Structural information on the deposited a-Si1-xCx films are essential when assessing issues such as the homogeneity of the material, the changes induced by a slight varia-tion of process condivaria-tions or the impact of the substrate material on the film growth.
For this purpose Si-rich a-Si1-xCx films (deposition conditions see chapter 4.2.3) with methane gas flows of 30 and 60 sccm were deposited on p-Fz silicon (100) and p-Cz germanium (111) substrates, respectively. The surfaces were prepared as usual for the passivation of the respective materials, that is a HF-dip (1 %) for the silicon sample and a sequence of H2SO4 and H3PO4 cleaning for the germanium sample on the day before the deposition (explicitly allowing the regrowth of a thin native oxide). For the critical removal of the GexOy (chapter 5.1), the Ge sample underwent an in-situ hydro-gen-argon plasma treatment (10 s) in the AK400M reactor prior to the a-Si1-xCx deposi-tion. From these samples, cross-sectional transmission electron microscopy (TEM) specimens were prepared by conventional standard sample preparation and subsequent thinning by Ar+-ion milling. For analytical TEM, a Zeiss 912 Ω TEM operating at a voltage of 120 kV was used for structural analysis exhibiting a point resolution of 0.37 nm. For bright field images and diffraction, the energy selective aperture (ΔE=15 eV) of the Omega filter was used to avoid background from inelastically scattered electrons.
The main qualitative conclusions that can be drawn from the TEM images of the respective samples shown in Fig. 5-2 are the following:
- Both films exhibit a very homogeneous structure, particularly no evidence of voids, tensions or cracks can be found irrespective of the substrate type.
- No apparent impact of the substrate type (and crystal orientation) on the film
Fig. 5-2: Cross sectional TEM images of 70 nm a-Si1-xCx (CH4=30 sccm) on Si (100) (left) and of 60 nm a-Si1-yCy (CH4=60 sccm) on Ge (111) (right) substrate.
60 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
growth is observable.
- The TEM image points to the existence of a 1-2 nm thick layer at the c-Si/
a-Si1-xCx interface which can be related to native SiOx present at the silicon sur-face prior to passivation. Apparently this layer is not (entirely) consumed during the deposition process, however, the passivation quality is not deteriorated by the remaining oxide since excellent lifetimes can be reached on equivalently proc-essed lifetime samples.
- No such interfacial layer can be observed at the c-Ge/a-Si1-yCy interface pointing to an effective removal of GexOy on the germanium surface through the in-situ plasma treatment.
- A thin layer on top of the amorphous film (Fig. 5-2 left) may be anticipated from the TEM image although a definitive distinction from the glue cannot be ascer-tained. This layer could be related to the oxidation of the a-Si1-xCx surface at air ambient. The latter might be responsible for oxide related peaks in FT-IR spectra as discussed in chapter 5.3.1.
The selected area electron diffraction patterns (SAEDP) acquired from the two sam-ples are depicted in Fig. 5-3 left. The two main features are the bright spots related to the crystalline substrate on the one hand and the diffuse rings around the (masked) direct beam typical for amorphous materials with a distinct short range order but a lack of long range order. The similarity of the diffraction patterns is remarkable and plot-ting the radial distribution of the diffraction intensity for the two samples (Fig. 5-3 right) confirms that the short range order of the two films is virtually identical. In other
Fig. 5-3: Selected area electron diffraction pattern of Si-rich amorphous silicon carbide on germanium and silicon substrate (left) and radial distribution of the diffraction in-tensity for the respective samples.
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 61 words, no structural dependence of the short range order on the substrate type or the methane gas flow under the given deposition conditions is found. The first peak at around 3.2 nm-1 in Fig. 5-3 right can unambiguously be related to the diffraction at (111) planes of tetrahedrally bond Si atoms (distance of adjacent planes d111=3.1 Å in crystalline silicon), the second broader peak centered at around 6.2 nm-1 is probably related to the superposition of diffraction signals from (022) and (113) planes of tetra-hedrally bond Si (d022=1.9 Å, d113=1.6 Å in c-Si, respectively [132]). The agreement of lattice plane spacings of the a-Si1-xCx network in the short range order with those in c-Si points to the fact that the observed bonding structure is very similar to amorphous silicon. The carbon fraction of 3-6 at. % in the film yielded during the low power deposition (chapter 4.2.3) is therefore not thoroughly incorporated into the network, that is the carbon atoms particularly do not undergo a tetrahedral bond structure with the silicon atoms. This finding is in agreement with the minor cracking of CH4 bonds under low power plasma conditions (chapter 3) and the observed absorption features related to carbon polyhydrides (CHn) in FT-IR spectra (chapter 3.1and 5.3).
Further structural information on the prepared a-Si1-xCx films was obtained by electron energy loss spectroscopy (EELS). This analytical technique is based on the measurement of the change in kinetic energy of electrons due to inelastic scattering in the specimen [134]. The comparison of the obtained energy loss spectra between the two Si-rich films deposited at different methane gas flows (CH4=30 and 60 sccm re-sulting in a composition of a-Si0.97C0.03 and a-Si0.94C0.06, respectively) is depicted in Fig. 5-4 for two different energy regions. Electron scattering due to plasmons involves small energy losses and the peaks centered around 17 eV (Fig. 5-4 left) are found to be
Fig. 5-4: Electron energy loss spectra for two Si-rich a-Si1-xCx passivation layers. Plasmon loss in the low-loss region (left) and Si L23-ionization edge (right). The EELS data for the pure a-Si:H layer was taken from [133].
62 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
typical for bulk plasmons in mono- and polycrystalline silicon as well as in a-Si:H [133]. The slight shift towards higher energies for the film deposited at increased CH4
flow agrees with data from literature indicating a strong dependence of the plasmon energy on the composition of a-Si:C:H alloys [135]. The silicon L ionization edge (excitation of electrons of the L-shell to empty states above the Fermi level by trans-mitted electrons) of the deposited films are very similar in shape to the one of pure a-Si:H (Fig. 5-4 right), emphasizing once again the similarity in chemical structure.
Furthermore, both a-Si1-xCx spectra exhibit the carbon K-edge at 291 eV (not shown), however, the core-loss intensities are small as could be expected from the low carbon content in the films.