High resolution XPS study of oxide layers grown on Ge substrates Page: 2 of 10
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T=380 C for 25minutes, under 400 mTorr of dry oxygen. The XPS measurements were
carried out at 9.3.2 beam line of the Advanced Light Source (ALS) of the Lawrence
Berkeley National Laboratory (LBNL, USA). The incident photon energy was varied
between 300 and 650eV. A state-of-the-art electron energy analyzer (type Scienta) was
used. Taking into account the resolving power of the monochromator and our
experimental conditions, the overall energy resolution was better than 0.2eV. Cls, Ge3d
and O1s lines were recorded from the as grown oxide layers and after argon etching
cycles of various durations. The Cis line of 284.5eV binding energy was used as a
reference to correct the binding energies for the charge shift. In situ heat treatments were
carried out under ultra high vacuum by electron bombardment of the back surface of the
samples in the XPS analysis chamber.
Analysis of the native oxide
Figure 1 shows the Ge3d line recorded from a layer of native oxide grown on a
chemically etched germanium surface during storage under air for one year. The incident
photon beam was focused down to 1mm diameter. The photon energy was 600 eV. The
broad peak located at 32.4 eV stems from the oxide layer. The second small peak located
at 28.9 eV originates from the non oxidized atoms of the substrate. The spectrum 2 in the
figure was obtained after heat treatment at T=400 C under ultra high vacuum (10-9
mbar) for 15min. One can clearly observe that the signal stemming from the oxide layer
has vanished while the peak originating from the substrate increases drastically
indicating that the native oxide has been removed as a result of the heat treatment. One
can also observe a 0.6 eV shift of the substrate signal towards higher binding energy
(from 28.9 eV to 29.5 eV). This shift results from the band bending that occurs at the
interface oxide/substrate. In N-type samples, the band binding leads to the reduction of
the distance between the Fermi level, which is flat, and the core level at the surface. It
should be pointed out that a 0.5 eV shift of the Ge3d line was observed in ref. 2 but in
the opposite direction, i.e. towards higher binding energy. As p-type samples were used
in ref. 2, it is expected that the band bending occurs in the opposite direction to that
observed in n-type samples. Therefore, our observations are qualitatively consistent with
those reported in ref.2.
Subsequently argon etching (curve 3) had no significant effect except a less visible
shoulder corresponding to the Ge3d3/2 line due to the structural damage resulting from the
ion bombardment. Notice that the shoulder is back in the picture after a heat treatment
at T=400 C (curve 4). Curve 5 was obtained with a better energy resolution using
300eV photon energy and shows clearly the two components of the Ge3d line.
A second sample with a native oxide layer grown in similar conditions was analyzed
after successive Ar etching cycles. Figure 2 shows the evolution of the Ge3d line. In
addition to the shift of the signal originating from the substrate (from 28.8 to 29.2 eV)
similar to that observed after the removal of the oxide layer in the previous sample, one
can observe a leV shift of the signal stemming from the oxide layer towards lower
binding energies (from 32. 4 to 31.4 eV, Fig. 2, curve 4). A simple explanation for this
observation is to assume a reduction of the oxide under the ion bombardment. In order
to check this interpretation, we have analyzed a GeO2 amorphous sample after successive
Ar etching cycle using a conventional XPS instrument (ESCALAB MKII). The sample
was prepared by a vapor condensation technique using a solar furnace . Surprisingly,
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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. (digital.library.unt.edu/ark:/67531/metadc781036/m1/2/: accessed October 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.