Amorphous Tm3+ doped sulfide thin films fabricated by sputtering

Optical Materials 33 (2010) 220–226

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Amorphous Tm3+ doped sulfide thin films fabricated by sputtering V. Nazabal a,⇑, A.-M. Jurdyc b, P. Neˇmec c, M.-L. Brandily-Anne a, L. Petit d, K. Richardson d, P. Vinatier e, C. Bousquet a, T. Cardinal e, S. Pechev e, J.-L. Adam a a

Chemical Sciences of Rennes, UMR-CNRS 6226, University of Rennes 1, Rennes, France LPCML, UMR-CNRS 5620, University Claude Bernard-Lyon 1, Villeurbanne, France c Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Pardubice, Czech Republic d School of Materials Science and Engineering, Clemson University, Clemson, SC, USA e Institute of Condensed Matter Chemistry of Bordeaux, CNRS UPR 9048, Pessac, France b

a r t i c l e

i n f o

Article history: Received 29 May 2010 Received in revised form 26 August 2010 Accepted 27 August 2010 Available online 25 September 2010 Keywords: Chalcogenide glasses Thin film RF sputtering Thulium

a b s t r a c t Amorphous chalcogenide films play a motivating role in the development of integrated planar optical circuits and their components. The aim of the present investigation was to optimize deposition conditions for preparation of pure and Tm3+ doped sulfide films by radio-frequency magnetron sputtering. The study of their composition, morphological characteristics and thermal properties was realized by scanning electronic microscope attached with energy dispersive spectroscopy, Rutherford backscattering, X-ray diffraction, and micro-thermal probe. Some optical properties, like transmission, index of refraction, optical band-gap, propagation modes from 633 to 1540 nm, were investigated on thin films. The whole results point out hopeful perspectives strengthened by the clear observation of the near-infrared photo-luminescence of Tm3+ doped sulfide films. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Because of their potential functionality in near-infrared (IR) and middle IR spectral range, chalcogenide glasses are attractive materials in the field of infrared optics, optical sensors, optical imaging, telecommunication and data storage [1,2]. Due to remarkable properties of the chalcogenide glasses – broad spectral range of transparency (up to 10–20 lm), low phonon energy, photosensitivity, high linear and non-linear refractive index –, amorphous chalcogenide films (pure or rare-earth (RE) ions doped) are attractive in the development of integrated optics [3–7]. Indeed, mentioned various properties allow combination of active and passive components on a single substrate for minimization of device size and cost. A key stage for integrated optic geometry for active waveguide devices is the elaboration of amorphous thin films with required chemical composition and suitable physical properties; for that the radio-frequency (rf) magnetron sputtering appears as one of the most efficient and flexible deposition method [8]. The chalcogenide glasses/films are useful hosts for RE doped amplifiers and lasers operating from the telecommunications bands wavelengths to middle IR. The low phonon energy of chalcogenide hosts warrants a low probability of multiphonon relaxation between the RE3+ energy levels, necessary for efficient Pr3+, Dy3+ or Tm3+ doped devices working in near-infrared spectral range and ⇑ Corresponding author. E-mail address: [email protected] (V. Nazabal). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.023

Pr3+, Dy3+, Er3+, Ho3+ or Tm3+ operating middle infrared domain [9–16]. These transitions are normally quenched in hosts with larger phonon energies such as silica or phosphate glasses; this fact is related to their high non-radiative relaxation rates which increase exponentially with the decrease of the energy gap between the excited and the next-lower level [17–20]. The aim of the present investigation was to optimize deposition conditions for fabrication of pure and Tm3+ doped sulfide films by magnetron rf sputtering for potential optical applications. The emission spectrum of Tm3+ ion is especially rich in non-oxide glasses, with transitions generally ranging from the UV to the IR [16]. For instance, the Tm3+ ion is well known for lasing at 2.3 lm. The lasing of Tm3+ at the 1.47 lm was recorded in doped fluorozirconate fiber which delivers 1 W, continuous-wave [21]. Selecting a chalcogenide glass host with lower phonon energy offers new radiative transitions in addition to the transitions available in fluoride and oxide glasses doped with Tm3+ ion; a midinfrared transition is expected to emit at 3.8 lm included in the atmosphere transparency window. An interest in sulfide glasses remains in the higher absorption cross-sections of Tm3+ transitions compared with fluoride glasses; this leads to better pumping conditions [22]. Glass target from Ge-Ga-Sb-S system was selected for this study due to: suitable thermo-mechanical properties for film deposition on various substrates as demonstrated in case of PLD process for Ga5Ge20Sb10S65 (2S2G) nominal composition [23] and presence of Gallium, which allows a better insertion of RE3+ ions than in

