Optical properties of thin semiconductor device structures with reflective back-surface layers

摘要

Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of (approximately) 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector.