Fabrication of Er3+ doped oxyfluoride-silicate glass film by pulsed laser deposition for planar amplifier

Fabrication of Er3+ doped oxyfluoride-silicate glass film by pulsed laser deposition for planar amplifier

Journal of Non-Crystalline Solids 355 (2009) 1893–1896 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage:...

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Journal of Non-Crystalline Solids 355 (2009) 1893–1896

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Fabrication of Er3+ doped oxyfluoride-silicate glass film by pulsed laser deposition for planar amplifier S. Shen a, W.H. Chow b, D.P. Steenson b, A. Jha a,* a b

The Institute for Materials Research, University of Leeds, Leeds LS2 9JT, UK Electronic and Electricity Engineering Department, University of Leeds, Leeds LS2 9JT, UK

a r t i c l e

i n f o

Article history: Available online 13 August 2009 PACS: 71.23.Cq 78.55.Qr Keywords: Laser deposition Luminescence

a b s t r a c t Pulsed laser deposition (PLD) technique has been employed to fabricate glassy films based on the Er3+ doped oxyfluoride lead-silicate glasses. Glassy films have been produced at temperatures between 300 and 400 °C with O2 pressures ranging from 25 to 175 mTorr. The film microstructure appears to be determined mainly by substrate temperature. The film refractive index increases with higher temperatures and lower O2 pressures. The photoluminescence intensity of Er3+ ions and lifetime are very much dependent on the film microstructure and defects. Fewer fabrication defects and less porous films produce stronger fluorescence intensity and longer lifetime of Er3+ ions. At an optimized temperature and O2 pressure, the Er photoluminescence properties of the film can be reproduced and are very close to those of target glass. Consequently the work reported here suggested a good candidate for further investigation as a thin film material for EDWA. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The increasing demand for data, voice, and video-on-demand requires large bandwidth amplifiers for high-speed (10 Gb/s for metro) local, metro- and wide-area networks. Although the Er3+ doped fiber amplifier (EDFA) has revolutionised the optical networks in 1990s, its cost and functionality is quite limiting for a widespread adoption for the metropolitan area networks. The amplifiers for such applications have to be cost effective and multifunctional, which means integrated Er3+ doped waveguide amplifiers (EDWA) are increasingly important. Recently EDWAs have been developed using a number of fabrication routes such as sol– gel [1], flame hydrolysis deposition [2], radiofrequency–sputtering [3], ion-implantation [4] and pulsed laser deposition (PLD) [5]. Pulsed laser deposition is one of the most successful laser technologies which have contributed significantly to the development of new functional materials and devices [6]. In this technique a high intensity pulsed laser beam is employed to deliver the energy required for ablation of the target materials. The most significant advantage of this technique is to produce thin films without dramatically altering the stoichiometry of the deposited films in comparison with the bulk glass target. The PLD offers extremely high

* Corresponding author. E-mail address: [email protected] (A. Jha). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.04.054

deposition rates, as well as the ability to control the deposition rate at a sub-nanometer level during each pulse. It is an extremely versatile tool to grow glass thin films for active waveguides, and this is further enhanced with the ability to incorporate multiple targets used sequentially or simultaneously. Recently, we demonstrated that oxyfluoride-silicate glass is a highly promising material host for EDWA applications [7]. This glass combines the structural stability and compatibility of a silicate glass with the attractive spectroscopic properties of a fluoride glass. Glass thin film and optical waveguide based on this glass has been fabricated using PLD technique equipped with ArF laser at 193 nm [8]. In this paper PLD system with KrF excimer laser at 248 nm was employed to fabricate glassy film. As the silicate glass is almost transparent at 248 nm, PbO and PbF2 were added in the glass in order to shift the glass UV edge to a longer wavelength. A range of temperatures and oxygen pressures have been exploited in the PLD process of a glassy thin film based on an Er3+ doped oxyfluoride lead-silicate glass. The film microstructure was analyzed by scanning electron microscopy, and the film thickness, refractive index and loss were all characterized using the prism coupling technique. The fluorescence properties of Er3+ ions were compared in the glass films and the target. The deposition temperature and O2 pressure in the process are shown to be critical factors for determining the optical quality of films. Low loss glassy films were fabricated by optimizing the deposition conditions. The relationship between the film microstructure and photoluminescence of Er ions was also investigated.

