Abstract- This document is about the determination of the semiconductor band gap energy and also to determine the type of Semiconductor material by using the Kelvin Probe. By studying the surface photo voltage as a function of incident photon energy useful information about important semiconductor properties may be extracted using a simple and qualitative analysis of the ensuing spectra. The properties include semiconductor band gap energy and the type of the semiconductor. The sample used are AlGaAs/GaAs
Keywords- Contact potential differences, surface potential, band gap energy.
The Kelvin Probe is an effective and non invasive tool for the mapping of surface potential of surfaces. Since surface potential includes a component due to work function and another due to trapped charge, this tool can be used to map either quantity on an surface when the other is kept constant. For example trapped charge is monitored in semiconductor integrated circuits(IC) fabrication because it has been correlated to the degradation of the device parameters. The ability to measure local charge distribution across patterns on production wafers can be critical asset in predicting yield and longevity of devices. This function can be performed by mapping the contact potential difference (CPD) between the Kelvin Probe and the sample wafer.
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The number of application of this versatile technique is constantly expanding: raging from absorption studies of metal, semiconductor and polymer surfaces, thin films, meta stable ,bulk recombination, spectroscopy, catalysis, surface charge imaging, surface roughness and bio potentials.
Another potential application for Kelvin Probe is the study of surface photovoltage as a function of incident photon enrgy. Useful information like band gap energy, types of semiconductor. This information have significant uses in the field of surface electronic structure, surface reaction, metal semiconductor interface, bulk defects, process control and more.
The Kelvin probe method which can be applied at room temperature and pressure, uses a vibrating capacitor to measure the Control Potential Difference. The concept is illustrated in Fig. 1 When two disconnected surfaces , such as the tip of the Kelvin probe and a sample are brought into proximity, the vacuum levels are aligned[Fig. 1(top)] and when the two sides are brought into an electron contact, the Fermi levels are aligned at equilibrium[Fig. 1(middle)]. Electron flow is necessary for the equilibrium to be achieved. As a result of this flow, the surface with the large work function acquires a negative charge and the other a positive one. An external bias may be used to compensate the electric field resulting from this charge transfer [Fig 1(bottom)]. When the system reaches equilibrium, the resulting potential difference is the Control Potential Difference.
Fig.1. Electron energy band diagram for two different metals separated by an air gap (top) before electrical contact(switch is open).(middle) after electrical contact(switch is closed).(bottom) after inclusion of the bias potential.
Where ï¹ï€±ï€ and ï¹ï€±ï€ are the work function of the sample and the probe tip respectively, and ï¹d is the potential due to trapped charge that may exist on the surface . As the probe, which is biased at -Vb with respect to the sample, is dithered in close proximity to it, an ac current is induced by the modulator of the capacitance (Ck) between them
Theoretically Vb is varied until the current is varied until the current goes to zero and is affected not only by noise, but by the finite input impedance and the leakage currents of the test instruments as well as the parasitic pathway in the device and setup. Thus, in practice the rms current is monitored, and the bias that minimizes it is Vcpd.
Electronic Structure of the Surface
A surface is defined as a boundary of media
with different physical properties. The surface between a
semiconductor and vacuum or gas often is referred to as a
'free surface' and the surface between a semiconductor and
another solid usually is referred to as an 'interface'.
In an ideal crystalline semiconductor the energy bands
is separated by forbidden energy gaps. The termination of a semiconductor at its free surface may form surface-localized electronic states within the semiconductor
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bandgap or surface dipole. The formation of surface atoms
with no upper atom to bind creates chemical
bonding configuration of surface atoms that minimizes the
surface energy and adds impurity atoms on the surface.
