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Applied Surface Science 282 (2013) 578– 582

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

nfluence of N2/Ar-flow ratio on crystal quality and electricalroperties of ScAlN thin film prepared by DC reactive magnetronputtering

ian-cang Yang ∗, Xiang-qin Meng, Cheng-tao Yang, Wu-jun Futate Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China

a r t i c l e i n f o

rticle history:eceived 10 April 2013eceived in revised form 30 May 2013ccepted 2 June 2013vailable online 13 June 2013

eywords:cAlN thin filmagnetron sputtering

a b s t r a c t

Scandium aluminum nitride alloy (ScAlN) thin films were prepared using DC reactive magnetron sput-tering with a scandium aluminum alloy (Sc0.06Al0.94) target on n-type silicon substrates. We haveinvestigated the influence of N2/Ar-flow ratio on the crystalline structure, the surface morphology andthe electrical properties of ScAlN thin films. Consequently, it was statistically proved that the N2/Ar-flowratio was an important control factor in the process of sputtering. According to the peak intensities in�/2� scans and rocking curve FWHM measurements of the (0 0 2) peaks, the crystalline quality of ScAlNthin film first increased and then decreased, reaching the best crystalline state at a N2/Ar-flow ratio of3.3:7. The best surface morphology of ScAlN thin film was obtained at N2/Ar-flow ratios of 3.2:7, 3.3:7 and

rystal qualitylectrical properties

3.4:7 and the surface roughness reached a minimum of 2.612 nm at 3.3:7. The resistivity and dielectricconstant first increased to maximum values of 3.35 × 1012 � cm and 13.6, and then decreased with theratio increasing. Moreover, ScAlN thin film exhibited a higher value of resistivity and dielectric constantwhen compared with un-doped AlN thin film. In addition, when the N2/Ar-flow ratio was 3.3:7, the high-est breakdown field strength and lowest leakage current were obtained, with values 1.12 MV/cm and3 × 10−8 A, respectively.

. Introduction

Presently, surface acoustic wave (SAW) devices, with the advan-ages of small scale, high frequency, large bandwidth and real-timeignal processing, are important for telecommunication and sensorpplications. However, with the rapid development of microelec-ronics, telecommunications and other fields, SAW devices withigh frequency ranges, low insertion loss, large electromechanicaloupling factor (k2), stable follow and integratability with com-lementary metal oxide semiconductor (CMOS) are necessary forhe above mentioned applications [1–3]. Among the piezoelectrichin film materials, zinc oxide (ZnO) is hard to access deposi-ion techniques that would guarantee repeatable results and itseactivity with other IC materials makes it hard to integrate withtandard CMOS processes. Furthermore, another common piezo-lectric material lead zirconate titanate (PZT), a material that isore difficult to process and integrate as a film on silicon, exhibits

uality factors that are too low for SAW application, and especiallys of concern to IC fabrication facilities because of contaminationisks in CMOS lines [4].

∗ Corresponding author. Tel.: +86 28 83208048; fax: +86 28 83202139.E-mail address: jcyang58@163.com (J.-c. Yang).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.06.014

© 2013 Elsevier B.V. All rights reserved.

Aluminum nitride (AlN) piezoelectric thin film is very attractivein the field of SAW applications because of its unique combina-tion of remarkable properties, such as high acoustic velocity, highthermal conductivity, high electrical resistivity and low acousticloss [5]. In addition, AlN is fully compatible with conventional sil-icon technology [6]. However, when compared with PZT and ZnOpiezoelectric thin films, AlN exhibits a lower electro-mechanicalcoupling coefficient (kt

2) and piezoelectric constant (d33), which,to some extent, limits AlN’s wide applications. Recently, it wasshown that Al substitution by Sc allows for an increase of thepiezoelectric response [7–11]. Experiments demonstrated a 400%increase in piezoelectric modulus d33 for ScxAl1−xN alloys withx = 0.43 [7]. Also, it was proven that the electro-mechanical cou-pling coefficient k2

t improved from 7% to 10% by alloying AlN withup to 20 mol% ScN, which is vital for AlN thin film’s use in SAWdevices [12].

As with the piezoelectric response, thin film’s crystal quality,resistivity and dielectric properties are also closely linked to SAWdevice’s performance [13,14]. However, there were few reportsabout the research on the deposition technology’s influence on

ScAlN film’s crystal quality and electrical properties. The N2/Ar-flow ratio is a critical factor in the process of depositing ScAlN filmby DC reactive magnetron sputtering, directly affecting film’s crys-tal quality and electrical properties [15]. In this paper, the effect of

face Science 282 (2013) 578– 582 579

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ScAlN film prepared at N2/Ar-flow ratio 3.3:7. It can be seen fromthe photograph that the thickness of the ScAlN film was about 2 �mand there was a clear columnar structure, which was identical withthe result XRD pattern indicated.

J.-c. Yang et al. / Applied Sur

2/Ar-flow ratio on the crystal structure, surface topography andlectrical properties were investigated systemically.

