Effect of substrate temperature on the structure and optical properties of ZnO
thin films deposited by reactive rf magnetron sputtering
Sukhvinder Singh a , R.S. Srinivasa b , S.S. Major a,⁎
a
b
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India
Abstract
Zinc Oxide films were deposited on quartz substrates by reactive rf magnetron sputtering of zinc target. The effect of substrate temperature on
the crystallinity and band edge luminescence has been studied. The films deposited at 300 °C exhibited the strongest c-axis orientation. AFM and
Raman studies indicated that the films deposited at 600 °C possess better overall crystallinity with reduction of optically active defects, leading to
strong and narrow PL emission.
Keywords: ZnO; rf magnetron sputtering; Microstructure; Optical properties
1. Introduction
ZnO is a versatile, wide band gap semiconductor with large
exciton binding energy (60 meV) and interesting piezoelectric
and ferroelectric properties. Its high chemical and thermal
stability and abundance make it an attractive material for a wide
variety of applications, such as, UV emitters and detectors,
SAW devices, gas sensors and transparent conducting electrodes [1]. Several growth techniques, such as, spray pyrolysis,
sputtering, pulsed laser deposition, MBE and MOCVD have
been extensively used for the deposition of un-doped and doped
ZnO films, among which, sputtering has been the most widely
used. Most of this work has been reviewed in Refs. [1] and [2].
High quality epitaxial films of un-doped ZnO have usually been
deposited on single crystalline substrates, such as sapphire by
MBE [3,4], MOCVD [5,6], PLD [7,8] and sputtering [9–11].
However, reports of sputtering of ZnO films on amorphous
substrates such as quartz and glass with properties comparable
to those of epitaxial films are limited [12–14]. In this work, we
report the sputtering of un-doped ZnO films of high optical
quality deposited on fused quartz substrates. In particular, the
⁎
effect of substrate temperature on the crystallinity and band
edge luminescence has been studied.
2. Experimental details
ZnO films were deposited on quartz substrates by reactive rf
magnetron sputtering. A 99.9% pure Zn target of 3-in diameter was
used. The target to substrate distance was 55 mm. The base
pressure was 1 × 10− 5 mbar. The flow rates of argon (24 sccm) and
oxygen (6 sccm) were controlled by mass flow controllers.
Deposition was carried out at a working pressure of 10− 2 mbar,
after pre-sputtering with argon for 10 min. The sputtering power
was maintained at 400 W during deposition. The depositions were
carried out in the temperature range of room temperature to 600 °C.
X-ray diffraction (XRD) studies were performed with a
PANalytical X'Pert PRO powder diffractometer using Cu Kα
radiation. Specular transmittance (T) and reflectance (R)
measurements at normal incidence were carried out with a
PerkinElmer Lambda-950 UV/Visible spectrophotometer.
Photoluminescence (PL) measurements were carried out at
room temperature using a 325 nm He–Cd laser and a JOBIN
YVON HR-460 monochromator. Raman spectra of the films
were recorded at room temperature in back scattering geometry,
using JOBIN YVON HORIBA HR-800 instrument equipped
with a 20 mW Ar+ laser (514.5 nm). Ambios XP-2 surface
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profilometer was used for thickness measurements. AFM studies
were carried out in contact mode with silicon nitride probes
using Digital Instruments Nanoscope IV Multimode SPM.
3. Results and discussion
The growth rate of films increased from ∼ 1.0 μm/h at room
temperature, to ∼ 1.4 μm/h at 100 °C and remained unchanged
at higher substrate temperatures. All the films were approximately of the same thickness (600 ± 50 nm). XRD patterns of
typical ZnO films are shown in Fig. 1. The films deposited at
room temperature and 100 °C showed a strong peak at 2θ ∼ 34°
and a weak peak ∼ 72°, which were identified respectively as
(0002) and (0004) reflections of hexagonal ZnO, indicating, a
strong c-axis orientation of crystallites. However, the films
deposited in the temperature range of 150–250 °C (a typical
case of 200 °C is shown in Fig. 1) exhibited multiple reflections
(as indexed in Fig. 1), though with a dominant (0002) peak.
Interestingly, the films deposited at 300 °C and higher
temperatures, again showed peaks corresponding to only
(0002) and (0004) reflections. The intensity variation of
Fig. 2. (a) Strain and (b) crystallite size along c-axis plotted against substrate
temperature.
Fig. 1. XRD patterns of ZnO films deposited at different substrate temperatures
as indicated in the figure.
(0002) peak with substrate temperature shown in Fig. 1 also
reveals an interesting pattern. The film deposited at 300 °C
exhibited (0002) peak intensity, nearly 40–60 times that from
the films deposited at lower temperatures. At higher temperatures, the intensity decreases gradually but remains within the
same order of magnitude.