V. Nazabal et al. / Optical Materials 33 (2010) 220–226

chalcogenide matrices free from this chemical element [15,24]. The thermal properties of the bulk glasses from Ge-Ga-Sb-S system were only investigated in term of glass transition temperature, expansion coefficient and viscosity in the topic of optical fibre drawing [15,24]. Due to the potential application of amorphous chalcogenide thin films, it is necessary to characterise more specifically the thermal properties of fabricated layers at least in term of thermal conductivity and softening or glass transition temperatures. In order to obtain appropriate films in Ga-Ge-Sb-S amorphous system which allows homogeneous Tm3+ doping, the fabrication of amorphous thin films and the analysis of their chemical and physical properties such as composition, morphology, thermal, optical properties and Tm3+ spectroscopy have been performed and reported in this article. 2. Experimental 2.1. Sample preparation The glass targets with nominal composition of Ga5Ge20Sb10S65 (2S2G) non-doped and doped with Tm3+ (0.1 and 0.5 mol.%) were prepared by means of conventional melting and quenching method. A target Ge25Sb10S65 (2S1G) was also prepared for the thermal analysis of the films with a chemical composition close to 2S2G without Gallium. High purity elements as Ga, Ge, Sb, and S (5–6 N) and Tm (3 N) were weighted in a dry glove box, incorporated in a fused silica ampoule and pumped under vacuum of 104 mbar for few hours. Then, the sealed tubes are heated in a rocking furnace to ensure the homogenization of the melt at 900 °C for 12 h. After the quenching, the glass rods were annealed below the glass transition temperature (Tg  310 °C) for 3 h. The targets for sputtering were cut and polished in the form of a glass cylinder of 2–3 mm for the thickness and 50 mm for the diameter. The deposition of thin films was carried out on chemically cleaned microscope glass, silicon or glassy carbon substrates at room temperature. The substrate-totarget distance was varied from 5 to 10 cm with an Ar pressure from 5  103 to 5  102 mbar and rf power varied between 20 and 50 W. The effects of rf power, deposition pressure and also bias substrate (25 V) were investigated with respect to physical and optical properties of films such as deposition rate, microstructure, and transmission. 2.2. Glass target and sputtered film characterizations The amorphous nature of the glass targets and amorphous films was verified by X-ray diffraction (Phillips PW 3710, Bragg–Brentano, Cu K alpha). The average film thicknesses were measured using a profilometer (PGI from Taylor Hobson), M-lines prism-coupling measurements (Metricon) or variable-angle spectroscopic ellipsometer (VASE, J.A. Woollan Co., Inc.). Thermal properties of bulk glasses were measured by differential scanning calorimetry (DSC 2010 from TA Instruments) with heating rate of 10 K/min (accuracy ±2 K). The SEM technique was also applied to observe the morphology of the thin films using a field-emission gun SEM (JSM 6301F). 2.2.1. Composition analysis The composition of the films and the targets was studied using scanning electron microscopy with energy dispersive X-ray analyzer (SEM-EDS, JSM 6400 – Oxford Link INCA). Moreover, for amorphous films, Rutherford backscattering spectroscopy (RBS) from a Van de Graff accelerator (2 MeV, 4He ions) of the Centre d’Etude Nucléaire de Bordeaux Gradignan (CENBG) was employed. For RBS analyses, thin films deposited on pure and well-polished

221

carbon substrates were used. All the elements with an atomic mass greater than that of carbon can be studied; a RUMP code was used for RBS simulation [25]. 2.2.2. Thermal properties The thermal properties (thermal conductivity and probe penetration temperature) of the thin films were measured using a micro-thermal analyzer (lTA 2990) from TA Instruments Corp. (http://www.tainstruments.com). The lTA device incorporates a U-shaped Pt/Rh Wollaston wire (5 lm dia.) probe which functions simultaneously as both a resistive heater and resistive temperature detector (RTD). For the measurement of the thermal conductivity, the lTA probe was heated to allow thermal energy to flow through the film sample when the probe was placed in contact with its surface. The film was maintained at a constant temperature on the sample stage while the probe scans the film surface. For each scan, both the topographic and the heat flow images were recorded. As heat can flow from the probe tip to both the sample and to the silver probe supports and surrounding air, there was a background heat flow which was unrelated to the sample. It was therefore important to measure this background heat loss by placing the probe at a set distance above the sample surface and repeating the thermal scan. In our experiment, this distance was found to be 200 lm. The power deviation between the probe and sample (Q) was estimated using the equation below [26]:

Pc  Pnc ¼Q Pc

ð1Þ

where Pc and Pnc are the probe power (mW) for contact and noncontact measurements respectively. From the power deviation (Q), it is possible to determine the thermal conductivity (k) using the following simplified relationship [26]:

Q¼

Ak Bþk

ð2Þ

where A and B are the fitting parameters which depend on the probe wire thickness and curvature. As proposed by Fischer et al. [27], it is possible through the use of Eq. (2) to estimate the thermal conductivity of unknown materials through the determination of the parameters A and B, which depend specifically on the set of experimental conditions used (scan rate as well as the probe and stage temperatures) and on the geometry of probe. For each new probe, the heat flow of the reference materials need to be measured again to determine the A and B constants of that new probe. The thermal conductivity of the glasses was determined using polycarbonate, fused silica, ZnSe, GaAs, silicon as reference materials with known thermal conductivity to determine A and B. Accurate measurement of thermal conductivity of thin films was obtained within ±10% by scanning a 50  50 lm area of the film surface heated to 27 °C at a scan rate of 100 lm/s with a probe temperature (Tp) of 127 °C. The probe penetration temperature, Tp, of the investigated glasses was measured by positioning the thermal probe at the surface of the glass (under a low, constant force), using a heating rate of 10 °C/s. Upon heating, an upward displacement of the probe was initially seen, caused by the thermal expansion of the film/substrate and of the probe itself. At some maximum, material-specific temperature, the upward motion of the probe ceased, and the probe begins to penetrate the specimen’s surface, yielding a marked change in slope the sensor position. The onset point of the slope change was defined as the probe penetration temperature (Tp). At 10 °C/s the accuracy in determining the onset temperature Tp was estimated to be ±10 °C. This large error bar is related to the high sensitivity of the micro-thermal analyzer to fluctuations associated

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with subtle system vibration and air currents (despite the probe/ specimen enclosure). 2.2.3. Optical and spectroscopic characterization of sputtered films The optical properties of thin films were obtained by a spectrophotometer (Perkin-Elmer 1050) in the spectral range of 400– 3200 nm. The refractive effective indices of the propagation modes were measured by means of the M-lines prism coupling configuration (Metricon-2010 instrument) and by VASE (J.A. Woollam Co., Inc.). A laser beam at 633, 1302 and 1540 nm wavelengths for both TE and TM polarization and rutile prism were used. For photoluminescence experiments, a Ti: Sapphire laser (2 GHz) pumped by an Ar+ laser, was used for the excitation of Tm3+-doped thin films and targets. A Jobin–Yvon monochromator (H25–1) and an IR photomultiplier (PM) cooled to liquid nitrogen temperature were used to analyze and detect the fluorescence in the spectral region of 950–1750 nm. The excitation laser wavelength (810 nm) was cut-up in front of the monochromator by a silicon filter. A mechanical chopper modulated the signal. Output data from the PM were amplified by a lock-in amplifier. The experimental conditions for measuring the PL lifetimes of the thin films were as follows: an optical parametric oscillator OPO (with pulse duration of few nanoseconds) pumped by a Nd3+:YAG laser, the excitation wavelength is set at 800 nm. A H25 monochromator (Jobin–Yvon) with a detection wavelength fixed at 1.48 lm, an infrared photomultiplier cooled with liquid nitrogen and a digital oscillator (Lecroy) were also used. The response time of the system is equal to 0.25 ls (R = 1 kX). 3. Results and discussion A detailed study of the morphological, compositional and thermal characteristics according to the deposition conditions was carried out in the final objective corresponding to the realization of single-mode waveguide satisfying a minimization of the optical losses. The Ga5Ge20Sb10S65 glass composition was chosen as the host matrix because of its good optical properties, its stability against crystallization, its capability of rare-earth ions adding and the possibility to develop fiber and rib/ridge waveguides [7,23,24]. 3.1. Chemical composition The compositions of the layers obtained by sputtering were analyzed by EDS and RBS (Table 1). The composition of the films was found to be comparable to the bulk glass target. Concerning the target, its composition is in perfect agreement with the nominal composition. The deflection ring area (corresponding to magnetron sputtering abrasion observed after several depositions) has in proportion less sulfur and antimony in favor of germanium from the rest of the target. The target is a homogeneous material composed of elements of different weight and presenting various

bond energies. The chemical nature of the materials is an important factor for sputtering deposition method in the mechanism for removal species from target under argon ions bombardment. When the sputtering begins, the plasma will be richer by one or several chemical elements leading to a specific surface concentration obtained after a given time of deposition which allows reaching of an equilibrium state. The surface composition of the target changes to achieve a representative balance of the sputtering yield combined with the concentration of species per unit surface. From the experimental results, the sputtering yield of sulfur is important compared with that of germanium inducing that the surface concentration of the target is richer in germanium and presents a deficit in sulfur. After the target composition reaches its equilibrium, the thin films present a deficit in sulfur compared with the theoretical composition. The proportion of germanium increases while those of gallium and antimony are more stable when sulfur decreases. Nevertheless for the gallium, changes are within the measurement uncertainty, but a trend showing a slight increase was observed more clearly for other deposition. The thin films may have a slight excess of antimony not consistently observed throughout the series. Overall, the stoichiometry of the deposited films is comparable to the composition of the target and these concentration variations can be compensated by a modification of the initial composition of the target. The RBS analyses have also allowed to check the composition of the films and to reveal the presence of thulium (Fig. 1). Even if the quantity of thulium is in the uncertainty of measurement (±0.5 at.%), this method makes it possible to highlight its presence by a shoulder located at the right side of the peak related to antimony. The gallium and germanium have very close atomic mass; their RBS peaks are not dissociable. For simulation, it is thus preferable to consider the sum of both and to refer to the value obtained for gallium by EDS to extrapolate the germanium concentration. The results obtained by RBS confirm the excess of germanium and the deficit of sulfur in same proportions as for EDS analyses. The influence of some deposition parameters on chemical composition of thin films was analyzed. The deposition pressure influences the composition of the films; the layers contain more sulfur and antimony when the pressure increases. The distance between target and substrate has only a minor impact on the stoichiometry of the sputtered films considering EDS analysis at the center of the thin film. The depositions under substrate bias present a shortage in sulfur to the benefit of others elements.

