Substrate temperature dependence of growth mode, microstructure and optical
properties of highly oriented zinc oxide films deposited by reactive sputtering
Sukhvinder Singh a, Tapas Ganguli b, Ravi Kumar b, R.S. Srinivasa c, S.S. Major a,⁎
a
b
c
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
Semiconductor Laser Section, RRCAT, Indore-452013, India
Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India
a b s t r a c t
Keywords:
ZnO
Reactive sputtering
Microstructure
Optical properties
X-ray diffraction
Transmission electron microscopy
Polycrystalline ZnO films were deposited on quartz substrates by reactive sputtering of zinc target. X-ray
powder diffraction, pole figure analysis and high resolution measurements along with transmission electron
microscopy, Raman and photoluminescence studies were carried out to study the microstructure,
crystallinity and optically active defects in the films. All the films deposited in the substrate temperature
range from room temperature to 600 °C exhibited strong c-axis preferred orientation. The changes in
preferred orientation of crystallites with substrate temperature were attributed to its being determined by
preferential nucleation at lower temperatures and surface diffusion at higher temperatures. A detailed
microstructural analysis showed that with increase in substrate temperature from 300 °C to 600 °C, a
significant reduction in micro-strain to ∼ 10− 3 takes place, along with a marginal increase in crystallite size.
Raman and photoluminescence studies have shown that the films deposited below 300 °C possessed poor
crystalline quality. The film deposited at 600 °C yielded the most intense and narrow (∼102 meV) band edge
luminescence at room temperature, though it did not exhibit the strongest c-axis orientation of crystallites.
This is attributed to its superior crystalline quality and absence of oxygen-deficiency related defects.
1. Introduction
ZnO is a wide band gap semiconductor with large exciton binding
energy (60 meV), making it one of the most potential materials to
realize the next generation optoelectronic devices, operating in short
wavelength region [1–3]. Several growth techniques, such as,
sputtering, pulsed laser deposition (PLD), metal-organic chemical
vapour deposition (MOCVD), Sol–gel process and spray pyrolysis have
been used for the deposition of polycrystalline ZnO films. In recent
years, high quality epitaxial ZnO films have also been grown on
crystalline substrates. Most of this work has been reviewed in Refs.
[1,2]. As a deposition technique, sputtering offers several advantages
due to its simplicity, versatility and scalability, and has been
extensively used for the deposition of ZnO films [4]. Epitaxial ZnO
films [5–9] as well as high quality polycrystalline ZnO films on noncrystalline substrates [10–12] have been deposited by various forms of
sputtering.
In a recent work [13] on reactively sputtered ZnO films on quartz
substrates, it has been shown that all the films deposited in the
substrate temperature range of room temperature to 600 °C exhibited
a single low order XRD peak due to (0002) reflection, except for the
film deposited ∼ 200 °C, which exhibited multiple peaks with a
dominant (0002) reflection. Thus all the ZnO films were c-axis
preferentially oriented. However, the intensity variation of (0002)
peak of the films deposited at different substrate temperatures
showed an interesting variation. The film deposited at 300 °C showed
the strongest (0002) peak. In contrast, all the films deposited at lower
temperatures showed much less intense (0002) peaks (by nearly two
orders), while those deposited at higher temperatures showed a small
and steady decrease in the intensity of (0002) peak with increase of
substrate temperature. The films deposited below 300 °C possessed
uniform strain (N10− 3), which became negligible in the films
deposited above 300 °C. Most interestingly, it was found that the
ZnO film deposited at 600 °C exhibited the sharpest absorption edge
and a strong and narrow (∼100 meV) room temperature band edge
photoluminescence (PL), though it exhibited a lesser degree of c-axis
preferred orientation than the film deposited at 300 °C.
In order to understand the above mentioned features, an extensive
microstructural investigation of ZnO films deposited on quartz
substrates at different temperatures has been undertaken, using Xray pole figure analysis, high resolution X-ray diffraction (HRXRD) and
transmission electron microscopy (TEM). Raman spectroscopy and
photoluminescence measurements were used to obtain additional
information on microstructure, crystallinity and defects in the films. A
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comprehensive attempt has thus been made to establish the role of
microstructural parameters in determining the optical quality of
polycrystalline ZnO films deposited by reactive sputtering of a zinc
target.
