Extremely low temperature growth of ZnO by atomic layer deposition

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Extremely low temperature growth of ZnO by atomic layer deposition
E. Guziewicz,1,a I. A. Kowalik,1 M. Godlewski,1,b K. Kopalko,1 V. Osinniy,1 A. Wojcik,1
S. Yatsunenko,1 E. Lusakowska,1 W. Paszkowicz,1 and M. Guziewicz2
1Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
2Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
Received 30 July 2007; accepted 19 November 2007; published online 11 February 2008
We report on the zinc oxide ZnO thin films obtained by the atomic layer deposition ALD method
using diethyl zinc and water precursors, which allowed us to lower deposition temperature to below
200 C. The so-obtained "as grown" ZnO layers are polycrystalline and show excitonic
photoluminescence PL at room temperature, even if the deposition temperature was lowered down
to 100 C. Defect-related PL bands are of low intensity and are absent for layers grown at 140
- 200 C. This is evidence that extremely low temperature growth by ALD can result in high quality
ZnO thin films with inefficient nonradiative decay channels and with thermodynamically blocked
self-compensation processes. (c) 2008 American Institute of Physics. DOI: 10.1063/1.2836819
to the high level of n-electron doping. This point was not
explored seriously until now since most of the efforts were
Zinc oxide ZnO crystals and structures have attracted
concentrated on obtaining ZnO films of high structural qual-
rapidly increased attention in the last few years. ZnO is a
ity, which requires higher growth temperature.
prospective material for a number of applications, such as for
On the other hand, the potential for flexible, lightweight
ultraviolet light emitters, gas sensors, piezoelectric transduc-
and mechanically strong electronics based on plastic sub-
ers, transparent electronics, surface acoustic wave devices,
strates has motivated intensive research on materials which
solar cells, and spintronics, when doped with transition metal
can be grown at low temperature. Plastic substrates, like
elements.1-5 This variety of applications derives from ZnO
polyethylene terephthalate, limit device processing to less
specific properties. This is a wide band gap semiconductor
than 150 C,7 which makes silicon-based electronics incom-
Eg 3.4 eV at room temperature with a high thermal and
patible with plastic substrates. In turn, organic semiconduc-
chemical stability. A large exciton binding energy 60 meV,
tors degrade under normal atmospheric conditions8 and re-
more than twice higher than exciton binding energy in GaN
quire application of protection layers. In this aspect, the
predestines ZnO for optoelectronic applications.
possibility of low-temperature processes make ZnO the at-
Conductivity of this material is strongly defect related
tractive material for transparent thin-film transistors TFTs .
and ranges from metallic to insulating. The control of defects
In addition, high electron mobility expected for ZnO-based
and associated free carrier concentration is of great impor-
TFT attracts considerable attention as it results in higher
tance for most of the earlier mentioned applications. Intrinsic
drive currents and a higher operating speed.9
free carrier concentration in bulk ZnO not intentionally
The most common methods used to grow zinc oxide thin
doped at room temperature should be at the level of
106 cm-3, which is about 11-15 orders of magnitude less
than values reported for ZnO material grown by different
sputtering.12-14 The former method requires a growth tem-
techniques Ref. 1, and references therein . A very high level
perature of 350- 450 C and is not appropriate for industrial
of doping in nominally undoped ZnO has been attributed to
purposes.10,11 Radio frequency magnetron sputtering, when
zinc interstitials, oxygen vacancies, and/or hydrogen Ref. 1,
applied at low temperature, requires postgrowth annealing
and references therein . These point defects, leading to
and does not lead to desired electrical parameters.12-14 The
heavily n-type doping, introduce very shallow donor levels at
atomic layer deposition ALD method has several advan-
10-50 meV below the conduction band minimum and are the
tages over other methods in controlling thin film character-
source of high free carrier concentration. However, the exact
istics and interface states.
value of free carrier concentration found in ZnO thin film
There are only a few papers which report on low-
strongly depends on the growth method and parameters used
temperature ZnO films fabricated by ALD.15-17 These papers
in the process. Probably the most important factor which
deal mainly with the growth window and structural proper-
influences the ZnO thin film electric parameters is growth
ties of ALD-ZnO thin films fabricated with diethylzinc
temperature as it controls the thermodynamics of the growth
DEZn and water precursors and are focused on growth
process.6 High temperature deposition processes can inten-
temperature above the limit defined by low thermal budget of
sify the oxygen vacancy formation and contribute in this way
organic electronics. Moreover, the light-emitting properties
of "as-grown" ALD-ZnO films are not good, which is as-
cribed to low temperature process.15,16 Ultraviolet band edge
Electronic mail: [email protected]
emission was observed only for ZnO layers after postgrown
Also at: Department of Mathematics and Natural Sciences College of Sci-
ence, Cardinal S. Wyszyski University, Warsaw, Poland.
annealing at temperature of 600 C and above.15,16 There is
0021-8979/2008/103 3 /033515/6/$23.00
103, 033515-1
(c) 2008 American Institute of Physics

Guziewicz et al.