3.2. Morphology of the sputtered thin films In principle, thin films on a variety of substrates can be prepared by magnetron RF sputtering, so that integration with other optical devices is simple. The thin films were proved to be amor-

Table 1 Chemical composition obtained by EDS (± 0.5 at.%) for Ga5Ge20Sb10S65 target and sputtered films (F-1 to F-4) and by RBS (± 0.5 at.%) for sputtered films (F-10 to F-30 ). Standard condition of deposition: pressure = 2  102 mbar, rf power = 30 W, substrate-to-target distance = 6 cm. Sample Theory Target F-1 F-2 F-3 F-4 F-10 F-20 F-30

Method EDS EDS

RBS

Deposition condition

S (at.%)

Ga (at.%)

Ge (at.%)

Sb (at.%)

Pulverization ring Standard Pressure: 5  103mbar rf Power: 15 W Subst. bias: 25 V Standard Pressure: 5  103mbar rf Power: 15 W

65.0 63.3 61.1 60.5 60.6 58.9 62.0 62.0 62.0

5.0 4.5 5.1 5.2 5.4 5.5 5.0 5.0 5.5

20.0 22.2 21.9 22.6 22.7 23.4 22.0 22.5 22.0

10.0 10.0 11.9 11.7 11.3 12.2 11.0 10.5 10.5

V. Nazabal et al. / Optical Materials 33 (2010) 220–226

Fig. 1. RBS spectrum of Ga5Ge20Sb10S65 sputtered films doped with thulium (black/ thick line: experimental curve, red/thin line: simulated curve).

phous by X-ray diffraction analyses whatever the substrate. The layers were found smooth and dense by SEM (Fig. 2); they do not have the same adhesion according to the deposition conditions and substrates used. Regarding the substrates, the layers appear to adhere better to silicon than to glass slides. For the deposition on glass slides, an easy layer peeling by scraping is obtained without leaving traces on the substrate, except in case of deposition with the substrate bias. In this case only, the layer seems to adhere better to the substrate. The substrate bias is expected to produce a dense film pushing away the molecules that are not enough connected into the growing thin film. To reduce rf power or to apply a bias on substrate allows decreasing the probability of appearance of film columnar structure on substrate. However, the effects of substrate bias seem rather negative, both in composition (net deficit in sulfur) and in terms of surface roughness as observed from SEM pictures. 3.3. Micro-thermal analysis of the chalcogenide thin films The thermal conductivity and the probe penetration temperature were measured using a micro-thermal analyzer. The main advantage of this novel technique is the ability to characterize the film’s thermal properties without removing it from its substrate. The thermal conductivity k was measured at (0.5 ± 0.1) and (0.4 ± 0.1) W/(m K), respectively, for the Ga5Ge20Sb10S65 and Ge25Sb10S65 films deposited on Corning 1737 glass, without any

223

variation with substitution of germanium by gallium (which is important for incorporation of rare earth within the amorphous chalcogenides). It is interesting to point out that the thermal conductivity of the bulks with related compositions to that of the film was measured at (0.2 ± 0.1) W/(m K). This variation in thermal conductivity between the bulk and the thin film with related composition can be explained by the influence of the film substrate, the Corning 1737 with thermal conductivity about 1.03 W/(m K); a substrate of comparable heat capacity to that of the film is expected to have little influence on the heat flow measurement as the heat is expected to be more slowly dissipated though the substrate. In this case, the film’s thermal behavior is not overshadowed by the heat from the underlying substrate. When the film is deposited on a substrate with high thermal conductivity, the thermal conductivity of a film is expected not only to be overshadowed by that of the substrate but also to vary strongly as a function of the film thickness, as the heat from the probe quickly penetrates into and through the thin film layer during the measurement of the thermal conductivity and is quickly dissipated in the substrate. Thus, to allow the measurement of a more representative value of the thermal conductivity of the film, the film should be deposited on a substrate with comparable heat capacity like As2S3 glass about 0.17 W/(m K) or selenide glasses with values slightly higher as Ge22As20Se58 (GASIR1) or As2Se3 with 0.28 and 0.25 W/(m K), respectively. The probe penetration temperature, Tp, of the Ga5Ge20Sb10S65 and Ge25Sb10S65 films with a thickness of 5 lm was measured to be 429 ± 10 and 486 ± 10 °C, respectively, using a scan rate of 10 °C/s. This indicates that this particular temperature is sensitive to chemical composition changes rather tenuous likewise glass transition temperature. The physical significance of this characteristic temperature of the films is not well defined yet and is thought to be related to the dilatometric softening temperature. As shown in Fig. 3, the Tp of these films are analogous to the Tp of bulk glasses with related composition. It is interesting to point out that the Tp of the Ga5Ge20Sb10S65 and Ge25Sb10S65 bulk glasses are higher than the glass transition temperatures obtained by the traditional DSC method (usual heating rate: 10 °C/min) and follow the variation of the glass transition temperature from one glass to another one, with a shift of 110–125 °C between these two characteristic temperatures. These results on the measurement of the thermal conductivity and probe penetration temperature are sufficiently encouraging that this method which used a micro-thermal analyzer can be accepted as a thermal characterization of chalcogenide thin film in order to monitor and compare the quality of the layers considering their thermal history, their deposition method or composition. As the probe penetration temperature of thin film seems to match well with the bulk glass, the thin films freshly deposited were annealed close to the glass transition temperature of the bulk glass of same composition. For such thermal conditions we found that the roughness and morphology of the film is not affected [28].

Fig. 2. SEM micrographs of Ga5Ge20Sb10S65 sputtered thin films: (a) oblique view of surface and section, (b) top view of surface, (c) side view of cross-section (note that cracks observed on the layer come from the cleavage only).

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V. Nazabal et al. / Optical Materials 33 (2010) 220–226

550

5µm

500

Temperature (°C)

Bulk (DSC) Bulk (µTA) Film (µTa)

450

5µm

400

350

300

Ge25Sb10S65

Ge20Ga5Sb10S65 Composition

Fig. 3. Temperature of thermal probe penetration into film and bulk glass compared with glass transition temperature of bulk glasses.

