HgCdTe (the current infrared material of choice) lacks a scalable, sufficiently lattice-matched substrate suitable for long
wave infrared focal plane array production. One possible alternative material is HgCdSe. Similar to HgCdTe, HgCdSe is
a ternary alloy which can be tuned across the infrared spectrum. Unlike HgCdTe, HgCdSe is nearly lattice matched to
the scalable (and commercially available) substrate GaSb. Thus long wave infrared focal plane arrays could potentially
be fabricated from HgCdSe grown on GaSb, with a ZnTeSe or CdTeSe buffer layer added to alleviate the slight
mismatch.
Samples of HgCdSe were grown via molecular beam epitaxy using a Se thermal cracker source to compare to those
grown using a simple Se valved source. This allowed us to study any differences between layers grown with
predominantly Se2 flux versus Se6 flux. Optimal growth parameters were explored using this new effusion source for Se.
All HgCdSe samples grown with the simple valved source were heavily n-type (n~1017 cm-3) despite being nominally
undoped. However, when the valved Se effusion cell was replaced with the Se cracker source, the electron concentration
was reduced and began to show significant temperature dependence below 100K. Subsequent experiments suggested
this may be more related to different purity in the source material between the sources than the cracking process itself.
Annealing under Hg raised the electron concentration, while annealing under Se lowered the concentration.
Much progress has been made in developing high quality HgCdTe/Si for large area focal plane array (FPA)
applications. However, even with all the material advances made to date, there is no guarantee that this technology will
be mature enough to meet the stringent FPA specifications required for long wavelength infrared (LWIR) systems. With
this in mind, the Army Research Laboratory (ARL) has begun investigating HgCdSe material for infrared (IR)
applications. Analogous to HgCdTe, HgCdSe is a tunable semiconductor that can detect any wavelength of IR radiation
through control of the alloy composition. In addition, several mature, large area bulk III-V substrates are nearly latticematched
to HgCdSe giving this system a possible advantage over HgCdTe in which no scalable, bulk substrate
technology exists. We have initiated a study of the growth of HgCdSe using molecular beam epitaxy (MBE). Growth
temperature and material flux ratios were varied to ascertain the best growth conditions and study defect formation.
Smooth surface morphology has been achieved using a growth temperature much lower than HgCdTe. Preliminary data
suggest a linear relationship between the Se/Cd flux ratio used during growth and the cut-off wavelength as measured by
FTIR.
Growth of ZnTe and HgCdSe on Si has been pursued using molecular beam epitaxy (MBE) as a new
class of IR materials. Besides, ZnTe/Si can also be used as a lattice-matching, large area and low cost
alternate substrate for other III-V and II-VI compound semiconductors, such as GaSb based type-II
superlattice materials around 6.1A. We report in this paper our systematic studies on MBE growth conditions
for ZnTe(211) on Si and highlights of MBE growth of HgCdSe on ZnTe/Si. A close to optimal growth
window has been established for MBE growth of ZnTe(211)/Si(211) to achieve high crystalline quality, low
defect and dislocation densities as well as excellent surface morphology. Using this baseline MBE growth
process, we are able to obtain ZnTe(211)/Si wafers with X-ray full-width at half-maximum (FWHM) as low
as 70 arcsec, low dislocation density (~105 cm-2) and defect density (1000 cm-2).
Mercury Cadmium Telluride (HgCdTe) is the material of choice for the majority of high performance infrared focal
plane array (IRFPA) systems fielded in the Army with state-of-the-art HgCdTe growth using a bulk Cadmium Zinc
Telluride (CdZnTe) substrate. However, as the push for larger array sizes continues, it has been recognized that an
alternative substrate technology will be required for HgCdTe IRFPAs. A major effort has been placed in developing
CdTe/Si as such a substrate. Although successful for short-wavelength (SWIR) and mid-wavelength (MWIR) focal
plane arrays, current HgCdTe/Si material quality is insufficient for long-wavelength (LWIR) arrays due to the high
density of dislocations present in the material. In this paper, we will discuss several processes being developed at the
U.S. Army Research Laboratory (ARL) to overcome this issue. Effort has been placed on both composite substrate
development and improvement, and on HgCdTe/Si post-growth processes. Recently, we have demonstrated HgCdTe/Si
material with dislocation density measuring 1 × 106 cm-2. This is a five times reduction in the baseline material
dislocation density currently used in the fabrication of devices.
