Growth of Er:Y2O3 thin films by pulsed laser ablation from metallic targets

Growth of Er:Y2O3 thin films by pulsed laser ablation from metallic targets

Applied Surface Science 186 (2002) 403±407 Growth of Er:Y2O3 thin ®lms by pulsed laser ablation from metallic targets Ph. Lecoeura,b,*, M.B. Korzensk...

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Applied Surface Science 186 (2002) 403±407

Growth of Er:Y2O3 thin ®lms by pulsed laser ablation from metallic targets Ph. Lecoeura,b,*, M.B. Korzenskia, A. Ambrosinia, B. Merceya, P. Camyc, J.L. Doualanc a

Laboratoire CRISMAT-ISMRA, UMR 6508, 14032 Caen Cedex, France Laboratoire LUSAC, EA 2607, Site Universitaire de Cherbourg, 50130 Octeville, France c Laboratoire CIRIL-ISMRA, UMR 6637, 14032 Caen Cedex, France

b

Abstract Structural and optical characterizations of Er:Y2O3 thin ®lms deposited by pulsed laser deposition (PLD) from metal targets have been undertaken. The background gas during deposition plays an important role on the crystalline quality of the ®lm. It was found that mixture of oxygen and an inert gas is needed in order to stabilize the Y2O3 phase. Emission spectroscopy of the plasma during deposition reveals that the oxidation of yttrium atoms during gas-phase transport is not necessary in this particular case. Using the pressure±distance scaling law, high quality ®lms can be obtained on sapphire (0 0 0 1) under a wide range of pressures. The 0.69 mm thick ®lms, deposited under an O2/Ar atmosphere of 6/244 mTorr and substrate±target distance of 45 mm, exhibit high levels of ¯uorescence and ef®cient guiding effects along the entire length of planar waveguide samples. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Y2O3 thin ®lms; Emission spectroscopy; Waveguide; Optical losses

1. Introduction Yttrium oxide is known to be an ef®cient matrix for optics applications in light ampli®cation and laser ®elds. For this reason, Y2O3 thin ®lms are good candidates as hosts for rare-earth ions in constructing active waveguides for telecommunication applications operating at 1.55 mm. We recently investigated the growth conditions of Y2O3 ®lms deposited from yttria and yttrium metal targets by conventional PLD [1]. The crystallinity and the surface morphology were found to be very sensitive to the background gas mixture and to the nature of the target. The best results were obtained with the metallic target ablated in an *

Corresponding author. Present address: Laboratoire CRISMATISMRA, UMR 6508, 14032 Caen Cedex, France. Tel.: ‡33-2-31-45-26-07; fax: ‡33-2-31-95-16-00. E-mail address: [email protected] (Ph. Lecoeur).

ambient background gas of 6 mTorr oxygen and 244 mTorr argon. Under such conditions, the effect of background gas and, more particularly, the reactivity of species during the gas-phase transport, differ from those usually found for PLD synthesis of high Tc superconductors and manganite ®lms [2,3]. In this study, the results obtained by the investigation of the gas-phase chemistry by emission spectroscopy and the effect of the deposition atmosphere on the crystallinity of Y2O3 ®lms are reported. Optical measurements of the guiding properties of ®lms obtained under these conditions are also discussed. 2. Experimental Laser ablation of metallic yttrium and erbium rotating targets was carried out in a high vacuum stainless steel chamber with a base pressure of 10 6 Torr. The target was irradiated at a 458 incidence angle by an

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 0 1 - 2

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excimer laser KrF (l ˆ 248 nm, t ˆ 25 ms, fluence ˆ 2 J=cm2 ). The substrate holder was placed at distance D ˆ 45 75 mm from the target and heated to 700 8C during the deposition. The background gas consisted of O2 and Ar in varying ratios. Films deposited on sapphire (0 0 0 1) were then cooled under 250 Torr oxygen at a rate of 10 8C/min. Selected substrates were etched before deposition in a 3:1 H2SO4:H3PO4 solution at 120 8C for 15 min and then heated at 750 8C for 30 min in an oxygen atmosphere. Film thickness measurements were obtained by interferometry using a Perkin Elmer spectrophotometer. To perform spectroscopic measurements, the emission light generated from the plume was collected at 908 from the target±substrate axis using a focal lens 5 cm in diameter and 10 cm in length. The light was focused at the entrance of a Jobin-Yvon 270M monochromator mounted with a 1800 lines/mm holographic grating. The detection was performed with a conventional charge coupled device (CCD) detector (Hamamatsu C4350/1024 pixels) connected to a computer. The base resolution of this set-up was determined to be 0.15 nm at the 398.4 nm mercury radiation wavelength. Typical spectra acquisition was performed at 15 s integration time with a laser working at a 3 Hz repetition rate. X-ray diffraction (XRD) in 2y±o-scan mode and rocking curve measurements were obtained using a Seiffert XRD-3000 diffractometer with Cu Ka1 radiation. The in-plane alignment and crystallinity measurements of the ®lms were carried out using a Philips X'Pert MRD diffractometer. Optical measurements on the Er:Y2O3 ®lms were performed using a Ti:sapphire laser operating around 800 nm. The incident beam was coupled into the waveguide section using a microscope objective and a cylindrical lens in order to reduce the beam divergence in the ®lm. The luminescence pro®le was recorded with a CCD camera placed on the top of the sample surface.