Table 2 Refractive index values of Ga5Ge20Sb10S65 bulk glass and sputtered films with different thickness (F-1, F-5 and F-6) obtained from M-lines prism coupling. Sample

Thickness

633 nm

1302 nm

1540 nm

Refractive index (±0.001) Target F-1 F-5 F-6

4 mm 910 nm 1160 nm 1260 nm

2.361 2.360 2.362 2.361

2.259 2.252 2.254 2.253

2.251 2.244 2.245 2.244

3.4. Refractive index and Tm3+ spectroscopic properties The refractive indices of the sputtered thin films obtained by VASE and M-lines method are close to those of the bulk glass; they only display slightly distinct dispersion curves certainly connected to the variation of composition compared with the glass target (Tables 1 and 2). The absorption cross-sections of the Tm3+ doped Ga5Ge20Sb10S65 bulk glass were compared in Fig. 4 with a classical fluorozirconate glass (ZBLAN) which presents relative low phonon energy (580 cm1), lower refractive index and higher ionic character than sulfide glass. In Fig. 4, the assignment of the transitions from 3H6 ground energy level of Tm3+ ion to the higher energy levels is indicated. The transitions to the 1G4 and 1D2 levels are hidden due to the fundamental absorption of the sulfide matrix which becomes important at 600 nm. The absorption cross-sections of the four observed transitions of Tm3+ ion are several times higher than those of oxide and fluoride glasses because of the covalent nature of chemical bonds in sulfide glasses which is also at the origin of the slight red shift of the transition energies [29]. Because of their strong covalent character and their large refractive indices, sulfide glasses give rise to intense absorption of rareearth ions, which is characterized by high absorption cross section compared with ionic materials such as fluoride glasses. The energy gap between the 3H4 level and the next-lower 3H5 level (about 4310 cm1) is large enough for both sulfide and fluoride glasses to present emission centered at 1.47 micron with excellent quantum efficiency. The fluorescence of the thin film (0.1 mol.% Tm3+ doped 2S2G) presents similar profiles as bulk materials: the emission spectra coming from thulium ions excited at 810 nm were recorded and normalized from 950 to 1750 nm (Fig. 5). The first band of lower intensity, which is centered at 1.25 lm, is

attributed to the 3H5 ? 3H6 transition. This transition is normally completely quenched in oxide glasses and can occur unlikely in fluoride glasses, but it could be detected in the chalcogenide bulk samples. The second band, centered at 1.47 lm, is much more intense; it corresponds to the 3H4 ? 3F4 transition. The peaks at 1070 and 1325 nm have been attributed to substrates; the fluorescence of the substrate becomes suppressed when the thickness of the sulfide thin film increases (Fig. 5). The lifetime of photoluminescence is an important parameter for characterization of the film quality and homogeneous dispersion of RE ions since it reflects the mechanisms of depopulation of energy levels which can be influenced by quenching or trapping relaxation. The lifetime of the excited levels is determined from the decay of fluorescence light intensity for a given transition; in the case of an exponential decay it is described by the relationship: IðtÞ ¼ I0 expðt s Þ. The objective of this study was to analyze the influence of rare earth concentration (0.1 and 0.5 mol.%) and layer thickness (800 nm, 2.0 and 4.5 lm) on the lifetime of the 3H4 level of Tm3+ ion in 2S2G thin film deposited by magnetron RF sputtering. As the decays are not purely exponential, the lifetime of 3RH4 leIðtÞdt vel has been determined using the following equation: s ¼ Ið0Þ ; the resulting lifetime values are shown in Table 3. In case of thin film with a thickness of 800 nm, the lifetime could not be measured as the fluorescence at 1.47 lm was too weak. The lifetime observed for the thin films is shorter (50 ls at maximum for the layers containing 0.1 mol.% of Tm3+ ions) than that of bulk samples (measured as powder in order to reduce reabsorption phenomenon), which was found to be 120 ls at room temperature for low concentration (0.05 mol.% of Tm3+ ions). The radiative lifetime value calculated by Judd–Ofelt approach from absorption spectra of bulk 2S2G samples is obviously about 120 ls even if the discrepancy between the calculated value of 120 ls and that one experimentally recorded at low temperature (155 ls) is not well understood yet [22]. At best, the thin films lifetimes should be close to that of bulk glass of the same composition. As it is not the case and because the decays of thin films are not exponential, it can be assumed that their lifetimes are disturbed by impurities or defects in the films, eventually by the non-uniformity in the distribution of RE ions. Theoretically, for low concentration and low excited state pump power, the energy transfer among rare-earth ions in a host and the upconversion phenomenon might be negligible. Thus, the lifetime of the 3H4 excited state should be totally radiative if we consider only the multiphonon relaxation process since ten phonons at least are required to deplete 3H4 level to the next-lower level. Sputtered 2S2G films did not present a lot of defects like homopolar bonds as it was determined by Raman spectroscopy and, moreover, their structure seems to be similar to the bulk glass one [28]. In sulfide glasses, the quenching of fluorescence lifetime can find its origin in non-radiative energy transfer to impurities ([OH] or [SH] entities) [15,30–32]. In used glass targets, the concentration of OH and SH entities was estimated from fiber attenuation and bulk absorption, respectively. The concentration of [OH] groups has been estimated to be less than 1 ppm (extinction coefficient of the absorption band is e = 5.0 dB/m/ppm at k = 2.92 lm) while the concentration of [SH] was evaluated to be around 50 ppm (from the absorption band due to vibration of the SH bond located at k = 4.01 lm, where e = 2.3 dB/m/ppm) [33,34]. The content of impurities in targets is thus quite low and should not influence significantly the decay lifetime of bulk glasses; this fact can favor the high quantum efficiency of emission from the 3H4 level. On the other hand, in case of thin films, the content of SH or OH impurities cannot be evaluated precisely and could be different from the bulk glasses as thin films present a high surface interaction. This eventuality may explain the decrease of 3H4 lifetime and moreover the discrepancy of intensity ratio of I(3H5)/I(3H4) between the target and the thin

