Optical and electrical properties of undoped ZnO films grown by

JOURNAL OF APPLIED PHYSICS
VOLUME 83, NUMBER 4
15 FEBRUARY 1998
Optical and electrical properties of undoped ZnO films grown
by spray pyrolysis of zinc nitrate solution
S. A. Studenikin,a) Nickolay Golego, and Michael Cocivera
Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Guelph,
Guelph, Ontario N1G 2W1, Canada
~Received 24 July 1997; accepted for publication 11 November 1997!
Undoped ZnO films were deposited by spray pyrolysis using aqueous zinc nitrate solution at
different substrate temperatures. The effect of the growth temperature on the structural, optical,
electrical, and relaxation properties has been studied. It was found that there was a critical
temperature T c 5180 °C below which the thermal decomposition to ZnO did not occur or was
incomplete. Films grown above T c showed strong preferred orientation of polycrystals along the
c-axis, while the films grown at T c or below showed a powder-like, non-oriented polycrystalline
structure when they were converted afterwards to zinc oxide by annealing. A slight increase of the
optical band gap was observed for as-prepared films as the substrate temperature was decreased near
the critical temperature. Annealing brought all the samples to the same band gap 3.30 eV measured
at a half height of the maximum absorption. After illumination, the steady-state photoconductivity
decayed very slowly with a time constant of about a week for as-grown samples. The steady-state
photoconductivity in daylight was very close to saturation. Steady-state photoconductivity in the
daylight can be as much as four orders in magnitude larger than the dark value. Annealing in
nitrogen at 400 °C brought all samples to the same conductivity of 10 23 (V cm!21 in daylight and
1024 ~V cm!21 in the dark. The photoconductivity transients were complicated and changed from
a power law to multiexponential time dependence after annealing. The data are discussed on the
basis of model in which hole traps located at the grain boundaries play the major role. © 1998
American Institute of Physics. @S0021-8979~98!04704-5#
I. INTRODUCTION
II. EXPERIMENT
Thin-film zinc oxide continues to attract attention because of its low toxicity and its many applications in solar
cell technology1 and as thin-film gas sensors,2–4 varistors,5,6
and a phosphor for color displays.7,8 A variety of methods
have been reported for the preparation of ZnO thin films. For
example, films have been deposited by thermal evaporation,9
rf-sputtering,3,10 chemical vapor deposition,11 laser ablation,12 and variations on these methods.13–16 In addition to
these techniques, spray pyrolysis has received a fair bit of
attention because of its simplicity and consequent economics
as it does not require a high vacuum apparatus. Another advantage of the spray pyrolysis technique is that it can be
adapted easily for production of large-area films, e.g., in the
case of display manufacturing. Previously zinc oxide films
have been prepared from spray solutions of zinc chloride,17,18
zinc acetate,19–23 and zinc acetylacetonate.24
Recently, it was reported25 that ZnO films can be deposited by spray pyrolysis in air at relatively low temperatures
from an aqueous solution of Zn~NO3) 2 . The main purpose of
the present work was a comprehensive study of the effects of
pyrolysis temperature on the decomposition of zinc nitrate to
form ZnO films as well as the structural, electrical, and optical properties of the as-deposited and annealed films.
The home-made spray pyrolysis apparatus used in this
work and the method were described previously.26,27 In brief,
an ultrasonic nebulizer created an aerosol, which was
sprayed vertically through a 0.9 mm diameter nozzle and
was rastered automatically across a Corning glass substrate
~25350 mm2) mounted on a heating block. Humid air was
used as the carrier gas, and the spray rate of the solution was
2–3 mL/h. Spraying times of 2–4 h provided films with
thicknesses between 0.2 and 0.4 m m. Thicker films can be
obtained in shorter times by increasing the concentration of
the solution. For all studies an aqueous solution of 0.1 M
Zn~NO3) 2 was used. A number of samples were annealed in
a nitrogen ambient at 400 °C in a tube furnace for 1 h before
physical characterization. The heating rate was 20 °C/min
and cooling took 2–3 h.
