Anna Liao

Demonstration of 1Kx1K long-wave and mid-wave superlattice infrared focal plane arrays

Jet Propulsion Laboratory is actively developing the III-V based infrared detector and focal plane arrays (FPAs) for remote sensing and imaging applications. Currently, we are working on Superlattice detectors, multi-band Quantum Well Infrared Photodetectors (QWIPs), and Quantum Dot Infrared Photodetector (QDIPs) technologies suitable for high pixel-pixel uniformity and high pixel operability large area imaging arrays. In this paper, we will discuss the demonstration of long-wavelength 1Kx1K QDIP FPA, 1Kx1K QWIP FPA, the first demonstration of the megapixelsimultaneously- readable and pixel-co-registered dual-band QWIP FPA, and demonstration of the first mid-wave and long-wave 1Kx1K superlattice FPA. In addition, we will discuss the advantages of III-V material system in the context of large format infrared FPAs.

GaN-based high temperature and radiation-hard electronics for harsh environments

We develop novel GaN-based high temperature and radiation-hard electronics to realize data acquisition electronics and transmitters suitable for operations in harsh planetary environments. In this paper, we discuss our research on AlGaN/GaN metal-oxide-semiconductor (MOS) transistors that are targeted for 500 °C operation and >2 Mrad radiation hardness. For the target device performance, we develop Schottky-free AlGaN/GaN MOS transistors, where a gate electrode is processed in a MOS layout using an Al2O3 gate dielectric layer. The AlGaN/GaN MOS transistors fabricated with the wide-bandgap gate oxide layer enable Schottky-free gate electrodes, resulting in a much reduced gate leakage current and an improved sub-threshold current than the current AlGaN/GaN field effect transistors. In this study, characterization of our AlGaN/GaN MOS transistors is carried out over the temperature range of 25°C to 500°C. The Ids- Vgs and Ids-Vds curves measured as a function of temperature show an excellent pinch-off behavior up to 450°C. Off-state degradation is not observed up to 400 °C, but it becomes measurable at 450 °C. The off-state current is increased at 500 °C due to the gate leakage current, and the AlGaN/GaN MOS HEMT does not get pinched-off completely. Radiation hardness testing of the AlGaN/GaN MOS transistors is performed using a 50 MeV 60Co gamma source to explore effects of TID (total ion dose). Excellent Ids-Vgs and Ids-Vds characteristics are measured even after exposures to a TID of 2Mrad. A slight decrease of saturation current (ΔIdss~3 mA/mm) is observed due to the 2Mrad irradiation.

Type II superlattice barrier infrared detector

Significant progress has been achieved in the antimonide-based type-II superlattices since the analysis by Smith and Mailhiot in 1987 first pointed out their advantages for infrared detection. In the long-wavelength infrared (LWIR), type-II InAs/Ga(In)Sb superlattices have been shown theoretically to have reduced Auger recombination and suppressed band-to-band tunneling. Suppressed tunneling in turn allows for higher doping in the absorber, which has led to reduced diffusion dark current. The versatility of the antimonide material system, with the availability of three different types of band offsets, provides great flexibility in device design. Heterostructure designs that make effective use of unipolar barriers have demonstrated strong reduction of generation-recombination (G-R) dark current. As a result, the dark current performance of antimonide superlattice based single element LWIR detectors is now approaching that of the state-of-the-art MCT detector. To date, the antimonide superlattices still have relatively short carrier lifetimes; this issue needs to be resolved before type-II superlattice infrared detectors can achieve their true potential. The antimonide material system has relatively good mechanical robustness when compared to II-VI materials; therefore FPAs based on type-II superlattices have potential advantages in manufacturability. Improvements in substrate quality and size, and reliable surface leakage current suppression methods, such as those based on robust surface passivation or effective use of unipolar barriers, could lead to high-performance large-format LWIR focal plane arrays.

Optical Studies on Antimonide Superlattice Infrared Detector Material

In this study the material quality and optical properties of type II InAs/GaSb superlattices are investigated using transmission and photoluminescence (PL) spectroscopy. The influence of the material quality on the intensity of the luminescence and on the electrical properties of the detectors is studied and a good correlation between the photodetector current-voltage (IV) characteristics and the PL intensity is observed. Studies of the temperature dependence of the PL reveal that Shockley-Read-Hall processes are limiting the minority carrier lifetime in both the mid-IR wavelength and the long-IR wavelength detector material studied. These results demonstrate that PL spectroscopy is a valuable tool for optimization of infrared detectors.

