The evolution of the effective lifetimes of c-Si and c-Ge lifetime samples passi-vated by a-Si1-xCx films deposited at various substrate temperatures ranging from Tdep=100-380°C (see chapter 4.2.2 for the temperature calibration of the AK400 reac-tor) is depicted in Fig. 5-9. The methane gas flow (CH4=30 sccm) was held constant for all depositions. Again, the samples were subject to an annealing sequence on a hotplate (air ambient) starting from 100°C up to 500°C. The annealing time at each specific temperature amounted to 30 min and was followed by a lifetime and a FT-IR measurement for the respective samples.
In the case of silicon (Fig. 5-9 left), the best overall performance in terms of elec-trical surface passivation is provided by the a-Si1-xCx film deposited at around 270°C (Topt). Here, no change in lifetime can be observed for annealing temperatures Tann up
Fig. 5-9: Evolution of the effective lifetimes τeff of a-Si1-xCx passivated c-Si (left) and c-Ge (right) lifetime samples with increasing annealing temperatures (Tann). Variation of the deposition temperature (Tdep) of the passivating film (see caption and respective arrow tags). Lines are guides to the eye.
68 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
to 300°C. However, exceeding this temperature leads to a significant degradation of the passivation quality. All of the a-Si1-xCx films deposited at temperatures below 270°C show the same features, that is an increase in lifetime as compared to the as-deposited values for Tdep<Tann<300°C, a maximum around 300°C and a degradation in lifetime for Tann >300°C analog to the one observed for Tdep=Topt. Most illustrative is the evolution of the lifetime for the sample passivated in a very low temperature proc-ess (100°C): under succproc-essive annealing, the as-deposited lifetime of a few µs peaks in a lifetime of a few hundred µs at around 300°C and then suffers from electrical degra-dation for Tann>300°C. It is worth mentioning that although lifetimes recover for films with Tdep<Topt under annealing, the performance of these samples generally lacks behind the one processed at Topt. This behavior is different from the one observed for high-quality a-Si:H passivation by de Wolf and Kondo [62]. The authors furthermore reported on a decrease in lifetime at moderate annealing temperatures for samples deposited at Tdep>220°C, a phenomenon which they essentially related to the growth of an epitaxial interface during deposition. In the case of the a-Si1-xCx films, lifetime degradation only occurs at elevated annealing temperatures while increasing or re-maining constant for Tann<Tdep. It may therefore be concluded that the carbon incorpo-ration inhibits epitaxial growth at the c-Si/ a-Si1-xCx interface.
The behavior of the lifetimes related to the germanium samples is similar to the one observed on silicon (Fig. 5-9 right). Again, the film deposited at about 270°C performs best which implies the highest lifetimes in the as-deposited as well as in the annealed state. Equally, the passivation quality of the other films improves under
Fig. 5-10: Left: evolution of the total Si-H bond density of various Si-rich a-Si1-xCx films deposited at different temperatures under annealing. Right: optical constants of the various a-Si1-xCx
films deposited at different substrate temperatures. Lines are guides to the eye.
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 69 successive annealing with respect to the as-deposited values, however peaking at an-nealing temperatures as high as 400-500°C.
The evaluation of the total Si-H bond density of the respective a-Si1-xCx films from IR spectroscopy measurements is presented in Fig. 5-10 left. The results obtained were again basically identical irrespective of the substrate type. Films grown at low sub-strate temperatures exhibit a large amount of hydrogen bonded to silicon predomi-nantly occurring in surroundings of clustered mono- and polyhydrides (absorption peak centered at 2085 cm-1, Fig. 5-11 right). Depositions at low temperatures therefore seem to result in void-rich structures giving rise to an electrically defect-rich ma-trix/interface. The reason for this is rather growth kinetics than an increased CH3 in-corporation, since the comparison of the absorption strength at 780 cm-1 for the various deposition temperatures (Fig. 5-11 left) shows that the density of CH3 groups (more precisely: Si-CH3 groups) is fairly constant for low temperatures. Elevated substrate temperatures result in increasingly dense structures as indicated by measurements of the optical properties of the a-Si1-xCx films (Fig. 5-10 right). This does not contradict the observed increase in carbon related bond concentrations in the IR spectrum (also observed in [139]) since the boost of CHn groups (peak at 1000 cm-1 in Fig. 5-11 left) for higher temperatures is independent from the Si-CH3 density (peak around 780 cm
-1) and can therefore be attributed to CH and CH2 groups. The latter are more readily incorporated into the film and give rise to less void formation. The electrical degrada-tion of the passivadegrada-tion quality for Tdep >270°C seems therefore to be linked to the reduced amount of Si-H bonds in the films (Fig. 5-11 right).
Fig. 5-11: IR absorption spectra for a-Si1-xCx films deposited at various substrate temperatures.
70 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
While this argumentation is plausible for the silicon samples, the same explanation is not straightforward for germanium samples since we previously found a certain decoupling of hydrogen and the passivation mechanism. This dilemma can be resolved when assuming a crucial role of hydrogen for the film growth and for the restructuring of the matrix under annealing. This would lead to a difference in the film structure (which is responsible for the passivation of germanium surfaces) depending on the incorporation of hydrogen during deposition. This could particularly explain the poor performance of the germanium samples passivated at 330°C.