Journal of Alloys and Compounds 739 (2018) 122e128
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Dependence of Al incorporation on growth temperature during laser
molecular beam epitaxy of AlxGa1 xN epitaxial layers on sapphire
(0001)
Prashant Tyagi a, b, Ch Ramesh a, b, S.S. Kushvaha a, Monu Mishra a, b, Govind Gupta a,
B.S. Yadav c, M. Senthil Kumar a, b, *
a
b
c
CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Academy of Scientific and Innovative Research (AcSIR), CSIR-NPL Campus, Dr. K.S. Krishnan Marg, New Delhi 110012, India
Solid State Physics Laboratory, Timarpur, Lucknow Road, New Delhi 110054, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 September 2017
Received in revised form
20 December 2017
Accepted 21 December 2017
Available online 22 December 2017
We report the successful growth of AlxGa1 xN (0 x 0.25) epitaxial films on c-plane sapphire substrate
by using laser molecular beam epitaxy technique. The role of growth temperature on the Al incorporation and the structural, electronic and optical properties of the AlxGa1 xN epitaxial layers grown in the
temperature range 500e700 C have been systematically studied. The atomic force microscopy analysis
shows that the grain size of AlxGa1 xN increases with increase in growth temperature and flat surface
epilayers are obtained at 600 C. The Al incorporation is confirmed with high resolution x-ray
diffraction, x-ray photo electron microscopy and photoluminescence studies. It is observed that the
growth temperature plays a critical role in determining the Al composition, which increases with
increasing growth temperature. AlxGa1 xN layer with about 23% of Al composition is obtained on sapphire (0001) substrate at a growth temperature of 700 C, which is about 100e150 C lower than the
conventional molecular beam epitaxy growth.
© 2017 Elsevier B.V. All rights reserved.
Keywords:
Group III-Nitrides
Hetero-epitaxy
Atomic force microscopy
High resolution X-ray diffraction
X-ray photoelectron spectroscopy
Photoluminescence spectroscopy
1. Introduction
In the past two decades, AlGaN epitaxial layers have gained a
great amount of research interest due to their large, direct and
tunable band gaps ranging from 3.4 to 6.2 eV. This ternary alloy of
group III-nitrides are being successfully employed for fabricating
high electron mobility transistor (HEMT) and solid state ultra violet
(UV) radiation sources and detectors in the wavelength range
210e365 nm [1e7]. AlGaN/GaN based HEMTs are known for their
high switching speed and high power density and require less
thermal management as compared to conventional silicon solid
state devices. AlGaN based UV light-emitting diodes (LEDs) and
laser diodes (LDs) could be used in lithography, bacteria disinfection and water purification. Moreover, they have a potential to
replace hazardous mercury based conventional UV sources used for
domestic as well as industrial applications. The crucial step in the
* Corresponding author. CSIR-National Physical Laboratory, Dr. K.S. Krishnan
Marg, New Delhi 110012, India.
E-mail address: senthilmk@nplindia.org (M. Senthil Kumar).
https://doi.org/10.1016/j.jallcom.2017.12.220
0925-8388/© 2017 Elsevier B.V. All rights reserved.
fabrication of III-nitride based UV devices is the growth of good
crystalline quality AlGaN epilayer that acts as active emission and
absorption medium. But, the growth of high quality ternary AlGaN
alloy, especially with high Al content, is very challenging due to
increased alloy scattering, phase segregation and the large lattice
and thermal mismatch with substrates. Mainly, Al adatoms are
much less mobile on the growth front as compared to Ga adatoms
due to higher sticking co-efficient and fail to incorporate at more
energetically favorable lattice sites, which results in higher defect
densities [8]. The straight forward approach to reduce the defect
density is to develop a low-temperature growth process for AlGaN
so as to employ closely lattice-matched uncommon substrates like
ZnO, Spinel, LiGa2O3, etc. that are vulnerable to high temperature
process [9e12].