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2. Experimental procedure

(a) 100

90

80

film 1 film 2 film 3

70

60 400

600

800

1000

1200

Wavelength nm

(b)

100

90

Transmission %

The Er doped oxyfluoride lead-silicate target glass is designed as SiPb: 54SiO2–20PbO–20PbF2–5NaF–1ErF3 (mol%). The glass target was prepared by melting in the temperature range of 1400– 1450 °C for 2 h in a N2 atmosphere then cast and annealed at 420 °C. The annealed sample was subsequently polished for PLD process and spectroscopic measurement. The glass target was ablated to deposit a thin film on the top of a silica glass slide using a KrF excimer laser (k = 248 nm, pulse length 20 ns). The laser beam was incident on the target at an angle of 45°, and the fluence was 7 J/cm2 with a pulse repetition rate of 5 Hz. The number of laser pulses was 90,000 for the films produced in our investigation.The distance between the substrate and target was 5.5 cm, and the substrate and target rotation rates were maintained at 10 rpm. The substrate temperature during the deposition process was maintained in the range of 300–400 °C. A dynamic flow of oxygen was used to maintain the chamber pressure of 25–175 mTorr to compensate for the eventual loss of the volatile oxygen elements during the ablation–deposition process. The morphology of the films was studied using a scanning electron microscopy (SEM), and the film thickness, refractive index and propagation loss measurements were carried out using a commercially available prism coupler (Metricon 2010 model). The Er3+ ion fluorescence spectra at 1.5 lm from the 4I13/2 ? 4I15/2 transition in both the films and ablation source target were characterized using a Fluometer F920 (Edinburgh Instrument). The lifetime decay was measured using a micro-second flash lamp as a pump source.

Transmission %

3+

80

film 4 film 5 film 6

70

3. Results 60

3.1. PLD film fabrication and transmittance spectrum measurement In the PLD process, the film thickness is dependent on the number of laser pulses, laser energy, and target to substrate distance. In this work, the PLD processing conditions were maintained at identical for each film deposition. A series of films were made with substrate temperatures 300, 350 and 400 °C and O2 partial pressure at 25, 75, 125 and 175 mTorr, respectively. The properties of films are summarized in Table 1. It is clear that no glassy state film can be deposited at 400 °C. When partial O2 pressure was reduced to 25 mTorr, the film was black 350 °C for deposition. Under such operating conditions, the color of the film changed from dark to light yellow and then to colorless when there was an increase in oxygen pressure. Six transparent films were chosen for measurements, and are labeled as film 1–6 in Table 1. The transmittance spectra of the film 1 to film 6 are shown in Fig. 1. It is obvious that all of the films have demonstrated strong interference patterns and the transmission range has increased with higher O2 partial pressure at both 300 and 350 °C. The films fabricated at 350 °C have more interference patterns than those films at 300 °C, which means the films have much higher density. The transmittance spectra of the films made at 400 °C are not displayed here due to their poor quality.

400

600

800

1000

1200

Wavelength nm Fig. 1. Transmittance spectra of PLD fabrication films: (a) films fabricated at 300 °C, (b) films fabricated at 350 °C.

used for all the measurements. Fig. 2 compares the prism coupling of the target glass and the six film samples. The film refractive index and thicknesses were calculated and are shown in Table 2. The instrument uses the coupling modes to determine both the thickness and refractive index for the measured films, as well as the coupling knee for the bulk target glass. The target glass refractive index is 1.7635, and it is quite clear that the refractive indices in most of the films are lower than the target glass except in film 4. With higher deposition temperature the film refractive index increased, and the index decreased with higher O2 partial pressure. All the film thicknesses are in the region of 1.8 lm, therefore the deposition rate is about 0.2 Å/pulse. By measuring the 1320 nm laser intensity decay in a typical coupling mode, the film propagation loss can be determined, and the loss analysed in all of the films is about 0.8–1.0 dB/cm.

