Long-Wavelength Infrared InAs/InGaSb Type-II Superlattice Photovoltaic Detectors K.A. Anselm, H. Ren, M. Vilela, J. Zheng, C.H. Lin Applied Optoelectronics, Inc. Sugar Land, TX 77478 V. Nathan AFRL/VSSS, Kirtland AFB, NM 87117 G. Brown AFRL/MLPO, Wright-Patterson AFB, OH 45433-7707 ABSTRACT The design and characteristics of very long wavelength InAs/InGaSb strained layer superlattice photodiodes are presented. These photodiodes have cutoff wavelengths ranging from 12 to longer than 15 microns, and are among the longest wavelengths reported for photovoltaic detectors made using this material system. Structural, optical and electrical properties are reported. Measured quantum efficiencies are as high as 10% at 10μm for a 0.7μm thick structure at 77K. The absorption coefficients are excellent, however, the electrical properties still need improvement. INTRODUCTION The detection of infrared signals in the long wavelength range, 8-12 microns and beyond, is important for commercial and military applications. HgCdTe has been the dominant material system for such applications for more than the past two decades. Despite considerable progress, this material system is difficult to work with and has problems with uniformity and stability of the epitaxially grown material as well as large dark currents and short lifetimes[1,2], especially at very long wavelengths. Infrared detectors using III-V semiconductor compounds instead of HgCdTe have been available through band-gap engineering techniques in recent years[3]. However, the inherent ultra-short lifetime due to phonon scattering (< 50 ps) and high thermal generation rates limit the quantum efficiency and operating temperature [3,4]. The intrasubband transitions for n-type quantum wells (QWs) are forbidden except for incident light with a z polarization parallel to the <100> growth direction, which complicates the fabrication techniques. Furthermore, the photoconductor structure strongly increases the dark current. The InAs/InGaSb type-II strained layer superlattice(SLS), was proposed in the late 80’s by Smith and Mailhiot[5] as an alternative for HgCdTe for long wavelength infrared detection. The superlattice consists of alternating thin (nm scale) layers of semiconductors in which the conduction band of one layer is below the valence band of the other (type II band alignment). This band alignment, in conjunction with thin layers that allow the electron and hole wavefunctions to overlap, lead to the formation of energy bands with transition energies that can be designed to correspond to a wide range of cutoff wavelengths including very long wavelength infrared. The epitaxial growth of these materials is not as mature as other III-V compounds, but this structure has shown promise as a long-wavelength IR photodiode material[6]. The absorption coefficients are comparable to HgCdTe and the structures promise smaller dark currents for long wavelength IR detectors due to a larger effective mass and longer minority carrier lifetimes due to suppressed Auger recombination[5]. Recently, significant progress has been made in the epitaxial growth of these superlattices[7] and the use of these superlattices in photodiodes[8,9] . Low background material has been grown by molecular beam epitaxy (MBE). It has also been shown that the reduction of crystalline defects in the material plays a significant role in the dark current. For small area diodes, the sidewall leakage also contributes significantly to the dark current, but can be improved with proper sidewall passivation. Photoconductors detecting at wavelengths longer than 20microns have been demonstrated [10], but most of the progress in photodiodes has been for structures less than 11microns[11]. In this paper, we report on the growth of photodiodes at wavelengths greater than 12 microns. Photodetectors: Materials and Devices VI, Gail J. Brown, Manijeh Razeghi, Editors, Proceedings of SPIE Vol. 4288 (2001) © 2001 SPIE · 0277-786X/01/$15.00 183