Highly oriented GaN films grown on ZnO buffer layer over quartz substrates by
reactive sputtering of GaAs target
Brajesh S. Yadav a, Sukhvinder Singh a, Tapas Ganguli b, Ravi Kumar b, S.S. Major a,⁎, R.S. Srinivasa c
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:
Gallium nitride (GaN)
Reactive sputtering
ZnO buffer layer
Microstructure
X-ray diffraction
Polycrystalline GaN films were deposited on quartz and ZnO buffer layers over quartz by reactive sputtering
of a GaAs target in 100% nitrogen at 550 °C and 700 °C. Micro-structural investigations of the films were
carried out using high resolution X-ray diffraction, atomic force microscopy and Raman spectroscopy. GaN
films deposited on ZnO buffer layers exhibit strongly preferred (0002) orientation of crystallites. In particular,
the film deposited at 700 °C on ZnO buffer layer over amorphous quartz substrate showed large crystallite
size, both along and perpendicular to growth direction, strong and nearly complete c-axis orientation of
crystallites with tilt of ~ 2.5° and low value of micro-strain ~ 2 × 10− 3. The significant improvement in
crystallinity and orientation of crystallites in the GaN film is attributed to the presence of the ZnO buffer layer
on quartz substrate and its small lattice mismatch (1.8%) with GaN.
1. Introduction
The development of GaN based high brightness light emitting
diodes and laser diodes has been followed by the realization of high
frequency power transistors, solar blind photodiodes, ultra-violet light
emitting diodes and detectors, high electron mobility transistors and
several other electronic devices [1–3]. GaN has received enormous
attention due to its applications in white light sources [4] and
spintronic devices [5]. Piezoelectric properties of GaN and its negative
electron affinity have also opened avenues of research, with promising
applications [1,6,7]. Device quality epitaxial GaN films have usually
been grown using metal organic chemical vapour deposition (MOCVD)
or molecular beam epitaxy (MBE) techniques [6,7]. However, owing to
its versatile properties and enormous application potential, there has
been a growing interest in polycrystalline GaN films deposited by
various versions of MBE [8–12] and MOCVD [13–17] and sputtering
[18–31]. Light emitting diodes [32–33] and field electron emitters
[14,34] based on polycrystalline GaN films have been demonstrated.
Potential applications of polycrystalline GaN in thin film transistors,
white lighting and electroluminescent devices for flat panel displays,
photovoltaics and photonic devices have driven the search for
alternative low cost deposition processes and substrates. It is also
desirable to explore low temperature processes for integration with
liquid crystal and photovoltaic device technologies. The inherent
ability of sputtering to deposit large area thin films on a variety of
substrates at relatively lower temperatures, opens up enormous
opportunities, not only for a robust semiconductor like GaN, but in
general, for III-nitrides. There have been several early reports on the
growth of GaN films by reactive sputtering of Ga target with nitrogen or
nitrogen–argon mixture [35–37], though problems of reproducibility
arising out of the low melting temperature of gallium have been
reported [38]. However, during the last few years, GaN films have been
deposited using both Ga [18–22] or GaN [23–28] targets. The growth of
GaN films has also been reported [29,39] by reactive sputtering of a
GaAs target with 100% nitrogen. Magnetron sputtering has also been
utilized to deposit epitaxial GaN films on sapphire substrates [40–42].
In an interesting approach, polycrystalline GaN films with
improved structural quality have been deposited over ZnO buffer
layers on silicon substrates, using MBE [43], MOCVD [44,45] and
pulsed laser deposition (PLD) [46,47] techniques. High quality
epilayers of GaN have also been deposited on ZnO buffer layer over
sapphire substrates, using this approach [44,48,49]. There are also a
few reports on the use of ZnO as a buffer layer in the growth of GaN
films by sputtering, using GaN [50] and Ga2O3 [51] targets. ZnO has a
hexagonal wurtzite structure with lattice constants close to those of
GaN (lattice mismatch ~ 1.8%) and polycrystalline ZnO films are known
to exhibit strong c-axis orientation of crystallites [52]. Hence, this
approach has resulted in very encouraging results.
In a recent work [53] on the growth and structure of hexagonal
GaN films deposited on quartz substrates by sputtering of a GaAs
target in 100% nitrogen, it has been shown that the tendency of (0002)
489
preferred orientation (c-axis orientation) of crystallites is significantly
enhanced at substrate temperatures above 500 °C, due to the
dominance of surface diffusion among grains [54]. It was also shown
that the films deposited above 500 °C possess ~1 at.% arsenic as
impurity, and those deposited at ~ 700 °C, possess practically
negligible arsenic. Hence, in order to explore the growth of highly caxis oriented GaN films with improved crystalline quality, a ZnO buffer
layer on amorphous quartz substrates has been used in this work.
