Deposition of microcrystalline silicon: Direct evidence for hydrogen-induced surface mobility of Si adspecies

Hydrogenated microcrystalline silicon thin films can be deposited at low su bstrate temperatures using plasma enhanced-or hot wire-chemical vapor depos ition using silane, reactive magnetron sputtering of silicon, or related te chniques. Microcrystalline silicon is deposited when a large quantity of mo lecular hydrogen is added to the process gas such that a large flux of atom ic hydrogen impinges on the film growth surface; otherwise, the films are a morphous. Three different microscopic mechanisms have been hypothesized to explain the formation of the microcrystalline phase. In essence, the hypoth eses are that atomic hydrogen: (i) enhances the surface diffusion of Si ads pecies, which in turn raises the probability of crystalline phase formation , (ii) promotes a subsurface transformation of amorphous into microcrystall ine Si, or (iii) preferentially etches amorphous regions such that only mic rocrystalline Si survives to produce film growth. In this work, we critical ly test mechanism (i) as follows. We deposit films using dc reactive magnet ron sputtering of a Si target in an argon-hydrogen plasma, which yields ver y poor adspecies mobility at low rates of hydrogen injection. We then incre ase the hydrogen injection and measure the increase in adspecies motion via the enhanced rate at which the surface smoothens for film growth on substr ates with a calibrated roughness of similar to 80 Angstrom. The dynamic sur face roughness and the structural phase are determined by real-time spectro scopic ellipsometry. The combination of high atomic hydrogen flux and high surface hydrogen coverage uniquely correlates with microcrystalline phase f ormation. Higher substrate temperatures do not increase adspecies mobility, and actually decrease it when the rate of thermal desorption becomes suffi cient to decrease the surface hydrogen coverage. These results also suggest that the original identity of the Si-bearing growth species is relatively unimportant, because the atomic hydrogen flux appears to produce mobile ads pecies via surface reactions. We have previously shown that subsurface tran sformations, mechanism (ii), can also occur. However, we find no evidence f or competitive etching, mechanism (iii), under our experimental conditions. (C) 2001 American Institute of Physics.