Surface Photo-Voltage Spectroscopy: A Versatile Technique …
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carriers builds up an electric field between the two plates, which results in drop of the
local vacuum level across the gap. The change in the potential is equal to the difference
between the work functions of the two materials, which is known as contact potential
difference (CPD) [12, 45]. It was suggested by Lord Kelvin that if the external d.c.
bias is applied in opposite direction to this parallel plate capacitor configuration, the
Fermi level of both the materials would separate again and when the applied external
d.c. bias matches exactly with the in-built CPD, the charge exchange between the two
materials will stop completely and the capacitor will be discharged. Hence, the CPD
can be measured simply by knowing the applied d.c. bias at which the capacitor gets
discharged completely even if the actual amount of the charge or the capacitance is
not known. The method, thus, is essentially a null method. The arrangement of this
type to measure the CPD is known as the Kelvin probe. Experimentally the Kelvin
probe is realized by means of a periodically vibrating plate hold in the close vicinity
of the illuminated surface of the sample in such a way that the effective a.c. capacitor
formed in this way establishes stable a.c. current [46]. This current becomes zero
only when the capacitor is completely discharged. Thus, the CPD can be determined
simply by finding out the value of d.c. bias at which the external a.c. current gets
nullified.
3.2 The Metal–Insulator–Semiconductor Approach
Another, recently more popular approach to carry out the measurements of SPV
signals directly is by using a Metal-Insulator-Semiconductor (MIS) configuration
with fixed metal electrode. When the MIS structure is under illumination, the surface
potential Vs changes means of the photovoltaic effect, which in turn triggers the same
amount of change in voltage across the capacitor [9]. Thus, the MIS structure offers
a relatively direct approach to measure the SPV. The process of gradual discharge
of MIS capacitor caused by the internal resistance of the system can be significantly
diminished by using modulated/chopped light instead of continuous light for illumination with sufficiently high chopping frequency. The use of chopped light for
illumination also allows the measurement through lock-in amplifier, which in turn
significantly enhances the signal-to-noise ratio. One must however take the parasitic
capacitance and resistance, particularly of the cables that reads the SPV signal from
the MIS structure and feeds it to the lock-in amplifier into account while using the
MIS structure for SPS measurements, as they may influence the measured signals.
Earlier, the MIS capacitor configuration was obtained by keeping a metallic grid
very close to the sample surface in such a way that the air or the vacuum between
the metallic grid and the semiconductor surface act as an insulator layer [47]. Subsequently, mica and other such insulator that do not temper with the surface properties were attempted as insulator layers in the MIS structure [48]. Further, Datta
et al. proposed a soft contact method for measurement of SPV in 2001 [49]. The
soft contact method involves a transparent conducting ITO coated glass (TCG) as
a probe electrode. The electrode is gently pressed onto the sample surface, while
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carriers builds up an electric field between the two plates, which results in drop of the
local vacuum level across the gap. The change in the potential is equal to the difference
between the work functions of the two materials, which is known as contact potential
difference (CPD) [12, 45]. It was suggested by Lord Kelvin that if the external d.c.
bias is applied in opposite direction to this parallel plate capacitor configuration, the
Fermi level of both the materials would separate again and when the applied external
d.c. bias matches exactly with the in-built CPD, the charge exchange between the two
materials will stop completely and the capacitor will be discharged. Hence, the CPD
can be measured simply by knowing the applied d.c. bias at which the capacitor gets
discharged completely even if the actual amount of the charge or the capacitance is
not known. The method, thus, is essentially a null method. The arrangement of this
type to measure the CPD is known as the Kelvin probe. Experimentally the Kelvin
probe is realized by means of a periodically vibrating plate hold in the close vicinity
of the illuminated surface of the sample in such a way that the effective a.c. capacitor
formed in this way establishes stable a.c. current [46]. This current becomes zero
only when the capacitor is completely discharged. Thus, the CPD can be determined
simply by finding out the value of d.c. bias at which the external a.c. current gets
nullified.
3.2 The Metal–Insulator–Semiconductor Approach
Another, recently more popular approach to carry out the measurements of SPV
signals directly is by using a Metal-Insulator-Semiconductor (MIS) configuration
with fixed metal electrode. When the MIS structure is under illumination, the surface
potential Vs changes means of the photovoltaic effect, which in turn triggers the same
amount of change in voltage across the capacitor [9]. Thus, the MIS structure offers
a relatively direct approach to measure the SPV. The process of gradual discharge
of MIS capacitor caused by the internal resistance of the system can be significantly
diminished by using modulated/chopped light instead of continuous light for illumination with sufficiently high chopping frequency. The use of chopped light for
illumination also allows the measurement through lock-in amplifier, which in turn
significantly enhances the signal-to-noise ratio. One must however take the parasitic
capacitance and resistance, particularly of the cables that reads the SPV signal from
the MIS structure and feeds it to the lock-in amplifier into account while using the
MIS structure for SPS measurements, as they may influence the measured signals.
Earlier, the MIS capacitor configuration was obtained by keeping a metallic grid
very close to the sample surface in such a way that the air or the vacuum between
the metallic grid and the semiconductor surface act as an insulator layer [47]. Subsequently, mica and other such insulator that do not temper with the surface properties were attempted as insulator layers in the MIS structure [48]. Further, Datta
et al. proposed a soft contact method for measurement of SPV in 2001 [49]. The
soft contact method involves a transparent conducting ITO coated glass (TCG) as
a probe electrode. The electrode is gently pressed onto the sample surface, while