Performance limits of the mid-wave InAsSb/AlAsSb nBn HOT infrared detector

P. Martyniuk A. Rogalski InAsSb ternary alloy is considered to be an alternative to HgCdTe (MCT) in mid-wavelength infrared spectral region. The high operation temperature conditions are successfully reached with AIIIBV bariodes, where InAsSb/AlAsSb system is playing dominant role. Since there is no depletion region in the active layer, the generation-recombination and trap-assisted tunneling mechanisms are suppressed leading to lower dark currents in comparison with standard photodiodes. As a consequence, the bariodes operate at a higher temperature than standard photodiodes which could be used in wide range of system applications, especially where the size, weight, and power consumption are crucial. The paper presents detailed analysis of the bariode's performance (such as dark and photocurrent, differential resistance area product, and detectivity) versus applied voltage, operating temperatures and structural parameters. The optimal working conditions are calculated. The theoretical predictions of bariode's performance are compared with experimental data published in the literature. Finally, the nBn InAsSb/AlAsSb performance is compared to the MCT Rule 07. - GR contribution may be successfully limited by the barriers incorporation to the detectors structure, while Auger GR mechanism could be suppressed either by the non-equilibrium conditions or designing the detectors with materials exhibiting lower Auger GR rates (Ashley and Elliott 1985; Maimon and Wicks 2006). The nBn architecture has been successfully implemented into AIIIBV bulk compounds and InAs/GaSb type-II superlatices (T2SLs). The InAs/GaSb T2SLs success has resulted from the physical properties of the artificial material and what is most important, the zero valence band offsets with advantageous band alignment slightly harder to attain in AIIIBV and AIIBVI bulk compounds (Ting et al. 2010). Although, T2SLs are considered to have advantage over bulk materials, there are indicators that, similarly to the technological problems related to the growth of self-organized quantum dot infrared detectors, T2SLs InAs/GaSb development is limited by technological issues related to the growth of uniform and thick enough SLs (Martyniuk and Rogalski 2008). Moreover, short carrier lifetimes (< 10 ns for T > 200 K) may hamper the development of the T2SLs IR devices (Wrbel et al. 2012). The nBn architecture was also implemented into HgCdTe alloy exhibiting type-I heterojunction where theoretical modeling indicates a potential advantages in order to circumvent p-type doping in Molecular Beam Epitaxy (MBE) growth (Itsuno et al. 2011, 2012; Martyniuk and Rogalski 2013b). Due to a nearly zero band valence offset with respect to AlAsSb in the valence band, InAsSb has emerged to play a dominant role in the designing of the nBn detectors (Klipstein 2008; Klem et al. 2010). Although theoretical prediction places T2SLs in front of the IR systems development, the better stability over large area, higher carrier mobility and developed technology favours InAsSb in MWIR range (Vincent et al. 1990; Klipstein et al. 2011; Plis et al. 2011; Weiss et al. 2012). In this paper we performed the detailed analysis of the InAsSb/AlAsSb nBn detector performance versus bias, operating temperatures, and structural parameters pointing out the HOT detectors optimal working conditions. Finally, the InAsSb/AlAsSb performance is compared to MCT Rule 07. 2 Simulation procedure The drift-diffusion (DD) model developed by Crosslight Software Inc. was used to simulate nBn InAsSb/AlAsSb detector. The material parameters are listed in Table 1. The electron affinity of both barrier layer (BL) and absorber layer (AL) are considered to be the most critical parameter to choose in nBn structure modeling. The valence band offset (VBO) varies from 80 to 270 meV for unbiased InAs1xSbx/AlAs1ySby structure (x y 0.09) at T = 300 K (Vurgaftman et al. 2001). The AlAsSb electron affinity was calculated using following dependence: with A = 5.72, similarly to the relation given by IOFFE Physical Technical Institute. The simulations include radiative (RAD), SRH GR and both tunneling mechanisms at barrierabsorber (BL-AL) heterojunction. Since the AlAsSbs barrier height was estimated to be in range of 2 eV, the GR mechanism in the BL is found to be negligible in assessing the bariode performance. In order to distinguish the intrinsic nBn performance, the n+-type contact layer while the InAsSbs electron affinity was calculated according to the relation: Table 1 Parameters taken in modeling of MWIR InAsSb/AlAsSb nBn detectors Trap energy level, ET rap Fig. 1 a Energy band diagram of the simulated nBn photodetector under reverse bias conditions. b The modelled nBn InAsSb/AlAsSb structure (Martyniuk and Rogalski 2013c) (CL) is incorporated to eliminate the holes generation contribution to the DD model at the n+ region (see Fig. 1a, b). The detailed description of the growth procedure and devices characterization could be found in the papers by (Klipstein et al. 2011) and (Weiss et al. 2012). The noise current is calculated using the expression including thermal Johnson-Nyquist noise and electrical shot noise: in (V ) = where: A is a detectors area and kB is the Boltzmann constant. The quantum efficiency is a function of the incident radiation wavelength and current responsivity, Ri , according to the relation (without electro-optical gain): Fig. 2 a Calculated energy band diagram for the nBn InAsSb/AlAsSb for V = 500 mV. b Ev for BL-AL interface versus applied voltage and temperature The detectors detectivity is defined by the expression: Ri D = in (V ) A. 3 nBn InAsSb/AlAsSb band alignment The calculated energy band diagram for biased conditions (V = 500 mV) is depicted in Fig. 2a. The InAsSb/AlAsSb system exhibits staggered type-II heterojunction, where VBO could be controlled by proper BL/AL compositions and doping levels. The nBn detector requires turn-on voltage to align the valence band (at BL-AL interface) allowing nearly unimpeded minority carrier transport to CL. It was estimated that applying V = 500 mV, the energy barrier for holes is being reduced to 80 meV in comparison with the equilibrium conditions. Figure 2b presents Ev (refer to Fig. 1a) versus voltage and operating temperature. The minority carriers in bariode are efficiently blocked for the Ev > 3kB T . The applied voltage mostly influences Ev , while Ec keeps nearly constant. It was found that for the BL-AL interface Ev 270 4 meV and for the CL-BL interface Ec 2032 2038 meV for V = 0 1V, respectively. The condition of unimpeded minority carrier transport to the CL (Ev < 3kB T = 78 meV at T = 300 K) is met for V > 500 mV. Ev slightly increases with temperature (see Fig. 2b) while Ec should be barely influenced by T . 4 Dark and photocurrent modeling The nBn detector operates in minority carrier manner. Dark current is driven mainly due to the hole transport from AL to CL. Figure 3a shows (...truncated)