Silicon nitride at high growth rate using hot wire chemical vapor deposition

Abstract

Amorphous silicon nitride (SiNx) is a widely studied alloy with many commercial applications. This thesis describes the application of SiNx deposited at high deposition rate using hot wire chemical vapor deposition (HWCVD) for solar cells and thin film transistors (TFTs). The deposition process of HWCVD SiNx is investigated. The obtained knowledge of the deposition process is used to increase the deposition rate of device quality SiNx to 7.3 nm/s. This deposition rate is much larger than offered by other deposition techniques. The compositional, structural, and optical properties of HWCVD SiNx films have been analyzed. . By comparing these properties with those of SiNx films deposited by plasma enhanced CVD, fundamental insight in the correlation between these properties is obtained. HWCVD SiNx films deposited at high deposition rate were applied on multi-crystalline Si solar cells, as passivating anti-reflection coating. The solar cells characteristics are compared to reference cells containing optimized microwave PECVD SiNx and low frequency PECVD SiNx. The best solar cell efficiency was obtained for SiN1.3 films and reached 15.7%. This value is comparable to the best solar cell in the reference group. A large value for the open circuit voltage combined with large internal quantum efficiencies and long charge carrier lifetimes demonstrate that good passivation is achieved. Since bonded H in SiNx films plays an important role in the degree of passivation that can be achieved, reliable quantification of the H-bond densities is crucial. A novel method to calibrate the FTIR proportionality factors is presented, allowing reliable quantification. This method was applied to HWCVD SiNx samples with a range of N/Si ratios. The values of the FTIR proportionality factors show a dependence on the N/Si ratio. By determining the FTIR proportionality factors for PECVD SiNx films with comparable N/Si ratio but different mass densities, it is shown that also the mass density influences the FTIR proportionality factors. The calibrated FTIR proportionality factors are used to study the H evolution in SiNx during an anneal treatment. Both the N/Si ratio and the mass density have a large influence on the H evolution pathways. HWCVD SiN1.3 was tested as dielectric medium in a-Si “all HWCVD” TFTs. These TFTs show a large electron mobility of 0.45 cm/Vs after an anneal treatment. From the analysis of capacitance-voltage and current-voltage measurements on metal-insulator-semiconductor structures, suggestions for further improvements for “all HWCVD” TFTs are given. Concluding, enhanced insight in the HWCVD SiNx deposition process is obtained, and used to achieve large deposition rate of 7.3 nm/s for device quality SiNx films. By comparing the compositional, structural and optical properties of HWCVD SiNx films with PECVD SiNx films, fundamental knowledge correlating these properties is obtained. Rapidly deposited HWCVD SiNx was applied on solar cells and reached an efficiency of 15.7%, comparable to the reference cells. A novel method to determine the FTIR proportionality factors is given, and this method is used to study the H evolution pathways in SiNx films. Finally, HWCVD SiN1.3 was tested as dielectric medium in “all HWCVD” TFTs, and showed good characteristics.