Experimental evidence for an inhomogeneous surface dangling bond limited growth mechanism in a-Si:H A.J. Flewitt * , J. Robertson, W.I. Milne Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK Abstract The growth mechanism of hydrogenated amorphous silicon (a-Si:H) deposited by radio frequency (rf) plasma en- hanced chemical vapour deposition (PECVD) has been studied by in situ scanning tunnelling microscopy (STM) at several stages during growth. The data showed that the surface has an island-like structure. The radially averaged Fourier transform of the topography reveals, quantitatively, how feature density scales with feature size. This yields a Fourier index of between 1.1 and 1.4 for the a-Si:H surface, which diers from the value of 4 which would be expected for a surface growing through surface diusion alone. We suggest, based on chemical arguments, that growth occurs by the diusion of SiH 3 radicals over the hydrogen terminated a-Si:H surface and their subsequent attachment to dangling bonds. We suggest that these dangling bonds preferentially form at kink-like and step-like surface sites, where hydrogen is sterically hindered. A simple computer simulation of this mechanism reveals that an island-like topography is indeed produced by this mechanism with a Fourier index of 1.55. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Hydrogenated amorphous silicon (a-Si:H) cur- rently dominates as the active layer of thin ®lm transistors (TFTs) in active matrix liquid crystal displays. It is also used in solar cells. The most common method of growth for large area appli- cations is plasma enhanced chemical vapour de- position (PECVD) produced by a radio frequency (rf) discharge. As a consequence of its technolog- ical importance, much eort has been spent on understanding the mechanisms at work during deposition. These have mostly concentrated on the gas phase reactions which take place in the plasma [1±3], and as a result, it is now generally accepted that, at smaller deposition rates, the dominant precursor to deposition is the SiH 3 radical, which is incorporated into the material at the growing surface. Scanning tunnelling microscopy (STM) allows the surface of a conducting material to be studied on an atomic scale in three-dimensions [4], and so is applicable to a study of the growth of a-Si:H. This work, which follows on from an earlier study [5] uses a STM operated in an ultra-high vacuum (UHV) chamber. This chamber is connected, via a gate valve, to a second chamber in which a-Si:H may be deposited by rf±PECVD, and this system has allowed the same sample topography to be studied at several stages during growth without exposing the a-Si:H to the laboratory atmosphere. Using a quantitative analysis, based on the Fourier transform of the topography, a growth mechanism is proposed. To evaluate the mechanism, a simple computer simulation of growth has been con- structed and the same quantitative tests applied. Journal of Non-Crystalline Solids 266±269 (2000) 74±78 www.elsevier.com/locate/jnoncrysol * Corresponding author. E-mail address: ajf@eng.cam.ac.uk (A.J. Flewitt). 0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 7 4 0 - 1