Effects of the defect structure on hydrogen transport in amorphous silicon

Abstract

Defect structure of carbon rich a -SiC:H films and the influence of gas and heat treatmentsHydrogen evolution transients were measured for hydrogenated amorphous silicon prepared by Si implantation of crystalline silicon and subsequent hydrogen implantation. The evolution curves are found to be similar for different H concentrations but with entirely different atomic and nanoscale structures, as was evidenced by small-angle x-ray scattering and infrared absorption investigations ͓Phys. Rev. B 53, 4415 ͑1996͔͒. This behavior is explained by a hydrogen-diffusion controlled effusion with a limited density of sites in the amorphous material that can be occupied by hydrogen. The experimental effusion curves are modeled by using diffusion coefficients in the implanted layers that were determined by secondary-ion mass spectrometry. Diffusion through a highly disordered material of low H content is found to have an activation energy of 2.26 eV.

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

“…It comes from desorption of the isolated or clustered monohydride in bulk. 17,18 The signature of peak AЈ ͑Fig. 9͒ had been previously established, 13 because in the low-quality films studied by Beyer 16 the EGA signal was not diffusion controlled.…”

“…Under these conditions the Si network is presumably much more stable than in most diffusion experiments carried out on as-grown films. 17,18,70 …”

“…Both weak bonds and dangling bonds act as traps for H. 4 Given our results, it is doubtful to assume that the density of traps remains unchanged during H diffusion. 18,67 In fact, the assumption of constant trap density relies on very particular experiments 4 where samples were annealed ͑and thus relaxed͒ before post-hydrogenation experiments. Under these conditions the Si network is presumably much more stable than in most diffusion experiments carried out on as-grown films.…”

“…From this and other experimental evidence, it is known that this lowtemperature dehydrogenation process is usually associated with clustered SiH groups on the surface of internal voids. 16 When dehydrogenation is diffusion controlled, desorption experiments have proven useful for the analysis of the diffusion mechanisms of hydrogen in a-Si: H. 17,18 The results are of special interest when done on device-quality material, where most hydrogen is in the isolated monohydride or clustered states in bulk. In this case, there is broad agreement that hydrogen diffuses in atomic form 16,19 until it reaches the external surface where it is desorpted as molecular hydrogen.…”

Dense restructuring of amorphous silicon network induced by non-bonded hydrogen

“…20,35,[44][45][46][47][48] The C NBH value was quantitatively determined using the total hydrogen content (C T ) [determined by Rutherford backscattering spectrometry=hydrogen forward scattering (RBS=HFS)], and C BH [determined using Fourier-transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) method]. [49][50][51] The C T value, determined using RBS=HFS was verified using thermal desorption spectrometry (TDS), 32,[52][53][54][55][56][57][58][59] which confirms the presence of NBHs.…”