Hydrogen energetics ina−Si:Has determined by a combination of mean-field modeling and experimental evolution data

Abstract

The energetics of hydrogen configurations in amorphous silicon ($a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$) and hydrogen populations associated with these configurations remain an important unresolved issue. To date, hydrogen evolution data have been difficult to analyze due to lack of models accurately describing the coupling of the thermally activated reaction/diffusion/desorption processes, that control evolution. A rigorous mathematical model for hydrogen transport in, and evolution from $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ is described and validated here. The model improves on other evolution models by including three discrete hydrogen energy levels in the film, one mobile level and two independent trap levels, and by explicitly simulating adsorption/desorption processes at the surface. This multienergy level model with a floating boundary condition at the surface is used to simulate two types of evolution experiments: temperature programed evolution and isothermal evolution. Results from both types of experiments for glow discharge deposited films are modeled using the same energetics, with trap depths of 1.5 and 1.8--1.9 eV below the transport level, and 20--30 % of hydrogen residing in the deep traps. These results contrast past experimental analysis, which suggested that hydrogen in deep traps is more tightly bound, 2.1--3.4 eV below the transport level. The difference of 0.3--0.4 eV we observed between the two trap energy levels agrees well with published bonding energies estimated using ab initio quantum-mechanical calculations. The difference in trap energies is also similar to the measured defect formation activation energy of 0.2--0.5 eV, suggesting that the defect formation mechanism in $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ may involve hydrogen motion between the two energy levels. Combination of our results with quantum-mechanical calculations of various hydrogen energy levels in crystalline silicon suggests that the interstitial hopping may not be the primary hydrogen transport mechanism.