Mechanical and physicochemical properties of coke produced by wet and dry slaking

Superlattices and Microstructures 36 (2004) 409–416

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Mechanical and physicochemical properties ofAlN thin films obtained by pulsed laser deposition

C. Ciberta, F. Tétardb,∗, P. Djemiab, C. Champeauxa,A. Catherinota, D. Tétarda

aSPCTS, Faculté des Sciences, UMR No 6638 CNRS, 123 Avenue A. Thomas, 87060 Limoges Cedex, FrancebLPMTM UPR 9001 CNRS, Institut Galilée, avenue J. B. Clément, 93430 Villetaneuse, France

Available online 2 November 2004

Abstract

AlN thin films have been deposited on Si(100) substrates by a pulsed laser deposition method.The deposition parameters (pressure, temperature, purity of target) play an important role inthe mechanical and physicochemical properties. The films have been characterized using X-ray diffraction, atomic force microscopy, Brillouin light scattering, Fourier transform infraredspectroscopy and wettability testing. With a high purity target of AlN and a temperature depositionof 750 C, the measured Rayleigh wave velocity is close to the one previously determined forAlN films grown at high temperature by metal–organic chemical vapour deposition. Growth ofnanocrystalline AlN at low temperature and of AlN film with good crystallinity for samples depositedat higher temperature is confirmed by infrared spectroscopy, as it was by atomic force microscopy, inagreement with X-ray diffraction results. A high hydrophobicity has been measured with zero polarcontribution for the surface energy. These results confirm that films made by pulsed laser depositionof pure AlN at relatively low temperature have good prospects for microelectromechanical systemsapplications.© 2004 Elsevier Ltd. All rights reserved.

∗ Corresponding address: Laboratoire PMTM, Université Paris Nord – Institut Galilée, 99 Avenue J. B.Clément, 93430 Villetaneuse, France. Tel.: +33 1 49403488; fax: +33 1 49403938.

E-mail address: [email protected] (F. Tétard).

0749-6036/$ - see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2004.09.055

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410 C. Cibert et al. / Superlattices and Microstructures 36 (2004) 409–416

1. Introduction

Piezoelectric materials have been of great interest since the first development ofacoustic wave devices (SAW, BAW, . . . ). Among these materials, aluminium nitride(AlN) possesses particularly good piezoelectric properties and the highest surface acousticwave velocity among all piezoelectric materials(6000 m· s−1). Furthermore, this III–Vcompound has a wide band-gap (6.2 eV) and good mechanical, dielectric, thermal andoptical properties. These properties make AlN a suitable material for use in wear-resistantcoatings [1], buffer layers for GaN growth in optoelectronic devices (UV emitting diodes[2]) and high power and high temperature electronic devices.

Aluminium nitride thin films have been deposited on different kinds of substrates byvarious techniques, such as MOCVD [3], PECVD [4], sputtering [5] and pulsed laserdeposition (PLD) [1], resulting in the growth of polycrystalline or well-oriented AlN thinfilms. PLD is a method well known to allow the growth of well-oriented complex oxides(YBaCuO, BSTO, . . . ) on various substrates. However, because of its non-thermodynamicequilibrium nature and its ability to reproduce the target composition in the growing film,PLD is also able to deposit various materials, such as diamond-like carbon (DLC), nitrides(BN, AlN) and metals.

Pulsed laser deposition has been used to determine AlN thin film characteristics with aview to introduction in acoustic wave devices.

2. Experimental procedure

The ablation process was performed in a high vacuum chamber (5× 10−8 mbarbase pressure) using a Lambda Physics LPX 210i pulsed laser (wavelength: 248 nm;pulse duration: 25 ns; repetition rate: 10 Hz). The laser beam was focused on a rotatingtarget with an energy density of 3 J cm−2. The material ejected from the AlN targetis collected onto a Si–SiO2 substrate located at a distance of approximately 4 cm fromthe target surface. The experiments were carried out under a nitrogen atmosphere (10−4

mbar pressure) with the substrate temperature from room temperature to 750C. Beforedeposition, substrates were cleaned in ultrasonic baths of trichlorethylene, acetone andethanol, rinsed with deionized water and dried under a nitrogen flow; targets were cleanedby a pre-ablation process in order to remove surface impurities.

The surface morphology of the AlN films was observed by AFM in tapping mode(Dimension 3100 from Digital Instrument) and films were characterized byθ–2θ XRD(Siemens D5000 Diffractometer).

The FTIR spectra were recorded using a Perkin-Elmer GX spectrometer at a resolutionof 2 cm−1 with a spectral range of 370–10000 cm−1. All IR spectra were the results ofthe averaging of 200 scans. The transmission spectra at normal incidence were normalizedwith the Si–SiO2 substrate.

Advancing contact angles are measured directly with small drops of the test liquids:1-bromonaphtalene, diiodomethane, water, glycerol and formamide placed on the surfaceof the sample. All liquids are Merck products of analytical grade purity,>99%. The wateris distilled twice. The thin films are cleaned with acetone before the measurements. Thedroplets are released in a controlled manner onto the surface of the samples from the tip of

C. Cibert et al. / Superlattices and Microstructures 36 (2004) 409–416 411

a microsyringe. For each sample, five drops are performed with each test liquid. The dropvolume of 1µl is used forα-bromonaphtalene and 5µl for the other three probe liquids.The angles of both sides of each droplet are measured and the mean values are used forcalculations. The standard deviation due to experimental error is calculated as being±0.5.

The Lifshitz–van der Waals (LW) and acid–base surface tension components andelectron donor/acceptor parameters are calculated from the contact angle data according tothe method of van Oss et al. [6,7]. The surface tension,γi , is the sum of two components:

γi = γ LWi + γ AB

i (1)

whereγ LWi is the Lifshitz–van der Waals component andγ AB

i is the Lewis acid–basecomponent. Acid–base activity of a material involves two functionalities, one for theelectron acceptor (Lewis acid) and the other for the electron donor (Lewis base). The ABcomponent of the surface tension is

γ ABi = 2

√γ +

i γ −i (2)

whereγ +i is the Lewis acid parameter andγ −

i is the Lewis base parameter (seeTable 1).The contact angleθ for solid–liquid systems can be related to the surface

thermodynamic properties of the solid (S) and liquid (L) via the Young–Dupre equation:

(1 + cosθ)γL = 2

(√γ LW

S γ LWL +

√γ +

S γ −L +

√γ −

S γ +L

). (3)

From Eq. (3), the surface free energy components of a solid (γ LWS , γ +

S , γ −S ) could be

determined.The Brillouin experiments are performed at room temperature. The light source is an

Ar+ laser tuned on a 514.5 nm single-mode line. Incident 200–400 mWp-polarized lightis focused on the surface of the sample. The scattered light is analysed by means of aSandercock-type 3+ 3-pass tandem Fabry–Perot interferometer characterized by a finesseof about 100 and a contrast ratio higher than 1010. The typical duration for the acquisitionof a Brillouin spectrum is 2–3 h. In the present work we use the backscattering geometry.In this condition the wave vector of the surface phonons involved, propagating along thesurface, is determined by the relation

QS = 2kI sin(i) (4)

wherekI denotes the optical wave vector in air and wherei is the angle of incidence. Thewavelength of the surface phonons probed in this experiment then typically lies around300 nm. The velocity(vS) is deduced from the measured frequency(Ω) by using therelation

vS = ΩQS

. (5)

3. Results

X-ray diffraction (XRD) experiments were performed on AlN thin films to determinetheir crystallinity and their orientation.Fig. 1 shows theθ–2θ XRD pattern for the film


Fig. 1. XRD spectra of an AlN film deposited at 750C qunder 10−4 mbar of nitrogen.

