Thin Solid Films 441(2003) 287–291

0040-6090/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0040-6090(03)00889-7

Piezoelectric coefficient of InN thin films prepared by magnetronsputtering

C.B. Cao *, H.L.W. Chan , C.L. Choya,b, b b

Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, PR Chinaa

Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,b

PR China

Received 28 August 2002; received in revised form 15 April 2003; accepted 16 May 2003

Abstract

Indium nitride (InN) thin films have been deposited on Si(1 0 0) and Pt(1 1 1)yTiySiO ySi(1 0 0) substrates by reactive r.f.2

magnetron sputtering. By using a plasma power of 25 W, nitrogen gas pressure of 5–7 mTorr and substrate temperature of 300–400 8C, InN films with (0 0 0 2) orientation were obtained. The relative permittivity and electrical resistivity of the InN film werecalculated from the measured electrical impedance. The piezoelectric coefficientd of the InN film was measured by a heterodyne33

interferometer and found to be 3.12"0.10 pm V .y1

� 2003 Elsevier B.V. All rights reserved.

Keywords: Indium nitride; Piezoelectric coefficient; Magnetron sputtering

1. Introduction

Recently, III–V nitrides have received considerableattention in the field of optoelectronic device applica-tions, especially in UV–blue light emitting devices andhigh-power, high-temperature electronic devicesw1–3x.Significant progress in the applications of III–V nitrideand their related alloys have been made. However, thereare relatively few reports on the piezoelectric propertiesof these nitridesw4–10x, and the published data aremainly on aluminum nitride and gallium nitride. Thepiezoelectrical parameters are important since III–Vnitrides have potential use in microactuators, microwaveacoustics and microelectromechanical device. To thebest of our knowledge, this is the first report on thepiezoelectric coefficientd of indium nitride.33

2. Experimental details

The InN films were deposited in a radio frequency(13.6 MHz) magnetron sputtering system equipped witha 99.99% pure indium target of diameter 50 mm. The

*Corresponding author. Tel.:q86-10-68913792; fax:q86-10-68915023.

E-mail address: cbcao@bit.edu.cn(C.B. Cao).

distance between the target and the substrate holder was100 mm. Si(1 0 0) wafers with an electrical resistivityof approximately 3=10 Vm and Pt(1 1 1)yTiySiO yy2

2

Si(1 0 0) wafers were used as substrates. Before depo-sition, both types of substrates were ultrasonicallycleaned with acetone and ethanol, and the Si(1 0 0)substrates were also etched in a 5% HF solution for 3min to remove the oxide on the surface. After thesputtering chamber had been evacuated to below5=10 Torr, high purity nitrogen(99.99%) was intro-y6

duced as the reactive gas. The indium target was thenpre-sputtered for 10 min before the shutter was opened.After sputtering, the film thickness was measured by aTencor P-10 surface profiler and the film structure wasexamined using a Philips X’pert X-ray diffractometer.The morphology of the film was observed in a LeicaStereoscan 440 scanning electron microscope(SEM).The piezoelectric coefficient was measured by a modelSH 120 Mach-Zehnder heterodyne interferometer man-ufactured by B. M. Industry, France.

3. Results

The deposition rate of InN films was studied as afunction of input power, nitrogen pressure and substrate

288 C.B. Cao et al. / Thin Solid Films 441 (2003) 287–291

Fig. 1. Deposition rate of InN film as a function of plasma power(nitrogen gas pressures7 mTorr) and as a function of nitrogen gaspressure(plasma powers40 W). Substrate temperatures300 8C.

Fig. 2. XRD patterns of InN films on Si(1 0 0) substrates prepared atdifferent plasma powers.

Fig. 4. XRD patterns of InN films on Si(1 0 0) prepared at differentsubstrate temperatures.

Fig. 3. XRD patterns of InN films on Si(1 0 0) substrates prepared atdifferent nitrogen gas pressures.

Table 1The summary of influence on form(0 0 0 2) orientation from XRDresults

Deposition conditions The XRD results

Plasma power Lower plasma power benefical to theformation of oriented films((25 W)

Substrate temperature Little effect, best orientation are inthe range of 300–4008C

N pressure2 7–10 mTorr benefical to the formationof oriented films

Substrate Pt coated substrate and Si(1 0 0)substrate has similar effect

Fig. 5. XRD patterns of InN films deposited on Pt(1 1 1)yTiySiO ySi2

substrates.

temperature. It was observed that the deposition rateincreased as the power increased but decreased as thenitrogen pressure increased(Fig. 1). It was also found

that the substrate temperature has no significant influ-ence on the growth rate of the film when varied between100 and 5008C.Fig. 2 shows the X-ray diffraction(XRD) patterns of

InN films deposited on Si(1 0 0) substrates at a substratetemperature of 3008C, nitrogen pressure of 7 mTorr andr.f. power varying from 25 to 50 W. For the filmprepared using a plasma power of 25 W, only the(0 0 0 2) and (0 0 0 4) peaks were observed, which

289C.B. Cao et al. / Thin Solid Films 441 (2003) 287–291

Fig. 6. Morphology of InN films on Si(1 0 0) substrates deposited at(a) 100 8C, (b) 200 8C, (c) 300 8C, (d) 400 8C, (e) 500 8C and morphologyof InN film on PtyTiySiO ySi substrate deposited at 4008C (f). Plasma powers25 W, nitrogen gas pressures7 mTorr.2

indicates that the film has an oriented hexagonal wurtzitestructure. If a higher power was used, other diffractionpeaks were observed, indicating that higher r.f. poweris not beneficial for(0 0 0 2) oriented film formation.The nitrogen gas pressure and substrate temperature alsoinfluence the InN film growth. Figs. 3 and 4 show theXRD patterns of InN films prepared at different temper-

atures and nitrogen gas pressures, respectively. From thediffraction pattern, we can see that the optimum pressureand temperature for the growth of(0 0 0 2) orientedfilms are 7–10 mTorr and 300–4008C, respectively.The optimum conditions for the deposition of InN filmson Pt coated Si substrates are similar to those forSi(1 0 0) substrates. As shown in Fig. 5, the films

290 C.B. Cao et al. / Thin Solid Films 441 (2003) 287–291

Fig. 7. Real and imaginary parts of the electrical impedance of theInN film as a function of frequency.

Fig. 8. Variation of the displacement of InN film with applied voltageat 10 kHz, the solid line is the regression curve.

deposited on Pt coated Si substrates under optimumconditions also exhibit(0 0 0 2) orientation. Table 1summarized the influence of the deposition conditionson the thin films orientation from XRD results.Fig. 6 shows the SEM micrographs of InN films

deposited at different temperatures. For deposition tem-peratures of 100–4008C, the films are very smooth.The size of the grains is smaller than 100 nm and varieslittle with the deposition temperature. For films depos-ited at 5008C, relatively large grains are observed(Fig.6e). A comparison of Fig. 6d and f shows that the filmdeposited on Pt coated Si has larger grains than thatdeposited on Si, indicating that the substrate has aneffect on the InN film growth. The physical mechanismis not clear.To prepare a InN film for the impedance and piezoe-

lectric coefficient measurements, a low substrate tem-perature (300 8C) was used such that less nitrogenvacancies(thus lower carrier concentration) were cre-ated. The plasma power and N pressure were 25 W2

and 7 mTorr, respectively. The 193 nm thick InN filmgrown on a Si(1 0 0) substrate was coated with analuminum (Al) top electrode of diameter 1 mm. Thereal and imaginary parts of the electrical impedance,Re(Z) and Im(Z), of the film were measured as afunction of frequencyf by a HP 4194A impedanceanalyser and are shown in Fig. 7. At the frequencyfmwhere Im(Z) has a maximum value Im(Z ),m

f s1y2pRC (1)m

Im(Z )sRy2 (2)m

whereR andC are the resistance and capacitance of thefilm, respectively. From the observedf , Im(Z ) andm m

geometrical factors, the relative permittivity and resistiv-ity of the film were estimated to be 4.5 and 9.7=104

