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Structural ordering, morphology and optical properties of amorphous AlxIn1−xN thin films grown by plasma-assisted dual source reactive evaporation
Accepted Manuscript Structural ordering, morphology and optical properties of amorphous Al x In1x
N thin films grown by plasma-assisted dual source reactive evaporation
M. Alizadeh, V. Ganesh, H. Mehdipour, N.F.F Nazarudin, B.T. Goh, A. Shuhaimi, S.A. Rahman PII: DOI: Reference:
S0925-8388(15)00314-X http://dx.doi.org/10.1016/j.jallcom.2015.01.216 JALCOM 33280
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
17 December 2014 18 January 2015 25 January 2015
Please cite this article as: M. Alizadeh, V. Ganesh, H. Mehdipour, N.F.F Nazarudin, B.T. Goh, A. Shuhaimi, S.A. Rahman, Structural ordering, morphology and optical properties of amorphous Al x In1- x N thin films grown by plasma-assisted dual source reactive evaporation, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/ 10.1016/j.jallcom.2015.01.216
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Structural ordering, morphology and optical properties of amorphous AlxIn1-xN thin films grown by plasma-assisted dual source reactive evaporation M. Alizadeh1*, V. Ganesh1, H. Mehdipour2, N.F.F Nazarudin1, B.T. Goh1, A. Shuhaimi1, S.A. Rahman1* 1
Low Dimensional Materials Research Centre (LDMRC), Department of Physics, Faculty of
Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
Plasma Nanoscience @ Complex Systems, The University of Sydney, New South Wales 2006,
Australia *
Corresponding authors: 1. M. Alizadeh, e-mail: [email protected], Tel: +601112279670. 2. S.A. Rahman, e-mail: [email protected], Tel: +60123290697.
Abstract Amorphous aluminum indium nitride (AlxIn1-xN) thin films were deposited on quartz substrates by plasma-assisted dual source reactive evaporation system. In-rich ( rich (
and
and
) and Al-
) films were prepared by simply varying an AC voltage applied to
indium wire. The X-ray-diffraction patterns revealed a small broad peak assigned to Al0.10In0.90N (002) plane, but no perceivable peaks assigned to crystalline AlxIn1-xN were observed for the films with
and
. The morphology of the film was changed from clusters of
small grains to uniformly shaped particles with decrease of . The band gap energy of the films increased from
to
as the Al composition varied from
to
. Also, Raman
results indicated that E2(high) and A1(LO) peaks of the AlxIn1-xN films are remarkably blueshifted by increasing x and the A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior . A bowing parameter of from bowing equation was
was obtained for AlInN films. The extrapolated value for band gap energy of InN.
Keywords: Amorphous AlInN, plasma-assisted deposition, XPS, Raman spectra, Band gap. 1
1. Introduction The III-nitride ternary semiconductors have become increasingly important in applications such as light-emitting diodes, laser diodes, solar cells, high electron mobility transistors and highly reflective Bragg reflectors in the ultraviolet region [1-4]. Among the IIInitride ternary alloys, Aluminum indium nitride (AlxIn1-xN) has a wide variable band gap of to
[5] depending on the indium (or aluminum) composition in the film structure. This
provides great degree of freedom in customizing the electronic structures of the alloy films for specific applications. For example, AlxIn1-xN thin films with tunable band gap of
to
can be used for multi-junction solar cell devices since this band gap energy range covers most part of solar spectrum [6]. Crystalline AlxIn1-xN (c-AlxIn1-xN) films have been fabricated over the years by various techniques such as molecular beam epitaxy (MBE) [7-10], metal organic chemical vapor deposition (MOCVD) [11,12], and sputtering-based deposition [13-15]. In general, except for sputtered films, c-AlxIn1-xN films are obtained at high substrate temperatures on specified substrates such as sapphire and GaN. Therefore, these methods are restricted by the type and temperature of substrate. On the other hand, it has been shown that amorphous III-nitride semiconductor films have several advantages such as cost-effectiveness, compositional flexibility, low temperature deposition, no dependency on substrates and greater control of electrical property [16-22]. S. H. Shim et al. [20] fabricated blue light emitting a-GaN films at room temperature. T. Itoh et al. [21] reported low temperature deposition of photoconductive aInxGa1-xN films and suggested that the films could to be utilized as photo-absorptive layer of top cell in multi-junction thin films solar cells. Amorphous InxGa1-xN films with considerable photosensitivity were obtained by T. Suzuki et al. [22] using simultaneous rf magnetron 2
sputtering. Therefore, much investigation should be considered on the properties of amorphous III-Nitrides alloys for the aim of effective use of these materials in semiconductor device applications considering the above-mentioned advantages. Band gap energy (
) of AlxIn1-xN is closely related to deposition technique and the
properties of the film. There have been remarkable disagreements among the band gap energies measured for AlInN films using various growth methods. One possible reason is that some of the measurements were carried out before the establishment of
[5] for band gap energy of
indium nitride (InN). The bowing parameter of ternary AlInN films as well as band gap energies of InN and AlN have been estimated in many reported works using different techniques. The reported experimental values for the AlInN bowing parameter are quite different varying in the wide range of obtained for
[23,24]. This is possibly due to the fact that different values were of InN films by various growth techniques [7,23]. Terashima et al. [8] reported a
bowing parameter of
for AlInN films by RF-MBE using values of
and
as
the optical band gap of InN and AlN, respectively. Wang et al. [12] employed MOCVD method for growth of AlInN films and obtained a bowing parameter of energy of InN at below
considering the band gap
. Moreover, it has been shown that highly crystalline films have
continuous band gap energy with respect to the films composition resulting in smaller bowing parameters [23]. H. He et al. [15] reported a bowing parameter of rf-magnetron-sputtered AlInN thin films. However, the obtained much larger than its intrinsic value
for highly crystalline value for InN was
,
.