Laser molecular beam epitaxy (LMBE) technique [also referred
as ultra high vacuum pulsed laser deposition or UHV-PLD] is being
developed as a low temperature growth technique for III-nitrides
since the laser power assists the kinetic energy of the growth
precursors for surface mobility and compound formation [13,14].
LMBE technique is relatively naive compared to conventional
P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128
techniques for III-nitrides such as metal organic chemical vapor
deposition (MOCVD) and molecular beam epitaxy (MBE) and the
influence of experimental parameters on LMBE growth of AlGaN is
not well understood yet. Huang et al. first grew AlGaN on c-plane
sapphire employing PLD using GaN and AlN sintered targets and
nitrogen gas as precursors [15]. But, the layers grown using sintered
target are characterized with a high level of impurities that trapped
air in the sintered targets. Masuyama et al. could solve the impurity
issue of AlGaN growth on sapphire substrates by employing high
purity metal based targets and nitrogen gas as precursor sources
but the crystalline quality of the grown layers remained an issue
due to lack of availability of high density nitrogen radicals [16].
These problems were addressed by Kobayashi et al. and Ueno et al.
by growing AlGaN layers by laser ablation of high pure metal targets in the presence of r.f. nitrogen plasma on lattice-matched ZnO
substrate [10,17]. Considering the commercial viability of sapphire
substrate for III-nitride device fabrication, it is important to develop
LMBE growth process for high quality AlGaN on sapphire. In this
paper, we report, for the first time to the best of our knowledge, on
the growth of AlxGa1 xN (0.0 < x < 0.25) epitaxial layers on the cplane sapphire substrate by using AlxGa1 x alloy target and r.f. nitrogen plasma as the precursors. Specifically, we discuss the effect
of growth temperature on the Al mole fraction and the quality of
LMBE grown AlGaN layers possess the epitaxial quality comparable
to that grown on lattice-matched ZnO substrate [10].
123
laser power and frequency were used in the range 3e5 J/cm2 and
45 Hz. The growth sequence followed for the AlGaN growth is
presented in Fig. 1(a) [18]. The growth rates are observed to be 0.22
and 0.1 mm/h for the growth temperatures of 500 and 700 C,
respectively.
The grown AlxGa1 xN layers have been characterized for their
structural, morphological, compositional, electronic and optical
properties using high resolution x-ray diffraction (HR-XRD), atomic
force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron
spectroscopy (XPS) and photoluminescence (PL) measurements. A
PANalytical X'Pert PRO MRD HR-XRD system, with CuKa1 source,
was employed to characterize the crystalline quality of AlGaN films
grown on sapphire substrate. A Multimode-V Veeco AFM was
employed in tapping mode using silicon tips of curvature radii less
than 10 nm for the surface morphology studies of AlxGa1 xN films.
The surface morphology of grown layers was also characterized
using field emission scanning electron microscopy (FE-SEM) (FIB,
ZEISS, Germany) at an operating voltage of 5 KV. The PL measurements were done using an excitation laser of wavelength 266 nm at
room temperature. The XPS measurements were performed in an
UHV based Omicron Multiprobe Surface Analysis System operating
2. Experimental
The growth of epitaxial AlxGa1 xN on sapphire (0001) substrates was carried out in an LMBE system equipped with reflection
high energy electron diffraction (RHEED), laser ablation target and
a radio-frequency (RF) nitrogen plasma source to supply nitrogen
radicals to the growth surface. The base pressure of the main
chamber was maintained at 2 10 10 Torr. The backside of sapphire wafer was coated with a layer of molybdenum of thickness
about 1 mm in order to increase the absorption of heat radiation and
uniform heating all around the substrate. The substrates were
cleaned with standard organic solvents and de-ionized water
before loading into the system. The AlxGa1 x alloy target was prepared using high purity Al and Ga elemental metals. Al and Ga
metals (20:80 at.%) were put together in a quartz ampoule and
sealed under a high vacuum of order 10 6 Torr. The ampoule was
then annealed in a hot furnace of temperature at 700 C for 60 h.
