1704 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 46, NO. 12, DECEMBER 2010 Photovoltaic MWIR Type-II Superlattice Focal Plane Array on GaAs Substrate Edward Kwei-wei Huang, Pierre-Yves Delaunay, Binh-Minh Nguyen, Siamak Abdollahi Pour, and Manijeh Razeghi, Fellow, IEEE Abstract— Recent improvements in the performance of type-II superlattice (T2SL) photodetectors has spurred interest in de- veloping low-cost and large-format focal plane arrays (FPAs) on this material system. Due to the limitations of size and cost of native GaSb substrates, GaAs is an attractive alternative with 8 in wafers commercially available, but is 7.8% lattice mismatched to T2SL. In this paper, we present a photovoltaic T2SL 320 × 256 FPA in the mid-wavelength infrared on GaAs substrate. The FPA attained a median noise equivalent temperature difference of 13 and 10 mK ( F# = 2.3) with integration times of 10.02 and 19.06 ms, respectively, at 67 K. Index Terms—Alternative substrate, focal plane array, GaAs infrared, mid-wavelength infrared, photodetectors, type-II super- lattice. I. I NTRODUCTION H IGH performance at low cost is often the main driving force for successful products, and it is by definition an attribute of third-generation infrared imaging [1]. In recent years, the type-II superlattice (T2SL) material platform has seen incredible growth in the understanding of its material properties, which has lead to unprecedented development in the arena of device design. Its versatility in band-structure engineering is perhaps one of the greatest hallmarks of the T2SL that other material platforms lack. Thus, as device performance improves with novel designs year after year, the natural progression in further enabling the ubiquitous use of this technology is to reduce cost and support the fabrication of large infrared imagers. From this viewpoint, larger substrates are required, especially as imager resolutions increase past the megapixel mark, which occupy substantially more real estate than their smaller predecessors. High-performance T2SL de- vices today are commonly grown on the native GaSb substrate that come in sizes up to 3 in. At this size, the wafers cannot accommodate more than four 1024 × 1024 focal plane arrays (FPAs) and can only fit a single 2048 × 2048 FPA die with 20 μm pitch. The use of an alternative substrate is therefore necessary for cost-effective third-generation infrared imaging with T2SLs. GaAs appears to be a good choice, as the lattice constant is close to that of the 6.1-Å family, and is widely Manuscript received April 20, 2010; revised July 9, 2010; accepted July 20, 2010. Date of current version October 6, 2010. This work was funded in part by the Defense Advanced Research Projects Agency and in part by the Missile Defense Agency. The authors are with the Center for Quantum Devices, Northwestern University, Evanston, IL 60208 USA (e-mail: razeghi@ece.northwestern.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2010.2061218 used in integrated circuit components. Most importantly, large diameter GaAs wafers up to 8 in are available, which could potentially accommodate 16 megapixel imagers [2]. The 7.8% lattice mismatch between GaSb and GaAs presents a growth challenge, as it is necessary to develop growth methods to reduce strain and the number of dislo- cations and spiral mounds. Prior to the recent availability of GaSb substrates, antimonide-based structures, including T2SLs, were achieved on GaAs substrates [3]. However, due to the complexity of the material growth, only single-element photoconductive detectors were demonstrated on GaAs. This is because photoconductive devices are able to compensate for defect limitation by collecting the current laterally along the growth plane. In contrast, photovoltaic detectors, which require very high material quality to avoid defect-related leakage current, were reported only when GaSb substrates became widely available [4]. Due to the recent progress of antimonide-based T2SLs, there is renewed interest in achieving similar material quality on GaAs substrates so that it would be feasible to produce high- quality large-format FPAs on T2SLs. Although the growth of GaSb and T2SL photoconductors on GaAs has been reported previously [5], there has been no successful demonstration of a photovoltaic T2SL camera on GaAs. In this paper, we will discuss the material growth and single-detector performance as well as demonstrate a mid-wavelength infrared (MWIR) T2SL 320 × 256 FPA on GaAs substrate for the first time. II. GROWTH OF MWIR SUPERLATTICE PHOTODIODES The material was grown using a Gen II Intevac solid source molecular beam epitaxy (MBE) reactor equipped with group- III SUMO cells and group-V valved crackers. A 2-in n-type GaAs wafer was loaded for the growth of GaSb on GaAs. Fur- ther details of the buffer layer growth are described in another publication [6]. Then, the 2 in wafer was retrieved from the MBE reactor and the surface morphology was characterized using an atomic force micrograph (AFM). For the 10 ×10 μm 2 scan, the root mean square (rms) roughness was 9.67 Å, while for the 5 × 5 μm 2 scan it was 6.44 Å. A quarter of the 2-in wafer was subsequently cleaved and loaded back into the reactor for the growth of T2SLs. The superlattice design used was 7 monolayers of InAs and 11 monolayers of GaSb, which was predicted to have a cut-off wavelength of 3.98 μm at 77 K. The device structure consisted of a 1.5-μm n-type (n∼1 × 10 18 cm -3 ) region, a 2-μm unintentionally doped 0018–9197/$26.00 © 2010 IEEE