Surface reaction probabilities and kinetics of H, SiH3, Si2H5, CH3, and C2H5 during deposition of a-Si:H and a-C:H from H2, SiH4, and CH4 discharges

Abstract

The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH3, and Si2H5 on a growing a-Si:H film, and CH3 and C2H5 on an a-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH3 on a-Si:H, β=0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si2H5, 0.1<β<0.3. For H on a-Si:H, 0.4<β<1 and mostly consists of surface recombination as H2, while the etching probability of Si as SiH4 is only ε≈0.03 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH4 in H2 the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surface recombination at the expense of sticking. For CH3 or C2H5 on a-C:H it is shown that β is not constant during the discharge afterglow, decreasing from about 0.01 down to 0.001. This reveals that chemisorption of these radicals on the H-saturated a-C:H surface is entirely governed by the competition between desorption and creation of active sites by ion bombardment or H atoms. The differences between the surface reaction kinetics of SiH3 on a-Si:H and CH3 on a-C:H are discussed within a unified model of precursor-mediated chemisorption.

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

“…The mechanism that is ruling the smoothening of the surface is activated by ϳ1.0 eV and this value is incompatible with surface smoothening by diffusion of physisorbed SiH 3 radicals, with assumed activation energies of ϳ0.2-0.3 eV, as often proposed in the literature. 4,8,11,18 This provides evidence that physisorbed SiH 3 is not responsible for the decreasing surface roughness of a-Si:H with increasing T sub in accordance with the earlier conclusion that not physisorbed SiH 3 but the position of the dangling bonds on the surface determines the surface roughness. 3,10 The dangling bonds act as growth sites for the SiH 3 radicals and ultimately the SiH 3 radicals will stick at these dangling bonds, even in the case that the physisorbed SiH 3 radicals have diffused over the surface.…”

“…2-9 More specifically, we will provide more evidence for the fact that surface diffusion of physisorbed SiH 3 radicals is not responsible for the a-Si:H surface smoothening mechanism, 10 as has often been suggested in the literature. 4,8,11 Family and Viscek 12 introduced a general scaling law that describes the evolution of the root-mean-square roughness W(L,t) of a growing film. Adopting the notation of Barabasi and Stanley, 1 the W(L,t) values of a so-called selfaffine surface obey the scaling laws W(L,t)ϰL ␣ F(u)ϰt ␤ for uӶ1 and W(L,t)ϰL ␣ F(u)ϰL ␣ for uӷ1; 12 with L the system size of the surface, t the mean film thickness, uϭt/t x a normalized scaling parameter, t x ϰL ␣/␤ the crossover thickness, ␣ the roughness scaling exponent, and ␤ the growth scaling exponent.…”

Time-resolved cavity ringdown study of the Si and SiH3 surface reaction probability during plasma deposition of a-Si:H at different substrate temperatures

“…Moreover, direct measurements of ␤ for the silane radicals have been carried out by means of the various diagnostics mentioned in Sec. I, revealing ␤ values of ϳ0.2-0.3, 6,8,[10][11][12][13]20,25 [40][41][42][43][44] and molecular dynamics simulations, 41,[45][46][47][48] which address the surface reactions at the atomistic scale. For SiH 3 , only indirect information is available about the dependence of ␤ on the substrate temperature from the early study of Matsuda et al 6 This study, though, suffers from the uncertainty that the measured ␤ values are averaged overall plasma species.…”

“…As a consequence, considerable effort has been put into the development of techniques and methods that measure directly the surface reactivity of a specific radical and which are generally applicable during plasma processing. These techniques are based on the measurement of the time evolution or the spatial profile of the radical density and utilize various plasma diagnostics, such as threshold ionization mass spectrometry, [12][13][14][15] laser-induced fluorescence spectroscopy, [12][13][14][15][16][17][18] actinometry, 19 and various kinds of absorption spectroscopy. [20][21][22][23][24] In addition to these techniques, we have reported recently that time-resolved density measurements by means of cavity ringdown laser absorption spectroscopy (CRDS) can also be used to measure the surface reactivity of plasma radicals.…”

High growth rate deposition of hydrogenated amorphous silicon-germanium films and devices using ECR-PECVD

“…The standard model of a-Si:H growth according t o Matsuda [37], Perrin [38], and Gallagher [39] states that the growth of a-Si:H and a-(Si,Ge):H is limited primarily by swface diffusion of a radical such SiH3 . When this radical finds an open site, it bonds, and H is eliminated by breaking of Si-H bonds and subsequent cross-linking of neighboring hydrogens' bonded to adjacent Si atoms.…”