In this work AlxIn1-xN thin films with high and low Al compositions were deposited by plasma-assisted dual source reactive evaporation technique and the effect of the applied voltage
3
to In source on the films properties was evaluated. The structural, compositional, morphological and optical characteristics of the films will be presented and discussed in the following section. 2. Material and Methods The AlxIn1-xN films were deposited on quartz substrate using a home-built plasmaassisted dual source reactive evaporation system. A schematic of the system is shown in fig.1. Prior to the deposition, the quartz substrate was ultrasonically degreased in decon 90 soap, rinsed in de-ionized water, ethanol and acetone and subsequently dried by nitrogen gas. Two separate tungsten wires were coiled and used as the hot filaments for evaporation of aluminum and indium wires (with
purity) and have been labeled FAl and FIn in fig. 1, respectively.
The filaments were clamped perpendicularly to each other between the grounded substrate holder and a radio-frequency (RF) powered stainless steel electrode. In order to activate the surface bonds (prior to the deposition), the substrates were treated in H2-plasma for hydrogen flow rate of
at a fixed
. The deposition process was conducted in three steps: 1) N2
plasma was generated in the chamber and stabilized for temperature required for evaporation of Al atoms (
min, 2) FAl was heated to the ) and, then cooled down to
to avoid high evaporation rates, 3) Immediately, an AC voltage was applied to FIn filament and the AlxIn1-xN film formation was initiated. Conditions for the film growth were maintained under N2 plasma for was varied in
. The applied voltage to FAl ( increment from
to
) was set at
, while FIn AC voltage (
)
in order to obtain AlxIn1-xN films with different
Al(In) compositions. Since the FIn was clamped close to the FAl (see fig. 1), deposition of the evaporated Al species on the FIn was unavoidable. Therefore, a voltage higher than the required for In evaporation was applied to the filament in order to generate sufficient amount of In atoms. The quartz substrates were initially heated to
and during the deposition process the final 4
substrate holder temperature was varied from
to
(depending on
) which was
monitored by a thermocouple inserted under of the substrates . After the deposition process, the FIn was switched off, FAl temperature was fixed at annealed in N2 plasma (RF power:
) for
and the obtained samples were . The deposition parameters for films
studied in this work are summarized in Table. 1. The structural properties of the grown AlxIn1-xN films were investigated by X-ray diffraction (SIEMENS D5000 X-ray diffractometer, Cu Kα X-ray radiation
) and
Raman spectroscopy (Renishaw inVia Raman Microscope, with a laser excitation wavelength of ). The compositional uniformity and bonding configuration of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu, Kratos Axis Ultra DLD). Field emission scanning electron microscopy (FESEM, FEI, Quanta 200) was employed to observe surface morphologies of the grown films. The thickness of the films was measured using surface profiler (KLA-Tencor). Also, the optical transmittance and reflectance for the fabricated AlxIn1-xN thin films were measured using an UV-vis-infrared spectrophotometer (Lambda 750, PerkinElmer). 3. Results and discussion The elemental Composition was analyzed from XPS spectra of the AlxIn1-xN films deposited at different applied voltage, AlxIn1-xN films prepared at
to
. The In3d5/2 core-level photoelectron spectra of the are shown in Fig. 2(a)-(d). A significant increase in
the intensity of In3d5/2 core-level is observed when
is increased, implying that higher
amounts of In adatoms were incorporated in AlxIn1-xN film structure at higher voltages. The core-level photoelectron spectra for all samples were deconvoluted into three main sub-peaks centered at
,
and
which are attributed to In-O, In-N and In-In bonds,
5
respectively [25,26]. The corresponding Al2p core-level photoelectron spectra of AlxIn1-xN films deposited at
to
are presented in inset of Fig. 2(a)-(d). The all spectra were also
deconvoluted into three main components at
,
and
which are assigned to Al-O, Al-N and Al-Al bonds, respectively [27,28]. The appearance of InIn, In-O, Al-Al and Al-O bonds is possibly due to higher concentration of reactive In and Al atoms reaching the growth sites compared to the reactive N atoms and high affinity of the metallic adatoms to oxygen. The analysis on the chemical composition of the films revealed that the estimated Al mole fractions (Al composition) in the AlxIn1-xN films, x, are and
,
,
which were determined from the ratio of the integrated intensity of the Al-N and In-N
peaks (Al – N/(Al – N + In – N) [22]) for the films deposited at
of
and
,
respectively. Fig. 3 displays dependence of FIn temperature and aluminum incorporation into the films composition on the applied voltage to FIn (
). An increase from
was recorded for FIn temperature by increasing the shows that
from
to to
which clearly
controlled the FIn temperature. The increase in FIn temperature results in enhanced
generation and deposition of reactive indium atoms at the substrate surface and, hence, decrease of aluminum mole fraction (x) in the films composition. Guo et al. [29] reported similar tuning capability of the Al composition in AlxIn1-xN films by varying the applied RF power to indium target using sputtering technique. Fig. 4 shows the X-ray diffraction (XRD) spectra of AlxIn1-xN films deposited at different . The spectra of the films grown at
,
and
show no diffraction peaks assigned
to crystalline AlxIn1-xN. The XRD pattern of the films deposited at the highest
6
of
V shows
a small broad peak at
which is very close to the position of (002) plane of wurtzite
Al0.10In0.90N based on Vegard’s law: , where
and
(1)
are the c lattice constant of crystalline AlN and InN,
respectively [30,31]. Interestingly, there is a good agreement between the Al composition (x) calculated from XPS analysis in this work and that obtained from Vegard’s law. Phases of In and Al crystalline grains are observed from XRD spectra of the films grown at
of
,
and
. The presence of these metallic phases was also deduced from the XPS spectra of the films. Basically, the films studied in this work are amorphous in structure with presence of In and Al crystalline grains scattered within the amorphous matrix of the AlInN structure. Fig. 5(a) shows Raman spectra for AlxIn1-xN films with different aluminum mole fraction, x. For reference and comparison, the Raman spectrum of the AlN films (grown at presented in fig. 5(b). Two major peaks centered at
and
) is
are observed in Raman
spectrum of Al0.10In0.90N films. These peaks are assigned to E2(high) and A1(LO) phonon modes of the In-rich AlInN films [32]. The peaks are slightly blue-shifted when the aluminum incorporation is increased to and and
. With further increase of the aluminum composition to
, the shift becomes more pronounced and the E2(high) peaks appear at , respectively. Considering the E2(high) phonon mode of the grown AlN films
which is centered at 665 [33] (see fig. 5(b)), it can be seen that the E2(high) Al0.60In0.40N and Al0.64In0.36N thin films are red-shifted by
and
modes of
, respectively.
