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Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering
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Original Research
Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering Shakil Khana,n, Muhammad Shahidb, A. Mahmoodd, A. Shahd, Ishaq Ahmede, Mazhar Mehmooda, U. Azizd, Q. Razad, M. Alamc a
Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan b Department of Nuclear Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan c Department of Applied Physics, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan d National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan e National Centre for Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan Received 27 August 2014; accepted 21 December 2014
Abstract DC reactive magnetron sputtering technique has been used for the preparation of AlN thin films. The deposition temperature and the flow ratio of N2/Ar were varied and subsequent dependency of the films crystallites orientation/texture has been addressed. In general, deposited films were found hexagonal polycrystalline with a (002) preferred orientation. The X-ray diffraction (XRD) data revealed that the film crystallinity improves, with the increase of substrate temperature from 300 1C to 500 1C. The dropped in full width half maximum (FWHM) of the XRD rocking curve value further confirmed it. However, increasing substrate temperature above 500 1C or reducing the nitrogen condition (from 60 to 30% in the environment) induced the growth of crystallites with (102) and (103) orientations. The rise of rocking curve FWHM for the corresponding conditions depicted that the films texture quality deteriorated. A further confirmation of the variation in film texture/orentation with the growth conditions has been obtained from the variation in FWHM values of a dominant E1 (TO) mode in the Fourier transform infrared (FTIR) spectra and the E2 (high) mode in Raman spectra. We have correlated the columnar structure in AFM surface analyses with the (002) or c-axis orientation as well. Spectroscopic ellipsometry of the samples have shown a higher refractive index at 500 1C growth temperature. & 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: DC reactive magnetron sputtering; XRD; FTIR and Raman spectroscopy
1. Introduction AlN thin film is a piezoelectric material with a high acoustic velocity. It is a superior applicant for high frequency devices, like surface acoustic waves devices (SAW), resonators, high frequency filters and pressure sensors [1–3]. The development of wurtzite (002) or c-axis oriented phase of aluminum nitride (AlN) thin film is vital in most of these applications for obtaining higher values of electromechanical coupling factor k2t [4].
n
Corresponding author. E-mail address: [email protected] (S. Khan). Peer review under responsibility of Chinese Materials Research Society.
It is known that the physical properties of the film are significantly influenced by its crystallographic orientation, which in turn is influenced by the sputtering process parameters [5]. The process parameters such as substrate temperature and nitrogen conditions strongly affect the AlN film microstructure and consequently affects SAW propagation velocity and piezoelectric response [6], i.e. the existence of phases other than the (002) oriented or c-axis oriented planes results in the deterioration of SAW propagation velocity and piezoelectric response. Surface morphology/roughness also plays an important role in applications such as metallization and wear-resistant coatings [7]. Therefore, to get AlN films with desire properties, it is essential to obtain a good control of the film crystal quality, preferred orientation and the transformation from one orientation to other with the variation of
http://dx.doi.org/10.1016/j.pnsc.2015.08.006 1002-0071/& 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
S. Khan et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]]
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growth conditions [8]. So far, many reports on the growth of AlN films have been published, but no conclusive optimal deposition parameters have been obtained, and the reports showed difficulties to determine consistent results [9,10]. For example, Verardi et al. using pulsed laser deposition at 200–450 1C on sapphire substrate [11]. The rise of (002) orientation with the increase in deposition pressure from 2 10–1 to 6 10–1 Pa and the transformation to (100) crystal orientation at 8 10–1 Pa is also reported. Okano et al. [12] concluded that the growth of c-axis orientated crystallites occurred with the decrease of nitrogen content in the environment, but Cho [13] and Cheng et al. [14] proposed the high N2 condition. As far as the substrate temperature is concerned, Medjani et al. [15] show that lower (below 400 1C) temperature is favorable for the formation of (002) plane, however Jin et al. [16] concluded the 430 1C being the optimum temperature for the maximum degree of c-axis preferred orientation. The X-ray diffraction (XRD) is a well known technique that is being frequently used to asses thin film crystalline orientation [17]. The other important tools for the microstructure analysis are the Raman and to some extent Fourier transform infra-red spectroscopy (FTIR) [8]. In this work, AlN thin films have been grown by DC reactive magnetron sputtering technique. The influence of various substrate temperatures and nitrogen conditions on film microstructure has been addressed. The films micro-structural analysis was performed by means of XRD at grazing angle of incidence, XRD rocking curves, FTIR and Raman spectroscopy. The results were correlated and obtained an empirical trend for the growth of (002) oriented AlN films. The surface feature of the grown film was analyzed by atomic force microscopy (AFM) and variation of morphology was also correlated with the growth conditions.
2. Experimental AlN thin films were deposited using a balanced DC reactive magnetron sputtering technique. In the synthesis chamber, a base vacuum of 10–4 Pa was achieved using a turbo-molecular pump (VARIAN Turbo-V-1000 HT) with an average speed of 800 l/sec. Electronic mass flow controllers (BROOKS mass flow controller 5850 series) was employed for controlled flow of gases. The substrate temperature during film growth was measured with the help of a thermocouple attached to the bottom of a substrate holder at a distance of 3 cm from target. The Al flux was obtained from sputtering of Al target (purity: 99.99%). The purity of argon (Ar) and nitrogen (N2) gases was 99.995%. A schematic diagram of the apparatus used in this work is shown in Fig. 1. To evade/reduce contamination of a growing film, silicon (111) and quartz substrates were ultrasonically cleaned in acetone/ trichloroethylene for 20 min. In a next step, the substrate was shadowed from the target by a shutter and its pre-sputtering wasperformed for 10 min. All the films were grown without substrate biasing, keeping the plasma current, cathode voltage and growth time at 200 mA, 800 V and 40 min, respectively. The deposition pressure and total gas flow were also kept constant at 6 10–1 Pa and 10 sccm, respectively. Samples were prepared at
Shutter control
Low Vacuum gauge
Water cooled cathode
High vacuum line
High Vacuum gauge
Substrate holder
Substrate rotation
Fig. 1. Experimental setup of magnetron sputtering.
