Ž .Applied Surface Science 138–139 1999 587–592

Vacuum ultraviolet annealing of thin films grown by pulsed laserdeposition

Valentin Craciun a,), Doina Craciun a, Pascal Andreazza b, Jacques Perriere c,Ian W. Boyd d

a Laser Department, National Institute for Laser, Plasma and Radiation Physics, Bucharest, PO Box MG-36, RO-76 900, Romaniab Centre de Recherches sur la Matiere DiÕisee, UniÕersite d’Orleans, BP 6759, 45067 Orleans Cedex 2, France

c Groupe de Physique des Solides, UniÕersites Paris VII et VI 2, place Jussieu, 75251 Paris Cedex 05, Franced Electronic and Electrical Engineering, UniÕersity College London, Torrington Place, London WC1E 7JE, UK

Abstract

Ž .The effect of a post-deposition annealing treatment in 1 bar of oxygen at moderate temperatures -4508C underŽ . Ž .illumination by vacuum ultraviolet VUV radiation emitted by an excimer lamp upon thin ZrO and hydroxyapatite HAp2

Ž .films grown by the pulsed laser deposition PLD technique was investigated. The optical and structural properties of thefilms were improved by this treatment, the lower the deposition temperature and, accordingly, the poorer the initialcharacteristics, the more significant the improvements. The combination of these two techniques allowed us to obtain at

Ž . 2temperatures below 3508C highly textured 020 ZrO films, exhibiting optical absorption coefficients lower than 5=102

cmy1 and high refractive index values of around 2.25 in the visible region of the spectrum. The VUV treatment was alsobeneficial for the partially crystalline HAp layers containing tetracalcium phosphate and calcium oxide phases grown by thePLD technique under a low pressure oxidising atmosphere of only 10y5 torr without any water vapours. After theVUV-assisted anneal, the crystalline structure and the stoichiometry greatly improved while the percentage of the othercrystalline phases initially present was many times reduced. q 1999 Elsevier Science B.V. All rights reserved.

PACS: 42.62.Cf; 78.20.Ci; 81.05.Je; 81.65.Mq

Keywords: Vacuum ultraviolet lamps; Laser ablation; Thin films; Zirconium dioxide; Hydroxyapatite

1. Introduction

Ž .The pulsed laser deposition PLD technique iswell known for the quality of the layers grown atrelatively lower substrate temperatures than other

) Corresponding author. E-mail: craciv@roifa.ifa.ro

w xthin film deposition methods 1 . However, there arestill many applications where a further reduction ofthe processing temperature is highly desirable. Previ-ous studies have indicated that the properties of thePLD grown layers degraded if the substrate tempera-ture employed was reduced below the optimal value,situated somewhere within the 5008–8008C range,depending on the characteristics of the depositedmaterial. In order to improve the properties of the

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00457-7

( )V. Craciun et al.rApplied Surface Science 138–139 1999 587–592588

films deposited at lower temperatures, a subsequentw xannealing treatment is generally performed 2–4 .

The effectiveness of this processing step depends onthe temperature and reactivity of the employedgaseous atmosphere.

Ž .It is known that vacuum ultraviolet VUV radia-tion, whose wavelengths are lower than 200 nm,produces the photodissociation of molecular oxygen,resulting in the formation of very reactive species

w xsuch as ozone and atomic oxygen 5,6 . It is thusexpected to obtain significant improvements of theannealed thin oxide film properties using these gases,even at moderate temperatures. The effect of a post-deposition VUV-assisted annealing treatment uponthin PLD grown layers of ZrO and hydroxyapatite2Ž .HAp , a simple and a complex oxide material, ispresented here to illustrate the potential of this newtechnique.

2. Experiment

The films were deposited from sintered home-Ž .made targets onto quartz or 100 Si substrates using

Ža KrF excimer laser ls248 nm, 20 ns pulse.duration, 5 Hz pulse repetition rate in an N O2

atmosphere. The deposition conditions employed aresummarised in Table 1. After deposition, the grownfilms were annealed for 1 h at different temperatures

Ž .in 1 bar of high purity oxygen 99.999% underVUV illumination. The source of the VUV radiationwas an excimer lamp working with Xe and emittingat 172 nm. More details about this VUV source and

w xits uses have been published elsewhere 5 . As theinteraction of 172 nm radiation with molecular oxy-gen produces ozone and atomic oxygen, this allowsthe oxidation reaction to proceed with a measurablerate even at temperatures not far higher than the

w xroom temperature 4,7,8 .ŽThe optical properties refractive index and ab-

.sorption coefficient, n and a , respectively of theZrO films were calculated following the method of2

w xSwanepoel 9 from the optical transmission curvesrecorded in the 190–1100 nm range with a double

