Stephen W. Kennerly

Normal-incidence, high-temperature, mid-infrared, InAs-GaAs vertical quantum-dot infrared photodetector

The growth, fabrication, and characterization of a normal-incidence, high-temperature, mid-wavelength infrared, InAs-GaAs vertical quantum-dot infrared photodetector with a single Al/sub 0.3/Ga/sub 0.7/As current-blocking barrier are described and discussed in detail. A specific detectivity /spl ap/3/spl times/10/sup 9/ cmHz/sup 1/2//W is measured for a detector temperature of 100 K at a bias of 0.2 V. Detector characteristics are measured for temperatures as high as 150 K. The superior low bias performance of the vertical quantum-dot infrared photodetector ensures its compatibility with commercially available silicon read-out circuits necessary for the fabrication of a focal plane array.

Comparison of HgCdTe and QWIP dual-band focal plane arrays

We report on results of laboratory and field tests of dual- band MWIR/LWIR focal plane arrays (FPAs) produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system and by BAE Systems using QWIP technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640 X 480-pixel single-color read-out integrated circuit (ROIC) to produce a dual-band 320 X 240 pixel array. The MWIR and LWIR pixels are co-located and have a high fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using MBE-grown III-V materials. The LWIR section consisted of GaAs quantum wells and AlGaAs barriers and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two color operation) which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The relative advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Dual band QWIP MWIR/LWIR focal plane array test results

We report on the results of laboratory and field tests on a pixel-registered, 2-color MWIR/LWIR 256 X 256 QWIP FPA with simultaneous integrating capability. The FPA studied contained stacked QWIP structures with spectral peaks at 5.1 micrometer and 9.0 micrometer. Normally incident radiation was coupled into the devices using a diffraction grating designed to operate in both spectral bands. Each pixel is connected to the read-out integrated circuit by three bumps to permit the application of separate bias levels to each QWIP stack and allow simultaneous integration of the signal current in each band. We found the FPA to have high pixel operability, well balanced response, good imaging performance, high optical fill factor, and low spectral crosstalk. We present data on measurements of the noise-equivalent temperature difference of the FPA in both bands as functions of temperature and bias. The FPA data are compared to single-pixel data taken on devices from the same wafer. We also present data on the sensitivity of this FPA to polarized light. It is found that the LWIR portion of the device is very sensitive to the direction of polarization of the incident light. The MWIR part of the device is relatively insensitive to the polarization. In addition, imagery was taken with this FPA of military targets in the field. Image fusion techniques were applied to the resulting images.

Self-mixing detector candidates for an FM/cw ladar architecture

The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARLs goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.

Comparison of HgCdTe and quantum-well infrared photodetector dual-band focal plane arrays

We report on results of laboratory and field tests of dual-band focal plane arrays (FPAs) in the medium-wave infrared (MWIR) and long-wave infrared (LWIR), produced under the Army Research Laboratorys Multidomain Smart Sensor Federated Laboratory program. The FPAs were made by DRS Infrared Technologies using the HgCdTe material system, and by BAE Systems using quantum-well infrared photodetector (QWIP) technology. The HgCdTe array used the DRS HDVIPTM process to bond two single-color detector structures to a 640×480-pixel single-color readout integrated circuit (ROIC) to produce a dual-band 320×240 pixel array. The MWIR and LWIR pixels are co-located and have a large fill factor. The images from each band may be read out either sequentially (alternating frames) or simultaneously. The alternating-frame approach must be used to produce optimal imagery in both bands under normal background conditions. The QWIP FPA was produced using III-V materials grown by molecular-beam epitaxy (MBE). The LWIR section consisted of GaAs quantum wells and AlGaAs barriers, and the MWIR section used InGaAs quantum wells with AlGaAs barriers. The detector arrays were processed with three ohmic contacts for each pixel, allowing for independent bias control over both the MWIR and LWIR sections. The arrays were indium bump-bonded to an ROIC (specifically designed for two-color operation), which puts out the imagery from both bands simultaneously. The ROIC has variable gain and windowing capabilities. Both FPAs were tested under similar ambient conditions with similar optical components. The FPAs were subjected to a standard series of laboratory performance tests. The advantages and disadvantages of the two material systems for producing medium-format dual-band FPAs are discussed.

