Surface Photo-Voltage Spectroscopy: A Versatile Technique …
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which are typically used in the SPS for different applications. Subsequently a wide
variety of various applications of SPS are described in detail.
2 Theoretical Framework for SPS
The semiconductors are characterized by energy bands with a well-defined forbidden
gap between them. This results from an assumption of infinite periodicity of their
crystalline structure. However, the termination of this periodicity at surfaces causes
formation of localized electronic states within the gap. It may also form a double
layer of charges at the surface, known as surface dipole. Similar localized electronic
states within the gap may also form at other interfaces, wherever the lattice symmetry
breaks. In addition to the lattice symmetry breaking, the surfaces are also prone to
quite a few other events. These include surface reconstruction, that is the change in the
surface atoms’ position or chemical bonding in order to minimize surface energy, the
formation of surface atoms with no neighbor atoms to bind to, known as ‘dangling
bonds’, steps or kinks at the surface or external impurity absorbed at the surface.
All these events may also contribute to the formation of localized electronic states
at the surfaces. These surface-localized states at the surface trigger charge transfer
between bulk and surface of the material to establish thermal equilibrium between
the two. Because of these charge transfers, the free carrier concentration at and near
the surface in the materials is generally different from its equilibrium value in the
bulk. Consequently an electrically non-neutral region is formed at the surface, which
is known as space–charge region (SCR). The SCR has a finite, non-zero electric field
in it and hence it allows a potential drop across it.
Thus, the surface potential Vs is different from the electric potential in the bulk
even under equilibrium. For instance, let us consider an n-type semiconductor with
a depleted surface, which means the majority carriers (i.e. electrons) are trapped in
surface states and hence their concentration near the surface is less than its equilibrium value but greater than that of the minority carriers. The potential drop across
such an SCR is manifested by bending of the semiconductor bands in such a way that
the free electrons are repelled from the surface and holes are attracted to it because
of those trapped electrons at the surface. The band bending as described here gives
rise to a finite, constant surface potential under normal conditions, even when the
semiconductor is not under any illumination, as shown in Fig. 1.
Now, when the surface of semiconductor is illuminated with photons of appropriate energy, the free charge carriers are generated because of the absorption of
these photons. These free charge carriers may result from either the creation of electron–hole pairs near the surface due to band-to-band transitions or by the excitation
of charge carriers captured in the trap states due to trap-to-band transitions. In any
case, this causes a significant charge transfer, which could be in form of movement of electrons and holes in the opposite directions under the built-in electric field
and/or diffusion between the surface and the bulk due to changed concentrations
and/or redistribution within the surface. In any case, these charge transfers change
607
which are typically used in the SPS for different applications. Subsequently a wide
variety of various applications of SPS are described in detail.
2 Theoretical Framework for SPS
The semiconductors are characterized by energy bands with a well-defined forbidden
gap between them. This results from an assumption of infinite periodicity of their
crystalline structure. However, the termination of this periodicity at surfaces causes
formation of localized electronic states within the gap. It may also form a double
layer of charges at the surface, known as surface dipole. Similar localized electronic
states within the gap may also form at other interfaces, wherever the lattice symmetry
breaks. In addition to the lattice symmetry breaking, the surfaces are also prone to
quite a few other events. These include surface reconstruction, that is the change in the
surface atoms’ position or chemical bonding in order to minimize surface energy, the
formation of surface atoms with no neighbor atoms to bind to, known as ‘dangling
bonds’, steps or kinks at the surface or external impurity absorbed at the surface.
All these events may also contribute to the formation of localized electronic states
at the surfaces. These surface-localized states at the surface trigger charge transfer
between bulk and surface of the material to establish thermal equilibrium between
the two. Because of these charge transfers, the free carrier concentration at and near
the surface in the materials is generally different from its equilibrium value in the
bulk. Consequently an electrically non-neutral region is formed at the surface, which
is known as space–charge region (SCR). The SCR has a finite, non-zero electric field
in it and hence it allows a potential drop across it.
Thus, the surface potential Vs is different from the electric potential in the bulk
even under equilibrium. For instance, let us consider an n-type semiconductor with
a depleted surface, which means the majority carriers (i.e. electrons) are trapped in
surface states and hence their concentration near the surface is less than its equilibrium value but greater than that of the minority carriers. The potential drop across
such an SCR is manifested by bending of the semiconductor bands in such a way that
the free electrons are repelled from the surface and holes are attracted to it because
of those trapped electrons at the surface. The band bending as described here gives
rise to a finite, constant surface potential under normal conditions, even when the
semiconductor is not under any illumination, as shown in Fig. 1.
Now, when the surface of semiconductor is illuminated with photons of appropriate energy, the free charge carriers are generated because of the absorption of
these photons. These free charge carriers may result from either the creation of electron–hole pairs near the surface due to band-to-band transitions or by the excitation
of charge carriers captured in the trap states due to trap-to-band transitions. In any
case, this causes a significant charge transfer, which could be in form of movement of electrons and holes in the opposite directions under the built-in electric field
and/or diffusion between the surface and the bulk due to changed concentrations
and/or redistribution within the surface. In any case, these charge transfers change