Measurement of absorption and external quantum efficiency of an InAs/GaSb Type II superlattice

•We investigated the absorption and the external quantum efficiency of a MWIR Type II super lattice (T2SL).•The external quantum efficiency is found to be consistent with the absorption quantum efficiency.•We also found the difference in the refractive index of GaSb and T2SL is 0.0014 from the thin-film interference.

We present an experimental result of absorption and external quantum efficiency measurements of an InAs/GaSb Type II superlattice (T2SL). The probed T2SL sample consisted of a 6 μm cutoff p–i–n diode having a 300-period superlattice with a 9-monolayer (ML) GaSb /0.9 ML InSb/7 ML InAs on a p-type GaSb substrate.The absorption quantum efficiency is estimated from the sample transmission at 87 K. We prepared three T2SL samples using GaSb substrates of different thicknesses: 150, 100, and 50 μm. By comparing the transmission spectra of these samples to that of a 100 μm thick GaSb substrate, we derived the absorption coefficient and the absorption quantum efficiency of the T2SL sample. We also discuss the refractive index of T2SL using a thin-film interference method together with the transmission measurements.The external quantum efficiency is measured with a p–i–n detector using the T2SL sample having the same design. The detector consists of a 50-period p-type (Be: 1 × 1017 cm−3) SL, a 200-period undoped SL as absorbing layer, and a 50-period n-type (Si: 1 × 1017 cm−3) SL. The detector responsivity is 0.4 A/W at 4.5 μm and the quantum efficiency is 17% at 30 K.The absorption quantum efficiency was 25% at 4.5 μm when we employed the complete T2SL (300-period T2SL). However, it became 18% when we used the absorption layer (200-period T2SL), which is consistent with the measured external quantum efficiency. This result suggests that almost all the carriers generated in the absorption layer are collected.