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50

3

F3,2

14

45

810 nm

3

H4

12

Energy (10 3 cm-1 )

40

2

cm )

35 3

-21

30

F2,3

σabs (10

25

β = 8%

2.3 µm β = 1% 3

8

3.8 µm β = 1%

6

1.8 µm

4

3

20

1.47 µm 10

H5

3

F4

1.2 µm

β = 99%

H4

3

2

H5

3

H6

0

3

Ga5Ge20Sb10S65

15

F4

10 1

5

ZBLAN

G4

0 400

600

800

1000

1200

1400

1600

1800

2000

Wavelength (nm) Fig. 4. Absorption cross-section of Tm3+ ion in Ga5Ge20Sb10S65 bulk glass compared with ZBLAN glass (Inset: Energy levels diagram of Tm3+ in 2S2G bulk glass with branching ratios calculated by Judd–Ofelt method).

1

0.1mol.% Tm for sputtered film 0.05 mol.% Tm for bulk glass

3+

2

0.1mol.% Tm 3+ 0.5 mol.% Tm

Intensity (a.u.)

Normalized intensity (a.u.)

substrate

Sputtered film 1.4μm

Sputtered film 1μm

0

1000

3

3

1200

1300

H5

bulk glass

1100

H6

3

3

1400

1500

0.01

F4

H4

1600

1700

Wavelength (nm)

1E-3 0.0

0.1

0.2

0.3

Time (ms)

Fig. 5. Photoluminescence spectra of Ga5Ge20Sb10S65:Tm3+ bulk glass and sputtered films of two different thicknesses (1000 and 1400 nm) on glass slide substrate.

Table 3 Lifetime of 3H4 level of Tm3+ ion in 2S2G sputtered films (Films 1, 2 and 3 are in asdeposited state with different thickness; Film 4 is annealed at 300 °C during 1 h). Sample

[Tm3+] mol.%

Thickness lm (±0.1 lm)

3

Film Film Film Film

0.5 0.5 0.1 0.1

4.5 2 2 2

27 28 47 43

1 2 3 4

0.1

H4 Lifetime ls (±3 ls)

films. The 3H5 level is probably more affected by impurity like SH (2495 cm1) as the DE (3H5 ? 3F4) = 2360 cm1 is not far to be perfectly resonant. Despite of the low concentration of RE ions within thin films, it is difficult to eliminate definitively the effect of RE ions clustering due to the deposition process. Reported cross-relaxation channels are involving several levels depending on excitation pumping scheme like for instance, c0 : (1G4,3H6) ? (3H4,3H5), c00 : (1G4,3H6) ? (3H5,3H4), c1/4 (3F2,3,3H6) ? (3F4,3H5), c2/4: (3F2,3,3F4) ? (3H4,3H5), c3/4: (3F2,3,3F4) ? (3H5,3H4), c4/4: (3F2,3,3H6) ? (3H5,3F4) or c

Fig. 6. Photoluminescence intensity decay of the 3H4 level of Tm3+ ions (0.1 and 0.5 mol.%) in 2S2G sputtered films.

(3H4,3H6) ? (3F4,3F4) [11,35]. Several cross-relaxations can have the 3H5 level as one of their final states and can not explain the difference in intensity ratio of I(3H5)/I(3H4) related to the decrease in population of that level for the thin films. The influence of Tm3+ concentration on 3H4 level lifetime was clearly observed (Fig. 6, Table 3) and likely related to the cross-relaxation phenomenon involving (3H4, 3H6 ? 3F4, 3F4) transitions for a pumping at 810 nm. For a better evaluation of ion-ion interaction between the thin film doped with 0.1 and 0.5 mol.% (6.0  1019 and 2.9  1020 ions/cm3, respectively), it could be interesting to determine the I(3H4)/I(3F4) intensity ratio. Nevertheless, the lifetime differences between the thulium doped thin films prepared by magnetron RF sputtering and the bulk glass is only about a factor 2 which is good result if we consider the lifetime of rare earth in non-annealed chalcogenide thin films [7]. Studies realized on evaporated or sputtered thin films showed that the annealing can have positive influence probably changing the environment around the rare earth ion [36,37]. Taking into account the results obtained by micro-thermal analysis, the films were annealed at temperatures near and above (about

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20 °C) the glass transition temperature under a controlled atmosphere. Unfortunately, the first annealing tests (300 and 320 °C) carried out in order to increase the lifetime of thulium in 2S2G thin film were inconclusive, particularly at 320 °C for which the luminescence became too weak for lifetime measurement; therefore, annealing treatments in these layers remain to be optimized.

