Imagery of long wavelength infrared HgCdTe and GaAs quantum well staring arrays in size 128 X 128 has been demonstrated. In this paper, we compare detector array performance characteristics, discuss the natural and technological limitations of both technologies and identify the improvements likely to be made in the near future. At this stage of feasibility demonstration in the spectral band 8 - 10 micrometers , the effective quantum efficiency in GaAs FPAs is 4% compared to 60% for HgCdTe and the responsivity is 0.08 A/W compared to 4.5 A/W. This value of 0.08 A/W is significantly below the value 2 A/W reported for single quantum well infrared photodetectors (QWIP) detectors. The peak detectivities and NE(Delta) T at 78 K are (5 X 10⁹ cm (root)Hz/W, 0.037 K) and (2 X 10¹¹, 0.005 K) for QWIP and HgCdTe, respectively. The residual nonuniformities after two-point correction are < 0.01% for QWIP arrays and 0.012% for HgCdTe. Crosstalk is currently unsatisfactory in QWIP detector arrays, but design concepts can be used to reduce this effect. For terrestrial imaging, GaAs quantum well detector arrays most likely will need to operate at temperatures below 80 K from fundamental considerations; HgCdTe detector arrays are background limited at operating temperatures <EQ 90 K. Since cooling can drive cost and reliability, and since significant progress has been made in producing high quality HgCdTe detector arrays with good yield, it is unlikely that HgCdTe will be displaced by this technology for terrestrial applications. For low background space applications at (phi) _b <EQ 10¹² ph/cm²-sec, QWIP detectors at 40 K are background limited. This observation plus their radiation hard characteristics suggest a possible niche in strategic applications.

Texas Instruments has developed a new thermal imaging technology based upon focal plane arrays (FPAs) using the pyroelectric effect in ceramic barium-strontium titanate (BST). These devices operate near the paraelectric-ferroelectric phase transition, which, for the selected composition of BST, is near room temperature. The detector elements operate in the voltage mode with a bias voltage applied to maintain and optimize the pyroelectric effect near the phase transition. The BST array attaches via bump-bonding to a CMOS readout circuit that filters, buffers and multiplexes the output signals. These FPAs have facilitated the development of a system technology capable of satisfying a wide variety of applications, including surveillance devices, weapons sights, missile seekers and driver's aids. Resulting systems are performance-competitive with scanned FLIRs in these applications, and they are smaller in size, lighter in weight, and require less power than scanned FLIRs. Simplicity and compactness of the system designs will result in production costs competitive with image intensification devices.

This paper presents two types of recent focal plane arrays manufactured in Laboratoire Infrarouge (LIR) of CEA-LETI: (1) long linear arrays in the 12 micron wavelength range operating at 50 K consisting of several submodules butted together, and (2) 128 X 128 two dimensional arrays operating at 200 K with a 4.2 micron cutoff wavelength. Furthermore, basic studies have been carried out and an improvement of a factor of ten for the RoA value is now obtained on new homojunction MCT detectors. First results are presented here.

Surface plasmon (SP) excitation in Schottky photodiodes was used both for increasing the efficiency and for creating sharply selective photodetectors. Theoretical and experimental research on the selective Schottky photodiodes with diffraction gratings at the interfaces were performed.

We present a systematic theoretical and experimental study on wavelength tuning and absorption lineshape of single bound state quantum well infrared photodetectors. We found that the absorption energy is determined by the energy level structure above the barriers as well as the shape of the quantum well ground state wave function. We calculated the absorption lineshape and show that it depends sensitively on the position of the final state relative to the global band structure of the detector. Using a quantum barrier as an electron energy high pass filter to discriminate against the lower energy dark current, we are able to increase the detectivity of the detector. The new device is referred as an IR hot-electron transistor. Its potential advantages in focal plane array applications will be discussed.