2. Experimental details
ZnO films were deposited on quartz substrates by reactive rf
magnetron sputtering using argon–oxygen mixture. 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 × 10−3 Pa. 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 ∼1 Pa, after presputtering 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. X-ray pole
figures were measured using a PANalytical MRD four-circle diffractometer using CuKα radiation. HRXRD measurements were carried out
in ω and ω–2θ scan geometries using PANalytical X'Pert MRD system.
The incident beam optics had a 4-bounce hybrid monochromator,
which ensured CuKα1 (1.54056A°) output collimated to about 20 arc
sec in the plane of scattering. A 1/2° slit was placed at the output
before the detector. Transmission electron microscopy (TEM) studies
were carried out using a PHILIPS Model CM200 Supertwin Microscope
operated at 200 kV. Raman spectra of the films were recorded at room
temperature in back scattering geometry, using JOBIN YVON HORIBA
HR-800 confocal micro-Raman spectrometer equipped with a 20 mW
Ar+ laser (514.5 nm). Photoluminescence (PL) measurements were
carried out at room temperature using a 325 nm He–Cd laser and a
JOBIN YVON HR-460 monochromator. Ambios XP-2 surface profilometer was used for thickness measurements.
3. Results and discussion
3.1. Microstructural studies
All the studies were carried out on films of approximately the
same thickness (650 ± 50 nm). The deposition rate was nearly the
same (∼ 1 μm/h) for all the films. Typical XRD patterns of the films
deposited at 100 °C, 150 °C, 200 °C, 250 °C, 300 °C and 600 °C are
shown in Fig. 1. As reported earlier [13], the films deposited upto
100 °C showed a single low order peak due to (0002) reflection, hence
only a typical case is shown in Fig. 1. No XRD peaks corresponding to
reflections other than (0002) and (0004) planes of hexagonal ZnO
were observed in these cases. However, all the films deposited in the
range of 150–250 °C, clearly showed the presence of low intensity
̄ and (1011)
̄ reflections, with a dominant (0002)
peaks due to (1010)
reflection. The film deposited at 200 °C showed relatively higher
̄ peaks. In contrast, the films deposited
intensity of (101̄ 0) and (1011)
at higher substrate temperatures again showed only (0002) peak
with much higher intensity, though the intensity decreased monotonically with increase of substrate temperature from 300 °C to
600 °C. Typical cases of the films deposited at 300 °C and 600 °C are
shown in Fig. 1. Thus all the ZnO films exhibit a strong c-axis
preferential orientation of crystallites, though the extent of preferred
orientation appears to be the highest in the film deposited at 300 °C. It
is also seen from Fig. 1 that the position of (0002) peak shifts
monotonically towards the bulk value with increase of substrate
temperature and the uniform strain in the films deposited above
300 °C is practically negligible, as reported earlier [13].
Fig. 2 shows the pole figures of (0002) reflection for ZnO films
deposited on quartz substrates at different temperatures. All the films
show a very pronounced (0002) texture, with rotational symmetry.
The films deposited at room temperature and 100 °C show broad pole
Fig. 1. XRD patterns of ZnO films deposited at different substrate temperatures (as
indicated in the figure). All the films were deposited at rf power of 400 W with 20%
oxygen in the sputtering atmosphere.
figures, which become sharper with initial increase in substrate
temperature. The films deposited at 300 °C and 400 °C thus show very
sharp and high intensity pole figures, confirming the presence of a
strong and nearly complete c-axis orientation of crystallites. Interestingly, at higher substrate temperatures of 500 °C and 600 °C, the pole
figures again become broader, indicating enhanced mis-orientation of
(0002) planes of crystallites with respect to the film surface. This
explains the steady decrease in the intensity of (0002) XRD peak with
increase in substrate temperatures above 300 °C, as mentioned above
and reported earlier [13].
