Calorimetry of dehydrogenation and dangling-bond recombination in several hydrogenated amorphous silicon materials

Abstract

Differential scanning calorimetry ͑DSC͒ 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 ͑EGA͒ has led to the identification of four dehydrogenation processes arising from polymeric chains ͑A͒, SiH groups at the surfaces of internal voids ͑AЈ͒, SiH groups at interfaces ͑B͒, and in the bulk ͑C͒. All of them are slightly exothermic with enthalpies below 50 meV/͑H atoms͒, 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 ͑A͒, 360 ͑AЈ͒, and 430°C ͑B͔͒ exhibit a kinetic-compensation effect characterized by a linear relationship between the activation entropy and enthalpy, which constitutes their signature. Their Siu H bond-dissociation energies have been determined to be E͑Siu H͒ 0 = 3.14 ͑A͒, 3.19 ͑AЈ͒, and 3.28 eV ͑B͒. In these cases it was possible to extract the formation energy E͑DB͒ of the dangling bonds that recombine after Siu H bond breaking ͓0.97 ͑A͒, 1.05 ͑AЈ͒, and 1.12 ͑B͔͒. It is concluded that E͑DB͒ increases with the degree of confinement and that E͑DB͒ Ͼ 1.10 eV for the isolated dangling bond in the bulk. After Siu H 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 600 meV per H atom. It is argued that the density of sites in the Si network for H trapping diminishes during atomic diffusion.

Quantification of the bond-angle dispersion by Raman spectroscopy and the strain energy of amorphous silicon

“…We should stress here that the hydrogen content cannot modify Q cryst substantially because, at the crystallization temperature, most hydrogen has already evolved from the sample and, additionally, any hydrogen-related group increases ͑slightly͒ the material energy. 37 Consequently, the high dispersion of our points in Fig. 3 probably corresponds to different contributions of structural defects that would be related to the deposition conditions.…”

Relaxation and derelaxation of pure and hydrogenated amorphous silicon during thermal annealing experiments

“…8,9 This signal disappeared when more inert conditions were used ͑higher N 2 flow rates͒ revealing a less intense signal containing endothermic contributions ͑Fig. 1͒.…”

Measurement of the specific heat and determination of the thermodynamic functions of relaxed amorphous silicon

“…Single vacancies have formation energy around 4 eV (Ref. 19), they should have a density of n d ¼ 1.7% to explain the excess crystallization enthalpy. This value agrees with the density of defects that remain in a-Si obtained by ion implantation after thermal annealing 20 and will be taken as representative of the contribution of defects to the a-Si thermodynamic functions.…”