Characterization of Y2SiO5:Ce thin films

Optical Materials 29 (2007) 1338–1343 www.elsevier.com/locate/optmat

Characterization of Y2SiO5:Ce thin films E. Coetsee a, H.C. Swart a

a,*

, J.J. Terblans a, O.M. Ntwaeaborwa a, K.T. Hillie W.A. Jordaan a,b, U. Buttner c

a,b

,

Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein ZA9300, South Africa b CSIR-NML, P.O. Box 395, Pretoria ZA0001, South Africa c Laser Research Institute, University of Stellenbosch, Stellenbosch, South Africa Received 9 May 2006; received in revised form 13 June 2006; accepted 29 June 2006 Available online 21 August 2006

Abstract Uncoated and SnO2-coated Y2SiO5:Ce thin film phosphors grown on Si (1 0 0) substrates by a pulsed laser deposition technique were characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-Ray analysis (EDS) and X-Ray diffraction (XRD). Cathodoluminescence (CL) of both the uncoated and SnO2-coated thin film phosphors was investigated for possible application in low voltage field emission displays (FEDs). Blue emission with peak values at 440 and 500 nm was from spherically shaped particles distributed unevenly on the surfaces of both the uncoated and coated thin film phosphors.  2006 Elsevier B.V. All rights reserved. Keywords: Y2SiO5:Ce; Thin films; Cathodoluminescence; SEM; AFM; EDS; XRD; PLD

1. Introduction The latest research studies on flat panel display (FPD) technology are aiming at improving luminescent efficiency of the phosphors used in field emission displays (FEDs) [1–3]. FEDs require higher efficiency at lower voltages (lower as 5 kV, in comparison with CRTs that require voltages between 20 and 30 kV). The lower voltages means FEDs operate with low energy electrons which have a shallower penetration depth for cathodoluminescence (CL). Higher current densities are required to maintain brightness and constant power, in the FEDs. However, the current density has been found to influence the degradation rate of CL intensity of traditional sulphide-based phosphors used in FEDs [4]. Compared to sulphide-based phosphors, oxide phosphors have been found to be more stable in high temperature, high pressure and high current densities needed for the FED environment [5,6].

*

Corresponding author. Tel.: +27 51 4012926; fax: +27 51 4013507. E-mail address: [email protected] (H.C. Swart).

0925-3467/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.06.009

Thin film phosphors have some advantages over powders in the FED environment, such as a reduction of light scattering and a good thermal contact between the screen and the faceplate [4,7]. Pulsed laser deposition (PLD) is a technique used to grow thin films with an important feature of maintaining the stoichiometry of the target material [4,7]. Surface morphology and thickness can be controlled by varying some of the growth parameters, such as the ambient gas pressure and the amount of pulses [7,8]. Shin et al. [9] investigated degradation of the CL intensity of ZnS:Mn phosphor coated with SnO2. Although the photo-luminescence (PL) emission intensities showed little change when SnO2 was coated on the surface, the CL emission intensity depends on the excitation energies. The degradation of the CL intensity of ZnS:Mn is consistent with a well known electron stimulated surface chemical reaction (ESSCR) [10]. Coating the surface of the phosphors is one possibility of decreasing the degradation rate. The coating should be thin enough to be transparent at low energies and it should not influence the chromaticity and brightness of the phosphor [11].