TEM studies were carried out on typical ZnO films, which were
lifted-off from quartz substrates. The film on quartz substrate was
immersed in dilute HF for few secs, which resulted in etching at the
film-substrate interface and thinning of the film. Subsequently, the
film on substrate was carefully transferred to a bath of de-ionized
water, which resulted in peeling-off of the film and floating on water
surface. The film was transferred onto a 200 mesh carbon coated
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Fig. 2. (0002) peak pole figures of ZnO films deposited at different substrate temperatures (as indicated in the figure).
copper grid for TEM studies. Both bright and dark field images were
examined in all the cases. Fig. 3 presents typical TEM results in the
form of bright and dark field images, along with the corresponding
diffraction patterns and crystallite size distributions for the films
deposited at room temperature, 300 °C and 600 °C. Electron
diffraction data obtained for all the films confirmed the presence of
polycrystalline hexagonal ZnO. The lattice constants (‘a’ and ‘c’)
obtained from the diffraction patterns are listed in Table 1 for typical
cases. The value of ‘c’, decreases with increasing substrate temperature and becomes close to the bulk value for the films deposited at
300 °C or higher temperatures. These observations are in agreement
with the above described XRD results. The lattice constant ‘c’ however,
remains close to the bulk value, irrespective of the substrate
temperature. The average lateral crystallite size of the films was
obtained from dark field TEM images. The values are listed in Table 1.
The results are in good agreement with the corresponding bright field
images. The crystallite size of the film deposited at room temperature
is (21 ± 14) nm, which increases slightly to (25 ± 20) nm for the film
deposited at 300 °C. However, for the film deposited at 600 °C, the
crystallite size increases substantially to (43 ± 30) nm.
The above described microstructural features of ZnO films,
especially the pattern of change in preferred orientation of crystallites
with substrate temperature is unusual and interesting. The unusual
reduction in the extent of c-axis preferred orientation in sputtered
ZnO films at intermediate temperatures and its subsequent enhancement at higher temperatures has been reported earlier [14], though
without much explanation. There are also some reports on ZnO films
deposited by sputtering [10,15] and MOCVD [16], showing a strong caxis preferred orientation near room temperature, followed by its
reduction with increase in substrate temperature to the range of 150–
300 °C. In a recent review, Kajikawa [17] has extensively analyzed the
issue of preferred orientation in sputtered ZnO films by compiling the
relevant experimental observations and explanations put forth for the
preferred orientation behaviour. A comprehensive understanding of
the complex dependence of c-axis preferred orientation of sputtered
ZnO films on processes such as, preferential nucleation, preferential
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[17,20,21] driven by minimization of surface energy. The preferred
orientation can also be significantly affected at the growth stage by
two processes, namely, sticking and surface diffusion of ad atoms.
Fujimira et al. [21] have proposed that the differences in the sticking
probabilities of growing species on different planes determine
preferred orientation. For example, in the case of sputtered ZnO
films [22,23], the existence of Zn and O in the sputtering gas causes aaxis orientation, while the higher sticking probability of Zn–O species
causes c-axis preferred orientation. In the present work, it has been
noticed that the preferred orientation effects show a strong
dependence on substrate temperature, but are practically independent of rf power (in the range of 300–400 W) and oxygen content (in
the range of 10–30%) of the sputtering atmosphere. Hence, effects due
to sticking coefficients of growing species have been ignored, while
explaining the preferred orientation behaviour.
The preferred orientation behaviour of sputtered ZnO films seen in
the present work can be explained in the light of the above discussion.
As shown in Fig. 1, the films deposited up to 100 °C, exhibit a single
XRD peak indicating (0002) preferred orientation, though the peak
intensity in these cases are nearly two orders of magnitude smaller,
compared to the strongly (0002) oriented film deposited at 300 °C.
Further, Fig. 2 shows that the corresponding pole figures for the films
deposited up to 100 °C are very broad, indicating significant misorientation of the crystallites. TEM studies of these films have also
indicated that the lateral crystallite size is relatively small (∼ 20 nm).
On the other hand, in these films, the crystallite size along the growth
direction (c-axis) is relatively larger (∼ 100 nm), as obtained from the
(0002) peak width. The broad and low intensity pole figures and small
lateral crystallite size indicate that although these films exhibit
preferred c-axis orientation, the degree of crystallinity and overall
structural order in these films is not very high. The presence of
disordered component in the films can also not be ruled out. This can
be explained by assuming negligible surface diffusion at lower
substrate temperatures. Thus, it is primarily due to preferential
(0002) nucleation that the c-axis oriented crystallites grow along with
some poorer crystallinity material.