. Experimental

ScAlN films were prepared on n-type (1 0 0) silicon substratesy DC reactive magnetron sputtering system with the N2/Ar-ow ratio varying from 3:7 to 3.5:7. The silicon substrates wereleaned successively by acetone, absolute ethyl alcohol and deion-zed water to ensure clean surfaces for film growth. PlatinumPt) thin films were prepared on substrates as bottom electrodeor electrical properties’ measurements. Because ScAl alloy tar-ets are effective for keeping scandium concentration constantn ScAlN thin film [9], therefore in this series of experiments,

99.99% pure scandium aluminum (ScAl) alloy target (110 mmn diameter, quality component proportion Sc:Al = 0.1:0.9) waspplied. The sputtering chamber was evacuated to a pressureelow 4.0 × 10−4 Pa, and then high-purity argon (99.999%) anditrogen (99.999%) were introduced. Before deposition, the ScAllloy target was cleaned under same deposition conditions for

min. All ScAlN films were controlled at 2 �m thick. The crystaltructures and crystal orientations of ScAlN films were investi-ated by X-ray diffraction (XRD, Bede D1) and Scanning Electronicroscope (SEM, JSM-6490LV). The surface topographies of ScAlN

lms were observed with Atomic Force Microscope (AFM, SPA-00HV). The electrical properties were respectively analyzed by

standard ferroelectric test system (Radiant Precision LC 2000),nsulate voltage test apparatus and Impedance Analyzer (Agilent294A).

. Results and discussion

.1. Crystal orientation

It is generally known that the piezoelectric response of piezo-lectric thin film strongly depends on crystal orientation [16–20].he crystal structures of ScAlN films were investigated by XRD tolear the influence of the N2/Ar-flow ratios. As shown in Fig. 1(a),he XRD intensity of (0 0 2) oriented peak increases with increasing2/Ar-flow ratio, which indicates a maximum value at a N2/Ar-flow

atio of 3.3:7. Above 3.4:7, the (0 0 2) peak intensity dramaticallyecreases. It probably means that the crystal growth is more highlyisordered above 3.4:7. On the other hand, the full width at halfaximum (FWHM) of X-ray rocking curves gradually decreases,

nd then increases above 3.4:7, as shown in Fig. 1(b). It indicateshat the crystal orientation is strongly influenced by the N2/Ar-flowatio.

According to the peak intensities in �/2� scans and rocking curveWHM measurements of the (0 0 2) peaks, the best crystalline qual-ty of ScAlN thin film was obtained at N2/Ar-flow ratio = 3.3:7, whichs to be expected due to an ample supply of reactant gas N2 andigher target-atom mobility allowing for better epitaxial growth of0 0 2) orientation.

Fig. 2 shows the XRD patterns of (0 0 2) oriented pure AlN filmnd Sc-doped AlN film prepared at N2/Ar-flow ratio 3.3:7. Only0 0 2) diffraction peak can be found in 20–60◦scanning range,hich means that the doping element Sc doesn’t exist in Sc or Scitride. The inset in Fig. 2 shows that (0 0 2) peak of ScAlN film has

relatively obvious deviation to the lower angle direction whenompared with the pure AlN film. The possible reason is that Sc ionsith larger ionic radius than Al ions have doped into the wurtzite

tructure of AlN lattice, causing the deviation [21]. In addition, theeviation to lower angle direction of ScAlN film’s peak indicates the

ncrease of c-parameter. This means a smaller lattice mismatch tohe silicon substrate, which will decrease the stress in films.

Fig. 1. (a) XRD patterns and (b) FWHM of X-ray rocking curves of ScAlN films pre-pared at various N2/Ar-flow ratios.

Fig. 3 shows SEM cross-sectional view of the microstructure for

Fig. 2. XRD patterns of (0 0 2) oriented AlN film and ScAlN film.

580 J.-c. Yang et al. / Applied Surface Science 282 (2013) 578– 582

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device. And dielectric constant affects the size, insertion loss

ig. 3. SEM cross-sectional view of ScAlN thin films deposited at the N2/Ar-flowatio of 3.3:7.

.2. Surface morphology

When applying piezoelectric films to SAW devices, the surfaceoughness of the piezoelectric film has a critical impact on the qual-ty of the device. As the surface acoustic wave is only propagated onhe surface, all the energy is concentrated almost within a wave-ength from the surface to the inside. So the surface acoustic waves not able to pass through when the surface roughness is more than

wavelength [22,23]. Generally, the surface roughness of the films required to be less than 30 nm [6].

Atomic force microscopy in tapping mode was employed to

haracterize the surface morphology of all layers. As shown inigs. 4 and 5, ScAlN thin films deposited at N2/Ar-flow ratio of.2:7.3.3:7 and 3.4:7 exhibit better surface morphology, of which

Fig. 4. AFM images of ScAlN film surfaces p

Fig. 5. Dependence of surface roughness of ScAlN films on N2/Ar-flow ratios.

the root-mean square (RMS) surface roughness are less than 3 nm.In particular, surface of the film grown at 3.3:7 consists compactlyof grains with uniform size and its surface roughness reaches aminimum of 2.612 nm.

