Double Layers of Ultrathin a-Si:H and SiNx for Surface Passivation of n-type Crystalline Si Wafers

Surface passivation by double layers made of hydrogenated amorphous silicon (a-Si:H) and hydrogenated silicon nitride (SiNx) on float zone n-type Si substrates was investigated. The thickness of the a-Si:H layers was varied from 0 to 4 nm and they were deposited at low temperature by plasma-enhanced chemical vapour deposition. The structure and composition of the double layers was determined by a combination of spectroscopic ellipsometry and glow discharge optical emission spectroscopy. The minority carrier effective lifetime and the uniformity of the surface passivation of the wafers were measured by calibrated photoluminescence imaging. The lifetime was observed to increase with increasing a-Si:H thickness. An excellent lifetime value of 7.3 ms was measured on the double layers with an a-Si:H layer thickness of 3.4 nm, corresponding to surface recombination velocity below 2 cm/s. Capacitance-voltage and conductance-voltage measurements were used to determine the densities of fixed charge and interface states at the interface between the Si substrate and the a-Si:H/SiNx stacks. The fixed charge density of the initial SiNx was positive, as expected. However, the overall fixed charge density of the double layers was observed to decrease when a layer of a-Si:H was inserted beneath the SiNx, and eventually reaches negative values when the a-Si:H thickness exceeds 2 nm. The conductance-voltage measurements revealed that the interface state density (Dit) is significantly lowered when adding the ultrathin a-Si:H film between the substrate and the SiNx layer, providing a significant improvement of the chemical passivation, thus reducing the overall surface recombination velocity.