Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 7, Issue 51, Pages 28215-28222Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.5b06606
Keywords
fixed charges; surface passivation; atomic layer deposition; interface modification; aluminum oxide; hafnium oxide; silicon oxide
Funding
- German Federal Ministry of Education and Research BMBF
- Federal Environment Ministry BMU (MWT-plus) [03SF0420A]
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A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes. Al2O3 nanolayers grown by atomic layer deposition contain negative fixed charges. Electrical measurements on slant-etched layers reveal that these charges are located within a 1 nm distance to the interface with the Si substrate. When inserting additional interface layers, the fixed charge density can be continuously adjusted from 3.5 X 10(12) cm(-2) (negative polarity) to 0.0 and up to 4.0 x 10(12) cm(-2) (positive polarity). A HfO2 interface layer of one or more monolayers reduces the negative fixed charges in Al2O3 to zero. The role of HfO2 is described as an inert spacer controlling the distance between Al2O3 and the Si substrate. It is suggested that this spacer alters the nonstoichiometric initial Al2O3 growth regime, which is responsible for the charge formation. On the basis of this charge-free HfO2/Al2O3 stack, negative or positive fixed charges can be formed by introducing additional thin Al2O3 or SiO2 layers between the Si substrate and this HfO2/Al2O3 capping layer. All stacks provide very good passivation of the silicon surface. The measured effective carrier lifetimes are between 1 and 30 ms. This charge control in Al2O3 nanolayers allows the construction of zero-fixed-charge passivation layers as well as layers with tailored fixed charge densities for future solar cell concepts and other field-effect based devices.
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