E. Coetsee et al. / Optical Materials 29 (2007) 1338–1343

Y2SiO5:Ce, is a blue emitting (double shoulder peak between 400 and 500 nm) rare earth phosphor. Light emission in rare earth phosphors is due to characteristic luminescence where electron hole pairs get created in the atom itself, emitting photons as they recombine. Ce3+ (trivalent cerium) has only one electron in the 4f shell. The 4f energy level splits into the 2F5/2 levels due to the electron having the ability to exhibit a + 1/2 or 1/2 spin [12]. Coetsee et al. [13] and Bosze et al. [12] reported on the cathodoluminescence of Y2SiO5:Ce powder phosphors and Zhang et. al. [5] reported on the photo-luminescence of Y2SiO5:Ce thin film phosphors with a light emission mechanism, due to the 5d ! 4f transition, resulting in the double shoulder peak between 400 nm and 550 nm. In this study, we report on the characterization of blue emitting Y2SiO5:Ce phosphor thin films prepared by the PLD technique. Results were compared with the same thin films coated with SnO2 through consecutive pulsed laser deposition method. Scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) were used to determine the surface morphology. Cathodoluminescence (CL) was used to investigate light emission from the thin films.

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substrate temperature was 400 C and the target to substrate distance was 3.7 cm. Rutherford backscattering (RBS) was used to measure the thickness of the thin films by using a 3.1 MeV 4He+ beam. SEM, AFM and EDS were used to monitor surface morphology and topography. SEM images were taken with the Gemini LEO 1525 Model at the CSIR – NML (Council for Scientific and Industrial Research – National Metrology Laboratory), Pretoria. AFM was done with the Digital Instruments Multi Mode with Nano Scope IV Controller and JV Scanner model and EDS was done with the Oxford 1525 model. The crystal structure of the thin films was determined with XRD by using a Siemens D5000 equipped with a Cu source. CL spectroscopy, excited by an electron beam with 2 keV energy electrons and a beam current density of 26 mA/cm2, was used to investigate the luminescence of the thin films. The CL measurements were made in an ultrahigh vacuum (UHV) chamber (base pressure of 4 · 109 Torr and backfilled with oxygen to a pressure of 1 · 106 Torr), with a PHI Model 549 system. Data were collected with a PC2000-UV Spectrometer using OOI Base32 computer software. 3. Results and discussion

2. Experimental Silicon (Si) (1 0 0) substrates were cleaned in Acetone for 5 min, in an ultrasonic water bath and then for another 5 min in methanol. The substrates were blown dry with nitrogen (N2) gas. Commercially available Y2SiO5:Ce standard phosphor powders from phosphor technology (UK) were pressed into a pellet and annealed at 600 C for about 16 h in air. The powder was heated to remove any water vapour and other gases that might be trapped in the pellet. The Lambda Physic 308 nm excimer XeCl laser was used to ablate the thin films. The laser energy was 81.81 mJ, repetition rate of 10 Hz, 6600 laser pulses were used to ablate the phosphor layer and 1200 pulses were used to ablate the SnO2 layer. The vacuum base pressure was 3 · 105 Torr before the system was backfilled with oxygen ambient gas to a pressure of 7.5 · 104 Torr, the

RBS results indicated that the coated thin films have a 58 nm thick SnO2 layer on the surface. It also reported a non-uniform layer which was shown by SEM, AFM and EDS to be the Y2SiO5:Ce phosphor layer consisting of spherical shaped particles not uniformly distributed. Fig. 1 shows the SEM images of the surface morphology for the uncoated (a) and coated (b) thin films. The surface is very rough with the particles not uniformly distributed that varied in micron sizes. Fig. 2 shows the AFM results done in contact mode. AFM results as shown in Fig. 2, (a three-dimensional image in which the colour intensity represents the altitude with dark for low and white for high.) indicated that the particle size distribution varies between 10 nm to micron size particles. The surface is not smooth, but covered with spherical particles. The 3D data or Z measurements provided by the AFM

Fig. 1. SEM images of the (a) uncoated and (b) coated phosphor thin films with a magnification of 1000·, 10 kV electrons, scale – 10 lm.

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Fig. 2. AFM results for the uncoated (a), (b) and for the coated (c), (d), thin films. Topographic images are shown in (a) and (c) and 3D images shown in (b) and (d). The 3D images have a z-scale of 4 lm per division.

is significant in this study since the phosphor films will be used in the high surface to volume FED environment. Fig. 3 shows the surface areas analyzed with EDS, the spectra for the uncoated and coated thin films are shown in Figs. 4 and 5 with the results listed in Table 1.

a

Si

X

Relative Intensity

Full scale 138 counts

C O 0

5

b Relative Intensity

Y

Fig. 3. SEM images of surface areas analyzed with EDS for the uncoated (a) magnification of 4270·, scale – 2 lm and coated (b) magnification of 22,500·, scale – 200 nm, thin films, 10 kV electrons.