As the substrate temperature is increased to the range of 150–
250 °C, the XRD patterns (Fig. 1) of the corresponding films show
multiple peaks. Though in all these cases, the (0002) peak remains
dominant, but it shows a significant increase in its half width. For the
film deposited at 200 °C, the (0002) peak shows the largest width
corresponding to a value of ∼ 50 nm for the crystallite size along
growth direction (c-axis). This behaviour can be explained by
considering that at these temperatures, the preferred orientation of
crystallites is no longer determined by preferred nucleation. Instead, it
is significantly affected by the growth process, controlled essentially
by enhanced surface diffusion due to increase in substrate temperature. However, at relatively low substrate temperatures, surface
diffusion may be limited to being within the grains, i.e., among planes
only. According to Kajikawa et al. [17,20], when surface diffusion takes
place among planes within a grain, higher surface energy planes may
become preferred orientation planes, because surface diffusion
essentially occurs to conceal the plane with higher surface energy.
Table 1
Characteristic parameters of ZnO films deposited at room temperature, 300 °C and
600 °C on quartz substrates
Fig. 3. Bright field and dark field TEM images of ZnO films deposited at different
substrate temperatures (as indicated in the figure). The diffraction pattern and the
analysis of the crystallite sizes are shown as insets.
crystallization, sticking, surface diffusion and grain growth appears to
be lacking. However, it is generally believed (though, this has also
been debated [18,19]) to originate from preferential (0002) nucleation
Substrate
temperature
Room temp.
300 °C
600 °C
From TEM
From HRXRD
Lateral
crystallite
size (nm)
‘c’ (Å)
(± 0.01)
‘a’ (Å)
(± 0.01)
Tilt
(degree)
Vertical
coherence
length (nm)
Micro-strain
21 ± 14
25 ± 20
43 ± 31
5.25
5.21
5.20
3.26
3.25
3.26
–
2.6
11.1
–
140 ± 15
65 ± 15
–
2.0 × 10− 3
1.1 × 10− 3
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Such a preferred orientation behaviour has been recently reported
[24] and discussed in detail for sputtered hexagonal GaN films. The
̄
̄
appearance of (1010)
and (1011)
reflections along with (0002)
reflections in the XRD patterns of these films, indicating the presence
of differently oriented crystallites with respect to the film surface, is
thus attributed to the surface diffusion among planes within the
grains. It is noteworthy that the AFM studies of these films reported in
Ref. [13], showed that the surface morphological features of two vastly
different sizes were present in the film deposited at 200 °C, unlike the
films deposited at other substrate temperatures. It is also noticed from
Fig. 2 that the pole figure for the film deposited at 200 °C is relatively
sharp, which indicates that there is an increased tendency of the
(0002) oriented crystallites to align with the growth direction. These
features, together with the decrease of crystallite size along the
growth direction to ∼ 50 nm indicate that a constrained growth of caxis oriented crystallites along with crystallites with other orientations takes place in this temperature range.
At higher substrate temperatures (300 °C and above), the
significantly increased surface diffusion coefficient is expected to
take the growth process to the regime where surface diffusion among
grains becomes dominant. In this temperature regime, the grains with
higher energy surface shrink and those with lower energy surface
grow laterally to determine the nature of preferred orientation.
Consequently, the lowest surface energy plane of ZnO, i.e., (0002)
acquires a preferential orientation, as has been observed in the
present case at substrate temperatures of 300 °C or higher. The
dominance of surface diffusion and tendency of lateral growth in these
films is also evident from the increase in lateral crystallite size (Fig. 3)
and the large surface features seen in their AFM images, reported
earlier [13]. However, an interesting feature of the films deposited at
and above 300 °C is the monotonic decrease in the intensity of (0002)
peak (Fig. 1) and the broadening of corresponding pole figures (Fig. 2)
with increase in substrate temperature. The films deposited at 300 °C
and 600 °C were analyzed by high resolution X-ray diffraction
(HRXRD) studies, which have provided additional insight into this
behaviour. The films deposited below 300 °C could not be studied,
owing to poor signal intensity. The HRXRD results are discussed
below.
