Influence of layer thickness on passivation properties in SiOx/Al2O3 stacks

Abstract

SiOx/Al2O3 passivation stacks have shown the potential to effectively passivate both n- and p-type surfaces as the charge density may be controlled via the SiOx layer thickness. Here, the passivation quality of the SiOx/Al2O3 stacks is investigated as the function of SiOx layer thickness and Al2O3 layer thickness. The SiOx/Al2O3 passivation stacks were deposited on n- and p-type Si using plasma-enhanced atomic layer deposition and the effective surface recombination velocity (Seff), interface defect density (Dit), and effective fixed charge density (Qeff) were measured. The level of passivation was highest when the surface was passivated with only Al2O3 (1.1 cm/s and 4.8 cm/s for n-type and p-type, respectively). Seff increased with increasing SiOx thickness, reached a maximum value, and then decreased for thicker SiOx layers. The low Seff values correlate with accumulation or inversion while maximum Seff appears at the depletion/edge of inversion. Increasing the Al2O3 thickness also lowered Seff, both when used on its own and in SiOx/Al2O3 stacks but did not affect where the maximum Seff was located. The defect density was low (Dit<1011eV−1cm−2) irrespective of SiOx thickness and the n-type wafers showed an overall lower defect density than p-type wafers. The SiOx layer lowered the defect density for n-type wafers and Dit<1010eV−1cm−2 could be observed. The SiOx layer thickness greatly affected the effective fixed charge density (Qeff) which became more positive as the thickness increased. The doping type had an influence on the resulting Qeff with the n-type Si becoming more negatively charged than p-type.