The substrates and targets were loaded inside the load-lock
chamber and degassed to remove any surface contaminants due
to their exposure to the atmosphere. The degassing was done under
a high vacuum of 10 7 Torr. The temperature was raised gradually
to 250 C and maintained for 2 h. After degassing, the target and
substrate were transferred into the growth chamber. Thermal
cleaning of sapphire substrate was carried out under UHV condition
by gradually increasing the temperature to 850 C by radiative
heating using infra-red (IR) heater for 10 min. Thermal cleaning
improves the surface quality with terraces suitable for epitaxial
growth. Afterwards, the substrate temperature was brought down
to 700 C for sapphire nitriadation that was carried out for 35 min
under RF nitrogen plasma ambient. The RF plasma power and N2
gas flow rate were kept constant at 400 W and 1.2 sccm with a
nitrogen partial pressure of 6.7 10 5 Torr. During the nitridation
process, a few monolayers of the sapphire substrate are converted
into AlN, which has a close lattice match to AlGaN and acts as a
buffer for AlGaN growth. We grew the AlGaN layer for 2 h on
nitridated sapphire substrate in the temperature range 500e700 C
under the nitrogen plasma power of 250 W and the nitrogen partial
pressure of 2.2 10 5 Torr. A KrF laser of 248 nm wavelength and
25 ns pulse width was used for the ablation of the alloy target. The
Fig. 1. (a). The growth sequence followed during growth of AlxGa1 xN layers by LMBE
technique (b). In-situ RHEED observations along [11e20] and [10e10] incident azimuths at various stages of AlxGa1 xN growth process: Sapphire cleaning, nitridation,
and growth under metal-rich and N-rich conditions.
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P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128
at a base pressure of 5 10 11 Torr. For XPS analysis, MgKa radiation source (1253.6 eV) was employed. An OMICRON EA125
hemispherical analyzer (operating at pass energy of 20 eV) equipped with a seven channeltron parallel detection unit was used to
collect the XPS spectra. The calibration of binding energy in
photoemission spectra was done referring to Au 4f7/2 emission line
[19].
3. Results and discussion
The entire growth sequence of AlGaN layer was monitored using
in-situ RHEED observations as shown in Fig. 1(b). A short streaky
RHEED pattern with kikuchi lines was observed for the bare sapphire at the end of UHV cleaning. The intensity of the kikuchi lines
became poor after sapphire nitridation and additional streaky
patterns belonging to AlN appeared along with sapphire features as
indicated in the respective images. It revealed the formation of AlN
on the sapphire surface, which is well known to improve the
crystalline quality of the epitaxial III-nitride layers [20]. The
epitaxial growth of AlGaN layers was examined under different III/
N flux conditions. A well defined streaky (2-dimensional growth) or
a spotty (3-dimensional growth) RHEED pattern was obtained
during AlGaN growth under metal-rich or N-rich condition,
respectively, as shown in Fig. 1(b). This observation is similar to the
conventional MBE growth process of III-nitrides. The surface
mobility of adatoms is reported to be relatively higher under Garich flux condition that leads to 2D growth of III-nitride epilayers
[21]. For the current study, all AlGaN samples were grown under
slightly metal-rich condition in the temperature range 500e700 C.