Table 1 General growth conditions for AlN films. Sample description
N2/(N2 þ Ar) (%)
Deposition temperature (1C)
Sample Sample Sample Sample Sample
60 60 60 60 30
300 400 500 600 500
A B C D E
Constant parameters (Deposition current and pressure¼200 mA and 6 10– 3 mbar, respectively).
different substrate temperatures in the range of 300–600 1C by means of resistive heating, while keeping the nitrogen fraction at 60%. Films were also deposited at lower nitrogen condition (30%) as shown in Table 1. Argon gas was increased in proportion to the decrease of nitrogen gas and the total pressure during film growth was kept constant through an adjustment of a high vacuum valve. XRD analysis was performed at grazing angle of incidence using D-8 Discoverer HR-XRD machine equipped with Cu-kα line (1.54 Å) source. A parallel beam geometry measurement was performed at grazing incidence diffraction. To get higher counts by detector of the XRD machine, the incidence angle (α) of primary beam was set as 41 with respect to film surface. At the secondary side, a long soller slit was used to limit the radial divergence to 0.121. To analyze the characteristic spectrum by functional group, FTIR spectroscopy was performed using Nicolet 6700 FTIR spectrophotometer at an oblique angle (451). Room temperature Raman spectra of the deposited films were obtained via spectroscopic system (Model: MST-4000A; DONG-WOO OPTRON Co., Ltd.). A IDUS-ANDOR software was use for collecting the emission spectra in the desired range of wavelength. The spectroscopic ellipsometric measurements of the films were carried out with SENTECH (SE-850) machine equipped with Xenon lamp. For the investigation of surface features/morphology of the deposited films, atomic force microscope (Model: QScope™
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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(002)
(103)
(002)
Intensity (a.u)
Intensity (a.u)
600 C
500 C
(102)
(103)
30%
400 C
60%
300 C
30
40
50
60
70
30
40
2θ (degree)
50
60
70
2θ (degree)
Fig. 2. XRD spectra of the AlN films prepared at various (a) substrate temperatures, (b) nitrogen conditions.
350) was employed. All the samples were scanned over an area of 2 2 mm2 in a taping mode.
3. Results and discussion 3.1. XRD analysis Investigation of crystalline structure of AlN films was conducted by means of XRD at grazing incidence angle of 41. The XRD patterns of films are shown in Fig. 2(a) and (b), as a function of substrate temperature and percentage nitrogen in the deposition chamber, respectively. The observed XRD peaks can be assigned to Wurtzite AlN (Card 01-070-2545) phase. The peak at 2θ¼ 36.0231 can be assign to the (002) orientation, though its position is slightly below the powder data ( 36.041), which exhibit residual compressive stress in the film. The preferred orientation of (002) plane along the surface is clearly evident for the samples prepared at 60% nitrogen, up to a temperature of 500 1C, above this the relative intensity of (103) increases. Furthermore, reducing the nitrogen fraction in the synthesis chamber induces the growth of (102) and (103) orientation as well. The XRD spectra also present that the full width at half maximum (FWHM) of (002) reflection changes with substrate temperature and is shown in Table 2. The FWHM of (002) peak was measured using X'pert high score analysis software. The measured FWHM values of (002) reflection are 0.5901, 0.301, 0.2951 and 0.4801 at growth temperature of 300, 400, 500 and 600 1C, respectively. Scherrer's formula of Eq. (1) could be used to calculate the grain size from XRD pattern [17] t ¼
0:9λ B cos θ
ð1Þ
where t is the grain size, λ is the wavelength of measured X-ray (1.54 Å), B is termed as the full width at half maximum (in radians) and θ is the half diffraction angle of crystalline orientation peak. The calculated average grain sizes of (002) crystallite for the
Table 2 XRD results for (002) orientation. Sample description
Temperature (1C)
N2/(N2 þ Ar) (%)
FWHM (1)
Crystallite size (Å)
Sample Sample Sample Sample Sample
300 400 500 600 500
60 60 60 60 30
0.590 0.30 0.295 0.480 0.246
142 279 284 174 341
A B C D E
samples prepared at 300, 400 and 500 1C are 142, 279 and 284 Å (shown in Table 2), respectively. For the reduced nitrogen conditions, the grain size of (002) orientation is slightly higher ( 341 Å) than the grain sizes for the former conditions. To determine the c-axis orientation distribution or textured quality of the films, rocking curve (RC) of (002) diffraction peaks were obtained at 2θ¼ 36.021 in the ω scan. The RCs of AlN (002) reflection at various temperatures are shown in Fig. 3(a). The rise of RC intensity could be seen with the increase of substrate temperature. The measured full width at half maximum (FWHM) values of the RCs against various temperatures are shown in Fig. 3(b). RC of the film prepared at reduced nitrogen fraction is shown in Fig. 3(c) with a 3.9270.151 FWHM value. The films prepared at 500 1C exhibited the lowest FWHM value depicting the decrease in orientation distribution and thus the highest textured film [18]. The higher texture reduced the native defects at grain boundaries and thus improved the preferred (002) oriented crystalline quality. The most stable phase of AlN thin films is the Wurtzite phase owing to its close packed structure and (002) orientation has also been reported to have lowest surface energy [19,20], i.e. surface of the film always has a tendency to lower its energy by growing in certain texture, which have the lowest possible surface energy and therefore may be a driving force for texture evolution [21]. At
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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12 11 10 FWHM (degree)
Intensity (a.u)
600 C
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9 8 7 6 5
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θ (degree)
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Fig. 3. Influence of temperature on (a) RCs of the AlN film, (b) FWHM of RCs and (c) influence of nitrogen fraction on the RC.
300 1C, enough energy was available for islands to nucleate and grow in (002) orientation and lower their surface energies. Since the adatoms energy strongly depends on the deposition temperature [22], increasing the growth temperature from 300 1C to 500 1C raised the adatoms mobility and surface diffusion length. Enhanced mobility enabled adatoms toward lattice sites of the (002) plane. At 500 1C, generation of vacancies may also occurred, which further grew the (002) preferred orientation [23]. The growth could also be related to the interface merging process induced by substrate temperature. The defects at the grain or crystallite boundaries exist due to the random orientation of the crystallites. At 600 1C, the reduction of (002) crystallites size may be due to the enhanced desorption rate of the species at substrate surface. The re-evaporation from the film surface decreased the adding probability of species to (002) orientation. Moreover, increasing the substrate temperature further enhanced the adatoms energy, which enables the (103) orientation to grow as well [23]. Abdallah et al. [24] reported the transformation of films texture from preferred (002) to (100) orientation with the increase of bias voltage. The growth of AlN films at different pressures, at various N2 þ Ar gas conditions and at different gas mixtures of nitrogen with other inert gases (neon, krypton and xenon) are also reported in the literature by different researchers [25,26]. In these reports, the texture of AlN films transformed when decreasing the nitrogen fraction in the gas mixture (N2 þ Ar) and with the increase in atomic number of the inert gas, the preferred orientation of AlN films changes from (002) to (101) orientation, and then to polycrystalline with random orientation and finally became
