Band diagram
Fig. 3 in the paper presents a band diagram for the n=6 Ba.72Sr.28O oxide grown on
silicon showing band-bending and the positions of the valence band edges for the oxide
and silicon. The figure is reproduced below in this supplement as Fig. 1.
The valence band energies on the band diagram have been determined using XPS, while
the conduction band energies have been determined by adding to the experimentally
determined valence band energies the previously determined band gap energies of silicon
and Ba.72Sr.28O. The following text describes how the positions of the valence band
edges (VBE) were determined from XPS data.
Experimental
XPS measurements were performed on five different overlayers: clean Si(001), n=6
Ba.72Sr.28O, n=6 SrO and n=6 BaO all on the 1/4 ML SrSi2 structure on Si(001). The
fifth is an n=6 Ba.72Sr.28O grown on 1/4 ML Be deposited on a 1/6 ML Sr -covered
Si(001) surface. All five silicon substrates were p-type and doped to 3x1015 cm-3 boron.
Each structure was prepared in the growth chamber starting from a new, RCA-cleaned
wafer and transferred within 5 minutes in 5-10 x10-9 Torr vacuum to the XPS system at
1x10-9 Torr. The XPS spectra were excited with MgKα radiation and the photo-ejected
electrons were analyzed using a hemispherical analyzer from the Physical Electronics’
Fig. 1. Valence Band diagram for n=6 Ba.72Sr.28O grown on Si(001) (solid
line) with the Fermi level indicated by the dashed line.
ESCA5000 system. The spectrometer was calibrated and the instrument zero potential
established using a gold standard. The Au 4f7/2 peak was located at 84.1 eV ± 0.05 eV
above the Au Fermi edge which was located at 0.00 ± 0.05 eV. X-ray satellites of the
MgKα radiation were subtracted from the data using the procedure outlined in Ref. 1.
All spectra were taken with a instrumental resolution of 1.6 eV as determined by
broadening of the Si 2p3/2 and Si 2p1/2 core levels. All spectra were taken using a 90°
take-off angle. In order to establish peak positions of the Si-2p core levels of the
substrate, we fit the peaks to a single Gaussian line shape. For clean silicon, only one
Gaussian peak was required for the fit. Two Gaussian peaks were fit to the Si 2p peak
profile and the location of the most intense determined the Si 2p peak position. Two
Gaussian peaks were fit to the Si-2p profile in order to take into account the possibility
that the silicon at the interface might have a different binding energy.
Fig. 2. Calculated valence band edge density of states for BaO (left) and SrO
(right). The DOS folded with a Gaussian with width 1.6 eV FWHM is shown
with a dashed line. This dotted line was the fitting function for determining the
position of the VBE.
The binding energies of all VBEs were determined by a fit to the spectra at the edge [2].
The fitting functions were calculated by convoluting the theoretical density of states
(DOS) at the VBE for BaO, SrO and Si (Figs. 2,3) with the Gaussian resolution function
of the instrument. The VBE DOS for BaO and SrO were calculated using a biaxial strain
so that the in-plane lattice parameter was equal to that of silicon. The silicon DOS was
taken from Ref. 3. Only the photoelectron intensity of the edge was used in the fit as was
done in Ref. 2. The fitting function consists of the calculated DOS convoluted with a
Gaussian having the resolution of the instrument and is shown as the dashed line in Figs.
2-3. For the oxides (see Fig. 2), the VBE DOS is cut off sharply at the VBE so that the
VBE of the convoluted fitting function is closer to the inflection point near the edge than
an extrapolated linear fit to zero photoelectron intensity. Locating the VBE at the
intersection of a linear fit to the edge with the background has been used for XPS
measurements of the band offset of SrTiO3 grown on silicon (4). The slope over 2 eV in
the VBE DOS of silicon means that a linear fit to the edge can be accurate.
Fig. 3. Calculated valence band edge density of states for bulk silicon.
Results
The binding energy of the VBE of clean silicon was measured directly by fitting as
described above. Measurements of clean silicon, see Fig. 4, placed the valence band edge
VBE
Si−2 p
at E CleanSi
=0.55 ± 0.1 eV. The Si-2p core level, Fig. 2, E CleanSi
, was measured to be 98.9
±0.1 eV above the VBE.
