Japanese Journal of Applied
Physics
REGULAR PAPER • OPEN ACCESS
Influence of surface nitridation and an AlN buffer layer on the growth of
GaN nanostructures on a flexible Ti metal foil using laser molecular
beam epitaxy
To cite this article: Chodipilli Ramesh et al 2019 Jpn. J. Appl. Phys. 58 SC1032
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REGULAR PAPER
Japanese Journal of Applied Physics 58, SC1032 (2019)
https://doi.org/10.7567/1347-4065/ab0f17
Influence of surface nitridation and an AlN buffer layer on the growth of GaN
nanostructures on a flexible Ti metal foil using laser molecular beam epitaxy
Chodipilli Ramesh1,2, Prashant Tyagi1,2, Govind Gupta1,2, Muthusamy Senthil Kumar1,2, and Sunil Singh Kushvaha1,2*
1
CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
2
*
E-mail: kushvahas@nplindia.org
Received December 29, 2018; revised February 23, 2019; accepted February 27, 2019; published online May 17, 2019
GaN nanorods (NRs) and hollow nanocolumns (HNCs) were grown on flexible Ti foils using laser-assisted molecular beam epitaxy at a growth
temperature of 700 °C. The shape, size and density of the GaN nanostructures were tuned by surface nitridation and AlN buffer layer growth
temperature on a Ti foil. Sparse (∼ 5.5 × 108 cm−2) GaN NRs were obtained on the bare surface whereas dense (∼3.47 × 109 cm−2) GaN NRs
were grown on the nitridated Ti foil. The shape of the GaN changed from NRs to HNCs by introducing an AlN buffer layer on nitridated Ti foil.
Raman spectroscopy showed the grown GaN nanostructures have a wurtzite crystal structure. Room-temperature photoluminescence
spectroscopy measurements show that the GaN nanostructures possess an intensive near band edge emission at ∼3.42 eV with a negligible
defect-related peak. The growth of tunable GaN nanostructures on flexible metal foils is attractive for flexible optoelectronics and sensor devices.
© 2019 The Japan Society of Applied Physics
1. Introduction
GaN nanostructures are attractive for various energy applications such as light-emitting diodes (LEDs), laser diodes, solar
cells and photocatalysts in chemical hydrogen production due
to their excellent optoelectronic properties and stability in
harsh environments.1–7) Recently, the growth of one-dimensional (1D) GaN nanostructures on metal substrates has
attracted special attention due to their direct back contact,
high optical reflectivity and good thermal and electrical
conductivities, which are limited with conventional substrates
such as sapphire and silicon.8–10) Few reports in the literature
have presented the growth of 1D GaN nanostructures on
various metal substrates.11–16) References 15 and 16 successfully fabricated highly efficient 1D GaN LED devices on
metal substrates. The utilization of thin metal foil is of special
interest for III-nitride growth due to its flexibility and
availability in a large size at low cost, as is required for
large-scale production. Reference 17 showed that there is no
change in the optical properties of 1D GaN nanostructures
after bending of metal foil with a positive radius of curvature
of 4 mm. Reference 18 successfully demonstrated AlGaNbased LEDs on flexible Ta metal foil.
Flexible metal foils possess randomly oriented grains with
polycrystalline structures. The growth of 1D GaN nanostructures on polycrystalline metal foils leads to the formation
of randomly orientated nanostructures due to the different
crystal orientations of the grains present in the metal foil.
Reference 18 observed the random formation of GaN
nanowires (NWs) at different orientations on a polycrystalline Ti metal foil. These randomly orientated 1D GaN
nanostructures are difficult to integrate when it comes to
large-scale production. From the point of view of applications, therefore, it is essential to obtain a well-aligned 1D
GaN nanostructure on a metal foil. Reference 19 reported that
the pre-nitridation of Ti metal foil favored the growth of wellaligned GaN NWs due to suppression of the influence of the
crystal orientation of Ti grains by terminating the Ti top layer
in TiN and TiOxNy. There are only a few reports on the
growth of 1D GaN nanostructures on flexible metal
foils.17–19) Still, there are many challenges involved in
improving and controlling the shape, size and density of
other GaN nanostructures on flexible metal foils for future
nitride-based flexible devices.
Here, we present the effects of substrate nitridation as well
as GaN and AlN buffer layer growth conditions on the
formation of various GaN nanostructures on flexible Ti metal
foils using laser-assisted molecular beam epitaxy (LMBE). In
this study, our motivation is to enhance the alignment of the
GaN nanostructures on the polycrystalline Ti metal foil and
to control the shape and density of the GaN nanostructures.