[6]

[7]

4. Conclusion

[8]

We report that chalcogenide Tm3+ ions doped Ga5Ge20Sb10S65 films have been successfully deposited by magnetron RF sputtering method on different substrates at room temperature. The morphological and compositional characteristics of the films have been characterized and compared considering different deposition conditions as pressure, rf power, substrate-to-target distance or substrate bias. The composition is mainly influenced by the deposition pressure – with the pressure increase, sulfur and antimony content in sputtered films is growing – and by substrate bias voltage – a shortage in sulfur to the benefit of others elements is observed–. The layers appear to adhere better to silicon than to glass slides; the substrate bias can improve it to the detriment of the surface roughness. The thermal properties of the films (thermal conductivity and probe penetration temperature) were evaluated by micro-thermal probe by comparison with bulk glass counterpart. The thermal conductivity of the Ga5Ge20Sb10S65 and Ge20Sb10S65 bulk glasses was measured at (0.2 ± 0.1) W/(m K) while the values for the films with related composition to that of the glass were higher (Dk  0.2 W/(m K)). It is still difficult to separate the influence of the substrate and the deposition method. The probe penetration temperature of the Ga5Ge20Sb10S65 and Ge25Sb10S65 bulk glasses are higher than the glass transition temperatures obtained by the traditional DSC method with an almost constant variation of about 110–125 °C. As the probe penetration temperature of thin film seems to match well with the bulk glass, the as-deposited layers were annealed just below the glass transition temperature without changing the roughness or morphology of the thin films. The refractive indices of the deposited films (2.244 ± 0.001 at 1.5 lm) show a good agreement with bulk glasses. The near-IR photoluminescence of amorphous Tm3+ doped 2S2G films was experimentally observed at 1.47 lm (3H4 ? 3F4 transition) with high enough intensity enabling decay measurements. The lifetime of the upper level around 50 ± 3 ls being not far from the bulk glass value of about 120 ls was recorded at room temperature. The presence of impurities is suspected to induce this discrepancy.

[9] [10] [11]

[12]

[13] [14] [15]

[16] [17] [18] [19] [20]

[21] [22]

[23]

[24]

[25] [26]

[27] [28] [29]

Acknowledgements [30]

This work was supported by CNRS (France) and the Ministry of Education, Youth and Sports of the Czech Republic (project MSM 0021627501).

[31]

References

[33] [34]

[1] A. Zakery, S.R. Elliott, Optical properties and applications of chalcogenide glasses: a review, J. Non-Cryst. Solids 330 (2003) 1–12. [2] X.H. Zhang, B. Bureau, P. Lucas, C. Boussard-Pledel, J. Lucas, Glasses for seeing beyond visible, Chemistry-a European Journal 14 (2008) 432–442. [3] S. Madden, D.Y. Choi, M.R.E. Lamont, V. Ta’eed, N. Baker, M. Pelusi, B. LutherDavies, B.J. Eggleton, Chalcogenide glass photonic chip, OPN (2008) 19–23. [4] J.A. Frantz, L.B. Shaw, J.S. Sanghera, I.D. Aggarwal, Waveguide amplifiers in sputtered films of Er3+-doped gallium lanthanum sulfide glass, Opt. Express 14 (2006) 1797–1803. [5] A.K. Mairaj, C. Riziotis, A.M. Chardon, P.G.R. Smith, D.P. Shepherd, D.W. Hewak, Development of channel waveguide lasers in Nd3+-doped chalcogenide

[32]

[35]

[36]

[37]

