Session E12: Mechanisms for Long Carrier Lifetimes and High Detectivities from Novel Ga-free Narrow Gap III-V Semiconductor Superlattices

Ga-free InAs/InAsSb type-II superlattice and its applications to IR lasers and photodetectors

This talk will review the research on Ga-free InAs/InAsSb type-II superlattices (T2SL), especially their growth, structural and electronic properties, and applications to IR lasers and photodetectors with the following highlights: 1) Review of the previous study of InAs/InAsSb T2SL and its application to IR lasers and photodetectors in the 90's. 2) Long minority carrier lifetime up to 12.8 $\mu $s in mid-wavelength infrared (MWIR) InAs/InAsSb T2SL was observed at 15 K, and 412 ns for long-wavelength infrared (LWIR) InAs/InAsSb T2SL were measured using time-resolved photoluminescence. The record long carrier lifetime in the MWIR range is due to carrier localization, which is confirmed by a 3 meV blue shift of the photoluminescence peak energy with increasing temperature from 15 K to 50 K, along with a photoluminescence linewidth broadening up to 40 K. In contrast, no carrier localization is observed in the LWIR T2SL. Modeling results show that carrier localization is stronger in shorter period (9.9 nm) MWIR T2SL as compared to longer period (24.2 nm) LWIR T2SL, indicating that the carrier localization originates mainly from InAs/InAsSb interface disorder. Although carrier localization enhances carrier lifetimes, it also adversely affects carrier transport. 3) Pressure-dependent photoluminescence (PL) experiments under hydrostatic pressures up to 2.16 GPa were conducted on a MWIR InAs/InAsSb T2SL structure at different pump laser excitation powers and sample temperatures. The results show a pressure coefficient of the T2SL transition was found to be 93 \textpm 2 meV\textbullet GPa$^{\mathrm{-1}}$; a clear change in the dominant photo-generated carrier recombination mechanism from radiative to defect related, providing evidence for a defect level situated at 0.18 \textpm 0.01 eV above the conduction band edge of InAs at ambient pressure. 4) LWIR InAs/InAsSb T2SL nBn photodetectors on GaSb substrates were demonstrated. The typical device consisted of a 2.2 micron thick absorber layer and has a 50{\%} cutoff wavelength of 13.2 $\mu $m, a measured dark current density of 5e-4 A/cm$^{\mathrm{2}}$ at 77 K under a bias of -0.3 V, a peak responsivity of 0.24 A/W at 12 $\mu $m and a maximum RA product of 300 ohm-cm$^{\mathrm{2}}$ at 77 K. The calculated generation-recombination noise limited specific detectivity (D*) and experimentally measured D* at 12 $\mu $m and 77 K are 1e10 (cm-Hz$^{\mathrm{1/2}})$/W and 1e8 (cm-Hz$^{\mathrm{1/2}})$/W, respectively.   

Identification of dominant recombination mechanisms in narrow-bandgap InAs/InAsSb type-II superlattices and InAsSb alloys

InAs/Ga(In)Sb type-II superlattices (T2SL) have been extensively studied for both advanced emitter and detector technologies associated with mid-wave (MWIR), long-wave (LWIR), and very-long-wave (VLWIR) infrared applications. The type-II band alignment, together with control of both the layer thicknesses and the alloy composition, provide a rich environment for band structure engineering, including band gap tuning and suppression of Auger recombination. Unfortunately, the InAs/Ga(In)Sb MWIR T2SLs have been found to have minority carrier lifetimes persistently below 100 ns, even at cryogenic temperatures. Such short lifetimes are problematic for detector applications and suggest that this material system will not compete with HgCdTe for IR detector applications. On the other hand, the report by Steenbergen, et al., [1] of much longer minority carrier recombination lifetimes (\textgreater 412 ns at 77K) in a longwave (8.2 \textmu m) InAs/InAsSb T2SL suggests that the ``Ga-free'' superlattices could be competitive for IR detector applications. We will discuss all-optical measurements of carrier lifetimes as a function of temperature and injected carrier density in InAs/InAsSb T2SLs with a broad range of sample designs based on variations in alloy composition and/or layer thickness. Minority carrier lifetimes ranging from 4.5 \textmu s for a 9.2 \textmu m-band-gap T2SL to 18 \textmu s for a 4.2 \textmu m-band-gap T2SL have been measured at 77 K. This research was performed in collaboration with Y. Aytac, B.V. Olson, J.K. Kim, E.A. Shaner, J.F. Klem, S.D. Hawkins, and M.E. Flatt\'{e}. [1] Steenbergen, et al., Appl. Phys. Lett. Vol. 99, 251110 (2011).   

Design of MWIR Type-II Superlattices for Infrared Photon Detectors

The Type II InAs/GaInSb and InAs/InAsSb superlattices are material systems for implementation as photodetector absorbers in infrared imaging applications. In addition to cutoff wavelengths spanning the infrared spectrum, they offer degrees of freedom in their materials design (e.g. layer thicknesses, alloy compositions, number of layers in one superlattice period) that permit the optimization of an infrared photon detector's figures of merit such as detectivity through the tuning of material properties like generation/recombination lifetimes and optical absorption.~ We describe efforts to obtain accurate electronic band structures of superlattice semiconductors with infrared energy gaps, and employing them to evaluate nonradiative minority carrier lifetimes. Simple device models are utilized to suggest potential performance enhancements that arise from employing superlattices as infrared absorber. We also discuss current efforts to simulate the molecular beam epitaxial growth of InAs/InAsSb superlattices to predict dominant native point defects and other growth nonidealities.   

Identification of Defect Candidates and their Effects on Carrier Lifetimes and Dark Currents in InAs/InAsSb Strained-Layer Superlattices for Infrared Detectors

The InAs/GaSb and InAs/InAsSb type-II strain-layer superlattices (T2SLS) are of great importance and show great promise for mid-wave and long-wave infrared (IR) detectors for a variety of civil and military applications. The T2SLS offer several advantages over present day detection technologies including suppressed Auger recombination relative to the bulk MCT material, high quantum efficiencies, and commercial availability of low defect density substrates. While the T2SLS detectors are approaching the empirical Rule-07 benchmark of MCT's performance level, the dark-current density is still significantly higher than that of bulk MCT detectors. One of the major origins of dark current is associated with the Shockley-Read- Hall (SRH) process in the depletion region of the detector. I will present results of ab initio electronic structure calculations of the stability of a wide range of point defects [As and In vacancies, In, As and Sb antisites, In interstitials, As interstitials, and Sb interstitials] in various charged states in bulk InAs, InSb, and InAsSb systems and T2SLS. I will also present results of the transition energy levels. The calculations reveal that compared to defects in bulk materials, the formation and defect properties in InAs/InAsSb T2SLS can be affected by various structural features, such as strain, interface, and local chemical environment. I will present examples where the effect of strain or local chemical environment shifts the transition energy levels of certain point defects either above or below the conduction band minimum, thus suppressing their contribution to the SRH recombination.