The samples were weighed before and after the heat
treatment to determine the mass loss. The structural and surface properties of the films before and after annealing were
determined by means of SEM ~Hitachi S570 scanning electron microscope!, x-ray diffraction ~Siemens D500 x-ray system using a 1.5418 Å Cu Ka source!, and a Sloan Dektak
surface profilometer. Optical absorption spectra were recorded at room temperature using a Shimadzu UV160U
spectrometer with a blank piece of 7059 glass in the reference beam. Conductivity of films was measured by means of
two-contact measurements using a Keithley 220 current
source and a Keithley 614 electrometer. For these measure-
a!
Permanent address: Institute of Semiconductor Physics, Siberian Branch of
Russian Academy of Sciences, Novosibirsk 630090, Russia.
0021-8979/98/83(4)/2104/8/$15.00
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© 1998 American Institute of Physics
J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
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FIG. 1. Percentage decrease in mass relative to the initial mass, caused by
annealing at 400 °C for films grown at different temperatures.
FIG. 2. Relative spray pyrolysis efficiency for growth of ZnO films from
zinc nitrate solution at different pyrolysis temperatures.
ments the samples were cut in 5310 mm2 rectangles, and
contacts were made with 1–2 mm strips using silver conductive paint from GC Electronics. Four-point probe measurements gave the same values.
In view of these results, the decomposition of zinc nitrate during spray pyrolysis is consistent with the formation
of oxides of nitrogen through some intermediate products
that can be written as follows:
H J
Zn~OH!2
III. RESULTS
A. Film deposition
The thermally induced decomposition of solid bulk zinc
nitrate was observed to start at 350 °C.28 However during
spray pyrolysis the decomposition of zinc nitrate was reported to begin at about 150–170 °C.25,29 This difference
may be due to lower stability of microcrystals relative to
bulk material. To better understand this decomposition process during spray pyrolysis, a set of samples were deposited
at temperatures between 130 and 400 °C in steps of 20–
40 °C. A measure of the completeness of the conversion
from the nitrate to the oxide was obtained by measuring the
decrease in the mass of the film after the as-deposited film
had been heated at 400 °C for 1 h in a nitrogen ambient.
Figure 1 shows the percentage decrease in mass relative
to the initial mass for the films grown at various temperatures. Three temperature regions were observed. Below
175 °C, annealing caused a large decrease in the mass, and
the monotonic decrease in the percentage change indicates
conversion to ZnO became more predominant as the substrate temperature was increased. Between 175 °C and
220 °C, the decrease was substantially less, amounting to no
more than about 7%. In this region, the nitrate was almost
completely converted to the oxide during spray pyrolysis. At
250 °C and above, annealing did not cause a decrease in
mass, indicating the conversion was complete at these temperatures. Based on molecular weights, the maximum decrease would be 57% for the conversion of pure, anhydrous
solid zinc nitrate film into zinc oxide.
Zn~NO3!2→
Zn2O~NO3!2
Zn~OH!~NO3!
→ZnO1yNw Ox 1zO2 .
~1!
Consistent with this suggestion are the color changes
observed during heat treatment of samples grown at temperatures lower than 175 °C. Initially after the 400 °C temperature had been attained the film was white; however, after 20
min the film became dark brown and, subsequently, transparent with a slight green color. The dark brown color is most
likely N2 O4 gas, which must diffuse out of the film to complete the process. Because it takes at least 20 min for pyrolysis of solid bulk zinc nitrate at 400 °C, the process is expected to take much longer at 250 °C. Furthermore, the
pyrolysis of zinc nitrate films resulted in cloudy ZnO films
whereas films prepared at T>220 °C were clear and transparent. As a result, spray pyrolysis at T>220 °C probably
occurred by way of small aggregates of zinc nitrate.