(Ga:La:S) glass through photoinduced material modification, Appl. Phys. Lett. 81 (2002) 3708–3710. N. Ponnampalam, R.G. DeCorby, Out-of-plane coupling at mode cutoff in tapered hollow waveguides with omnidirectional reflector claddings, Opt. Express 16 (2008) 2894–2908. V. Nazabal, P. Nemec, A.M. Jurdyc, M.L. Anne, F. Charpentier, H. Lhermite, J. Charrier, J.P. Guin, A. Moreac, M. Frumar, J.L. Adam, Optical waveguide based on amorphous Er3+ doped Ga-Ge-Sb-S(Se) pulsed laser deposited thin films, Thin Solid Films 518 (2010) 4941–4947. S. Ramachandran, S.G. Bishop, Photoinduced integrated-optic devices in rapid thermally annealed chalcogenide glasses, IEEE J. Sel. Top. Quantum Electron. 11 (2005) 260–270. J. Heo, Emission and local structure of rare-earth ions in chalcogenide glasses, J. Non-Cryst. Solids 353 (2007) 1358–1363. J. Heo, Y.B. Shin, Absorption and mid-infrared emission spectroscopy of Dy3+ in Ge-as(or Ga)-S glasses, J. Non-Cryst. Solids 196 (1996) 162–167. T. Schweizer, B.N. Samson, J.R. Hector, W.S. Brocklesby, D.W. Hewak, D.N. Payne, Infrared emission and ion-ion interactions in thulium- and terbiumdoped gallium lanthanum sulfide glass, J. Opt. Soc. Am. B-Opt. Phys. 16 (1999) 308–316. J.S. Sanghera, L.B. Shaw, I.D. Aggarwal, Chalcogenide glass-fiber-based mid-IR sources and applications, IEEE J. Sel. Top. Quantum Electron. 15 (2009) 114– 119. M.F. Churbanov, I.V. Scripachev, V.S. Shiryaev, V.G. Plotnichenko, S.V. Smetanin, Y.N. Pyrkov, B.I. Galagan, J. Non-Cryst. Solids 326 (2003) 301–305. P. Sourkova, B. Frumarova, M. Frumar, P. Nemec, M. Kincl, V. Nazabal, V. Moizan, J.L. Doualan, R. Moncorge, J. Lumines. 129 (2009) 1148–1153. V. Moizan, V. Nazabal, J. Troles, P. Houizot, J.L. Adam, J.L. Doualan, R. Moncorge, F. Smektala, G. Gadret, S. Pitois, G. Canat, Er3+-doped GeGaSbS glasses for midIR fibre laser application: synthesis and rare earth spectroscopy, Opt. Mater. 31 (2008) 39–46. J.L. Adam, Lanthanides in non-oxide glasses, Chem. Rev. 102 (2002) 2461– 2476. L.A. Riseberg, M.J. Weber, (Eds.), Progress in Optics, E. Wolf, Amsterdam, North-Holland, 1976. C.B. Layne, W.H. Lowdermilk, M.J. Weber, Phys. Rev. B 16 (1977) 10. R.S. Quimby, B.G. Aitken, Multiphonon energy gap law in rare-earth doped chalcogenide glass, J. Non-Cryst. Solids 320 (2003) 100–112. V.G. Truong, B.S. Ham, A.M. Jurdyc, B. Jacquier, J. Leperson, V. Nazabal, J.L. Adam, Relaxation properties of rare-earth ions in sulfide glasses: Experiment and theory, Phys. Rev. B 74 (2006). Y. Miyajima, T. Komukai, T. Sugawa, Electron. Lett. 29 (1993) 660. V.G. Truong, A.M. Jurdyc, B. Jacquier, B.S. Ham, A.Q. Le Quang, J. Leperson, V. Nazabal, J.L. Adam, Optical properties of thulium-doped chalcogenide glasses and the uncertainty of the calculated radiative lifetimes using the Judd-Ofelt approach, J. Opt. Soc. Am. B-Opt. Phys. 23 (2006) 2588–2596. V. Nazabal, P. Neˇmec, J. Jedelsky´, C. Duverger, J. Le Person, J.L. Adam, M. Frumar, Dysprosium doped amorphous chalcogenide films prepared by pulsed laser deposition, Opt. Mater. 29, (2006) 273–278. Y. Guimond, J.L. Adam, A.M. Jurdyc, H.L. Ma, J. Mugnier, B. Jacquier, Optical properties of antimony-stabilised sulphide glasses doped with Dy3+ and Er3+ ions, J. Non-Cryst. Solids 257 (1999) 378–382. L.R. Doolittle, Nucl. Instrum. Meth. 344 (1985). S. Lefevre, S. Volz, J.-B. Saulnier, C. Fuentez, N. Trannoy, Thermal conductivity calibration for hot wire based DC scanning thermal microscopy, Rev. Sci. Instrum. 74 (2003) 2416–2423. H. Fischer, Thermochim. Acta 425 (2005) 69–74. M.L. Anne, V. Nazabal, V. Moizan, et al., Proc. SPIE 6475 (2007) 647508. J.H. Song, Y.G. Choi, K. Kadono, K. Fukumi, H. Kageyama, J. Heo, EXAFS investigation on the structural environment of Tm3+ in Ge-Ga-S-CsBr glasses, J. Non-Cryst. Solids 353 (2007) 1251–1254. D.R. Simons, A.J. Faber, H. Dewaal, GeSx glass for Pr3+-doped fiber amplifiers at 1.3 lm, J. Non-Cryst. Solids 185 (1995) 283-288. M. Naftaly, A. Jha, W.G. Jordan, 1.3 lm fluorescence quenching in Pr-doped glasses, J. Appl. Phys. 84, (1998) 1800–1804. M. Scheffler, J. Kirchhof, J. Kobelke, K. Schuster, A. Schwuchow, Increased rare earth solubility in As-S glasses, J. Non-Cryst. Solids 257 (1999) 59–62. G. Fonteneau, Mater. Res. Bull. 19 (1984) 685. M.F. Churbanov, V.S. Shiryaev, S.V. Smetanin, V.G. Pimenov, E.A. Zaitseva, E.B. Kryukova, V.G. Plotnichenko, Effect of oxygen impurity on the optical transmission of As2Se3.4 glass, Inorg. Mat. 37 (2001) 1188–1194. Y.B. Shin, W.Y. Cho, J. Heo, Multiphonon and cross relaxation phenomena in Ge-As(or Ga)-S glasses doped with Tm3+, J. Non-Cryst. Solids 208 (1996) 29–35. V. Nazabal, P. Camy, P. Nemec, H. Lhermite, J. Charrier, J.L. Doualan, S. Zhang, M. Frumar, J.L. Adam, Erbium-doped germanium-based sulphide optical waveguide amplifier for near- and mid-IR, Proc. SPIE 7366 (2009) 73661T. J. Fick, E.J. Knystautus, A. Villeneuve, F. Schiettekatte, S. Roorda, K.A. Richardson, High photoluminescence in erbium-doped chalcogenide thin films, J. Non-Cryst. Solids 272 (2000) 200–208.