Infrared absorption near the bandgap in the InAs/InAsSb superlattice

We have studied infrared absorption near the bandgap energy in mid-wavelength (MWIR) III-V photon detectors built in the nBn configuration. The absorbing material is the InAs/InAsSb superlattice. We show that in a practical device near the infrared (IR) cutoff, the spectral response curve as a function of photon energy is proportional to the absorption coefficient, to a good approximation. Thus, in the near-gap range, the energy dependence of the device spectral response is a reliable proxy for the energy dependence of the absorption coefficient. We demonstrate this by means of an expansion of the Hovel equations in powers of the product of the absorption coefficient and hole diffusion length. One application of this result is that the point of maximum slope of the spectral response curve can be used to locate the true bandgap energy. This result also facilitates a study of absorption in the Urbach tail, which occurs at sub-gap energies. The temperature dependence of the Urbach steepness parameter was found to be consistent with the dominant phonon energy of InAs.