Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth

Abstract

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

Surface Smoothening Mechanism of Amorphous Silicon Thin Films

“…Smets and co-workers studied the scaling behavior of the growth surface morphology of a-Si:H films and derived an activation energy barrier of 1:0 eV for the smoothening mechanism [8]. However, Kondo and co-workers performed an ex situ study of a-Si:H surface morphology over the range 50 C < T < 450 C, and indicated a weakly activated smoothening process [10], in agreement with the results of Ref.…”

“…The T dependence of the surface roughening evolution of a-Si:H films has been analyzed in several experiments, under conditions where the dominant deposition precursor is the SiH 3 radical [7][8][9][10][11]. Smets and co-workers studied the scaling behavior of the growth surface morphology of a-Si:H films and derived an activation energy barrier of 1:0 eV for the smoothening mechanism [8].…”

“…Reported barriers for precursor diffusion on a-Si:H surfaces are dependent on the model employed and vary between 0.2 eV [7] to 1 eV [8]. In addition, density functional theory (DFT) calculations performed on the hydrogen-terminated crystalline Si(110) surface have been used to propose the existence of a mobile state of the adsorbed SiH 3 radical, in which the radical diffuses with an energy barrier of 0.4 eV between nearestneighbor surface Si atoms on this crystalline surface [14].…”

“…Based on the exceptionally low surface roughness of a-Si:H films grown under these conditions over the temperature (T) range 450 K < T < 750 K [4,5], a ''surface valley-filling'' mechanism has been postulated; according to this mechanism, a mobile surface species, generally assumed to be the SiH 3 radical [5], passivates preferentially dangling bonds (DBs) of Si atoms located in surface valleys [4,6]. Experimental data for the roughness evolution of a-Si:H films provide only indirect interpretations for the smoothening mechanism [7][8][9][10][11] and yield controversial results for the diffusion barrier of the SiH 3 radical on the a-Si:H surface [7,8,12,13]. Moreover, there exists no detailed account of either the role of surface morphology in affecting the incorporation of SiH 3 radicals into the film, or of the preferential locations for growth on the a-Si:H surface.…”

“…In experimental studies of a-Si:H film surface evolution, additional models for surface diffusion, like the solid-onsolid model [8], or assumptions such as that of a weakly adsorbed precursor state [9], or of a self-similar surface morphological evolution [7] typically need to be invoked to connect measured data for the surface topography to kinetic parameters. Reported barriers for precursor diffusion on a-Si:H surfaces are dependent on the model employed and vary between 0.2 eV [7] to 1 eV [8].…”