For the structures with overlayers, we determined the position of the valence band edge
Si− 2 p
, and the Si-2p/VBE offset measured
of silicon using the Si-2p core level position, E overlayer
SiVBE
for clean silicon so that the VBE for an overlayer structure, E overlayer
, is given by (2)
SiVBE
VBE
Si−2 p
Si−2 p
E overlayer
= ECleanSi
+ (E overlayer
− ECleanSi
),
1)
The binding energy of the VBE for the fully developed O-2p state in the oxide overlayer
is required to determine the energy of the top of the valence band in Fig. 1. To do this,
we compare the measured spectra for clean silicon with the n=6 oxide overlayers to
obtain the difference spectra representing the VBE of the oxide alone:
VBE
VBE
IDifference
(BE) = In=
6Oxide (BE) −
Si−2 p
∫ In=
6Oxide VBE
ICleansi (BE − ∆),
Si−2 p
∫ ICleanSi
2)
VBE
VBE
(BE) is the valence band spectrum for the oxide alone, ICleanSi
( BE ) and
where IDifference
VBE
In= 6Oxide ( BE ) are the measured spectra from clean silicon and the oxide overlayer and
Si−2 p
Si−2 p
∫ ICleanSi
and ∫ In=
6Oxide are, respectively, the integrated Si-2p intensities measured from
the clean silicon and n=6 overlayer structures. ∆ is the shift in energy required to
overlap the peak position of the Si-2p peak measured for clean silicon with the n=6 oxide
Fig. 4. Data for the Si-2p core level (left) and valence band (right). Red is a fit to the
valence band edge and black is for the clean 2x1Si(001).
overlayer.
The result of the subtraction in Eqn. 2 is represented by the curve plotted with open
circles in Figs. 5-8 for the four thin film structures studied. For n=6 Ba.72Sr.28O on SrSi2
on Si, a fit to the spectrum derived from the BaO VBE DOS locates the top of the valence
band at 1.8 ± 0.1 eV for Ba.72Sr.28O grown on the coulomb buffer (See Fig. 9). The
valence band offset is the difference in the Si VBE, 0.46 eV in Table I, and the BaSrO
VBE and is 1.3 ± 0.1 eV. Table I summarizes the parameters of Eqn. 2 that went into
determining the valence band offset. The results of the fits on the remaining thin films
are shown in Figs. 9-12.
Due to the ~50 Å escape depth for photoejected electrons, the length-scale of Fig. 1 is
also ~50 Å from the surface of the thin-film structure. The band bending of the silicon
shown in Fig. 1 takes place over the Debye-length of the silicon substrate which is 8 µm
for 3x1015 cm-3, p-type doping (5). The valence band is located 0.3 eV below the Fermi
level (5).
Table I. Parameters in Eqn. 2 used for determining the VB offsets for four different
oxide films grown on silicon.
∆ (eV)
Si/BeSi2/ Ba.72Sr.28O
Si/SrSi2/ Ba.72Sr.28O
Si/SrSi2/SrO
Si/SrSi2/BaO
0.05
0.08
0.02
0.06
Attenuation
of Si
0.33
0.30
0.22
0.37
VBE Oxide
VBE Si
VB Offset
2.4 ± 0.2
1.8 ± 0.1
2.5 ± 0.2
1.8± 0.1
0.50
0.46
0.53
0.49
1.9 ± 0.2
1.3 ± 0.1
2.0 ± 0.2
1.3 ± 0.1
References
1) Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (1979) Handbook of
X-ray photoelectron spectroscopy, Perkin-Elmer Corp, Eden Prarie, MN
2) Kraut EA, Grant RW, Waldrop JR, andKowalczyk SP (1980) Phys. Rev.
Lett. 44, 1620
3) Papaconstantopoulos DA, Keegan M, Akdim B, Coley C (2003), “Electronic
Structures Database”, http://manybody.nrl.navy.mil/esdata/database.html
4) Chambers SA, Liang Y, Yu Z, Droopad R, Ramdani J, Eisenbeiser K (2000), Appl.
Phys. Lett. 77, 1662-1664
5) Sze SM (1981), “Physics of Semiconductor Devices”, (J. Wiley & Sons, New York
Fig. 7. Valence band edge spectrum for n=6 BaO showing data taken from
oxide/silicon structure (open squares), the silicon substrate contribution (open
triangles) to the spectra and the difference (open circles) as calculated using Eqn. 2.
Fig. 8. Valence band edge spectrum for n=6 SrO showing data taken from
oxide/silicon structure (open squares), the silicon substrate contribution (open
triangles) to the spectra and the difference (open circles) as calculated using Eqn. 2.
Fig. 9. Valence band edge spectrum for n=6 Ba.72Sr.28O showing difference (open
squares) and fit at edge (solid red line).
Fig. 10. Valence band edge spectrum for n=6 Ba.72Sr.28O with the SrSi2 at the
interface replaced by BeSi2 showing difference (open circles), and the fit to the
valence band edge (solid red line).
Fig. 11. Valence band edge spectrum for n=6 BaO showing difference (open circles)
and fit to valence band edge (solid red line).
Fig. 12. Valence band edge spectrum for n=6 SrO showing difference (open circles)
and fit to valence band edge (solid red line),