620
A. Jariwala et al.
combination with the SPV magnitude in order to understand the charge-dynamics in
the materials.
4.2 Frequency Dependence of SPS
In the same experimental configuration, that is MIS structure with light chopping
mode, one can easily employ measurements of SPV spectra by changing the chopping frequency of the light. The chopping frequency can affect the different SPV
generation mechanisms present in the samples differently, depending on the effective time constant of the processes. Thus, the chopping frequency can not only help
understanding these mechanisms but also at times used to eliminate contribution of
certain process in order to probe the other mechanisms at work in samples. There are
quite a few reports on use of chopping frequency dependence of the SPV generation
to understand the charge-dynamics of semiconductor materials and devices.
As discussed earlier, the SPV spectrum relies on the absorption of photons,
followed by redistribution of excess charges. Typically, the chopping frequency
dependent features in the SPS are mostly governed by the excess charge dynamics
and not to the absorption mechanisms. The redistribution of these excess charges
is, in principle, caused by (i) separation of charges in the surface SCR, (ii) the
Dember potential and (iii) the differences in rates of emission and recombination
processes from the mid–gap levels in both, bulk region and the surface [9, 70–76].
All these three processes have different time-constants. The first two processes are
generally fast processes with very small time constant, whereas the third one is relatively a slow process. By changing the chopping frequency, it is possible to change
the contribution of these different processes in the measured SPV signals and thus,
by correlating the data, one can better understand the carrier generation and transfer
mechanism prevailing in the concerned materials system.
Sharma et al. [72] have reported the use of chopping frequency dependence of
SPV measurements on SI-GaAs samples. They measured the SPV spectra at 28 Hz,
128 Hz, 228 Hz, 1 kHz, and 3 kHz, respectively, and have identified various features of
SPV spectra in the sub-bandgap regions by their different responses to these different
chopping frequencies. We have also employed the chopping frequency dependent
SPV measurements extensively. For example, the CdTe/CdS samples as described
above (in the Phase Scan section) were also subjected to the SPV spectroscopy
at different chopping frequency. The SPS were carried out at different chopping
frequencies from 100 to 2000 Hz at some discrete intervals. The reliance of the
SPS signal on frequency stems from the dynamics of excess charges, which can be
modeled by [42].
SPV = A
τ
√
1 + ω 2 τ 2
(3)
A. Jariwala et al.
combination with the SPV magnitude in order to understand the charge-dynamics in
the materials.
4.2 Frequency Dependence of SPS
In the same experimental configuration, that is MIS structure with light chopping
mode, one can easily employ measurements of SPV spectra by changing the chopping frequency of the light. The chopping frequency can affect the different SPV
generation mechanisms present in the samples differently, depending on the effective time constant of the processes. Thus, the chopping frequency can not only help
understanding these mechanisms but also at times used to eliminate contribution of
certain process in order to probe the other mechanisms at work in samples. There are
quite a few reports on use of chopping frequency dependence of the SPV generation
to understand the charge-dynamics of semiconductor materials and devices.
As discussed earlier, the SPV spectrum relies on the absorption of photons,
followed by redistribution of excess charges. Typically, the chopping frequency
dependent features in the SPS are mostly governed by the excess charge dynamics
and not to the absorption mechanisms. The redistribution of these excess charges
is, in principle, caused by (i) separation of charges in the surface SCR, (ii) the
Dember potential and (iii) the differences in rates of emission and recombination
processes from the mid–gap levels in both, bulk region and the surface [9, 70–76].
All these three processes have different time-constants. The first two processes are
generally fast processes with very small time constant, whereas the third one is relatively a slow process. By changing the chopping frequency, it is possible to change
the contribution of these different processes in the measured SPV signals and thus,
by correlating the data, one can better understand the carrier generation and transfer
mechanism prevailing in the concerned materials system.
Sharma et al. [72] have reported the use of chopping frequency dependence of
SPV measurements on SI-GaAs samples. They measured the SPV spectra at 28 Hz,
128 Hz, 228 Hz, 1 kHz, and 3 kHz, respectively, and have identified various features of
SPV spectra in the sub-bandgap regions by their different responses to these different
chopping frequencies. We have also employed the chopping frequency dependent
SPV measurements extensively. For example, the CdTe/CdS samples as described
above (in the Phase Scan section) were also subjected to the SPV spectroscopy
at different chopping frequency. The SPS were carried out at different chopping
frequencies from 100 to 2000 Hz at some discrete intervals. The reliance of the
SPS signal on frequency stems from the dynamics of excess charges, which can be
modeled by [42].
SPV = A
τ
√
1 + ω 2 τ 2
(3)