William E. Tennant

Planar p‐on‐n HgCdTe heterostructure photovoltaic detectors

We report a process to fabricate planar Hg1−yCdyTe/Hg1−xCdxTe (x<y) heterostructure photodiodes with the p‐on‐n configuration. The material used for this demonstration was grown by molecular beam epitaxy. The p‐on‐n planar devices consist of an arsenic‐doped p‐type epilayer (y=0.28) on top of a long wavelength infrared n‐type epilayer (x=0.225, λ=10 μm). The planar junctions were formed by selective pocket diffusion of arsenic deposited by ion implantation. The detailed analysis of the current‐voltage characteristics of these diodes as a function of temperature show that they have high performance and that their dark currents are diffusion limited down to 52 K. The results also show that the R0A values for these devices are highly uniform at 77 K.

Molecular‐beam epitaxy growth and in situ arsenic doping of p‐on‐n HgCdTe heterojunctions

In this paper we present, results on the growth of in situ doped p‐on‐n heterojunctions on HgCdTe epilayers grown on (211)B GaAs substrates by molecular‐beam epitaxy (MBE). Long wavelength infrared (LWIR) photodiodes made with these grown junctions are of high performance. The n‐type MBE HgCdTe/GaAs alloy epilayer in these structures was grown at Ts=185 °C and it was doped with indium (high 1014 cm−3 range) atoms. This epilayer was directly followed by the growth, at Ts=165 °C, of an arsenic‐doped (1017–1018 cm−3 ) HgTe/CdTe superlattice structure which was necessary to incorporate the arsenic atoms as acceptors. After the structure was grown, a Hg annealing step was needed to interdiffuse the superlattice and obtain the arsenic‐doped p‐type HgCdTe layer above the indium‐doped layer. LWIR mesa diodes made with this material have 77 K R0A values of 5×103, 81, 8.5, and 1.1 Ω cm2 for cutoff wavelengths of 8.0, 10.2, 10.8, and 13.5 μm, respectively; the 77 K quantum efficiency values for these diodes were gre...

Attainment of high sensitivity at elevated operating temperatures with staring hybrid HgCdTe-on-sapphire focal plane arrays

Cost-effective high-performance IR imaging cameras need affordable staring focal plane arrays (FPAs) that can operate effectively at temperatures compatible with inexpensive long-life coolers. We report on staring hybrid 128 x 128 and 256 x 256 Hg1-xCdxTe FPAs that have requisite yield, sensitivity, operability, and reliability at a medium-wavelength IR (MWIR) cutoff wavelength (λc ~4.6 μm at 180 K) and elevated operating temperatures. Mean 256 x 256 FPA noise-equivalent temperature differences (NEΔT) using broadband f/1.7 optics were 4.3, 7.7, and 55 mK at 120, 140, and 180 K, respectively. We extrapolate that camera NEΔT ≤ 0.02 K can be achieved at 190 K using optimized (λc of ~4.4 μm (180 K), a 3.4- to 4.2-μm bandpass filter, and f/1 optics. Because the CMOS multiplexers have a low-power dissipation and need little ancillary circuitry in the dewar, a viable thermoelectrically-cooled FPA technology is thus implied once the λc is optimized for MWIR imaging.

Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

High-performance 5-um 640 x 480 HgCdTe-on-sapphire focal plane arrays

A high-performance 5-μm 640 X 480 HgCdTe/CdTe/Al2O3 infrared focal plane array (FPA) that offers full TV-compatible resolution with excellent sensitivity at temperatures below 120 K has been developed. Mean FPA D* at 95 K and background of 1014 photons/cm2 s is background-limited at ~1 x 1012 cm Hz1/2/W for the typical mean quantum efficiency of 60 to 70%. The key technology making this large, high-sensitivity device producible is the epitaxial growth of HgCdTe on a rugged CdTe-buffered sapphire substrate. Mean camera noise-equivalent temperature difference NEΔT of 13 mK has been achieved at ≤ 120 K operating temperature and 3.4- to 4.2-μm passband; this is about an order of magnitude better than similar currently available cameras, which use PtSi FPAs and require cooling to ≤ 77 K to maintain performance at low scene temperatures.

Molecular beam epitaxy (MBE) HgCdTe flexible growth technology for the manufacturing of infrared photovoltaic detectors

In this paper we present p-on-n heterostructure HgCdTe photovoltaic device data that illustrates the high performance and flexibility in band gap control of the molecular beam epitaxy (MBE) technology. This flexibility demonstration was carried out by growing material for operation in the following cut-off wavelength ((lambda) co) ranges of interest: LWIR [(lambda) co(77 K) equals 9-11 micrometers ], MLWIR [(lambda) co(77 K) equals 6-7 micrometers ], and VLWIR [(lambda) co(40 K) equals 20 micrometers ]. Detailed analyses of the current-voltage characteristics of these diodes as a function of temperature show that their dark currents are diffusion-limited down to 80 K, 50 K, and 30 K for the MLWIR, LWIR, and VLWIR photodiodes, respectively. In general, the RoA device values were uniform for the three band gap ranges when operating under diffusion limited conditions. The planar MBE HgCdTe technology has been further validated with the successful fabrication and operation of 64 X 64 hybrid FPAs.

