Hydrogenated amorphous silicon deposited under accurately controlled ion bombardment using pulse-shaped substrate biasing

Abstract

We have applied pulse-shaped biasing to the expanding thermal plasma deposition of hydrogenated amorphous silicon at substrate temperatures ∼200 °C and growth rates around 1 nm/s. Substrate voltage measurements and measurements with a retarding field energy analyzer demonstrate the achieved control over the ion energy distribution for deposition on conductive substrates and for deposition of conductive materials on nonconductive substrates. Presence of negative ions/particles in the Ar–H2–SiH4 plasma is deduced from a voltage offset during biasing. Densification of the material at low Urbach energies is observed at a deposited energy <4.8 eV/Si atom and attributed to an increase in surface mobility of mobile species as well as well as surface atom displacement. The subsequent increase in Urbach energy >4.8 eV/Si atom is attributed to bulk atom displacement in subsurface layers. We make the unique experimental abservation of a decreasing Tauc band gap at increasing total hydrogen concentration—this allows to directly relate the band gap of amorphous silicon to the presence of nanovoids in the material.

Structural origins of intrinsic stress in amorphous silicon thin films

“…Fig. 4b (in agreement with results observed by Hamers,33,34 Smets,23 and Wank 35 indicates that increasing ion momentum results in an initially rapid decrease in film porosity in the tensile region, followed by a nearly invariant low porosity in the compressive region.…”

“…From these results, in combination with recent electrical transport 43,44 and optical measurements 35 reported elsewhere, a more complete picture of a-Si:H film processstructure-property relations begins to emerge. With the ability to predict film microstructure structure using empirical models, controllable a-Si:H properties should be achievable on a wide range of deposition systems.…”

Small-Angle X-Ray Scattering of Amorphous Germanium: Numerical Modeling

“…It is well known that such feature in the small-angle scattering data mirrors the heterogeneity of the amorphous material at the nanoscale. By correlating small-angle scattering results with those derived from other experiments [14][15][16][17][18][19][20], the mass-density measurements principally [19], many authors have postulated the existence of nanovoids in a-Si and a-Ge films. Indeed, such structural inhomogeneities are commonly believed to result in the mass-density of a-Si and a-Ge being lower than their corresponding crystalline phases; they also generate significant fluctuations in the density at the nanoscale which can be at the origin of the intense small-angle scattering in these materials.…”

Rapid calculation of the ion energy distribution on a plasma electrode

“…The waveform had to be given the right slope to account for charging of nonconductive substrates, and still yield a nearly constant sheath potential, and thus a nearly monoenergetic IED, in the limit s i /s rf ( 1. A similar approach was used by Kudlacek et al 15 who also studied the influence of substrate charging on the required voltage waveform, 16,17 so that monoenergetic IEDs are obtained. Agarwal and Kushner 18 and Rauf 19 conducted computational investigations of the effect of non-sinusoidal bias voltage waveforms on the IED, etching rate and selectivity.…”