Optimized nc-Si:H thin films with enhanced optoelectronic properties prepared by micro-waves PECVD used as an effective silicon surface passivation layer

Abstract

In this work we investigated a novel approach to elaborate hydrogenated nanocrystalline silicon (nc-Si:H) thin films using microwave plasma enhanced chemical vapor deposition (MW-PECVD) system, used as an efficient silicon surface passivation layer. We studied the effect of the hydrogen flow rate on the structural, morphological and optical properties of nc-Si:H thin films. Obtained results confirm its crucial role on the properties of the resulted thin films. Optimized hydrogen flow rate allowed an hydrogenated nanocrystallites (nc-Si:H) with around 80% of a crystallization phase which is a high percentage compared to previous works. The XRD analysis was supported by Raman spectroscopy, indicating the pure phase of cubic structure with a preferred orientation along (111) plane. The average grain size was found to vary between 7 and 16 nm. The surface morphology of the deposited thin films was carried out using atomic force microscopy (AFM), showing a clear dependence of surface roughness with the hydrogen flow rate. The refractive index of the obtained thin films was evaluated by means of the Bruggeman effective medium approximation (BEMA). The optical band gap and the porosity of the films were deduced from spectroscopic ellipsometry measurements and results were found strongly correlated with XRD and AFM analysis respectively. Moreover, the minority carrier lifetime measurements (MCL) prove the effectiveness of the subjected treatment for a high quality nc-Si:H thin film and its crucial role as a passivation layer of crystalline silicon substrate, revealing a significant improvement of their optoelectronic properties.