Fig. 1. Emission spectra from interaction of species ablated from yttrium metal target showing molecular YO …A2 P3=2;1=2 ! X2 S‡ † and atomic Y …2 D05=2;3=2 !2 D3=2 † transitions under: (a) 250 mTorr O2; (b) 6 mTorr O2; (c) gas mixture of 6 mTorr O2 and 244 mTorr argon.

3. Results and discussion

mechanism for production of yttrium monoxide during the gas-phase transport between the target and the substrate in the PLD process [4]. The signature of this exothermic reaction (2:29  0:11 eV [5]) is observed in the emission of yttrium monoxide radicals (YO) formed in the plume. In order to understand why the optimum value of the oxygen partial pressure in the background gas seems to be 6 mTorr [1], an investigation by emission spectroscopy was undertaken. Fig. 1a shows the emission spectrum of the YO radical measured in a pure oxygen background gas of 250 mTorr. This spectrum exhibits both YO and Y emissions with the YO/Y ratio following the oxygen pressure, as explained by Dye et al. [4]. The same acquisition was performed under 6 mTorr of pure oxygen. Fig. 1b shows the absence of the YO emission signature, within experimental limits under these conditions. Upon the addition of argon gas, bringing the total pressure to 250 mTorr, the YO emission does not increase. Under such conditions, the absence of the YO signature indicates that the oxidation of the yttrium does not occur in the gas-phase transport, but instead at the ®lm surface.

3.1. Emission spectroscopy of the plume

3.2. Plasma range

The reaction of ground state yttrium atoms with molecular oxygen is known to be the dominant

Another way to distinguish between the oxidation process in the gas phase and the oxidation process on

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the surface is the use of the P±D scaling law, were P is the working pressure and D the target±substrate distance. It has been established that the scaling law observed in the growth of thin ®lms by laser ablation is related to the plasma dynamics occurring within the laser plume. In the presence of a small fraction of oxygen (2%), a change in the working distance is expected to affect only the plasma dynamics and not the surface reactions. A quantitative approach to this P±D scaling law can be obtained by the ``blast wave model'' [6]. In such a model the shock front propagates with the distance±time relationship of  0:2 E D/ t0:4 (1) r0 where E is the explosion energy and r0 the gas density. Assuming that the best ®lms are obtained at a certain optimum value of the velocity of the species arriving at the surface, the scaling law for PLD can be expressed as [7] PD3 ˆ constant

(2)

Fig. 2a shows the evolution of the diffraction intensity of different ®lms deposited with a target±substrate distance of 45 mm with the oxygen partial pressure kept constant at 6 mTorr. The optimum value of the X-ray intensity is obtained for a total pressure of 250 mTorr. The sharp decrease of this intensity when the pressure is increased is due to the lack of surface activation by moderately energetic ions and atoms. On the other hand, a decrease of the total pressure results in highly energetic ions and atoms of yttrium which can cause damage to the ®lm and subsequently affect the crystallinity. Similar experiments were performed for various distances, D. The optimum pressure for each distance D, determined by XRD, is plotted in Fig. 2b. It can be seen that the scaling law ®ts the data reasonably well. These results, coupled with those from the previous section, imply that under the conditions used for the growth of Y2O3, plasma dynamics dominate over gasphase reactions in determining the crystallinity of the ®lms. 3.3. Structural characterization Fig. 3 shows the diffraction pattern of an Er:Y2O3 ®lm (Er3‡ 1.6 at.%) grown on [0 0 0 1] sapphire

Fig. 2. (a) Optimal total pressure for a ®xed distance determined by XRD intensity; (b) optimal total pressure as function of target distance. Dotted line: PD3 ˆ constant.

substrate at 700 8C by alternate target PLD technique from metal targets. In addition to the (0 0 6) sapphire peak located at 41.668 2y, there is a peak at 29.148 (d spacing ˆ 3:07 Ð) which corresponds to the 2 2 2 re¯ection of Y2O3. Diffraction peaks associated with other Y2O3 orientations could not be detected in the 2y range between 58 and 708, clearly indicating a ®lm uniaxially textured along the [1 1 1]-axis. The inset in Fig. 3 shows the o-scan rocking curve of the 2 2 2 diffraction peak and yields a full width at half maximum (FWHM) of 0.878, implying some degree of tilt of the (2 2 2) planes with respect to the substrate. This large FWHM value is associated with the large lattice mismatch between Y2O3 (1 1 1) and sapphire (0 0 0 1) and is the result of compression within the epilayer at the ®lm±substrate interface. Epitaxy was checked using a full f-scan. The pattern exhibits two sets of peaks which indicate two different in-plane orientations, as discussed in a previous paper [1].