The formation of surface-localized states causes charge
transfer between bulk and surface in order to establish
thermal equilibrium between the two. The free carrier
density in the surface therefore deviates
from its equilibrium value in the bulk. This results in a
surface space-charge region (SCR),which is electrically not neutral and thus creates a non-zero electric field in it . So, even under equilibrium conditions the surface potential is different from the electric potential far away in the bulk. The
potential drop across the SCR is manifested by the bending
of the semiconductor bands, it is such that electrons are
repelled from the surface and holes are attracted to it, due to
the trapped surface electrons. Higher the electrical potential
lower is the energy band, so that a positive potential leads to down-ward band bending. The surface dipole is another important phenomenon of semiconductor material . The surface barrier is usually characterized
by means of the electron affinity, defined as the energy
needed to release an electron from the conduction band into
a vacuum. At a semiconductor surface or interface,
an effective surface barrier may be determined by additional
microscopic dipole contributions.The difference between the effect of surface states andthe surface dipole on the semiconductor band diagram is
best illustrated by the concept of a local vacuum level,
that is the energy of an electron at a given point if it is
at rest and free from the influence of the crystal potential
(which determines the underlying band structure) but not
from macroscopic potentials. The local vacuum level therefore must follow any changes in the electric potential along the sample.At the surface, the effective electron affinity may differ from that of the bulk due to the various surface dipole effects.
Surface Photovoltaic Effect
The photovoltaic effect is typically the result of some charge transfer or redistribution of charge within the device due to the illumination of the sample.A specific variant of the photovoltaic effect is the surface photovoltaic effect.
It is important to note that the formation of a SPV occurs
only if carrier generation per second is followed by net charge recombination. Normally, no significant driving force for such
redistribution is found beyond the SCR and the underlying
bulk remains quasi-neutral. Only in the presence of
significantly non-uniform generation or recombination may
this assumption break down .
The SPV mechanism depends strongly on whether the
incident photon energy is super-bandgap or sub-bandgap,
i.e. on the dominant carrier excitation mechanism.
The electric field in the SCR causes excess electrons to be
swept away from the surface and excess holes to be swept
towards it. This serves to reduce the density of surface trapped
electrons and decreases the band-bending. In a
second mechanism, either electrons or holes are preferentially trapped at surface defects. This
effectively charges the surface and thus modifies the surface
potential. This second mechanism is usually apparent in bulk
samples only if the equilibrium surface band-bending is fairly
small, but increases in importance in polycrystall inematerial
with decreasing crystallite size because of an increasing
surface to volume ratio.
The most common mechanism for sub-bandgap SPV
involves the direct modification of the surface charge, and
hence potential, by excitation of trapped carriers. Illumination by photons with energy hf > Ec _ Et
may produce electron transitions from a surface state at an
energy Et into the conduction band, where they are swept
quickly to the semiconductor bulk by means of the surface
electric field. Hence, the negative surface charge is reduced
and the band-bending is decreased. Conversely, illumination
by photons with energy hv > Et _ Ev may produce electron
transitions from the valence band into a surface state situated
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at an energy Et above the valence band maximum Ev (which
are equivalent to hole transitions from the surface state
to the valence band). Such transitions increase the surface
negative charge and therefore the surface band-bending. For
the SPV to develop in this case it is necessary to have a
significant diffusion of the excess holes into the bulk or
a significant recombination of electrons and holes inside the
SCR, because there is no field-assisted driving force of holes
into the bulk. Other mechanisms for sub-bandgap SPV are
based on band-to-band transitions, made possible with subbandgap photon energies. It has been note that because the optically induced SPVgenerating
electron transitions take place against a background
of thermal transitions, the magnitude of the SPV
effect generally increases with decreasing temperature, for
both super-bandgap and sub-bandgap illumination.
IVExperimental set up
Kelvin Control setup
The Kelvin Control 07 is designed to operate the Kelvin Probe S. The LO output of the Kelvin Control is grounded. The Signal monitor and the reference terminal of the Kelvin Control is connected to the oscilloscope and the CPD terminal of the Kelvin Control is connected to the HP34401A multimeter. The multimeter is connect to a PC using GPIB cable.
Light source setup
Oriel 500-watt halogen lamp is used to lit the sample and a Orial monochromators is used to change the wavelength of the halogen lamp. The monochromators was connected to a PC using Parallel Port cable.
Fiber optic cable is used to shine light on the sample under the Kelvin Probe.
Automated Data Acquisation
All the instruments were controlled by using Labview from a PC.
