Surface Photo-Voltage Spectroscopy: A Versatile Technique …
625
very feeble signal also to be detected, as the external electric field assists the photogenerated carriers in their transport process and such carriers eventually contribute
to the measured SPV.
4.6.2 Pump-Probe SPS
The pump-probe SPS is a combination of pump-probe spectroscopy with the conventional SPS measurements. The technique essentially involves a pump beam, generally
in d.c. but sometimes in pulsed form in order to control the charge dynamics in the
material being probed. The major role of the pump beam, generally, is to saturate the
sub-bandgap localized states in order to distinguish the contribution of such localized
states from the band-to-band transitions. The pump beam can also affect the Dember
effect in the materials. Thus, the pump-probe SPS technique enables us to determine
the bandgap of semiconductor epitaxial layers accurately, even with very high density
of localized sub-bandgap states. The pump beam can also effectively suppress the
effects of surface/interface states beside the bulk trap states. Similarly, an above
bandgap pump beam can also be used to distinguish the signatures of sub-bandgap
states by suppressing the band edge related feature. D. Jana et al. have employed
[92] pump-probe SPS measurements on semiconductor epitaxial layers. They used
a sub-bandgap cw pump laser beam in a conventional chopped light geometry SPS
setup under the pump-probe configuration. They successfully established usefulness
of the pump-probe SPS technique by unambiguously determining the bandgap of
p-GaN epilayers grown on Sapphire substrates, SI-InP wafers, and p-GaAs epitaxial
layers grown on SI-GaAs substrates.
5 Applications of SPS
5.1 Determination of Optical Bandgap and Type
of the Materials
The band gap energy, Eg, is one of the vital parameters that characterize the semiconductor materials. Normally, the bandgap of a nondegenerate bulk semiconductor
can be easily obtained by finding the optical absorption threshold using transmission/absorption spectroscopy or modulated specular reflectance spectroscopies.
These techniques, however, may not be suitable for powder samples, mainly because
of their significant scattering of light. The transmission spectrum is also not possible
in thick wafers or relatively thick films with very high absorption coefficients. Further,
the light scattering and reflections may make the determination of the optical bandgap
difficult in case of diffused reflectance spectroscopy or transmission spectroscopy,
respectively. SPS is a versatile tool for determination of band gap of various semiconductor structure including powders. Since the SPS relies on the absorption of
625
very feeble signal also to be detected, as the external electric field assists the photogenerated carriers in their transport process and such carriers eventually contribute
to the measured SPV.
4.6.2 Pump-Probe SPS
The pump-probe SPS is a combination of pump-probe spectroscopy with the conventional SPS measurements. The technique essentially involves a pump beam, generally
in d.c. but sometimes in pulsed form in order to control the charge dynamics in the
material being probed. The major role of the pump beam, generally, is to saturate the
sub-bandgap localized states in order to distinguish the contribution of such localized
states from the band-to-band transitions. The pump beam can also affect the Dember
effect in the materials. Thus, the pump-probe SPS technique enables us to determine
the bandgap of semiconductor epitaxial layers accurately, even with very high density
of localized sub-bandgap states. The pump beam can also effectively suppress the
effects of surface/interface states beside the bulk trap states. Similarly, an above
bandgap pump beam can also be used to distinguish the signatures of sub-bandgap
states by suppressing the band edge related feature. D. Jana et al. have employed
[92] pump-probe SPS measurements on semiconductor epitaxial layers. They used
a sub-bandgap cw pump laser beam in a conventional chopped light geometry SPS
setup under the pump-probe configuration. They successfully established usefulness
of the pump-probe SPS technique by unambiguously determining the bandgap of
p-GaN epilayers grown on Sapphire substrates, SI-InP wafers, and p-GaAs epitaxial
layers grown on SI-GaAs substrates.
5 Applications of SPS
5.1 Determination of Optical Bandgap and Type
of the Materials
The band gap energy, Eg, is one of the vital parameters that characterize the semiconductor materials. Normally, the bandgap of a nondegenerate bulk semiconductor
can be easily obtained by finding the optical absorption threshold using transmission/absorption spectroscopy or modulated specular reflectance spectroscopies.
These techniques, however, may not be suitable for powder samples, mainly because
of their significant scattering of light. The transmission spectrum is also not possible
in thick wafers or relatively thick films with very high absorption coefficients. Further,
the light scattering and reflections may make the determination of the optical bandgap
difficult in case of diffused reflectance spectroscopy or transmission spectroscopy,
respectively. SPS is a versatile tool for determination of band gap of various semiconductor structure including powders. Since the SPS relies on the absorption of