Broadband and incoherent thermal radiation from hot surfaces may be turned into narrowband and quasi-coherent light using selective thermal emitters. Selective thermal emitters operating at high temperatures enable many applications including gas sensing and thermophotovoltaic energy conversion. The narrow emission bandwidth of selective emitters is accomplished by some resonant phenomenon, and often built using refractory metals because of their thermal stability. However, refractory metals have high optical losses at high temperatures and shorter wavelengths limiting the spectral selectivity of the emitter. Thus, high quality factors (> 100) are not possible for metallic resonators when operating at temperatures > 700 K and center wavelengths < 4000 nm. On the other hand, nearly lossless semiconductors provide high spectral selectivity, but with very small thermal emission. Thus, there remains an open question – what material makes the best platform for high temperature (> 700 K) selective thermal emitters? Answering this question requires us to evaluate temperature dependent optical properties of metals and semiconductors, and identify the right material platform for any given application.
In this work, we model the temperature dependent optical properties of materials using the physics of microscopic processes. Modeling metals is relatively straightforward as their optical properties are dominated by free carriers. However, semiconductors pose a tougher challenge as their sub-bandgap optical properties are dominated by the interaction between free carriers, phonons, impurities and bandstructure changes. We develop a semi-empirical model to predict temperature dependent optical constants of semiconductors for any doping concentration and at any temperature. We show that our model predicts the optical constants with reasonable accuracy by comparing our predictions against experiments and more complicated theoretical methods such as variational method and second order perturbation theory. Using the temperature dependent optical properties of metals and semiconductors, we attempt to answer the question of what makes a good material platform for high temperature selective thermal emitters. Our analysis shows that in the temperature range 300-1200 K, semiconductors with tunable optical properties are promising and for high temperatures, refractory metals are promising. Our investigation guides the choice of materials and their processing condition such as doping concentration, and provides insight into new material discovery for high temperature selective thermal emitters.