Background Limited Performance Research Articles

Background limited performance in p-doped GaAs/Ga0.71In0.29As0.39P0.61 quantum well infrared photodetectors

Background limited infrared photodetection has been achieved up to 100 K at normal incidence with p-type GaAs/Ga0.71In0.29As0.39P0.61 quantum well intersubband photodetectors grown by low-pressure metalorganic chemical vapor deposition. Photoresponse covers the wavelength range from 2.5 μm up to 7 μm. The device shows photovoltaic response, the cutoff wavelength increases slightly with bias, and the responsivity increases nonlinearly with bias. These effects are attributed to an asymmetric quantum well profile.

Temperature dependence of Hg/sub 0.68/Cd/sub 0.32/Te infrared photoconductor performance

An experimental and theoretical study has been carried out of the temperature dependent noise and responsivity performance of n-type x=0.32 Hg/sub 1-x/Cd/sub x/Te (corresponding to a 4.6 /spl mu/m cutoff wavelength at 80 K) photoconductors. The fundamental noise sources that ultimately limit the specific detectivity, D*/sub /spl lambda//, at the three main temperatures of interest (i.e., 80 K, 200 K, and 300 K) are identified and correlated with the experimental material parameters of the device. A device model is presented for the responsivity and noise voltage which takes into account surface effects such as surface recombination and accumulation layer shunting. For a given set of device and material parameters this model is well able to account for the observed experimental values of responsivity and noise voltage over the full temperature range from 80-300 K. Using a theoretical model, it is shown that under ideal conditions it is possible to achieve background limited performance at temperatures up to 210 K. Experimental results are presented for responsivity, noise voltage, semiconductor surface charge density and D*/sub /spl lambda// for a frontside-illuminated Hg/sub 1-x/Cd/sub x/ Te photoconductive detector, as a function of temperature in the range 80-300 K. The devices were fabricated on Liquid Phase Epitaxially (LPE) grown n-type Hg/sub 0.68/Cd/sub 0.32/Te, and were passivated with anodic oxide/ZnS on the front side. The detector area is 250/spl times/250 /spl mu/m/sup 2/ and has a cut-off wavelength of 4.6 /spl mu/m at 80 K. For a signal wavelength of 4 /spl mu/m and a 40/spl deg/ field of view, a background limited D*/sub /spl lambda// of 3.8/spl times/10/sup 11/ cm Hz/sup 1/2 / W/sup -1/ was obtained for temperatures up to 180 K, while D*/sub /spl lambda// of 1.4/spl times/10/sup 11/ and 2/spl times/10/sup 9/ cm Hz/sup 1/2 / W/sup -1/ were measured at 200 K and 300 K, respectively. These figures are comparable to the highest reported D*/sub /spl lambda// for n-type x=0.32 Hg/sub 1-x/Cd/sub x/Te mid-wavelength infrared photoconductive detectors operating at these temperatures. >

Theoretical performance of very long wavelength InAs/InxGa1−xSb superlattice based infrared detectors

Optimal detectivities of very long wavelength (11–19 μm) photovoltaic infrared detectors based on ideal InAs/InGaSb superlattices are calculated. Accurate K⋅p band structures are used to obtain radiative, electron–electron and hole–hole band-to-band Auger, and for the first time shallow acceptor level assisted Auger recombination rates for n-on-p photodiodes. The suppression of band-to-band Auger by ‘‘band gap engineering’’ is predicted to lead to improved background-limited operating temperatures just as it does in long-wave InAs/InGaSb infrared detectors.

Stray-light issues for background-limited far-infrared telescope operation

The detector in a background-limited infrared telescope receives less power due to the thermal emission from telescope components than from the background. The Arizona Program for the Analysis of Radiation Transfer has been used to set the temperature of the telescope components so that each component contributes less than 10-13 W of thermal power to the detector in the Space Infrared Telescope Facility long-wavelength configuration. The secondary mirror assembly, seen from the detector, is the most critical emitter even at 3 K.

Ultralow dark current p-type strained-layer InGaAs/InAlAs quantum well infrared photodetector with background limited performance for T≤100 K

An ultralow dark current normal incidence p-type strained-layer In0.3Ga0.7Al/In0.52Al0.48As quantum well infrared photodetector (PSL-QWIP) grown on (100) semi-insulating InP substrate by molecular beam epitaxy technique for 8–12 μm infrared detection was demonstrated for the first time. This PSL-QWIP shows background limited performance (BLIP) for T≤100 K, which is the highest BLIP temperature ever reported for a QWIP. Due to a 1.5% lattice mismatch between the substrate and quantum well, a biaxial tensile strain was created in the In0.3Ga0.7As well layers. As a result, the light-hole state becomes the ground state for the free hole with small effective mass. The dramatic increase of optical absorption can be attributed to the large in-plane density of states and the small light-hole effective mass as a result of heavy- and light-hole state inversion. The dark current density and BLIP detectivity for this PSL-QWIP were found to be 7×10−8 A/cm2 and 5.9×1010 cm−√Hz/W, respectively, at λp=8.1 μm, Vb=2 V, and T=77 K.

