Type-II superlattices (T2SLs) have several fundamental advantages over bulk infrared-sensitive materials due to larger band edge effective masses and the ability to have their band structures engineered to suppress Auger recombination, leading to lowering tunneling currents, longer carrier lifetimes and higher ideal sensitivity. Realizing in practice the potential performance gains relies heavily on reducing the number or efficacy of defects that form Shockley-Read-Hall (SRH) recombination centers, which otherwise limit carrier lifetimes. InAs/GaInSb T2SLs typically have relatively short minority carrier lifetimes in comparison with bulk HgCdTe, which has limited the detectivities of photodetectors based on these T2SLs at both cryogenic and ambient operating temperatures. Studies have shown that InAs/InAsSb T2SLs lattice matched to GaSb substrates are comparable in ideal photodiode performance to InAs/GaInSb ones. Reducing the electrical activity of defects by passivating them with hydrogen is equivalent to lowering their density, and has proven successful in other semiconductor systems. We report here results from Ga-free and Ga-containing T2SLs exposed to inductively-coupled plasmas (ICPs). Our technical approach consisted of characterizing the basic material properties of LWIR InAs/InAsSb T2SL wafers and device performance of LWIR InAs/GaSb T2SL photodiodes that were bulk-passivated with atomic hydrogen, and comparing with unpassivated samples. On average, the in-plane Hall electron mobility increased from 1800 cm2/Vs to 6800 cm2/Vs after hydrogenation. ICP hydrogenation also improved the minority carrier lifetime for each of the explored ICP conditions. Lifetime values increased from an average of 80 ns before hydrogenation to almost 200 ns, a relative increase of over 200%, suggest that some recombination-mediating defects have been at least partially passivated. The Hall mobility improvements were found to be rather stable over the considered short periods of room temperature storage.
The detection of infrared radiation is of great importance for many defense and civilian applications. Eyesafe short-wavelength infrared (SWIR) spectral range is particularly interesting due to atmospheric propagation through obscurants. Applications include low-cost, long-range target identification, identification of heavily obscured targets, obstacle avoidance, and high resolution imaging from a variety of platforms including hand-held devices, unmanned air vehicles, or ground vehicles. HgCdTe grown on CdTe/Si by molecular beam epitaxy (MBE) was processed into mini-arrays for 1.55 μm LADAR applications. Low-capacitance photodiodes (<10 pF) were demonstrated at room temperature with frequency responses exceeding 100 MHz. This paper discusses the device architecture and device performance results.
Spatial noise and the loss of photogenerated current due material non-uniformities limit the performance of long
wavelength infrared (LWIR) HgCdTe detector arrays. Reducing the electrical activity of defects is equivalent to
lowering their density, thereby allowing detection and discrimination over longer ranges. Infrared focal plane arrays
(IRFPAs) in other spectral bands will also benefit from detectivity and uniformity improvements. Larger signal-to-noise
ratios permit either improved accuracy of detection/discrimination when an IRFPA is employed under current operating
conditions, or provide similar performance with the IRFPA operating under less stringent conditions such as higher
system temperature, increased system jitter or damaged read out integrated circuit (ROIC) wells. The bulk passivation of
semiconductors with hydrogen continues to be investigated for its potential to become a tool for the fabrication of high
performance devices. Inductively coupled plasmas have been shown to improve the quality and uniformity of
semiconductor materials and devices. The retention of the benefits following various aging conditions is discussed here.
Inductively coupled plasma (ICP) chemistry based on a mixture of CH4, Ar, and H2 was investigated for the purpose of delineating HgCdTe mesa structures and vias typically used in the fabrication of second and third generation infrared
photo detector arrays. We report on ICP etching uniformity results and correlate them with plasma controlling
parameters (gas flow rates, total chamber pressure, ICP power and RF power). The etching rate and surface morphology
of In-doped MWIR and LWIR HgCdTe showed distinct dependences on the plasma chemistry, total pressure and RF
power. Contact stylus profilometry and cross-section scanning electron microscopy (SEM) were used to characterize the
anisotropy of the etched profiles obtained after various processes and a standard deviation of 0.06 &mgr;m was obtained for
etch depth on 128 x 128 format array vias. The surface morphology and the uniformity of the etched surfaces were
studied by plan view SEM. Atomic force microscopy was used to make precise assessments of surface roughness.
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.
The optimal performance of HgTe/CdTe superlattice-based LWIR (8-12 μm cutoff wavelengths) and VLWIR (greater than 12um cutoff wavelength) photovoltaic detectors is assessed theoretically. The electronic band structures and optical absorption spectra are computed with a fourteen-band restricted-basis envelope function Hamiltonian. Auger and radiative lifetimes are computed with these accurate band structures. Vertical carrier mobilities are obtained from a Monte Carlo transport methodology. Photon detectors are modeled by solving current continuity and Poisson's equations. Predictions are compared with those for HgCdTe-alloy based detectors. We find that the superlattice-based two-color detector promise sharp rises in quantum efficiencies near the cutoff wavelengths, reflecting the quasi-2-dimensional nature of their density of states.
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.
The cost and performance of hybrid HgCdTe infrared focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, are available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an infrared sensor technology based on monolithically integrated infrared focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts, (2) the low temperature cleaning of Si(001) wafers, (3) growth of CdTe and HgCdTe layers on read-out integrated circuits, (4) array fabrication, interconnection between focal plane array and read-out integrated circuit input nodes and demonstration of the photovoltaic operation, and (5) maintenance of the read-out integrated circuit characteristics after substrate cleaning, molecular beam epitaxy growth and device fabrication. Crystallographic, optical and electrical properties of the grown layers are presented. Electrical properties for diodes fabricated on misoriented Si and read-out integrated circuit substrates are discussed. The fabrication of arrays with demonstrated I-V properties show that monolithic integration of HgCdTe-based infrared focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the 3rd generation of infrared systems.
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.
The advantages of mercury cadmium telluride for 'HOT' IR detector applications are discussed. Molecular beam epitaxy (MBE) is used to grow advanced device structures for this purpose. MBE offers the potential to grow HgCdTe heterostructure layers on large silicon substrates leading to very large format and high performance IR focal plane array sin the future. Preliminary material and device properties achieved p+-v-n+ device structures grown on 3 inch oriented silicon wafers are discussed.
The annealing and electrical properties of extrinsic in situ doped mercury cadmium telluride epilayers grown by molecular beam epitaxy (MBE) on B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers of CdTe mole fraction in the range of mid-wavelength IR are grown at substrate temperatures of 175-185 degrees C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. The analysis of the Hall coefficient in the temperature range of 40-300 K with a fitting based on a three-band non-parabolic Kane model, a fully ionized compensating donor concentration, and tow independent discrete acceptor levels is reported. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed-type characteristics whereas annealed samples showed p-type characteristics. Activation annealing at different temperatures was performed. Conversion to p-type at lower than conventional annealing temperatures was achieved. Theoretical models are utilized to understand the dependence of the activated arsenic concentration on the annealing temperature.