The analysis of the degradation behavior of the effective lifetimes of passivated samples on successive annealing at a constant temperature allows further insight re-garding the mechanism responsible for passivation.
Maeda et al. [147] studied the thermal dissociation process for H-atoms in PECVD SiNx by isothermal experiments, where the Si-H and N-H bond densities measured by FT-IR were considered. They concluded that the dissociation reaction for the Si-H bonds was a first-order reaction (the rate of the decrease in concentration is propor-tional to the concentration itself) and estimated the activation energy for Si-H bond rupture to 0.60 eV. Lauinger and Kerr [33, 39] adopted the same isothermal procedure for SiNx films, however evaluating the decay of the electrical surface passivation (effective lifetime). They again assumed a single-exponential decay behavior of the form C1+C2 exp(-t/τd) and extracted the activation energy Ea for the electrical degrada-tion mechanism by a linear fit to the strongly temperature dependent decay time con-stants τd in Arrhenius representation. In good agreement both authors found a value of Ea≈1 eV for the lifetime degradation of SiNx passivated c-Si samples. Kerr attributed the discrepancy between his value and the value of Maeda to the fact that the “mecha-nism responsible for the thermal degradation of the SiNx passivation is probably more complicated than previously assumed” [33]. In other words, the Si-H bond rupture process (and the electronically active defects created thereby at the c-Si/SiNx interface and in the film) cannot be directly linked to the degradation in lifetime in the case of SiNx. In fact, it is straightforward to attribute the increased activation energy to a (ad-ditional) thermally more stable passivation mechanism which can be related to the fixed charges (K+ centers) in SiNx films (chapter 2.5.4).
74 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
At this point it is important to mention that neither the activation energies deduced from the electrical decay nor the ones deduced from decreasing bond densities can be directly related to the respective bond dissociation energies – the extracted values are always considerably lower (Table 5-3). This discrepancy can be attributed to the exis-tence of energy gain mechanisms present in the film such as the (exothermal) forma-tion of hydrogen molecules from atomic hydrogen and the reconstrucforma-tion of the net-work after out-diffusion of hydrogen resulting in energetically stable Si-Si, Si-C or Si-N configurations [148]. The extracted activation energies are therefore linked to a cooperative phenomenon in the film. At the same time, this implies that the absolute activation energies are somewhat dependent on the actual network structure.
Fig. 5-15 shows the lifetime degradation of a-Si1-xCx passivated c-Si samples at various temperatures. The successive annealing steps were performed on a hotplate and the lifetime measurements were again done by QSS-PC. As indicated by the dashed lines (left), the assumed single-exponential function fits well the decay behav-ior of the lifetime samples. The fitted τd values are plotted in Arrhenius representation (right) and an activation energy of around 0.5 eV can be extracted. This value is in fairly good accordance with the previously mentioned Si-H bond rupture energy in hydrogenated SiNx. The results therefore suggest that the passivation quality of a-Si1-xCx is directly linked to the decrease of Si-H bonds and thus we do not find any indication for a passivation mechanism other than the saturation of recombination active Si dangling-bonds. Furthermore the accordance with the activation energies observed by Biegelsen et al. [140] in ESR studies regarding the decrease of the
Fig. 5-15: Isothermal lifetime degradation at various temperatures for crystalline silicon samples passivated by Si-rich a-Si1-xCx (left) and extracted decay time constants in Arrhenius representation (right).
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 75 Si-dangling bond density by post-deposition annealing for hydrogenated a-Si films points to the fact that “positive” (increase of electronic properties) and “negative”
(decrease of electronic properties) annealing is linked to the same effect, that is the saturation and creation of Si dangling-bonds.
A similar isothermal annealing experiment for a-Si1-xCx passivated c-Ge substrates was performed. Within the first few minutes under annealing, the measured lifetime typically increases considerably as compared to the as-deposited values, probably due to relaxation processes at the a-Si1-xCx/c-Ge interface. From then on, the assumed single-exponential fit function again displays well the observed degradation behavior (Fig. 5-16 left). However, the transition from slow to fast electrical degradation occurs in a significantly smaller temperature range as compared to its silicon counterpart, resulting in an elevated activation energy of about 2 eV. This finding again confirms that the creation of Si dangling-bonds in the a-Si1-xCx matrix is not directly linked to a decrease in lifetime in the case of germanium substrate. A different passivation mechanism is likely to be involved.
