Optical constants of digital alloy InGaAs

The digital alloy (DA) growth technique has been widely reported to implement band structure engineering for deterministic optical and electronic properties and to overcome growth limitations imposed by miscibility gaps. Random alloy (RA) InGaAs lattice-matched to InP with a bandgap of 0.74 eV is widely used as the absorption material for photodetectors in the short-wavelength infrared spectral range. In this work, the InGaAs is grown on InP substrates as digital alloy, short-period InAs/GaAs superlattices, with six monolayer periodic thickness to extend its cut-off wavelength. The effective extension of the absorption spectral range makes DA InGaAs a promising candidate for absorption at longer wavelengths than the cutoff of RA InGaAs, motivating the study of the optical characteristics of this material system. Variable-angle spectroscopic ellipsometry measurements were carried out for both DA and RA InGaAs samples from 193 nm to the cut-off wavelength. After the multi-layer model building, the optical constants were extracted via the Kramers-Kronig consistent B-Spline fitting method. The results can be used to design new optoelectronic devices. The absorption coefficient at 2 μm of six monolayer DA InGaAs was found to be 398 cm^-1. The extracted optical constants of RA InGaAs were compared with the published values, and a good agreement was obtained, corroborating the effectiveness of extracting optical constants via ellipsometry for the InGaAs material system. These optical constants are beneficial for the future utilization of DA InGaAs in optoelectronic devices with extended spectral response.