Intrinsic carriers play a dominant role especially in the long wavelength (8-12 μm cut-off) HgCdTe material near ambient temperatures due to high thermal generation of carriers. This results in low minority carrier lifetimes caused by Auger recombination processes. Consequently, this low lifetime at high temperatures results in high dark currents and subsequently high noise. Cooling is one means of reducing this type of detector noise. However, the challenge is to design photon detectors to achieve background limited performance (BLIP) at the highest possible operating temperature; with the greatest desire being close to ambient temperature operation. We have demonstrated a unique planar device architecture using a novel approach in obtaining low arsenic doping concentrations in HgCdTe. Results indicate Auger suppression in P+/π/N+ devices at 300K and have obtained saturation current densities of the order of 3 milli Amps-cm2 on these devices.
At the Army Research Laboratory (ARL), a new ternary semiconductor system CdSexTe1-x/Si(211) is being investigated as an alternative substrate to bulk-grown CdZnTe substrates for HgCdTe growth by molecular beam epitaxy. Under optimized conditions, best layers show surface defect density less than 400 cm-2 and full width at half maximum of X-ray double crystal rocking curve as low as 100 arc-sec with excellent uniformity over 3 inch area. LW-HgCdTe layers on these compliant substrates exhibit comparable electrical properties to those grown on bulk CZT substrates. Photovoltaic devices fabricated on these LWIR material shows diffusion limited performance at 78K indicating high quality material. Measured RoA at 78K on λco = 10 μm material is on the order of 340 Ω-cm2. In addition to single devices, we have fabricated 256x256 2-D arrays with 40 μm pixel pitch on LW-HgCdTe grown on Si compliant substrates. Data shows excellent QE operability of 99% at 78K under a tactical background flux of 6.7x1015 ph/cm2sec. Most probable dark current at the peak distribution is 5.5 x 109 e-/sec and is very much consistent with the measured RoA values from single devices. Initial results indicate NETD of 33 mK for a cut-off wavelength of 10 μm with 40 micron pixels size. This work demonstrates CdSexTe1-x/Si(211) substrates provides a potential road map to more affordable, robust 3rd generation FPAs.
High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation.
In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures.
We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.
For the first time, cathodoluminescence of CdSexTe1-x (with x = 0-1) films grown by molecular beam epitaxy on (211) Si substrates were systematically studied and compared with photoluminescence. The Se mole fraction was consistently determined by x-ray rocking-curve diffraction, wavelength-dispersive spectroscopy, and Rutherford backscattering. The band gap energy, as determined by both cathodoluminescence and photoluminescence, was found consistent with literature. The band gap energy varied parabolically with composition as predicted by theory. The results suggest cathodoluminescence can be used to conveniently map composition fluctuations such as Se segregation in CdSexTe1-x films, with higher spatial resolution than photoluminescence.
Mid wavelength infrared (MWIR) HgCdTe heterostructures were grown on 3-inch dia Si (211) substrates by the molecular beam epitaxy technique and p+n format devices were fabricated by arsenic ion implantation. Very long wavelength infrared (VLWIR) layers have been employed as interfacial layers to block the propagation of detects from the substrate interface into the HgCdTe epilayers. Excellent material characteristics including the minority carrier lifetime of 7.2 usec at 200K and 2 usec at 80K in the n-HgCdTe absorber layer with 5 um cut-off wavelength at 80K were achieved. The photovoltaic detectors fabricated on these MWIR heterostructures show excellent zero-bias resistance-area product (R0A) on the order of 108 ohm-cm2 and peak dynamic impedances on the order of 109 ohm-cm2. A two-step arsenic activation anneal followed by the 'Hg' vacancy filling anneal (third step) is shown to produce the best R0A values, since the intermediate temperature annealing step seems to control the diffusion of arsenic, assisted by the implantation-induced defects. The experimental R0A values are compared with that predicted by theory based on a one-dimensional model, indicating g-r limited performance of these MWIR devices at 80K.
