Chemistry Reference
In-Depth Information
band-gap light illumination on
n
-Si in 1 M HCl or KCl solutions is very low, below
0.9 eV. At photoenergy above 0.9 eV, a relatively large increase in the current occurs
due to the onset of band gap transition. It is increased by a factor of 5 by changing the
pH from 1 to 4.7. However, the subband photocurrent is still several orders of magni-
tude smaller than the dark current.
A similar effect is observed in acetonitrile solutions.
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Subband photocurrent
slowly increases with time due to the development of surface states by the slow oxi-
dation of the surface. It is greatly enhanced by monolayer quantity of metal deposits.
Also, it is affected by the redox couples present in the solution. According to Chaza-
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the subband photocurrent is caused by the presence of surface states associ-
ated with adsorbed ions which lie 0.7-0.9 eV below the conduction band. The surface
generation by subband photon excitation can arise via two processes: either by elec-
tron excitation from the valence band into the empty surface states or by electron
excitation from the occupied surface states into the conduction band. The relative con-
tribution of these two processes is different for
p-
Si and
n
-Si and different quantum
yields for the two materials are found.
In the absence of surface recombination and with a fast rate of electron transfer,
the photocurrent increases with increasing potential when the depletion layer starts to
form and a saturation current is quickly reached. On the other hand, with a fast surface
recombination or in the case of slow electron transfer reactions, the apparent onset of
the photocurrent is shifted to higher bias and the saturation current is only reached at
larger band bending.
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Among other factors, surface treatment strongly affects the
photocurrent onset potential due to its effect on surface states which determine the
recombination process.
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Surface recombination for a well prepared silicon surface in HF solutions is very
low. The surface recombination velocity of silicon/HF electrolyte, which can be as low
as 0.25 cm/s, is lower than those of oxide-covered surface, in air and in solutions.
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For example, surface recombination velocity of an
tion is about 100 cm/s.
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Such low values found in HF solutions are due to surface ter-
mination by the covalent Si-H bonds, which leaves virtually no surface dangling bonds
to act as recombination centers. The hydrogen-terminated surface is rather stable as it
shows low recombination velocities in different acids. Figure 5.17 shows that the
recombination velocity increases with acid concentration and is largely similar for the
different acids, indicating molarity rather than pH is responsible for the change in veloc-
lviel,
solu-