GaAs/AlGaAs quantum well infrared photodetector arrays for thermal imaging applications

The performance of GaAs/AlGaAs multiple quantum well infrared detectors is studied theoretically and experimentally, with special emphasis on 8-12 μm thermal imaging applications. The dependence of detector performance on various factors like light coupling configurations (one and two dimensional reflection gratings or 45° polished edge), detector temperature, response wavelength and quantum well doping density is dealt with. An absorption quantum efficiency of 87% is demonstrated using a crossed grating and a waveguide (CGW). It is also found that an optimised 34 μm × 34 μm detector (a detector size suitable for large staring arrays, i.e. 256 × 256 or larger) with 9.0 μm cut-off wavelength, f# = 2 optics and 70% optical transmission reaches background limited operation at 74 K detector temperature.The potential of making highly uniform staring arrays utilising the mature GaAs material and processing technology is demonstrated by uniformity measurements of detector dark current. The experiments show that a metalorganic vapour phase epitaxy (MOVPE) grown structure can have a dark current standard deviation to mean value ratio over a 10mm long linear detector array of less than 2%.The staring array performance in terms of noise equivalent temperature difference (temporal NETD) is calculated to NETD < 20 mK at 77 K detector temperature and NETD < 10 mK at 70 K detector temperature.

Background-Limited Performance in the Submillimeter

We have been awarded NASA funds to construct a Submillimeter (SMM) Array Photometer for use at the Caltech SMM Observatory (CSO) on Mauna Kea. The array will consist of 5×5 composite bolometers illuminated through light cones. On the CSO, each cone will cover 25 arcsecs on the sky, implying a total field-of-view for the array of just over 2×2 arcmins. Our aim is to achieve background-limited performance in the 730 and 870 μm atmospheric windows, thus obtaining a Noise Equivalent Flux Density (NEFD) of approximately 0.1 Jy/√Hz at these wavelengths. This sensitivity level will enable 5-sigma measurements of 10 mJy sources (such as radio-quiet quasars) in about one hour. We also will have the capability to operate at 1.1 mm, but the photometer will not be optimized at this wavelength. This system is expected to be operational on the CSO in January 1994.

Temperature limits on infrared detectivities of InAs/InxGa1−xSb superlattices and bulk HgxCd1−xTe

Detailed theoretical calculations of Auger and radiative recombination rates for an optimized InAs/InxGa1−xSb superlattice (SL) and bulk HgxCd1−xTe (MCT) show that 300 K background limited operation for a 60° field of view can be theoretically achieved up to 130 K for the 11 μm SL and up to 185 K for 5 μm MCT. The SL structure is theoretically superior to MCT for 11 μm operation. The converse is true at 5 μm.

The theory of multiple quantum-well GaAs-AlGaAs infrared detectors

The theory of multiple quantum-well GaAs-AlGaAs IR detectors based on the principle of quantum well photoionization is presented. The expression for the photoionization probability of the symmetric quantum-well is found to be exact in the effective mass approximation. Depolarization effects in the optical absorption spectra are discussed. We demonstrate that the depolarization shift of the spectrum maximum and its broadening lead to non-monotonic dependence of the photodetector detectivity on the electron concentration in wells in the background limited IR performance condition. The optimal factor of quantum-well filling, corresponding to the maximum of detectivity is rather small, and for the photoresistor with boundary wavelength λ 1 = 10 μm, is θ =0.10. We have also performed calculations for the concentration dependence of the transition temperature for background limited IR performance and found that for optimal concentrations it is about T = 83 K (for λ 1 = 10 μm). The lifetime of non-equilibrium electrons is determined by capturing processes into the wells accompanied by emission of polar optical phonons and is τ ≅ 2 × 10 −11 s. The gain, along with the photoresistor sensitivity, is maximal for structures with a single quantum well.

Experimental and theoretical studies of the performance of quantum-well infrared photodetectors

We report on the performance of GaAs/AlGaAs quantum-well infrared detectors exhibiting intersubband absorption in the 8–14 μm band. The dark current characteristics have been investigated as a function of barrier width and electron density in the well and compared with a model which takes into account thermionic emission and thermally assisted tunneling in the Wentzel–Kramers–Brillouin approximation. The model gives a good description of the data and is extended to predict the variation of detectivity and background limited operating temperature with structural parameters.