A strongly oriented polycrystalline film on amorphous substrates
can be considered to consist of crystallites, with certain mean vertical
and lateral dimensions. For materials having hexagonal structure,
usually the crystallites are oriented with their b0002N axis along the
growth direction and are rotated randomly about this direction.
Information about lateral and vertical coherence lengths (crystallite
sizes), crystallite tilt and micro-strain can be estimated from full width
at half maximum (FWHM) of the omega (ω) and ω–2θ scans of (000 l)
reflections, using Williamson–Hall plots [25]. The terms vertical and
lateral refer respectively, to the growth direction and a direction
perpendicular to it, in the plane of the film. The finite crystallite size,
tilt and micro-strain cause the broadening of reciprocal space points
[26]. The broadening due to finite size is invariant in reciprocal space
with higher orders of reflection (increasing scattering vector, q), but
the broadening due to both tilt and micro-strain increase with
increasing magnitude of q [26]. The finite vertical coherence length
and micro-strain cause the broadening, Δqz of (000l) reflections in ω–
2θ scans, which can be respectively, estimated from the intercept and
slope of the linear Williamson–Hall plots of Δqz (FWHM along qz
direction) vs. q. Here, q is the magnitude of the position of (000 l)
point in the reciprocal space. Similarly, the finite lateral coherence
length and tilt cause broadening, Δqx in the qx–qy plane, which can be
measured by the spread of (000l) reflections in ω-scan. The lateral
coherence length and tilt can be estimated respectively, from the
intercept and slope of the linear Williamson–Hall plots of Δqx (FWHM
along qx direction) vs. q.
ω–2θ scans of symmetric (0002), (0004) and (0006) reflections from
ZnO films deposited at substrate temperatures of 300 °C and 600 °C are
shown respectively, in Fig. 4(a). For all the cases, the values of FWHM in
reciprocal space (Δqz) are also indicated. As discussed above, the finite
vertical crystallite size and micro-strain were calculated from Williamson–Hall plots (Δqz vs. q), shown in Fig. 4(b). The values of these
parameters are also listed in Table 1. It may be noted that the vertical
crystallite size of the film deposited at 300 °C is ∼140 nm, as compared to
the value of ∼65 nm for the film deposited at 600 °C. The micro-strain
(non-uniform strain) is found to be 1.1 × 10− 3 in the film deposited at
600 °C, which is nearly half the value of 2.0×10− 3, measured in the film
deposited at 300 °C.
Fig. 4. (a) ω–2θ scans for ZnO films deposited at substrate temperatures of 300 °C and
600 °C for (0002), (0004) and (0006) reflections. (b) Broadening in the reciprocal space
as obtained from the symmetric scans along ω–2θ-axis (qz) for (0002), (0004) and
(0006) reflections.
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Fig. 5(a) shows the ω scans of symmetric (0002), (0004) and
(0006) reflections from ZnO films deposited at 300 °C and 600 °C. For
all the cases, the values of FWHM in reciprocal space (Δqx) are also
indicated. The ω scans of the film deposited at 600 °C were found to be
much broader compared to the film deposited at 300 °C. The corresponding Williamson–Hall plots of Δqx vs. q are shown in Fig. 5(b),
for both the films. The linear fits for the two films show significantly
different slopes, from which the values of tilt have been estimated and
listed in Table 1. The average tilt value of ∼ 2.6° for the film deposited at
300 °C is significantly smaller than the value of ∼11.1° for the film
deposited at 600 °C. These results are in tune with the qualitative
information obtained from the corresponding pole figures (Fig. 2). The
intercepts of Δqx vs. q plots however, possess large uncertainties
owing to the broad ω curves, especially in case of the film deposited at
600 °C. This implies the presence of a large error in the estimation of
lateral coherence length, hence the lateral crystallite size in these
films has been taken as measured by TEM, and listed in Table 1.
