Study of Surface Treatments on InAs/GaSb Superlattice LWIR Detectors

We report on the comparison of mesa sidewall profiles of InAs/GaSb strained-layer superlattice (SLS) detector structures (λ50% cutoff ≈ 14 μm at Vbias = 0 V and T = 30 K) obtained after (a) a conventional BCl3-based inductively coupled plasma etch, (b) a chemical etch (H2O2:HCl:H2O, 1:1:4), and (c) a combination of both etches. We found that the smoothest sidewall profile with reasonable undercut (~5 μm) was obtained after chemical etch only. The chemical etch was optimized primarily using an n-type GaSb substrate. During this process, numerous chemical etchants were examined. GaSb n-type substrates were chosen for this study in preference over InAs substrates due to their high chemical reactivity and the complicated composition of the native oxide. In addition, SLS detectors are usually grown on GaSb substrates and, after hybridization of the focal-plane array to the readout integrated circuit, the GaSb substrate is etched away using a combination of wet and dry etching techniques. We found that H2O2:HCl:H2O (1:1:4) etching solution provided the smoothest etched surface of GaSb, with a root-mean-square roughness of 1.59 nm.