HgZnTe for very long wavelength infrared applications

HgZnTe has been of considerable interest in the last few years as an alternative infrared (IR) detector material to HgCdTe because of HgZnTe’s greater mechanical hardness, higher Hg vacancy formation energies, and other desirable properties which may lead to greater producibility (yield) and reliability (operation life). We are developing very long wavelength IR (VLWIR) HgZnTe detectors (17 μm at ≥65K) for longterm Earth Observing System NASA missions. In recent months we have been able to achieve, nearly routinely, outstanding quality HgZnTe on lattice-matched substrates (Cd0.84Zn0.16Te). By careful screening of substrate quality and refinement of the growth system and growth parameters, we overcame previous melt retention problems observed when using these high Zn% substrates. X-ray rocking curve widths (full width at half-maximum) as narrow as 26 arc-sec (for a 1×8 mm spot) for these epilayers have been achieved which is comparable to that for the best HgCdTe ever reported. In addition, we have, for the first time, fabricated and demonstrated high performance VLWIR (λco&amp;gt;17 μm at 80K) photoconductors made from liquid phase epitaxy HgZnTe which exhibit near background limited performance (peak D* up to 1011 cm(Hz)1/2/W) at high background. From these device results we see a clear correlation between the layer/substrate lattice match and the resulting device performance.

QUANTUM WELLS IN OPTOELECTRONICS: Optimisation threshold parameters of multiple quantum well infra-red photodetector

Detectivity of the multiple quantum well photodetector has been calculated in the background limited infra-red performance. Depolarisation effects have been taken into account. It is shown that detectivity depends nonmonotonically on the concentration of electrons in wells, a phenomenon resulting from depolarisation effects. As a result, an optimal concentration of a doping impurity which corresponds to the maximum of detectivity exists.

Optimal sampling and sensitivity limits of integrating detector arrays in a space environment

The performance of integrating detectors with either destructive read-out or non-destructive direct read-out (N-DRO) capabilities are investigated for an environment subject to charged particle and radiation hits. The optimal integration time for maximizing performance in the presence of a given hit rate is determined. This optimization is a result of striking a balance between achieving a sufficiently long integration time to reduce the effects of the “read noise” of the device while remaining short enough to keep the probability of a hit relatively low. The performance relative to that expected for background limited performance (BLIP) is evaluated using a combination of analytic techniques and Monte Carlo simulations. N-DRO arrays are considered in the context in which over a fixed clock time: (a) all data between hits is good and kept; (b) only the longest integration time for a given detector is kept; (c) only the data before the first hit is kept; (d) the performance of the detectors degrades with each hit; (e) the detector requires some time after a hit to recover. Best performance is obtained with case (a) of the N-DRO array in which all data is used. For instance whenever the time between hits is comparable to the time to become background limited, case (a) is a factor of 1.7 better in performance than the destructive read-out array (a factor of 2.9 in time). When degradation due to hits (case d) or due to a finite recovery time after a hit (case e) occurs, the performance lies between that expected for cases (a) and (c). These results are applied to estimating the performance degradation of the long-wavelength spectrometer of the Space Infrared Telescope Facility due to cosmic rays. The performance may be a factor of 1–5 worse than BLIP depending on the read noise and how performance is degraded due to particle hits.

Photon-counting solid-state photomultiplier

The solid state photomultiplier is a silicon device capable of continuous detection of individual photons in the wavelength range from 0.4 to 28 mu m. Operated with an applied bias near 7 V the device responds to the absorption of an incident photon with a submicrosecond-risetime current pulse with a narrow amplitude distribution well above the electronic readout noise levels. Optimal photon-counting performance occurs between 6 and 10 K and for count rates less than 10/sup 10/ counts/s/cm/sup 2/ of detector area. A 60% counting quantum efficiency has been demonstrated at 20 mu m, and near 60% was observed in the visible light region. The underlying mechanism involves extremely fast internal charge amplification by impact ionization of impurity-band electrons and results in a pulse for each photoelectrically or thermally induced free carrier. The thermally induced dark pulse rate at 7 K is sufficiently low that background-limited detector performance is obtained at a background of less than 10/sup 6/ photons/cm/sup 2/ s.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Atomic resonance filters