A narrow gap semiconductor layer grown directly on a Si-substrate is the preferable approach to realize large IR-focal plane arrays. We report on our new work on lead chalcogenide photovoltaic IR-detector arrays, grown monolithically on Si (111) substrates using a stacked CaF₂/BaF₂ buffer layer. The sensor fabrication process is described, and a simple thermal camera system is used to verify the functionality of our arrays. An epitaxial narrow gap lead chalcogenide layer of only 3 micrometers thickness is grown on an 0.3 micrometers thick CaF₂/BaF₂ buffer layer on Si (111), both using Molecular Beam Epitaxy. Photovoltaic IR-detectors are formed by deposition of a blocking Pb contact on the p-type semiconducting surface. We fabricated staggered linear sensor arrays with up to 2 X 128 pixels and with the cut off ranging from 3 to 12 micrometers . For demonstration, we built up a simple thermal camera using our detector arrays as the IR sensitive element. The read out is done using a new multiplexed direct injection device, capable to store large charge packages and offering individual biasing for each diode. The IR-diodes are fabricated monolithically on the completely finished readout chip.

The SOFRADIR IRFPA technology is now used in production and there is no question mark about the producibility of such IRFPA. Nevertheless the competition is still worldwide open at the level of the performances as the existing technologies are different. Indeed, SOFRADIR technology is based on an homojunction approach together with a fully planar technique which allows very good yields in production. The competing technologies are mainly based on an heterojunction principle using a mesa technology. The goal of this paper is to present, after a brief overview of the SOFRADIR IRFPA technology the last results in comparison with the standard application needs and to give an idea of the SOFRADIR existing products line.

We discovered and explained a new effect--'Two Peak Effect' (TPE) in the spectral responses of a p-n heterojunction infrared detector. That means there is two spectral peaks in only one p- n junction. This is a universal and important effect in semiconductor heterojunctions. In this paper, we further presented a new two color detector and its array on the basis of TPE, such as for HgCdTe, PbSnTe, etc. This new multicolor device structure has more advantages and applications than ordinary ones.

Recent developments in metal-organic chemical vapor deposition (MOCVD) growth of the infrared detector material Hg_1-xCd_xTe can be subdivided into three main areas: reaction kinetics, in situ monitoring of growth and doping. The growth regime for most laboratories is kinetic/catalytic, and a good understanding of these growth mechanisms is essential for adequate control of growth rate and composition. In situ monitoring using laser reflectance can measure growth rate and composition and can be sensitive to morphology throughout a multilayer structure. This new information on the kinetic processes is a powerful diagnostic tool for the crystal grower. Doping the epitaxial layers with low volatility organometallics, or in the case of As using the diluted hydride, has been demonstrated by a number of laboratories, but problems with memory doping from indium and incorporation/activation efficiencies for arsenic will be resolved by understanding the surface chemistry. Arsenic implanted double layer heterostructure diode results show comparable 80 K R_OA values to LPE p/n diodes.

A theoretical investigation of electrical crosstalk induced by surface channel was made for Hg_1-xCd_xTe n-on-p photodiode arrays, by using a model based on diode surface channel current theory. An analytic expression of crosstalk, which is an exponential function of the inversion layer charge density, element spacing, p-n junction resistance-area product, and surface electron mobility, was deduced. The dependence of crosstalk on some related device parameters, such as the positive fixed interface charge density, etc., was calculated and discussed. The results show that better control of the substrate surface potential, as well as careful choice of some device parameters, is significant for reducing electrical crosstalk.

Liquid-phase epitaxy (LPE) has emerged as the predominant materials growth technology for the fabrication of HgCdTe infrared (IR) detectors in the IR community over the past decade. This paper reviews one of the most successful LPE technologies developed for HgCdTe, specifically, 'infinite-melt' vertical LPE (VLPE) from Hg-rich solutions. A historical perspective and the current status of VLPE technology are reported. Extensive statistics of performance and producibility of the VLPE technology are elaborated to show its maturity and manufacturing readiness. Particular emphasis is placed on the key role of the double-layer heterojunction (DLHJ) detectors realized by the VLPE technology for high-performance second-generation focal plane arrays. Recent developments in the successful use of the VLPE technology for epitaxial growth on Si-based alternative substrates and for growth of triple- layer heterostructures for two-color applications, which further demonstrate the versatility of the technology, are also reported. The review concludes with a discussion of the prospects for use of the VLPE technology for fabricating advanced device structures of high performance as well as investigating fundamental material properties of HgCdTe.