High-Performance LWIR Superlattice Detectors and FPA Based on CBIRD Design

We report our recent efforts on advancing of antimonide superlattice based infrared photodetectors and demonstration of focal plane arrays based on a complementary barrier infrared detector (CBIRD) design. By optimizing design and growth condition we succeeded to reduce the operational bias of CBIRD single pixel detector without increase of dark current or degradation of quantum efficiency. We demonstrated a 1024×1024 pixel longwavelength infrared focal plane array utilizing CBIRD design. An 11.5 μm cutoff focal plane without anti-reflection coating has yielded noise equivalent differential temperature of 53 mK at operating temperature of 80 K, with 300 K background and cold-stop. Imaging results from a recent 10 μm cutoff focal plane array are also presented. These results advance state-of-the art of superlattice detectors and demonstrated advantages of CBIRD architecture for realization of FPA.

Growth and characteristics of type-II InAs/GaSb superlattice-based detectors

We report on band engineering, growth and device performance of infrared photodetectors based on type II InAs/Ga(In)Sb strain layer superlattices (SLs) using the complementary barrier infrared detector (CBIRD) design. The unipolar barriers on either side of the absorber in the CBIRD design in combination with the type-II InAs/GaSb superlattice material system are expected to outperform traditional III-V LWIR imaging technologies and offer significant advantages over the conventional II-VI material based FPAs. The innovative design of CBIRDS, barrier and band offset engineering, low defect density material growth, and robust fabrication processes have resulted in the development of high performance long wave infrared (LWIR) focal plane arrays at JPL.

GaN-based micro chemical sensor nodes for early warning chemical agents

We are developing micro chemical sensor nodes that can be used for real time, remote detection and early warning of chemical agent threats. The chemical sensors in our sensor nodes utilize GaN HEMTs (High Electron Mobility Transistors) fabricated with catalytically active transition metal gate electrodes. The GaN HEMT chemical sensors exhibit high sensitivity and selectivity toward chemical agent simulants such as DECNP (Diethyl cyano phosphonate), and this is the first time that chemical agent simulants have been detected with GaN micro sensors. Response time of the GaN HEMT sensor to a chemical species is within a second, and the maximum electronic response speed of the sensor is ~3 GHz. A prototype micro chemical sensor node has been constructed with the GaN sensor, a micro controller, and an RF link. The RF sensor node is operated with a single 3V Li battery, dissipating 15 mW during the RF transmission with 5 dBm output power. The microcontroller allows the operation of the RF sensor nodes with a duty cycle down to 1 %, extending lifetime of the RF sensor nodes over 47 days. Designed to transmit RF signals only at the exposures to chemical agents and produce collective responses to a chemical agent via a sensorweb, the GaN micro chemical sensor nodes seem to be promising for chemical agent beacons.

Toward high-performance infrared imaging

Focal plane arrays (FPAs) operating in the 3–5 m and 8– 12 m atmospheric transmission windows have many applications, such as imaging spectroscopy of land and coastal surfaces. Currently, the commercial market for FPAs operating in the 3–5 m mid-wavelength infrared (MWIR) spectral region is dominated by indium antimonide (InSb), which enjoys cost and large-format advantages over mercury cadmium telluride (MCT), but operates only below 80K. In the 8–12 m longwavelength infrared (LWIR) spectral range, MCT grown on cadmium zinc telluride (CZT) substrates can produce excellent imaging FPAs, but high-quality, large-area CZT substrates are expensive and in short supply. Alternative material systems are under development, including lead salts, indium arsenide antimonide (InAsSb), and a type-II indium arsenide/gallium antimonide (InAs/GaSb) superlattice (SL), but so far none has demonstrated the desired performance. In contrast, the nearly-lattice-matched antimonide material system offers tremendous flexibility in realizing highperformance infrared detectors. Antimonide-based SL detectors, where the absorber is a periodic structure of layers of two or more materials, can be tailor-made to have cutoff wavelengths in the range 1–11.5 m. SL detectors have been predicted to have suppressed Auger (i.e., electron-hole) recombination rates and low interband tunneling, reducing dark currents (that is, the current that flows through the photodetector when not exposed to light and a source of noise in the detectors that limits their performance).1 Moreover, this material system, consisting of InAs, GaSb, aluminum antimonide (AlSb), and their alloys, allows for the construction of SL heterostructures. In particular, it is possible to implement unipolar barriers in their design. These Figure 1. Spectral responsivity of barrier photodetectors without quantum dots (QD) (A) and with QD (B) at temperature T D 225K at applied bias Vb D 0:1V . The photoluminescence (PL) intensity for sample B (dashed blue line) shows a correspondence between the two photoluminescence peaks and the short/long wavelength sections of the photoresponsivity curve.