Semiconductors for high selectivity thermal emitters

Selective emitters limit thermal radiation to a narrow spectral band enabling applications including energy conversion and sensing. While applications demand high spectral selectivity, the materials used for selective emitters limit the ultimate selectivity. At high temperatures (>600 K), the optical properties of materials degrade, resulting in poor spectral selectivity, and thus leaving an open question - what material platform makes a good selective thermal emitter? Here, we show that doped semiconductors make a good choice by predicting their temperature dependent optical constants in a wide doping range. We build semi-empirical models to predict the temperature dependences of microscopic processes in GaAs and Si, a polar and a non-polar semiconductor respectively, and demonstrate that semiconductors with small and tunable optical losses enable very high spectral selectivity in thermal emitters. Our predictions agree reasonably well with more complex and accurate methods such as variational and second order perturbation theories, and serve as a guide in the discovery and development of materials for high temperature nanophotonics.