial fluxes that are present in the diode-formation process. This leads to minimal diode spread, limited primarily to the n + region and, hence, a potential for use in really high-density infrared focal planes. At the same time, the Hg interstitials generated in the diode-formation process should purge the photodiode volume of fast diffusing species, resulting in a high-quality, diode-depletion region devoid of many Shockley-Read recombination centers. These aspects of diode formation in this material are discussed.

An overview on DRS' approaches towards realization of HgCdTe photonic infrared detectors based on DRS's proven HDVIP technology is given. The first approach involves the use of a silicon microlens array attached to the detector array, and the second reduction of dark currents in each detector itself. Recent progress is presented.

Excellent high-operating-temperature infrared photodiodes in the medium-wavelength infrared spectral band with cutoff wavelengths ∼5 μm at 77 K were fabricated on Hg1−xCdxTe samples (x∼0.30) prepared by liquid-phase epitaxy in a tellurium-melt reactor. The samples were doped with indium to ∼1×1014 cm−3 and gold to ∼5×1015 cm−3. Thick planar diodes (∼80 μm thick) and thin cylindrical diodes (∼10 μm thick) of the n-on-p type were fabricated, and they gave comparable, excellent detector dark values within a factor of two. At 130 K, dark currents as low as 5×10−7 A/cm2 were obtained.

We report on results of laboratory and field tests of dual-band MWIR/LWIR focal plane arrays (FPAs) produced under the Army Research Laboratory's Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system and by BAE Systems using QWIP technology. The HgCdTe array used the DRS HDVIP(TM) process to bond two single-color detector structures to a 640x480-pixel single-color read-out integrated circuit (ROIC) to produce a dual-band 320x240 pixel array. The MWIR and LWIR pixels are co-located and have a high fill factor. The images from each band maybe read out either sequentially (alternating frames) or simultaneously. The alternating frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using MBE-grown III-V materials. The LWIR section consisted of GaAs quantum wells and AlGaAs barriers and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two color operation) which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The relative advantages and disadvantages of the two material systems for producing medium format dual-band FPAs are discussed.

Gold-doped Hg1−xCdxTe samples of x=0.2067 (in the very-long-wavelength infrared spectral band, with cutoff wavelengths ∼13.2 μm at 77 K) were prepared by tellurium-melt liquid-phase epitaxy. The samples were doped with indium to ∼2×1014 cm−3 and gold to ∼7×1015 cm−3, and were characterized by secondary ion mass spectroscopy, Hall measurements, and minority carrier lifetime measurements. State-of-the-art minority carrier lifetime of ∼0.82 μs was obtained.