A THEORETICAL STUDY OF HYDROGEN EXODIFFUSION IN a-Si : H, COMPARISON WITH CONDUCTIVITY MEASUREMENTS

Abstract
Hydrogen evolution in a-Si : H prepared by glow discharge decomposition of silane has been studied previously as a function of annealing temperature, using nuclear reaction, conductivity (σ) and Electron Paramagnetic Resonance (EPR) measurements. Conductivity measurements show the existence of a surface dehydrogenated layer (SDL) for T ≥ 500 °C. We presented a theoretical kinetic model which takes into account the existence of two kinds of site for hydrogen in the amorphous network : one site is a center from which hydrogen can diffuse, the other is a tightly bound center (corresponding to isolated Si-H bonds). This theoretical model is used here to interpret our conductivity measurements (which are sensitive to the departure of H in the Si-H configuration) for annealing temperatures above 500 °C. This allows us to describe quantitatively the SDL in the same temperature range. We then deduce the thermodynamic parameters which characterize the breaking of the isolated Si-H bond, and the possibility of trapping of a hydrogen by a Si-dangling bond. We obtain the corresponding activation energies respectively equal to 3.3 eV and 4 eV. We thus confirm the value of the Si-H bond energy and give the capture energy of a hydrogen in an isolated Si-dangling bond, which has not been previously determined.