Based on the results presented in Ref. [53], GaN films have been
deposited at two substrate temperatures, namely, 550 °C and 700 °C,
on quartz substrates, with and without the ZnO buffer layer. A detailed
micro-structural characterization of these films has been carried out
by high resolution X-ray diffraction (HRXRD) and atomic force
microscopy (AFM). Additional information about the crystallinity of
the films has been obtained from Raman spectroscopy. These studies
have shown that highly c-axis oriented, polycrystalline GaN films with
large crystallite size, small crystallite tilt and low micro-strain values
can be deposited on ZnO buffer layer over amorphous quartz
substrates by reactive sputtering of a GaAs target.
2. Experimental details
GaN films were deposited by 13.56 MHz, radio frequency (rf)
magnetron reactive sputtering of a 3 inch GaAs target with 100%
nitrogen as sputtering-cum-reactive gas, as described in detail,
elsewhere [42]. An rf shielded heater was used to control substrate
temperature up to (700 ± 10) °C. Films were deposited at a nitrogen
pressure of 8 × 10− 1 Pa and flow rate of 25 SCCM (SCCM denotes cubic
centimeter per minute at STP). The thickness of the films was
measured by Ambios XP-2 surface profilometer. All the films were in
the thickness range of 500–600 nm. The ZnO buffer layer was
deposited on quartz substrates by reactive sputtering of a zinc target.
The thickness of the buffer layer was in the range of 100–200 nm and
in this thickness range, the effect of buffer layer on the structure of
GaN films was found to be reproducible.
The structure of the films was studied using PANalytical X'Pert PRO
powder diffractometer using Cu Kα radiation. HRXRD measurements were
carried out in omega (ω) and ω–2θ scan geometries using PANalytical
X'Pert MRD system. The incident beam optics had a 4-bounce hybrid
monochromator, which ensured Cu Kα1 (1.54056 Å) output collimated to
about 20″ in the plane of scattering. A 1/2° slit was placed at the output
before the detector. AFM studies were carried out in contact mode with
silicon nitride probes using Digital Instruments Nanoscope IV multimode
SPM. 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).
3. Results and discussion
The XRD patterns of GaN films deposited at substrate temperatures of
550 °C and 700 °C on quartz and ZnO buffer layer over quartz are presented
in Fig. 1. XRD pattern of the ZnO buffer layer on quartz is also shown in the
same figure, for comparison. The pattern of the film deposited at 550 °C on
quartz shows several peaks corresponding to hexagonal GaN, among
̄ and (0002) are the most prominent. In comparison, the film
which (1011)
deposited on quartz at 700 °C shows a strong (0002) peak, along with a
̄ shoulder, clearly showing the tendency towards c-axis orientation
(1011)
of crystallites, with increase in substrate temperature. The film deposited
on ZnO buffer layer at 550 °C shows a strong (0002) peak, though a small
̄ plane is also present. In comparison, the
peak corresponding to (1011)
films deposited at 700 °C on ZnO buffer layer shows a single and strong
low order peak due to (0002) reflection, along with a (0004) peak,
indicating a nearly complete c-axis orientation. It is also seen from Fig. 1
that the intensity of (0002) peak in the films deposited on ZnO buffer layer
(d and e) are significantly enhanced, as compared to the corresponding
films on quartz substrate (a and b).
Fig. 1. XRD patterns of GaN films deposited on quartz at (a) 550 °C and (b) 700 °C. The
XRD patterns of (c) ZnO buffer layer on quartz and GaN films over ZnO buffer layers
deposited at (d) 550 °C and (e) 700 °C are also shown.
In order to assess the crystalline quality of highly c-axis oriented
GaN films deposited at 700 °C on quartz and ZnO buffer layer, high
resolution X-ray diffraction measurements were carried out to study
the microstructure of these films. These studies 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 (non-uniform strain) can be obtained from full
width at half maximum (FWHM) of the omega (ω) and ω–2θ scans of
(000l) reflections, using Williamson–Hall plots [55]. 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 [56].
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 [56]. The finite vertical coherence length and microstrain cause the broadening, Δqz of (000l) reflections in ω–2θ scans,
which can be estimated respectively, from the intercept and slope of the
490
linear Williamson–Hall plots of Δqz (FWHM along qz direction) vs. q.