The microstructural features of the films deposited at 300 °C and
600 °C can now be compared. The average vertical (along growth
direction) and lateral crystallite sizes of the film deposited at 300°C
are ∼140 nm and ∼25 nm, respectively. This film also exhibits a very
sharp (0002) pole figure (Fig. 2) and correspondingly a small value
(∼2.6°) of tilt. It clearly possesses the highest quality in terms of c-axis
crystallite orientation, in comparison to all the other films. As the
substrate temperature is increased, the pole figures become broader,
indicating that the mis-orientation of (0002) planes of crystallites
with respect to the film surface increases. Consequently, the film
deposited at 600 °C shows a much larger value of tilt (∼11.1°). Further,
the average vertical and lateral crystallite sizes for this film are
∼65 nm and ∼43 nm, respectively, indicating that with increase in
substrate temperature, the vertical crystallite size decreases, but the
lateral crystallite size increases. It can thus be inferred that the film
deposited at 600 °C consists of relatively equiaxed crystallites with
slightly larger overall crystallite volume compared to the film
deposited at 300 °C. These observations can be explained in
accordance with the structure-zone model, proposed originally by
Thornton [27]. According to this model [28], surface diffusion
controlled columnar growth usually takes place at intermediate
temperatures. At higher substrate temperatures, lattice and grain
boundary diffusion processes begin to dominate, leading to formation
of equiaxed re-crystallized grains. The results presented above show
that a transition from columnar crystallites to equiaxed crystallites
takes place with increase of substrate temperature. The increase in tilt
seen in the films deposited above 300 °C is thus indicative of misorientation amongst equiaxed crystallites, which is attributed to the
absence of columnar growth in sputtered films deposited on
amorphous substrates, at relatively higher temperatures.
The HRXRD studies also show that the micro-strain present in the
film deposited at 600 °C is nearly half of that in the film deposited at
300 °C. The higher micro-strain in the film deposited at 300 °C is
indicative of a larger presence of point defects in this film. It may be
recalled from earlier work [13], that the uniform strain in the films
deposited below 300 °C was considered intrinsic to the growth
process. The intrinsic strain was attributed to the O− ions formed at the
target, which may have sufficient energies to bombard the growing
film [29], causing implantation, displacement or removal of surface
atoms. The relatively large micro-strain present in the film deposited
at 300 °C is also attributed to bombardment due to O− ions. The
significant decrease in micro-strain seen in the film deposited at
600 °C is attributed to annealing effects, leading to reduction in point
defects.
3.2. Raman spectroscopy studies
Fig. 5. (a) ω-scan for ZnO films deposited at substrate temperatures of 300 °C and 600 °C
for (0002), (0004) and (0006) reflections. (b) Broadening in the reciprocal space as
obtained from the symmetric scans along ω-axis (qx) for (0002), (0004) and (0006)
reflections.
Fig. 6 shows the Raman spectra of ZnO films deposited at different substrate temperatures. The spectrum of the quartz substrate is
also shown for comparison. All the ZnO films deposited in the
substrate temperature range of room temperature to 250 °C show two
very broad peaks centered ∼400 cm− 1 and ∼ 570 cm− 1. The broad peak
∼400 cm− 1 is attributed to a combination of A1(TO) and E1(TO)
modes (TO denotes transverse optical) of ZnO, which are reported at
380 cm− 1 and 407 cm− 1, respectively [30,31]. This broad peak has a
shoulder on the higher frequency side at ∼440 cm− 1, which is assigned
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compared to the film deposited at 300 °C, shows a much larger tilt
(mis-orientation of crystallites). This result clearly shows that the
mis-orientation of crystallites does not significantly affect the
intrinsic quality of crystallites in terms of point defects and strain.
Raman results have thus shown that the mere appearance of a single
(0002) XRD peak in ‘a film’, which is usually taken as indicative of a
strong c-axis orientation of crystallites and hence its ‘high’ crystalline quality, can be misleading. It is clear that additional data is
required to make such assertions.