3.3. Electrical properties

Resistivity and dielectric constant are important indicators ofmeasuring piezoelectric film’s electrical performance. The higherthe resistivity, the lower the film’s dielectric loss and relaxationfrequency, which is beneficial to reducing insertion loss of SAW

and electromechanical coupling coefficient of the device. Highdielectric constant can reduce the size of piezoelectric device anddielectric loss, which will be conducive to realization of device

repared at various N2/Ar-flow ratios.

J.-c. Yang et al. / Applied Surface S

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ig. 6. Dependence of resistivity and dielectric constant on N2/Ar-flow ratios.

iniaturization and low loss, but the dielectric constant and elec-romechanical coupling coefficient are inversely proportional, thus,o gain medium dielectric constant has important significance tomprove SAW device’s performance.

Fig. 6 shows the varying pattern of the ScAlN films’ resistivitynd dielectric constant at the N2/Ar-flow ratio. With N2/Ar-flowatio increasing, the resistivity first increases and then decreases,eaching a maximum of 3.35 × 1012 � cm at N2/Ar-flow ratio 3.3:7.ielectric constant changes in the same rule and gains a peak of3.6 at N2/Ar-flow ratio 3.3:7.

We think the possible reason is that when N2/Ar-flow ratio isow, the film is amorphous and rich in Al, Sc atom. Incompleterystallization results in the appearance of vacancy and interstitialefects, leading to a low resistivity and dielectric constant. Alongith the increase of N2/Ar-flow ratio is the high quality of film crys-

allization, that is, uniform size and less defects, which leads to highesistivity and low leakage current [24]. Accordingly, the dielectriconstant is high. When the N2/Ar-flow ratio is too large, the leakageurrent of film increases because of excessive crystallization, grainoarsening and the increasing number of grain boundary, whichesults in the decreasing of resistivity and dielectric constant.

An electrical properties testing on pure AlN film with (0 0 2) pre-erred orientation was also performed with the same equipment,

btaining its resistivity 2.0 × 1011 � cm and dielectric constant 8.9.inally, we came to a conclusion that Al substitution by Sc alsollows for an increase of resistivity and dielectric constant.

ig. 7. Dependence of breakdown field strength and leakage current on N2/Ar-flowatios.

cience 282 (2013) 578– 582 581

In addition, the determinations of the leakage current andthe dielectric breakdown field strength are both important fordielectrics. Fig. 7 shows the varying pattern of the ScAlN films’leakage current and dielectric breakdown field strength at theN2/Ar-flow ratio. With N2/Ar-flow ratio increasing, the breakdownfield strength first increases and then decreases, reaching a maxi-mum of 1.12 MV/cm at N2/Ar-flow ratio 3.3:7. The leakage currentfirst decreases to a minimum of 3 × 10−8 A at N2/Ar-flow ratio 3.3:7and then increases. However, when the gas ratio is 3:7, the film isamorphous and rich in Al, Sc atoms and therefore the film has highleakage current and low breakdown field strength.

4. Conclusion

A series of ScAlN films was prepared by DC reactive magnetronsputtering at different N2/Ar-flow ratios varying from 3:7 to 3.5:7.The influence of the N2/Ar-flow ratio on the crystalline structure,the surface topography and the electrical properties was systemat-ically studied. The XRD intensity of (0 0 2) oriented peak increaseswith increasing N2/Ar-flow ratio, reaching a maximum value at aN2/Ar-flow ratio of 3.3:7. Above 3.4:7, the (0 0 2) peak intensitydramatically decreases. According to the peak intensities in �/2�scans and rocking curve FWHM measurements of the (0 0 2) peaks,the crystalline quality of ScAlN thin film first increases and thendecreases, reaching the best crystalline state at a N2/Ar-flow ratioof 3.3:7. The SEM photograph of film deposited at 3.3:7 shows acolumnar structure, which is in accord with the result XRD pat-tern indicates. The best surface morphology of ScAlN thin film wasobtained at N2/Ar-flow ratio of 3.2:7, 3.3:7 and 3.4:7 and the surfaceroughness reaches a minimum of 2.612 nm at 3.3:7. The resistivityand dielectric constant change in the same rule, first increasing to amaximum value of 3.35 × 1012 � cm and 13.6, and then decreasingwith the ratio increasing. Moreover, ScAlN thin film exhibits ahigher value of resistivity and dielectric constant when comparedwith un-doped AlN thin film. In addition, when the N2/Ar-flow ratiowas 3.3:7, the highest breakdown field strength and lowest leak-age current were obtained, with values 1.12 MV/cm and 3 × 10−8 A,respectively.

Acknowledgement

This article is supported by Project supported by the Funda-mental Research Funds for the Central Universities of Ministry ofEducation of China.

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