As indicated in the heat treatment studies above, films
prepared at temperatures of 180 °C or higher lost little or no
mass. Consequently, the stoichiometry and density were very
nearly the same at these temperatures, and it is possible to
compare the relative efficiency of the deposition at each temperature by comparing the ratio film thickness relative to the
number of milliliters of solution sprayed. These data are
shown in Fig. 2. It is seen that the ratio is constant within
10% over temperature range from 180 to 400 °C. Thus the
spray pyrolysis efficiency is very nearly the same for deposition at all temperatures in this range. In summary, there is
a critical temperature T c 5180 °C below which the pyrolysis
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J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
FIG. 3. SEM image ~20320m 2) of ZnO film grown by spray pyrolysis at
180 °C .
FIG. 4. X-ray diffractograms ~Cu Ka, l51.5418 Å! of ZnO films pyrolyzed
at different substrate temperatures.
of zinc nitrate to ZnO did not occur readily, and the deposition efficiency remained the same between 180 and 400 °C.
perature decreased. The main peak ~002! in diffraction spectra had full width at a half maximum intensity of about 0.008
rad which corresponds to polycrystal dimension d.20 nm,
estimated using the Sherrer’s formula.30 As a result, x-ray
and SEM data indicated that polycrystal dimensions were in
20–100 nm range that has been observed by different authors
for sprayed ZnO.21–24
B. Structural SEM and x-ray study
The films were very smooth to the eye and very difficult
to distinguish from the glass substrates. Figure 3 shows an
SEM image of a ZnO film grown at 180 °C. Films grown at
higher temperatures had similar images. Very small spots
that can be distinguished are polycrystals with dimensions
about 100 nm or less. In more thorough SEM and atomic
force microscope ~AFM! studies the polycrystallites were reported to be approximately 50 nm in diameter.25 A roughness
of about 10 m m in lateral direction could be seen in Fig. 3.
The same average length of the lateral roughness was obtained by means of Sloan Dektak surface profile measurements. All the films in the study had similar thicknesses of
about 0.3 m m. Vertical roughness of the films was about 0.03
m m. The measured values of the horizontal and vertical
roughness were consistent with the deposition model in
which water droplets containing 0.1 M of Zn~NO3) 2 evaporate and then pyrolyze at the hot substrate. Using this concentration and estimating the size of the droplet gave a 10
m m ZnO disk of 0.01 m m thickness.
Examples of x-ray diffraction patterns of three ZnO
films grown at different temperatures are shown in Fig. 4.
The theoretical pattern for ZnO wurtzite structure (a
53.2495 Å, c55.2069 Å! also is depicted in this figure. It is
evident from the figure that ZnO crystallizes in wurtzite type
structure during pyrolysis. Films grown at critical temperature T c 5180 °C has a powder like pattern with no preferred
orientation. The same pattern was observed on the samples
grown at lower than the critical temperature and converted to
ZnO by annealing at 400 °C. Films grown at higher than
200 °C exhibit predominantly one peak showing a very high
level of orientation along the c-axis perpendicular to the substrate. The degree of orientation decreased slightly as tem-
C. Optical band gap
Figure 5 illustrates that an as-deposited ZnO film grown
below the critical temperature ~T c 5180 °C! did not exhibit
any absorption in the UV/visible range. An absorption spectrum appeared after this film was annealed at 400 °C, con-
FIG. 5. Absorption spectra of as-grown and annealed zinc oxide film deposited below the critical temperature.
J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
FIG. 6. Absorption spectra of ZnO film pyrolyzed at T g 5210 °C and annealed under the following conditions: in N2 ~400 °C!, in air ~400 °C! and in
N2 ~520 °C!.
sistent with the conclusion that zinc nitrate films were
formed below 180 °C and subsequently converted to ZnO by
annealing. Figure 6 shows the typical absorption spectrum
for films grown between 180 °C and 360 °C. The distinct
absorption edge existed in both as-prepared and annealed
films. Noticeable shifts of the edge were observed after annealing in nitrogen at 400 °C, even for samples grown at
360 °C. Subsequent annealings in air at 400° and again in
nitrogen at 520 °C did not change the absorption.
