Determination of the defect density in thin film amorphous and microcrystalline silicon from ESR measurements: The influence of the sample preparation procedure

Accurate evaluation of the defect density (N-D) is of high relevance for the optimization of thin film silicon. The spin density (N-s) measured in ESR experiments is often used as a measure for the density of deep defects in the material, assuming that all defects are in a paramagnetic charge state. However, exposure to air, water, or acid during ESR sample preparation can potentially change the N-s in a sample and lead to misinterpretation of N-D. We have investigated how the preparation procedures of a Si thin film ESR sample may affect the properties of its ESR spectrum. Samples of different structural composition from highly crystalline pc-Si:H to a-Si:H deposited by PECVD on Mo-foil. Al-foil and ZnO:Al were studied for different states of exposure to ambient conditions and annealing. N-s measured directly after sample preparation and after air exposure was found to be higher than N-s measured in the annealed state. Particularly in highly crystalline material this discrepancy may reach one order of magnitude. On the other hand in a-Si:H and medium crystalline pc-Si:H relevant for applications, the difference in N-s between air-exposed and annealed conditions is smaller. ESR measurements performed at 40 K suggest that atmospheric exposure leads to charging of the defect states, which in turn influences the evaluated spin density. (C) 2012 Elsevier B.V. All rights reserved.