J. Appl. Phys. 103, 033515 2008
also no systematic study on the relation between electrical
properties of undoped ZnO films and growth temperature.
Free carrier concentration was investigated for films grown
at 170 C and above17 and reported as 1020 cm-3, which is
to high for most applications.
We report here on ZnO films grown by the ALD method
with DEZn and water precursors at temperature ranging from
100 to 200 C. We did not perform any postgrowth treat-
ment as such a procedure is incompatible with the low ther-
mal budget requirements of hybrid organic/inorganic struc-
We have grown the ZnO thin films in the ALD process at
very low temperature using Savannah-100 reactor from
FIG. 1. The growth rate of the ZnO film as a function of deposition
Cambridge NanoTech. The atomic layer deposition technique
is frequently regarded as a kind of chemical vapor deposition
CVD or metalorganic chemical vapor deposition method.
In fact, both these methods of thin film deposition differ
to 180 C. Inside the growth window we achieved a growth
significantly. In the ALD process precursors are introduced
rate of 1.8- 1.9 A per cycle which is similar to the growth
sequentially into the growth chamber and cycles of precursor
rate reported by the other groups.15,17 Such a growth rate
dosing are interrupted by periods of purging with an inert
seems to be near the limit that can be achieved in the ALD
gas. Therefore, chemical processes in a volume of the reac-
process with a DEZn precursor.
tion chamber are effectively avoided and reactants meet only
at the surface of the growing film. This is a serious superi-
ority of the ALD over the CVD method which allows using
very reactive precursors and thus a lower deposition tem-
Structural properties of the ZnO samples obtained in the
three series described earlier were determined by atomic
In the series of deposition processes we adopted water
force microscopy AFM and x-ray diffraction XRD . Opti-
and DEZn Zn C
cal properties were investigated by room temperature PL and
2H5 2
precursors and high purity nitrogen
as a purging gas. ZnO thin films were deposited on glass and
electrical properties were obtained from Hall measurements.
silicon 100 substrates in the processes in which both pre-
cursors were kept at room temperature, whereas the substrate
A. Surface morphology and structural quality
temperature was varied between 90 and 200 C. Such ex-
tremely low growth temperature is of great importance for
All the so-obtained ZnO films show atomically flat sur-
some special applications as was described in the previous
faces, as was derived from the AFM studies. The root-mean-
paragraph. Low temperature growth is also a basic factor
square rms of surface roughness varied between 4 and 0.4
which limits the processes of spinodal decomposition and
nm depending on the growth temperature. For lower deposi-
foreign phases formation when transition metal atoms are
tion temperature
between 90 and 140 C
rms values
incorporated into the ZnO lattice.18,19 We concentrated on the
strongly decrease with the temperature. The rms value for
possibility of controlling free carrier concentration by lower-
ZnO layers grown at 150 C and above is below 1 nm Fig.
ing growth temperature, i.e., by blocking the formation of
2 and the surface morphology does not further improve with
some intrinsic defects such as vacancies.
the temperature.
Here we compare results obtained for three series of
Structural properties of ZnO layers were obtained from
ZnO-ALD processes in which we applied very short pulsing
XRD in a full angular range using an X'Pert MPD diffracto-
and long purging times. The pulse time of the water precur-
meter. The investigated angular region 2
was 25 -70
sor of 15 ms was the same for all three deposition series,
and included the ZnO related diffraction maxima corre-
while the purging time after the H2O precursor was 8 s in
sponding to 10.0 , 00.2 , 10.1 , and 11.0 crystallographic
series I and II and 20 s for series III. The pulsing time for the
orientations. The XRD spectra show the polycrystalline
DEZn precursor was 20 ms for series I, 90 ms for series II,
structure of ZnO films grown at low temperatures Fig. 3 .