To determine the energy gap, most authors use the
model for direct interband transitions:
a n•h n 5C ~ h n 2E g ! 1/2,
~2!
in which h n is photon energy, n is index of refraction, and a
is the absorption coefficient. In this approximation, ( a nh n ) 2
should be a linear function of h n . This model may not be
suited to wide band gap materials because the optical absorption band edge is strongly disturbed by a coulombic electronhole interaction leading to the excitonic effect. For all of the
films in this study, (ahn n ) 2 was a linear function of energy
over a range of only about 0.05 eV, which is very small
compared to the band gap. Because of the sharpness of the
absorption edge, the photon energy at the half height of the
maximum absorption gave a band gap value very close to the
extrapolated one. Consequently the half height values are
presented in Fig. 7, which shows the optical band gap of
ZnO films as a function of growth temperature, and after
annealing at 400 °C. It is seen in this figure that as-prepared
films grown at or below 220 °C had a slightly bigger band
gap. Annealing at 400 °C brought all films to the same value
~E g 53.30 eV!, except for those films grown below the critical temperature. The lower values for these films may be
related to their cloudy appearance. An analogous blue shift
of the band edge was observed for ZnO films prepared by
SILAN ~successive ion layer adsorption and reaction!.10 The
observed shift might be due to the presence of unreacted
2107
FIG. 7. Optical band gap of ZnO films measured at a half height of the
maximum absorption as a function of the deposition temperature.
intermediate products if the growth temperature is not sufficiently high. Their presence could cause mechanical strains
and structural distortions, which are known to alter the band
gap and the excitonic effect.
D. Conductivity
Figure 8 presents the conductivity of as-deposited films
in daylight in the lab measured either by two-contact or fourpoint probe methods. The difference in time between these
two measurements was more than 1 month, and the good
agreement between two sets of measurements indicates the
films were stable under these conditions. In general, the conductivity of the as-deposited films in daylight decreased as
FIG. 8. Conductivity of as-deposited ZnO films in daylight in the lab measured either by two-contact and four-point probe methods.
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FIG. 9. Conductivity of as-deposited ZnO films in daylight and in the darkness 8 days after removing the light.
the substrate temperature increased from 180 to 400 °C.
Films grown below T c 5180 °C were dielectric, which supports conclusions presented above concerning the poor conversion to ZnO below this temperature.
In daylight the conductivity of the as-grown film was, in
fact, the steady-state photoconductivity, which was very
close to saturation and had a very long relaxation time, of the
order of days. Saturation was tested using an arc lamp to
provide higher intensity, and the steady state photoconductivity did not increase substantially relative to daylight intensity. For as-deposited samples prepared at various temperatures, Figure 9 illustrates the photoconductivity in daylight
and the dark conductivity at room temperature 8 days after
removing the light. As seen in this figure, the light and dark
conductivities differed by up to four orders of magnitude,
with the lowest dark values around 1028 V 21 cm21. The
magnitude of this change depended on the preparation temperature, with the T c 5180 °C showing the smallest decrease.
As might be expected the rate of relaxation from the steadystate photoconductivity could be increased by raising the
temperature. Thus, the steady-state photoconductivity of the
as-grown samples decreased to ~2–6!31029 V 21 cm21 in 2
h heated at T5100 °C in the dark. Only the sample grown at
T c 5180 °C did not exhibit a temperature effect over this
range. Relaxation back to the stationary dark values shown
in Fig. 9 took several days. A similar effect was observed
also for another wide band gap material, TiO2 . 31
Annealing at 400 °C in nitrogen brought all the films
~including the dielectric films grown below T c ) to the same
dark conductivity of 1024 V 21 cm21 and about 1.331023
V 21 cm21 in daylight. Consequently, except for the 180 °C
film, the dark conductivity increased for films grown at all
temperatures. In comparison to the as-deposited films, annealed samples had substantially shorter relaxation times,
reaching the equilibrium ‘‘dark’’ state in about 10 h after
illumination was stopped. After annealing, the 100 °C treat-
Studenikin, Golego, and Cocivera
FIG. 10. Photoconductivity relaxation of ZnO film ~T g 5360 °C! annealed 1
hour in nitrogen at 400 °C. Experimental data are fitted with four exponents
by sequential subtraction method.
ment did not influence their dark conductivity. Daylight illumination increased the conductivity by about an order of
magnitude for all annealed samples.
