Abstract
The use of time modulated gas flows in plasma enhanced chemical vapor deposition (PECVD) gives an additional degree of flexibility to the growth process to control film nucleation and modify the properties of the deposited films. Specifically, we show that time modulated flow of silane into a hydrogen plasma allows for substrate selective nucleation and enhanced crystallinity of microcrystalline silicon films. We have used Raman spectroscopy, infrared absorption spectroscopy and scanning tunneling microscopy (STM) to investigate reactions between atomic hydrogen and the growth surface that result in crystallite formation and selective deposition. We show that exposing the growth surface to atomic hydrogen results in: 1) breaking of highly strained surface bonds; and 2) etching of silicon from the surface. We find that substrate dependent nucleation, and the different etch rates of the different nucleii, lead to selective deposition. However, we show that etching is not sufficient to account for microcrystallite formation, and that bond strain relaxation by atomic hydrogen promotes motion of adatoms on the surface and leads to the nucleation of crystalline silicon. The selective deposition technique has been applied to the formation of thin film transistor devices, and these results are also presented.