Vertical Hole Transport and Carrier Localization inInAs/InAs1−xSbxType-II Superlattice Heterojunction Bipolar Transistors

Abstract

Heterojunction bipolar transistors are used to measure vertical hole transport in narrow-bandgap InAs/InAs 1-x Sb x type-II superlattices (T2SLs). Vertical hole mobilities (µ h) are reported and found to decrease rapidly from 360 cm 2 /V-s at 120 K to ≈ 2 cm 2 /V-s at 30 K, providing evidence that holes are confined to localized states near the T2SL valence miniband edge at low temperatures. Four distinct transport regimes are identified: (1) pure miniband transport, (2) miniband transport degraded by temporary capture of holes in localized states, (3) hopping transport between localized states in a mobility edge, and (4) hopping transport through defect states near the T2SL valence miniband edge. Region (2) is found to have a thermal activation energy of ε 2 = 36 meV, corresponding to the energy range of a mobility edge. Region (3) is found to have a thermal activation energy of ε 3 = 16 meV, corresponding to the hopping transport activation energy. This description of vertical hole transport is analogous to electronic transport observed in disordered amorphous semiconductors displaying Anderson localization. For the T2SL, we postulate that localized states are created by disorder in the group-V alloy of the InAs 1-x Sb x hole well causing fluctuations in the T2SL valence band energy.

Temperature-Dependent Minority-Carrier Mobility inp-TypeInAs/GaSbType-II-Su

“…In region 2, the increase in the mobility with the temperature can be associated with the carriers being set free from the impurity states, with an activation energy of 115 meV. A similar observation has been reported previously in ntype InAs/InAsSb T2SL structures [40], in which results show that above 120 K, the minority carrier hole transport occurs through minibands, while for lower temperatures, the holes are localized and transport is only due to phonon-assisted tunneling.…”

InAs/InAsSb Strained-Layer Superlattice Mid-Wavelength Infrared Detector for High-Temperature Operation

“…Since the first demonstration of SLSs for infrared (IR) detectors [12], InAs/Ga(In)Sb SLSs continue to improve [19], while InGaAs/InAsSb SLSs [15,20] have also shown promising results. Researchers are currently addressing the poor hole mobility [21] and carrier localization [22,23] effects in n-type SLS to improve the diffusion length. They have also explored p-type SLS detectors, but surface passivation leading to surface leakage current [24,25] remains an insurmountable problem.…”

Carrier Transport in the Valence Band of nBn III–V Superlattice Infrared Detectors

“…With the PL technique, 15 however, the experimental manifestation of the localization sites occurred mainly for T < 60 K, whereas our photocurrent results were obtained for T > 70 K. We suggest that the sensitivity of our photocurrent technique in the higher temperature range is possible because of the lower level of injection (dp % 10 12 cm À3 ) as compared with the elevated level (dp % 10 16 cm À3 ) in the PL technique, keeping the localizations sites far from saturation. In the other investigation, Olson 16 found that the hole mobility exhibits different behavior in four separate temperature intervals. Their interval number 2 (67-110 K) best corresponds to the interval (72-98 K) of our results in Fig.…”

“…The first, by Steenbergen et al, 15 used temperature-dependent and excitation-dependent photoluminescence (PL) spectroscopy in MWIR samples to demonstrate the existence of the sites and to measure their energy depth. The second, by Olson et al, 16 used the characteristics of a specially made bipolar heterojunction transistor with a base consisting of LWIR (12.5 lm cutoff) InAs/InAsSb to measure the hole mobility in the growth direction as a function of temperature. The third is the present work, in which we have used the temperature dependence of the photocurrent in MWIR nBn devices to measure the energy depth of the localization sites and demonstrate that they are distributed throughout the absorber volume.…”

“…[12][13][14] Evidence for localization sites has been reported by Steenbergen et al 15 based on temperaturedependent photoluminescence, finding an average depth of about 8 meV for the sites in MWIR InAs/ InAsSb superlattice samples. Further study of the localization phenomenon, in long wavelength infrared (LWIR) InAs/InAsSb, has been reported by Olson et al 16 in terms of the temperature dependence of the hole mobility in the growth direction. Our results, obtained from the temperature dependence of the MWIR nBn photocurrent, are consistent with theirs.…”