Here, q is the magnitude of the position of (000l) point in the reciprocal
space. Similarly, the finite lateral coherence length and tilt cause
broadening, Δqx in qx–qy plane, which can be measured by the spread of
(000l) reflections in ω scan. Lateral coherence length and tilt can be
Fig. 2. ω–2θ scans for GaN films deposited at substrate temperature of 700 °C on quartz
(———) and ZnO buffer layer over quartz (———) for (a) (0002), (b) (0004) and (c) (0006)
reflections. The broadening in the reciprocal space as obtained from the symmetric
scans along ω–2θ-axis (qz) for (0002), (0004) and (0006) reflections are shown in (d) for
the two films.
Table 1
Micro-structural parameters of GaN films deposited on quartz and ZnO buffer layer over
quartz at a substrate temperature of 700 °C
Substrate Lateral
crystallite
size (nm)
Crystallite Vertical
Microtilt (°)
crystallite strain
size (nm)
Quartz
10
5.4
40
ZnO/
quartz
N limit of
2.5
measurement
200
Lateral size Surface
of surface
roughness,
features
rms (nm)
from AFM
(nm)
5.9 × 10− 3 40–50 and
80–120
2.3 × 10− 3 100–200
7
11
estimated respectively, from the intercept and slope of the linear
Williamson–Hall plots of Δqx (FWHM along qx direction) vs. q.
ω–2θ scans for the symmetric (0002), (0004) and (0006)
reflections for GaN films deposited at 700 °C on quartz and ZnO
buffer layer over quartz are shown respectively, in Fig. 2(a–c). For all
the cases, the FWHM in reciprocal space (Δqz) are also indicated. The
(0006) reflection for GaN film on quartz substrate could not be
recorded due to very low signal intensity. As discussed above, the
vertical crystallite size and micro-strain were calculated from
Williamson–Hall plots (Δqz vs. q), shown in Fig. 2(d). The values of
these parameters are listed in Table 1. It may be noted that the vertical
crystallite size of the film deposited on ZnO buffer layer is ~200 nm as
compared to the size of ~40 nm for the film deposited on quartz
substrate. The micro-strain is found to be 2.3 × 10− 3 in the film
deposited on ZnO buffer layer, which is nearly one third of the value of
5.9 × 10− 3, found in the film deposited on quartz substrate. These
results show that the presence of ZnO buffer layer results in a drastic
increase in the crystallite size (along growth direction) of GaN film to a
large value ~ 200 nm and a significant reduction in the micro-strain to
a value ~ 2 × 10− 3. There are no reported values of micro-strain in
polycrystalline GaN films deposited by MBE, MOCVD, PLD or
sputtering, over any substrate. Micro-strain values of (2–4) × 10− 4
have been reported for epitaxial GaN films on sapphire substrates,
grown by MOCVD [55]. Considering the polycrystalline nature of
sputtered GaN films deposited in the present work, the micro-strain
value of ~2 × 10− 3 is quite low. It indicates the high crystalline quality
of the GaN film on ZnO buffer layer, which is attributed to the large
crystallite size and limited presence of point defects in the film.
Fig. 3(a–e) shows the ω scans for the symmetric (0002), (0004)
and (0006) reflections for the GaN films deposited at 700 °C on quartz
and ZnO buffer layer over quartz. For all the cases, the FWHM in reciprocal
space (Δqx) are also indicated. The ω scans of the film deposited on quartz
were found to be much broader (FWHM of (0002) ~6.76°) compared to the
film deposited on ZnO buffer layer (FWHM of (0002) ~2.60°). The
corresponding Williamson–Hall plots of Δqx vs. q are shown in Fig. 3(d), for
both the films. The linear fits for the two films show significantly different
slopes, from which the values of crystallite tilt have been estimated and
listed in Table 1. The film deposited on ZnO buffer layer exhibits a much
lower slope as compared to the film deposited on quartz. The
corresponding value of tilt ~2.5° for the film on ZnO buffer layer is
significantly smaller than the value of ~5.4° for the film deposited on
quartz. The intercept of the Δqx vs. q plot for the film deposited on quartz
was found to be much higher compared to the film deposited on ZnO
buffer layer. This indicates that the lateral crystallite size of the film on
quartz is much smaller compared to that of the film on ZnO buffer layer.
