Surface Photo-Voltage Spectroscopy: A Versatile Technique …
611
strong surface recombination of photo-generated electrons - holes at the surface
cause a significant reduction in the SPV signal as the photon energy increases far
above the bandgap.
In the above discussion, we only considered the charge carrier separation at the
surface. A photovoltage, however, can also develop due to redistribution of the charge
carriers in the quasi-neutral bulk, even though there is no electric field to cause the drift
there. The carrier separation in bulk is generally driven by the diffusion of carriers due
to their non-uniform generation along the depth. As we know, the intensity of the light
falls exponentially as it penetrates in the bulk, the resultant photo-generation also has
an exponentially decaying concentration along the depth of the sample. The gradient
in the concentration of these photo-generated carriers trigger the diffusion. Further,
the electrons and holes diffuse at different speeds due to their different mobilities
in the materials, causing an effective separation of positive and negative chargecenters. This separation of charges in bulk also produces photovoltage and this effect
is known as Dember effect. The voltage thus developed is known as Dember voltage,
which also contributes to the measured SPV signals in SPS. The Dember voltage
is prominent in samples with very low band bending at the surface such as semiinsulating GaAs (SI-GaAs) [42]. A special case of generation of Dember voltage also
arises when the optical transitions occur between a defect level and one of the bands
in the bulk region. Such transitions generate only one type of free carriers, which
diffuse through the sample, whereas the other type of carriers remains captured in
the defect state. The sign of the Dember voltage in such a case depends on the type
of free carriers.
The SPV signal can also be measured in case of the nano-structured materials.
However, in these materials, the separation of carriers within the potential well does
not suffice [43, 44]. It is important for the carriers to escape from the well to be able
to contribute to the SPV signals. It has been demonstrated that the escape mechanism of photo-generated carriers from a quantum well (QW) located in the SCR
are predominantly result from the thermal emission rather than the field-assisted
tunnelling [43]. It is obvious that optically induced SPV signal is generated in the
background thermally induced transitions. Hence, the magnitude of the SPV signals
goes up at lower temperatures in both super- and sub-bandgap regimes [9, 15]
Sensitivity Beyond the Surface
So far we have considered a terminated surface along with the thick bulk of the
semiconductor in our discussions. Actual semiconductor devices, however, may use
materials in form of thin-films with their thicknesses of the order of the diffusion
length of the carriers. The devices may employ multilayered structures as well with
several junctions and more than one SCR involved. Although the SPV is obtained
by means of measuring the changes in surface potential at the illuminated surface,
it can probe the buried junctions and interfaces far from the surface as well. This is
because the optical absorption occurring in any space charge region, irrespective of
whether it is at surface or at the buried interfaces within the sample, may contribute
to the generation of SPV signal. This results from the fact that the energy bands
are connected serially in the sample and hence the potential difference developing
611
strong surface recombination of photo-generated electrons - holes at the surface
cause a significant reduction in the SPV signal as the photon energy increases far
above the bandgap.
In the above discussion, we only considered the charge carrier separation at the
surface. A photovoltage, however, can also develop due to redistribution of the charge
carriers in the quasi-neutral bulk, even though there is no electric field to cause the drift
there. The carrier separation in bulk is generally driven by the diffusion of carriers due
to their non-uniform generation along the depth. As we know, the intensity of the light
falls exponentially as it penetrates in the bulk, the resultant photo-generation also has
an exponentially decaying concentration along the depth of the sample. The gradient
in the concentration of these photo-generated carriers trigger the diffusion. Further,
the electrons and holes diffuse at different speeds due to their different mobilities
in the materials, causing an effective separation of positive and negative chargecenters. This separation of charges in bulk also produces photovoltage and this effect
is known as Dember effect. The voltage thus developed is known as Dember voltage,
which also contributes to the measured SPV signals in SPS. The Dember voltage
is prominent in samples with very low band bending at the surface such as semiinsulating GaAs (SI-GaAs) [42]. A special case of generation of Dember voltage also
arises when the optical transitions occur between a defect level and one of the bands
in the bulk region. Such transitions generate only one type of free carriers, which
diffuse through the sample, whereas the other type of carriers remains captured in
the defect state. The sign of the Dember voltage in such a case depends on the type
of free carriers.
The SPV signal can also be measured in case of the nano-structured materials.
However, in these materials, the separation of carriers within the potential well does
not suffice [43, 44]. It is important for the carriers to escape from the well to be able
to contribute to the SPV signals. It has been demonstrated that the escape mechanism of photo-generated carriers from a quantum well (QW) located in the SCR
are predominantly result from the thermal emission rather than the field-assisted
tunnelling [43]. It is obvious that optically induced SPV signal is generated in the
background thermally induced transitions. Hence, the magnitude of the SPV signals
goes up at lower temperatures in both super- and sub-bandgap regimes [9, 15]
Sensitivity Beyond the Surface
So far we have considered a terminated surface along with the thick bulk of the
semiconductor in our discussions. Actual semiconductor devices, however, may use
materials in form of thin-films with their thicknesses of the order of the diffusion
length of the carriers. The devices may employ multilayered structures as well with
several junctions and more than one SCR involved. Although the SPV is obtained
by means of measuring the changes in surface potential at the illuminated surface,
it can probe the buried junctions and interfaces far from the surface as well. This is
because the optical absorption occurring in any space charge region, irrespective of
whether it is at surface or at the buried interfaces within the sample, may contribute
to the generation of SPV signal. This results from the fact that the energy bands
are connected serially in the sample and hence the potential difference developing