Enhanced Coherent Thermal Emission From SiO₂ on a Porous Silicon Photonic Crystal

We show that coherent thermal emission from an amorphous SiO₂ thin film can be significantly enhanced by placing it on top of a photonic crystal (PC). To demonstrate this principle, we simulated the reflectance and transmittance of a 1 micron thick layer of SiO₂ on a 20 layers PC using the scattering matrix method and finite difference numerical computations. Emissivity, calculated using Kirchhoff’s law, reaches unity at a peak wavelength around 10 microns due to overlapping of the PC’s forbidden band with bulk phonon-polariton modes in SiO₂. This region of the electromagnetic spectrum is of particular interest for many technological applications as it corresponds roughly to maximum thermal emission from a blackbody at room temperature. In order to validate these theoretical predictions, a PSi multilayer was fabricated by electrochemical dissolution of p-type Si in HF with current density modulated as a function of time to produce alternating layers of two different porosities with different refractive indices. A 1 micron thick layer of SiO₂ was then deposited on top of the resulting PC by plasma enhanced chemical vapor deposition (PECVD). Reflectance of the PC was measured at normal incidence using a Fourier Transform Infrared Spectrometer (FTIR) before and after SiO₂ deposition and matches the simulations.