Progress in MOVPE of HgCdTe for advanced infrared detectors

This paper reviews the significant progress made over the past five years in the development of metalorganic vapor phase epitaxy (MOVPE) for the in situ growth of HgCdTe p-n junction devices for infrared detector arrays. The two basic approaches for MOVPE growth of HgCdTe, the interdiffused multilayer process (IMP), and direct alloy growth (DAG) are compared. The paper then focuses on the progress achieved with the IMP approach on lattice-matched CdZnTe substrates. The benefits of the precursors ethyl iodide (EI) and tris-dimethylaminoarsenic (DMAAs) for controlled iodine donor doping and arsenic acceptor doping at dopant concentrations relevant for HgCdTe junction devices are summarized along with the electrical and lifetime properties of n-type and p-type HgCdTe films grown with these precursors. The relative merits of the two CdZnTe substrate orientations we have used, the (211)B and the (100) with 4°–8° misorientation are compared, and the reasons why the (211)B is preferred are discussed. The growth and repeatability results, based on secondary ion mass spectrometry analysis, are reported for a series of double-heterojunction p-n-N-P dual-band HgCdTe films for simultaneous detection in the 3–5 μm and 8–10 μm wavelength bands. Finally, the device characteristics of MOVPE-IMP in situ grown p-on-n heterojunction detectors operating in the 8–12 μm band are reviewed and compared with state-of-the-art liquid phase epitaxial grown devices.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Dual-band infrared detectors

IntroductionMulticolor capabilities are highly desirable foradvance infrared(IR) systems.Systems that gatherdata in separate IR spectral bands can discriminateboth absolute temperature and unique signatures ofobjects in the scene.By providing this new dimensionof contrast,multiband detection also enables ad-vanced color processing algorithms to further im-prove sensitivity above that of single- color devices.This is extremely important for the process of identi-fying temperature difference b…     

Dark current generating mechanisms in short wavelength infrared photovoltaic detectors

The current-voltage characteristics and quantum efficiencies of double layer planar heterostructure photodiodes were investigated. Results are reported on devices with cutoff wavelengths of 1.8, 2.4, and 3.3 μm. For these respective devices, the dominant currents for temperatures >250,>200,>150K are diffusion currents limited by shallow Shockley-Hall-Read (SHR) processes. The remarkable result is that the electrical and optoelectronic properties of these devices of diverse cut-off wavelength can be explained by simple models using independently measured layer parameters such as the minority carrier lifetimes. For all three cases, the analysis suggests that the same shallow (SHR) centers located at 78% of the energy gap are causing the observed effects. These traps located in then-type base of the device are not influenced by the magnitude of n-type doping and this observation was used to significantly improve the performance of the devices and validate the predictive capability of the models used in the analysis. The shallow centers appear to be process induced rather than grown-in. This assertion is based on the observation that changes in the annealing process led to an order of magnitude improvement in the minority carrier lifetime.     

HgCdTe/Si materials for long wavelength infrared detectors

The heteroepitaxial growth of HgCdTe on large-area Si substrates is an enabling technology leading to the production of low-cost, large-format infrared focal plane arrays (FPAs). This approach will allow HgCdTe FPA technology to be scaled beyond the limitations of bulk CdZnTe substrates. We have already achieved excellent mid-wavelength infrared (MWIR) and short wavelength infrared (SWIR) detector and FPA results using HgCdTe grown on 4-in. Si substrates using molecular beam epitaxy (MBE), and this work was focused on extending these results into the long wavelength infrared (LWIR) spectral regime. A series of nine p-on-n LWIR HgCdTe double-layer heterojunction (DLHJ) detector structures were grown on 4-in. Si substrates. The HgCdTe composition uniformity was very good over the entire 4-in. wafer with a typical maximum nonuniformity of 2.2% at the very edge of the wafer; run-to-run composition reproducibility, realized with real-time feedback control using spectroscopic ellipsometry, was also very good. Both secondary ion mass spectrometry (SIMS) and Hall-effect measurements showed well-behaved doping and majority carrier properties, respectively. Preliminary detector results were promising for this initial work and good broad-band spectral response was demonstrated; 61% quantum efficiency was measured, which is very good compared to a maximum allowed value of 70% for a non-antireflection-coated Si surface. The R₀A products for HgCdTe/Si detectors in the 9.6-μm and 12-μm cutoff range were at least one order of magnitude below typical results for detectors fabricated on bulk CdZnTe substrates. This lower performance was attributed to an elevated dislocation density, which is in the mid-10⁶ cm⁻² range. The dislocation density in HgCdTe/Si needs to be reduced to <10⁶ cm⁻² to make high-performance LWIR detectors, and multiple approaches are being tried across the infrared community to achieve this result because the technological payoff is significant.     