Table 1Values of the surface tension components in mJ m−2 of test liquids at 20C (from [7])

Liquid γ WL γ LW

L γ +L γ −

L

Diiodomethane 44.4 44.4 0 0Glycerol 64.0 34.0 3.92 57.4Water 72.8 21.8 25.5 25.5Formamide 58.0 39.0 2.28 39.6α-Bromonaphtalene 44.4 44.4 0 0

grown at 750C under a nitrogen pressure of 10−4 mbar. The pattern clearly shows theexpected Si (400) plane with a hexagonal AlN (002) reflection. This result means that thec-axis of the film is perpendicular to the surface substrate. No other AlN reflection canbe seen in the pattern, indicating that AlN crystals are predominantly oriented along the[001] direction. On the other hand, in XRD scans of AlN thin films deposited at lowersubstrate temperature we could not detect any AlN reflection, indicating that the films areamorphous or nanocrystallized. It appears that the growth ofc-axis oriented AlN thin filmson Si–SiO2 substrate needs a minimum substrate temperature of about 750C. This resultis in good agreement with other works [8].

Contact angles and surface tension components for the five probe liquids measuredon the surfaces of samples are listed inTables 2and 3, respectively. For all samples,a generally high contact angle is measured for polar liquids. A very low or a zeropolar surface tension is expected. Thin films of AlN present a high hydrophobicity. Thetemperature of deposition had a great influence on the contact angle with all liquids. Aminimum of contact angle is observed for the sample made at 500C. Effectively, theglobal surface tension is maximum around 40.1 mJ m−2.

For the other samples, a zero acceptor electron component (i.e. a zero acidobasiccomponent) is found.γ −

S is reduced significantly and so the hydrophobicity increases[9]. The transition between hydrophobicity and hydrophilicity is calculated forγ −

S at29 mJ m−2 andγ LW

S at 42 mJ m−2. The critical angle with water for this transition is55.6.


Fig. 2. AFM observations of AlN thin films at (a)T = 350C (b) T = 500C (c) T = 750C.

Table 2Contact angles (in degrees) on different AlN thin films

Liquid AlN RT AlN 350 AlN 500 AlN 750

Diiodomethane 60.0 51.0 44.5 51.3Glycerol 85.7 81.3 71.7 82.0Water 81.2 82.9 57.3 73.1Formamide 79.5 69.5 49.6 71.2α-bromonaphtalene 42.2 35.8 28.5 34.2

Fig. 2 shows the different microstructures obtained with AFM. The temperatureincreases the grain size and the percentage of crystallinity. The size of the crystallitesincreases from 20 to 60 nm at respectively 350 and 750C. The amount of crystallinephase is from around 65% at 350C to 100% at 750C.


Fig. 3. Transmission infrared spectra with normal incidence on AlN thin films at different temperatures ofdeposition.

Table 3Values ofγ LW

S , γ +S , γ −

S for on different AlN thin films

Surface tension component AlN RT AlN 350 AlN 500 AlN 750

γS (mJ/m2) 31.1 (0.1) 35.1 (0.1) 40.1 (0.1) 35.3 (0.1)

γ LWS (mJ/m2) 31.1 (0.1) 35.1 (0.1) 38.3 (0.1) 35.3 (0.1)

γ +S (mJ/m2) 0.00 (0.01) 0.00 (0.01) 0.02 (0.01) 0.00 (0.01)

γ −S (mJ/m2) 10.1 (0.5) 8.9 (0.2) 25.0 (0.4) 22.4 (0.5)

Infrared spectra are represented inFig. 3. The transmission spectra show broad, intensebands in the TO vibrations, with that for AlN between 580 and 860 cm−1. The bandwidthhas high values at room temperature around 360 cm−1 and decreases to around 185 cm−1

for 750 C. The TO(E1) mode frequency tends to decrease from 694 cm−1 at roomtemperature to 675 cm−1 at 750C. Moreover, at 750C, other modes appear at 605and 850 cm−1 and could be associated respectively with TO(A1) and LO(A1) [10]. Forthis sample, the IR data confirmed that this film has a greater degree ofc-axis orientationperpendicular to the surface.

In the case of a hard thin film (AlN) on a soft substrate (Si/SiO2 (900 nm), only theRayleigh surface wave (RW) can propagate along the film plane. The bulk transverse (TA)


Fig. 4. Brillouin spectra from AlN film samples: (a) substrate Si/SiO2 (900 nm), (b) room temperature,(c) T = 350C and (d)T = 750 C. The angle of incidence is 50. RW denotes the Rayleigh surface waveof the film. In (d) the Rayleigh wave is dispersive, as the film thickness (160 nm) is lower than the acousticwavelength (around 300 nm).

and the longitudinal (LA) bulk waves could be observed for thick enough film (typicallyaround or higher than 1000 nm).