Ž .beam spectrophotometer Perkin-Elmer Lambda 14 .The surface morphology was investigated by scan-

Ž .ning electron microscopy SEM while the crys-talline structure by grazing incidence X-ray diffrac-

Ž .tion GIXD . The chemical composition and molecu-lar structure of the HAp films were investigated by

Ž .Rutherford backscattering spectrometry RBS andŽ .Fourier transform infrared spectroscopy FTIR .

3. Results

3.1. ZrO films2

Typical examples of the improvement of the opti-cal transmittance of ZrO layer deposited at 45082

Ž .and 3008C samples no. 3 and no. 5 after the VUVŽ .anneal samples no. 4 and no. 6 are shown in Fig. 1.

The spectral dependence of the calculated refractiveindexes and absorption coefficients of the as-de-posited and VUV-annealed ZrO films is presented2

in Figs. 2 and 3, respectively. From these graphs one

Table 1Ž .Deposition conditions for the growth of ZrO and HAp thin films T is the substrate temperature2 s

Ž .Sample no. Fluence T P N O No. of Thickness E UV anneals 2 g2Ž . Ž . Ž . Ž . Ž .Jrcm 8C mbar pulses nm eV

y41 7 550 1=10 1800 140 5.92y42 7 550 1=10 1800 130 5.94 1 h at 4508Cy53 5 450 5=10 4500 334 5.68y54 5 450 5=10 4500 290 5.71 1 h at 4508Cy55 5 300 5=10 4550 – 5.63y56 5 300 5=10 4550 420 5.64 1 h at 3508Cy57 4 650 1=10 9000y58 4 650 1=10 9000 1 h at 4508Cy19 4 650 1=10 9000y110 4 650 1=10 9000 1 h at 4508C

( )V. Craciun et al.rApplied Surface Science 138–139 1999 587–592 589

Fig. 1. Optical transmittance of ZrO films grown at 4508 and2

3008C.

can clearly see that the VUV treatment has improvedthe optical properties of the films, the lower thedeposition temperature, the more substantial the im-provement. The increase of the refractive index afterannealing, from values below 2.0 up to values above2.25 in the visible range, corresponding to highquality bulk ZrO , is most likely due to the densifi-2

w xcation of the film 10 . This is in agreement with thesimultaneous slight decrease in the film thicknessŽ .see Table 1 .

It is worth noting that after the VUV treatment thevalues of the optical absorption coefficients shown in

ŽFig. 3 are very low, similar to the best values of 4 to. 2 y1 w x6 =10 cm yet reported 11 . The optical band-

Fig. 2. Spectral dependence of the refractive index of ZrO films;2

sample numbers are those shown in Table 1.

Fig. 3. Spectral dependence of the absorption coefficient of ZrO2

films; the same symbols as those shown in Fig. 2 were used.

w xgap energy, E , determined from the Tauc plot 12 ,g

found to be around 5.6–5.9 eV for the as-depositedfilms, slightly increased by a few tenths of an elec-tron volt after the anneal as one can see in Fig. 4.

The grown films were textured, exhibiting onlyŽ . Ž .the 020 and 040 XRD patterns. However, the

intensity of the diffraction peaks for the as-depositedfilms were rather low, probably because films grownat these substrate temperatures were not very crys-talline. After the VUV treatment, the intensity of thediffraction peaks increased dramatically. The mea-sured positions of the diffraction angles, corrected

Ž . Ž .using the Si 200 and Si 400 peaks as standards,were 34.37 and 72.49 degrees, which compares wellwith the published positions for baddeleyite phase at

Ž .2Fig. 4. Plot of the a hn vs. hn .

( )V. Craciun et al.rApplied Surface Science 138–139 1999 587–592590

w x34.3838 and 72.4438 13 . The morphology of thefilms, as investigated by SEM, was typical for PLDgrown layers, with a smooth, featureless surfaceexhibiting a low density of droplets.

