Surface Photo-Voltage Spectroscopy: A Versatile Technique …
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5.2 Determination of the Type of Semiconductor
The SPS may also allow determining the type of conductivity of the semiconductor
samples. This may be achieved by observing the sign of the knee linked with the
onset of SPV signal. This stems from the fact that the most semiconductor surfaces
are depleted/inverted, which means that the bands of n-type semiconductors are bent
upwards and vice versa. The super-bandgap illumination generally decreases the
surface band-bending, this would result in a positive SPV in n-type semiconductors
and a negative SPV in p-type semiconductors Although this determination of type
of semiconductor may look trivial, in many cases where unknown samples are being
characterized, it could come handy. For example, the SPS measured on free-standing
porous Si films made from p-type Si has shown to have either of the conductivity
type. The type of the conductivity was found to be dependent on the resistivity of
the Si substrate used. In another instance, SPS was employed to determine the type
of conductivity in GaN films [99], which is crucial due to the fact that the p-doping
of GaN may be subject to compensation by its native n-type doping.
5.3 Study of Surface States, Surface Passivation Treatments
and Monitoring
A key feature of SPS lies in its sensitivity to the surface states and its ability to
determine the energy position and type of surface states in semiconductors. It comes
from the fact that the charge carriers may transit from a surface state to a band, or vice
versa on illumination of photons with sufficient energy. Thus, a knee is observed in
the SPS spectrum as and when the incident photon energy matches with the threshold
energy of a certain transition. If electrons excite to the conduction band from surface
state by absorbing the equivalent energy from a photon, a positive SPS is observed.
Similarly if holes transit to the valence band, the surface charge becomes more
negative, which gives rise to a negative SPV. Thus, by observing the SPV threshold
energy and the sign of the slope on SPV signal, it is possible to determination an
approximate energy and type of surface state within the bandgap. There are a large
number of reports on application of SPS for understanding the surface states within
the bandgap and the subsequent evaluation of the passivation treatments for the same.
For example, SPS has been extensively used to monitor metal-induced surface states
[100–102], on GaAs(110) and InP(110) surfaces. Since, the SPS is a contactless, nondestructive technique; it can be easily applied to a variety of surfaces in any ambience.
Due to this advantage, SPS offers possibilities to extract direct correlation of surface
electronic structure with surface passivation treatments [68, 103–105]. Asquini et al.
have used the SPS for evaluation of effects of the post deposition treatments on
a-Si:H/c-Si interface quality [106]. They attempted two different treatments, viz. a
thermal annealing for 10 min at of 250 °C temperature in nitrogen atmosphere and
627
5.2 Determination of the Type of Semiconductor
The SPS may also allow determining the type of conductivity of the semiconductor
samples. This may be achieved by observing the sign of the knee linked with the
onset of SPV signal. This stems from the fact that the most semiconductor surfaces
are depleted/inverted, which means that the bands of n-type semiconductors are bent
upwards and vice versa. The super-bandgap illumination generally decreases the
surface band-bending, this would result in a positive SPV in n-type semiconductors
and a negative SPV in p-type semiconductors Although this determination of type
of semiconductor may look trivial, in many cases where unknown samples are being
characterized, it could come handy. For example, the SPS measured on free-standing
porous Si films made from p-type Si has shown to have either of the conductivity
type. The type of the conductivity was found to be dependent on the resistivity of
the Si substrate used. In another instance, SPS was employed to determine the type
of conductivity in GaN films [99], which is crucial due to the fact that the p-doping
of GaN may be subject to compensation by its native n-type doping.
5.3 Study of Surface States, Surface Passivation Treatments
and Monitoring
A key feature of SPS lies in its sensitivity to the surface states and its ability to
determine the energy position and type of surface states in semiconductors. It comes
from the fact that the charge carriers may transit from a surface state to a band, or vice
versa on illumination of photons with sufficient energy. Thus, a knee is observed in
the SPS spectrum as and when the incident photon energy matches with the threshold
energy of a certain transition. If electrons excite to the conduction band from surface
state by absorbing the equivalent energy from a photon, a positive SPS is observed.
Similarly if holes transit to the valence band, the surface charge becomes more
negative, which gives rise to a negative SPV. Thus, by observing the SPV threshold
energy and the sign of the slope on SPV signal, it is possible to determination an
approximate energy and type of surface state within the bandgap. There are a large
number of reports on application of SPS for understanding the surface states within
the bandgap and the subsequent evaluation of the passivation treatments for the same.
For example, SPS has been extensively used to monitor metal-induced surface states
[100–102], on GaAs(110) and InP(110) surfaces. Since, the SPS is a contactless, nondestructive technique; it can be easily applied to a variety of surfaces in any ambience.
Due to this advantage, SPS offers possibilities to extract direct correlation of surface
electronic structure with surface passivation treatments [68, 103–105]. Asquini et al.
have used the SPS for evaluation of effects of the post deposition treatments on
a-Si:H/c-Si interface quality [106]. They attempted two different treatments, viz. a
thermal annealing for 10 min at of 250 °C temperature in nitrogen atmosphere and