On the formation of blisters in annealed hydrogenated a-Si/a-Ge multilayers

The amorphous SiGe alloy is employed in solar cells as the low band gap cell in association with the high band gap a-Si. One way to obtain the SiGe alloy is through annealing of a-Si/a-Ge multi-layers (MLs) so as to intermix Si and Ge [e.g. T. Sameshima, Thin Solid Films 487 (2005) 67]. H is added to passivate the dangling bonds to reduce the defects in the band gap. We report on the evolution of the structure of hydrogenated a-Si/a-Ge MLs upon annealing as a function of the incorporated H. The MLs were obtained by RF sputtering in Ar with addition of H at different flow rates. The MLs consisted of 50 couples of alternating 3 nm thick a-Si and a-Ge layers. Single layers were sputtered under the same conditions to be used as reference samples. Annealing was done at 350 and 400 °C for 1, 4, 10 h. Analyses were performed by AFM, ERDA, IR Absorption. The H incorporation increases with increasing flow rates up 17 and 7 at % in Si and Ge, resp. Upon annealing of the MLs formation of blisters and, at the same time, release of H from its bonds to Si and Ge were observed. Blisters density increases with the increase of liberated H. The H is first released from the mono-hydride bonds to Si and Ge suggesting that this occurs primarily at nanocavities. Unbonding from the di-hydrides, likely located in the bulk, occurs for longer annealing times. The growth of the nanocavities containing the released H leads to the formation of the blisters and should occur by in-diffusion of other H liberated in the annealing from the next- neighboring regions and/or by Ostwald ripening. Calculations of the H2 molecule density in the blisters by the Wan's lenticular crack model [K.-T. Wan, J. Mater. Res. 8 (1993) 1126] show that it is 9.9 and 5.7 % of the H remained in a-Si layers and a-Si/a-Ge MLs, resp., after annealing.