M. B. Reine

Growth and characterization of P‐on‐n HgCdTe liquid‐phase epitaxy heterojunction material for 11–18 μm applications

The capability of growing long‐wavelength infrared HgCdTe liquid‐phase epitaxy P‐on‐n heterojunction films with state‐of‐the‐art photodiode performance, with excellent thickness uniformity (±10%), and with excellent cutoff wavelength uniformity (e.g., 10.5±0.1 μm) across 2.5 cm×4.0 cm wafers has been demonstrated. In addition, we have extended the region of HgCdTe photodiode operation to wavelengths of 18–19 μm at 80 K. Both measured carrier lifetime and photodiode data show that the n‐type HgCdTe base layers are of excellent quality, with 77 K carrier lifetimes at the calculated Auger‐1 limit for film carrier concentrations above 4×1014 cm−3. The R0A products for large‐area diodes (10−3 cm2 ) with cutoff wavelengths of 11–19 μm are consistent with n‐side diffusion current calculated using the film Auger‐1 lifetime. Smaller diodes of area 1×10−5 cm2 have typical R0A values of 12 Ω cm2 at 80 K for a 12.2 μm cutoff wavelength. Large area diodes with an 80 K cutoff wavelength of 18–19 μm have R0A products of...

Independently accessed back-to-back HgCdTe photodiodes: a new dual-band infrared detector

We report the first data for a new two-color HgCdTe infrared detector for use in large dual-band infrared focal plane arrays (IRFPAs). Referred to as the independently accessed back-to-back photodiode structure, this novel dual-band HgCdTe detector provides independent electrical access to each of two spatially collocated back-to-back HgCdTe photodiodes so that true simultaneous and independent detection of medium wavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) infrared radiation can be accomplished. This new dual-band detector is directly compatible with standard backside-illuminated bump-interconnected hybrid HgCdTe IRFPA technology. It is capable of high fill factor, and allows high quantum efficiency and BLIP sensitivity to be realized in both the MW and LW photodiodes. We report data that demonstrate experimentally the key features of this new dual-band detector. These arrays have a unit cell size of 100 x 100 μm2, and were fabricated from a four-layer p-n-N-P HgCdTe film grown in situ by metalorganic chemical vapor deposition on a CdZnTe substrate. At 80K, the MW detector cutoff wavelength is 4.5 μm and the LW detector cutoff wavelength is 8.0 μm. Spectral crosstalk is less than 3%. Data confirm that the MW and LW photodiodes are electrically and radiometrically independent.

Simultaneous MW/LW dual-band MOVPE HgCdTe 64x64 FPAs

We report results for 64 X 64 simultaneous MW/LW dual-band HgCdTe Focal Plane Arrays (FPAs). The MW and LW average cutoff wavelengths at 78 K are in the 4.27 - 4.35 micrometer and 10.1 - 10.5 micrometer ranges respectively. The unit cell size is 75 X 75 micrometer2. These staring dual-band FPAs exhibit high average quantum efficiencies (MW: 79%; LW:67%), high median detectivities (MW: 4.8 X 1011 cm- (root)Hz/W; LW: 7.1 X 1010 cm-(root)Hz/W), low median NE(Delta) Ts (MW: 20 mK; LW: 7.5 mK for TSCENE equals 295 K and f/2.9), large dynamic ranges (MW: 77 dB; LW: 75 dB), and 87% stare efficiencies for both the MW and LW spectral bands. The dual-band HgCdTe detector array is fabricated from a four- layer P-n-N-P film grown in situ by MOVPE. The dual-band silicon CMOS input circuit utilizes a unique floating-direct- injection approach to achieve separate and simultaneous integration of both bands within each unit cell. There are two Compact Signal Averager circuits in each unit cell, to average subframes within every frame for each spectral band, allowing full stare efficiency in both spectral bands, as well as variable band-independent transimpedance gains. These data confirm that all key features of our P-n-N-P dual-band HgCdTe detector and our dual-band input circuit function as designed.

HgCdTe photodiodes for IR detection : A review

This paper reviews recent advances in photovoltaic (PV) HgCdTe infrared detector technology. Recent advances have enabled a new generation of spaceborne multispectral instruments for remote sensing applications, and have led to the practicality of dual-band (or two-color) IR focal plane array technology. The focus of this paper is on the back-illuminated HgCdTe PV arrays that have made this new generation of spaceborne instruments possible.