For HgCdTe photoconductive and photovoltaic detectors, the surface passivation is required. The surface physics and chemistry characteristics could be dominated by the technology of passivation. This paper reports the effect of HgCdTe anodic oxide interface on the performance of HgCdTe photoconductive detectors. Using the formulas obtained in the present paper, considering the effect of surface recombination velocity, the responsibility, noise and detectivity of Hg_0.8Cd_0.3Te photoconductive detectors operating at 77 K are calculated with 300 K background and 2 (pi) field of view. In order to understand the effect of anodic oxide interface and improve the performance of HgCdTe infrared detectors, the relation among surface fixed positive charges, surface recombination velocity and the thickness of detector is discussed.

We discuss the influence of the Hg flux on defect formation and we show that under optimized growth parameters the crystal quality of HgCdTe epilayer is similar to that of the CdZnTe substrate. We confirm the MBE growth of HgCdTe requires stringent control in growth conditions and occurs under Te saturated conditions. We show also that diffusion of impurities originating from the substrates is a very serious problem. Indium doped HgCdTe layers have been found to exhibit excellent structural and electrical characteristics.

A hybrid HgCdTe 640 X 480 infrared (IR) focal plane array (FPA) that meets the sensitivity, resolution, and field-of-view requirements of high-performance medium wavelength infrared (MWIR) imaging systems has been developed. The key technology making this large, high sensitivity device producible is the epitaxial growth of HgCdTe on a CdTe-buffered, sapphire substrate (referred to as PACE, for Producible Alternative to CdTe for Epitaxy; PACE-I refers to sapphire). The device offers TV resolution with excellent sensitivity at temperatures below 120 K. Mean NE(Delta) T as low as 13 mK has been achieved at operating temperatures < 130 K, which is about an order of magnitude better than has been achieved with PtSi 640 X 480 FPAs. In addition, the latter require cooling to <EQ 77 K. Mean PACE-I FPA D^* at 78 K and background of 10¹⁴ photons/cm²-sec is BLIP-limited at 1 X 10¹² cm-Hz^1/2/W for the typical mean quantum efficiency of 60 - 70%. Imagery having excellent quality has been obtained using simple two-point nonuniformity compensation.

Normal incidence spectral response and bias dependent responsivity measurements on p-type Ge_xSi_1-x/Si quantum well infrared photodetectors reveal anomalous long wavelength photoresponse, extending out to 18 micrometers depending on quantum well parameters. Room temperature FTIR absorptance measurements do not reveal significant absorption in this wavelength range; aside from the shorter wavelength intersubband optical transitions near 6 - 12 micrometers in these Ge_0.25Si_0.75 quantum wells. Dark current induced filling of the higher-lying split-off hole state, with subsequent optical absorption, would give rise to infrared absorption in the 14 - 18 micrometers range; however the split-off ground state to heavy-hole ground state lifetime required is -10 microsecond(s) ecs. In two-hole state structures having 300 angstroms barriers we observe dramatic increases in responsivity for bias voltage above 3 V, attributed to hot-hole transport enhancement of the photoconductive gain. This behavior is not observed in 500 angstroms barrier width structures.

The strained layer Si_1-xGe_x/Si heterostructures and multiple quantum well structures have created a great deal of interest due to the potential of monolithic integration with the conventional silicon VLSI signal processing technology. For infrared detection application, the intersubband transition using Si_1-xGe_x/Si offers normal incident detection possibilities in contrast with that of AlGaAs/GaAs. In this paper, we will first discuss the principle and applications of Si and SiGe quantum well structures for infrared detection using intersubband transition. The physics of intersubband transition in Si-based quantum wells and superlattices will then be described for both n-type and p-type materials. Several normal incidence detection mechanisms will be illustrated: namely, from nonvanishing off-diagonal elements of the effective mass tensor in n-type and intervalence band transition for p-type in addition to free carrier absorption. The effect of the strain in determining the occupancy of the valleys will be described. The experimental results of normal incidence intersubband transition in SiGe/Si quantum wells and (delta) -doped Si layers are compared with the theoretical calculation of oscillator strength. The importance of many-body effects in determination of transition energy in very heavily doped structures and (delta) -doped layers, will also be shown for tuning of a wide range of transition energy (from 2 micrometers to tens of micrometers and perhaps longer). Finally, infrared detector structures using SiGe/Si multiple quantum well structures will be illustrated.

