Controlling the opto-electronic properties of nc-SiOx:H films by promotion of ⟨220⟩ orientation in the growth of ultra-nanocrystallites at the grain boundary

A systematic development of undoped nc-SiOx:H thin films from (SiH4 + CO2) plasma diluted by a combination of H-2 and He has been investigated through structural, optical and electrical characterization and correlation. Gradual inclusion of O into a highly crystalline silicon network progressively produces a two-phase structure where Si-nanocrystals (Si-nc) are embedded into the a-SiOx:H matrix. However, at the intermediate grain boundary region the growth of ultra-nanocrystallites controls the effectiveness of the material. The ultra-nanocrystallites are the part and portion of crystallinity accommodating the dominant fraction of thermodynamically preferred < 220 > crystallographic orientation, most favourable for stacked layer device performance. Atomic H plays a dominant role in maintaining an improved nanocrystalliny in the network even during O inclusion, while He in its excited state (He*) maintains a good energy balance at the grain boundary and produces a significant fraction of ultra-nanocrystalline component which has been demonstrated to organize the energetically favourable < 220 > crystallographic orientation in the network. The nc-SiOx:H films, maintaining proportionally good electrical conductivity over an wide range of optical band gap, remarkably low microstructure factor and simultaneous high crystalline volume fraction dominantly populated by ultra-nanocrystallites of < 220 > crystallographic orientation mostly at the grain boundary, have been obtained in technologically most popular 13.56 MHz PECVD SiH4 plasma even at a low substrate temperature similar to 250 degrees C, convenient for device fabrication. (C) 2017 Elsevier B.V. All rights reserved.