and 60 ms for series III. The purging time after DEZn was
We found that the relative intensities of the individual peaks
always 8 s. Such a long purging time can be applied in some
depend on the deposition temperature and other growth pro-
technological processes because the ZnO layer thickness is
cess parameters such as pulsing and purging times. We ob-
usually very low. We performed deposition processes with
served that long purging times after water precursor and
500 and 1000 cycles and the thickness of ZnO films was
growth temperatures above 140 C privilege 00.2 crystal-
between 70 and 200 nm.
lographic orientation, i.e., the situation when the c axis is
Under the earlier conditions we estimated that the
perpendicular to the layer surface. The short pulse time of
growth window of ALD process see Fig. 1 ranges from 100
the DEZn precursor and short purging time after water pre-

Guziewicz et al.
J. Appl. Phys. 103, 033515 2008
FIG. 2. Color online The AFM image of a 2
m region of the ZnO/Si layers from series III and deposited at a 100, b 130, and c 160 C.
cursor privilege
crystallographic orientation, i.e.,
defect centers. The latter means that intensity of the edge PL
growth when the c axis is parallel to the surface Fig. 3 .
usually anticorrelates with intensity of some "deep" defect
Relative good structural quality of our ZnO films was
related bands.
quite an unexpected feature. It was generally assumed that
Temperature dependence of the PL is also an important
low temperature LT films should be rather amorphous. In
indication of film quality. At low temperature excitons are
contrast, we found that by selecting long purging time films
usually localized by potential fluctuations or trapped at im-
grown at very low temperature are still ordered and are poly-
purities in ZnO predominantly donor impurities commonly
present in thin films obtained by various methods. Such ex-
citon localization or trapping helps to avoid exciton migra-
tion to the centers of nonradiative decay. The edge PL is then
B. Optical properties
readily observed. Once the temperature is increased excitons
Photoluminescence is a commonly used characterization
become mobile and can approach centers of nonradiative de-
method of layers grown by different techniques. Generally
cay. Therefore, observation of the edge PL at room tempera-
speaking, the most important observation is the detection or
ture is so difficult in most of the cases. On the other hand,
not of the PL at the spectral region close to the band gap
observation of edge PL leads to the statements on the high
value called further on "edge" PL . This PL is only seen in
quality of the obtained films. For the purpose of the present
films of good structural quality and also at relatively low free
work we also looked for deep defect related bands, the in-
carrier concentration. The latter relates to efficient Auger
tensity of which correlates with a high doping level with
processes of nonradiative recombination in which energy of
unintentional impurities.
the recombination of free excitons is transferred to free car-
PL experiments presented in this study were performed
riers, which are then excited high into continuum states of
on a set of as grown ALD-ZnO samples deposited at a tem-
the conduction band in n-type samples or valence band in
perature range of 90- 200 C. PL investigations were carried
p-type samples . These processes can totally kill edge PL in
out at room temperature using SM2203 Spectrofluorymeter
metalliclike samples.
with built in two double monochromators, 150 W high pres-
As already mentioned, ZnO has an atypically large value
sure Xe lamp as the excitation source, and the R-928
of free exciton binding energy, which is more than two times
Hamamatsu photomultiplier for the PL detection.
larger than the kT value at room temperature. As a conse-
In most of the cases studied the PL spectra consist of two
quence free excitonic PL can be seen in ZnO up to room
bands see Fig. 4 . One of these is the so-called edge emis-
temperature. However, this is only possible in samples of a
sion likely with free excitonic contribution located near the
high quality, in which nonradiative processes are inefficient
energy of the band gap, i.e., in the vicinity of 384 nm. In the
and in which carriers are not dominantly trapped by some
red and green spectral regions 550-750 nm the defect re-
FIG. 3. The results of XRD measure-
ments of ZnO/Si films grown at a
similar temperature but different depo-
sition and purging time series I and

Guziewicz et al.
J. Appl. Phys. 103, 033515 2008
FIG. 4. left PL observed at room temperature RT for the ZnO layer from series III grown at 100 C on Si substrate right PL observed at RT for the LT
ZnO layer from the series II grown on the Si substrate at 140 C.
lated emission broadband appears, but only for some of stud-
emissions is still disputed, as reviewed recently in Refs. 2
ied films. The intensity and full width at half maximum
and 20. For example, the green ZnO PL seen only in thin
FWHM of the edge PL strongly depends on growth condi-
films grown at higher temperatures was first "unambigu-
tions. First of all, this PL is stronger and narrower for lower
ously" attributed to copper impurities. Then this PL was re-
temperatures of the growth in thicker films and increases
lated to the oxygen vacancy VO ,21-26 i.e., the green PL
with increasing temperature, reaching optimum growth at a
would be an analog of F center emission in ionic crystals.