Resonant cavity-enhanced mercury cadmium telluride detectors

The next-generation mercury cadmium telluride (HgCdTe) detectors will need to be able to spectrally resolve images to a degree far exceeding that currently available in two or even three color techniques. However, narrow, spectral pass bands will result in very low photon flux impinging on a detector. This paper investigates the use of resonant cavity-enhanced (RCE) detectors as a means of improving signal-to-noise performance of narrow spectral-width infrared (IR) detection systems.     

A comparative study and performance characteristics of ion-implanted and heterojunction short-wave infrared HgCdTe focal-plane arrays

Short-wave infrared (SWIR) HgCdTe focal-plane arrays (FPAs) with a cutoff wavelength of 2.5 μm have been produced using both planar ion-implanted and heterojunction-mesa device structures. The two-dimesnional FPAs are comprised of a 320×256 format with 30-μm pixel pitch and are cooled by a multistage thermo-electric (TE) cooler. Measured R₀A values of the two types of device structures show similar results below about 130 K because of the performance-limiting effect of the surface passivation of the heterojunction. However, a substantial difference is seen above 130 K and up to 300 K between the two structures types, with the heterojunction-mesa p-on-n device having an order of magnitude higher R₀A value than the planar ion-implanted n-on-p configuration. The difference in the R₀A values is reflected in the FPA images of the two different device types, where at 200 K, both FPAs display a clear picture with the n-on-p implanted device having a somewhat lesser resolution. However, no image can be seen from the planar-implanted FPA at 300 K, whereas the heterojunction-mesa FPA still exhibits a notable image at this temperature. These differences are examined and are attributed largely to higher diffusion and generation-recombination (g-r) currents that are thought to be prevalent in the ion-implanted n-on-p device structure. Yet, baking studies carried out show the ion-implanted diodes to be slightly more robust, as experiments reveal that they tend to survive a 120°C heat treatment longer than the mesa devices, which tend to degrade after a certain period of time. The nature of n-type donors in ion-implanted diodes is discussed, and a new theory based on Te antisites is proposed to explain recent experimental findings.     

Development and fabrication of two-color mid- and short-wavelength infrared simultaneous unipolar multispectral integrated technology focal-plane arrays

We report the development and fabrication of two-color mid-wavelength infrared (MWIR) and short-wavelength infrared (SWIR) HgCdTe-based focalplane arrays (FPAs). The HgCdTe multilayers were deposited on bulk CdZnTe (ZnTe mole fraction ∼3%) using molecular beam epitaxy (MBE). Accurate control of layer composition and growth rate was achieved using in-situ spectroscopic ellipsometry (SE). Epilayers were evaluated using a variety of techniques to determine suitability for subsequent device processing. These techniques included Fourier transform infrared (FTIR) spectroscopy, Hall measurement, secondary ion mass spectroscopy (SIMS), defect-decoration etching, and Nomarski microscopy. The FTIR transmission measurements confirmed SE’s capability to provide excellent compositional control with run-to-run x-value variations of ∼0.002. Nomarski micrographs of the as-grown surfaces featured cross-hatch patterns resulting from the substrate/epilayer lattice mismatch as well as various surface defects (voids and “microvoids”), whose densities ranged from 800–8,000 cm⁻². A major source of these surface defects was substrate particulate contamination. Epilayers grown following efforts to reduce these particulates exhibited significantly lower densities of surface defects from 800–1,700 cm⁻². Dislocation densities, as revealed by a standard defect-decoration etch, were 2–20×10⁵ cm⁻², depending on substrate temperature during epitaxy. The FPAs (128×128) were fabricated from these epilayers. Preliminary performance results will be presented.     

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.     