3.2. Film thickness and refractive index measurement

3.3. SEM measurement of the films

The prism coupling technique was employed to characterize the film properties. The coupling source was a 1320 nm laser and was

The microstructure of the films 1–6 was by SEM. It is clear from Fig. 3 that at 300 °C, the film consists of many small particles with

Table 1 Fabrication conditions, label and transparency of PLD films. Oxygen/temperature (°C)

25 mTorr

75 mTorr

125 mTorr

175 mTorr

300 350 400

Light black Yellowish Black

Deep yellowish film 1 Very light yellowish film 4 Milky and rough

Light yellowish film 2 Colorless and almost transparent film 5 Milky and rough

Light yellowish film 3 Colorless and transparent film 6 Milky and rough

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2.0

Table 3 Metastable lifetimes of Er3+ ion at 1540 nm in the PLD films and the glass target.

film1

1.8

film2

1.6

film4

film3

Intensity

film5

1.4

Sample

Film 1

Film 2

Film 3

Film 4

Film 5

Film 6

Target

Lifetime s ± 10 ls

740

638

764

3242

7437

5928

7550

film6 SiPb glass

1.2 1.0 0.8 0.6 0.4 0.2 1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

Refractive index Fig. 2. Prism coupling measurement of PLD films and target at 1320 nm wavelength.

Table 2 Film thickness and refractive index results calculated from prism coupling measurements. Sample

Film 1

Film 2

Film 3

Film 4

Film 5

Film 6

Refractive index ± 0.001 Thickness ± 0.005 lm

1.732 1.773

1.703 1.767

1.675 1.825

1.783 1.784

1.754 1.882

1.725 1.823

sizes between 50 and 200 nm. The films formed at 350 °C are much denser with continuous structure than those at 300 °C, and consequently much less porous. O2 pressure has also played a significant role in the formation of the film microstructure at 350 °C, which demonstrated less porosity at lower O2 partial pressure. At 175 mTorr the film structure formed with continuous pores as shown in Fig. 3. 3.4. Er3+ ion Photoluminescence measurement in the PLD films and target glass The Er3+ ion photoluminescence spectra and intensity decay at 1.5 lm in the above PLD films and glass target are shown in Figs.

4 and 5, respectively. The lifetime results fitted from a single exponential function are shown in Table 3. It is quite clear that the films formed at 300 °C exhibit very weak PL and much shorter lifetimes comparing to films formed at 350 °C. Deposition temperature is a critical parameter of determining the Er3+ ion photo-luminescent properties in the PLD films. Notably the Er3+ ion lifetime in all of the films formed by PLD is shorter than that in the glass target SiPb. From the comparison of the results of photoluminescence decay the O2 partial pressure has more significantly affected the Er3+ ion lifetime in the films formed at 350 °C than those at 300 °C. It is also obvious that the fluorescence intensity decay in the films is not a single exponential, compared with a target glass. 4. Discussion 4.1. Film quality with PLD processing conditions From the measurements of film refractive index, thickness, and the analysis of SEM microstructure, the deposition temperature is clearly the most important factor for controlling the film quality. At too high a temperature, the film becomes milky with noticeable crystallization, and at too low a temperature the film is formed with particles and voids, which is not suitable for making good quality waveguides. At an optimum temperature such as 350 °C, the O2 partial pressure plays a significant role in determining the film quality. At too low a pressure such as at 25 mTorr, the film is reduced to a dark and opaque colour. By comparison at too high O2 pressure, the film became porous (film 6) which also compromised the quality. The optimum deposition parameter for producing a colorless and transparent glassy film is an oxygen partial pressure of 125 mTorr and substrate temperature of 350 °C (film5). It is interesting to note the difference of the refractive index in the film and the target glass. The index has increased with higher

Fig. 3. SEM measurement of the PLD films.

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90000 80000 film1 film2 film3 film4 film5 film6 SiPb target

Intensity

70000 60000 50000 40000 30000 20000 10000 0 1450

1500

1550

1600

1650

Wavelength nm Fig. 4. Er3+ ion photoluminescence spectra at 1.5 lm in the PLD films and the glass target.