An Al_xGa_1-xAs/GaAs quantum well infrared photodetector (QWIP) 128 X 128 focal plane array, incorporating integral grating structures for efficient optical coupling have been hybridized to silicon CMOS multiplexes. The demonstrated high uniformity, excellent detectivity, and high yield of operating pixels (99% plus) has resulted in excellent imagery in the 8 - 12 micrometers range, with a low noise equivalent temperature difference (NE(Delta) T) of less than 10 mK, and high image contrast signal to noise ratios (CSNR). We will discuss the figures of merit that govern the image quality in our QWIP focal plane arrays, such as D^*, uniformity, CSNR, maximum input charge to the CMOS multiplex, etc. Our latest results will also be reported.

We demonstrate that GaAs/AlGaAs multiple-quantum-well (MQW) structures grown by atmospheric pressure metalorganic vapor phase epitaxy have state-of-the-art structural, optical, and electrical properties. The 50-well MQW structures, with well thicknesses ranging from 14 to 90 angstroms, were analyzed by atomic resolution transmission electron microscopy, photoluminescence, and deep-level transient spectroscopy with the aid of a theoretical model for the eigenstates of the MQWs. It is shown that the MQW structures have a layer-to-layer thickness uniformity, interface roughness, and heterojunction abruptness of only one monolayer. A selectively doped MQW structure shows an infrared absorption efficiency of 15% at a wavelength of 11 micrometers .

This paper presents the preliminary analysis of quantum-well IR detectors grown on both GaAs and Si substrates.

A new type of asymmetric stepped GaAs/AlGaAs multi quantum well infrared detectors is reported. These asymmetric detectors utilize the usual bound to continuum transition. The current responsivity is remarkably asymmetric with regard to the voltage polarity. In contrast with rectangular wells, in which responsivity is saturated in both bias polarities, these wells exhibit saturation only for negative bias. The responsivity increases monotonously with positive electric field. The difference between polarities for the noise is much smaller at low temperatures. As a result, the highest D^* in positive polarity is much higher than in the negative one. This is attributed to changes induced by the field on the transport properties of the excited electrons. In particular, the bias affects the dwell time spent by the carrier wave packet in the well region. Employing this model, we achieve a very good fit with experimental data. The transport asymmetry is further studied using identical asymmetric wells which were grown in opposite sequences. It is shown that the effect of the asymmetry in the interfaces is of the same order of magnitude as the structural asymmetry.

Excitation of surface plasmons by attenuated total reflection is used to optically characterize platinum silicide films and to produce enhanced photosignals from PtSi-Si Schottky barrier devices. In this work the Otto configuration (prism-air gap-sample) for attenuated total reflection is used to excite the mode at the air-PtSi interface on PtSi-Si structures; surface plasmon excitation is evidenced as a prominent dip in measurement of reflectance as a function of internal angle of incidence in the prism. Coupling efficiencies of 90 - 95% between incoming p-polarized radiation and surface plasmons on PtSi have been achieved at wavelengths ranging from the visible to the infrared (0.633 micrometers to 3.39 micrometers ). The surface plasmon energy is dissipated by absorption through the creation of electron-hole pairs in the lossy silicide layer. Subsequent hot carrier emission over the Schottky barrier can thus yield an enhanced photosignal associated with the enhanced absorption due to surface plasmon excitation--initial results demonstrating this phenomenon are reported. here. Finally, reflectivity calculations show also the possibility of infrared surface plasmon excitation in PtSi grating structures with periodicity of the same order as the wavelength.