Estimation of the p-n junction depth in LWIR HgCdTe detectors from the spatial profile of the lateral photocurrent and transverse photovoltage induced by an infrared small spot

A new, simple nondestructive procedure for the estimation of the junction depth in planar long wavelength HgCdTe photodiodes is presented. The technique uses a combination of scanning light beam-induced current data with a transversal photovoltage measurement to extract the junction depth. The technique is applicable to both homojunction and heterojunction diodes. It is assumed that in the cross-section perpendicular to the device surface, the junction has a two-dimensional structure, and the length of the vertical junction is treated as the junction depth. The spatial profile of the lateral photocurrent produced by the spot scan marks the boundaries of the horizontal junction. Of these boundaries, only one of the vertical junctions contributes to the photocurrent. Using weakly absorbed infrared light insures the homogeneous profile of the photogenerated carriers along the vertical junction. A uniformly irradiated vertical junction produces the transverse photovoltage between its terminals. When the remote contacts are grounded, this photovoltage is dependent on the series resistance between the junction and the contacts. In the case when the circuit between the remote contacts is opened, the current is zero, and the photovoltage reaches its maximum value, equal to the junction open circuit voltage. From comparison of the two voltages, we extract the value of the series resistance. We have derived an analytical expression for the series resistance, which includes the junction depth. Comparison between the observed R_s values and those of the analytical expression, enable us to extract the junction depth value. The validity of the method is shown using a two-dimensional numerical simulation, and experimentally from ion-implanted and diffused p-n junctions.     

Percolation problem in boron—Implanted mercury cadmium telluride

We used high-resolution x-ray diffraction to measure precisely structural modifications in variously composed Hg_1−xCd_xTe layers which were fabricated by different growth techniques and subjected to boron implantation to form p-n junctions. Analysis of implantation-induced features in the diffraction profiles allowed us to deduce the interstitials concentration remaining in the sample interior and, thus, to obtain important information on post-implantation defect migration. As a result, a percolation problem in the migration of Cd interstitials was discovered in samples with x<x_c (x_c=0.265 is the percolation threshold). Due to the percolation problem, the implanted samples having Cd content below and above x_c exhibited very different surface recovery, which was visualized by high resolution scanning electron microscopy. It was found that additional annealing at 250–300°C stimulates diffusion of formerly locked Cd interstitials and leads to the change in the conductivity type (n-p) at the expense of remaining non-compensated vacancies. The percolation problem in samples with x < x_c seems to be responsible for limited mobility of implanted boron and difficulties in boron activation in Hg_1−xCd_xTe-based devices for 8–12 μm atmospheric transparency window.     

Molecular beam epitaxial growth and performance of HgCdTe-based simultaneous-mode two-color detectors

Molecular beam epitaxy was employed for the growth of HgCdTe-based n-p⁺-n device structures on (211)B oriented CdZnTe substrates. The device structures were processed as mesa isolated diodes, and operated as back-to-back diodes for the simultaneous detection of two closely spaced sub-bands in the mid-wave infrared spectrum. The devices were characterized by R₀A values in excess of 5 × 10⁵ Ω cm² at 78K, at f/2 fov and quantum efficiencies greater than 70% in each band. Infrared imagery from a focal plane array with 128 × 128 pixels was acquired simultaneously from each band at temperatures between 77 to 180K, with no observable degradation in the image quality with increase in temperature.     

Isolation and control of voids and void-hillocks during molecular beam epitaxial growth of HgCdTe

Formation of small voids and defect complexes involving small voids during the molecular beam epitaxial growth of mercury cadmium telluride on cadmium zinc telluride was investigated. Some of these defects were demonstrated to form away from the substrate-epi interface. Other defects were demonstrated to close before reaching the top surface without leaving any perturbations on the surface, thus remaining completely hidden. The voids, which formed away from the substrate-epifilm fixed interface, nucleated on defects introduced into the film already grown, leading to the formation of defect complexes, unlike the voids which nucleated at the substrate-epifilm fixed interface. These defect complexes are decorated with high density dislocation nests. The voids which closed before reaching the film surface usually also nucleated slightly away from the film-substrate interface, continued to replicate for a while as the growth progressed, but then relatively rapidly closed off at a significant depth from the film surface. These voids also appeared to form defect complexes with other kinds of defects. Correlations between these materials defects and performance of individual vertically integrated photodiode (VIP) devices were demonstrated, where the relative location of these defects with respect to the junction boundary appears to be particularly important. Elimination or reduction of fluctuations in relative flux magnitudes or substrate temperature, more likely during multi-composition layer growth, yielded films with significantly lower defect concentrations.     