From HRXRD 2q-u scan measurements, a shift of XRD peak
positions towards higher angle was noticed for the AlGaN layers as
compared to GaN. The XRD peak shift increased with growth
temperature and was minimal for the AlGaN layers grown up to
600 C due to the close lattice constants of GaN and AlN (lattice
mismatch~2.4%) [22]. An apparent peak shift in the XRD position
has been obtained for the layer grown at 700 C as presented in
Fig. 2(a). It implies that the Al incorporation in the AlGaN layer
increases with the increase of growth temperature from 500 to
700 C. The XRD peaks corresponding to only (0001) crystalline
planes are obtained exhibiting that the AlGaN layer is grown along
c-axis with a wurtzite hexagonal crystalline structure. Moreover,
the shift in XRD peak position towards higher angle when
compared to GaN peaks indicates a decrease in lattice parameter
due to the incorporation of Al in the grown layer. Using Vegard's
Law [23], we could further calculate the Al composition from the
peak shift of (0004) plane to be ~24% for the layer grown at 700 C,
which is close to the composition of Al20Ga80 target alloy [24].
In order to examine the tilt and twist mosaic structures, the xray rocking curve measurements were employed along symmetric
(0002) and asymmetric (10e12) reflections of AlGaN layers. It is
found that the full width at half maximum (FWHM) of both curves
decreases with increasing growth temperature as given in Fig. 2(b).
The inset shows the (0002) rocking curve of AlGaN layer grown at
various temperatures. The rocking curves are symmetric that indicates a minimal of any Al inhomogenity in the grown layer. A
rocking curve FWHM of 0.4 and 0.95 is obtained along the (0002)
and (10e12) planes of AlGaN layer, respectively, grown at 500 C.
However, the FWHM values of (0002) and (10e12) plane rocking
curves decreased to 0.27 and 0.68 when the AlGaN growth temperature is increased to 700 C. These are the lowest values reported for AlGaN layer grown on sapphire by employing LMBE
technique with an Al composition >20% and are comparable with
that grown on closely lattice-matched ZnO substrate [10]. Here, it
should be noted that the crystalline quality of the AlGaN layers was
retained even with increased Al mole fraction with increasing
Fig. 2. (a) HRXRD 2q-u scan of GaN and AlxGa1 xN layer grown on sapphire (0001) by
LMBE at 500 and 700 C, respectively. (b) X-ray rocking curve FWHM values of
symmetric and asymmetric plane reflections of AlxGa1 xN layer as a function of growth
temperature. The inset shows the normalized (0002) plane x-ray rocking curves of
AlxGa1 xN layers grown at various temperatures.
growth temperature. In case of MOCVD process, Kucukgok et al.
obtained a (0002) x-ray rocking FWHM of 0.146 for 600 nm thick
of Al0.23Ga0.77N layer grown on c-sapphire using AlN buffer [25].
Monroy et al. reported ~0.2 of x-ray rocking curve FWHM from the
(0002) reflection of AlGaN layer (0.5e0.7 mm thick) with Al content
of 23% grown by plasma-assisted MBE on GaN/sapphire template
[26].
The surface morphology of LMBE grown AlGaN layers at various
temperatures as imaged by SEM and AFM measurements are presented in Fig. 3. As seen in Fig. 3(a), at 500 C, the AlGaN layer was
grown as a high density of nano-sized islands and localized
agglomeration of islands are also found across the surface as bright
spots. With increase of growth temperature, the grain size
increased and a flat surface growth is observed for the layers grown
at and above 600 C without any particle-like agglomerations. For
the layer grown at 600 C, deep valley-like features are observed on
the surface due to incompletion of coalescence process of AlGaN
layer growth. AlGaN nucleates and grows as islands at the
P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128
125
Fig. 3. SEM and AFM surface morphologies of AlxGa1 xN layer grown on sapphire (0001) at (a) 500, (b) 600 and (c) 700 C.
beginning and with the progress of growth the islands coalescence
due to enhanced lateral growth and form a smooth layer surface.
The rms surface roughness values are noted to be less than 2 nm for
these layers. It should be noted that several micron-sized Aldroplets are found on the surface of AlGaN layers grown at
600 C. For the AlGaN layer grown at 700 C, smooth surface with a
high density of small-sized surface pits is observed in the AFM
morphology. A surface roughness (rms) value of ~1.2 nm is obtained
for 2 mm 2 mm scan-area. The surface pits are inverted hexagonal
in feature and normally occur as a result of strain relaxation. In
many cases, the pits are associated with threading dislocations
arise from the interface due to large lattice and thermal mismatch
between AlGaN and sapphire substrate [27]. The density of surface
pits generally increases as the Al composition increases in the layer
due to increased strain [28].