Metalorganic chemical vapor deposition of HgCdTe p/n junctions using arsenic and iodine doping

We report new results on metalorganic chemical vapor deposition (MOCVD)in situ growth of long wavelength infrared (LWIR) P-on-n and medium wavelength infrared (MWIR) n-on-P HgCdTe heterojunction photodiodes using the interdiffused multilayer process (IMP). The n-type regions are doped with iodine using the precursor ethyl iodide (El). I-doped HgCdTe using El has mobilities higher than that obtained on undoped background annealed films and are comparable to LPE grown In-doped HgCdTe. The p-type layers are doped with arsenic from either tertiarybutylarsine (TBAs) or a new precursor,tris-dimethylaminoarsenic (DMAAs). The substrates used in this work are lattice matched CdZnTe oriented (211)B or (100)4°→«110». Junction quality was assessed by fabricating and characterizing backside-illuminated arrays of variable-area circular mesa photodiodes. This paper presents four new results. First, carrier lifetimes measured at 80K on arsenic doped single HgCdTe layers are generally longer for films doped from the new precursor DMAAs than from TBAs. Second, we present data on the first P-on-n HgCdTe photodiodes grownin situ with DMAAs which have R0A products limited by g-r current at 80K for λco = 7–12 μm, comparable to the best R0A products we have achieved with TBAs. Third, we report the first experimental data on a new HgCdTe device architecture, the n-on-P heterojunction, with a wide gap p-type layer which allows radiation incident through the substrate to be absorbed in a narrower gap n-type layer, thereby eliminating interface recombination effects. With the n-on-P architecture, MWIR photodiodes were obtained reproducibly with classical spectral response shapes, high quantum efficiencies (70-75%) and R0A products above 2 x 105 ohm-cm2 for λco = 5.0 μm at 80K. Fourth, we report 40K data for LWIR P-on-n HgCdTe heterojunction photodiodes (using TBAs), with R0A values of 2 x 104 ohm-cm2 for λco = 11.7 μm and 5 x 105 ohm-cm2 for λco- 9.4 μm. These are the highest R0A values reported to date for LWIR P-on-n heterojunctions grownin situ by MOCVD.

Improved arsenic doping in metalorganic chemical vapor deposition of HgCdTe and in situ growth of high performance long wavelength infrared photodiodes

Controlled and effective p-type doping is a key ingredient forin situ growth of high performance HgCdTe photodiode detectors. In this paper, we present a detailed study of p-type doping with two arsenic precursors in metalorganic chemical vapor deposition (MOCVD) of HgCdTe. Doping results from a new precursortris-dimethylaminoarsenic (DMAAs), are reported and compared to those obtained from tertiarybutylarsine (TBAs). Excellent doping control has been achieved using both precursors in the concentration range of 3 × 1015-5 × 1017 cm−3 which is sufficient for a wide variety of devices. Arsenic incorporation efficiency for the same growth temperature and partial pressure is found to be higher with DMAAs than with TBAs. For doping levels up to 1 × 1017 cm−3, the alloy composition is not significantly affected by DMAAs. However, at higher doping levels, an increase in the x-value is observed, possibly as a result of surface adduct formation of DMAAs dissociative products with dimethylcadmium. The activation of the arsenic as acceptors is found to be in the 152–50% range for films grown with DMAAs following a stoichiometric anneal. However, a site transfer anneal increases the acceptor activation to near 100%. Detailed temperature dependent Hall measurements and modeling calculations show that two shallow acceptor levels are involved with ionization energies of 11.9 and 3.2 meV. Overall, the data indicate that DMAAs results in more classically behaved acceptor doping. This is most likely because DMAAs has a more favorable surface dissociation chemistry than TBAs. Long wavelength infrared photodiode arrays were fabricated on P-on-n heterojunctions, grownin situ with iodine doping from ethyl iodide and arsenic from DMAAs on near lattice matched CdZnTe (100) substrates. At 77K, for photodiodes with 10.1 and 11.1 (im cutoff wavelengths, the average (for 100 elements 60 × 60 µm2 in size) zero-bias resistance-area product, R0A are 434 and 130 ohm-cm2, respectively. Quantum efficiencies are ≥50% at 77K. These are the highest R0A data reported for MOCVDin situ grown photodiodes and are comparable to state-of-the-art LPE grown photodiodes processed and tested under identical conditions.

