High resolution XPS study of oxide layers grown on Ge substrates Page: 3 of 10
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the results showed a very stable oxide since no shift of the Ge3d line was observed after
20min. argon sputtering using 3 keV ion energy. Therefore, we conclude that the inner
layers of the native oxide may include lower oxidation states of germanium such as Ge3+
and Ge2+. However, considering the small thickness of the oxide layers, one cannot
completely exclude a possible effect of the substrate that would contribute to the
formation of lower oxidation states during the ion bombardment.
In the following section we present similar results obtained by analyzing an oxide layer
formed by dry oxidation of a germanium sample.
Figure 3 shows two XPS spectra of the valence band region. The spectrum 1 was
obtained from the surface covered by the native oxide. The second curve was recorded
after Ar etching of 160 minute duration using 1.5 keV ion energy followed by a heat
treatment under ultra high vacuum at T=315 C for 20 min. The three germanium
bands, commonly called I, II and III [10] are clearly visible, preceded by a strong
broad peak. The spectrum 2 obtained after removal of the oxide layer, shows that the
three germanium bands remained while the broad strong peak vanished. The latter was
assigned to electron states located at the oxide/substrate interface. These are the charged
states that are responsible of the band bending. The high intensity of the peak indicates a
high density of the electron states. The absence of the signal stemming from these states
after the removal of the oxide layer is consistent with the shift of the Ge3d core level
stemming from the substrate that was discussed above.
Analysis of an oxide layer obtained by dry oxidation at T=380 0C, under dry oxygen:
Figure 4 shows the evolution of Ge3d line after successive Ar etching cycles. The first
four cycles were carried out using 1 keV ion energy and a gas pressure PAr=107 mbar.
After 2 hour etching the conditions were changed into 1.5 keV ion energy and PAr =
5 x 10-6 mbar. One can notice similar results to that observed in the case of the native
oxide, in particular, the 1 eV shift of the signal stemming from the oxide layer (Ge3dox)
However, we observe a more progressive shift of the signal originating from the
substrate. This could be a result of the non uniformity of the oxide layer that leads to a
progressive removal of islands of oxide during sputtering.
We have also systematically recorded the evolution of the Cls spectrum as it was used to
correct the binding energies for the charge shift (Fig.5). The counts rate was very low
as the ion etching progressed. Unfortunately we did not increase the counting time to
improve the statistics. However, the spectra show a clear shoulder located at 283 eV
binding energy as the interface oxide/substrate is reached. This shoulder could be
assigned to carbides particles present at the oxide/substrate interface. This is a surprising
result because it contradicts a common belief that carbon does not form carbides by
reacting with germanium [2, 11] Further investigations are needed in order to confirm a
possible formation of carbides as a result of our conditions of surface treatment and
oxidation.
Conclusion
High Resolution XPS was used to analyze thin oxide layers grown on germanium
substrates. The results revealed the presence of a high density of electron states located at
the oxide/germanium interface that leads to about 0.6 eV band bending. The composition
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Tabet, N.; Faiz, M.; Hamdan, N.M. & Hussain, Z. High resolution XPS study of oxide layers grown on Ge substrates, article, July 29, 2002; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc781036/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.