Optical EngineeringCoherent thermal radiation effects on temperature-dependent emissivity of thin-film structures on optically thick substrates
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Many studies on thermal processing of thin-film structures assume an incoherent thermal emission with an emissivity value that is constant or varies according to bulk properties. However, thermal radiation has a coherence length that varies with temperature. Thus, coherent thin-film interference affects the radiative properties of heterostructures when the wavelengths of radiation are of the same order of magnitude as the film thicknesses. For semiconducting and dielectric thin films on optically thick substrates, there are distinct trends in these microscale radiative effects. Numerical simulations of the temperature-dependent optical characteristics of multilayer thin-film structures were conducted to evaluate the thin-film interference effects. These results can be applied to thermal processes of thin-film structures where a spatial or temporal temperature distribution exists. An example of these effects is shown for radiative cooling of silicon-on-insulator wafers.