We have introduced GaN and AlN buffer layers at different
growth temperatures on nitridated Ti metal foils. Thin AlN
film was most commonly used as a buffer layer, especially
with a Si substrate.8,20–22) The AlN buffer layer worked as a
blocking layer for suppression of unwanted interfacial layer
formation and also as a seed layer for growth of 1D GaN
nanostructures.23–27) Reference 26 showed that the AlN
buffer layer improves the alignment of GaN NWs on a Si
(111) substrate. Reference 29 reported the enhancement of
GaN nanocolumn alignment when a thin AlN buffer layer
was introduced on a graphene-layered Si substrate. Here, we
report on tuning of the shape of GaN nanostructures from
randomly oriented NRs to a well-aligned GaN hollow
nanocolumn (HNC) array on a nitridated Ti metal foil
when the growth temperature of the AlN buffer layer is
raised from 600 to 700 °C. The possible growth mechanism
of various GaN nanostructures obtained on Ti foil is also
discussed. The structural and optical properties of LMBEgrown GaN nanostructures on flexible Ti metal foils were
studied systematically.
2. Experimental details
Various GaN nanostructures have been grown by the LMBE
growth technique on 0.127-mm-thick Ti metal foils (Alfa
Aesar, purity 99.99%) on an area of 10 mm × 10 mm by
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Jpn. J. Appl. Phys. 58, SC1032 (2019)
C. Ramesh et al.
introducing surface pre-nitridation as well as GaN and AlN
buffer layers in the growth sequence. Initially, the Ti metal
foil was chemically cleaned with standard organic solvents
and degassed in a load lock chamber (base pressure of
2.3 × 10−7 Torr) at 230 °C. The degassed Ti metal foil was
further thermally cleaned in a growth chamber at 850 °C for
30 min under ultra-high-vacuum (base pressure 2.0 × 10−10
Torr) conditions. Nitridation of the Ti foil was performed at
800 °C under a RF nitrogen plasma with a nitrogen flow rate
of 1.1 sccm at 400 W plasma power. Here, we prepared two
sets of GaN samples: in the first set, GaN was grown directly
on bare (sample A0) and pre-nitridated (sample A1) Ti foil.
In the second set, various buffer layers were grown on prenitridated Ti foils. For example, low-temperature (LT, 600 °
C) GaN (sample B0), a LT (600 °C ) AlN buffer layer
(sample B1) and high-temperature (HT) AlN at 700 °C
(sample B2) were grown on the nitridated Ti foil before the
growth of the main GaN. A KrF excimer pulsed laser
(wavelength 248 nm, pulse 25 ns) was used to ablate the
target materials. The GaN was grown by ablating a hydride
vapor phase epitaxy-grown polycrystalline solid GaN target
(purity 99.9999%) with a laser repetition rate of 20 Hz and
laser energy density of about 3 J cm−2, whereas the AlN
buffer layer was grown by ablating a high-temperature
sintered AlN solid target with a laser repetition rate of
10 Hz and a laser energy density of about 5 J cm−2. During
the growth, additional nitrogen plasma was supplied with a
nitrogen flow rate of 0.4 sccm at an applied forward power of
250 W to maintain the nitrogen-rich conditions. The main
GaN was grown at 700 °C under nitrogen-rich growth
conditions for a duration of 2 h.
The surface morphology of LMBE-grown GaN samples
was investigated using field emission scanning electron
microscopy (FESEM) (FIB, Zeiss, Germany) with an operating voltage of 5 kV. Room-temperature Raman spectroscopy was employed to find the crystal phase of the GaN
nanostructures on Ti metal foil by exciting the samples with
an excitation laser source of wavelength 514.5 nm. The
optical emission properties were analyzed using photoluminescence (PL) spectroscopy measurements at room temperature with a 325 nm He–Cd laser as the excitation source.