The enhancement of c-axis orientation with increasing
substrate temperature above 100 °C is attributed to increase in
surface diffusion of the adsorbed species and is in tune with
most of the earlier studies [1,2]. The unusual reduction in the
extent of c-axis orientation at intermediate temperatures and its
subsequent enhancement at higher temperatures seen in the
present work, has been reported earlier in sputtered ZnO films
[15]. There are also a few reports on ZnO films deposited by
sputtering [12,16] and MOCVD [17], showing a strong c-axis
preferred orientation near room temperature followed by its
reduction with increase in substrate temperature to the range
150–300 °C. The present results also show that the film
deposited at 300 °C is of the highest quality in terms of c-axis
orientation of crystallites. The marginal decrease in intensity of
(0002) peak seen at higher substrate temperatures has not been
reported earlier and is attributed to mis-orientation of (0002)
planes with substrate surface.
A comprehensive understanding of the processes leading to
c-axis orientation over the complete range of substrate
temperatures, which is required to explain all the above
features, seems to be lacking. Recently, Kajikawa [18] has
extensively reviewed and analyzed the complex dependence of
c-axis preferred orientation in sputtered ZnO films on processes
such as, preferential nucleation, preferential crystallization,
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Fig. 3. AFM images of ZnO films deposited at (a) room temperature, (b) 200 °C, (c) 300 °C and (d) 600 °C.
sticking, surface diffusion and grain growth and concluded that
the validity of each process under given experimental conditions cannot be ensured, due to lack of reliable data on sticking
coefficients and surface diffusivities.
Fig. 1 also reveals shifts in the positions of (0002) and (0004)
peaks, with substrate temperature. The position of (0002) peak
shifts monotonically from 34.18° for the film deposited at room
temperature to 34.44° for the film deposited at 400 °C, the later
being quite close to the corresponding value of 34.467° for bulk
ZnO (JCPDS file No. 75-1526). The position of the (0002) peak
remains unchanged for the films deposited at higher substrate
temperatures. The lattice constant ‘c’ was obtained from the
position of the (0002) peak for all the films, and was used to
estimate the strain (ε) in the films along c-axis given by, ε =
(cfilm − cbulk) / cbulk. Fig. 2(a) shows the variation of strain with
substrate temperature. The film deposited at room temperature
showed the maximum compressive strain (∼ 8 × 10− 3), which
decreased monotonically to nearly zero at ∼400 °C. The
expected thermal strain introduced by differences in thermal
expansion coefficients of the film (αZnO = 4 × 10− 6 K− 1) and
quartz substrate (αquartz = 0.5 × 10 − 6 K − 1 ) at a substrate
temperature of 300 °C is ∼ 10− 3 and is expected to be lesser
at lower substrate temperatures. The higher values of strain (2–
8 × 10− 3) in the films deposited below 400 °C and its decrease
with increasing substrate temperature indicate that thermal
stress does not contribute significantly to the observed strain.
Further, the amorphous nature of quartz substrate rules out
strain due to lattice mismatch. Hence the compressive strain in
the films deposited below 400 °C is not attributed to extrinsic
stress but is considered intrinsic to the growth process. It has
been reported in the case of ZnO films that O− ions formed at
the target have sufficient energies to bombard the growing film
and cause implantation or displacement of surface atoms deeper
into the film, resulting in compressive strain [19]. The decrease
in strain at higher substrate temperatures is attributed to
annealing effects and consequent reduction in defects.
The FWHM of (0002) peak was used to estimate the
crystallite size along c-axis (growth direction) by Scherrer's
relation. The variation of crystallite size with substrate
temperature is shown in Fig. 2(b). It is seen that in the films
deposited up to 100 °C, the crystallite size along the growth
direction is ∼ 100 nm. A noticeable decrease is seen in
crystallite size for films deposited in the temperature range of
150–250 °C, which had earlier exhibited reduced c-axis
Fig. 4. Raman spectra of ZnO films deposited at different substrate temperatures
along with the spectrum of the quartz substrate.
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peak ∼ 569 cm− 1 is assigned to a combination of A1(LO) and
E1(LO) modes [20,21]. The appearance of LO modes has been
attributed to the presence of oxygen defects and zinc interstitials
[22]. These observations, including the weak E2-high mode as
a shoulder, indicate the presence of structural defects, misorientation of (0002) planes and disorder in the film [22–24], in
spite of its c-axis preferred orientation, seen in the XRD pattern.
The film deposited at 300 °C shows an enhancement of E2-high
intensity, though a broad band of TO modes is still seen. The
increase in the prominence of E2-high peak corroborates well
with the significant improvement in c-axis orientation of the film.
Interestingly, the film deposited at 600 °C shows only an intense
E2-high peak along with weak A1(LO) mode ∼575 cm− 1, which
is characteristic of strong c-axis orientation [1,21]. The
reduction in intensity as well as shift of LO modes to higher
frequencies seen in this film, are attributed to decrease in point
defects [21]. The decrease in defects correlates well with the
increase in crystallite size and decrease in stress, leading to
overall improvement in crystallinity of the film deposited at
600 °C. It is interesting to note that such features are seen in the
films deposited at 600 °C, which shows a lesser degree of c-axis
orientation, as compared to that deposited at 300 °C.