The Labview program written in G-language changed the wavelength of the light shined on the sample and recorded the reading from the HP34401A multimeter.
Light from the halogen lamp is signed on the sample and the after every 1000ns the wavelength of the light sighed on it is changed.
The CPD out put from the Kelvin Control was recorded and the recorded out put was drawn against change of wavelength.
Initially the bandgap of the sample semiconductor is found out. This determination is based on the large increase in
absorption coefficient near the bandgap energy Eg found in
most semiconductors. This increase brings about a significant
change of the SPV signal ,which is identified easily as a sharp
change in slope of the SPV curve and often is the most
significant one in a given spectrum. An example is shown in [Fig 2]where the slope changes related to the bandgaps of
GaAs and InP are identified clearly.
Fig. 2 Typical SPV spectra super band gap
The study of studied of silicon-on-sapphire films showed that aside from the fundamental Si bandgap at .1.1 eV, a second,
distinct, knee in the SPV spectrum is found at .1.4 eV. This
value typically marks the onset of significant absorption at
amorphous silicon films and therefore partial amorphization
of the films was deduced.
The use of SPS for extraction of Eg is, at heart,
nothing more than an emulation of an absorption spectrum.
However, as opposed to transmission spectroscopy, for
example, SPS does not require light collection and therefore
can be performed on arbitrarily thick
sandwiched within a heterostructure) and does
not require the sample to be removed from the substrate or
grown on a transparent one. It is also inherently insensitive
to reflection and scattering and thus is eminently useful for
micro- and nanocrystallites
The obtained value of Eg is only approximate. The nominal bandgap indeed is nearly always found within the onset of the largest SPV signal.
However, this onset usually is relatively broad, the exact
position of Eg within it is by no means obvious and the
error in Eg is often .0.1 eV and may be as large as 0.2 eV
in some cases. This observation is true even for the clean
surfaces of high-quality GaAs and InP single crystals .The broad onset of the super-bandgap SPV is due to
the absorption of photons with an energy slightly below Eg,
resulting photo-assisted charge transfer between shallow
states extending from the bandgap (also known as 'tail
states') and one of the bands. Because SPS is inherently
much more sensitive to sub-bandgap effects in the surface
and SCR than absorption spectroscopy, 'tail state' effects
become much more noticeable and an accurate value of
Eg can seldom be obtained from the SPS curve simply by
Detailed experimental comparisons between SPV and
absorption spectra revealed that the two are often similar
but never identical. It can be shown that a linear
dependence of the SPV on the illumination intensity
(achieved if the latter is low enough), an absorption
length much larger than the diffusion length and an
effective surface recombination velocity that is only weakly
dependent on the illumination intensity allow for the
super-bandgap SPV spectrum to emulate properly the
absorption spectrum.If these conditions are met, Eg may
be extrapolated quantitatively from the data, just as in
To determining the type
(p or n) of semiconductor the knee associated with the SPV
Onset is taken into account. Most of the semiconductor surfaces are depleted/inverted,
which means that the bands of p-type semiconductors
are bent downwards towards the surface, whereas the
bands of n-type semiconductors are bent upwards. Because
super-bandgap illumination typically tends to decrease the
surface band-bending, this would result in a positive SPV
in n-type semiconductors and a negative SPV in p-type
semiconductors. The SPV spectra of n-type GaAs
and p-type InP clearly feature opposite
onset signs that obey the above rules.
Although in many cases the type of semiconductor is
known a priori, this is not always the case and SPS can
become very useful in determining the semiconductor type.
SPS studies on free-standing porous Si films
made from p-type Si revealed that the films may be of
either conduction type, depending on the resistivity of the Si
substrate used.In another example, SPS was used for fast
and non-destructive verification of the semiconductor type in
GaN films.This is very important because the p-doping of
GaN is known to be non-trivial and subject to compensation
by its native n-type doping if the band-bending is small, super-bandgap SPV mechanisms may dominate. It was seen that in CdSe quantum dot films were found to exhibit a
p-type response in a humid ambient and an n-type response
in a dry ambient. Using photoelectron
spectroscopy, however, these films were found to remain
n-type at all times in terms of the position of the Fermi
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