High Performance LWIR Focal Plane Arrays are critical for many space applications. Reliable LWIR focal plane arrays are needed for these applications that can operate in space environment without any degradation.
In this paper, we present various LWIR detector array architectures currently being evaluated for LWIR applications. These include backside-illuminated configurations for HgCdTe fabricated on CdZnTe and Silicon substrates. To optimize the LWIR device performance, minimize the anti-reflection losses, and significant reduction in the effects of solarization in space, innovative Anti-reflection coatings are needed, that will enhance the performance of the LWIR detector / focal plane arrays.
We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 8-14 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.
At the Army Research Laboratory (ARL), a new ternary semiconductor system CdSexTe1-x/Si(211) is being investigated as an alternative substrate to Bulk-grown CdZnTe substrates for HgCdTe growth by molecular beam epitaxy. Under optimized conditions, best layers show surface defect densities less than 400 cm-2 and full width at half maximum as low as 100 arcsec with excellent uniformity over 3 inch area. LWIR HgCdTe on CdTe/Si substrates have also been grown and characterized with optical, x-ray diffraction, etch pit etching and Hall effect measurements. Photo Voltaic devices fabricated on these LWIR material shows G-R limited performance at 78K indicating detector performance is not limited by the bulk properties of the grown material.
Research on silicon based composite substrates is being conducted at the Army Research Laboratory. These substrates can be used to deposit HgCdTe alloys to fabricate large-format infrared photodetector arrays. Traditionally, composite structures are fabricated by growing CdZnTe buffer layers on Si substrates using molecular beam epitaxy process. Recently, we have demonstrated that composite structures using CdSeTe can also be used. The CdSeTe compound offers better surface morphology and control of composition. In this work we present our results on the Si-based substrate technology and its application in the use of substrate material for LWIR HgCdTe detector development. In this paper we also present our study of molecular beam epitaxy and characteristics of CdSexTe1-x ternary films on Si. A detailed study of the alloy composition and lattice structures were investigated. In general, we find that the crystalline quality of CdSeTe films on Si is superior to CdZnTe on Si. Best CdSeTe/Si samples had EPD as low as 1.4x105 cm-2. This study also discusses a comparison of cation versus anion mixing in chalcogenide compounds. Results of LWIR detectors on CdTe/Si are also presented as a precursor and rational for a need of better lattice-matched substrates other than the conventional CdZnTe/Si substrates.
The epitaxial growth of Hg1-xCdxTe in the composition range 0.40 < x < 0.17 has been carried out on 3-inch CdTe/Si substrates mounted on indium-free molybdenum substrate holders. Because this mounting configuration prevents the effective use of a direct thermocouple contact to control the sample temperature, and because a dramatic change in the surface emissivity of the sample occurs during the onset of HgCdTe nucleation, an alternative method for controlling the surface temperature is developed. We utilize reflection high-energy electron diffraction (RHEED) and a thermocouple ramping sequence to maintain a constant HgCdTe surface temperature. Due to the narrowness of the HgCdTe growth window, small variations in the surface temperature produce a slight but observable change in the RHEED pattern. Through careful observation of the RHEED images, an optimized thermocouple ramping process is obtained such that the RHEED pattern remained constant from the onset of HgCdTe nucleation. Structural and electrical characterization of these samples demonstrate the usefulness of the temperature ramping methodology. For middle wavelength IR (MWIR) material, mobility measurements made on several n-type samples at 77 K range give values in the 2 X 104 - 4 X 104 cm2/Vsec range with doping levels in the low 1014 cm-3. Additionally, preliminary lifetime measurements made on one MWIR sample gives 2.8 microsecond(s) ec. For long wavelength IR material, mobility measurements made on several n-type samples at 77 K give values in the 3 X 105 to 5 X 105 cm2/Vsec range with doping levels in the mid 1015 cm-3. Electrical, structural and defect characterization along with device results are presented with a focus on the optimization of the thermocouple ramping process. In addition, the efficacy of Si-based composite substrates for the technological advancement of large format IR focal plane arrays will be discussed.
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