SWIR staring FPA performance at room temperature

We report significant improvements in the performance of short wavelength infrared 128 X 128 focal plane arrays at room temperature. Using InGaAs and HgCdTe detector materials coupled to readout multiplexers having gate modulated detector interface, sensitivity that is near the theoretical detector-limited levels has been achieved via both low detector dark current and self-adjusting readout current gain. Extrapolating to nocturnal imaging conditions, the uncooled FPA-level sensitivities of 1.68 micrometers InGaAs and 1.86 micrometers HgCdTe arrays are shown to be within 35% and 80% of theoretical, respectively.

Molecular beam epitaxy (MBE) HgCdTe infrared focal plane array (IRFPA) flexible manufacturing

Extensive material and device statistics of performance and reproducibility are presented to show the maturity of this technology. The demonstration vehicles to monitor yields during this demonstration were long-wavelength infrared (LWIR) HgCdTe multilayer wafers with 128 X 128 detector arrays. The heterostructure photodetectors were of the p-on-n planar configuration. Device data show that MBE LWIR diode test structures have performance that equals that of p-on-n double heterostructure photodiodes made by LPE. Due to the special attention given to understanding and reducing epilayer growth-induced defects, we have achieved improvements in FPA operability values from 92% to 98%. These improvements have resulted in the demonstration of a 128 X 128 FPA hybrid that had detectivity (D*) background limited performance when operating at 80 K in a tactical background environment. Mean D* was 1.28 X 1011 cmHz1/2/W. The corresponding mean NE(Delta) T was an excellent 5.9 mK.

128 x 128 PACE-I HgCdTe hybrid FPAs for thermoelectrically cooled applications

Staring 128 X 128 hybrid HgCdTe FPAs have been demonstrated with very good sensitivity and operability at temperatures compatible with thermoelectric cooling (> 160 K). The FPAs consist of HgCdTe/sapphire (PACE-I; producible alternative to CdTe for epitaxy) detector arrays hybridized to a CMOS readout having a gate modulation input circuit. FPAs with SWIR (2.5 micrometers at 78 K) and MWIR (4.56 micrometers at 180 K) cutoff wavelengths ((lambda) co) were made and evaluated. The SWIR arrays were ZnS passivated; the MWIR arrays were CdTe-passivated. Though the (lambda) co of the MWIR devices was not specifically optimized for terrestrial imaging at TE-cooled temperatures in the preferred 3.4 to 4.1 micrometers band, very good sensitivity was achieved, particularly relative to other technologies at temperatures >= 120 K. Mean laboratory noise equivalent temperature differences (NE(Delta) T) at 120 K, 170 K, and 180 K were 0.0048 K, 0.053 K, and 0.061 K respectively, for the MWIR device. While the NE(Delta) T was measured without a spectral filter, the sensitivity for 3.4 to 4.1 micrometers bandpass extrapolates to camera NE(Delta) T <EQ 0.05 K, if f/1.5 or faster optics are used. Near BLIP Detectivity (D*) of 1.62 X 1013 cm-Hz1/2/W and mean NE(Delta) T of 0.04 K were measured on the SWIR hybrid at 22.5 msec integration time and operating temperatures <EQ 162 K. Imagery of corresponding quality was subsequently generated. Since the CMOS multiplexer dissipates little power and needs a minimum of support circuitry, a viable thermoelectrically cooled FPA technology is implied.

Isothermal vapor‐phase epitaxy of Hg1−xCdxTe on CdTe and Al2O3 substrates

The isothermal vapor‐phase‐epitaxial (ISOVPE) growth of device‐quality HgCdTe layers on both CdTe and CdTe/Al2O3 substrates has been demonstrated. The material and device properties on both types of substrates have been studied and compared with reported values for HgCdTe grown by other techniques. The as‐grown ISOVPE Hg1−xCdxTe (x≂0.3) epilayers are always p type with carrier concentrations of ∼5×1015 to 3×1016 cm−3 and mobilities of ∼230–260 cm2/V s at 77 K. The temperature and compositional dependence of electrical properties of ISOVPE Hg1−xCdxTe are somewhat different from those of liquid‐phase epitaxy (LPE) and bulk HgCdTe. In particular, the acceptor ionization energy, EA =7 meV, is about half that obtained in midwavelength infrared LPE or bulk HgCdTe, and nearly independent of composition x. The R0A product of a representative photodiode (λc ≂4.65 μm, 77 K) is 2×106 and 4 Ω cm2 at 77 and 195 K, respectively, with comparable device qualities seen on both CdTe and CdTe/Al2O3 substrates.