temperature of about 160- 180 C see Fig. 5 , but also for
However, some results suggest that the green PL may as well
the selected growth conditions and for the Si substrate. For a
be related to another native point defect, to a zinc vacancy
temperature above 160 C FWHM of the edge PL is identi-
VZn .27-30 If the latter identity is correct, weakness of this PL
cal for all types of films, indicating similar microstructure
in the spectra obtained for our films indicates that by lower-
distribution of grain sizes and strain conditions in the stud-
ing the growth temperature we can practically block the self-
ied series of LT ALD ZnO films.
compensation mechanism in our films. Vacancies are not
The broad PL band is clearly due to an overlap of two or
even three known ZnO defect related bands--ZnO red PL,
In the present study we also found that the relative in-
ZnO yellow, and ZnO green PL. Origin of these deep PL
tensity of the edge PL emissions depends on the purging time
FIG. 5. a , b Intensity of the band edge PL observed at RT for the two series of LT ZnO layers--series II
a left
and series III
b right grown by ALD
on the Si substrate at specified temperatures.

Guziewicz et al.
J. Appl. Phys. 103, 033515 2008
TABLE I. The results of Hall measurements for ZnO layers grown with the following growth parameters: time
H2O = 15 ms, purging time= 20 s, time DEZn = 60 ms, and purging time= 8 s.
Growth temperature
cm2 / V s
-1 cm-1
after supplying the water precursor. From our secondary ion
perature where edge PL appears only after annealing.31 Fur-
mass spectrometry measurements we found that OH groups
thermore, for ZnO layers grown at temperatures between 130
are abundant in such films in particular in those grown with
and 200 C we did not observe defect related bands in the
short purging times , which suggests that OH groups are very
PL spectra. This is evidence that extremely low temperature
effective centers of nonradiative recombination in ZnO.
growth by ALD can result in the high quality ZnO thin films
with inefficient nonradiative decay channels and with ther-
C. Electrical properties of LT ZnO films
modynamically blocked self-compensation processes. Elec-
trical properties of these films were only briefly summarized
Electrical parameters of LT ZnO films were obtained
here and will be described in detail elsewhere.
from the Hall effect measurements, which were carried out
By comparison of structural, optical, and electrical prop-
by the direct current dc four probe method at room tem-
erties of our LT ZnO films we noticed that LT ZnO growth
perature in the magnetic field up to 1.3 T. The good ohmic
correlates with good structural and optical properties and
contact was ensured by indium soldering the thin gold wires
also with a low doping level, which in some films was below
to the sample surface. It is observed the linear current-
1017 cm-3 for as grown samples.
voltage characteristics of these contacts in the wide range of
Summarizing, the present work opens a new way of
the current values. The Hall effect measurement setup con-
achieving controlled electrical properties of ZnO films. An
sists of the current source Keithley current/voltage source
unintentional level of n-type doping was reduced by two
238 , the voltmeter Keithley multimeter 2000 , and the tem-
orders of magnitude by reducing the growth temperature by
perature controller Lake Shore 331 . The typical values of
about 100 C from 200 to 100 C and selecting a rela-
electrical current flown through the sample were Idc= 0.1
tively long, as for ALD processes, purging time. In our opin-
- 1 mA for samples with resistance about 1 k
low resis-
ion these very encouraging properties of extremely LT ZnO
tance samples and Idc= 0.1- 1
A for 1 M
and greater
films open chances for several new applications, including
high resistance samples .
formation of hybrid polymer/ZnO heterostructures, where LT
For all investigated ZnO samples free carrier concentra-
growth is crucial considering the temperature stability of
tion varied between 1019 and 1017 cm-3 and strongly corre-
conductive p-type polymer films.
lated with deposition temperature, i.e., low free carrier con-
centration and low conductivity were observed for samples
grown at lower temperature. The lowest observed free carrier
concentration measured for as grown samples was achieved
The work was supported by Polish Grant SPUB No. 180/
for samples grown at 90- 100 C and was at the level of 2
6.PR UE/2006/7
6 Framework Programme-priorytet 2
1017 cm-3.
and European Project No. FP6 026714 VERSATILE.
The best electrical parameters, i.e., the lowest free car-
rier concentration we obtained for ZnO films grown with a
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