O

Full scale 155 counts

10 keV

L

Y Si

C

0

5

10 keV

Fig. 4. EDS spectrums for the uncoated thin film on the marked areas (a) X and (b) L.

E. Coetsee et al. / Optical Materials 29 (2007) 1338–1343

a

O

c

T

Full scale 107 counts

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Y O

Sn

Full scale 100 counts

Y

Relative Intensity

Relative Intensity

Y Si

Sn Si

0

5

Sn Sn

10 keV 0

b

Full scale 180 counts

d

X

Si Full scale 140 counts

10 keV

L

Relative Intensity

Relative Intensity

Si

5

O C

O

Sn

0

5

10 keV

0

5

10 keV

Fig. 5. EDS spectrums for the coated thin film on the market areas (a) T, (b) X, (c) Y and (d) L.

Table 1 Listed EDS results for the uncoated and coated thin films Wt%

C

O

Si

Y

(a) Uncoated X L

41 21

22

59 8

49

31 14 28 12

2 41 9 88

(b) Coated T X Y L

10

37

Sn

67 35 26

Areas marked with X and L in Fig. 3a and T, Y, L, X in (b) are the areas analyzed with EDS. The electron beam used for the analysis (10 kV with a resolution of 5 nm), produced X-rays that escaped from a depth of 1–2 lm [14]. For the uncoated thin film, Fig. 3a, the area marked X resulted in the thin film surface containing 59% Si and 41% adventitious C, see also Fig. 4a. L, Fig. 4b, was measured on the spherical particle and the results proved it to be the Y2SiO5:Ce phosphor particle with 49% Y and 8% Si. (Note – It must be pointed out that the measured X-rays for EDS are also coming from layers below the surface (up to 2 lm), which is from deeper layers than the deposited layer itself. The concentrations are therefore only an indication of the elements present at the specific measured position.) The Ce concentration was too low to be measured. Figs. 4b and 5c shows an overlapping of the Si and Y peaks at about 1.75 keV. Fig. 4b also shows a high concentration of Y from the particle having a diameter of

about 4 lm. The area marked with T in Figs. 3b and 5a is agglomerated SnO2 nano-particles. The area marked with X, Figs. 3b and 5b, is the thin film surface consisting of 41% Si contribution due to the substrate, 10% adventitious C, 14% O and 35% Sn coated layer. Y, Figs. 3b and 5c, was measured on the spherical shaped particle and the results showed Si, Y and Sn, thus proving the particles to be the Y2SiO5:Ce phosphor particles coated with SnO2. The area marked with L, Figs. 3b and 5d resulted in a high concentration of Si, 88% and 12% O, which is the Si substrate. This is an area on the surface not completely covered with the SnO2 coating. XRD was done on both the uncoated and coated thin films to determine the crystal structure. Fig. 6a shows the XRD results for the uncoated and (b) for the coated thin films. Fig. 6b shows the [1 0 1] plane for tetragonal SnO2 and both figures (a) and (b) show the [0 2 0], [0 1 3], [2 2 2] and [5 1 4] planes of the monoclinic Y2SiO5 and the [4 0 0] plane for the Si substrate. Y2SiO5 is a good host material for CL phosphors. Two different monoclinic structures have been found in literature [15], a low temperature phase X1 and a high temperature phase X2. The X1 phase has the space group P21/c, whereas the space group B2/c is assigned to the X2 phase. Both X1 and X2 phases have two different Y+3 sites, the coordination numbers of which are 7 and 9 for the X1 phase and 6 and 7 for the X2 phase. The Y2SiO5 peaks as measured in the XRD spectrum are in agreement with the X1 phase. No evidence of the X2 phase was found in this study. Fig. 7 shows the

