Al vacancies, O interstitials, and H interstitials are proposed as the reasons for the negative Q f of Al2O3[23, 24]. The measured Q f in Figure 3 and information on Al vacancies in Figure 7 were considered in analyzing the effect of Al selleck vacancy density
on the negative fixed charge Q f. With increased annealing temperature from 300°C to 500°C, the increase in Q f was opposite to the decrease in Al vacancy in the bulk film. Thus, Q f may not be related with Al vacancies in the Al2O3 films. The measured minimum effective lifetime in Figure 3 and S parameters of SiO x interface in Figure 7 were correlated, and the decrease in vacancy of SiO x was coincident with the enhanced chemical passivation at annealing temperatures lower than 500°C. However, the chemical passivation breakdown at 750°C cannot be explained: among the samples annealed at 300°C and 750°C, the chemical passivation at 750°C was the poorest, but the defect density at the interface region still decreased. The functions of interstitial atoms (O or H) near the interface require further investigation. Conclusions Q f did not significantly affect the passivation at a low annealing temperature (300°C). The interface trap density
markedly increased at a high annealing temperature (750°C) and failed at surface passivation even at a high Q f. Positron annihilation techniques were used to probe QNZ purchase the vacancy-type defects. A three-layered microstructure of thermal ALD Al2O3 films on Si substrate was found. The Al defect density in the bulk film and the vacancy density near the interface decreased with increased temperature based on the fitted S parameter at different positions in the Al2O3 films. The Al vacancy of the bulk film was not related to Q f based on the Q f measurement results. The effects of interstitial atoms on Q f need further investigation. The defect density in the SiO x region may affect chemical passivation, but other factors enough may also influence chemical passivation particularly beyond 500°C. Acknowledgments This study was supported by the National
High Technology Research and Development Program of China (grant no. 2011AA050515) and the National Basic Research Program of China (grant no. 2012CB934204). The authors are grateful to Dr. Cao for the DBAR measurements at the Beijing Slow Positron Beam, Institute of High Energy Physics, Chinese Academy of Sciences. References 1. Schmidt J, Werner F, Veith B, Zielke D, Bock D, Brendel R, Tiba V, Poodt P, Roozeboom F, Li A, Cuevas A: Surface passivation of silicon solar cells using industrially relevant Al 2 O 3 deposition techniques. Photovoltaics Int 2010, 10:42–48. 2. Rothschild A, Vermang B, Goverde H: Atomic layer deposition of Al 2 O 3 for industrial local Al back-surface field (BSF) solar cells. Photovoltaics Int 2011, 13:92–101. 3. Schmidt J, Merkle A, Brendel R, Hoex B, van de Sanden MCM, Kessels WMM: Surface passivation of high-efficiency silicon solar cells by atomic-layer-deposited Al 2 O 3 .