In Fig. 4we show spectra from the AlN samples deposited at different temperatures on aSi–SiO2 (900 nm) substrate, obtained at an angle of incidencei = 65 with p polarization(TM) of the incident electric field and no polarization of the scattered electric field. Thepeak at the lowest frequency corresponds to the Rayleigh surface wave (RW) of the AlNfilm travelling parallel to the film surface and no higher frequencies that correspond toguided surface acoustic waves in the film are seen, as expected. The broad peak around35 GHz corresponds to the bulk longitudinal acoustic mode of SiO2. Due to the largefilm thickness near to or higher than 0.3 µm, except for the sample deposited at 750C(160 nm) the Rayleigh wave is entirely confined within the film. So, its phase velocityis mainly characterized by the shear elastic constant C44 of the film and not dispersive[10]. Measurements of the frequency position of the RW line enable us to determine itsphase velocityvR, according to Eq. (5). We find velocities ranging from 5000 m s−1

(RT sample) to 5600 m s−1 (fully crystallized). This is in agreement with earlier work on


thick samples (1000 nm) made by magnetron sputtering or metal–organic chemical vapourdeposition (MOCVD) [11]. No more information on elastic properties can be gained fromBrillouin spectra for such samples, as only the Rayleigh wave is observed. Nonetheless,a temperature of 750C for PLD deposited AlN films seems to lead to good enoughproperties for SAW device use, as indicated by our results on the Rayleigh surface wavepropagation velocity.

4. Conclusion

AlN films have been produced by PLD on Si–SiO2 substrate at temperatures rangingfrom room temperature to 750C. AlN thins films present ac-axis preferential orientationnormal to the surface and hexagonal structure at 750C. IR spectra and AFM observationsconfirm a high degree of crystallization. A high hydrophobicity is found for AlN thinfilm. Brillouin light scattering has been used for determining the phase velocity of theRayleigh surface wave. It compares fairly well for 750C temperature deposition withthose measured in AlN films deposited at high temperature by MOCVD or magnetronmethods [12].

References

[1] V.P. Godbole, J. Narayan, Mater. Sci. Eng. B 39 (1996) 153–159.[2] Y. Inoue, H. Nagasawa, N. Sone, K. Ishino, A. Ishida, H. Fujiyasu, J.J. Kim, H. Makino, T. Yao,

S. Sakakibara, M. Kuwabar, J. Cryst. Growth 265 (2004) 65–70.[3] K. Dovidenko, S. Oktyabrsky, J. Narayan, M. Razeghi, J. Appl. Phys. 79 (1996) 2439.[4] J.W. Soh, S.S. Jang, I.S. Jeong, W.J. Lee, Thin Solid Films 279 (1996) 17.[5] H.P. Löbl, M. Klee, R. Milsom, R. Dekker, C. Metzmacher, W. Brand, P. Lok, J. Eur. Ceram. Soc. 21 (2001)

2633.[6] C.J. van Oss, M.K. Chaudhury, R.J. Good, Chem. Rev. 88 (1988) 927.[7] C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994.[8] R.D. Vispute, J. Narayan, J.D Budai, Thin Solid Films 299 (1997) 94.[9] C.J. van Oss, Colloids Surf. B: Biointerfaces 5 (1995) 91.

[10] V.Yu. Davydov, Yu.E. Kitaev, I.N. Goncharuk, A.N. Smirnov et al., Phys. Rev. B 58 (1998) 12899–12907.[11] G. Carlotti, D. Fioretto, G. Socino, B. Rodmacq, V. Pelosin, J. Appl. Phys. 71 (1992) 4897.[12] G. Carlotti, G. Gubbiotti, F.S. Hickernell, H.M. Liaw, G. Socino, Thin Solid Films 310 (1997) 34.

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Mechanical and physicochemical properties of coke produced by wet and dry slaking