We studied the compositional and electronic properties of LMBE
grown AlGaN layers using XPS measurements. Two AlGaN samples
grown at 500 and 700 C were taken for the XPS analysis. The core
level (CL) spectra have been plotted after necessary carbon
correction and the position of valence band maxima (VBM) was
calculated by extrapolating a linear fit to the leading edge of the
valence band photoemission to the baseline [29,30]. Fig. 4(a) shows
the survey scans of both the AlGaN samples displaying the presence
of aluminum, gallium and nitrogen with small amount of oxygen
and carbon contaminations at their respective binding energy positions [31]. The angle dependent XPS analysis revealed that oxygen
and carbon contaminations are present only on the layer surface
where the intensity of these peaks increased with variation in TakeOff Angle (TOA) from 0 to 60 .
The Al (2p) CL spectra are shown in Fig. 4(b) where the intensity
of AlGaN layer grown at 700 C was observed to be significantly
higher than that grown at 500 C. The Al (2p) CL peak is observed at
a binding energy of 73.8 and 74.0 eV for the AlGaN layers grown at
500 and 700 C, respectively, attributed to the Al-N bonding
[32,33]. The stoichiometry of both the AlGaN samples were calculated from survey scan using respective sensitivity factors [34] and
are obtained to be about 3 and 23% for the layers grown at 500 and
700 C, respectively. The survey scan as well as the CL spectra
confirms that the Al incorporation into the layer increases at higher
growth temperature. Fig. 4(c) shows the valence band (VB) spectra
of the AlGaN layers grown at 500 and 700 C. The VBM was
calculated to be positioned at 2.9 ± 0.1 and 3.6 ± 0.1 eV below the
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P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128
Fig. 4. XPS survey scans of LMBE grown AlxGa1 xN layers at 500 and 700 C. (a) Core
level survey scans, (b) Al (2p) spectra and (c) Valence Band Spectra.
Fermi Level for the AlGaN layers grown at 500 and 700 C,
respectively. It indicates that the as-grown layers have n-type
conductivity. It is also noted that the VBM shift towards the higher
binding energy with increase of Al content and is attributed to the
increase of band gap of AlxGa1 xN. The bandgap of GaN is 3.4 eV
while that of AlN is 6.2 eV. The bandgap of AlxGa1 xN ternary
compound increases with increment of Al content (x). Hence, the
position of Fermi level shifts towards higher binding energy with
increase in Al content or Bandgap or simply the growth temperature in the present scenario.
Fig. 5 represents the room temperature PL spectra recorded for
Fig. 5. Room temperature PL Spectra of AlxGa1 xN layers grown on sapphire (0001) by
LMBE at various growth temperatures along with GaN PL spectrum.
the LMBE AlGaN layers grown on sapphire (0001) at various temperatures. The PL spectra were acquired with the same incident
beam aperture size and excitation for all the samples. A strong UV
emission is seen for all layers with a weak deep band yellow
luminescence (YL) emission under the measurement conditions
adopted. The UV peak corresponds to the near band edge (NBE)
emission due to band-to-band transition. The NBE peak position
blue shifts with increasing growth temperature implying that the
Al composition increases in the AlGaN layer as a function of growth
temperature. It is also noticed that the PL emission peak broadens
with increase of Al composition, which is normally attributed to the
band tail states, which arise due to the shallow energy states of
point defects present in AlGaN ternary alloy [35e38]. Moreover, the
increase of Al mole fraction in the AlGaN layer can also cause
further broadening of the PL emission peak because of the alloy
broadening effect [39]. The alloy fluctuations occur due to inhomogeneities present in the grown ternary compound at nanoscale level. The PL spectra of 700 C grown sample exhibits an
additional low intensity peak at 3.27 eV, which is generally
accepted to the transitions associated with point defects such as VIII
and its complexes [40]. The strong NBE emission with less intensity
defect peaks indicates the good optical quality of the AlGaN
epitaxial layers grown by LMBE technique. Fig. 6 shows the graph of
band gap obtained by room temperature PL versus Al composition
in the AlxGa1 xN layer estimated by using XPS and EDX analyses.