AlGaN UV focal plane arrays

This paper presents characterization data, including UV imagery, for 256 x 256 AlGaN UV Focal Plane Arrays (FPAs). The UV-FPAs have 30 x 30 μm 2 unit cells, and use back-illuminated arrays of AlGaN p-i-n photodiodes operating at zero bias voltage. The photodiode arrays were fabricated from multilayer AlGaN films grown by MOCVD on sapphire substrates. Data are also presented for individual AlGaN photodiodes and variable-area diagnostic arrays.

HgCdTe MWIR back-illuminated electron-initiated avalanche photodiode arrays

This paper reports performance data for back-illuminated planar n-on-p HgCdTe electron-initiated avalanche photodiode (e-APD) 4×4 arrays with large-area unit cells (250×250 μm2). The arrays were fabricated from p-type HgCdTe films grown by LPE on CdZnTe substrates. The arrays were bump-mounted to fanout boards and were characterized in the back-illuminated mode. Gain increases exponentially with reverse bias voltage, and gain versus bias curves are quite uniform from element to element. The maximum gain measured is 648 at -11.7 V for a cutoff wavelength of 4.06 μm at 160 K. For the same reverse bias voltage, the gain at 160 K for elements with two different cutoff wavelengths (3.54 and 4.06 μm at 160 K) increases exponentially with increasing cutoff wavelength, in agreement with Becks empirical model for gain versus voltage in HgCdTe e-APDs. Spot scan data show that both the V=0 response and the gain at V=-5.0 V are quite uniform spatially over the large junction area. To the best of our knowledge, these are the first spot scan data for avalanche gain ever reported for HgCdTe e-APDs. Capacitance versus voltage data are consistent with an ideal abrupt junction having a donor concentration equal to the indium counterdoping concentration in the as-grown LPE film. Calculations predict that bandwidths of 500 MHz should be readily achievable in this vertical collection geometry, and that bandwidths as high as 3 GHz may be possible with careful placement of the junction relative to the compositionally interdiffused region between the HgCdTe LPE film and the CdZnTe substrate.

Advances in 15-um HgCdTe photovoltaic and photoconductive detector technology for remote sensing

There has been significant progress during the past several years in photovoltaic (PV) HgCdTe technology for advanced long wavelength remote sensing applications. Useful cutoff wavelengths have been extended to beyond 17.0 micrometer. Junction quality has been improved to the point that D* greater than 3 multiplied by 1011 cm-(root)Hz/W can be achieved in arrays at temperatures of 60 - 65 K. The atmospheric infrared sounder (AIRS) instrument, scheduled for launch in the year 2000 as part of the NASA EOS Program, uses over 4000 PV HgCdTe detector elements organized into ten linear multiplexed arrays, with cutoff wavelengths extending as far as 15.0 micrometer at 60 K. The AIRS instrument also uses two long linear arrays of photoconductive (PC) HgCdTe detectors for the 13.7 - 15.4 micrometer band. These PC detector arrays have cutoff wavelengths of 16.0 - 17.0 micrometer and achieve D* values of 3 - 5 multiplied by 1011 cm-(root)Hz/W at 60 K. PV HgCdTe offers many advantages over PC HgCdTe for advanced remote sensing instruments: negligible 1/f noise, much higher impedance so that cold preamps or multiplexers are possible, configurational versatility with backside- illuminated two-dimensional arrays of closely spaced elements, 10X - 100X better linearity, dc coupling for measuring the total incident photon flux, and a (root)2 higher BLIP D* limit. In this paper we compare the relative merits of PV and PC HgCdTe for advanced remote sensing instruments, and we review recent data for both PV and PC HgCdTe arrays with cutoff wavelengths as long as 17.5 micrometer.

Numerical Simulation of InAs nBn Back-Illuminated Detectors

We are developing a numerical simulation model for back-illuminated nBn detectors. This model is based on the simultaneous numerical solution of the carrier continuity equations and Poisson equation on a three-dimensional finite-element grid. This paper reports our initial results for one-dimensional simulations of a back-illuminated nBn device with an InAs absorber layer and an AlAsSb barrier layer. These results include spatial profiles for the energy bands and the hole concentration profiles for a wide range of bias voltages, temperatures, and illumination fluxes. These results help to illustrate the underlying physics of the nBn device. In particular, these results show the influence of the doping type and concentration in the barrier layer on the dark current and the photocurrent, and demonstrate that the quantum efficiency at zero bias voltage can be degraded significantly by barrier layer doping concentrations that are too high.