3. Results and discussions
Figure 1(a) shows the 45° tilt-view FESEM image of LMBEgrown GaN on thermally cleaned bare Ti metal foil without
any pre-nitridation. It reveals the growth of randomly
distributed GaN NRs on untreated Ti metal foils at the kink
sites or steps of thermally cleaned Ti foil.19) The length,
width and density of GaN NRs were statistically analyzed
using several FESEM images and were obtained as 170–
650 nm, 20–60 nm and ∼ 5.5 × 108 cm−2, respectively. In
Ref. 19, randomly oriented GaN NWs were obtained on
bare Ti foil that had a rough surface; in our case, vertically
oriented growth of GaN NWs was observed on steps or kinks
on bare Ti foil. High-density, well-aligned GaN NRs were
obtained when GaN was grown on pre-nitridated Ti metal foil
[Fig. 1(b)]. The grown GaN NRs are closely packed and the
NRs were found to coalesce with neighbors at a few
locations. The inset of Fig. 1(b) shows the cross-sectional
FESEM image. The length, width and density of GaN NRs
were in the ranges 220–320 nm, 90–150 and
Fig. 1. (Color online) 45° tilt-view FESEM images of GaN NRs grown on
(a) bare (sample A0) and (b) nitridated (sample A1) Ti metal foils. The inset
of (b) shows the cross-sectional FESEM image of sample A1.
∼3.47 × 109 cm−2, respectively. These observations reveal
that the adsorption of laser-ablated Ga and growth of GaN1–x
adatoms are highly enhanced on nitridated Ti metal foil
compared with bare Ti foil. Reference 19, using ex situ X-ray
photoelectron spectroscopy (XPS), reported the formation of
TiN and TiOxNy in the uppermost layers after pre-nitridation
of Ti metal foil. Reference 28 reported that native oxide
layers on TiN caused by exposure to air only disappear after
isothermal annealing at 1000 °C in an ultra-high vacuum.
These observations show that the possibility of surface
oxidation of nitridated Ti foil cannot be ignored after
exposure to ambient conditions and it requires systematic
in situ XPS measurements to better understand the chemical
composition and species present on the uppermost layer of
nitridated Ti metal foil.
Further, we have studied the effect of GaN and AlN buffer
layers on GaN nanostructure formation and its surface
modification when grown on pre-nitridated Ti foil.
Figure 2(a) shows GaN growth on the LT-GaN buffer layer
(sample B0). A GaN film consisting of nano-islands was
grown and the lateral sizes of these islands were found to be
in the range of 50–170 nm. This reveals that the LT-GaN
layer suppresses the 1D elongated growth. For the LT-AlN
buffer layer on nitridated Ti foil, GaN NRs were obtained
after GaN growth at 700 °C, as shown in Fig. 2(b). The crosssectional FESEM image presented in the inset of Fig. 2(b)
shows the randomly oriented growth of GaN NRs. The GaN
NRs grown on LT-AlN have a length, width and density in
the range of 80–130 nm, 20–50 nm and ∼5.66 × 109 cm−2,
respectively. For GaN grown on pre-nitridated Ti metal with
a HT-AlN buffer layer, the surface morphology changes from
inclined NRs to vertically aligned HNC structures as seen in
Fig. 2(c). The height, top width and density of GaN HNCs
are in the range of 220–330 nm, 110–200 nm and
∼3.78 × 109 cm−2, respectively. The density of the GaN
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© 2019 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 58, SC1032 (2019)
C. Ramesh et al.
Fig. 2. (Color online) (a) FESEM image of GaN island film (sample B0) grown on a LT-GaN buffer layer on nitridated Ti metal foil. 45° tilt-view FESEM
image of LMBE-grown GaN on: (b) a LT-AlN buffer layer on nitridated Ti (sample B1); (c) a HT-AlN buffer layer on nitridated Ti (sample B2). Insets in (b)
and (c) show the cross-sectional FESEM images of sample B1 and B2, respectively.
HNCs is close to the directly grown aligned GaN NRs on
nitridated Ti metal foil. The FESEM studies clearly reveal
that the AlN buffer growth temperature plays an important
role in the shape transition of GaN nanostructures from NRs
to HNCs.