E. Photoconductivity relaxation
These studies employed white light from a Welch Allyn
arc lamp with an intensity of 250 W/m2 at the sample, and
about 0.5% of total power had photon energy h n .E g ~ZnO!.
About one hour was required to reach the steady-state photoconductivity at this intensity. The relaxation rate in the
dark depended on the treatment of the sample, e.g., for an
as-grown sample (T g 5360°) semi-logarithmic and doublelogarithmic plots indicated that the relaxation did not have an
exponential dependence on time. A double-logarithmic plot
indicated the relaxation had two power-law time dependencies with the exponent changing from 20.75 to 21.0 after
103 s of the relaxation. As discussed above, the conductivity
decreased about four orders in magnitude over a period of
days, and the most rapid decrease from 1024 to 10 26
V 21 cm21 occurred in the first 5 h. The stationary dark value
of 231028 V 21 cm21 was reached in about 1 week at room
temperature.
After annealing in nitrogen at 400 °C, the samples relaxed more rapidly and took about 10 h to reach the ‘‘dark’’
equilibrium conductivity. An example of the conductivity relaxation for annealed sample (T g 5360°) is plotted in Fig.
10. This decay consists of four exponential transients, which
could be separated unambiguously by sequential subtraction
~starting with the slowest conductivity transient! to give time
constants 1.233104, 2.223103, 534 and 135 s ~Fig. 11!. Except for the slowest transient, each time constant was obtained by an exponential fit of the D s i data over a range of
two orders of magnitude or more.
J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
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FIG. 11. Four exponential transients obtained by sequential subtraction ~starting with the slowest conductivity transient! to give time constants ~a! 1.23
310 4, ~b! 2.223103, ~c! 534, and ~d! 135 s.
IV. DISCUSSION
As presented above, the films are polycrystalline with
grain dimensions lying between 20 and 100 nm. Because
grain boundary states can create depletion layers in grains,
they can cause barriers to carrier transport. This aspect has
been discussed earlier in regard to the dark carrier transport
of polycrystalline ZnO having large grain size ~resulting in a
large barrier!32 and small grain size ~resulting in a small
barrier!.33 Since the size of the barrier is related to the
amount of depletion in the grain, the thickness of the depletion layer will be addressed first. Undoped ZnO films are
known to exhibit n-type conductivity that is usually attributed to oxygen vacancies in the crystallites. In the crystallite
the depletion width can be estimated according to:
W5
S
2 e 0e V i
eN D
D
1/2
~3!
in which e 0 is the permittivity of free space, e is the dielectric constant ( e 58.59 for ZnO!,34 N D is the concentration of
uncompensated donor impurities, and V i is the band bending.
In the dark, N D does not equal the equilibrium free carrier
concentration because of partial or complete depletion of the
crystallites. However, N D can be estimated from the steady
state photoconductivity at saturation. As discussed above,
saturation of the photoconductivity was confirmed by the
nearly identical values found for illumination in daylight
(I'1 W/m2) and with the arc lamp (I5250 W/m2) 0.073
and 0.1 V 21 cm21, respectively. Saturation of the photoconductivity implies the bands are flat, and the conductivity is
governed by the concentration of donors N D , as discussed
below. Using a carrier mobility m ' 15 cm2/V s,21,25,35 we
get N D < 431016 cm23. The calculated value of W depends
also on the value of V i . The large difference between the
light and dark conductivity indicates this value should be
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J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
substantial. However, even a relatively small value of V i
'0.1 eV gives a total depletion width of 100 nm, considering two interfaces for the grain. Thus, even this small barrier
causes a depletion width that is twice the grain size, and
most likely the grain is fully depleted, especially since the
trap energies are likely to be larger as described below.
