Hydrogen Diffusion and Microstructure in Undoped and Boron-Doped Hydrogenated Amorphous Silicon: AN IR and SIMS STUDY*

Hydrogenated amorphous silicon (a-Si:H) prepared by rf sputtering of a polycrystalline Si target at various rf powers 50 <= P <= 550 W (0.27-2.97 W/cm^2), target to substrate distance 1 <= d <= 2^{' '}, and varying hydrogen partial pressures. Doping was accomplished by introducing diborane (B_2H_6) in the plasma. Hydrogen diffusion was studied from the depth profiles obtained from the SIMS on multilayered a -Si:H/a-Si:(H,D)/a-Si:H samples. The properties of the samples were characterized by IR absorption, optical gap measurements and ESR. IR yielded quantitative and qualitative information total hydrogen content and the nature of hydrogen bonding, respectively. Hence the hydrogen microstructure of the samples could be varied in a systematic manner and monitored from the hydrogen vibrational modes. The ESR gave information on the number of paramagnetic defects per unit volume in the samples. The IR absorption of both as-deposited and annealed samples were closely monitored and the results clearly demonstrate a strong correlation between hydrogen diffusion and its microstructure. It is shown that microvoids in a-Si:H play a critical role in the process of diffusion by inducing deep hydrogen trapping sites that render them immobile. Consequently, as the microvoid density increases beyond a critical density (corresponding to N_ {rm d} >= 6.5 at. %) hydrogen diffusion is totally quenched. The diffusion results are discussed both in the context of multiple trapping transport of hydrogen in an exponential distribution of trapping sites and the floating bond model. As expected the hydrogen diffusion in boron-doped a-Si:H is faster than undoped material by a few orders of magnitude. The diffusion results have shown a significant departure from the power-law time dependence upon prolonged annealing. Such a departure has not been reported before. It is suspected that this deviation is caused by structural relaxations of the silicon network. ftn*DOE Report IS -T-1506. This work was performed under contract No W-7405 -Eng-82 with US Department of Energy.