amorphous. It appeared that when relatively heavier mass ions impinge on substrate surface during film growth, it imparts more energy to the depositing species. The adatoms energy thus plays a key role in determining texture of the deposited AlN films. As far as the transformation of a highly textured orientation to a mixed one at lower nitrogen content is concerned, the impact of greater numbers of relatively heaver argon ions imparts higher energy to the depositing species, enabling them to grow in other directions as well, such as [102] and [103]. 3.2. FTIR and Raman analysis FTIR and Raman spectroscopy are important spectroscopic techniques to probe vibrational states of molecules and their local order. In this study, the FTIR and Raman analysis were performed to observe the longitudinal optical (LO) and transverse optical (TO) phonon vibration modes of AlN thin film and their results were compared to obtain an empirical relation between them. Group theory predicts eight optical phonon modes (2A1 þ 2E1 þ 2E2 þ 2B modes) at the Γ point of Brillouine zone of hexagonal AlN. These modes split into the longitudinal optical (LO) and transverse optical (TO) components due to their polar behavior. Among these modes, A1 and E1 are infrared active modes, E2 along with the former modes are Raman active, while B modes are silent in both. The LO modes are active only in the oblique angles analysis [27,28]. The IR spectral analysis was conducted in a reflection mode at oblique angle (451 off normal). The spectra of the films were
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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Absorbance (arb units)
5
500
600
700
Wavenumber
800
900
500
600
700
800
900
Wavenumber (cm-1)
(cm-1)
obtained in the range of 400 to 1000 cm 1. The effects of deposition temperature and nitrogen conditions on the FTIR spectra are presented in Fig. 4(a) and (b), respectively. FTIR spectra of each figure depict a dominant peak at 673 cm 1 accompanied by a shoulder peak at 613 cm 1 and a mode at 895 cm 1. Absorption peaks at 611 cm 1, 672 cm 1 and 895 cm 1 correspond to the A1 (TO), E1 (TO) and A1 (LO) mode of aluminum nitride film, respectively [29]. The 620 cm 1 peak arises due to silicon substrate. The E1 (LO) mode is missing in the FTIR pattern that is expected to appear at 912 cm 1. The E1 (TO) mode is dominant in FTIR spectra irrespective of substrate temperature, which demonstrate that the films contain mostly (002) or c-axis oriented crystallites [30]. However, the appearance of a shoulder peak for A1 (TO) mode depicts that crystallites of other phases are also exist and the (002) crystallites are slightly tilted from surface normal [31]. The decrease in FWHM of FTIR absorption peaks is observable with the rise in temperature up to 500 1C, as shown in Fig. 4(a). The decrease in FWHM indicates an improvement in the AlN film crystallinity (improvement in the short-range ordering of atoms) [32]. However, further raising the substrate temperature to 600 1C reverses the trend of peak width, depicting a relatively disordered structure. In effect, observation of FTIR spectra confirms 500 1C being a relatively more suitable growth temperature for c-axis oriented AlN films grown by DC magnetron sputtering. Observations regarding different nitrogen growth conditions are shown in Fig. 4(b), depicting a relatively sharpened peak at 30% nitrogen condition. It appears that at lower nitrogen content, the crystallites growth of AlN film occurs. As the total pressure during deposition was kept constant, reducing the nitrogen content means a rise of Ar þ concentration in the plasma gas. When relatively heavier argon ions impinge on substrate surface, it transfers more energy to the adatoms, thereby increasing their mobility. Thus, more coalescent energy is available for relatively defect-free crystallites in the film. Raman spectroscopy is a technologically more important spectroscopic analysis to probe the vibrational states of crystals or semiconductors. The measurement was performed using a high resolution (4 cm 1) Raman spectroscopy system (model MST4000A). To avoid overlapping of the AlN peak with silicon substrate, the Raman analysis was conducted on AlN films grown on quartz substrate. Fig. 5 offers the Raman spectra of AlN films
Intensity (a.u)
Fig. 4. Observed FTIR spectra of AlN film deposited at different (a) temperatures, (b) nitrogen fractions.
500
600
700
800
900
1000
-1
Raman Shift (cm )
Fig. 5. Raman analysis at different growth temperatures.
grown at different substrate temperatures. The spectra of AlN film consist of vibrational optical phonon modes at 659 cm 1 and 892 cm 1 corresponding to the transverse optical E2 (high) mode and A1 (LO) of AlN, respectively [18]. The peak position of E2 mode appears very close to the reported value 657.4 cm 1 [33] for the un-strained AlN layer. The rise of peak intensity with the increase of substrate temperature up to 500 1C suggests the local ordering of crystal structure. The appearance of E2 (high) phonon mode irrespective of substrate temperature showed that deposited films are preferably (002) oriented AlN film [34] and is in agreement with XRD and FTIR results. The decrease in FWHM of observed peak can be seen with the increase in substrate temperature up to 500 1C that demonstrated lower phonon scattering effect at grain boundaries and thus lower defects in film [26]. 3.3. AFM analysis Atomic force microscopy of the deposited films was performed with Model: QScope™ 350 scanning probe microscope. Fig. 6 shows AFM images of AlN films depicting the variation of surface morphology with the increase in substrate temperature. Film prepared at 300 1C (Fig. 6(a)) depicts only small number of nano-sized clusters. The number of clusters and packing density increases with
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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Fig. 6. AFM images of AlN films prepared at (a) 300 1C, (b) 400 1C, (c) 500 1C, and (d) 600 1C.
increase in the substrate temperature up to 400 1C, as shown in Fig. 6 (b). Increasing the substrate temperature to 500 1C has resulted in a substantially modified surface morphology where a highly textured surface profile is evident. Computed RMS surface roughness values were 7, 4, 5 and 6 nm for the films prepared at 300, 400, 500 and 600 1C respectively. Although RMS roughness does not differ appreciably, clear indication of substantial surface modification might result from the change in growth mode. The appearance of columnar structure in AFM images implies that films are c-axis oriented as shown in Fig. 6(c). However, at 600 1C some grooves are produced within the columnar structure. At lower substrate temperature ( 300 1C), the constituent particles may not have enough thermal energy to accommodate themselves at proper position and the growth of crystallites is therefore multidirectional. Such type of disordered structure contains higher density of structural defects. At enhanced substrate temperature of 500 1C, the formation of columnar structure corresponds to the (002) orientation [32]. The higher substrate temperature enhances the adatoms mobility and surface diffusion length. As the most stable phase of AlN films is the wurtzite, in a higher adatoms energy configuration, the most suitable growth for the islands is to grow in c-axis direction, which is due to its close packed structure and (002) orientation have the lowest surface energy [19,20]. However, increasing temperature beyond 500 1C may causes desorption
of the adatoms. The desorption of pits/grooves results in the formation of clusters as it is clear from Fig. 6(d) within the caxis oriented columns. AFM images of AlN films prepared at different N2 conditions are shown in Fig. 7(a) and (b). AlN film deposited at 30% nitrogen condition (Fig. 7(b)) exhibits a mixed structure of columns and clusters, suggesting a c-axis orientation along with other phases as well. The departure of columnar surface structure to mixed structure of Fig. 7(b) may be attributed to the difference of argon and nitrogen atomic masses. From the above analysis, it can be concluded that AFM analysis is consistent with XRD results. 3.4. Ellipsometry analysis Ellipsometric measurements of AlN films were performed to measure deposited film thickness and refractive index. The obtained data was in a form of Psi (ψ) and Delta (Δ) function, where ψ represents amplitude ratio while Δ represents the phase difference between the reflected and incident waves. The measurements were performed at an angle of 701 over a spectral range from 300 nm to 900 nm. A three layer model was used as an input in ellipsometry measurement and is shown in Fig. 8(a). The obtained experimental curves were simulated theoretically by using multilayered-Cauchy-Urbach model. Cauchy–Urbach
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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Fig. 7. AFM images of AlN film prepared at N2 condition (a) 60% and (b) 30%.
400 350
Psi / Delta
300
Ambient air
Psi Exp Psi Fitted Delta Exp Delta Fitted
250 200 150 100 50
AlN(312 nm) SiO 2
Si (111)
0 -50 300
400
500 600 700 800 Wave Length (nm)
900
Fig. 8. (a) Multi-Layer model used in ellipsometry, (b) experimental and simulated Psi and Delta curves.
dispersion model utilizes complex refractive index (N¼ nþ ik) whose components i.e. refractive index n(λ) and extinction coefficients κ(λ), are governed by nðλÞ ¼ n0 þ C0 nλ21 þ C 1 nλ42 þ :::
ð2Þ
k1 k2 þ C 1 4 þ ::: ð3Þ 2 λ λ where n0, n1, and n2 are the refractive indices of ambient, 1st and 2nd layers respectively, while k0, k1 and k2 represent extinction coefficient of respective layers. C0 and C1 are constants with values 102 and 107 respectively. Fig. 8(b) shows the spectroscopic ellipsometry data together with the simulated curves for a sample prepared at 400 1C. From Fig. 8(b), it is clear that there is a good agreement between the simulated and experimental data. After sufficiently accurate fitting was obtained, the film thickness and refractive indeces were obtained from fitted model. The fitting was analyzed using the mean square error (χ2) given in Eq. (4) to see the difference of experimental and simulated data.
kðλÞ ¼ k 0 þ C 0
χ2 ¼
N 1 X ðMesi Thi Þ2 N i¼1 σ 2i
ð4Þ
where N is the number of data points, Mesi is the ith experimental
Table 3 Ellipsometery results. Sample description
Temperature N2/ (N2 þ Ar) (oC) (%)
χ2
Thickness Refractive index (nm) at 2.3 eV
Sample Sample Sample Sample Sample
300 400 500 600 500
4.3 3.7 3.8 3.2 4.1
322 312 296 273 369
A B C D E
60 60 60 60 30
1.87 1.89 1.90 1.88 1.86
data point, Thi is the ith calculated data point from theoretical model and σi is the standard deviation. The values of mean square errors are shown in Table 3. Deposition rates of the deposited films were calculated from the measured film thickness and deposition time. The effect of substrate temperature on the deposition rate and refractive indeces is shown in Fig. 9(a) and (b), respectively. It appears that deposition rate of the deposited films continuously decreases. The desorption of adatoms from film surface depend on substrate temperature. Increasing the temperature enhances the adatoms
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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9
1.92 Refractive index (n)
Deposition rate (nm/min)
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6 1.84 5 300
350
400 450 500 Temperature ( C)
550
600
1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 Photon energy (eV)
Fig. 9. Influence of substrate temperature on (a) deposition rate, (b) refractive index.
energy and re-evaporation from film surface. The re-evaporation from film surface, lowers the growth rate[23]. The other possibility that lowers the deposition rate is the increase of film density with the increase in substrate temperature [35]. Refractive index lies in the range of 1.85–1.9and is close to the reported values for AlN [36,37]. The AlN film prepared at 500 1C exhibited a 1.9 refractive index (n) at 2.3 eV. It remains higher than the refractive index of samples deposited at 400 1C for whole range of photon energy. At higher substrate temperature, surface diffusion length of the depositing species is greater, which improve the film density and refractive index [38]. 4. Conclusions In summary, films texture under the influence of substrate temperature and nitrogen fraction has been addressed. XRD and FTIR analysis have shown that the (002) orientation growth occurs with the increase of substrate temperature from 300 1C to 500 1C. It was also found that at 30% nitrogen condition, the crystallites grow in other direction as well. The appearance of E1 (TO) mode peak at 672 cm 1 in the IR spectra and E2 (high) mode in the Raman spectra demonstrated the c-axis oriented crystallites growth. A shoulder absorption band around 611 cm corresponding to the A1 (TO) optical mode depicted that the crystallites of other phases are also exist and the (002) crystallites are slightly tilted from surface normal. Surface analysis of the deposited films indicated the columnar structure, suggesting a c-axis oriented growth. Ellipsometry results have shown a 1.9 refractive index at 2.3 eV. From the above results, it can be concluded that 500 1C substrate temperature and 60% N2 fraction are the optimum deposition conditions for the growth of c-axis oriented films. Acknowledgments The authors are thankful for the financial support of Department of Metallurgy and Materials Engineering (PIEAS) and National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan. References [1] V. Mortet, A. Vasin, P.Y. Jouan, O. Elmazria, M.A. Djouadi, Surf. Coat. Technol. 176 (2003) 88–92. [2] A.F. Belyanin, L.L. Bouilov, V.V. Zhirnov, A.I. Kamenev, K.A. Kovalskij, B.V. Spitsyn, Diam. Relat. Mater. 8 (1999) 369–372.
[3] Kyungsoo Jang, Kwangsoo Lee, Junsik Kim, Sunghyun Hwang, Jeongin Lee, Suresh Kumar Dhungel, Sungwook Jung, Junsin Yi, Mater. Sci. Semicond. Process. 9 (2006) 1137–1141. [4] L. Vergara, M. Clement, E. Iborra, A. Sanz-Herva´s, J. Garcia Lopez, Y. Morilla, J. Sangrador, M.A. Respaldiza, Diam. Relat. Mater. 13 (2004) 839–842. [5] Cheng-Liang Huang, Kok-Wan Tay, Long Wu, Solid-State Electron. 49 (2005) 219–225. [6] X.-H. Xu, H.-S. Wu, C.-J. Zhang, Thin Solid Films 388 (2001) 62–67. [7] B.W. Karr, I. Petrov, D.G. Cahill, J.E. Greene, Appl. Phys. Lett. 70 (1997) 1703. [8] A. Sanz-Hervás, E. Iborra, M. Clement, J. Sangrador, M. Aguilar, Diam. Relat. Mater. 12 (2003) 1186–1189. [9] Kuan-Hsun Chiu, Jiann-Heng Chen, Hong-Ren Chen, RueyShing Huang, Thin Solid Films 515 (2007) 4819–4825. [10] M.A. Moreira, I. Doi, J.F. Souza, J.A. Diniz, Microelectron. Eng. 88 (2011) 802–806. [11] P. Verardi, M. Dinescu, C. Stanciu, C. Gerardi, L. Mirenghi, V. Sandu, Mater. Sci. Eng. B 50 (1997) 223. [12] H. Okano, Y. Takahashi, T. Tanaka, K. Shibata, S. Nakano, Jpn. J. Appl. Phys. 31 (1) (1992) 3446. [13] Shinho Cho, J. Cryst. Growth 326 (2011) 179–182. [14] C.C. Cheng, Y.C. Chen, H.J. Wang, W.R. Chen, J. Vac. Sci. Technol. A: Vac. Surf. Films 14 (1996) 2238. [15] F. Medjani, R. Sanjine, G. Allidi, A. Karimi, Thin Solid Films 515 (2006) 260–265. [16] Hao Jin, Bin Feng, Shurong Dong, Changjian Zhou, Jian zhou, Yi yang, Tianling Ren, Jikui Luo, Demiao Wang, J. Electron. Mater. 41 (2012). [17] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, 3rd ed., Prentice-Hall, Englewood Cliffs, NJ, 2001. [18] N. Itoh, K. Okamoto, J. Appl. Phys. 63 (1988) 1486. [19] M. Clement, E. Iborra, J. Sangrador, J. Appl. Phys. 94 (3) (2003) 1495–1500. [20] S. Ohuchi, P.E. Russel, J. Vac. Sci. Technol. A 5 (1987) 1630. [21] G. Knuyt, C. Quaeyhaegens, J. D'Haen, L.M. Stals, Surf. Coat. Technol. 76-77 (1995) 311–315. [22] M. Ohring, The Materials Science of Thin Films, first edition, Academic Press, San Diego, 1992. [23] Kuan-Hsun Chiu, Jiann-Heng Chen, Hong-Ren Chen, RueyShing Huang, Thin Solid Films 515 (2007) 4819–4825. [24] B. Abdallah, A. Chala, P.-Y. Jouan, M.P. Besland, M.A. Djouadi, Thin Solid Films 515 (2007) 7105–7108. [25] Hwan-Chul Lee, Guen-Hong Kim, Soon-Ku Hong, ki-Young Lee, YoonJoong Yong, Chang-Hwan Chunb, Jai-Young Lee, Thin Solid Films 261 (1995) 148–153. [26] H. Cheng, Y. Sun, J.X. Zhang, Y.B. Zhang, S. Yuan, P. Hing, J. Cryst. Growth 254 (2003) 46–54. [27] T. Prokofyeva, M. Seon, J. Vanbuskirk, M. Holtz, Phys. Rev. B 63 (2001) 125313. [28] Y.F. Lu, Z.M. Ren, T.C. Chong, B.A. Cheong, S.K. Chow, J.P. Wang, J. Appl. Phys. 87 (2000) 1540.
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Original Research
Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering Shakil Khana,n, Muhammad Shahidb, A. Mahmoodd, A. Shahd, Ishaq Ahmede, Mazhar Mehmooda, U. Azizd, Q. Razad, M. Alamc a
Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan b Department of Nuclear Engineering, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan c Department of Applied Physics, Pakistan Institute of Engineering & Applied Sciences, Islamabad, Pakistan d National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan e National Centre for Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan Received 27 August 2014; accepted 21 December 2014
Abstract DC reactive magnetron sputtering technique has been used for the preparation of AlN thin films. The deposition temperature and the flow ratio of N2/Ar were varied and subsequent dependency of the films crystallites orientation/texture has been addressed. In general, deposited films were found hexagonal polycrystalline with a (002) preferred orientation. The X-ray diffraction (XRD) data revealed that the film crystallinity improves, with the increase of substrate temperature from 300 1C to 500 1C. The dropped in full width half maximum (FWHM) of the XRD rocking curve value further confirmed it. However, increasing substrate temperature above 500 1C or reducing the nitrogen condition (from 60 to 30% in the environment) induced the growth of crystallites with (102) and (103) orientations. The rise of rocking curve FWHM for the corresponding conditions depicted that the films texture quality deteriorated. A further confirmation of the variation in film texture/orentation with the growth conditions has been obtained from the variation in FWHM values of a dominant E1 (TO) mode in the Fourier transform infrared (FTIR) spectra and the E2 (high) mode in Raman spectra. We have correlated the columnar structure in AFM surface analyses with the (002) or c-axis orientation as well. Spectroscopic ellipsometry of the samples have shown a higher refractive index at 500 1C growth temperature. & 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: DC reactive magnetron sputtering; XRD; FTIR and Raman spectroscopy
1. Introduction AlN thin film is a piezoelectric material with a high acoustic velocity. It is a superior applicant for high frequency devices, like surface acoustic waves devices (SAW), resonators, high frequency filters and pressure sensors [1–3]. The development of wurtzite (002) or c-axis oriented phase of aluminum nitride (AlN) thin film is vital in most of these applications for obtaining higher values of electromechanical coupling factor k2t [4].
n
Corresponding author. E-mail address: [email protected] (S. Khan). Peer review under responsibility of Chinese Materials Research Society.
It is known that the physical properties of the film are significantly influenced by its crystallographic orientation, which in turn is influenced by the sputtering process parameters [5]. The process parameters such as substrate temperature and nitrogen conditions strongly affect the AlN film microstructure and consequently affects SAW propagation velocity and piezoelectric response [6], i.e. the existence of phases other than the (002) oriented or c-axis oriented planes results in the deterioration of SAW propagation velocity and piezoelectric response. Surface morphology/roughness also plays an important role in applications such as metallization and wear-resistant coatings [7]. Therefore, to get AlN films with desire properties, it is essential to obtain a good control of the film crystal quality, preferred orientation and the transformation from one orientation to other with the variation of
http://dx.doi.org/10.1016/j.pnsc.2015.08.006 1002-0071/& 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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growth conditions [8]. So far, many reports on the growth of AlN films have been published, but no conclusive optimal deposition parameters have been obtained, and the reports showed difficulties to determine consistent results [9,10]. For example, Verardi et al. using pulsed laser deposition at 200–450 1C on sapphire substrate [11]. The rise of (002) orientation with the increase in deposition pressure from 2 10–1 to 6 10–1 Pa and the transformation to (100) crystal orientation at 8 10–1 Pa is also reported. Okano et al. [12] concluded that the growth of c-axis orientated crystallites occurred with the decrease of nitrogen content in the environment, but Cho [13] and Cheng et al. [14] proposed the high N2 condition. As far as the substrate temperature is concerned, Medjani et al. [15] show that lower (below 400 1C) temperature is favorable for the formation of (002) plane, however Jin et al. [16] concluded the 430 1C being the optimum temperature for the maximum degree of c-axis preferred orientation. The X-ray diffraction (XRD) is a well known technique that is being frequently used to asses thin film crystalline orientation [17]. The other important tools for the microstructure analysis are the Raman and to some extent Fourier transform infra-red spectroscopy (FTIR) [8]. In this work, AlN thin films have been grown by DC reactive magnetron sputtering technique. The influence of various substrate temperatures and nitrogen conditions on film microstructure has been addressed. The films micro-structural analysis was performed by means of XRD at grazing angle of incidence, XRD rocking curves, FTIR and Raman spectroscopy. The results were correlated and obtained an empirical trend for the growth of (002) oriented AlN films. The surface feature of the grown film was analyzed by atomic force microscopy (AFM) and variation of morphology was also correlated with the growth conditions.
2. Experimental AlN thin films were deposited using a balanced DC reactive magnetron sputtering technique. In the synthesis chamber, a base vacuum of 10–4 Pa was achieved using a turbo-molecular pump (VARIAN Turbo-V-1000 HT) with an average speed of 800 l/sec. Electronic mass flow controllers (BROOKS mass flow controller 5850 series) was employed for controlled flow of gases. The substrate temperature during film growth was measured with the help of a thermocouple attached to the bottom of a substrate holder at a distance of 3 cm from target. The Al flux was obtained from sputtering of Al target (purity: 99.99%). The purity of argon (Ar) and nitrogen (N2) gases was 99.995%. A schematic diagram of the apparatus used in this work is shown in Fig. 1. To evade/reduce contamination of a growing film, silicon (111) and quartz substrates were ultrasonically cleaned in acetone/ trichloroethylene for 20 min. In a next step, the substrate was shadowed from the target by a shutter and its pre-sputtering wasperformed for 10 min. All the films were grown without substrate biasing, keeping the plasma current, cathode voltage and growth time at 200 mA, 800 V and 40 min, respectively. The deposition pressure and total gas flow were also kept constant at 6 10–1 Pa and 10 sccm, respectively. Samples were prepared at
Shutter control
Low Vacuum gauge
Water cooled cathode
High vacuum line
High Vacuum gauge
Substrate holder
Substrate rotation
Fig. 1. Experimental setup of magnetron sputtering.
Table 1 General growth conditions for AlN films. Sample description
N2/(N2 þ Ar) (%)
Deposition temperature (1C)
Sample Sample Sample Sample Sample
60 60 60 60 30
300 400 500 600 500
A B C D E
Constant parameters (Deposition current and pressure¼200 mA and 6 10– 3 mbar, respectively).
different substrate temperatures in the range of 300–600 1C by means of resistive heating, while keeping the nitrogen fraction at 60%. Films were also deposited at lower nitrogen condition (30%) as shown in Table 1. Argon gas was increased in proportion to the decrease of nitrogen gas and the total pressure during film growth was kept constant through an adjustment of a high vacuum valve. XRD analysis was performed at grazing angle of incidence using D-8 Discoverer HR-XRD machine equipped with Cu-kα line (1.54 Å) source. A parallel beam geometry measurement was performed at grazing incidence diffraction. To get higher counts by detector of the XRD machine, the incidence angle (α) of primary beam was set as 41 with respect to film surface. At the secondary side, a long soller slit was used to limit the radial divergence to 0.121. To analyze the characteristic spectrum by functional group, FTIR spectroscopy was performed using Nicolet 6700 FTIR spectrophotometer at an oblique angle (451). Room temperature Raman spectra of the deposited films were obtained via spectroscopic system (Model: MST-4000A; DONG-WOO OPTRON Co., Ltd.). A IDUS-ANDOR software was use for collecting the emission spectra in the desired range of wavelength. The spectroscopic ellipsometric measurements of the films were carried out with SENTECH (SE-850) machine equipped with Xenon lamp. For the investigation of surface features/morphology of the deposited films, atomic force microscope (Model: QScope™
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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(002)
(103)
(002)
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Intensity (a.u)
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(103)
30%
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60%
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40
50
60
70
30
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Fig. 2. XRD spectra of the AlN films prepared at various (a) substrate temperatures, (b) nitrogen conditions.
350) was employed. All the samples were scanned over an area of 2 2 mm2 in a taping mode.
3. Results and discussion 3.1. XRD analysis Investigation of crystalline structure of AlN films was conducted by means of XRD at grazing incidence angle of 41. The XRD patterns of films are shown in Fig. 2(a) and (b), as a function of substrate temperature and percentage nitrogen in the deposition chamber, respectively. The observed XRD peaks can be assigned to Wurtzite AlN (Card 01-070-2545) phase. The peak at 2θ¼ 36.0231 can be assign to the (002) orientation, though its position is slightly below the powder data ( 36.041), which exhibit residual compressive stress in the film. The preferred orientation of (002) plane along the surface is clearly evident for the samples prepared at 60% nitrogen, up to a temperature of 500 1C, above this the relative intensity of (103) increases. Furthermore, reducing the nitrogen fraction in the synthesis chamber induces the growth of (102) and (103) orientation as well. The XRD spectra also present that the full width at half maximum (FWHM) of (002) reflection changes with substrate temperature and is shown in Table 2. The FWHM of (002) peak was measured using X'pert high score analysis software. The measured FWHM values of (002) reflection are 0.5901, 0.301, 0.2951 and 0.4801 at growth temperature of 300, 400, 500 and 600 1C, respectively. Scherrer's formula of Eq. (1) could be used to calculate the grain size from XRD pattern [17] t ¼
0:9λ B cos θ
ð1Þ
where t is the grain size, λ is the wavelength of measured X-ray (1.54 Å), B is termed as the full width at half maximum (in radians) and θ is the half diffraction angle of crystalline orientation peak. The calculated average grain sizes of (002) crystallite for the
Table 2 XRD results for (002) orientation. Sample description
Temperature (1C)
N2/(N2 þ Ar) (%)
FWHM (1)
Crystallite size (Å)
Sample Sample Sample Sample Sample
300 400 500 600 500
60 60 60 60 30
0.590 0.30 0.295 0.480 0.246
142 279 284 174 341
A B C D E
samples prepared at 300, 400 and 500 1C are 142, 279 and 284 Å (shown in Table 2), respectively. For the reduced nitrogen conditions, the grain size of (002) orientation is slightly higher ( 341 Å) than the grain sizes for the former conditions. To determine the c-axis orientation distribution or textured quality of the films, rocking curve (RC) of (002) diffraction peaks were obtained at 2θ¼ 36.021 in the ω scan. The RCs of AlN (002) reflection at various temperatures are shown in Fig. 3(a). The rise of RC intensity could be seen with the increase of substrate temperature. The measured full width at half maximum (FWHM) values of the RCs against various temperatures are shown in Fig. 3(b). RC of the film prepared at reduced nitrogen fraction is shown in Fig. 3(c) with a 3.9270.151 FWHM value. The films prepared at 500 1C exhibited the lowest FWHM value depicting the decrease in orientation distribution and thus the highest textured film [18]. The higher texture reduced the native defects at grain boundaries and thus improved the preferred (002) oriented crystalline quality. The most stable phase of AlN thin films is the Wurtzite phase owing to its close packed structure and (002) orientation has also been reported to have lowest surface energy [19,20], i.e. surface of the film always has a tendency to lower its energy by growing in certain texture, which have the lowest possible surface energy and therefore may be a driving force for texture evolution [21]. At
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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12 11 10 FWHM (degree)
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9 8 7 6 5
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3
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30
300
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θ (degree)
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θ (degree)
Fig. 3. Influence of temperature on (a) RCs of the AlN film, (b) FWHM of RCs and (c) influence of nitrogen fraction on the RC.
300 1C, enough energy was available for islands to nucleate and grow in (002) orientation and lower their surface energies. Since the adatoms energy strongly depends on the deposition temperature [22], increasing the growth temperature from 300 1C to 500 1C raised the adatoms mobility and surface diffusion length. Enhanced mobility enabled adatoms toward lattice sites of the (002) plane. At 500 1C, generation of vacancies may also occurred, which further grew the (002) preferred orientation [23]. The growth could also be related to the interface merging process induced by substrate temperature. The defects at the grain or crystallite boundaries exist due to the random orientation of the crystallites. At 600 1C, the reduction of (002) crystallites size may be due to the enhanced desorption rate of the species at substrate surface. The re-evaporation from the film surface decreased the adding probability of species to (002) orientation. Moreover, increasing the substrate temperature further enhanced the adatoms energy, which enables the (103) orientation to grow as well [23]. Abdallah et al. [24] reported the transformation of films texture from preferred (002) to (100) orientation with the increase of bias voltage. The growth of AlN films at different pressures, at various N2 þ Ar gas conditions and at different gas mixtures of nitrogen with other inert gases (neon, krypton and xenon) are also reported in the literature by different researchers [25,26]. In these reports, the texture of AlN films transformed when decreasing the nitrogen fraction in the gas mixture (N2 þ Ar) and with the increase in atomic number of the inert gas, the preferred orientation of AlN films changes from (002) to (101) orientation, and then to polycrystalline with random orientation and finally became
amorphous. It appeared that when relatively heavier mass ions impinge on substrate surface during film growth, it imparts more energy to the depositing species. The adatoms energy thus plays a key role in determining texture of the deposited AlN films. As far as the transformation of a highly textured orientation to a mixed one at lower nitrogen content is concerned, the impact of greater numbers of relatively heaver argon ions imparts higher energy to the depositing species, enabling them to grow in other directions as well, such as [102] and [103]. 3.2. FTIR and Raman analysis FTIR and Raman spectroscopy are important spectroscopic techniques to probe vibrational states of molecules and their local order. In this study, the FTIR and Raman analysis were performed to observe the longitudinal optical (LO) and transverse optical (TO) phonon vibration modes of AlN thin film and their results were compared to obtain an empirical relation between them. Group theory predicts eight optical phonon modes (2A1 þ 2E1 þ 2E2 þ 2B modes) at the Γ point of Brillouine zone of hexagonal AlN. These modes split into the longitudinal optical (LO) and transverse optical (TO) components due to their polar behavior. Among these modes, A1 and E1 are infrared active modes, E2 along with the former modes are Raman active, while B modes are silent in both. The LO modes are active only in the oblique angles analysis [27,28]. The IR spectral analysis was conducted in a reflection mode at oblique angle (451 off normal). The spectra of the films were
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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Absorbance (arb units)
5
500
600
700
Wavenumber
800
900
500
600
700
800
900
Wavenumber (cm-1)
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obtained in the range of 400 to 1000 cm 1. The effects of deposition temperature and nitrogen conditions on the FTIR spectra are presented in Fig. 4(a) and (b), respectively. FTIR spectra of each figure depict a dominant peak at 673 cm 1 accompanied by a shoulder peak at 613 cm 1 and a mode at 895 cm 1. Absorption peaks at 611 cm 1, 672 cm 1 and 895 cm 1 correspond to the A1 (TO), E1 (TO) and A1 (LO) mode of aluminum nitride film, respectively [29]. The 620 cm 1 peak arises due to silicon substrate. The E1 (LO) mode is missing in the FTIR pattern that is expected to appear at 912 cm 1. The E1 (TO) mode is dominant in FTIR spectra irrespective of substrate temperature, which demonstrate that the films contain mostly (002) or c-axis oriented crystallites [30]. However, the appearance of a shoulder peak for A1 (TO) mode depicts that crystallites of other phases are also exist and the (002) crystallites are slightly tilted from surface normal [31]. The decrease in FWHM of FTIR absorption peaks is observable with the rise in temperature up to 500 1C, as shown in Fig. 4(a). The decrease in FWHM indicates an improvement in the AlN film crystallinity (improvement in the short-range ordering of atoms) [32]. However, further raising the substrate temperature to 600 1C reverses the trend of peak width, depicting a relatively disordered structure. In effect, observation of FTIR spectra confirms 500 1C being a relatively more suitable growth temperature for c-axis oriented AlN films grown by DC magnetron sputtering. Observations regarding different nitrogen growth conditions are shown in Fig. 4(b), depicting a relatively sharpened peak at 30% nitrogen condition. It appears that at lower nitrogen content, the crystallites growth of AlN film occurs. As the total pressure during deposition was kept constant, reducing the nitrogen content means a rise of Ar þ concentration in the plasma gas. When relatively heavier argon ions impinge on substrate surface, it transfers more energy to the adatoms, thereby increasing their mobility. Thus, more coalescent energy is available for relatively defect-free crystallites in the film. Raman spectroscopy is a technologically more important spectroscopic analysis to probe the vibrational states of crystals or semiconductors. The measurement was performed using a high resolution (4 cm 1) Raman spectroscopy system (model MST4000A). To avoid overlapping of the AlN peak with silicon substrate, the Raman analysis was conducted on AlN films grown on quartz substrate. Fig. 5 offers the Raman spectra of AlN films
Intensity (a.u)
Fig. 4. Observed FTIR spectra of AlN film deposited at different (a) temperatures, (b) nitrogen fractions.
500
600
700
800
900
1000
-1
Raman Shift (cm )
Fig. 5. Raman analysis at different growth temperatures.
grown at different substrate temperatures. The spectra of AlN film consist of vibrational optical phonon modes at 659 cm 1 and 892 cm 1 corresponding to the transverse optical E2 (high) mode and A1 (LO) of AlN, respectively [18]. The peak position of E2 mode appears very close to the reported value 657.4 cm 1 [33] for the un-strained AlN layer. The rise of peak intensity with the increase of substrate temperature up to 500 1C suggests the local ordering of crystal structure. The appearance of E2 (high) phonon mode irrespective of substrate temperature showed that deposited films are preferably (002) oriented AlN film [34] and is in agreement with XRD and FTIR results. The decrease in FWHM of observed peak can be seen with the increase in substrate temperature up to 500 1C that demonstrated lower phonon scattering effect at grain boundaries and thus lower defects in film [26]. 3.3. AFM analysis Atomic force microscopy of the deposited films was performed with Model: QScope™ 350 scanning probe microscope. Fig. 6 shows AFM images of AlN films depicting the variation of surface morphology with the increase in substrate temperature. Film prepared at 300 1C (Fig. 6(a)) depicts only small number of nano-sized clusters. The number of clusters and packing density increases with
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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500nm 2000nm 0
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Fig. 6. AFM images of AlN films prepared at (a) 300 1C, (b) 400 1C, (c) 500 1C, and (d) 600 1C.
increase in the substrate temperature up to 400 1C, as shown in Fig. 6 (b). Increasing the substrate temperature to 500 1C has resulted in a substantially modified surface morphology where a highly textured surface profile is evident. Computed RMS surface roughness values were 7, 4, 5 and 6 nm for the films prepared at 300, 400, 500 and 600 1C respectively. Although RMS roughness does not differ appreciably, clear indication of substantial surface modification might result from the change in growth mode. The appearance of columnar structure in AFM images implies that films are c-axis oriented as shown in Fig. 6(c). However, at 600 1C some grooves are produced within the columnar structure. At lower substrate temperature ( 300 1C), the constituent particles may not have enough thermal energy to accommodate themselves at proper position and the growth of crystallites is therefore multidirectional. Such type of disordered structure contains higher density of structural defects. At enhanced substrate temperature of 500 1C, the formation of columnar structure corresponds to the (002) orientation [32]. The higher substrate temperature enhances the adatoms mobility and surface diffusion length. As the most stable phase of AlN films is the wurtzite, in a higher adatoms energy configuration, the most suitable growth for the islands is to grow in c-axis direction, which is due to its close packed structure and (002) orientation have the lowest surface energy [19,20]. However, increasing temperature beyond 500 1C may causes desorption
of the adatoms. The desorption of pits/grooves results in the formation of clusters as it is clear from Fig. 6(d) within the caxis oriented columns. AFM images of AlN films prepared at different N2 conditions are shown in Fig. 7(a) and (b). AlN film deposited at 30% nitrogen condition (Fig. 7(b)) exhibits a mixed structure of columns and clusters, suggesting a c-axis orientation along with other phases as well. The departure of columnar surface structure to mixed structure of Fig. 7(b) may be attributed to the difference of argon and nitrogen atomic masses. From the above analysis, it can be concluded that AFM analysis is consistent with XRD results. 3.4. Ellipsometry analysis Ellipsometric measurements of AlN films were performed to measure deposited film thickness and refractive index. The obtained data was in a form of Psi (ψ) and Delta (Δ) function, where ψ represents amplitude ratio while Δ represents the phase difference between the reflected and incident waves. The measurements were performed at an angle of 701 over a spectral range from 300 nm to 900 nm. A three layer model was used as an input in ellipsometry measurement and is shown in Fig. 8(a). The obtained experimental curves were simulated theoretically by using multilayered-Cauchy-Urbach model. Cauchy–Urbach
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
S. Khan et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]]
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Fig. 7. AFM images of AlN film prepared at N2 condition (a) 60% and (b) 30%.
400 350
Psi / Delta
300
Ambient air
Psi Exp Psi Fitted Delta Exp Delta Fitted
250 200 150 100 50
AlN(312 nm) SiO 2
Si (111)
0 -50 300
400
500 600 700 800 Wave Length (nm)
900
Fig. 8. (a) Multi-Layer model used in ellipsometry, (b) experimental and simulated Psi and Delta curves.
dispersion model utilizes complex refractive index (N¼ nþ ik) whose components i.e. refractive index n(λ) and extinction coefficients κ(λ), are governed by nðλÞ ¼ n0 þ C0 nλ21 þ C 1 nλ42 þ :::
ð2Þ
k1 k2 þ C 1 4 þ ::: ð3Þ 2 λ λ where n0, n1, and n2 are the refractive indices of ambient, 1st and 2nd layers respectively, while k0, k1 and k2 represent extinction coefficient of respective layers. C0 and C1 are constants with values 102 and 107 respectively. Fig. 8(b) shows the spectroscopic ellipsometry data together with the simulated curves for a sample prepared at 400 1C. From Fig. 8(b), it is clear that there is a good agreement between the simulated and experimental data. After sufficiently accurate fitting was obtained, the film thickness and refractive indeces were obtained from fitted model. The fitting was analyzed using the mean square error (χ2) given in Eq. (4) to see the difference of experimental and simulated data.
kðλÞ ¼ k 0 þ C 0
χ2 ¼
N 1 X ðMesi Thi Þ2 N i¼1 σ 2i
ð4Þ
where N is the number of data points, Mesi is the ith experimental
Table 3 Ellipsometery results. Sample description
Temperature N2/ (N2 þ Ar) (oC) (%)
χ2
Thickness Refractive index (nm) at 2.3 eV
Sample Sample Sample Sample Sample
300 400 500 600 500
4.3 3.7 3.8 3.2 4.1
322 312 296 273 369
A B C D E
60 60 60 60 30
1.87 1.89 1.90 1.88 1.86
data point, Thi is the ith calculated data point from theoretical model and σi is the standard deviation. The values of mean square errors are shown in Table 3. Deposition rates of the deposited films were calculated from the measured film thickness and deposition time. The effect of substrate temperature on the deposition rate and refractive indeces is shown in Fig. 9(a) and (b), respectively. It appears that deposition rate of the deposited films continuously decreases. The desorption of adatoms from film surface depend on substrate temperature. Increasing the temperature enhances the adatoms
Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006
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10
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1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 Photon energy (eV)
Fig. 9. Influence of substrate temperature on (a) deposition rate, (b) refractive index.
energy and re-evaporation from film surface. The re-evaporation from film surface, lowers the growth rate[23]. The other possibility that lowers the deposition rate is the increase of film density with the increase in substrate temperature [35]. Refractive index lies in the range of 1.85–1.9and is close to the reported values for AlN [36,37]. The AlN film prepared at 500 1C exhibited a 1.9 refractive index (n) at 2.3 eV. It remains higher than the refractive index of samples deposited at 400 1C for whole range of photon energy. At higher substrate temperature, surface diffusion length of the depositing species is greater, which improve the film density and refractive index [38]. 4. Conclusions In summary, films texture under the influence of substrate temperature and nitrogen fraction has been addressed. XRD and FTIR analysis have shown that the (002) orientation growth occurs with the increase of substrate temperature from 300 1C to 500 1C. It was also found that at 30% nitrogen condition, the crystallites grow in other direction as well. The appearance of E1 (TO) mode peak at 672 cm 1 in the IR spectra and E2 (high) mode in the Raman spectra demonstrated the c-axis oriented crystallites growth. A shoulder absorption band around 611 cm corresponding to the A1 (TO) optical mode depicted that the crystallites of other phases are also exist and the (002) crystallites are slightly tilted from surface normal. Surface analysis of the deposited films indicated the columnar structure, suggesting a c-axis oriented growth. Ellipsometry results have shown a 1.9 refractive index at 2.3 eV. From the above results, it can be concluded that 500 1C substrate temperature and 60% N2 fraction are the optimum deposition conditions for the growth of c-axis oriented films. Acknowledgments The authors are thankful for the financial support of Department of Metallurgy and Materials Engineering (PIEAS) and National Institute of Laser and Optronics (NILOP), Islamabad, Pakistan. References [1] V. Mortet, A. Vasin, P.Y. Jouan, O. Elmazria, M.A. Djouadi, Surf. Coat. Technol. 176 (2003) 88–92. [2] A.F. Belyanin, L.L. Bouilov, V.V. Zhirnov, A.I. Kamenev, K.A. Kovalskij, B.V. Spitsyn, Diam. Relat. Mater. 8 (1999) 369–372.
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Please cite this article as: S. Khan, et al., Texture of the nano-crystalline AlN thin films and the growth conditions in DC magnetron sputtering, Progress in Natural Science: Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.08.006