Prospects of uncooled HgCdTe detector technology

Over the last several years cooled applications of HgCdTe at low temperatures have proliferated. Having low fundamental dark current at any given wavelength and temperature makes HgCdTe attractive for high temperature applications as well. We are exploring detectors with cut off wavelengths from the near to middle infrared region (∼1.5 to ∼4 μm). Theory allows applications from low light level imaging in starlight and “nightglow” to thermal imaging, both with useful sensitivities at room temperature. The demonstrated possibility of reducing or eliminating traditional recombination processes (radiative and Auger) further increase the attractiveness of HgCdTe. Current materials technology shows some evidence that these sensitivities can be attained. Current detector technology, being limited by SRH traps, appears to require modest cooling (to about 250K). Improved materials and processes should eliminate the need for even this cooling.     

Study of the pixel-pitch reduction for HgCdTe infrared dual-band detectors

The third generation of HgCdTe infrared-detector focal-plane arrays (FPAs) should be able to detect simultaneously in two spectral bands. The feasibility of this type of dual-band detectors has already been shown in our laboratory with a pixel size of 50 μm in the 3–5-μm wavelength range. To improve the detector resolution, it is necessary to decrease the pixel pitch. Dry etching is a key process technology to fulfill this goal because of the high aspect-ratio structures needed (typically 10–15-μm deep and 2–5-μm wide trenches). In this paper, we present results of a parametric study on HgCdTe dry etching, as well as results obtained on detector arrays made with the dry-etching technique. The etching study has been done in a microwave plasma reactor with the aim of controlling the surface roughness, the etch rate, and the slope of the trench side. We show how these parameters are influenced by the reactive gas-mixture composition (based on CH₄, H₂, and Ar) and the substrate self-bias. We show how polymer film deposition can prevent etching from occurring but can improve anisotropy. We show some examples of results obtained when manufacturing the trenches that separate the pixels, keeping a high fill factor, and anisotropic etching. We also show results of the material surface characterizations done with scanning electron microscopy (SEM) and Hall effect measurements. These studies allow us to evaluate and compare the damages done to the HgCdTe surface with different etching conditions. Our best process allows us to make a light electrical damage, confined to less than a micron deep in the material. Using the dry-etching process, we have developed detector arrays fabricated with a pixel pitch as low as 30 μm. We finally present the results of the first electrical characterizations made on these arrays, showing promising results for the development of high-resolution dual-band detectors.     

Comparison of In1−xTlx Sb and Hg1−x Cdx Te as long wavelength infrared materials

Cohesive energies, elastic constants, band structures, and phase diagram are calculated to evaluate the In_1−xTl_xSb alloy (ITA) as a long-wavelength infrared (LWIR) material compared to Hg_1−xCd_xTe (MCT). To obtain a 0.1 eV gap at zero temperature, the x value for ITA is estimated to be x=0.083 as compared to x=0.222 for MCT. At this gap, ITA is more robust than MCT because the cohesive energies order as InSb>TlSb>CdTe>HgTe, and ITA has the stronger bonding InSb as the majority component. Although TlSb is found to favor the CsCl structure, ITA is a stable alloy in the zincblende structure for low x values. However, our phase diagram indicates that it is difficult to grow the 0.1 eV gap ITA from the melt, because above the eutectic the liquidus curve is flat, and the solidus drops rapidly. Moreover, the width of the stable concentration range of the zincblende solid phase shrinks at low temperatures due to the presence of the CsCl structure.     