Uniquely, the Raman spectra of Al0.60In0.40N and Al0.64In0.36N films reveal one hitherto unknown Raman feature located at around
(labeled as *) which was detected by repeated micro-
7
Raman spectroscopies at various test points. Because it was also observed in Raman spectrum of AlN samples (fig. 5(b)), it cannot be assigned to any E2(high) mode. This peak does not seem to correspond to the allowed Raman modes of hexagonal AlN based on group theory and may be related to acoustic phonon modes of AlN [34]. A broad peak at around from Raman spectra of the AlInN films with Al compositions of
was detected and
which may
be formed from overlapping of some Raman structures. In order to identify each of the contributions, we have analyzed the Raman spectrum of Al0.64In0.36N films in range using curve fitting method (fig. 5(c)). The observed broad peak at around was deconvoluted into two main components at around
and
which are
assigned to InN-like A1(LO) and AlN-like A1(LO) phonon modes of AlInN, respectively [35,36] as shown in fig. 5(c). This is in good agreement with Raman results reported by T. T. Kang et al. [35] and Jiang et al. [36] who showed that A1(LO) phonon mode of AlxIn1-xN films (x ≥ 30) exhibits two-mode behavior. Fig. 6 displays the FESEM images of the surface of AlInN films with different composition. It is obviously seen that the surface morphology of AlxIn1-xN films strongly depends on Al composition of the films, x. The surface morphology of Al0.64In0.36N thin film (Fig. 6(a)) shows a smooth and compact microstructure of nanoparticles with an average size of . Agglomeration of these nanoparticles on the surface results in formation of grains of larger dimension
. A decrease in the Al composition x to
, enhances the
agglomeration of the nanoparticles and a few excessively large grains with the size of are formed as a result (see Fig. 6(b)). The growth of these grains perpendicular to the surface forms a rough surface. Distribution of pebble-like particles is observed in the FESEM image of the more Al-diluted Al0.18In0.82N films (Fig. 6(c)). However, some relatively large 8
grains are also seen from the morphology. The surface morphology of AlxIn1-xN with Al composition of
(Fig. 6(d)) exhibits features of near-spherical-shaped nanoparticles
distributed uniformly over the entire surface. This kind of morphology is favorable in solar cell application as the spherical particles are good channels for charge carrier transport [15]. The optical properties of AlxIn1-xN films with different compositions were studied by obtaining transmittance and reflectance spectra of the films. Fig 7(a) displays the transmittance curves of AlxIn1-xN films with Al composition of (
and
transmittance (e. g.,
,
,
, and
. In-rich films
) show an absorption in near infrared (NIR) region and an insignificant ) in the range of
. As
is increased toward Al-rich
region, the absorption edge of the films shifts toward shorter wavelength and AlxIn1-xN films with higher Al content become more transparent in visible and near infrared (NIR) regions. To further clarify the optical results, we have measured transmittance of the AlN film (see inset in Fig. 7(a)). Absorption edge of the sample occurred at around
which is in good
agreement with reported absorption edge wavelength of AlN films [37-40]. Fig. 7(b) depicts the wavelength dependence of optical reflectance spectra of the AlInN films with different aluminum compositions x. It can be clearly observed that the minimum of the reflectance is shifted toward longer wavelength when the aluminum incorporation (x) is reduced from
to
. Since minimum value of the reflection occurs in the vicinity of the wavelength related to the optical bandgap energy, the results in fig. 7(b) show that the bandgap value of the AlInN films increases by increasing x. A insignificant reflectance (e. g.,
) at around
is
obtained from optical reflectance spectrum of AlN films (see the inset of fig. 7(b)) which is in good agreement with the corresponding transmittance result shown in the inset of fig. 7(a).
9
The absorption coefficient ( ) was also calculated for each film with the measured transmittance , reflection
and film thickness
from equation [23]:
.
(2)
In order to obtain direct band gap of the AlxIn1-xN films, we have plotted photon energy (
as a function of
) (Fig. 8). It is known that the squared absorption coefficient of a
semiconductor with a direct band gap is a function of photon energy: , where
and
(3)
are proportional constant and band gap energy, respectively [24]. Direct band
gap of an AlxIn1-xN film is determined by extrapolating the liner part of
vs
curves
down to the energy axis. This method has been employed to measure band gap energy of amorphous semiconductors [41]. As seen in the Fig. 8, the optical band gap of the AlxIn1-xN film changes with the variation of the Al composition within the films structure. The optical band gap of grown Al0.64In0.36N thin film is estimated to be
. As x decreases to
the band gap energies of the films are obtained at
,
and
,
and
, respectively. Such
controlled tuning of the band gap energy is very crucial for materializing the application of AlInN alloys in multi-junction solar technology. The Fig. 8 is estimated to be
value of pure AlN shown in the inset of
, which is smaller than the value
reported in the previous
works [42]. It has been shown that the band gap of an AlN film can be reduced by several factors such as high carrier concentration, nonstoichiometric composition, and impose of residual stresses as well as presence of structural defects and oxygen impurity in the film structure [43,44]. The AlN band gap energy obtained in this work is very close to the values reported in other similar works [37,39,45]. It is also much larger than the 10
(of AlN films) reported by Cho
et al. [46] and Wu et al. [47] who prepared the AlN films by reactive magnetron sputtering and laser molecular beam epitaxy methods, respectively. For optoelectronic applications envisaged for AlxIn1-xN film, it is essential to estimate the band gap energy based on the film composition . Fig. 9 shows dependence of Eg values of our AlxIn1-xN films on the aluminum composition (dotted line). Obviously, the relationship between the obtained band gap values and Al composition is not linear. This phenomenon is named as “bowing effect” which is due to unstable nature of AlxIn1-xN within the miscibility gap, charge exchange between forming bonds in disordered solid solution and relaxation of bond length [48]. The bandgap energy of AlxIn1-xN (
) as a function of Al composition (x) is determined
using the following equation [10]: Eg,AlInN = xEg,AlN + (1 – x)Eg,InN – bx(1 - x), where b is bowing parameter and
and
(4)
are the band gaps of AlN and InN films,
respectively. Fitting the Eq. (4) to the experimental data obtained in this work, gives the bowing parameter b of
, which is close to
and
reported by Goldhahn et al. [49] and T.-
S. Yeh et al. [50] respectively, and is much smaller than By extrapolation the band gap energy curve to the the InN film, which is close to the reported value (
obtained by of Wang et al. [12].