The change in GaN nanostructure morphology can be
understood on the basis of the influence of surface modification by pre-nitridation and GaN and AlN buffer layer growth
on Ti metal foil. The GaN grown on bare Ti metal foil forms
sparse GaN NRs with a wider size variation, probably due to
limited nucleation of GaN at a specific location such as Ti
grain boundaries or kink sites [Fig. 1(a)].19) The chemical
reaction of Ti metal atoms with impinging GaN adatoms
suppresses GaN nucleation on Ti metal.19) As growth
progresses under nitrogen plasma, new nucleation sites can
be formed on Ti foil and can lead to a wider size distribution
(170–650 nm) of GaN NRs on bare Ti metal foil. With prenitridation of Ti metal foil, the nitridated layer is likely to
reduce the chance of a direct reaction between Ti metal and
the impinging adatoms and thus enhances the GaN nucleation
sites on Ti metal foil, resulting in high-density growth of
GaN NRs. In addition to the surface nitridation of Ti metal
foil, the introduction of a HT-AlN (700 °C) seed layer
modifies the morphology of grown GaN from NRs to
HNCs. The shape transition may occur due to the formation
of stacking faults and partial stress, which originates at the
HT-AlN nucleation sites.27) For a LT-AlN (600 °C) buffer
layer, inclined GaN NRs were formed on Ti metal foil [seen
in Fig. 2(b)]. The AlN buffer layer grown at low temperature
forms a smooth-grained AlN film due to the short diffusion
length of AlN growth adatoms.27) The GaN grown on the LTAlN buffer layer forms a coalesced dense film as a wet layer
and further promotes GaN NRs at the grain edges under
nitrogen-rich conditions.23,27) A similar behavior is observed
when LT-GaN was used as a buffer layer on nitridated Ti
metal foil, as we can see in Fig. 2(a), promoting dense growth
of nano-islands.
Structural analysis of GaN nanostructures grown on Ti foil
was carried out by micro-Raman spectroscopy measurements
under backscattering geometry at room temperature. Raman
spectroscopy is a convenient technique for identifying the
crystal phase of GaN nanostructures.30–32) For example, the
E2 (high) Raman peak for GaN is dominant for the wurtzite
GaN structure whereas shift of the E2 (high) peak position
shows the stress present in GaN.30) Figure 3(a) illustrates the
Raman spectra of GaN nanostructures for samples A1, B1
and B2. The Raman spectra were fitted with the Lorentzian
function for identification of various peaks and estimation of
full width at half maximum (FWHM) values for E2 (high)
peaks [Fig 3(b)].30) Here, the prominent Raman E2 (high)
phonon peak was observed for samples A1, B1 and B2 at
566.37, 566.57 and 566.96 cm−1, respectively. These E2
(high) peak values are close to the stress-free wurtzite GaN
E2(high) peak value of 567.6 cm−1.32–34) The presence of a
Raman E2 (high) phonon peak reveals that the GaN
nanostructures grown on Ti foils possess a wurtzite crystalline phase. The redshift values of the Raman E2 (high) peak
of GaN samples A1, B1 and B2 are 1.23, 1.03 and
0.64 cm−1, respectively, with respect to stress-free GaN.
The stress (σ) present in GaN nanostructures can be estimated
from the following equation:34,35)
w - w0
s=
(1 )
4.3
where ω and w0 are the E2 (high) peak positions of the
measured samples and stress-free GaN (567.6 cm−1), respectively. The estimated values of tensile stress for samples A1,
B1 and B2 are 0.28, 0.24 and 0.15 GPa, respectively. Here,
sample B2 has a lower tensile stress than samples A1 and B1.
These observations reveal that the AlN buffer layer affects
the stress present in GaN and it is lower for a HT-AlN buffer
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Jpn. J. Appl. Phys. 58, SC1032 (2019)
C. Ramesh et al.
Table I. Raman and PL data for LMBE-grown GaN nanostructures on
various buffer layers grown on Ti metal foils (NBE, near band edge).
Raman data
Sample
index
A1
B0
B1
B2
PL data
E2 (high)
peak position (cm−1)
E2 (high)
peak FWHM
value (cm−1)
NBE peak
position
(eV)
NBE
FWHM
value
(meV)
566.37
—
566.57
566.96
6.0
—
11.0
9.5
3.42
3.36
3.43
3.43
230
340
280
330
Fig. 3. (Color online) (a) Room-temperature Raman spectra of LMBEgrown GaN nanostructures on Ti metal foils (samples A1, B1 and B2). (b)
Lorentzian fitted Raman spectrum of GaN NR sample A1 showing the
presence of A1 (TO) and E1 (TO) peaks.
layer. Reference 32 showed that GaN NRs grown on a
graphene-layered Si substrate with a thin AlGaN buffer layer
had a tensile stress at the bottom of the NR and this tensile
stress decreased from the bottom to the top.
Figure 3(b) shows the Raman spectrum of GaN NR sample
A1 fitted with the Lorentzian function. Apart from the E2
(high) peak, we can observe forbidden E1 (TO) and A1 (TO)
peaks at ∼556.93 and ∼529.2 cm−1, respectively. The
obtained E1(TO) peak position for LMBE-grown GaN NRs
on Ti foil is very close to previous reports on the freestanding GaN NRs grown (557.8 cm−1) by MBE36) The
appearance of Raman E1 (TO) and A1 (TO) peaks is due to
either sidewall scattering of the incident beam or inclined
GaN nanostructures.30–33) We have also estimated the
FWHM value for the Raman E2 (high) peak to find out the
structural quality of samples A1, B1 and B2 and their FWHM
values; the results are listed in Table I. Sample A1 has a
lower FWHM value, indicating the good structural quality of
GaN NRs.
