Passivation effects of atomic-layer-deposited aluminum oxide

Atomic-layer-deposited (ALD) aluminum oxide (Al2O3) has recently demonstrated an excellent surface passivation for both n- and p-type c-Si solar cells thanks to the presence of high negative fixed charges (Qf ~ 1012−1013 cm-2) in combination with a low density of interface states (Dit). This paper investigates the passivation quality of thin (15 nm) Al2O3 films deposited by two different techniques: plasma-enhanced atomic layer deposition (PE-ALD) and Thermal atomic layer deposition (T-ALD). Other dielectric materials taken into account for comparison include: thermally-grown silicon dioxide (SiO2) (20 nm), SiO2 (20 nm) deposited by plasma-enhanced chemical vapour deposition (PECVD) and hydrogenated amorphous silicon nitride (a-SiNx:H) (20 nm) also deposited by PECVD. With the above-mentioned dielectric layers, Metal Insulator Semiconductor (MIS) capacitors were fabricated for Qf and Dit extraction through Capacitance-Voltage-Conductance (C-V-G) measurements. In addition, lifetime measurements were carried out to evaluate the effective surface recombination velocity (SRV). The influence of extracted C-V-G parameters (Qf,Dit) on the injection dependent lifetime measurements τ(Δn), and the dominant passivation mechanism involved have been discussed. Furthermore we have also studied the influence of the SiO2 interfacial layer thickness between the Al2O3 and silicon surface on the field-effect passivation mechanism. It is shown that the field effect passivation in accumulation mode is more predominant when compared to surface defect passivation.