Vm, respectively.The surface displacement induced in the InN sample

by an applied a.c. field was measured by a Mach-

Zehnder interferometer. The voltage applied to the sam-ple was measured by an oscilloscope with a 50V

termination. The Al electrode also served as a mirror toreflect the probe beam from the interferometer. It wasfound that sample resonance occurred at a frequencyhigher than 50 kHz, hence the measurement was per-formed at 10 kHz. The variation of the measureddisplacement with applied voltage for the InN sample isshown in Fig. 8. We only obtained the data of voltagelarger than 5 V because below 5 V the displacement istoo small to be measured for it is lower than theinterferometer’s sensitivity. From the slope of the bestfitted line, thed coefficient of InN was calculated:33

d sSyE sDLyL=1yE sDLyV33 3 3y1s3.12"0.10 pm V (3)

Here theS stands for the strain caused by electricfield and E stands for electric field performed on the3

film. The DL stands for displacement andV is voltagecharged on sample.

4. Discussion

The quality of the deposited InN films depends onthe plasma power, N pressure and substrate temperature2

as these parameters affect the film growth rate and theion concentration ratio of In and N. At lower plasmapower, the In ion concentration should be relatively low,and the growth rate is slower as it is mainly determinedby the In ion concentration. Under this condition, atomshave enough time to migrate on the growth surfacethereby facilitating growth in the(0 0 0 2) orientation.The reaction pressure determines the N concentrationand InyN ion ratio. The decrease in deposition rate withincreasing N pressure shows that the InyN ion ratio2

plays a critical role in the film growth. XRD patternsalso show that there exists an optimal N pressure, as2

too high or too low a InyN ratio may introduce In or Ndefects into the films and affect the film orientation. It

291C.B. Cao et al. / Thin Solid Films 441 (2003) 287–291

has been shownw11x that elemental indium can exist infilm grown in pure nitrogen in a d.c. reactive sputteringsystem if the InyN ratio becomes excessive. Temperaturehas little influence on the growth rate, but it caninfluence the atom migration rate in the film. If thesubstrate temperature is too high, e.g. 5008C, the Nions can escape, thereby creating N vacancies in thefilm. This leads to an increase in the carrier concentra-tion and hence a decrease in the resistivity of the film.The effective piezoelectric coefficient of InN film

obtained in this work can be compared with our previousmeasurements on AlN and GaN films also deposited onSi substratesw9x. The results are:d s3.12"0.1033

pm V for InN, d s3.90"0.10 pm V for AlN andy1 y133

d s2.38"0.10 pm V for GaN. It is seen that InNy133

has a piezoelectric coefficient lower than that of AlNbut higher than that of GaN, which is in agreement withthe order of the calculated piezoelectric coefficiente .33The theoretical value ofe is 1.46 C m for AlN, 0.73y2

33

C m for GaN and 0.93 C m for InN, respectivelyy2 y2

w10x.

5. Summary

InN films with (0 0 0 2) orientation have been depos-ited by reactive r.f. magnetron sputtering on Si(1 0 0)substrates and Pt coated Si(1 0 0) substrates. Low plas-ma power(25 W), medium N pressure(7–10 mTorr)2

and 300–4008C substrate temperature are optimaldeposition conditions to form(0 0 0 2) orientated InN

thin films. The prepared InN thin films has a resistanceof 9.7=10 Vm and its relative permittivity is 4.5. The4

piezoelectric coefficientd of InN film as measured by33

a laser interferometry is 3.12"0.10 pm V .y1

Acknowledgments

This work was supported by the Industrial supportFund of the Hong Kong Special Administrative region(Project No. AFy147y98) and the Center for SmartMaterials of the Hong Kong Polytechnic University.

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