Formation and Transient Photovoltaic Properties of ZnO/Si …
315
4 Transient Surface Photovoltage
Surface photovoltage (SPV) is formed by the separation of nonequilibrium carriers
in the near-surface sample region [52]. In particular, the separation exists in the
space charge regions that occur on the sample surfaces or at grain boundaries and
interfaces. The magnitude and direction of the appropriate band bending in these
regions determine the magnitude and sign of the SPV signal formed under light
illumination. The band bending itself depends on the concentration and type of
defects in surface and interface areas. These concentrations and type also determine
the generation and recombination velocity of nonequilibrium free charges.
In our case of ZnO/SiO 2 /Si heterostructures, the SPV amplitude and decay times
taken after the light is turned off can, in general, be related to varying defect content
in ZnO films and in a narrow subsurface region of the Si substrate. The SPV characteristics would therefore depend on the deposition method and thickness of the ZnO
layer. Measuring SPV can furthermore be interesting for improving the photovoltaic
properties of ZnO/SiO 2 /Si and NiO/ZnO/Si heterostructures.
Varying the light wavelength, one can vary the light penetration depth into the
semiconductor structure, thus appropriately changing the carrier generation depth
spanning the junction regions of the heterostructures. This is particularly true in case
of small pulse widths of the illuminating light, when the width remains much smaller
than the carrier lifetime. Otherwise, if the light pulse is of the order of or greater than
the lifetime of nonequilibrium carriers, then the carriers from a thick subsurface layer
comparative with their diffusion length contribute to the SPV signal, even for the
strongly absorbed light [53].
Taking the bandgap E g = 3.2–3.3 eV in ZnO [54] and E g = 1.12 eV in the Si
substrate [55] one gets that exciting light with λ > 375 nm correspond to E g of
ZnO [54]. Therefore, the SPV signal generated by a visible light is formed only
by free carriers excited in the Si substrate. For the light with a wavelength of less
than ≈375 nm, the signal will come from nonequilibrium carriers generated both in
the ZnO film and Si substrate. Strictly speaking, for both excitation conditions, one
should take into account the carrier diffusion and drift from the Si substrate into the
ZnO film and in the opposite direction.
The SPV data given below were measured in the capacitance arrangement and the
measurement technique was described in detail in [56]. SPV decay curves taken in
different ZnO-on-Si samples are shown in Fig. 10. The samples are excited by light
pulses of 1 μs duration with three different wavelengths λ = 405 nm, 470 nm, and
860 nm. At these conditions, nonequilibrium charge carriers are generated only in the
Si substrate at depths of 1/α ≈ 98 nm, 312 nm, and 21 μm, respectively, where α is
the light absorption coefficient in Si for the above three excitation wavelengths [55].
Negative SPV signals were experimentally detected in the circuit schematics used
and, for convenience, they are shown in Fig. 10 as well as in the text that follows,
the SPV decay curves are inverted.
It is shown in Fig. 10 that regardless of the wavelength of the exciting light, the
SPV decay rate decreases monotonically with increasing the ZnO thickness from 11
315
4 Transient Surface Photovoltage
Surface photovoltage (SPV) is formed by the separation of nonequilibrium carriers
in the near-surface sample region [52]. In particular, the separation exists in the
space charge regions that occur on the sample surfaces or at grain boundaries and
interfaces. The magnitude and direction of the appropriate band bending in these
regions determine the magnitude and sign of the SPV signal formed under light
illumination. The band bending itself depends on the concentration and type of
defects in surface and interface areas. These concentrations and type also determine
the generation and recombination velocity of nonequilibrium free charges.
In our case of ZnO/SiO 2 /Si heterostructures, the SPV amplitude and decay times
taken after the light is turned off can, in general, be related to varying defect content
in ZnO films and in a narrow subsurface region of the Si substrate. The SPV characteristics would therefore depend on the deposition method and thickness of the ZnO
layer. Measuring SPV can furthermore be interesting for improving the photovoltaic
properties of ZnO/SiO 2 /Si and NiO/ZnO/Si heterostructures.
Varying the light wavelength, one can vary the light penetration depth into the
semiconductor structure, thus appropriately changing the carrier generation depth
spanning the junction regions of the heterostructures. This is particularly true in case
of small pulse widths of the illuminating light, when the width remains much smaller
than the carrier lifetime. Otherwise, if the light pulse is of the order of or greater than
the lifetime of nonequilibrium carriers, then the carriers from a thick subsurface layer
comparative with their diffusion length contribute to the SPV signal, even for the
strongly absorbed light [53].
Taking the bandgap E g = 3.2–3.3 eV in ZnO [54] and E g = 1.12 eV in the Si
substrate [55] one gets that exciting light with λ > 375 nm correspond to E g of
ZnO [54]. Therefore, the SPV signal generated by a visible light is formed only
by free carriers excited in the Si substrate. For the light with a wavelength of less
than ≈375 nm, the signal will come from nonequilibrium carriers generated both in
the ZnO film and Si substrate. Strictly speaking, for both excitation conditions, one
should take into account the carrier diffusion and drift from the Si substrate into the
ZnO film and in the opposite direction.
The SPV data given below were measured in the capacitance arrangement and the
measurement technique was described in detail in [56]. SPV decay curves taken in
different ZnO-on-Si samples are shown in Fig. 10. The samples are excited by light
pulses of 1 μs duration with three different wavelengths λ = 405 nm, 470 nm, and
860 nm. At these conditions, nonequilibrium charge carriers are generated only in the
Si substrate at depths of 1/α ≈ 98 nm, 312 nm, and 21 μm, respectively, where α is
the light absorption coefficient in Si for the above three excitation wavelengths [55].
Negative SPV signals were experimentally detected in the circuit schematics used
and, for convenience, they are shown in Fig. 10 as well as in the text that follows,
the SPV decay curves are inverted.
It is shown in Fig. 10 that regardless of the wavelength of the exciting light, the
SPV decay rate decreases monotonically with increasing the ZnO thickness from 11