, the band gap of
is obtained for
) [5]. However, this extrapolation-led
estimation of the InN band gap has to be confirmed experimentally by growing a pure InN using this technique. 4. Conclusion Amorphous In-rich (
and
) and Al-rich (
and
) AlxIn1-xN thin
films were successfully deposited using plasma-assisted dual source reactive evaporation 11
method. It was shown that the Al composition in the films can be adjusted by simply varying the voltage applied to the indium wire (
). Raman results indicated that E2(high) and A1(LO) peaks
of the AlxIn1-xN films are considerably blue-shifted by increasing x and the A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior. The morphology of the film was changed from clusters of small grains to uniformly shaped particles with decrease of characterizations showed that the band gap energy of the films increased from as the Al composition varied from
to
. A bowing parameter of
. Optical to
was calculated for
the grown AlxIn1-xN films and the extrapolated value from the bowing equation was 0.85 eV for band gap energy of for InN. Thus, plasma-assisted dual source reactive evaporation offers a plausible approach towards facile tuning the band gap of the amorphous AlxIn1-xN thin film for specific applications envisaged for such materials. Acknowledgment This work was supported by University of Malaya, High impact research (HIR) fund of UM.C/HIR/MOHE/SC/06 and the Ministry of Higher Education, Fundamental Research Grant Scheme (FRGS) of FP009-2013B.
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Krischok, J.A. Schaefer, O. Ambacher, Correlation between structural and electrical properties of InN thin films prepared by molecular beam epitaxy, Superlattices Microstruct. 40 (2006) 289-294. [27] Y. You, A. Ito, R. Tu, T. Goto, Effects of laser power on the growth of polycrystalline AlN films by laser chemical vapor deposition method, Surf. Coat. Technol. 232 (2013) 1-5. [28] M. Alizadeh, H. Mehdipour ,V. Ganesh, A. N. Ameera, B. T. Goh, A. Shuhaimi, S. A. Rahman, Plasma-assisted hot filament chemical vapor deposition of AlN thin films on ZnO buffer layer: toward highly c-axis-oriented, uniform, insulative films, Appl. Phys. A 117 (2014) 2217-2224. [29] Q.X. Guo, Y. Okazaki, Y. Kume, T. Tanaka, M. Nishio, H. Ogawa, Reactive sputter deposition of AlInN thin films, J. Cryst. Growth 300 (2007) 151-154. [30] H. Angerer, D. Brunner, F. Freudenberg, O. Ambacher, M. Stutzmann, R. Höpler, T. Metzger, E. Born, G. Dollinger, A. Bergmaier, S. Karsch and H.-J. Körner, Determination of the Al mole fraction and the band gap bowing of epitaxial AlxGa1−xN films, Appl. Phys. Lett. 71 (1997) 1504-1506. [31] V. Darakchieva, M.Y. Xie, F. Tasnádi, I. A. Abrikosov, L. Hultman, B. Monemar, J. Kamimura and K. Kishino, Lattice parameters, deviations from Vegard’s rule, and E2 phonons in InAlN, Appl. Phys. Lett. 93 (2008) 261908. [32] H.F. Liu, C.G. Li , K.K. Ansah Antwi, S.J. Chua, D.Z. Chi, Electron-beam- induced carbon deposition on high-indium-content AlInN thin films Grown on Si by sputtering at elevated temperature, Mater. Lett. 128 (2014) 344-348. [33] L.E. McNeil, Vibrational Spectroscopy of Aluminum Nitride, J Am. Ceram. Soc. 76 (1993) 16
1132-1136. [34] H. Hobert, H.H. Dunken, J. Meinschien, H. Stafast, Infrared and Raman spectroscopic investigation of thin films of AlN and SiC on Si substrates,Vib. Spectrosc. 19 (1999) 205211. [35] T.T. Kang, A. Hashimoto, and A. Yamamoto, Raman scattering of indium-rich AlxIn1−xN: Unexpected two-mode behavior of A1(LO), Phys. Rev. B 79 (2009) 033301-4. [36] L.F. Jiang, J.F. Kong, W.Z. Shen, and Q.X. Guo, Temperature dependence of Raman scattering in AlInN, J. Appl. Phys. 109 (2011) 113514. [37] H.T. Wang, C.H. Jia, J.K. Xu, Y.H. Chen, X.W. Chen, W.F. Zhang, Epitaxial growth of non-polar m-plane AlN film on bare and ZnO buffered m-sapphire, J. Cryst. Growth 391 (2014) 111-115. [38] Q.X. Guo, T. Tanaka, M. Nishio, H. Ogawa, Growth properties of AlN films on sapphire substrates by reactive sputtering, Vacuum 80 (2006) 716-718. [39] J.C. Sanchez-Lopez, L. Contreras, A. Fernandez, A.R. Gonzalez-Elipe, J.M. Martin, B. Vacher, AlN thin films prepared by ion beam induced chemical vapour deposition, Thin Solid Films 317 (1998) 100-104. [40] C. Ozgit, I. Donmez, M. Alevli, N. Biyikli, Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition, Thin Solid Films 520 (2012) 2750-2755. [41] R. Weingartner, J.A. Guerra Torres, O. Erlenbach, G. Galvez de la Puente, F. De Zela, A. Winnacker, Bandgap engineering of the amorphous wide bandgap semiconductor (SiC)1−x(AlN)x doped with terbium and its optical emission properties, Mat. Sci. Eng. B-
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19
Table 1. Plasma-assisted dual source reactive evaporation conditions used for preparing AlxIn1xN
thin films.