Device physics and focal plane array applications of QWIP and MCT

Infrared sensor technology is critical to many commercial and military defense applications. Traditionally, cooled infrared material systems such as indium antimonide, platinum silicide, mercury cadmium telluride, and arsenic doped silicon (Si:As) have dominated infrared detection. Improvement in surveillance sensors and interceptor seekers requires large size, highly uniform, and multicolor IR focal plane arrays involving medium wave, long wave, and very long wave IR regions. Among the competing technologies are the quantum well infrared photodetectors based on lattice matched or strained III-V material systems. This paper discusses cooled IR technology with emphasis on QWIP and MCT. Details will be given concerning device physics, material growth, device fabrication, device performance, and cost effectiveness for LWIR, VLWIR, and multicolor focal plane array applications.

Laboratory and field imaging test results on single-color and dual-band QWIP focal plane arrays

Abstract We report on recent laboratory and field measurements on quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). The results of laboratory measurements of imaging performance such as noise-equivalent temperature difference (NEΔT), minimum resolvable temperature, conversion efficiency, uniformity of response and dark current and their dependence on operating temperature are presented on large format (640×480 pixels) single-color long-wavelength infrared (LWIR) and dual-band (256×256 pixels) mid-wavelength infrared (MWIR)/LWIR arrays. We found that under optimum operating conditions (65 K and 1 V bias), the NEΔT of the LWIR FPA was approximately 30 mK and was limited by the optics and noise from the closed-cycle cooler. We will show field imagery of military targets taken with this FPA. In addition, field imagery taken with the LWIR QWIP camera of a large-transient event will be compared with that taken with an InSb MWIR camera. We will also show imagery of a recent total eclipse of the moon. We will also present laboratory results on a simultaneously integrating, pixel-registered dual-band FPA showing excellent operability and response uniformity with NEΔT of approximately 30 mK in both bands (T=60 K). The dual-band FPA was also taken to the field and we will present imagery of various targets acquired with this FPA.

Characteristics of QWIPs at low background

Abstract Quantum well infrared photodetectors (QWIPs) rapidly become mature in high background applications. On the other hand, the detector properties at low temperature and low background conditions are less well characterized. We have studied the current–voltage characteristics of QWIPs under this regime. We found that the current level in a fixed experimental condition can change slowly and substantially with time. The slow time dependence leads to large current hysteresis upon thermal cycling at a finite cycling rate. Under certain conditions, this time constant can be exceeding long, which leads to different metastable current states, depending on the experimental history. We also observed that in a long time scale, the dc current level of a detector need not be directly proportional to the background radiation. Specifically, we observed that the dc current level of some detectors under F /16 can be larger than that under F /2 at 10 K and with 290 K background. We attribute these phenomena to the effects of unintentional dopants in the QWIP barriers. In addition, we found that the dominant noise in this regime is the 1/ f noise caused by the DX centers in the barriers.

Bias-dependent tunable response of normal incidence long wave infrared quantum dot detectors

We report development of array compatible individual high-performance quantum dot pixel structures that allow for spectral tuning by the application of an external bias. Custom-made post-processing algorithms have been used to further enhance and optimize their tuning and spectral-separation capability far beyond the device limit. The dots-in-a-well (DWELL) detector consists of 10 layers of InAs dots placed in a thin InGaAs well, which in turn is placed in a GaAs matrix. The DWELL structure provides good confinement for the carriers trapped in the quantum dots.

NIR small arms muzzle flash

Utilization of Near-Infrared (NIR) spectral features in a muzzle flash will allow for small arms detection using low cost silicon (Si)-based imagers. Detection of a small arms muzzle flash in a particular wavelength region is dependent on the intensity of that emission, the efficiency of source emission transmission through the atmosphere, and the relative intensity of the background scene. The NIR muzzle flash signature exists in the relatively large Si spectral response wavelength region of 300 nm-1100 nm, which allows for use of commercial-off-the-shelf (COTS) Si-based detectors. The alkali metal origin of the NIR spectral features in the 7.62 × 39-mm round muzzle flash is discussed, and the basis for the spectral bandwidth is examined, using a calculated Voigt profile. This report will introduce a model of the 7.62 × 39-mm NIR muzzle flash signature based on predicted source characteristics. Atmospheric limitations based on NIR spectral regions are investigated in relation to the NIR muzzle flash signature. A simple signal-to-clutter ratio (SCR) metric is used to predict sensor performance based on a model of radiance for the source and solar background and pixel registered image subtraction.