Computer simulation of the influence of hydrogen on stress–order correlations in amorphous silicon

Abstract

This paper reports the computational simulation of the correlations between atomic-level stress and local structure fluctuations in a computational model of amorphous silicon. A single parameter has been identified, which uniquely characterises the structural order in these structures. This parameter is the linear combination of the SDs of the first and second nearest neighbour separations. The stress fluctuations, under progressive hydrogen incorporation, show two clear dependences on this parameter, and therefore on the structural order. This dual dependency clearly alludes to structural network configurations that contain low and high hydrogen concentrations. The implications of the results on the local geometry of tetrahedrally bonded amorphous solids are discussed.

Tight-binding molecular dynamics study of the hydrogen-induced structural modifications in tetrahedral amorphous carbon

“…The region of accessible data also shows linear trend-line dependence, and quite interestingly, the data is also oriented at an angle of 36 to the 1 -axis, though the structural fluctuations is within a narrow range of 0.15 Å . This range is noticeably smaller than the 0.40 Å range observed earlier for similar structural fluctuations in a-Si:H [25]. This is most likely due to the high Young's modulus of diamond-like carbon, 850 GPa [37], compared with 150 GPa [38] for silicon.…”

“…It is therefore clear that stress information can only be available for points in the domain of the order parameter space where the coordinates are welldefined. The coupling of the changes in inter-atomic distances, and the preferred orientation within the domain of structural order suggest that it is, again, possible to use a single parameter to characterize order in the structure as originally proposed in [25]. In the diamond structure, the number of first (Z 1 ) and second (Z 2 ) nearest-neighbours in a perfectly-ordered carbon network is always 4 and 12, respectively.…”

“…The decrease in stress clearly alludes to structure relaxation, which in tetrahedral amorphous carbon, culminates in more structurally disordered network. In a-Si:H, an opposite structural response to hydrogen-saturation was observed [25]. There is a noticeable step-like dependence of the rms stress with hydrogen concentration.…”

“…However, the presence of the dihydride configuration in structural configurations, which contain hydrogen concentrations between 15% and 60% H cannot be ruled out, because of the removal of the restrictions on H-H interaction in the simulations at high hydrogen concentrations. Figure 4 shows the order parameter field [25] of the simulated ta-C:H networks. This 2D domain is spanned by the standard deviation of the first and second-neighbour separations ( 1 , 2 ), for C-C interaction.…”

“…Such simple indices have a long history of providing direct means of comparing material properties. The effectiveness of the recently proposed order parameter [25] to quantify the structural order in ta-C:H structures is investigated in this study, by directly determining the ordering rule in the order parameter domain of tetrahedral amorphous carbon structures. Tight-binding molecular dynamics simulations are also performed to investigate the response of the local network in ta-C materials to dynamic hydrogen saturation, and to determine the ordering rule within the domain of the order-parameter field under the influence of stress.…”