Deposition parameters
Values
N2 flow rate (sccm)
40
RF power (W)
250
FAl-substrate distance (mm)
10
FIn-substrate distance (mm)
15
Substrate holder
320-360
temperature (°C) FAl temperature (°C)
1450
FIn temperature (°C)
1000-1100
Process time (min)
5
20
Fig. 1. Schematic of the experimental setup of plasma-assisted dual source reactive evaporation system.
21
Fig. 2. In3d5/2 core-level photoelectron spectra of AlxIn1-xN films deposited at (a), (b), (c) and (d). The inset of (a), (b), (c) and (d) are the corresponding Al2p core-level photoelectron spectra.
22
Fig. 3. Dependence of Al composition in the AlxIn1-xN films and FIn temperature on the applied AC voltage to indium wire.
23
Fig. 4. XRD pattern of the AlxIn1-xN thin films grown at different VIn.
24
Fig. 5. Raman spectra of the grown AlxIn1-xN (a) and AlN (b) thin films. (c) Fitted curve of Raman spectrum of Al0.64In0.36N films in the range of 650-900 cm-1. 25
Fig. 6. FESEM images of the AlxIn1-xN films with Al compositions of (c) and (d).
26
(a),
(b),
Fig. 7. Transmittance (a) and Reflectance spectra (b) of AlxIn1-xN films with different Al compositions. The inset of (a) and (b) are transmittance and reflectance spectra of AlN (x = 1), respectively.
27
Fig. 8. Plots of
versus
for AlxIn1-xN films with different x. The inset is the plot of versus for AlN films.
28
Fig. 9. Band gap energy of AlxIn1-xN film as a function of Al composition .
29
Highlights:
In-rich and Al-rich AlxIn1-xN films were grown by Plasma-aided reactive evaporation.
The A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior.
The band gap of the films was tuned from
A bowing parameter of
The morphology was changed from clusters to uniformly shaped grains by decreasing .
to
.
was calculated for the grown AlxIn1-xN films.
30
N thin films grown by plasma-assisted dual source reactive evaporation
M. Alizadeh, V. Ganesh, H. Mehdipour, N.F.F Nazarudin, B.T. Goh, A. Shuhaimi, S.A. Rahman PII: DOI: Reference:
S0925-8388(15)00314-X http://dx.doi.org/10.1016/j.jallcom.2015.01.216 JALCOM 33280
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
17 December 2014 18 January 2015 25 January 2015
Please cite this article as: M. Alizadeh, V. Ganesh, H. Mehdipour, N.F.F Nazarudin, B.T. Goh, A. Shuhaimi, S.A. Rahman, Structural ordering, morphology and optical properties of amorphous Al x In1- x N thin films grown by plasma-assisted dual source reactive evaporation, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/ 10.1016/j.jallcom.2015.01.216
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Structural ordering, morphology and optical properties of amorphous AlxIn1-xN thin films grown by plasma-assisted dual source reactive evaporation M. Alizadeh1*, V. Ganesh1, H. Mehdipour2, N.F.F Nazarudin1, B.T. Goh1, A. Shuhaimi1, S.A. Rahman1* 1
Low Dimensional Materials Research Centre (LDMRC), Department of Physics, Faculty of
Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
Plasma Nanoscience @ Complex Systems, The University of Sydney, New South Wales 2006,
Australia *
Corresponding authors: 1. M. Alizadeh, e-mail: [email protected], Tel: +601112279670. 2. S.A. Rahman, e-mail: [email protected], Tel: +60123290697.
Abstract Amorphous aluminum indium nitride (AlxIn1-xN) thin films were deposited on quartz substrates by plasma-assisted dual source reactive evaporation system. In-rich ( rich (
and
and
) and Al-
) films were prepared by simply varying an AC voltage applied to
indium wire. The X-ray-diffraction patterns revealed a small broad peak assigned to Al0.10In0.90N (002) plane, but no perceivable peaks assigned to crystalline AlxIn1-xN were observed for the films with
and
. The morphology of the film was changed from clusters of
small grains to uniformly shaped particles with decrease of . The band gap energy of the films increased from
to
as the Al composition varied from
to
. Also, Raman
results indicated that E2(high) and A1(LO) peaks of the AlxIn1-xN films are remarkably blueshifted by increasing x and the A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior . A bowing parameter of from bowing equation was
was obtained for AlInN films. The extrapolated value for band gap energy of InN.
