Optimal absorber thickness in long-wave multiple-stage detector
The detectivity characteristics of interband cascade infrared type-II superlattice detectors for long-wave infrared detection (λcut-off = 8 μm at room temperature) are discussed. We present comparison of two superlattices: InAs/GaSb and InAs/InAsSb, assuming the characteristic parameters—absorption coefficients α and carrier lifetimes τ published in literature. Dependence of the Johnson-noise limited detectivity on the absorber thickness for a different number of stages is reported. Higher detectivity D* value can be achieved by increasing the carrier lifetime. However, for detectors based on type-II InAs/GaSb superlattice increasing the carrier lifetime up to 25 ns leads to a situation in which one stage is preferred, i.e. for detector with a single absorber, we obtain the highest detectivity value. In the case of InAs/InAsSb material, the situation is similar for τ ≥ 80 ns. We have shown that the optimal absorber thickness at which the highest detectivity values are obtained depends not only on the absorption coefficient α and the number of stages NS, but also on the carrier diffusion length L. According to a calculations, cascade detectors based on Ga-free material should have much higher optimal absorber thicknesses than materials based on InAs/GaSb.
KeywordsInterband cascade infrared detectors IB CID T2SL InAs/GaSb T2SL InAs/InAsSb LWIR detection
The search for novel infrared detector materials, as well as new detector architectures, is motivated by the need to increase background limited operating temperatures, thereby lowering impact of bulky and expensive coolers. The commonly used and known material for infrared detectors, which is HgCdTe in contrast to the materials from group III-V requires a reduced operating temperature. Type-II superalttices (T2SLs) InAs/GaSb (Rogalski et al. 2017; Ye et al. 2015; Lei et al. 2016; Huang et al. 2017) have recently gained a lot of interest and are considered as a viable alternative HgCdTe technology in the long-wave infrared (LWIR) detection. The advantages of the type-II superlattices (T2SLs) include the suppressions of Auger recombination (Ashley and Elliott 1985) and tunneling current (Nguyen et al. 2007; Bogdanov et al. 2011), and high uniformity (Nguyen et al. 2011). Increasing minority carrier lifetimes relative to bulk HgCdTe will result in increased detectivities at the same operating temperature, or the same detectivity at higher operating temperature. Short minority carrier lifetime in T2SLs InAs/GaSb has recently been partially attributed to acceptor like defects in GaSb rather than the interfaces. With Ga being the suspected origin of the short minority carrier lifetime (Steenbergen et al. 2012), the “Ga-free” T2SLs InAs/InAsSb have been proposed as another alternative material that has the potential for longer lifetimes (Rogalski et al. 2017; Belenky et al. 2011; Grein et al. 1995; Pellegrino and DeWames 2009; Connelly et al. 2010; Donetsky et al. 2010; Steenbergen et al. 2011).
Nevertheless, T2SLs have their own important shortcomings affecting the performance of the detector, which is limited by diffusion length. This issue can be overcome utilizing a multiple-stage photodetector with multiple short absorbers—interband (IB) cascade infrared detectors (CIDs). IB CIDs contain multiple-discrete absorber connected in series using interband tunneling heterostructure. Each absorber can be shorter than the diffusion length, so the incident photons can be effectively collected. The thin discrete absorbers and series connections make the device resistance high so that noise is suppressed and the photocurrent flows more easily to an external circuit, resulting in the ability to high-temperature and high-speed operation (Hinkey and Yang 2013; Yang et al. 2010).
In this work, we report on the dependence of the Johnson-noise limited detectivity on the absorber thickness for a different number of stages, assuming performance of LWIR T2SLs, which can be useful to find the best detector architecture to reach high value of detectivity. We apply well known general theory for photovoltaic cells to multiple-stage interband detectors and obtain the aforementioned dependences.
In this work, we reported on the dependence of the Johnson-noise limited detectivity on the absorber thickness for a different number of stages, assuming performance of LWIR T2SLs. We have shown that the optimal absorber thickness at which the highest detectivity values are obtained depends not only on the absorption coefficient α and the number of stages NS, but also on the carrier diffusion length L. A comparison of the two types of the superlattice showed that the detector based on InAs/InAsSb reaches higher detectivity than the detectors based on InAs/GaSb, even for fewer cascades. High temperatures and lower carrier lifetimes lead to the diffusion length shortening. Thus, in such cases, multiple-stage approach should be beneficial. Nevertheless, even if the detectivity is greater for a single absorber detector, the cascade detectors are characterized by a higher speed of operation. Thus, the normalized signal-to-noise ratio is only one of the parameters that can be optimized. In order to improve the speed of the detector’s operation, its architecture should also be optimized, including the thickness of individual absorbers.
We acknowledge support by The National Science Centre—the Grant No. OPUS/UMO-2015/19/B/ST7/02200.
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