3.3. Photoluminescence studies
Absorption studies on ZnO films have shown [13] that all the films
exhibited band gaps ∼3.3 eV. It was however, observed that with
increase in substrate temperature, the absorption edge became
sharper and sub-band gap absorption was significantly reduced. It
was also shown that the ZnO film deposited at 600 °C exhibited a
strong and narrow band edge photoluminescence peak. In this
section, a detailed study of the near band edge and defect level
luminescence at room temperature is presented for the ZnO films
deposited at different substrate temperatures.
Fig. 6. Raman spectra of ZnO films deposited at different substrate temperatures (as
indicated in the figure), along with the spectrum of the quartz substrate.
to E2-high mode [30,31]. These features are rather unexpected, since
in a c-axis oriented ZnO film in backscattering geometry, only a strong
E2-high mode and a weak A1(LO) mode (LO denotes longitudinal
optical) are expected to be present [1,30]. The broad peak ∼ 570 cm− 1
is assigned to a combination of A1(LO) and E1(LO) modes, which are
reported at 574 cm− 1 and 583 cm− 1, respectively [30,31]. These
features of Raman spectra of ZnO films are indicative of poor
crystallinity, arising from lattice strain, structural defects and disorder
[32–34]. These observations, especially the presence of weak E2-high
mode as a shoulder, indicate that though most of the films deposited
below 300 °C showed [13] a single and intense (0002) peak, indicating
c-axis preferred orientation of crystallites, these films actually possess
poor crystalline quality. The features seen in the Raman spectra are
thus consistent with the significantly low intensity of (0002) XRD
peaks (Fig. 1), broad pole figures (Fig. 2), small lateral crystallite size
(Fig. 3) and the high uniform strain (N10− 3) exhibited by these films
(reported in Ref. [13]).
The film deposited at 300 °C however, shows drastically different spectral features. An enhancement of E2-high intensity is
clearly seen, though a shoulder due to TO modes and a broad band
due to LO modes continue to exist. The increase in the prominence of
E2-high peak corroborates well with the significant improvement in
c-axis orientation of crystallites, as also seen from the corresponding
XRD patterns and pole figures. The films deposited at higher
substrate temperatures show a monotonic increase in the intensity
and sharpness of the E2-high peak and decrease in the intensity of TO
and LO mode peaks. Interestingly, the film deposited at 600 °C shows
only an intense E2-high peak along with a very weak and broad A1
(LO) mode at ∼ 575 cm− 1. As mentioned above, such features are
characteristic of strong c-axis orientated ZnO films [1,31]. The small
shift of LO modes towards higher frequencies and the reduction in
their intensity are attributed to decrease in point defects [31]. The
decrease in point defects correlates well with the significant
decrease in micro-strain and the small increase in crystallite size,
for the film deposited at 600 °C. It is interesting to note that such
features are seen in the ZnO film deposited at 600 °C, which, as
Fig. 7. PL spectra of ZnO films deposited at different substrate temperatures (as
indicated in the figure).
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Fig. 7 shows the room temperature PL spectra of ZnO films
deposited at different substrate temperatures. All the films show
strong emission at ∼ 376 nm, attributed to near band edge luminescence of ZnO [35]. Weak and broad bands are also seen in visible
region, which are known to arise from oxygen vacancies and zinc
interstitials [36,37]. The intensity of band edge luminescence is found
to increase with substrate temperature, accompanied by narrowing of
the peak, as reported earlier for sputtered ZnO films on Si [38]. The
films deposited at room temperature, 100 °C and 200 °C showed very
weak band edge emission and negligible luminescence in the visible
region. This is attributed to poor crystallinity and large disorder in the
films, as seen earlier from UV–visible absorption studies [13], which
showed large sub-band gap absorption and is in tune with the Raman
results. It has also been reported that ZnO films with smaller
crystallites show poorer luminescence, which has been attributed to
non-radiative relaxation through surface states [38,39]. With increase
in substrate temperature, the intensity of band edge emission begins
to increase together with the defect related luminescence in the
visible region. The film deposited at substrate temperature of 400 °C
thus showed comparable intensities of band edge and defect
luminescence. The increase in both band edge as well as defect
luminescence together, can be attributed to decrease in non-radiative
relaxation processes. With further increase in substrate temperature
to 500 °C, the intensity of band edge emission begins to increase faster
than the defect related luminescence. The band edge luminescence
intensity however, shows a drastic increase for the film deposited at
600 °C, while the defect related luminescence bands remain
practically unchanged and weak. The half width of the band edge
luminescence peak is also significantly reduced to ∼ 102 meV. The
sharp and intense band edge PL from the film deposited at 600 °C is in
line with the microstructural and Raman studies, which indicated
reduction of point defects in this film. This is further supported by the
presence of weak defect related luminescence in the visible region. It
has been reported [36] that the visible emission in ZnO originates
from oxygen vacancy or zinc interstitial related defects, and gets
significantly reduced by the improvement in stoichiometry of ZnO
films.
