
H. A. THJEEL ET AL.
76
The silicon nanospikes tips may be working as a com-
pensator reducing the carrier concentration in the ZnO
deposited film which changes its polarity from n-type to
p-type. The same result was found by Vanmaekelbergh
and Liljeroth for ZnO film deposited on silicon substrate
[20]. The capturing of the excess charge carriers in the
ZnO film by the silicon nanospikes layer reduced the
surface charge density leading to increase the surface resis-
tivity of the deposited ZnO film. The ZnO nanocrystals
grow around the silicon nanospikes having a size similar
to the nanospikes dimension. Thus the size of ZnO
nanocrystals are reduced when the film is deposited on
porous silicon compare to its size when the film depos-
ited on glass. The nanocrystalline size reduction may
help in improvement of the speed of response of the ZnO
UV detector. The reduction in nanocrystalline dimension
helped in maximizing the surface to volume ratio. The
increase of the surface to volume ratio led to the increase
in the overlap of the electron and hole wave functions.
Since the increasing of the overlap functions account for
the reduction in the carriers recombination life time, the
speed of response of the ZnO UV detector is improved
for the ZnO nanofilm deposited on nanospikes silicon
layer. The charge carriers recombination mechanisms in
semiconductor nanocrystals were intensively studied by
many authors [21-23].
4. Conclusions
The fast response ZnO UV detector prepared by chemi-
cal spray pyrolysis technique was fabricated. The ZnO
films deposited on chemically etched silicon substrate
show a p-type behavior rather than n-type when they are
deposited on glass substrates .The p-ZnO film show high
carrier mobility leading to high speed UV detector. The
functionalization of the ZnO film surface by polyamide
nylon highly improved the photoconductive gain of the
detector. The fabricated detector was tested to detect fast
nitrogen laser pulses. The output signal was character-
ized by 180 µs rise time and 750 µs fall time. These re-
sults indicate that the deposition of ZnO nanofilm on
porous silicon is recommended for improving the re-
sponse time of the fabricated ZnO UV Photoconductive
detectors.
5. Acknowledgements
The author would like to thank Prof. Dr. R. A. Radhi,
chairman of Physics Department, College of Science of
Baghdad University for his interest and useful discussion
during the work. Also, we would like to appreciate the
contribution of Dr. M. T. Hussein for his help in using
the nitrogen laser.
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