Studies on the Impedance Characteristics, Dielectric Relaxation, and ac Conductivity in Silicon Oxycarbide (SiO_xC_y:H) Films at the Amorphous-to-Nanocrystalline Transition Zone

Abstract

The impedance characteristics and ac conductivity behavior of the silicon oxy-carbide (SiO x C y :H) network have been investigated during its transition from an amorphous dominated to a crystalline populated structure, grown by varying only the growth temperature (T S) from 160 to 180 °C. At elevated T S, the Si nanocrystals (Si-ncs) increased in size and number density, with corresponding elevation in crystallinity from 28 to 45%. Two distinct dielectric relaxations for the individual a-SiO x C y :H and nc-Si components were identified in the low-frequency and high-frequency zones, respectively, for the film deposited at higher T S (180 °C). In contrast, for the lower T S (160 °C)-grown film, a significant fraction of the ultra-nanocrystalline (unc-Si) component contributed only one consolidated relaxation peak at a moderate frequency. The scaling measurements of the imaginary part of impedance (−Z Im) revealed two distinct physical events for a-SiO x C y :H and nc-Si, both being temperature scale-independent. The activation energy of relaxation is low for the ordered nc-Si component and high for the disordered a-SiO x C y :H matrix. According to the Bode plot analysis, the presence of several defects in the grain boundary contributed a poor carrier lifetime to the nc-Si component, while the a-SiO x C y :H matrix with lesser defects had a longer carrier lifetime. An equivalent circuit model formed by parallel resistors and constant phase elements (R||CPE), following Nyquist plot analysis, demonstrated an intense association between the microstructural and charge carrier characteristics. The temperature-dependent ac-conductivity behavior of the mixed-phase material is well explained by the nonoverlapping small polaron tunneling model, at two individual stages across a transition from an amorphous to crystalline configuration around a small span of growth temperature.