3.2. HAp films

The FTIR investigations have shown that the PLDgrown layers were basically HAp, exhibiting all the

w xmajor absorption bands 14 , the spectra being simi-lar to that recorded from the HAp target. The VUVanneal had a spectacular effect upon the layers struc-ture, as revealed by the FTIR spectra shown in Fig.5. The intensity of the absorption bands increasedsignificantly while their width reduced considerably,indicated the transformation of a disordered layerinto a highly crystalline HAp layer. A new absorp-tion band around 1430 cmy1 appeared which is

w xusually assigned to the surface carbonate ions 14 . Itis also worth mentioning that after 1 h the transfor-mations taking place inside the grown layer duringthe anneal were completed, as a further 1 h annealdid not produce any measurable modifications.

A similar improvement of the crystallinity wasseen from the GIXD spectra displayed in Fig. 6. The

Ž .as-deposited film sample no. 7 , grown in a verylow pressure oxidising atmosphere, exhibited a mix-

Žture of HAp as0.9418 nm, cs0.6884 nm, similar.to the structure shown in JCPDS 09-0432 card ,

Žtetracalcium phosphate and calcium oxide noted ),.P1 and P2, respectively, on Fig. 6 phases. After the

Fig. 5. FTIR transmission spectra of an as-deposited and a VUV-Ž .annealed HAp thin films sample no. 7 and no. 8, respectively .

Ž .Fig. 6. GIXD spectra of samples no. 7 continuous line and no. 8Ž .dashed-line .

Ž .annealing sample no. 8 , the tetracalcium phosphateand calcium oxide phases were reduced four fold andtwo fold, respectively, while the content of the crys-talline HAp phase increased up to around 70%. Theaverage grain dimension, estimated to be around 40nm and the microstrain, es0.30%, did not changeafter the treatment.

w xRBS simulations 15 of the spectra acquired fromthe same samples no. 7 and no. 8 and displayed inFig. 7 showed an improvement of the CarP ratiovalue, from 3.0 to 2.5–2.2. Also, the oxygen contentincreased, being closer to that corresponding to stoi-

( )V. Craciun et al.rApplied Surface Science 138–139 1999 587–592 591

Ž . Ž .Fig. 7. RBS spectra of the samples a no. 7 and b no. 8.

chiometric HAp. Some marginal improvements afterthe VUV anneal were also noticed for the layersdeposited under a high pressure of the oxidising gasŽ .sample no. 10 with respect to sample no. 9 . TheCarP ratio, measured by RBS, decreased fromaround 1.72 to the stoichiometric value of around1.67. According to the GIXD results, the as-de-posited sample already consisted of almost 100%crystalline HAp and remained practically unchangedafter the VUV anneal.

4. Discussion

The combination of these two techniques allowedus to growth crystalline HAp layers without usingwater vapours or a high pressure oxidising atmo-sphere. This is particularly important when one con-sider that Ti substrates get oxidised during the usualdeposition conditions employed to obtain crystalline

w xHAp layers 16 . It is also important to note thatannealing treatments below 5008C have been foundto be ineffective for the crystallisation of HAp lay-ers, while those performed at 6008C or higher tem-peratures caused a decrease of the adhesion of layers

w xto the substrate 17,18 . Another important advantageof this method is that it allows one to control thecrystallinity of the HAp layer. Recent investigationshave pointed out that 100% crystalline HAp layersare not the most useful coatings for metallic implantsas their solubility and surface reactivity are very low

w xand therefore inhibit early bone formation 19 .The possibility to obtain high quality optical lay-

ers at low temperatures opens new perspective to the

use of complex or plastic material substrates, whichhave many important applications in integrated op-tics.

The processes taking place during the VUV treat-ment are not yet very clear understood. It is knownthat 172 nm radiation dissociates molecular oxygenforming atomic oxygen and then ozone which areadsorbed on the surface of the film. The atomicoxygen can easily diffuse deep into the film depthand react with any atoms which are not completelyoxidised. This is confirmed by an intake of oxygeninside the annealed samples detected by the RBSanalysis. It is more difficult to explain the crystallinetransformations occurring at these rather modesttemperatures. It is very likely that some of thedefects, and especially those located at or near grainboundaries were effectively annealed by reactionwith the oxidising species.

5. Conclusions

In conclusion, high quality ZrO and HAp films2

can be obtained from layers grown by PLD by afurther VUV irradiation in oxygen at modest temper-atures. The film characteristics are improved by theanneal, becoming similar to some of the highestquality films yet reported. The use of low substratetemperatures and low pressure oxidising atmospheresduring the laser ablation process increased the depo-sition rate and limited the surface oxidation of thesubstrate. Further research is underway to understandthe underlying mechanisms of the annealing stepand, based on this, to optimise the process.

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