Keywords: Amorphous AlInN, plasma-assisted deposition, XPS, Raman spectra, Band gap. 1
1. Introduction The III-nitride ternary semiconductors have become increasingly important in applications such as light-emitting diodes, laser diodes, solar cells, high electron mobility transistors and highly reflective Bragg reflectors in the ultraviolet region [1-4]. Among the IIInitride ternary alloys, Aluminum indium nitride (AlxIn1-xN) has a wide variable band gap of to
[5] depending on the indium (or aluminum) composition in the film structure. This
provides great degree of freedom in customizing the electronic structures of the alloy films for specific applications. For example, AlxIn1-xN thin films with tunable band gap of
to
can be used for multi-junction solar cell devices since this band gap energy range covers most part of solar spectrum [6]. Crystalline AlxIn1-xN (c-AlxIn1-xN) films have been fabricated over the years by various techniques such as molecular beam epitaxy (MBE) [7-10], metal organic chemical vapor deposition (MOCVD) [11,12], and sputtering-based deposition [13-15]. In general, except for sputtered films, c-AlxIn1-xN films are obtained at high substrate temperatures on specified substrates such as sapphire and GaN. Therefore, these methods are restricted by the type and temperature of substrate. On the other hand, it has been shown that amorphous III-nitride semiconductor films have several advantages such as cost-effectiveness, compositional flexibility, low temperature deposition, no dependency on substrates and greater control of electrical property [16-22]. S. H. Shim et al. [20] fabricated blue light emitting a-GaN films at room temperature. T. Itoh et al. [21] reported low temperature deposition of photoconductive aInxGa1-xN films and suggested that the films could to be utilized as photo-absorptive layer of top cell in multi-junction thin films solar cells. Amorphous InxGa1-xN films with considerable photosensitivity were obtained by T. Suzuki et al. [22] using simultaneous rf magnetron 2
sputtering. Therefore, much investigation should be considered on the properties of amorphous III-Nitrides alloys for the aim of effective use of these materials in semiconductor device applications considering the above-mentioned advantages. Band gap energy (
) of AlxIn1-xN is closely related to deposition technique and the
properties of the film. There have been remarkable disagreements among the band gap energies measured for AlInN films using various growth methods. One possible reason is that some of the measurements were carried out before the establishment of
[5] for band gap energy of
indium nitride (InN). The bowing parameter of ternary AlInN films as well as band gap energies of InN and AlN have been estimated in many reported works using different techniques. The reported experimental values for the AlInN bowing parameter are quite different varying in the wide range of obtained for
[23,24]. This is possibly due to the fact that different values were of InN films by various growth techniques [7,23]. Terashima et al. [8] reported a
bowing parameter of
for AlInN films by RF-MBE using values of
and
as
the optical band gap of InN and AlN, respectively. Wang et al. [12] employed MOCVD method for growth of AlInN films and obtained a bowing parameter of energy of InN at below
considering the band gap
. Moreover, it has been shown that highly crystalline films have
continuous band gap energy with respect to the films composition resulting in smaller bowing parameters [23]. H. He et al. [15] reported a bowing parameter of rf-magnetron-sputtered AlInN thin films. However, the obtained much larger than its intrinsic value
for highly crystalline value for InN was
,
.
In this work AlxIn1-xN thin films with high and low Al compositions were deposited by plasma-assisted dual source reactive evaporation technique and the effect of the applied voltage
3
to In source on the films properties was evaluated. The structural, compositional, morphological and optical characteristics of the films will be presented and discussed in the following section. 2. Material and Methods The AlxIn1-xN films were deposited on quartz substrate using a home-built plasmaassisted dual source reactive evaporation system. A schematic of the system is shown in fig.1. Prior to the deposition, the quartz substrate was ultrasonically degreased in decon 90 soap, rinsed in de-ionized water, ethanol and acetone and subsequently dried by nitrogen gas. Two separate tungsten wires were coiled and used as the hot filaments for evaporation of aluminum and indium wires (with
purity) and have been labeled FAl and FIn in fig. 1, respectively.
The filaments were clamped perpendicularly to each other between the grounded substrate holder and a radio-frequency (RF) powered stainless steel electrode. In order to activate the surface bonds (prior to the deposition), the substrates were treated in H2-plasma for hydrogen flow rate of
at a fixed
. The deposition process was conducted in three steps: 1) N2
plasma was generated in the chamber and stabilized for temperature required for evaporation of Al atoms (
min, 2) FAl was heated to the ) and, then cooled down to
to avoid high evaporation rates, 3) Immediately, an AC voltage was applied to FIn filament and the AlxIn1-xN film formation was initiated. Conditions for the film growth were maintained under N2 plasma for was varied in
. The applied voltage to FAl ( increment from
to
) was set at
, while FIn AC voltage (
)
in order to obtain AlxIn1-xN films with different
Al(In) compositions. Since the FIn was clamped close to the FAl (see fig. 1), deposition of the evaporated Al species on the FIn was unavoidable. Therefore, a voltage higher than the required for In evaporation was applied to the filament in order to generate sufficient amount of In atoms. The quartz substrates were initially heated to
and during the deposition process the final 4
substrate holder temperature was varied from
to
(depending on
) which was
monitored by a thermocouple inserted under of the substrates . After the deposition process, the FIn was switched off, FAl temperature was fixed at annealed in N2 plasma (RF power:
) for
and the obtained samples were . The deposition parameters for films
studied in this work are summarized in Table. 1. The structural properties of the grown AlxIn1-xN films were investigated by X-ray diffraction (SIEMENS D5000 X-ray diffractometer, Cu Kα X-ray radiation
) and
Raman spectroscopy (Renishaw inVia Raman Microscope, with a laser excitation wavelength of ). The compositional uniformity and bonding configuration of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu, Kratos Axis Ultra DLD). Field emission scanning electron microscopy (FESEM, FEI, Quanta 200) was employed to observe surface morphologies of the grown films. The thickness of the films was measured using surface profiler (KLA-Tencor). Also, the optical transmittance and reflectance for the fabricated AlxIn1-xN thin films were measured using an UV-vis-infrared spectrophotometer (Lambda 750, PerkinElmer). 3. Results and discussion The elemental Composition was analyzed from XPS spectra of the AlxIn1-xN films deposited at different applied voltage, AlxIn1-xN films prepared at
to
. The In3d5/2 core-level photoelectron spectra of the are shown in Fig. 2(a)-(d). A significant increase in
the intensity of In3d5/2 core-level is observed when
is increased, implying that higher
amounts of In adatoms were incorporated in AlxIn1-xN film structure at higher voltages. The core-level photoelectron spectra for all samples were deconvoluted into three main sub-peaks centered at