Typical specular R and T spectra of the films are shown in
Fig. 5(a). All the films exhibit high transmittance in the visible
region, limited by 10–15% reflectance. The sharp fall in
transmittance below 400 nm is due to the onset of fundamental
Fig. 5. (a) Reflectance (R) and Transmittance (T) spectra and (b) α2 vs. hν plots
for ZnO films deposited at different substrate temperatures.
preferred orientation. However, at substrate temperature of
300 °C, the crystallite size increases to ∼ 80 nm and remains
practically unchanged till ∼600 °C, whence a slight increase to
∼ 110 nm is noticed.
Typical AFM images of the films are shown in Fig. 3. Fig. 3(a)
shows surface features of uniform lateral size of 50–70 nm
for room temperature deposited film. In contrast, the film
deposited at 200 °C shows a mixture of small (40–80 nm) and
large (150–200 nm) surface features (Fig. 3(b)). However, the
film deposited at 300 °C again shows uniform features similar to
the room temperature deposited films, but with larger size of
60–100 nm (Fig. 3(c)). The film deposited at 600 °C, shows
distinctly different surface morphology, as large features of
∼ 250 nm along with smaller features of 60–100 nm are seen.
The increase in the lateral size of morphological features with
substrate temperature is attributed to the increase in lateral size
of crystallites.
Fig. 4 shows the Raman spectra of ZnO films along with that
of quartz substrate. The ZnO film deposited at room temperature shows two very broad peaks centered ∼ 398 cm− 1 and
∼ 569 cm− 1. The broad peak ∼ 398 cm− 1 is attributed to a
combination A1(TO) and E1(TO) modes and the small shoulder
∼ 440 cm− 1 is assigned to E2-high mode [20,21]. These features
are rather unexpected, since in a highly c-axis oriented ZnO
film in backscattering geometry, only strong E2 modes and
weak A1(LO) mode are expected to be present [1,21]. The broad
Fig. 6. PL spectra of ZnO films deposited at different substrate temperatures.
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absorption of ZnO. Absorption coefficient (α) was obtained
from specular R and T data and thickness (d) of the films, using
the expression,
T ðkÞcð1 RðkÞÞ2 eaðkÞd
for a self supporting film, with high absorption and low
reflectance. The corresponding plots of α2 vs. hν are shown in
Fig. 5(b), from which, the direct band gap of the films has been
estimated. The band gaps for all the films are ∼ 3.30 eV, which
agrees well with bulk band gap of ZnO. It is however observed
that with the increase in substrate temperature, the α2 vs. hν
plots become sharper, indicating a reduction of defects in the
films. The presence of optically active defects in films deposited
at room temperature and 300 °C is evident from the relatively
enhanced sub-band gap absorption in the films.
Fig. 6 shows room temperature PL spectra of the films. All
the films show strong emission ∼ 377 nm, attributed to near
band edge UV luminescence of ZnO [25]. Weak and broad
bands are seen in green–yellow region, which are known to
arise from oxygen defects and zinc interstitials [1,26]. The
intensity of band edge luminescence is found to increase with
substrate temperature, accompanied by narrowing of the peak,
as reported earlier for sputter deposited ZnO films on Si [27].
The PL spectrum of the ZnO film deposited at 600 °C showed a
high intensity band edge emission with FWHM of 102 meV.
This value of PL peak width is comparable to reported values
for ZnO films deposited by MBE [3], MOVPE [25] and PLD
[26] on single crystalline substrates. The PL results are in line
with optical absorption, which indicate reduction of optically
active defects in the films deposited at 600 °C. It may be noted
that the most intense and narrow PL emission does not come
from the ZnO film deposited at 300 °C, which exhibited
strongest c-axis orientation. The improved band edge luminescence of the film deposited at 600 °C is attributed to its better
overall crystallinity resulting from reduction of strain and
defects as well as improvement in crystallite size (both lateral
and along growth direction) as indicated by the structural
studies presented above and supported by Raman spectra.
4. Conclusions
ZnO thin films of high optical quality have been deposited
on quartz substrates by reactive sputtering. Though XRD
studies showed the strongest c-axis orientation for the films
deposited at 300 °C, AFM and Raman studies indicated that the
films deposited at 600 °C possess better overall crystallinity
resulting from reduction of strain and defects and improvement
in crystallite size. As a consequence of reduction in optically
active defects, the ZnO film deposited at 600 °C showed a high
intensity band edge luminescence with FWHM of ∼102 meV,
which is comparable to the reported values for films deposited
on single crystalline substrates.
Acknowledgements
The financial support from MHRD (Govt. of India) for this
work is gratefully acknowledged. Sukhvinder Singh is thankful
to CSIR, New Delhi (India) for Senior Research Fellowship.
FIST (Physics)-IRCC central SPM Facility and CRNTS of IIT
Bombay are respectively acknowledged, for providing AFM
and Raman facilities.
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