The value of ~10 nm for the lateral crystallite size in GaN film deposited on
quartz agrees well with the value obtained by transmission electron
microscopy and reported earlier [53]. The nearly zero intercept seen in the
case of the GaN film deposited on ZnO buffer layer shows that the lateral
crystallite size in this case is larger than the limit of measurements. The
crystallite tilt of ~2.5° in sputtered GaN film on ZnO buffer layer over
quartz substrate, is comparable to the reported value of ~2° for GaN films
sputter deposited on ZnO/Si substrates from a Ga2O3 target [51]. The
491
constant at all values of ϕ. This indicates the absence of any in-plane
orientation of crystallites in these films, which is attributed to the
amorphous nature of quartz substrate and the absence of in-plane
orientation in ZnO buffer layer, which was also independently checked.
The surface morphology of GaN films deposited at 700 °C on quartz
and ZnO buffer layer was investigated by atomic force microscopy and
the results are shown in Fig. 4. The corresponding values of lateral size of
surface features and surface roughness are also included in Table 1. The
GaN film deposited on ZnO buffer layer exhibits significantly different
surface features compared to the film deposited on bare quartz
substrate. It shows very large lateral surface features ~100–200 nm,
which are distinctly different from the mixture of small (~40–50 nm)
and large (~80–120 nm) sized surface features, seen in the GaN film on
quartz substrate. It may be pointed out that the surface features may not
necessarily represent single crystallites, but the larger size seen in case of
GaN film on ZnO buffer layer is indicative of the increase in crystallite
size, associated with the strong c-axis orientated growth. This result
corroborates well with the ω-scan measurements, which also indicated
a large increase in lateral crystallite size of the GaN film deposited on
ZnO buffer layer. The film deposited on ZnO buffer layer also has a larger
surface roughness of ~11 nm, which is somewhat higher than the films
grown on quartz substrate (~7 nm). The increase in surface roughness
appears to be associated with the enhancement of c-axis orientation of
crystallites.
Fig. 5 shows a comparison of Raman spectra of GaN films deposited
at 700 °C on quartz and ZnO buffer layer over quartz. The Raman
spectrum of bare quartz substrate has also been shown as reference.
The spectrum of GaN film deposited on quartz substrate is dominated
by the peak centred at ~ 728 cm− 1 along with a broad peak ~560 cm− 1.
The peaks ~560 cm− 1 and ~ 728 cm− 1 are attributed to E2 (high) and A1
(LO) modes (LO denotes longitudinal optical), which are reported at
567.6 cm− 1 and 734 cm− 1 for epitaxially grown GaN films [58],
Fig. 3. ω-scans for GaN films deposited at substrate temperature of 700 °C on quartz
(———) and ZnO buffer layer over quartz (———) for (a) (0002), (b) (0004) and (c) (0006)
reflections. The broadening in the reciprocal space as obtained from the symmetric
scans along ω-axis (qx) for (0002), (0004) and (0006) reflections are shown in (d) for the
two films.
measured value of tilt is about an order of magnitude higher than the
reported values (from (0002) rocking curve measurements) for epitaxial
GaN films grown over ZnO buffer layers on sapphire substrates by MBE
[48], MOCVD [44] and PLD [50,57] techniques.
Phi (ϕ) scans of asymmetric (101̄1) reflections were also carried out
on both the films. But the intensity of these scans was found to be
Fig. 4. AFM images of GaN films deposited at substrate temperature of 700 °C on (a)
quartz and (b) ZnO buffer layer over quartz.
492
hexagonal systems such as polycrystalline ZnO films [52], has not been
frequently observed in sputtered GaN films, though it may be mentioned
that the data available on sputtered/polycrystalline GaN films is rather
limited. The c-axis orientation in ZnO films is believed to originate from
preferential (0002) nucleation [52,54,62], driven by minimization of
surface energy. It has been recently shown [53] that sputtered GaN films
exhibit (0002) preferential orientation of crystallites in early stages of
̄ and
growth, though crystallites with other orientations, such as, (1010)
̄
(1011)are
also present. This result indicates that (0002) plane in the GaN
system may also possess the lowest surface energy. However, the limited
data [63,64] for GaN indicate, nearly equal values of surface energy of
different crystallographic planes [63], which possibly results in the
preferred orientation of crystallites in thicker GaN films to be strongly
influenced at the growth stage by factors, such as, surface diffusion and
sticking. It has been shown for sputtered GaN films [53] that, governed
̄ or
essentially by substrate temperature, crystallites with either (1011)
(0002) plane grow, depending upon, whether surface diffusion takes
place between planes within the crystallites or between the crystallites.