Theoretical band gap values for AlxGa1 xN are calculated by using
Vegard's Law that is assumed to be linear for semiconductor alloy
system [41].
Eg ðxÞ ¼ xEg ðAlNÞ þ ð1
xÞEg ðGaNÞ
(1)
where Eg(GaN) ¼ 3.4 eV and Eg(AlN) ¼ 6.2 eV [42]. The experimental values are near to the theoretical calculation of band gap
based on equation (1) indicating the absence of any bowing for the
Al incorporation up to 23% in the AlGaN layers grown by LMBE. This
observation is similar to the one reported by Ochalski et al. on the
MOVPE and MBE grown AlGaN layers [43].
In general, GaN requires a lower growth temperature while AlN
demands a high temperature due to the strong Al-N bond-strength
compared to Ga-N [44]. Hence, AlGaN layers are grown at an
optimal temperature between GaN and AlN. Using LMBE, we have
shown the growth of good quality GaN epitaxial layers at temperatures as low as 500 C and the growth rate decreased drastically
Fig. 6. Theoretical and experimental Band gap energy E(g) versus Aluminum
composition incorporated into AlxGa1 xN thin films.
P. Tyagi et al. / Journal of Alloys and Compounds 739 (2018) 122e128
when the growth temperature is raised above 600 C due to
increased Ga desorption from the growth surface [45,46]. In case of
AlGaN growth by LMBE, the Al incorporation becomes difficult due
to relatively less surface mobility of Al atoms and poor formation of
Al-N bond at low growth temperatures of 600 C irrespective of
the high Al content in the target material. The excess Al remains in
the form of droplets on the layer surface at the end. Increase of
growth temperature promotes more Al incorporation in the
growing layer due to enhanced Al-N bond formation. On the other
hand, Ga surface desorption becomes very significant at higher
temperatures above 600 C. In addition, when the layer is grown
under metal-rich condition, the lifetime of the physisorbed groupIII adatoms is increased leading to longer diffusion length and thus
enhanced Ga desorption rate at high temperature [47]. As a result,
the Al to Ga flux ratio is increased on the growth surface at higher
temperatures favouring more Al incorporation in the AlGaN layer.
In LMBE process, the growth at 700 C has led to a strong increase of
Al composition in the AlxGa1 xN layer even higher than the
composition of the alloy target. These observations reveal that, in
LMBE AlGaN growth, the designated Al content cannot simply be
achieved by appropriately choosing the target composition but the
growth temperature should also be taken into account as it plays a
very critical role in determining the degree of Al incorporation.
4. Conclusion
We have successfully grown epitaxial AlGaN layers on prenitridated sapphire (0001) using LMBE technique. The effect of
growth temperature on the Al incorporation and physical properties of grown AlGaN layers have been studied in detail. It is found
that the Al mole fraction increases with increase of growth temperature and a maximum Al content of about 23% is achieved at a
growth temperature of 700 C. The growth of good quality AlGaN
layers has been achieved in the temperature range of 600e700 C
in LMBE technique, which is 100e150 C lower than the typical
AlGaN growth temperature of conventional MBE growth.
Acknowledgements
The authors thank Dr.K.K. Maurya for the help in XRD measurements. The authors would like to acknowledge the financial
support from Council of Scientific and Industrial Research (CSIR)
through network project (PSC-0109). Prashant Tyagi and Ch.
Ramesh are grateful to CSIR for the award of senior research
fellowship.
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