Abstract
Differential scanning calorimetry (DSC) was used to study the dehydrogenation processes that take place in three hydrogenated amorphous silicon materials: nanoparticles, polymorphous silicon, and conventional device-quality amorphous silicon. Comparison of DSC thermograms with evolved gas analysis (EGA) has led to the identification of four dehydrogenation processes arising from polymeric chains , SiH groups at the surfaces of internal voids , SiH groups at interfaces , and in the bulk . All of them are slightly exothermic with enthalpies below ∕(H atoms), indicating that, after dissociation of any SiH group, most dangling bonds recombine. The kinetics of the three low-temperature processes [with DSC peak temperatures at around 320 , 360 , and ] exhibit a kinetic-compensation effect characterized by a linear relationship between the activation entropy and enthalpy, which constitutes their signature. Their bond-dissociation energies have been determined to be , 3.19 , and . In these cases it was possible to extract the formation energy of the dangling bonds that recombine after bond breaking [0.97 , 1.05 , and 1.12 ]. It is concluded that increases with the degree of confinement and that for the isolated dangling bond in the bulk. After dissociation and for the low-temperature processes, hydrogen is transported in molecular form and a low relaxation of the silicon network is promoted. This is in contrast to the high-temperature process for which the diffusion of H in atomic form induces a substantial lattice relaxation that, for the conventional amorphous sample, releases energy of around per H atom. It is argued that the density of sites in the Si network for H trapping diminishes during atomic diffusion.
4 More- Received 8 July 2005
DOI:https://doi.org/10.1103/PhysRevB.73.085203
©2006 American Physical Society