The atomic resonance filter (ARF) is an ultranarrowband (Q approximately 10/sup 5/-10/sup 6/), large-acceptance-angle, isotropic optical filter. These features make the device ideally suited for applications in which weak laser signals are detected against a continuum background. The filter properties arise from the physical processes of absorption, emission, and internal energy conversion in atomic vapors. The characteristics of the ARF are described and the underlying physics that governs the operation is discussed. Representative examples of passive, active, and IR filters are presented. A metastable ARF that offers improved solar background-limited performance by filtering signals at Fraunhofer wavelengths is described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phenomena in silicon photodiodes doped with Zn and S

The electrophysical and photoelectrical properties of Si〈S〉 were investigated and the noise and threshold characteristics of photoresistors made of this material were considered. Such photoresistors had detectivity D* (5.3; 570.1)= 5.15 × 10 10 and 2.0 × 10 −11cm Hz W −1, near to the background limited performance for a field of view 40° and 10° respectively, at a temperature 78 K and an electrical field of 300 V cm −1. Paraphotosensitivity of a diode made of Si doped with Zn was investigated. Some peculiarities of the current-voltage characteristics of p + nn + and p + np + Si〈Zn〉 structures were observed. Generation-recombination noise spectral density calculations are given for the case of doping the semiconductors with double-charged deep centres using the “impedance field” method.

Stressed Ge:Ga infrared detectors: performance and operational parameters

A stressed Ge:Ga infrared detector was tested in various operating conditions. Background-limited performance was demonstrated at photon backgrounds down to less than 10 to the 8th photon/s sq cm. The optimal operational parameters were determined.

High performance HgCdTe photoconductive devices grown by metalorganic chemical vapor deposition

Photoconductive detector arrays fabricated from metalorganic chemical vapor deposited epitaxial films of Hg0.8Cd0.2Te on both CdTe and Al2O3 substrates are reported for the first time. These devices operate in the 8–12-μm spectral region and have detectivities of 3.6×1010 and 2.6×1010 cm Hz1/2/W for CdTe and Al2O3 substrates, respectively. These results approach background limited performance, and as such establish the feasibility of metalorganic chemical vapor deposited HgCdTe for use in advanced infrared detection systems.

Recent developments in cadmium mercury telluride infrared detectors

A II–VI compound, cadmium mercury telluride, has dominated recent advances in the detection of infrared radiation. Although the main application is in thermal imaging, other applications include instrumentation and guidance. In this paper we describe the history of the development of cadmium mercury telluride as a detector material, with emphasis on the importance of material parameters and the role of the material scientist. High purity material, free from defects and of good crystal quality, is needed in order to ensure good minority carrier lifetime. Because of the segregation of impurities during solidification, material produced by the Bridgman technique offers considerable advantages over material produced by cast recrystallize techniques. Detectors based on a principle established by C.T. Elliott of the RSRE, Malvern, have led to families of thermal imagers that produce near perfect imagery. These detectors incorporate signal processing in the element by providing the time delay and integration functions that are normally performed off the focal plane in conventional serial scanned systems. A particular requirement of these SPRITE detectors is for long minority carrier lifetime. This has called for improvements in both material and fabrication technology which have led to an advanced technology that has benefited all aspects of the manufacture of cadmium mercury telluride detectors. Near background limited performance will be described in both the 8–14 and the 3–5 μm atmospheric transmission bands. There is, however, more image blurring in the 3–5 μm band than occurs in the 8–14 μm band. The direction of present work is towards detectors combining cadmium mercury telluride elements with advanced integrated circuits to provide more complex signal processing in the focal plane. This has been driven by the need to improve the sensitivity of thermal imaging systems where the scan rates are too low to allow useful time delay and integration in SPRITE detectors and by future needs for simplified missile guidance without mechanical scanning. Although the approaches have included extrinsic silicon, IR sensitive Schottky barriers and silicon hybridised with a variety of IR sensitive materials, the main thrust of present work in Europe and the United States is towards cadmium mercury telluride-silicon hybrids and intrinsic cadmium mercury telluride CCD focal planes. In the United Kingdom, the emphasis has been on two dimensional staring arrays of cadmium mercury telluride photodiodes combined with silicon substrates that incorporate either CCD circuits or MOSFET switch addressing. The switched approach has led to a novel line scanned array (LSA) that has enabled the first demonstrations of staring array imagery in the 8–14 μm band. Recent work will be described both on LSA and on CCD focal planes.

Transport in photo-conductors—II: Sampling from a finite device

The simplified analysis given in an earlier paper—for a class of photo-conductors in which charge accumulation performs a signal processing function on the focal plane—is extended to examine the combined effects of sampling length and finite device length. In particular, the influence of sampling upon bandwidth is described and compared for both the MTF and noise spectrum, under background limited performance. Further sources of signal degradation and noise, associated with thermal transport, are also discussed.