Carrier recombination at silicon–silicon nitride interfaces fabricated by plasma-enhanced chemical vapor deposition

Abstract

Using the light-biased microwave-detected photoconductance decay method, injection level dependent measurements of the effective surface recombination velocity $S_{\mathrm{eff}}$ at silicon surfaces passivated by plasma-enhanced chemical vapor deposited (PECVD) silicon nitride $({\mathrm{SiN}}_x)$ films are performed on monocrystalline silicon wafers of different resistivities and doping types. In order to theoretically simulate the measured dependences of $S_{\mathrm{eff}}$ on the bulk injection level $\mathrm{\Delta n,}$ the extended Shockley-Read-Hall formalism is used. Simulation input parameters are the energy dependent interface state densities and capture cross sections of the involved interface defects as well as the positive insulator charge density $Q_f.$ The energy dependent properties of the interface defects are experimentally determined by means of small-pulse deep-level transient spectroscopy. These measurements reveal the existence of three “deep” silicon dangling bond defects at the ${\mathrm{Si-SiN}}_x$ interface with similar interface state densities but very different capture cross sections and hence recombination rates. Another defect is found very close $\text{(⩽0.1\hspace{0.05em}}\mathrm{eV})$ to the edge of the silicon conduction band. This defect is identified with the ${\mathrm{K}}^{\mathrm{+}}$ center which is responsible for the large positive $Q_f$ values ${\text{(∼10}}^{12} {\mathrm{cm}}^{\mathrm{-2}})$ at ${\mathrm{Si-SiN}}_x$ interfaces obtained from standard dark capacitance-voltage measurements. In order to get a good agreement between measured and calculated $S_{\mathrm{eff}}\mathrm{(\Delta n)}$ dependences, a reduction of $Q_f$ by one order of magnitude is found to be necessary. The explanation for this reduction is the capture of electrons from the silicon conduction band into the ${\mathrm{K}}^{\mathrm{+}}$ centers. The comparison of ${\mathrm{Si-SiN}}_x$ interfaces fabricated by different PECVD techniques shows that the dominant interface defect is produced by the ion bombardment during the ${\mathrm{SiN}}_x$ deposition. Thus, avoidance of the ion bombardment leads to a strongly reduced interface recombination and hence a better surface passivation quality.