This paper will review the current state of the art of internal photoemission infrared detectors. That is, detectors which sense in a multi-step process, beginning with photon absorption in an electrode and terminating in emission of an excited carrier over a potential barrier into a semiconductor depletion region. Internal photoemission (PE) is a surface barrier sensing process similar to vacuum photoemission. This differs from competing bulk detection processes where both photoabsorption and carrier measurement takes place in the semiconductor. The PtSi/p-Si Schottky photodiode is the most advanced and best documented infrared PE device. Our interest is more general however and we will include devices where the photoemission 'electrode' may be a metal, a metal silicide or a degenerate semiconductor.

The results of experimental investigations of the photovoltage generated in Schottky barrier under CO₂ laser radiation are presented. We demonstrate for the first time the nonlinear dependence of photovoltage on the incident power density. This dependence indicates the multi-photon light absorption in the semiconductor-metal interface.

Field-effect transistors based on InP have many advantages over those based on GaAs. The major difficulty in realizing MESFETs based on InP, however, lies in low barrier height and breakdown voltage of Schottky gate. To overcome this, a pseudomorphic insulating layer instead of a conventional insulator on gate for a MIS-like FET structure is proposed in this paper. The computed I-V characteristics for such a structure having semitransparent gate metallization under dark and optically illuminated gate conditions are presented.

A monolithic infrared focal plane array where photo-diodes and field effect transistors are integrated is presented. The photodiode is connected directly to the integrating capacitor while the transistor controls the integrated signal-charge transfer to the video line. The operating modes of such array are discussed and especially the pseudo-staring mode. Such arrays were produced and their system performance were investigated. The detectors average specific detectivity D^*_(lambda )(f/no equals 1, (lambda) equals 3.83 micrometers ) was equal to 1.3 X 10¹¹ [cm-Hz^1/2/W] and was used to calculate the Noise Equivalent Temperature Difference (NETD) as a function of the number of detectors and vectors. The NETD was estimated to be of 0.039 K for an optimal 5 vectors array.

512 and 1024 elements linear In_xGa_1-xAs detector arrays have been fabricated for cutoff wavelengths of 1.7 micrometers (x equals 0.53, E_g equals 0.73 eV), 2.2 micrometers (x equals 0.71, E_g equals 0.56 eV), and 2.6 micrometers (x equals 0.82, E_g equals 0.47 eV) using hydride vapor phase epitaxy (VPE) and metallorganic chemical vapor deposition (MOCVD). The 25 X 500 micrometers s pixel sizes have a center to center spacing of 25 micrometers and currently exhibit typical leakage currents of 6 pA (300 K, -10 mV) for 1.7 micrometers cutoff arrays, 500 pA (300 K, -10 mV) for 2.2 micrometers cutoff arrays, and 20 nA (300 K, -10 mV) for 2.6 micrometers cutoff arrays. Improved crystal growth, n-type sulfur doping (-1 X 10¹⁷ cm^-3) of the semiconductor 'cap' layer and the active InGaAs layer, and post-crystal growth thermal cyclings of the detector wafers have helped to reduce the leakage current in these detectors. Furthermore, these techniques have produced a remarkable increase in the photodetector yields, which is essential for the commercial viability of these arrays. The relationship between the photodetector bandgap (E_g) and the theoretically lowest attainable leakage current is discussed. The design of a new infrared (IR) multiplexer operating at near zero bias is also discussed.

Nominally identical multiple-quantum-well detectors have been fabricated on GaAs and Si substrates, and the performance of these detectors has been compared at a temperature of 77 K and a wavelength of 10.2 micrometers . The two different substrates yield practically identical absorption characteristics, but the detector on the GaAs substrate has an approximately 60% higher specific detectivity because of its higher photoconductive gain and lower dark-current density.

Several photodetector applications require optimizing pulse response at moderate bandwidths. With current technology, such detector circuits can be op-amp based transimpedance amplifiers. Unfortunately, the usual transimpedance topology, having a single feedback resistor, does not give enough control over pole placement for design optimization. If the single resistor is replaced by a tee-network, however, the transimpedance amplifier can be made to approximate any desired second-order network response. We present an analysis that shows how to choose resistor values to achieve desired second-order parameters: damped natural frequency and damping coefficient. Experimental results are presented that confirm the validity of our design method.