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.