This is also evident from the XRD patterns of GaN films in Fig. 1, which
show that even at relatively higher substrate temperatures of 700 °C, the
(0002) preferred orientation of GaN films on bare quartz substrates is
̄ orientation remain present
not facile enough and crystallites with (1011)
in the film. However, the presence of a ZnO buffer layer with (0002)
oriented crystallites clearly makes a major difference to this situation.
Being lattice matched and aligned with the growing GaN film, the buffer
layer prevents the formation of mis-oriented nuclei and crystallites of
GaN and thus limits the influence of factors, such as, surface diffusion
and possibly sticking, which become increasingly important at the
growth stage. The presence of ZnO buffer layer clearly has a very strong
influence and is effective in reducing the dislocation density of the
growing GaN film, resulting in the formation of large and c-axis oriented
GaN crystallites with small tilt. It is also clear that the ZnO buffer layer
absorbs significant part of the strain due to the amorphous nature of
quartz substrate and the much smaller lattice mismatch between GaN
and ZnO (1.8%) results in the reduction in micro-strain and thus the
density of point defects in the GaN film.
4. Conclusions
Fig. 5. Raman spectra of (a) quartz substrate, (b) GaN film deposited on quartz, (c) ZnO
buffer layer over quartz and (d) GaN film deposited on ZnO buffer layer over quartz. The
GaN films in both cases were deposited at 700 °C.
respectively. Two humps were also observed at ~260 cm− 1 and
~ 320 cm− 1, with the latter being identified as acoustic overtone [59].
The small low frequency peak ~ 260 cm− 1 has been attributed to As
clusters [60] or a zone boundary phonon mode [61] in GaN. The
Raman spectrum of the film deposited on ZnO buffer layer has all the
features similar to those described above, including a relatively
intense A1 (LO) mode peak and two very small peaks at ~258 and
~ 320 cm− 1, but most interestingly, it is dominated by a strong and
sharp E2 (high) mode at ~560 cm− 1. It is known that in the z(x,y)z̄
geometry, with z-axis along the c-axis of the hexagonal structure, the
Raman spectrum of GaN is expected to show only the E2 (high) and A1
(LO) modes [59]. This result is thus in tune with the micro-structural
studies of GaN film deposited on ZnO buffer layer, and confirms the
significant improvement in its crystalline quality, particularly c-axis
orientation of crystallites.
The above results can be summarized and explained as follows. The
significant increase in crystallite size (both lateral and along growth
direction) and the small values of tilt (~2.5°) and micro-strain (~2 × 10− 3)
seen in GaN film deposited at 700 °C on ZnO buffer layer over amorphous
quartz substrate show that this film possesses high crystalline quality, in
terms of crystallite size and orientation as well as absence of point
defects, in comparison to the film deposited on quartz substrate. The
strong c-axis or (0002) preferred orientation commonly reported in
GaN films were deposited on quartz and ZnO buffer layer on quartz
by reactive sputtering of GaAs target in 100% nitrogen at 550 °C and
700 °C. HRXRD, AFM and Raman spectroscopy studies together show
that the GaN film deposited at 700 °C on ZnO buffer layer possesses a
much improved crystallinity. In particular, it showed the presence of
large crystallites (~ 200 nm) along, as well as, perpendicular to growth
direction, strong and nearly complete c-axis orientation of crystallites,
with average tilt of ~ 2.5°. The micro-strain of ~2 × 10− 3 present in the
film deposited on ZnO buffer layer at 700 °C was nearly one third of
the corresponding value for the GaN film deposited on bare quartz
substrate. The significant improvement in crystallite size and
orientation, and the reduction of micro-strain in this film are
attributed to the presence of the ZnO buffer layer on quartz substrate
and its small lattice mismatch (1.8%) with GaN. The buffer thus
prevents the formation of mis-oriented nuclei or crystallites of GaN
and does not allow factors like surface diffusion and sticking to change
the nature of preferred orientation during growth stage, as observed
in the case of GaN films grown on bare quartz substrates.
Acknowledgements
The financial support for this work received from the Board of
Research in Nuclear Sciences, Department of Atomic Energy (GoI) is
gratefully acknowledged. Tapas Ganguli and Ravi Kumar are thankful to
Dr. S. M. Oak for his support and encouragement. FIST (PHYSICS)-IRCC
SPM Facility and CRNTS of IIT Bombay are respectively, acknowledged
for providing the facilities for AFM and Raman studies.
493
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