612
A. Jariwala et al.
across any SCR, no matter how far it is from the surface inside the sample as long
as it receives the illumination, may add up to the potential at the free surface. The
effective SPV signal would then depend on the relative directions of photo-induced
fields in the SCR involved.
Thus, although the SPS is a highly sensitive technique to the surface, it can also
probe the effects whose physical origin may not necessarily be at the surface. Hence,
the SPS can be effectively employed to characterize various buried interfaces and
bulk properties of semiconductors beside its usual applications to probe the surfaces.
3 Experimental Implementation of SPS
The experimental measurement of SPV single essentially relies on injection of free
carriers in the materials by means of illumination induced photo-generation followed
by the separation of charges and finally the determination of surface potential. In
order to obtain the spectral dependence of the SPV signal, one needs to control the
wavelength of incident light on the surface, typically by means of a monochromator.
The next task is to measure the voltage developed across the free surface with respect
to the bulk of the materials, that is the SPV. The measurement of SPV signal is nontrivial process as it involves detection of built-in potential across the SCR. The SPV
signal cannot be simply measured by connecting a volt-meter across the front and the
back surface of the sample. Further, the application of contact electrode to carry out
electrical measurements on free surface may also affect the surface characteristics and
thus, deviates the properties being measured. In view of these challenges, different
techniques have been developed to determine the surface potential without applying
a direct electrical contact. Two major techniques used extensively to measure the
SPV signals have been described here along with various controlled parameters or
experimental variants and their effects on the measurements.
3.1 The Kelvin Probe
If the surface of a material is not photo-chemically active, it is evident that the any
change in the band-bending at the surface would be equally reflected in corresponding
change in the work function Ws at the surface as well. This implies that the measurement of changes in work function under such condition at the surface is an indirect
way of measuring of the absolute magnitude of the SPV signal. The difference of the
work functions between two materials can be measured by a parallel plate capacitor
configuration. When not in contact, the two materials in a parallel plate capacitor
are electrically neutral and the two materials have a same common vacuum level.
However, when the two materials are short-circuited, it allows the charges to flow
from the materials with smaller work function to that with larger work function in
order to achieve the equilibration of the Fermi levels. This movement of the charge
A. Jariwala et al.
across any SCR, no matter how far it is from the surface inside the sample as long
as it receives the illumination, may add up to the potential at the free surface. The
effective SPV signal would then depend on the relative directions of photo-induced
fields in the SCR involved.
Thus, although the SPS is a highly sensitive technique to the surface, it can also
probe the effects whose physical origin may not necessarily be at the surface. Hence,
the SPS can be effectively employed to characterize various buried interfaces and
bulk properties of semiconductors beside its usual applications to probe the surfaces.
3 Experimental Implementation of SPS
The experimental measurement of SPV single essentially relies on injection of free
carriers in the materials by means of illumination induced photo-generation followed
by the separation of charges and finally the determination of surface potential. In
order to obtain the spectral dependence of the SPV signal, one needs to control the
wavelength of incident light on the surface, typically by means of a monochromator.
The next task is to measure the voltage developed across the free surface with respect
to the bulk of the materials, that is the SPV. The measurement of SPV signal is nontrivial process as it involves detection of built-in potential across the SCR. The SPV
signal cannot be simply measured by connecting a volt-meter across the front and the
back surface of the sample. Further, the application of contact electrode to carry out
electrical measurements on free surface may also affect the surface characteristics and
thus, deviates the properties being measured. In view of these challenges, different
techniques have been developed to determine the surface potential without applying
a direct electrical contact. Two major techniques used extensively to measure the
SPV signals have been described here along with various controlled parameters or
experimental variants and their effects on the measurements.
3.1 The Kelvin Probe
If the surface of a material is not photo-chemically active, it is evident that the any
change in the band-bending at the surface would be equally reflected in corresponding
change in the work function Ws at the surface as well. This implies that the measurement of changes in work function under such condition at the surface is an indirect
way of measuring of the absolute magnitude of the SPV signal. The difference of the
work functions between two materials can be measured by a parallel plate capacitor
configuration. When not in contact, the two materials in a parallel plate capacitor
are electrically neutral and the two materials have a same common vacuum level.
However, when the two materials are short-circuited, it allows the charges to flow
from the materials with smaller work function to that with larger work function in
order to achieve the equilibration of the Fermi levels. This movement of the charge