,
and
which are attributed to In-O, In-N and In-In bonds,
5
respectively [25,26]. The corresponding Al2p core-level photoelectron spectra of AlxIn1-xN films deposited at
to
are presented in inset of Fig. 2(a)-(d). The all spectra were also
deconvoluted into three main components at
,
and
which are assigned to Al-O, Al-N and Al-Al bonds, respectively [27,28]. The appearance of InIn, In-O, Al-Al and Al-O bonds is possibly due to higher concentration of reactive In and Al atoms reaching the growth sites compared to the reactive N atoms and high affinity of the metallic adatoms to oxygen. The analysis on the chemical composition of the films revealed that the estimated Al mole fractions (Al composition) in the AlxIn1-xN films, x, are and
,
,
which were determined from the ratio of the integrated intensity of the Al-N and In-N
peaks (Al – N/(Al – N + In – N) [22]) for the films deposited at
of
and
,
respectively. Fig. 3 displays dependence of FIn temperature and aluminum incorporation into the films composition on the applied voltage to FIn (
). An increase from
was recorded for FIn temperature by increasing the shows that
from
to to
which clearly
controlled the FIn temperature. The increase in FIn temperature results in enhanced
generation and deposition of reactive indium atoms at the substrate surface and, hence, decrease of aluminum mole fraction (x) in the films composition. Guo et al. [29] reported similar tuning capability of the Al composition in AlxIn1-xN films by varying the applied RF power to indium target using sputtering technique. Fig. 4 shows the X-ray diffraction (XRD) spectra of AlxIn1-xN films deposited at different . The spectra of the films grown at
,
and
show no diffraction peaks assigned
to crystalline AlxIn1-xN. The XRD pattern of the films deposited at the highest
6
of
V shows
a small broad peak at
which is very close to the position of (002) plane of wurtzite
Al0.10In0.90N based on Vegard’s law: , where
and
(1)
are the c lattice constant of crystalline AlN and InN,
respectively [30,31]. Interestingly, there is a good agreement between the Al composition (x) calculated from XPS analysis in this work and that obtained from Vegard’s law. Phases of In and Al crystalline grains are observed from XRD spectra of the films grown at
of
,
and
. The presence of these metallic phases was also deduced from the XPS spectra of the films. Basically, the films studied in this work are amorphous in structure with presence of In and Al crystalline grains scattered within the amorphous matrix of the AlInN structure. Fig. 5(a) shows Raman spectra for AlxIn1-xN films with different aluminum mole fraction, x. For reference and comparison, the Raman spectrum of the AlN films (grown at presented in fig. 5(b). Two major peaks centered at
and
) is
are observed in Raman
spectrum of Al0.10In0.90N films. These peaks are assigned to E2(high) and A1(LO) phonon modes of the In-rich AlInN films [32]. The peaks are slightly blue-shifted when the aluminum incorporation is increased to and and
. With further increase of the aluminum composition to
, the shift becomes more pronounced and the E2(high) peaks appear at , respectively. Considering the E2(high) phonon mode of the grown AlN films
which is centered at 665 [33] (see fig. 5(b)), it can be seen that the E2(high) Al0.60In0.40N and Al0.64In0.36N thin films are red-shifted by
and
modes of
, respectively.
Uniquely, the Raman spectra of Al0.60In0.40N and Al0.64In0.36N films reveal one hitherto unknown Raman feature located at around
(labeled as *) which was detected by repeated micro-
7
Raman spectroscopies at various test points. Because it was also observed in Raman spectrum of AlN samples (fig. 5(b)), it cannot be assigned to any E2(high) mode. This peak does not seem to correspond to the allowed Raman modes of hexagonal AlN based on group theory and may be related to acoustic phonon modes of AlN [34]. A broad peak at around from Raman spectra of the AlInN films with Al compositions of
was detected and
which may
be formed from overlapping of some Raman structures. In order to identify each of the contributions, we have analyzed the Raman spectrum of Al0.64In0.36N films in range using curve fitting method (fig. 5(c)). The observed broad peak at around was deconvoluted into two main components at around
and
which are
assigned to InN-like A1(LO) and AlN-like A1(LO) phonon modes of AlInN, respectively [35,36] as shown in fig. 5(c). This is in good agreement with Raman results reported by T. T. Kang et al. [35] and Jiang et al. [36] who showed that A1(LO) phonon mode of AlxIn1-xN films (x ≥ 30) exhibits two-mode behavior. Fig. 6 displays the FESEM images of the surface of AlInN films with different composition. It is obviously seen that the surface morphology of AlxIn1-xN films strongly depends on Al composition of the films, x. The surface morphology of Al0.64In0.36N thin film (Fig. 6(a)) shows a smooth and compact microstructure of nanoparticles with an average size of . Agglomeration of these nanoparticles on the surface results in formation of grains of larger dimension
. A decrease in the Al composition x to
, enhances the
agglomeration of the nanoparticles and a few excessively large grains with the size of are formed as a result (see Fig. 6(b)). The growth of these grains perpendicular to the surface forms a rough surface. Distribution of pebble-like particles is observed in the FESEM image of the more Al-diluted Al0.18In0.82N films (Fig. 6(c)). However, some relatively large 8
grains are also seen from the morphology. The surface morphology of AlxIn1-xN with Al composition of
(Fig. 6(d)) exhibits features of near-spherical-shaped nanoparticles
distributed uniformly over the entire surface. This kind of morphology is favorable in solar cell application as the spherical particles are good channels for charge carrier transport [15]. The optical properties of AlxIn1-xN films with different compositions were studied by obtaining transmittance and reflectance spectra of the films. Fig 7(a) displays the transmittance curves of AlxIn1-xN films with Al composition of (
and
transmittance (e. g.,
,
,
, and
. In-rich films
) show an absorption in near infrared (NIR) region and an insignificant ) in the range of
. As
is increased toward Al-rich
region, the absorption edge of the films shifts toward shorter wavelength and AlxIn1-xN films with higher Al content become more transparent in visible and near infrared (NIR) regions. To further clarify the optical results, we have measured transmittance of the AlN film (see inset in Fig. 7(a)). Absorption edge of the sample occurred at around
which is in good
agreement with reported absorption edge wavelength of AlN films [37-40]. Fig. 7(b) depicts the wavelength dependence of optical reflectance spectra of the AlInN films with different aluminum compositions x. It can be clearly observed that the minimum of the reflectance is shifted toward longer wavelength when the aluminum incorporation (x) is reduced from
to
. Since minimum value of the reflection occurs in the vicinity of the wavelength related to the optical bandgap energy, the results in fig. 7(b) show that the bandgap value of the AlInN films increases by increasing x. A insignificant reflectance (e. g.,
) at around
is
obtained from optical reflectance spectrum of AlN films (see the inset of fig. 7(b)) which is in good agreement with the corresponding transmittance result shown in the inset of fig. 7(a).
9
The absorption coefficient ( ) was also calculated for each film with the measured transmittance , reflection
and film thickness
from equation [23]:
.
