Surface roughness evolution of a-Si:H growth and its relation to the growth mechanism
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Surface roughness evolution of a-Si:H growth and its relation to the growth mechanism A.H.M. Smets1, D.C. Schram and M.C.M. van de Sanden2 Eindhoven University of Technology, Department of Applied Physics, Center for Plasma Physics and Radiation Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands 1 [email protected], [email protected] ABSTRACT The post-initial growth dynamic scaling exponent β, which describes the surface roughness evolution in time, is determined for a-Si:H growth using in situ single wavelength (632.8 nm) rotating compensator ellipsometry. β is measured as function of the substrate temperature for three different growth rates 2, 5 and 22 Ås-1 under conditions where SiH3 dominantly contributes to growth. β (≤ 0.5) decreases with increasing substrate temperature and does not strongly depend on the growth rate within the range of growth rates. A roughness evolution model is proposed, based upon a random generation of active growth sites and a subsequent site dependent surface diffusion process. The measured β temperature dependence can be simulated with an activated site hopping activation energy of about 1.0 eV. This activation energy is much higher than what would be expected from a model based upon the diffusion of physisorbed silyl SiH3 radical and suggests therefore another mechanism which is responsible for the surface smoothening during a-Si:H growth. INTRODUCTION Many studies in the past have addressed the surface roughness evolution during plasma enhanced chemical vapour deposition of hydrogenated amorphous silicon [1-4]. Under optimized conditions for high quality a-Si:H, it has been recognized that a-Si:H is extremely smooth, a property which has been related to either low sticking probability radicals or a long diffusion length of the surface adsorbed precursor for growth. Actually, many studies have argued that the small roughness of a-Si:H during growth under optimized conditions is a consequence of the dominance of the SiH3 radical during PECVD of a-Si:H. In general the root mean square roughness (rms) w evolution of a self-affine surface during deposition obeys the scaling law introduced by Family and Viscek [5]: w ∝ Lα F (d α / β / L) ∝ d β for d α / β / L > 1
(1b)
where L is a length scale parallel to the growth surface and approximately equal to the square root of the sampling area, d is the film thickness, α is the static scaling exponent and β is the dynamic scaling exponent. For constant deposition rate w will increase with dβ until the surface features become of the same order of the sampling area and the resulting roughness scales with
A7.6.1
Lα. β is directly linked to the growth mechanism and is therefore more interesting than α from a physical point of view. Both in situ optical techniques as well as intrusive atomic force and scanning tunneling microscopy (AFM and STM) [1,3,6,7] have been used to probe the roughness w of the growth surface of a-Si:H. Here we will discuss the measurement of the surface roughness in the postinitial growth phase
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