614
A. Jariwala et al.
illuminating by chopped light through the conducting glass window. Generally, the
oxide layer on the surface itself serves as an insulator in the MIS structure in this
case. The soft contact approach can drastically enhance the strength of SPV signals
due to extremely thin insulating layer in the MIS structure.
Due to different operating principles involved in case of Kelvin probe approach
and the MIS approach, their suitability depend on the materials and the properties
being probed. The Kelvin probe approach is more suitable when the surface states
under investigations are slow with relatively large relaxation times. The method can
extract the information related to the work function without modifying the surface
band bending. The minus on part of the Kelvin probe is the inherent noise generated
by the vibrating electrode during measurements. Also the involvement of moving
part in the system makes it difficult to implement under conditions such as high
vacuum or low temperatures. The MIS approach, on the other hand, has a distinct
advantage under such conditions of high vacuum or low temperatures, as there are no
moving parts involved. The measurements of SPV in MIS configuration may respond
better to the transitions with very low relaxations times. It allows relatively quicker
SPS scans and offers a better signal to noise ratio, which make it more convenient in
industrial environments [50].
3.3 The SPS Set-up
Now that the mechanisms to pick up SPV signals have been discussed, let us see how
we can experimentally implement these set-ups to carry out the surface photovoltage
spectroscopy. The sample under test is generally mounted inside a metallic box, which
should be suitable to encompass the probe depending on whether it is a vibrating
metal plate in case of Kelvin probe or the MIS structure. The box, with just a tiny
window for the light beam to enter and illuminate the probed surface, can serve as
a Faraday cage as well as a dark box. In the current work, our prime focus is on the
MIS structure based measurements of SPV. Hence, the experimental setup for the
has been discussed in detail in the perspective of the MIS structure.
The broadly tunable light required for the spectroscopic measurement is generally
obtained by combining a broad band light source with a suitable monochromator.
Also, since the strength of the SPV signals may vary drastically from sub-bandgap
regime to the super-bandgap regime, a several orders of magnitude at times, it is
important to prevent the spurious contribution of stray lights and other high-order
diffraction peaks coming from the monochromator. This requires use of appropriate
cut-off filters in the path of illumination beam. Again, since time SPV signal may
be very feeble particularly in the sub-bandgap regime, the signal to noise ratio may
not be very appreciable. Hence measurements are generally taken with the help of
a lock-in amplifier with chopped light beam configuration. The reference for the
lock-in is typically obtained from optical chopper. In case of Kelvin probe approach,
the vibration of the plate itself may provide reference to the lock-in and hence the
chopping of the light beam is not required. In some cases, an auxiliary light source
A. Jariwala et al.
illuminating by chopped light through the conducting glass window. Generally, the
oxide layer on the surface itself serves as an insulator in the MIS structure in this
case. The soft contact approach can drastically enhance the strength of SPV signals
due to extremely thin insulating layer in the MIS structure.
Due to different operating principles involved in case of Kelvin probe approach
and the MIS approach, their suitability depend on the materials and the properties
being probed. The Kelvin probe approach is more suitable when the surface states
under investigations are slow with relatively large relaxation times. The method can
extract the information related to the work function without modifying the surface
band bending. The minus on part of the Kelvin probe is the inherent noise generated
by the vibrating electrode during measurements. Also the involvement of moving
part in the system makes it difficult to implement under conditions such as high
vacuum or low temperatures. The MIS approach, on the other hand, has a distinct
advantage under such conditions of high vacuum or low temperatures, as there are no
moving parts involved. The measurements of SPV in MIS configuration may respond
better to the transitions with very low relaxations times. It allows relatively quicker
SPS scans and offers a better signal to noise ratio, which make it more convenient in
industrial environments [50].
3.3 The SPS Set-up
Now that the mechanisms to pick up SPV signals have been discussed, let us see how
we can experimentally implement these set-ups to carry out the surface photovoltage
spectroscopy. The sample under test is generally mounted inside a metallic box, which
should be suitable to encompass the probe depending on whether it is a vibrating
metal plate in case of Kelvin probe or the MIS structure. The box, with just a tiny
window for the light beam to enter and illuminate the probed surface, can serve as
a Faraday cage as well as a dark box. In the current work, our prime focus is on the
MIS structure based measurements of SPV. Hence, the experimental setup for the
has been discussed in detail in the perspective of the MIS structure.
The broadly tunable light required for the spectroscopic measurement is generally
obtained by combining a broad band light source with a suitable monochromator.
Also, since the strength of the SPV signals may vary drastically from sub-bandgap
regime to the super-bandgap regime, a several orders of magnitude at times, it is
important to prevent the spurious contribution of stray lights and other high-order
diffraction peaks coming from the monochromator. This requires use of appropriate
cut-off filters in the path of illumination beam. Again, since time SPV signal may
be very feeble particularly in the sub-bandgap regime, the signal to noise ratio may
not be very appreciable. Hence measurements are generally taken with the help of
a lock-in amplifier with chopped light beam configuration. The reference for the
lock-in is typically obtained from optical chopper. In case of Kelvin probe approach,
the vibration of the plate itself may provide reference to the lock-in and hence the
chopping of the light beam is not required. In some cases, an auxiliary light source