Semiconductor
interfaces strongly affect the electrical characteristics of semiconductor
produces such as solar cells, TFT, and LSI.
Interface states, i.e., defect states at SiO2/Si interfaces,
seriously degrade electrical characteristics in spite of the low state
density. In Kobayashi laboratory, we
have succeeded in observation of interface states directly using spectroscopic
method, i.e., XPS measurements under bias, for the first time.
Figure 1 Diagram of XPS measurements under bias for
observation of energy distribution of interface states at oxide/semiconductor
interfaces.
Figure 1 shows the diagram of
measurements of XPS spectra under bias.
An oxide layer with thickness in the range between 1 and 2.5 nm is
formed on semiconductors such as Si, followed by deposition of a ~3 nm Pt
layer. The Pt layer is earthed and a
bias voltage is applied to the rear semiconductor surface during X-ray irradiation,
and emitted photo-electrons are detected in the surface-normal direction.
Figure 2 Principle of observation of interface states by means of XPS measurements
under bias: (a) band diagram of MOS structure at zero bias; (b) that under
negative bias V applied to the semiconductor with respect to the metal
layer.
Figure 2 shows the principle for determination of energy distribution of interface
states. At zero bias (Fig. 2a), the metal Fermi level and semiconductor Fermi level are located at the
same energy, and interface states below the Fermi level is occupied by
electrons, while those above it are unoccupied.
Under bias, V, applied to the semiconductor with respect to the metal layer
(Fig. 2b), the semiconductor Fermi level deviates from the metal Fermi level.
Consequently, interface states located between the metal Fermi level and
the semiconductor Fermi level are newly occupied by electrons. These charges in the interface states, ΔQi, change the potential gradient across the oxide layer by the
magnitude given by
ΔVox= ΔQit / Cox
where Cox is the capacitance of the oxide layer. Since the energy difference between the semiconductor valence band maximum
and the semiconductor core levels (e.g., Si 2p level) is constant, the
semiconductor core levels shift by the magnitude the same as ΔVox, which is detectable by means of XPS. Therefore, the energy distribution of
interface states can be obtained from observation of the energy shift of the
semiconductor core level as a function of the bias voltage.
The energy distribution of interface
states is usually elucidated from electrical measurements such as
capacitance-voltage measurements.
Determination of the energy distribution of interface states from
electrical measurements requires various assumptions: 1) equivalent circuit, 2)
assumption that all interface states follow (or do not follow) the low
frequency (or high frequency) AC signal, 3) ignorance of edge effect and
interface roughness, 4) uniform distribution of dopants, etc. Moreover, electrical measurements cannot be
performed on an ultrathin oxide layer through which a high density leakage
current flows. XPS measurements under
bias, on the other hand, can determine the energy distribution of interface
states without these assumptions even for an oxide layer with a high leakage
current density.
Figure 3 Energy distribution of interface states
obtained from XPS measurements under bias for the ultrathin thermal SiO2/Si
structures.
Figure 3
shows the energy distribution of interface states for the ~1.5 nm thermal oxide
layer formed on Si at various temperatures.
The energy distribution of interface states strongly depends on the
temperature of thermal oxidation. It is
found that the atomic density of the oxide layer increases with the thermal
oxidation temperature, leading to the observed variation of interface state
spectra.
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