Kinetic analysis of hydrogen evolution from reactively sputtered amorphous silicon-hydrogen alloys

Hydrogen evolution from annealed samples of amorphous silicon-hydrogen alloys ($a$-Si: H) is discussed. From the results of isothermal-annealing experiments, it is demonstrated that at fairly low temperatures ($T\approx 225°$C) the rate-limiting step for hydrogen evolution is a singly activated desorption process with a free energy of activation of 1.7 eV. We show that this low-temperature evolution is not limited by the diffusion of hydrogen through the silicon network. For this low-temperature desorption process, we measure in isochronal-annealing experiments an activation enthalpy of 0.4 eV and an activation free energy of 1.7 eV. Approximately one third of the hydrogen in $a$-Si: H resides in the relatively weakly bound states investigated in these low-temperature evolution studies. Also observed is the evolution of hydrogen associated with crystallization that occurs near 600 °C, and (in some samples) an intermediate temperature desorption process near 500 °C that involves a free energy (enthalpy) of activation of 2.4 (1.6) eV.