Although the grains are probably depleted in the undoped films, the prospect of some band bending due to partial surface field screening by the charged impurities should
be considered. For nearly depleted spherical grains with a
uniform impurity distribution that is much larger than the
free carrier concentration, the Poisson equation gives the
band bending V i :
V i5
eN D r 2
,
6 e 0e
~4!
in which r is the radius of the crystallite. For N D <4
310 16 cm23 and r'25 nm, the bend bending is V i ' 10
meV, which is comparable to the thermal energy at room
temperature. Consequently crystallites are fully depleted
with almost flat bands. These results support the conclusion
that the conductivity is completely governed by charging the
grain boundary states. This value of V i is comparable to that
calculated for barrier of thin film ZnO from the temperature
dependence of the mobility.33 Consequently, fully depleted
crystallites are consistent with this mobility study.
The strong dependence of the dark conductivity on the
growth temperature ~Fig. 10! indicates that the density of
uncompensated acceptor states at the grain boundary increased as the pyrolysis temperature increased. These results
are consistent with an increase in Zn vacancies as the films
were grown at higher temperatures in air. The change in
conductivity due to heat treatment in nitrogen at 400 °C depended on the film growth temperature. Consider the high
temperature films, which exhibited an increase in conductivity after heat treatment. Although bulk diffusion of oxygen is
possible, little oxygen was present in the nitrogen ambient.
Furthermore, incorporation of oxygen in the bulk should result in a decrease in conductivity rather than an increase
because the decrease in oxygen vacancies ~donors! would
cause a decrease in the free carrier density. An explanation
for the increase in conductivity after annealing could be that
heat treatment removed oxygen from the grain boundary and
caused a decrease in the density of acceptor states at the
grain boundary, resulting in the capture of a smaller number
of bulk electrons by these acceptor states. Support for this
conclusion is that ZnO at 400 °C has a pale green color indicative of an oxygen deficiency.7 In the case of the 180 °C
film, annealing caused a small decrease in the dark conductivity. The higher conductivity of the film grown at this temperature may be due to passivation of grain boundaries by
gettering of incomplete reaction products at the grain boundaries. Annealing brought all samples to the same conductivity, indicating the same density of grain boundary states.
This grain boundary state model can also be used to
describe the photoconductivity and its transients. The effect
of illumination was to reverse the process described above
by neutralizing negative charge in the surface states. The use
of optical filters confirmed that only photon energy greater
than the band gap caused the photoconductivity. Electronhole pairs created in the bulk of crystallites mainly recombine via radiative or Shockley-Read-Hall mechanisms. Holes
also can be captured by deep traps at the grain boundaries,
resulting in an increase in number of free electrons which are
unable to recombine with these holes. When the photoconductivity is saturated, the bands are essentially flat, and the
free electron density is equal to the donor density because
hole traps play the role of sensitizing centers36 that prevent
holes from recombining with electrons. After excitation has
ceased, the holes slowly escape from the traps and recombine
with free electrons, and the conductivity slowly decreases as
the electron density drops. Because this process is thermally
activated, deeper levels have slower relaxation rates. The
long relaxation times are consistent with a large density of
deep traps. The shorter relaxation times found for the annealed films is consistent with a decreased density of deep
interface states.