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Fig. 3. XRD pattern of 0.69 mm thick Er:Y2O3 ®lm on (0 0 0 1) sapphire substrate. Inset: o-rocking curve of the (2 2 2) peak of the ®lm.

3.4. Optical characterization The ¯uorescence of erbium transitions in Er:Y2O3 was measured from 400 nm to the mid-infrared region, 1600 nm. The spectra are presented in a previous publication [10]. They are in good agreement with other works involving the growth of Y2O3 thin ®lms by sputtering techniques [8,9], and suggest that the rare-earth ion is properly incorporated into the crystal structure. Moreover, the structure of the emission spectra corresponds well with results observed in bulk erbium-doped Y2O3 materials. The guiding properties of the 0.69 mm thick Er:Y2O3 samples were investigated by inducing green

luminescence with an incident laser beam (800 nm) and recording, through an interferometric ®lter (bandwidth 550  30 nm), the output with a CCD camera. The exponential decay was ®tted using the following expression: I ˆ I0 e …Ns‡a†x , where s is the erbium absorption cross-section [8], N the erbium concentration, a represents the losses and x is the position in the ®lm along the propagation line. Fig. 4a illustrates the waveguiding properties of an Er doped ®lm deposited on sapphire without any prior treatment of the substrate. Fig. 4b shows the waveguiding properties of a sample whose substrate was pre-etched. Both ®lms were grown under optimized conditions described in the previous section.

Fig. 4. Green ¯uorescence transverse of the Er:Y2O3 ®lms dimensions 2 mm  5 mm recorded with a CCD camera showing: (a) exponential decay of the ®lm without pre-etching of the sapphire substrate; (b) exponential decay of the ®lm with the pre-etching treatment of the substrate.

Ph. Lecoeur et al. / Applied Surface Science 186 (2002) 403±407

The mean roughness (Rms) measured by AFM of the ®lm deposited without the pre-etching procedure Ê . The use of the pre-etching leads to a mean is 34.5 A Ê . This demonstrates that in addition roughness of 5 A to high crystallinity, the surface roughness of a ®lm is an important parameter for its waveguiding properties. In the case of the second ®lm, losses were estimated to be lower than 1 dB/cm at 800 nm [10]. In summary, Y2O3 thin ®lms of up to 0.69 mm thickness were grown on (0 0 0 1) sapphire substrates. From the emission spectroscopy measurements and the P±D scaling law, it has been determined that the oxidation of yttrium atoms in the gas phase is not necessary to obtain high quality ®lms on sapphire substrates. Instead, the dominant oxidation process occurs at the ®lm surface. Moreover, the guiding properties of Er:Y2O3 ®lms are not only dependent on the crystalline quality of the samples but also on the surface roughness of the ®lm, which can be signi®cantly improved by a preetching treatment of the substrate before ®lm deposition.

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Acknowledgements The authors wish to thank Mr. B. Pouderoux for technical support with the electronic set-ups. References [1] M.B. Korzenski, Ph. Lecoeur, B. Mercey, D. Chippaux, B. Raveau, R. Desfeux, Chem. Mater. 12 (2000) 3139. [2] A. Gupta, J. Appl. Phys. 73 (1993) 7877. [3] Ph. Lecoeur, A. Gupta, P.R. Duncombe, G. Gong, G. Xiao, J. Appl. Phys. 80 (1996) 513. [4] R.C. Dye, R.E. Muenchausen, N.S. Nogar, Chem. Phys. Lett. 181 (1991) 531. [5] J.B. Pedley, E.M. Marshall, J. Phys. Chem. Ref. Data 12 (1983) 967. [6] D.B. Geohegan, Appl. Phys. Lett. 60 (1992) 2732. [7] H.S. Kim, H.S. Kwok, Appl. Phys. Lett. 61 (1992) 2234. [8] T.P. Hoekstra, P.V. Lambeck, H. Albers, Th.J.A. Popma, Electron. Lett. 29 (1993) 581. [9] M. Beukema, H.J. van Weerden, P.V. Lambec, J. Broeng, J.L. Philipsen, A. Bjarklev, J.E. Pedersen, Proceedings of the European Conference on Integrated Optics, Vol. 97, 1997, p. 397. [10] M.B. Korzenski, Ph. Lecoeur, B. Mercey, P. Camy, J.L. Doualan, Appl. Phys. Lett. 78 (2001) 1210.