K. Imamura et al. / Surface Science 603 (2009) 968–972
971
increases the energy difference between the bonding states (i.e.,
valence band) and anti-bonding states (i.e., conduction band).
The distinct difference in the electronic structures between the
oxides formed at 350 and 500 °C is mainly due to the variation of
-
-
-
-
-
-
-
40
42
44
46
48
50
(
(
a)
b)
the SiO
absence of trap-states in SiO
tively. Namely, for the former SiO
tunneling may occur from the Si conduction band to the SiO
2
atomic density. Another reason may be the presence and
formed at 350 and 500 °C, respec-
layer, the Fowler–Nordheim
2
2
2
trap-states, leading to underestimation of the conduction band
off-set energy.
The SiO
tration OH species [20]. OH species disturbs SiO
tion, probably resulting in a decrease in the band-gap energy. For
this reason, the SiO layer formed with HNO vapor at 500 °C pos-
2
layer formed with HNO
3
vapor contains high concen-
2
network forma-
2
3
sesses a band-gap energy smaller than the thermal oxide layer in
spite of the higher atomic density. (The atomic densities are higher
in the order of (b) > (c) > (a).)
(
c)
2
In the present study, the effective mass for all the SiO layers is
52
0
assumed to be identical in spite of the different electronic struc-
.12
0.13
0.14
0.15
0.16
*
tures. In the present paper, we use m /m = 0.5, and the barrier
1
/Eox (cm/MV)
heights are estimated to be 2.40 and 2.76 eV, respectively, for
*
Fig. 4. Fowler–Nordheim plots for the hAl/SiO
layer formed with the following conditions: (a) in HNO
HNO vapor at 500 °C; and (c) in oxygen at 900 °C.
2
/Si(100)i MOS diodes with the SiO
2
HNO
3
vapor oxidation at 350 and 500 °C. When m /m is assumed
3
vapor at 350 °C; (b) in
to be unity, the barrier heights are calculated to be 1.90 and
3
2
.19 eV, respectively, for 350 and 500 °C oxidation.
ues, the conduction band offset energy at the SiO
be determined as described below.
2
/Si interfaces can
4
. Conclusions
Fig. 5 shows the band diagrams of the SiO
conduction band (or valence band) offset energies are determined
to be 2.13 eV (4.48 eV) for 350 °C HNO vapor oxidation (Fig. 5a),
.49 eV (or 4.80 eV) for 500 °C HNO vapor oxidation (Fig. 5b),
and 3.00 eV (or 4.81 eV) for 900 °C thermal oxidation (Fig. 5c).
The conduction band off-set energies are estimated using the bar-
2
/Si structure. The
The SiO
2
layer with ꢀ9 nm thickness is formed in HNO
3
vapor at
temperatures lower than 500 °C, and the valence and conduction
band offset energies are determined from measurements of va-
lence band spectra and the Fowler–Nordheim plots, respectively.
The band-gap energies of the SiO
50 °C are estimated to be 8.39 and 7.71 eV, respectively. The dif-
ference in the band-gap energy is attributable to (i) presence and
3
2
3
2
layer formed at 500 and
(
3
rier height obtained from the Fowler–Nordheim plots minus the
energy difference between the bulk Si conduction band and the
absence of trap-states in the SiO
respectively, and (ii) various SiO
2
layer formed at 350 and 500 °C,
Si Fermi level.) Therefore, the SiO
to be 7.71 eV for 350 °C HNO vapor oxidation, 8.39 eV for 500 °C
HNO vapor oxidation, and 8.91 eV for 900 °C thermal oxidation.
2
band-gap energies are estimated
2
atomic density.
3
3
The estimation method of the conduction band offset energy from
the Fowler–Nordheim plots (i.e., indirect method in contrast to the
valence band offset energy estimation method using photoelectron
spectroscopy) may give lower values than the real values, leading
to underestimation of the band-gap values. We think that these
variations of the band-gap energies mainly arise from the differ-
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Fig. 5. Band diagrams of the SiO
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(