HgCdTe/CdTe/Si infrared photodetectors grown by MBE for near-room temperature operation

Conventional HgCdTe infrared detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. Although the need for cooling has long been thought to be fundamental and inevitable, it has been recently suggested that Auger recombination and generation rates can be reduced by using the phenomena of exclusion and extraction to produce nonequilibrium carrier distributions. The devices with Auger suppressed operation requires precise control over the composition, and donor and acceptor doping. The successful development of the molecular beam epitaxy (MBE) growth technique for multi-layer HgCdTe makes it possible to grow these device structures. Theoretical calculations suggest that the p n+ layer sequence is preferable for near-room temperature operation due to longer minority carrier lifetime in lightly doped p-HgCdTe absorber layers. However, because the low doping required for absorption and nonequilibrium operation is easier to achieve in n-type materials, and because Shockley-Read centers should be minimized in order to obtain the benefits of Auger suppression, we have focused on p⁺ n structures. Planar photodiodes were formed on CdTe/Si (211) composite substrates by As implantation followed by a three step annealing sequence. Three inch diameter Si substrates were employed since they are of high quality, low cost, and available in large areas. Due to this development, large area focal plane arrays (FPAs) operated at room temperature are possible in the near future. The structures were characterized by FTIR, x-ray diffraction, temperature dependent Hall measurements, minority carrier lifetimes by photoconductive decay, and in-situ ellipsometry. To study the relative influence of bulk and surface effects, devices with active areas from 1.6 10⁻⁵ cm² to 10⁻³ cm² were fabricated. The smaller area devices show better performance in terms of reverse bias characteristics indicating that the bulk quality could be further improved. At 80 K, the zero bias leakage current for a 40 m 40 m diode with 3.2 m cutoff wavelength is 1 pA, the R₀A product is 1.1 10⁴-cm² and the breakdown voltage is in excess of 500 mV. The device shows a responsivity of 1.3 10⁷ V/W and a 80 K detectivity of 1.9 10¹¹ cm-Hz^1/2/W. At 200 K, the zero bias leakage current is 5 nA and the R₀A product 2.03-cm², while the breakdown voltage decreases to 40 mV.     

InAsSb/InTISb superlattice: A proposed heterostructure for long wavelength infrared detectors

A novel superlattice (SL) heterostructure, comprising of InTlSb well and InAsSb barrier lattice matched to InSb, is proposed for long wavelength 8−12 urn detectors. Improvements in the InTlSb epilayers’ structural quality are expected, as it will be sandwiched between higher quality zinc-blende InAsSb epilayers. Preliminary energy band calculations of 30? InAs_0.07Sb_0.93/100? In_0.93Tl_0.07Sb SL show the band alignment favorable to type I with three heavy-hole subband confinement in the valence band and a partial electron subband confinement in the conduction band due to the small conduction band offset. Including the effect of strain indicates significant changes in the band offsets, with optical bandgap essentially unaltered. The optical band gap of this SL was computed to be 0.127 eV (9.7 μm) at OK, indicating its potential for long wavelength applications.     

Molecular beam epitaxial growth and performance of integrated two-color HgCdTe detectors operating in the mid-wave infrared band

The first report of molecular beam epitaxial growth and performance of HgCdTe two-color detectors for the simultaneous detection of radiation at 4.1 and 4.5 μm is presented. In-situ doped devices with the n-p-n architecture were grown by molecular beam epitaxy on (211)B CdZnTe substrates. Representative structures exhibited x-ray rocking curves with full width at half-maxima of 40–60 arcs. The typical near surface etch pit density in these structures were 4−7 × 10⁶ cm⁻². The devices were processed as mesa diodes and electrical contacts were made to the two n-type layers and the p-type layer to facilitate simultaneous operation of the two p-n junctions. The spectral response characteristics of the devices were characterized by sharp turn-on and turn-off for both bands, with R₀A values >5 × 10⁵ ωcm² at 77K. The detectors exhibited quantum efficiencies >70% in both bands.     

Bake stability of long-wavelength infrared HgCdTe photodiodes

The bake stability was examined for HgCdTe wafers and photodiodes with CdTe surface passivation deposited by thermal evaporation. Electrical and electrooptical measurements were performed on various long-wavelength infrared HgCdTe photodiodes prior to and after a ten-day vacuum bakeout at 80°C, similar to conditions used for preparation of tactical dewar assemblies. It was found that the bakeout process generated additional defects at the CdTe/ HgCdTe interface and degraded photodiode parameters such as zero bias impedance, dark current, and photocurrent. Annealing at 220°C under a Hg vapor pressure following the CdTe deposition suppressed the interface defect generation process during bakeout and stabilized HgCdTe photodiode performance.