Figure 4(a) shows the PL spectra of GaN nanostructure
samples grown on pre-treated Ti foil. Intense near band edge
(NBE) emission is observed for all the GaN samples. The PL
NBE peak positions of the GaN nanostructures are 3.42, 3.36,
Fig. 4. (Color online) (a) Room-temperature PL spectra of samples A1,
B0, B1 and B2. (b) The respective normalized PL spectra; the dashed line
indicates the band gap of bulk GaN (3.4 eV).
3.43 and 3.43 for samples A1, B0, B1 and B2, respectively.
Compared with the NBE position of bulk GaN (3.4 eV),37)
GaN samples A1, B1 and B2 show a blue shift of 20, 30 and
30 meV, respectively, whereas sample B0 shows a redshift of
40 meV, as we can see in the normalized PL spectrum in
Fig. 4(b). The blue shift of GaN nanostructure samples A1,
B1 and B2 mainly occurs due to variation of stress from the
bottom to the top of the GaN nanostructures.32,38) The
redshift of the PL NBE peak in sample B0, i.e. GaN film,
may due to the generation of point defects during the
coalescence of GaN islands.39)
Here, sample A1 exhibits a high PL intensity with a
negligible yellow luminescence (YL) peak compared with the
other samples [Fig. 4(a)]. However, the GaN nanostructures
with an AlN buffer layer showed a significant YL peak of the
PL spectrum, with a maximum of ∼2.26 eV. The GaN PL
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C. Ramesh et al.
spectrum indicates that the transition of optically excited
electrons from the conduction band to deep acceptor states is
negligible for samples A1 and B0.40–42) The prominent YL
defect band in samples B1 and B2 is probably due to the
presence of surface point defects.31,43) The broadening of the
PL NBE peak was resolved by Lorentzian fitting analysis,
and the FWHM values are listed in Table I. Samples B0, B1
and B2 show high PL NBE peak broadening (280–340 meV)
whereas sample A1 shows a slightly lower PL NBE peak
broadening (230 meV). The broadening of the PL NBE peak
is due to variation of tensile stress in the nanostructures
together with the superimposed luminescence intensity.30,32)
Overall, the Raman and PL spectroscopy studies imply that
the GaN sample A1 has good structural and optical properties
compared with the other samples. These results demonstrate
that the shape and size of the GaN nanostructure can be tuned
on Ti metal foil by pre-growth surface modifications such as
surface nitridation and a AlN buffer layer.
4. Conclusions
GaN NRs and HNCs have been successfully grown on
flexible Ti metal foil using the LMBE growth technique by
tuning the surface conditions of the Ti metal. The surface
morphology studies with FESEM measurement revealed that
surface pre-nitridation and an AlN buffer layer play an
important role in determining the size, shape and density of
GaN nanostructures. Pre-nitridation of the Ti metal foil helps
to increase the density of GaN NRs and also enhances their
uniformity in size and shape. The growth temperature of the
AlN buffer layer on nitridated Ti metal is found to change the
morphology from NRs to HNCs. Raman spectroscopy
analysis indicates that the grown GaN nanostructures are in
the wurtzite crystal phase and possess minimal tensile biaxial
stress. Room-temperature PL spectroscopy shows that the
GaN NRs grown on nitridated Ti metal foil have a high NBE
peak intensity compared with the other GaN nanostructures.
The growth of controllable GaN nanostructures on flexible Ti
metal can be attractive for flexible high-efficiency lightemitting diodes, laser diodes, solar cell and photo-electrochemical water-splitting applications.
Acknowledgments
The authors are grateful to the Director, NPL for his constant
encouragement and support. The authors would like to thank
Dr N. Dilawar, Dr M. Saravanan and M. Kaur for their help
in characterizations. CR and PT would like to thank CSIR for
the senior research fellowship (CSIR-SRF) award. CR thanks
SERB-DST (file no. ITS/2018/005054) for financial support
to present this work in IWN-2018, Japan.
ORCID iDs
Sunil Singh Kushvaha
7486
https://orcid.org/0000-0003-0698-
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