R. E. Dewames

Origin of void defects in Hg 1-x Cd x Te grown by molecular beam epitaxy

Characterization of defects in Hg1−xCdxTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg1−xCdxTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm−2. Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.

Performance of HgCdTe, InGaAs and quantum well GaAs/AlGaAs staring infrared focal plane arrays

The ability to hybridize various detector arrays in disparate technologies to an assortment of state-of-the-art silicon readouts has enabled direct comparison of key IR detector technologies including photovoltaic (PV) HgCdTe/Al2O3, PV HgCdTe/CdZnTe, PV InGaAs/InP, and the photoconductive (PC) GaAs/AlGaAs quantum well IR photodetector (QWIP). The staring focal plane arrays range in size from 64 X 64 to 1024 X 1024; we compare these IR detector technologies versus operating temperature and background flux via hybrid FPA test at operating temperatures from 32.5 K to room temperature and photon backgrounds from mid-105 to approximately equals 1017 photons/cm2-s. Several state-of-the-art IR FPAs are included: a 1.7 micrometers 128 X 128 InGaAs hybrid FPA with room temperature D of 1.5 X 1013 cm-Hz1/2/W and 195K D of 1.1 X 1015 cm-Hz1/2/W; a 3.2 micrometers 1024 X 1024 FPA for surveillance; a 4.6 micrometers 256 X 256 HgCdTe/Al2O3 FPA for imaging with BLIP NE(Delta) T of 2.8 mK at 95K; and a 9 micrometers 128 X 128 GaAs QWIP with 32.5 K D > 1014 cm-Hz1/2/W at 32.5K and 8 X 1010 cm-Hz1/2W at 62K.

Variable-area diode data analysis of surface and bulk effects in MWIR HgCdTe/CdTe/sapphire photodetectors

The authors investigate the separate dark current components which are dominant in the diffusion-limited regime in MWIR n/p HgCdTe/CdTe/sapphire photodetectors. Both mesa and planar configurations of variable-area diodes were fabricated and evaluated over the temperature range from 78 to 250 K. Simple analytical expressions are used to calculate the contributions of bulk, lateral and surface effects from the perimeter/area dependence of R0A and measurement of the minority carrier diffusion length. The analysis indicates that at 180 K the mesa diode results can be accounted for by bulk and lateral currents, but that the planar diodes are limited by surface currents. The 180 K median R0A for the mesa diodes ranges from 63 Omega cm2 for 500*500 mu m2 diode areas to 14 Omega cm2 for 30*30 mu m2 diodes at a cut-off wavelength of 4.64 mu m. Scanning laser microscope measurements determine the 180 K electron minority carrier diffusion length to be 17-18 mu m.

Current mechanisms in VLWIR Hg1−xCdxTe photodiodes

VLWIR (c∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance. Id−Vd curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range. At 30 K the detectors are limited by tunneling currents in reverse bias. Since the detectors are diffusion limited near zero bias down to 40 K, the R0Aimp versus temperature data represents the diffusion current performance of the detector as a function of temperature. The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy Ea optical. The activation energy, Ea electrical, obtained from the measured diffusion limited R0Aimp versus temperature data is not equal to the activation energy, Ea optical, obtained from the spectral response measurement for all x values measured. Ea electrical=*Ea optical, where ranges between 0.64 and 1.0 For cutoff wavelengths in the 9 m at 78 K, Ea electrical=Ea optical. Ea electrical=0.65* Ea optical have been measured forc=17 m at 78 K detectors. As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system. In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor. Such a picture could explain the observed behavior. Further probing experiments and modeling efforts will help clarify the physics of this behavior.

Noise in large-area CrlS Hg1-xCdxTe photovoltaic detectors

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is a Fourier Transform interferometric sensor that measures earth radiances at high spectral resolution. Algorithms use the data to provide pressure, temperature, and moisture profiles of the atmosphere. The CrIS instrument contains photovoltaic detectors with spectral cut-offs denoted by SWIR, MWIR and LWIR. The CrIS instrument requires large-area, photovoltaic detectors with state-of-art detector performance at temperatures attainable with passive cooling. For example, detectors as large as 1 mm in diameter are required. To address these needs, Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg1-xCdxTe on lattice matched CdZnTe. The p-side is obtained via arsenic implantation followed by appropriate annealing steps.

Cross-track infrared sounder FPA performance

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is an interferometric sensor that measures earth radiances at high spectral resolution, using the data to provide pressure, temperature and moisture profiles of the atmosphere. The pressure, temperature and moisture sounding data are used in weather prediction models that track storms, predict levels of precipitation etc. The CrIS instrument contains SWIR ((λc approximately 5 μm at 98K), MWIR (λc approximately 9 μm at 98K) LWIR (λc approximately 16 μm at 81K) Focal Plane Array (FPA) modules. A critical CrIS design selection was the use of photovoltaic (PV) detectors in all three spectral bands. PV detectors have the important benefits of high sensitivity and linearity. Each FPA modules consists of nine large (1000 μm diameter) photovoltaic detectors with accompanying cold preamplifiers. This paper describes the performance for all the modules forming the CrIS Detector Preamplifier Module (DPM). Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg1-xCdxTe on lattice matched CdZnTe. SWIR, MWIR and LWIR 1000 μm diameter detectors have been manufactured using the Lateral Collection Diode (LCD) architecture. Custom pre-amplifiers have been designed to interface with the large SWIR, MWIR and LWIR detectors. The operating temperature is above 78K, permitting the use of passive radiators in spacecraft to cool the detectors. Recently fabricated 1000 micrometers diameter photovoltaic detectors have the measured performance parameters listed in the Table below. Expected D* performance from the detector/pre-amplifier models are also listed in the table. The D* values are calculated at the CrIS program peak wavelength specified for each spectral band.