Returning to silicon substrates, further isothermal annealing experiments were per-formed with different layer systems and again the temperature dependence of their decay time constants are analyzed (Fig. 5-17). All layers featured an intrinsic (Si-rich) a-Si1-xCx film next to the c-Si surface. Layer A and B exhibited single layers with different thicknesses (35 and 70 nm), layer C consisted of a stack system of intrinsic and phosphorous doped a-Si1-xCx (35/35 nm) and layer D consisted of an intrinsic a-Si1-xCx/a-Si1-yCy (y ≈ 0.5) stack (35/200 nm). The very similar slopes of the various
Fig. 5-16: Isothermal lifetime degradation at various temperatures for crystalline germanium samples passivated by Si-rich a-Si1-xCx (left) and extracted decay time constants in Arrhenius representation (right).
76 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
graphs indicate that the same activation energy and hence the same passivation mecha-nism is involved for all samples. This is not surprising since the fairly thick intrinsic a-Si1-xCx film next to the interface is expected to be responsible for the electrical passi-vation. However, what is the reason for the shift in τd to higher values for layers B and C? A possible issue, already addressed by Maeda et al., may be the recombination of Si dangling-bonds with hydrogen that was released beforehand by bond-rupture [147].
An equilibrium between bond-rupture and recombination (Si-H↔Si + H) can therefore be assumed, which is dependent on the bond-rupture rate (temperature) and the out-diffusion of hydrogen (diffusivity). The latter is clearly related to the structure of the films. Layer A exhibits the lowest thickness of the considered layer systems, the diffu-sion to the surface and hence the out-diffudiffu-sion of hydrogen occurs relatively fast. The
“pressure” of non-bonded hydrogen in the film is reduced and the equilibrium is shifted towards bond-rupture. Layers B and C are thicker as compared to A while the deposited phosphorous doped a-Si1-xCx is supposed to have an equally dense structure as its intrinsic counterpart. The increased diffusion path results in an increase in hy-drogen pressure in the matrix therefore effectively delaying the overall bond-rupture and shifting the decay time constants towards higher values. The capping layer (a-Si1-yCy) in the stack system D with increased carbon content (y ≈ 0.5) deposited at high power densities exhibits a less dense structure allowing for a fast out-diffusion and/or absorption of the hydrogen released in the film below (see section 5.3.5), there-fore effectively having no impact on the Si-H bond kinetics in the underlying dense a-Si1-xCx film. The presented model brings about the conclusion that although a certain sequence of deposited films does not necessarily change the passivation mechanism itself, its impact on the thermal behavior of the passivation quality can be significant.
This phenomenon was already observed in literature for low-temperature PECVD a-Si in combination with PECVD SiOx [149] and SiNx [150, 151]. Gatz et al. identi-fied the density of the deposited SiNx film and the ability of transferring hydrogen to the a-Si layer during annealing as the crucial issues regarding the thermal stability of the passivating system. However, for the a-Si/SiNx system deposited on c-Si surfaces, Saint-Cast et al. found evidence of an altered passivation mechanism as compared to a-Si single layers [152], revealing that the dissociation kinetics of Si-H bonds in this case are probably not (the only) responsible for the observed increase in thermal stabil-ity.
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 77
Fig. 5-17: Comparison between several decay time constants for silicon lifetime samples passivated by different layer systems on the basis of Si-rich a-Si1-xCx.
Table 5-3: Overview of relevant single bond energies. In the case of differing literature data, various values for the same bond are presented.
bond bond energy (eV)
source
Si-H 3.06 K.P. Huber "AIP Handbook of Physics"
Si-H 3.10 J. Robertson, Philos. Magaz. B 69, 307 (1994) Si-H 3.29 R. Tsu, Phys. Rev. B 35 (1986)
Si-H 3.34 F.W.Smith and Z.Yin, J. of Non-Cryst. Solids, 137&138 (1991) Si-H 3.50 L.Paulin, The Nature of Chemical Bonds (Cornell University,
Ithaca, NY, 1960), p.65
C-H 3.47 K.P. Huber "AIP Handbook of Physics"
C-H 4.31 F.W.Smith and Z.Yin, J. of Non-Cryst. Solids, 137&138 (1991) N-H 3.59 K.P. Huber "AIP Handbook of Physics"
N-H 4.05 F.W.Smith and Z.Yin, J. of Non-Cryst. Solids, 137&138 (1991) Ge-H 2.99 R. Tsu, Phys. Rev. B 35 (1986)
Si-Si 2.34 J. Robertson, Philos. Magaz. B 69, 307 (1994)
Si-C 3.21 F.W.Smith and Z.Yin, J. of Non-Cryst. Solids, 137&138 (1991) Si-Ge 3.12 http://www.webelements.com/silicon/bond_enthalpies.html Si-N 3.45 J. Robertson, Philos. Magaz. B 69, 307 (1994)
Si-O 4.82 F.W.Smith and Z.Yin, J. of Non-Cryst. Solids, 137&138 (1991) N-N 1.70 J. Robertson, Philos. Magaz. B 69, 307 (1994)
78 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates