Influence of the nitrogen fraction on AlN thin film deposited by cathodic arc ion

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Influence of the nitrogen fraction on AlN thin film deposited by cathodic arc ion Shakil Khan a,n, Ishaq Ahmed b, Mazhar Mehmood a, Gulfam Sadiq a a Department of Metallurgy and Materials Engineering (DMME), Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan b National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistan

a r t i c l e i n f o

Keywords: AlN Cathodic arc ion XRD FTIR spectroscopy

abstract Thin films of aluminum nitride (AlN) have been grown, using the cathodic arc ion deposition technique. The effects of nitrogen fractions in the discharge on synthesized films growth rate, stoichiometric ratio (N/Al), crystal orientation and molecular mode of vibration have been investigated. AlN films have been studied by means of Rutherford backscattering (RBS) spectroscopy, X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM) and the four probe method. In RBS results, it has been found that growth rate and stoichiometric ratio decrease while reducing the nitrogen content in the synthesized chamber. XRD patterns indicated that films prepared in 100–85% nitrogen condition exhibit mixed phase of wurtziteþFFC, with preferential orientation along (002) corresponding to the hexagonal phase. It also demonstrated that at lower nitrogen environment, the transformation from mixed phase of wurtziteþFCC to a single phase of FCC–AlN occurs. FTIR spectroscopic analysis was employed to determine the nature of chemical bonding and vibrational phonon modes. Its spectra depicted a dominant peak around 850 cm  1 corresponding to the longitudinal optical (LO) mode of vibration. A shift in the LO mode peak toward lower wavenumbers was noticed with the decrease of nitrogen fraction, illustrating the decline of nitrogen concentration in the deposited AlN films. The 75% nitrogen fraction appeared critical for AlN film properties, such as shifting of mixed (wurtziteþ FCC) phase to single FCC–Al(N), a sharp drop of stoichiometric ratio and deposition rate. Measurements of resistivity recorded by the four probe method depicted a sharp decline in the corresponding growth condition. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction Aluminum nitride (AlN) thin film with a wurtzite crystalline structure is of immense scientific importance due to its distinctive properties. AlN thin film, being a piezoelectric material with a high acoustic velocity, is a superior applicant for high frequency devices, like surface acoustic wave (SAW) devices, resonators, high frequency

n

Corresponding author. E-mail address: [email protected] (S. Khan).

filters and pressure sensors working in an aggressive environment [1–3]. AlN thin films have been prepared by several deposition techniques including chemical vapor deposition [4], reactive evaporation [5], molecular beam epitaxy [6,7], ion beamassisted deposition [8], metal organic chemical vapor deposition [9] and reactive sputtering [10]. In most of these deposition techniques, process temperature is quite high, which limits the choice of substrate and may cause film deterioration due to thermal stress [11]. Cathodic arc ion, like magnetron sputtering has the advantage of low temperature and conformal coating process. However, the former

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technique has the advantages of high throughput and considerable quantities of ions rather than neutral species of the cathode material. A disadvantage is the production of cathode material macro-particles in the cathodic arc ion process [12]. The development of c-axis orientated AlN films is important for surface acoustic wave devices owing to its piezoelectric property. The process parameter such as the nitrogen gas fraction plays an important role in the orientation of the AlN film [13] and is not yet fully exploited, particularly in a cathodic arc ion process. In the present work, (002) oriented AlN thin films have been prepared by cathodic arc ion deposition technique. Nitrogen gas fraction: N2/(ArþN2)% was varied and the subsequent dependency of material characteristics has been investigated. Characterizations of the deposited AlN films have been performed using Rutherford backscattering (RBS) spectrometry, X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), scanning electron microscope (SEM) and the four probe method. 2. Experimental Cathodic arc ion technique was employed for the growth of aluminum nitride thin films. Deposition was made in a commercial cathodic arc ion system having dual cathodes and Kaufman ion sources as shown in a schematic diagram of Fig. 1. The base pressure of 10–4 Pa was obtained in the deposition chamber using a turbo-molecular pump. Two aluminum discs (purity: 99.99%) were simultaneously used as targets. The distance between the two targets was 120 mm. To obtain higher throughput of depositing species/particles, both targets were utilized. Samples were

deposited without heating the substrates during film growth. During deposition, the substrate temperature was measured with the help of a thermocouple inserted through a ceramic tube that was screwed to the bottom of the substrate holder. Glass substrates were used for thin film growth. Substrates were ultrasonically cleaned in trichloroethylene for about 15 min with subsequent rinsing in de-mineralized water and acetone. To avoid/minimize contamination of growing films by oxygen and other impurities, a three-stage procedure was adopted. First, the chamber was evacuated and the substrates were degassed at 150 1C temperature in order to remove adsorbed species on the substrates. Next, the substrates were sputter etched for 15 min using two Kauffman argon ion sources (1 keV each). Lastly before aluminum nitride film growth, targets were pre-sputtered in pure argon atmosphere for 10 min to clean the surface of the targets. Before removing the shutter, the arc was created to settle the ArþN2 flow at pressure  3  10–1 Pa. The plasma ignition current of each target was kept constant at 50 A, while bias voltage was set at  300 V. Once the parameters settled at their respective values, the shutter was removed and species were allowed to deposit on the substrate. Initially sample S1 was deposited at 100% nitrogen flow (100 sccm). In the next case, various controlled nitrogen fractions (N2/(N2 þAr)%) were injected in the deposition chamber during film growth. The samples were deposited by mixing increasing argon content in the deposition chamber at constant pressure. Details of the flow ratio are listed in Table 1. During deposition, the spontaneous heating of the DC plasma arc raises the temperature of the substrate.

Compound Vacuum Monitor

Shutter Handle

Kauffman Ion Source 1

Target1 Substrate Heating Element

+

Shutter

Kauffman Ion Source 2

Target 2 Substrate Ar Gas Negative Bias Voltage

High Vacuum System

Rotary Assembly

N2 Gas

Cold Water Circulation

Fig. 1. Schematic diagram of deposition chamber.

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Owing to the high throughput of cathodic arc ion, the temperature was controlled in a range of 80–100 1C. AlN thin films' atomic percent composition and thickness have been investigated employing Rutherford back scattering (RBS) spectroscopy. Composition and thickness of all the samples were determined from the experimental results and theoretical simulations by Rutherford universal manipulating program (RUMP). Planer size of each sample subjected to RBS measurement was 2  2 cm2. A collimated 2.0 MeV 4He þ 2 beam having charge of 15 mC and current 35 nA produced by 5UDH-2 Pelletron was employed for RBS measurements. The sample was mounted on a high precision (0.011) five-axis goniometer in a vacuum chamber, so that the orientation of this sample relative to the 4He þ 2 beam could be precisely controlled. The backscattered particles were collected by an Au–Si barrier detector. The detection angle was 1701 and energy resolution of the detector was about 25 keV. The crystalline structure of the deposited films was determined via X-ray diffraction. X-ray diffraction was performed using Bruker D8 Discover with Cu Kα radiation. The measurement was taken in α-2θ mode (grazing incidence diffraction) with parallel beam geometry. To maximize the counts from the AlN films, grazing incidence angle (α) of the primary beam was set as 0.5–11 with respect to the sample. On the secondary side, a long Soller slit was utilized to limit

Table 1 Deposition parameters. Constant parameters during film growth (time of deposition for AlN layer: 15 min, deposition pressure  3  10–1 Pa). Sample #

Biasing (V)

Nitrogen fraction (%) N2/(Arþ N2)

AlN thickness (nm)

Stoichiometric (N/Al) ratio

S1 S2 S3 S4 S5 S6 S7

 300  300  300  300  300  300  300

100 92 88 85 75 60 50

520 513 512 505 430 390 385

0.666 0.666 0.612 0.612 0.388 0.25 0.176

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the radial divergence to 0.121. To analyze the characteristic spectrum by the functional group, FTIR spectroscopy using Nicolet 6700 FTIR spectrophotometer at an oblique angle (451) was employed. For the investigation of surface feature of the deposited films, a scanning electron microscope was used. Resistivity of the AlN film was measured by means of the four-probe method. 3. Results and discussion 3.1. RBS study Rutherford backscattering spectroscopy was performed to estimate the film thickness and stoichiometric ratio of nitrogen and aluminum in the films. Fig. 2(a) shows typical RBS results for the specimen S1 prepared at a bias voltage of 300 V and 100% nitrogen environment. Simulated spectrum (for a film thickness of 520 nm and Al to N atomic ratio of 60:40) is also shown in the figure, exhibiting an excellent match with the measurements. Aluminum concentration in the deposited film is greater than the nitrogen concentration, which is the characteristic of cathodic arc ion processes [12,14]. Counts from O and Si arise from the glass substrate, while the counts from N and Al arise from the deposited film. The marked positions of O and Si correspond to calculated energies if both lie at the surface. In the measurements, also in agreement with the simulated spectra, the edge has been markedly shifted to lower energies confirming that the substrate is completely covered by the film. Fig. 2(b) shows typical measured RBS spectra for the samples synthesized in different nitrogen environments. The ratio of Al to N remains almost the same (62:38) for a sample prepared at 92% nitrogen fraction, though the film thickness is reduced to 488 nm. By contrast, sample S7 prepared at a bias voltage of  300 V and 50% nitrogen condition exhibits a marked decrease in nitrogen content, i.e., Al to N atomic ratio of 87:13, with a film thickness of about 390 nm. Stoichiometric variation of aluminum and nitrogen in the films with nitrogen conditions is shown in Fig. 3(a).

20

20 50%

15

Normalized yield

Normalized yield

60%

15

10

5

75%

10

85%

88%

5 92%

N

0 400

O

600

Al Si

800

Channel No.

1000

100%

1200

0 400

600

800

1000

1200

Channel No.

Fig. 2. (a) Simulated and experimental graph of RBS for sample S1 and (b) combined RBS graphs for samples deposited at different nitrogen fraction.

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36

Depositon rate (nm/min)

0.7

Stiochoimetry (N/Al)

0.6

0.5

0.4

0.3

34

32

30

28

26

0.2

0.1

24 50

60

70

80

90

100

50

60

70

Nitrogen %

80

90

100

Nitrogen %

Fig. 3. Influence of nitrogen conditions on the (a) atomic ratio of AlN films and (b) deposition rate.

3.2. XRD study X-ray diffraction (XRD) of the AlN specimens prepared at different nitrogen conditions was performed for structural analysis. The XRD patterns are shown in Fig. 4 depicting the polycrystalline nature of the films. It may

Al(N)(111)

AlN(002)

Al(N)(200)

50%

AlN(103)

60% 75%

Intensity (a.u)

As N2 fraction in the chamber decreases, concentration of nitrogen in the film also falls. The decrease rate of N/Al ratio appears low in the 100–85% range, however, it drops sharply at 75% nitrogen fraction. The deposition rate has been estimated from the film thickness obtained in a deposition time of 15 min, as shown in Fig. 3(b). The film prepared at 100% nitrogen fraction exhibits a growth rate of 35 nm/min, which declines to 34 nm/min for a film prepared at 85% nitrogen fraction. However, it drops sharply to 29 nm/min for a film grown at 75% nitrogen condition. Apparently, 75% nitrogen condition is a critical point in the growth of AlN films, below which the growth rate as well as the nitrogen content of the AlN film decreases sharply. Changes in the atomic ratio and deposition rate could be related to the changes in the sputtering mode of the aluminum target, pace of AlN formation and resputtering of the deposited species on the film's surface. All these mainly depend on the composition of the gas mixture. The sputtering yield for Ar þ ions should be generally greater than that for N þ [15], as related with their atomic mass. On the other hand, argon is a monoatomic gas that needs only to be ionized to enter the plasma state. However, N2 is a diatomic gas that is largely dissociated before entering the plasma phase. It thus needs a larger energy input to enter the plasma state. This enhanced energy causes increased enthalpy of the plasma and thus higher heat transfer rates as well [16], for relatively higher nitrogen condition. As a result, the plasma is hotter and the flux of active ions increases, which possibly facilitates the nitride formation with a higher growth rate of the film. As far as resputtering is concerned, the formation of AlN must increase the binding force of atoms that should generally better resist resputtering and contribute to increased growth rate of the film.

85%

88%

92%

100%

36

40

44

48

52

56

60

64

68

2θ Fig. 4. Influence of various nitrogen fractions on the XRD pattern of AlN thin film.

be interesting to note that in the samples prepared in 100– 85% nitrogen environment, a pronounced peak appearing at 2θ¼36.51 can be clearly assigned to (002) reflection of hexagonal AlN (wurtzite). However, the lower intensity peak appearing at 2θ¼ 38.61 corresponds to the facecenter cubic structure of Al (N). Attempts to tune the peak positions of (a single-phase) wurtzite AlN by EVA software of Bruker did not provide any match with this peak. This result is very different from our previous work on AlN films [14], where we have observed only a w-AlN phase. For the second group of films prepared in 75% nitrogen environment and below, XRD patterns exhibit a pronounced difference from the samples prepared at 100–85% nitrogen condition. For the former, all the observed XRD peaks can be

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3.3. FTIR analysis

FTIR spectroscopy is based on absorption in the infrared range. FTIR absorption spectra of all the samples were obtained ranging from 2000 to 400 cm  1 in a reflection mode at an oblique angle (451 off normal). The aim was to observe longitudinal optical (LO) and transverse optical (TO) phonon modes of A1 and E1. The transverse nature of electromagnetic (EM) radiation corresponds to the splitting of these modes. In conventional transmission spectroscopy at normal incidence, interaction of only TO mode (parallel to the surface) of vibrations occurs with E.M. waves. However, the impact of E.M radiation at oblique angle on thin film’s surface interact parallel and perpendicular to the film surface and can excite TO and LO phonon modes, respectively (Berreman effect) [22]. Among the two modes, LO phonon peak position is an indicator of film stoichiometry [23]. FTIR normalized spectra of Fig. 5 depict a dominant absorption peak around 850 cm  1 corresponding to the longitudinal optical mode A1 (LO) [24], whereas the absorption peak at 620 cm  1 owing to A1 transverse optical (TO) mode of aluminum nitride [11]. The appearance of A1 mode peaks in the spectra reveals that films contain wurtzite AlN grains in which c-axis is tilted at an angle with respect to the normal surface [25]. The films prepared at 100%, 92%, 88% and 85% nitrogen conditions have shown LO mode peak position at 857 cm  1, 853 cm  1, 850 cm  1 and 847 cm  1, respectively. As LO mode peak is an indicator of film stoichiometry, the possibility of this observed shift can be attributed to the deficiency of the nitrogen content in the deposited films [14]. Spectra of films synthesized at 75% and 60% nitrogen fractions are shown in Fig. 5, depicting a broad band centering at 850 cm  1. The appearance of A1 mode implies that the films still contain AlN wurtzite phase [25]. For films deposited at lower N2 condition, it is observed that

50%

60%

75%

Transmittance %

assigned to an FCC structure. These positions of the peaks are close to those of metallic aluminum. The nitrogen content of these samples was also low as determined from RBS measurements. It appears that non-stoichiometric aluminum nitride formed at r75% nitrogen condition has predominantly an FCC structure in which the aluminum atoms occupy the FCC structure while the nitrogen atoms occupy interstitial voids without any particular ordering to give rise to any additional superlattice peaks. Although, apparently only one phase (FCC) is revealed by XRD patterns, the formation of a second nitrogen-rich amorphous phase in the films cannot be over-ruled. The appearance of the FCC structure at lower nitrogen condition was also reported by Ou et al. [17] while depositing AlN film by rf sputtering. Anyway, the wurtzite AlN grew with a preferential orientation of (002). For device application, (002) orientation of wurtzite AlN is desirable to obtain higher values of electromechanical coupling factor k2t [18]. The (002) AlN plane is the closed-packed basal plane with either all aluminum (þve) or nitrogen (  ve) atoms, whose formation may possibly be related with energy of the adatoms [19]. Cheng et al. [11] also reported the disappearance of wurtzite structure (002) peak with the decrease of nitrogen fraction. As far as FCC structure in the films is concerned, macroparticles of aluminum are known to be formed by cathodic arc ion deposition. Increasing the argon content raises not only the energy of the species but also the throughput of aluminum from target material owing to its higher atomic mass (relative to nitrogen atoms). Apparently, the atomic aluminum complexes reaching the substrate form wurtzite AlN by reacting with nitrogen in the plasma and the environment, while the nitrided aluminum macroparticles retain their FCC structure. EDX spectroscopy of these macroparticles in the deposited films reveals only a small difference of nitrogen content from the other regions of the films, suggesting that the macroparticles are easily nitrided due to nitrogen ions in the plasma and bias applied to the coating/substrate. Nevertheless, in the metallic aluminum macroparticles, the phase transformation to hexagonal AlN did not occur and tends to retain their FCC structure in spite of substantial intake of nitrogen from the plasma. Thus the plasma potential of nitrogen and the energy of bombarding particles/species is insufficient for the conversion of a metastable FCC aluminum nitride (formed by nitriding of intermediate FCC metallic phase) to a stable wurtzite AlN.

AlN with wurtzite structure belongs to the space group 6 of C4V (C63mc) with two-formula units per primitive cell. The zone-center optical modes are represented as follows:

85%

88%

92%

620 cm-1

Γ OP ¼ A1 þ E1 þ 2E2 þ 2B1 In this representation, A1, E1, and E2 mode are infrared and Raman active, while B1 mode is inactive [20,21]. Furthermore, A1 and E1 modes split into longitudinal optical (LO) and transverse optical (TO) components due to their polar behavior.

5

100%

550

600

650

700

750

800

850

900

950

Wavenumber (cm-1) Fig. 5. Depicting the FTIR spectra of AlN film deposited at different nitrogen conditions.

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FWHM of IR mode is small compared to those synthesized at higher nitrogen fractions. The appearance of broader peaks depicts that AlN phase in the deposited film moves from an order structure to a short range order texture [9] and is consistent with XRD results. Hence, it can be concluded that nitrogen incorporated in this non-stoichiometric AlN occupies interstitial voids in FCC aluminum. However, the film grown at 50% nitrogen fraction shows a relatively flat spectrum. 3.4. SEM analysis Micrographs of scanning electron microscope (SEM) of AlN thin films deposited at different nitrogen fractions were taken. The typical scanning electron micrographs of AlN thin films deposited at 88% and 75% N2 conditions are shown in Figs. 6(a) and 7(a), respectively. The SEM images did not reveal any undulations, fractures or cracking on the surface of the deposited films. From these micrographs, it is clear that the AlN thin films are dense and uniform.

They are also continuous over the whole substrate having almost spherical small and large crystalline particles/ clusters. At different N2 flow rates, the momentous number of tiny bright spots can be seen. It illustrates that the layer deposited at 75% nitrogen condition consists of rather densely distributed crystalline particles compared to the film prepared at 88% nitrogen fraction. The concentration variation was confirmed by Rutherford backscattering spectroscopy for these samples as already described. Further verification has been performed via EDX analysis depicted in the form of aluminum and nitrogen peaks in Figs. 6(b) and 7(b). The peaks of other elements arise from the glass substrate. Comparing the two analyses, the film synthesized at 75% nitrogen fraction depicts a higher intensity Al peak (relative to nitrogen peak). This investigation showed that the AlN films deposited at lower N2 fraction contain greater amounts of Al. The macroparticles in the deposited films were also analyzed by EDX spectroscopy that revealed only a small difference of nitrogen content from the other regions of the films, suggesting that

Fig. 6. (a) SEM images and (b) EDX analysis for a sample prepared at 88% N2 environment.

Fig. 7. (a) SEM images and (b) EDX analysis of a specimen grown at 75% N2 nitrogen flow.

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Resistivity ( Ω m)

10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 50

60

70

80

90

100

Nitogen (%) Fig. 8. Influence of various nitrogen conditions on the resistivity of AlN film.

the macroparticles are easily nitrided due to nitrogen ions in the plasma and bias applied to the coating/substrate.

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arc ion. The AlN film prepared in pure nitrogen environment showed a higher growth rate and stoichiometric (N/ Al) ratio. The decreasing rate of N/Al ratio appeared low, while decreasing the nitrogen flow from 100% to 85% range. However, it drops sharply at 75% nitrogen fraction. Growth of preferred (002) orientation was observed for a film prepared at 88% nitrogen flow, though at 75% nitrogen condition it disappeared completely. The results obtained depicted a mixed phase of FCCþwurtzite AlN-type, followed by a gradual transition toward FCC–AlN type crystalline phase. The longitudinal optical (LO) mode was observed around 850 cm  1 corresponding to the Al (LO) mode of the AlN films. Further analysis showed that the LO mode peak shifted to lower wave numbers, while decreasing the nitrogen content in the deposition chamber. This shift was attributed to the reduction of the nitrogen content in the deposited films. From the analysis, it has been noticed that AlN films show a metallic behavior when the N2 gas ratio is r75%, whereas when N2 condition is higher than 75%, the conducting property of the film changes to a dielectric behavior.

3.5. Resistivity

Acknowledgments

The electrical resistivity of the samples was obtained from the sheet resistances measured by the four-point probe method. Fig. 8 depicts the resistivity behavior of AlN films against percent nitrogen fraction. The films deposited at 100% and 92% nitrogen have shown 0.31 Ω m and 2.53 Ω m resistivities, respectively. A maximum resistivity value of 4.21 Ω m appears for a film synthesized at 88%, though it slightly decreases for 85% nitrogen fraction. These values are in close agreement with the reported value of Al-rich AlN film resistivity [26]. A sharp fall in resistivity to 9.9  10–4 Ω m appears for a film prepared at 75% nitrogen condition. A similar trend was also reported by Mientus et al. [27] for the AlN film deposited at different nitrogen pressures. The total resistivity of the films is a contribution of several independent electron scattering processes due to phonons, impurity atoms and defects [28]. XRD patterns have shown the growth of (002) preferred orientation while reducing the nitrogen fraction upto 88%. The initial increase of the AlN films resistivity can be assigned to the growth of (002) orientation. As (002) preferred orientation is a close packed structure. Increasing the content of (002) crystallites has increased the scattering of electron with phonons, which consequently leads to higher film resistivity. As RBS analyses have shown a lower N/Al ratio at r75% nitrogen condition, the sharp fall of the resistivity at r75% is thus attributed to the content of the synthesized AlN films. It appears that films show a metallic behavior when the N2 gas ratio is r75%, whereas the conducting property of the film changes to a dielectric behavior above the critical N2 conditions.

We thank Dr. Shaukat Saeed (PIEAS) for FTIR measurements and Dr. Husnain (NCP) for RBS analysis. Special thanks to my grateful wife (Mrs. Hafsa Shakil).

4. Conclusions In summary, preferred c-axis oriented aluminum nitride (AlN) thin films have been prepared, using cathodic

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