Replacing dielectric barriers with semiconductors: generating atmospheric-pressure surface plasmas with help from electron-hole plasmas

In air at atmospheric pressure, surface dielectric barrier discharges (SDBD) often suffer from poor homogeneity or low levels of ionization. Only under certain circumstances can quasi-uniformity be achieved in the form of densely packed streamers. However, plasma-surface interactions with barriers composed of alternative materials may overcome these inherent limitations resulting from the exclusive use of bulk dielectrics. The semiconducting barrier discharge (SeBD), featuring silicon within a dielectric-semiconductor-dielectric barrier architecture, is one such example. Starting from a pin electrode, the SeBD propagates uniformly in the radial direction at all times, both in positive and negative voltage polarities, never branching into streamers. Furthermore, the apparent conduction current is considerably higher than for a comparable SDBD. We hypothesize that this unique discharge behavior results from strong photonic and electric field coupling between the air plasma and an electron-hole plasma in the silicon layer. To confirm the presence of a photonic effect, we illuminated the surface with a laser, which causes the plasma to increase in emission intensity, propagation distance, and apparent electric field as deduced from optical emission spectroscopy. To understand these results, we have developed a 2-D axisymmetric numerical model based on a drift-diffusion formulation to describe both the gas-phase and electron-hole plasmas. The simulation reproduces the salient features of air plasma propagation and current found experimentally. This validation has enabled us to gain insight into the character of the electron-hole plasma, including the impact of photogeneration of electron-hole pairs due to irradiation by photons from the air plasma. In addition, the distributions of both free and fixed charge carriers in silicon, as well as charge transport parameters, influence the electric field in air. These experimental and modeling results have informed the exploration of different barrier materials and discharge geometries.