Arezou Khoshakhlagh

Type-II Superlattice Infrared Detectors

Publisher Summary This chapter provides an overview of type-II superlattice infrared detectors. The type-II InAs/GaSb superlattices have several fundamental properties that make them suitable for infrared detection: (1) their band gaps can be made arbitrarily small by design, (2) they are more immune to band-to-band tunneling compared with bulk material, (3) the judicious use of strain in type-II InAs/GaInSb strained layer superlattice (SLS) can enhance its absorption strength over that of the type-II InAs/GaSb superlattice to a level comparable with HgVdTe (MCT), and (4) type-II InAs/Ga(In)Sb superlattices also reduce Auger recombination. In addition, the dark current characteristics of type-II superlattice-based single element long-wavelength infrared (LWIR) detectors are currently approaching state-of-the-art MCT detector. Noise measurements highlight the need for surface leakage suppression, which can be tackled by improved etching, passivation, and device design. The chapter also describes the principles behind advanced superlattice infrared detectors based on heterostructure designs. It also explores some aspects of device fabrication and characterization.

High performance long-wave type-II superlattice infrared detectors

The authors report on growth, material characterization, and device performance of infrared photodetectors based on type II InAs/GaSb superlattices using the complementary barrier infrared detector (CBIRD) design. In this paper, control steps for improvement of material quality in terms of surface, structural, and optical properties of infrared detectors grown at Jet Propulsion Laboratory are described. For a specific CBIRD studied, these quality control steps indicate high structural and optical quality of the grown material. Furthermore, single-element detector from the optimized growth conditions exhibit dark current density less than 1 × 10−5 A/cm2 at applied biases up to Vb = 0.36 V (T = 77 K), so this material can be utilized for focal plane arrays development.

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.

Minority carrier lifetime and photoluminescence studies of antimony-based superlattices

In this work, we have used the optical modulation response technique to investigate the minority carrier lifetimes in (42 A, 21 A) InAs/GaSb superlattices. The feasibility of using a visible 643 nm excitation source with short penetration depth was investigated by comparing the results to reference measurements performed with a 1550 nm IR laser. Minority carrier lifetimes in the range of 33 – 38 ns were observed, in good agreement with the reference measurements. Furthermore, when comparing superlattices with essentially the same PL peak wavelength, correlation between the minority carrier lifetime and the PL intensity was observed. This shows that the PL intensity serves as a good indicator of the material quality.

Antimonide type-II superlattice barrier infrared detectors

We provide a brief overview of recent progress in III-V semiconductor infrared photodetectors resulting from advances in infrared detector materials, including type-II superlattices (T2SL) and InAsSb alloy, and the unipolar detector architecture. We summarize T2SL unipolar barrier infrared detector and focal plane array development at the NASA Jet Propulsion Laboratory in support of the Vital Infrared Sensor Technology Acceleration (VISTA) Program. We also comment on the connection of T2SL barrier infrared detector to MCT infrared detectors.

Carrier transport in unipolar barrier infrared detectors

We examine carrier transport in unipolar barrier infrared photodetectors and discuss aspects of barrier, contact, and absorber properties that can affect minority carrier collection. In a barrier infrared detector the unipolar barrier should block only the majority carriers while allowing the un-impeded flow of the minority carriers. Under the right conditions, unipolar barrier doping can reduce generation-recombination dark current without affecting minority carrier extraction. In an nBn structure, ideally with an electron unipolar barrier, improper barrier doping or barrier-absorber valence band offset could also block minority carriers and result in higher turn-on bias. We also examined the temperature-dependent turn-on bias in an n+Bn device and showed that observed behavior may be attributed to contact doping. Hole mobility in n-doped type-II superlattice (T2SL) is believed to be very low because of the extremely large effective mass along the growth direction. In practice MWIR and LWIR barrier infrared detectors with n-type T2SL absorbers have demonstrated good optical response. A closer inspection of the T2SL band structure offers a possible explanation as to why the hole mobility may not be as poor as suggested by the simple effective mass picture.

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.

Superlattice barrier infrared detector development at the Jet Propulsion Laboratory

We report recent efforts in achieving state-of-the-art performance in type-II superlattice based infrared photodetectors using the barrier infrared detector architecture. We used photoluminescence measurements for evaluating detector material and studied the influence of the material quality on the intensity of the photoluminescence. We performed direct noise measurements of the superlattice detectors and demonstrated that while intrinsic 1/f noise is absent in superlattice heterodiode, side-wall leakage current can become a source of strong frequency-dependent noise. We developed an effective dry etching process for these complex antimonide-based superlattices that enabled us to fabricate single pixel devices as well as large format focal plane arrays. We describe the demonstration of a 1024×1024 pixel long-wavelength infrared focal plane array based the complementary barrier infrared detector (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.

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.