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 Seff at silicon surfaces passivated by plasma-enhanced chemical vapor deposited (PECVD) silicon nitride (SiNx) films are performed on monocrystalline silicon wafers of different resistivities and doping types. In order to theoretically simulate the measured dependences of Seff on the bulk injection level Δn, 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 Qf. 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 Si-SiNx interface with similar interface state densities but very different capture cross sections and hence recombination rates. Another defect is found very close (⩽0.1 eV) to the edge of the silicon conduction band. This defect is identified with the K+ center which is responsible for the large positive Qf values (∼1012 cm−2) at Si-SiNx interfaces obtained from standard dark capacitance-voltage measurements. In order to get a good agreement between measured and calculated Seff(Δn) dependences, a reduction of Qf 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 K+ centers. The comparison of Si-SiNx interfaces fabricated by different PECVD techniques shows that the dominant interface defect is produced by the ion bombardment during the SiNx deposition. Thus, avoidance of the ion bombardment leads to a strongly reduced interface recombination and hence a better surface passivation quality.