0

e

film1 film2 film3 film4 film5 film6 SiPb target

Intensity decay

-1

e

longer lifetime have also demonstrated stronger PL intensity. From EDX and a Rutherford Backscattering Spectrometry (RBS) measurement in our previous work [9], the films made from the PLD process has maintained the glass stoichiometry compositions of the target glass. From the prism coupling measurement, all of the PLD films have shown to have a similar film thickness. Therefore we can assume that the Er3+ ion concentration is very close in all of the films. PL quenching is usually due to Er3+ ion concentration quenching, energy transfer, OH concentration, and structural defects. Therefore in this case the primary reason for the PL change in the above PLD films is from the glass film microstructure, which has many small particles and porous structure defects. These structural defects are known to produce significant quenching effects on Er3+ ion fluorescence. The non-single exponential decay of PL at 1.5 lm in the PLD films has provided clear evidence for these defects. Comparing the Er3+ ion lifetime in the films with that of the target glass, the lifetime in all of the films is shorter than that in the target glass, which is understandable due to the fabrication defects in the films. The higher the porosity and particulates present in the films, the stronger is the quenching effect. Film 5 has the longest lifetime amongst all the films characterised herein and is very close to the target lifetime. On the basis of comparison of PL data and SEM results, we conclude that the film 5 identifies the optimum condition for deposition. 5. Conclusions

-2

e

-3

e

0

5000

10000

15000

20000

Time µs Fig. 5. Er3+ ion photoluminescence intensity decay at 1540 nm in the PLD films and the glass target.

deposition temperature, which means the film formed with higher density, as has been confirmed from the SEM result and film transmittance spectra. The refractive index has decreased with higher O2 pressure, which can be explained by the presence of more O2 rich defects in the film structure. The refractive index in film 5 is the closest to that of the target, which means the composition in this film is very close to the target glass. The film 5 has relatively the least structural and physical defects among all of the PLD films. 4.2. Discussion of Er3+ ion photoluminescence property in the PLD films For making Er3+ doped waveguide amplifier, Er3+ ion photoluminescence (PL) at 1.5 lm from the 4I13/2 ? 4I15/2 transition is a critical property in the glassy film. Strong emission intensity and a long lifetime at 1.5 lm are required. In the PLD films discussed above, the Er3+ ion PL is also significantly changed by deposition temperature and O2 partial pressure, which are shown in Figs. 4 and 5 and Table 3. It is clear that the PL in the films formed at 350 °C is much stronger and the lifetime is much longer (about 5–10 times) than that of the films formed at 300 °C. Films with

The PLD deposition temperatures and O2 partial pressures are found to be the most important parameters in the formation of high quality of glassy films. Film microstructure is mainly determined by the deposition temperature. The refractive index of film increased with higher temperature and lower O2 pressure. Er3+ ion photoluminescence intensity and lifetime in the films are very much dependent on the film microstructure and fabrication defects. Fewer defects and a less porous film give stronger fluorescence intensity and longer lifetime of excited Er3+ ions. At an optimized temperature and O2 partial pressure, the Er3+ ion photoluminescence properties in the films can be controlled to be very close to that of the target glass. Acknowledgement The authors would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for the financial support. References [1] W. Huang, R.R.A. Syms, E.M. Yeatman, M.M. Ahmad, V.T. Clapp, S.M. Ojha, IEEE Photonic Technol. Lett. 14 (2002) 959. [2] K. Hattori, T. Kitagawa, M. Oguma, M. Wada, J. Temmyo, M. Horiguchi, Electron. Lett. 29 (1993) 357. [3] J. Shmulovich, A. Wong, Y.H. Wong, P.C. Becker, A.J. Bruce, Electron. Lett. 28 (1992) 1181. [4] D. Barbier, M. Rattay, F. Saint Andre, G. Clauss, M. Trouillon, A. Kevorkian, J.M.P. Delavaux, E. Murphy, IEEE Photonic Technol. Lett. 9 (1997) 315. [5] M. Martino, A.P. Caricato, M. Fernandez, G. Leggieri, A. Jha, M. Ferrari, M. Mattarelli, Thin Solid Films 433 (2003) 39. [6] J.J. Dubowski, RIKEN Rev. 32 (2001) 47. [7] S. Shen, A. Jha, Opt. Mater. 25 (2004) 321. [8] R.R. Thomson, H.T. Bookey, A.K. Kar, M.R. Taghizadeh, A. Klini, C. Fotakis, F. Romano, A.P. Caricato, M. Martino, S. Shen, A. Jha, Electron. Lett. 41 (2005) 1376. [9] A.P. Caricato, A. Fazzi, A. Jha, A. Kar, G. Leggieri, A. Luches, M. Martino, F. Romano, S. Shen, M. Taghizadeh, R. Thomson, T. Tunno, Opt. Mater. 29 (2007) 1166.