Date of Award
Doctor of Philosophy
Physics and Astronomy
Long-range H motion and desorption in low hydrogen concentration undoped hydrogenated amorphous silicon (a-Si:H) and germanium (a-Ge:H) was studied by deuterium secondary ion mass spectrometry (SIMS) depth profiles and IR absorption of a-Si:H/a-Si:(H,D)/a-Si:H and a-Ge:H/a-Ge:(H,D)/a-Ge:H. SIMS monitors deuterium motion (assumed similar to that of H), while IR yields information on hydrogen content and bonding. The diffusion constant was found to be dispersive with time, and depended on H content C[subscript] H, diffusion length L, and microvoid content, at temperatures T ≤ 400°C for a-Si:H and T ≤ 310°C for a-Ge:H. It exhibited a power-law D(t) = D[subscript]oo([omega] t)[superscript]-[alpha] relation in both systems. In a-Si:H, [alpha] generally deviates from the 1 - T/T[subscript] o dependence on the temperature T expected from a multiple trapping mechanism. The diffusion constant at constant diffusion length D(t[subscript] L) then deviates from an Arrhenius dependence on the temperature. The "apparent" activation energy E[subscript] a and prefactor D[subscript] o, defined by the linear best-fit of lnD(t[subscript] L) vs 1/T, strongly increase with L at low C[subscript] H. The Meyer-Neldel relation (MNR) D[subscript] o = [macron] A[subscript]ooexp(E[subscript] a/T[subscript] o[superscript]'), where [macron] A[subscript]oo ~eq 3.1 x 10[superscript]14 cm[superscript]2/s and T[subscript] o[superscript]' ~eq 730 K, holds for all 1.3≤ E[subscript]a≤ 2.4 eV and 2.5x 10[superscript]-5≤ D[subscript] o≤ 3100 cm[superscript]2/s;The a-Ge:H, [alpha] is essentially temperature and composition independent, but increases with microvoid content. The activation energy E[subscript] a ranges from 0.7 to 1.2 eV among the various films. The Meyer-Neldel relation is observed, with [macron] A[subscript]oo~eq 5.5x 10[superscript]-16 cm[superscript]2/s and T[subscript] o, ~eq 530 K. These values are lower than the corresponding values in a-Si:H. Hydrogen desorption temperature is as low as 180°C. Yet the significance of the MNR is questionable in both a-Si:H and a-Ge:H;The diffusion results for both a-Si:H and a-Ge:H are discussed in relation to the microstructure of the films. The nature of the microvoid-induced deep H-trapping sites is also discussed. Finally, a possible relation between the dispersive diffusion and a percolation model is presented. ftn*DOE Report IS-T-1559. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Wu, Xiao-Lin, "Experimental studies of long-range atomic H motion and desorption in hydrogenated amorphous silicon and germanium " (1991). Retrospective Theses and Dissertations. 10089.