الفهرس 
GENERAL INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
In this thesis Zn Cd1S/znTe heterojunctions were inten-
x -x
sively investigated.
Our aim.was to relate the terminal elec-
trical and optoelectronic characteristics with the internal
physical and technological parameters of the device. The efforts
were devoted to work out the optimum performance of this hetero-
j·unction as a sol ar ce 11, as we 11 as to explore the phy 5 ical
mechanisms responsible for the observed low solar energy conver-
sion efficiency of such heterojunctions.
In this way one can propose methods for enhancing the device
performance regarding its application as a solar cell.
The substrate was single crystalline ZnTe while a new method
has been utilized to deposit the required Cd Zn S films. This
method is the r.f. sputtering technique ..
The electrical, photoelectrical and structural characteri-
stics of these films were ~tudied as a function of the technologi-
cal parameters, namely, film thickness, substrate tempe~ature,
and postpreparation annealing.
It was found that the resistivity
values of the CdS films can be controlled within a certain range
by controlling the substrate temperature and film thickness.
Also the resistivity of the films decreased after annealing in
H2, and no aging effect after 12 months was observed.
vation energies for the temperature dependence of electrical
conductivity for Cd S films are in the same order of magn~tude for
iii
The actiCd S films prepared by other methods.
Appreciable photosensi-
tivity was obtained for low deposition temperatures and larger
thicknesses of the films.
Therefore, for solar cell appli-
cations where smaller sheet resistance is required, moderate
deposition temperature and relatively small thickness are
preferable.
The effect of increasing Zn concentration x of Zn Cdl S
x -x
films resistivity was studied. The resistivity values obtained
are lower than those of films prepared by other techniques. They
fall wi thili the ran ge 0 f sol arc e 11 qua lit y fi 1m s .
The composition dependence of the optical band gap agrees
wi th that represented for Zn Cd S single crystals up to x = 0.6.
x l-x
Improvement in the C-axis preferential orientation was observed
after annealing and the grain size increased.
The electrical and optical properties of undoped and phos-
phorous doped Zn Te crystals were measured.
It was found that
doping Zn Te with phosphoro~s reduces the room temperature elec-
trical resistivity from 55 Jlcm (for undoped Zn Te) to O.l6Jl.. cm
(for P doped ZnTe).
Also a broad absorption peak at about 1.06eV
was observed for the P doped Zn Te with the increase of hole con-
centration.
The effect of alloying ZnTe with Mg was also examined to
explore the origin of the infrared band absorption that emerges
in the absorption spectra of doped crystals.
The results show
that the absorption coefficient is directly related to the number
of vacant states near the top of the valence bands A self consistent electrical and opto-electrical characteri-
zation of Zn Cd S/ZnTe junction as a solar cell was carried
x I-x
out. It was generally observed that the properties of the Zn Cd SI
ZnTe junctions depends largely on the technological para-
meters.
Heat treatment was found necessary to improve the cells
efficiency, while heat treatment for longer times (>3 h) deterio-
rates both V and J
oc sc
To determine the properties of the space charge region
formed at the interface of the two materials C-V and G-V
measurements was carried out at different frequencies from 10 KHz
to 1 MHz.
For some samples, the measured junction capacitance
was reduced appreciably by increasing the frequency.
To account
for .this behaviour we developed a small signal model taking into
account the series resistance of the junction and the shunting
effect of the conductance on the measured value of the transition
capacitance. It has been found that the junction capacitance
decreases with the composition x of Zn concentration.
Consequently the width of the space charge increases with the
composition x of zinc in the film. The effect of heat treatment
on capacitance voltage characteristics was also studied.
After excluding the effects of the series and shunt
resistances from the measured dark I-V characteristics, it was
found that the conduction in the junctions is dominated by
recombinations inside .the space charge region The reverse saturation current J increases firstly with Zn
o
content in the film then decreases. This behaviour of J 0 was
explained for the first time. It was also generally noticed that
the absolute value of Jo are relatively large for solar cell
applications.
This large value is due to the very small lifetime
of minority carriers in the film because of traps at boundaries
of the very small grains of the films.
It has been found that photocurrent decreases monotonically
with
Zn concentration in the film. Its maximum measured value is
2
about
rnA/cm which
is
less than
that estimated under AMI illu-
mination (3.5 rnA/cm
) .
This difference is due to the very small
2
lifetime of minority carriers in the film and substrate as well
as the reduction in the collection effeiciency of the photogene-
rated carriers in the space charge region. The variation of short
circuit current J open circuit voltage V efficiency Tj and
fill factor
FF
sc oc
as a function of composition x of the Zn Cd S/
0.55
ZnTe (~x~O.2) cells showed high open circuit voltage of
V for the composition x = 0.2 and short circuit current of
2
rnA/cm for x = 0.05. It should be noted that the maximum measured
value of 0.55 V for open circuit voltage is practical for solar
cell applications.