(2)
In order to obtain direct band gap of the AlxIn1-xN films, we have plotted photon energy (
as a function of
) (Fig. 8). It is known that the squared absorption coefficient of a
semiconductor with a direct band gap is a function of photon energy: , where
and
(3)
are proportional constant and band gap energy, respectively [24]. Direct band
gap of an AlxIn1-xN film is determined by extrapolating the liner part of
vs
curves
down to the energy axis. This method has been employed to measure band gap energy of amorphous semiconductors [41]. As seen in the Fig. 8, the optical band gap of the AlxIn1-xN film changes with the variation of the Al composition within the films structure. The optical band gap of grown Al0.64In0.36N thin film is estimated to be
. As x decreases to
the band gap energies of the films are obtained at
,
and
,
and
, respectively. Such
controlled tuning of the band gap energy is very crucial for materializing the application of AlInN alloys in multi-junction solar technology. The Fig. 8 is estimated to be
value of pure AlN shown in the inset of
, which is smaller than the value
reported in the previous
works [42]. It has been shown that the band gap of an AlN film can be reduced by several factors such as high carrier concentration, nonstoichiometric composition, and impose of residual stresses as well as presence of structural defects and oxygen impurity in the film structure [43,44]. The AlN band gap energy obtained in this work is very close to the values reported in other similar works [37,39,45]. It is also much larger than the 10
(of AlN films) reported by Cho
et al. [46] and Wu et al. [47] who prepared the AlN films by reactive magnetron sputtering and laser molecular beam epitaxy methods, respectively. For optoelectronic applications envisaged for AlxIn1-xN film, it is essential to estimate the band gap energy based on the film composition . Fig. 9 shows dependence of Eg values of our AlxIn1-xN films on the aluminum composition (dotted line). Obviously, the relationship between the obtained band gap values and Al composition is not linear. This phenomenon is named as “bowing effect” which is due to unstable nature of AlxIn1-xN within the miscibility gap, charge exchange between forming bonds in disordered solid solution and relaxation of bond length [48]. The bandgap energy of AlxIn1-xN (
) as a function of Al composition (x) is determined
using the following equation [10]: Eg,AlInN = xEg,AlN + (1 – x)Eg,InN – bx(1 - x), where b is bowing parameter and
and
(4)
are the band gaps of AlN and InN films,
respectively. Fitting the Eq. (4) to the experimental data obtained in this work, gives the bowing parameter b of
, which is close to
and
reported by Goldhahn et al. [49] and T.-
S. Yeh et al. [50] respectively, and is much smaller than By extrapolation the band gap energy curve to the the InN film, which is close to the reported value (
obtained by of Wang et al. [12].
, the band gap of
is obtained for
) [5]. However, this extrapolation-led
estimation of the InN band gap has to be confirmed experimentally by growing a pure InN using this technique. 4. Conclusion Amorphous In-rich (
and
) and Al-rich (
and
) AlxIn1-xN thin
films were successfully deposited using plasma-assisted dual source reactive evaporation 11
method. It was shown that the Al composition in the films can be adjusted by simply varying the voltage applied to the indium wire (
). Raman results indicated that E2(high) and A1(LO) peaks
of the AlxIn1-xN films are considerably blue-shifted by increasing x and the A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior. The morphology of the film was changed from clusters of small grains to uniformly shaped particles with decrease of characterizations showed that the band gap energy of the films increased from as the Al composition varied from
to
. A bowing parameter of
. Optical to
was calculated for
the grown AlxIn1-xN films and the extrapolated value from the bowing equation was 0.85 eV for band gap energy of for InN. Thus, plasma-assisted dual source reactive evaporation offers a plausible approach towards facile tuning the band gap of the amorphous AlxIn1-xN thin film for specific applications envisaged for such materials. Acknowledgment This work was supported by University of Malaya, High impact research (HIR) fund of UM.C/HIR/MOHE/SC/06 and the Ministry of Higher Education, Fundamental Research Grant Scheme (FRGS) of FP009-2013B.
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19
Table 1. Plasma-assisted dual source reactive evaporation conditions used for preparing AlxIn1xN
thin films.
Deposition parameters
Values
N2 flow rate (sccm)
40
RF power (W)
250
FAl-substrate distance (mm)
10
FIn-substrate distance (mm)
15
Substrate holder
320-360
temperature (°C) FAl temperature (°C)
1450
FIn temperature (°C)
1000-1100
Process time (min)
5
20
Fig. 1. Schematic of the experimental setup of plasma-assisted dual source reactive evaporation system.
21
Fig. 2. In3d5/2 core-level photoelectron spectra of AlxIn1-xN films deposited at (a), (b), (c) and (d). The inset of (a), (b), (c) and (d) are the corresponding Al2p core-level photoelectron spectra.
22
Fig. 3. Dependence of Al composition in the AlxIn1-xN films and FIn temperature on the applied AC voltage to indium wire.
23
Fig. 4. XRD pattern of the AlxIn1-xN thin films grown at different VIn.
24
Fig. 5. Raman spectra of the grown AlxIn1-xN (a) and AlN (b) thin films. (c) Fitted curve of Raman spectrum of Al0.64In0.36N films in the range of 650-900 cm-1. 25
Fig. 6. FESEM images of the AlxIn1-xN films with Al compositions of (c) and (d).
26
(a),
(b),
Fig. 7. Transmittance (a) and Reflectance spectra (b) of AlxIn1-xN films with different Al compositions. The inset of (a) and (b) are transmittance and reflectance spectra of AlN (x = 1), respectively.
27
Fig. 8. Plots of
versus
for AlxIn1-xN films with different x. The inset is the plot of versus for AlN films.
28
Fig. 9. Band gap energy of AlxIn1-xN film as a function of Al composition .
29
Highlights:
In-rich and Al-rich AlxIn1-xN films were grown by Plasma-aided reactive evaporation.
The A1(LO) phonon mode of the Al-rich films exhibits two-mode behavior.
The band gap of the films was tuned from
A bowing parameter of
The morphology was changed from clusters to uniformly shaped grains by decreasing .
to
.
was calculated for the grown AlxIn1-xN films.
30