In the case of the annealed films ~Fig. 10!, the multiexponential relaxation indicates discrete levels participated in
the relaxation process. The time required to escape from a
trap at energy E t can be described by the well-known
expression36
t5
S
D
E t 2E v
1
exp
,
n
kT
~5!
in which (E t 2E v ) is the hole trap energy above the valance
band ~we consider that the main effect is due to captured
holes!, n is the attempt-to-escape frequency, which is usually
accepted to be the lattice vibration frequency ~1013 s21 ). In
the case of fast recombination, hole re-trapping is insignificant after holes have escaped from the traps, and the excess
free electron density is Dn5SDp t , in which summation
runs over all hole traps. The rate of change of density of
trapped holes in states at energy E t is
pt
dpt
52 ,
dt
t
~6!
in which t is given by Eq. ~5!. Because the samples have
been illuminated for a long time before the light is shut off,
the traps have been filled completely with holes so the free
electron density at saturation can be equated to hole trap
densities, and the trap density N i was obtained by fitting the
experimental s transient curve using
s 5e m
(i N i e 2t/ t ,
i
~7!
in which t i is the release time from the trap related to its
energy by Eq. ~5!.
The fitting procedure of the transient for the annealed
sample involved first and exponential fit of the slowest decay
followed by successive subtraction as illustrated in Fig. 11.
This procedure worked unambiguously because of the large
differences in the time constants. Parameters base on these
results ~Table I! were calculated using m 515 cm2/V s. It is
seen from the table that the traps are located about one-third
of the band gap from the valence band as has usually been
observed for sensitizing centers in bulk semiconductors.36
J. Appl. Phys., Vol. 83, No. 4, 15 February 1998
Studenikin, Golego, and Cocivera
TABLE I. Parameters of hole traps in annealed ZnO film ~T g 5360 °C!
obtained by fitting the experimental transient curve in Fig. 10 using Eq. ~7!
and the electron mobility m 515 cm2/V s.
a
sI
(V 21 cm21)
ti
~s!
Ni
~cm23)
Ei
~eV!
1.031024
`a
4.231013
•••
2.0931024
1.233104
8.831013
0.99
1.0331023
2.223103
4.331014
0.94
2.2831023
5.343102
9.631014
0.91
3.831024
1.353102
1.631015
0.87
Equilibrium state in the dark.
This analysis cannot distinguish between traps in the bulk
and at the grain boundaries. On the other hand, grain boundary states are consistent with the large photosensitivity observed for these films as well as the large effect of moderate
annealing on the conductivity and relaxation kinetics. Also,
these values of E t are consistent with the grain boundary
barriers found for large grain ZnO in the dark.32 Finally, the
high sensitivity of polycrystalline ZnO to the ambient annealing atmosphere indicates that the analyses used in the
present study could be useful in gaining insight into mechanisms and energy level changes associated with gas sensors
such as ZnO films. This aspect is currently under study.
V. CONCLUSION
Structural, optical, conductivity, and relaxation properties of undoped ZnO films deposited by spray pyrolysis at
different temperatures have been studied. The critical temperature for spray pyrolysis of zinc nitrate solutions was
found to be T c 5180 °C. The films grown above T c show
strong preferred orientation of polycrystals along the c axis,
while the films grown at T c or below and converted to zinc
oxide by annealing show powder-like non-oriented polycrystalline structure. A slightly larger optical band gap was observed for films grown between 180 and 240 °C. Annealing
brought all the samples to the same band gap, 3.30 eV. Photoconductivity showed very slow relaxation kinetics after illumination had been stopped, and the time constant was
about a week for as-grown samples and about 10 h for
samples annealed in nitrogen. Conductivity of as-deposited
films in the daylight and in the darkness could differ by more
than four orders in magnitude. Annealing in nitrogen at
400 °C brought all samples to the same conductivity, which
was 1023 V cm21 in daylight and 1024 V cm21 in the dark.
The relaxation had a complicated character, which changed
from a power-law time dependence for as-grown films to a
multiexponential transient for annealed samples. The transients were used to extract information about trap densities
and their energy distribution.
2111
ACKNOWLEDGMENTS
This work was supported in part by a grant to M.C. from
the Natural Sciences and Engineering Research Council of
Canada. One of the authors ~S.S.! would like to thank Serguei Grabtchak ~Department of Chemistry! for fruitful remarks during manuscript preparation.
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