Advances in large-area Hg 1-x Cd x Te photovoltaic detectors for remote sensing applications

State-of-the-art large area photovoltaic detectors fabricated in HgCdTe grown by Molecular Beam Epitaxy have been demonstrated for the Crosstrack Infrared Sounder instrument. Large area devices (1 mm in diameter) yielded excellent electrical and optical performance operating at 81K for LWIR band and at 98K for MW and SWIR bands. LWIR and MWIR detectors have near-theoretical electrical performance, and AR-coated quantum efficiency is greater than 0.70. Measured average RoA at 98K is 2.0E7 W-cm2 and near-theoretical quantum efficiencies greater than 0.90 were obtained on SWIR detectors. These state-of-the-art large area photovoltaic detector results reflect high quality HgCdTe grown by Molecular Beam Epitaxy on CdZnTe substrates in all three spectral bands of interest.

1/f noise in Hg1-xCdxTe detectors

This paper investigates 1/f noise performance of Hg1-xCdxTe photovoltaic detectors when detector current is varied by changing detector area, bias, temperature and incident flux. Holding detector bias and temperature constant, measured 1/f noise current is proportional to the detector current. However for all detector areas measured, non-uniformity is observed in the noise current due to the varied quality of the detectors. Even for the λc=16μm , 4-μm-radius, diffusion-limited detectors at 78K held at reverse bias, the average and standard deviation in dark current is Id=9.76+/- 1.59x10-8A while the average and standard deviation in noise current at 1 Hz in a 1 Hz bandwidth is in=1.01+/- 0.63x10-12A. For all detector areas measured at 100 mV reverse bias, the average and standard deviation in dark current to noise current ratio is α D=in/Id=1.39+/- 1.09x10-5. Defects are presumed resident in the detectors that produce greater non- uniformity in the 1/f noise as compared to the dark current at 100 mV reverse bias. Noise was also measured as a function of temperature for two λ c=16 micrometers detectors from 55 K to 100 K. The average and standard deviation in the noise current to dark current ratio is αD=in/Id=2.36+/- 0.83x10-5 for the 26-micrometers -diameter detector and (alpha) D=1.71+/- 0.69x10-5 for the 16-micrometers -diameter detector. Dark and noise current were measured while changing the bias applied to a detector. In the diffusion-limited portion of the detector I-V curve, 1/f noise is independent of bias with α D=in/Id=1.51+/- 0.12x10-5. When tunneling currents dominated, αT=in/Id=5.21+/- 0.83x10-5. The 1/f noise associated with tunneling currents is a factor of three greater than the 1/f noise associated with diffusion currents. In addition, 1/f noise was measured on detectors held at -100 mV and 78 K under dark and illuminated conditions. The average noise to current ratio αD was approximately 1.5 x 10-5 for dark and photon-induced diffusion current. However, detector-to-detector variations exist even within a single chip. The two most important points are that non-uniformities in material/fabrication need to be addressed and that each individual type of current component has an associated 1/f noise current component, the magnitude of the relationship being different depending on the source current.

VLWIR HgCdTe photovoltaic detectors performance

Very Long Wavelength InfraRed (VLWIR; (lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detector operating in the 78 K range are needed for remote sensing applications. This temperature range permits the use of passive radiators in spacecraft to cool the detectors. VLWIR ((lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detectors in a range of sizes (8 micrometer diameter to 1000 micrometer diameter) have been fabricated and their performance measured as a function of temperature. Molecular Beam Epitaxy (MBE) was used to grow n-type VLWIR Hg1-xCdxTe on lattice matched CdZnTe. Arsenic was implanted and the wafer was annealed to provide the p-type regions. All the material was grown with wider bandgap cap layers and consequently the detector architecture is the Double Layer Planar Heterostructure (DLPH) architecture. Id - Vd versus temperature curves for 8 and 1000 micrometer diameter, (lambda) c equals 17 micrometer at 78 K detectors indicate that the 8 micrometer diameter detector is diffusion limited for temperatures greater than 63 K even at a -200 mV bias. There is no appreciable tunneling at T equals 50 K and at -200 mV applied bias. At T equals 40 K tunneling commences at a bias approximately equals -80 mV. Below T equals 30 K, the diode is tunneling limited. The 1000 micrometer diameter detector is diffusion limited at bias values less than -50 mV at 78 K. At zero bias, the detector impedance is comparable to the series/contact resistance. Interfacing with the low (comparable to the contact and series resistance) junction impedance detector is not feasible. Therefore a custom pre- amplifier was designed to interface with the large VLWIR detectors in reverse bias. The detector is dominated by tunneling currents at temperatures less than 78 K. The 1000 micrometer diameter, (lambda) c approximately equals 17 micrometer at 78 K detectors have dark currents approximately equals 160 (mu) A at a -100 mV bias and at 78 K. Detector non-AR coated quantum efficiency > 60% was measured at -100 mV bias in these large detectors and the response was constant across the (lambda) equals 7 micrometer to 15 micrometer spectral band. With AR- coating the quantum efficiency will be > 70%. Response was measured and non-linearity < 0.15% was calculated for the 1000 micrometer detectors. The flux values were in the 1017 ph/cm2/sec range and were changed by varying the blackbody temperature. In addition, a linear response was measured while varying the spot size incident on the 1000 micrometer detectors. This excellent response uniformity measured as a function of spot size implies that, low frequency spatial response variations are absent, for the 1000 micrometer detectors.