It may be pointed out that the band edge PL peak width of 102 meV
at room temperature is quite comparable to the reported values for
ZnO films deposited by MBE [40], MOVPE [35,41] and PLD [37], on
single crystalline substrates. It is also interesting to note that the most
intense and narrow PL does not come from the ZnO film deposited at
300 °C, which exhibited the strongest and nearly complete c-axis
preferred orientation of crystallites. This result is important and
interesting, as it indicates that the main cause of the enhancement and
narrowing of band edge luminescence in polycrystalline ZnO films is
the decrease of micro-strain, which results from the reduction in point
defects, such as oxygen vacancies and zinc interstitials. It may also be
mentioned that the film deposited at 600 °C possessed equiaxed
crystallites with a slightly larger overall size, as compared to the film
deposited at 300 °C. This may also partly contribute to its superior
optical quality.
4. Conclusions
Polycrystalline ZnO films were deposited on quartz substrates by
reactive sputtering of zinc target. The films deposited below 300 °C
showed preferred c-axis orientation of crystallites but Raman and PL
studies indicated their poor crystallinity and optical quality. The films
deposited up to substrate temperatures of 100 °C, show c-axis
preferred orientation due to preferential nucleation. The appearance
of multiple orientations in the substrate temperature range of 150–
250 °C is attributed to surface diffusion between planes within the
grains. The film deposited at 300 °C and higher temperatures showed a
strong c-axis orientation of crystallites due to the dominance of surface
diffusion between grains. However, with increase in substrate
temperature from 300 °C to 600 °C, the c-axis orientation was again
found to steadily decrease, owing to transition from columnar
structure to equiaxed structure of crystallites. The micro-strain present
in the film deposited at 600 °C was nearly half of that in the film
deposited at 300 °C, which is attributed to lesser point defects. Raman
and PL studies also show that the film deposited at 600 °C possesses
lesser oxygen-deficiency related point defects, which correlates well
with the observed decrease in its micro-strain to ∼ 10− 3. As a
consequence of reduction in optically active point defects, the ZnO
film deposited at 600 °C showed a high intensity and narrow band edge
luminescence with FWHM of ∼ 102 meV at room temperature.
Polycrystalline ZnO films deposited by reactive sputtering of a metallic
zinc target have not been usually reported to possess such structural
and optical quality. The reduction in defects is attributed to the
presence of energetic zinc ad atoms having high surface mobility as
well as annealing effects. It is also inferred that the micro-strain
present within the crystallites plays the most crucial role compared to
other microstructural parameters, in determining the optical quality of
polycrystalline ZnO films.
This work has shown that the mere appearance of a single and
intense (0002) peak in ‘a film’ is not a sufficient indicator of its ‘high’
crystalline quality. This is particularly relevant to the case of
polycrystalline ZnO films, which under most conditions of deposition
exhibit a single or intense (0002) XRD peak. It is also clear that in
general, there is a need to examine other aspects of microstructure,
such as, crystallite size, crystallite tilt and uniform as well as nonuniform strain (micro-strain) and also independently assess the
presence of optically active defects, before drawing meaningful
conclusions about the ‘quality’ of polycrystalline semiconductor films.
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. Tapas Ganguli and Ravi
Kumar are thankful to Dr. S. M. Oak for support and encouragement.
The National OIM Texture Facility of IIT Bombay is gratefully
acknowledged for X-ray pole figure measurements. SAIF and CRNTS,
IIT Bombay are respectively acknowledged, for providing TEM and
Raman facilities.
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