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E. Coetsee et al. / Optical Materials 29 (2007) 1338–1343 x 10 Y SiO 2 5 -2 2 2

a 60

Uncoated

40

20 10

1 0 0 Si

30

-5 1 4 Y SiO 2 5

0 2 0 Y SiO 5 013 2

Intensity (au)

50

0 0

20

30 40 2 theta

50

60

70

Coated

-2 2 2

Y SiO 2 5

x 10

1 0 0 Si

-5 1 4 Y SiO 2 5

1 0 1 SnO

20

2

40

0 2 0 Y SiO 5 013 2

Intensity (au)

b 60

10

0 0

10

20

30 40 2 theta

50

60

70

The CL intensity of the uncoated thin film was about 60% higher than for the coated thin film see Fig. 7. The lower CL intensity of the coated thin film is mainly due to the energy loss of primary electrons penetrating and moving through the SnO2 layer leaving the electrons with less energy to excite the phosphor particles. Scattering effects of the exited photons due to the SnO2 layer may also contribute to the CL intensity loss. A uniform layer covering the surface of the spherical particles results in a small critical angle for transmission and a large fraction of the light being totally internally reflected [19]. Both the coated and uncoated thin films were degraded in oxygen while exposed to an electron beam. The coated thin film resulted in a lower but constant CL intensity during the degradation process. This means that the SnO2 coating prevented the Y2SiO5:Ce to degrade during electron exposure. The degradation process will however be discussed in more detail in a next paper. Light emission from the spherical shaped phosphor particles as excited by the electron beam is more intense due to the fact that much lesser photons get totally internal reflected. Fig. 8 shows a schematic diagram of the photons with energy hv, excited by the electron beam in a spherical shaped particle compared to a uniform layer. The spherical shape of the particles results in more photons leaving the surface and contributing to the high CL intensity of the thin films.

Fig. 6. XRD results for the uncoated (a) and coated (b) thin films.

Electron beam

100 uncoated coated

CL Intensity (au)

80

hv

60 40 20 0 300

Spherical particle. 400

500

600 700 800 Wavelength (nm)

900

1000

Electron beam

Fig. 7. CL intensity against wavelength for both the uncoated and coated thin films.

hv

CL intensity against wavelength (nm) for both the uncoated and coated thin films. The CL spectrum is the characteristic double shoulder peak for the blue (between 400 and 550 nm) light emission from the Y2SiO5:Ce phosphor [5,12,16–18]. The light emission is due to the 5d ! 4f transitions, with the two maxima due to the splitting of the 2F term of the ground 4f1 configuration of the Ce3+ ion into two spin-orbit coupled levels, 2 F7/2 and 2F5/2.

Uniform layer Fig. 8. Schematic diagram of photons exciting a spherical particle compared to the total internal reflection effects in a uniform layer.

E. Coetsee et al. / Optical Materials 29 (2007) 1338–1343

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4. Conclusion Electron beam

hv

Y2SiO5:Ce Phosphor particles

Thin Film

Fig. 9. Schematic illustration of light emission from the phosphor thin films with spherical particles on the surface, as stimulated by the electron beam with a diameter of 220 lm.

The spherical outside curve of the particles results in more photons exiting the surface and contributing to the high CL intensity of the non-uniform thin films. Fig. 9 illustrates the light emission effect on the thin films. It must however be pointed out that the relative intensity for uniform thin films was however not compared to the intensities obtained in this study. Electrons get impinged on the thin film surface in an electron beam with a diameter of about 220 lm, which results in photon excitation from an area on the surface containing a whole group of the spherical particles that vary in size from a few nano-metres to micron size particles. The photons are scattered from the particles in all directions thus creating the bright blue light emission visible with the human eye. More particles will result in a higher intensity photon excitation. Chen [8] reported a chapter on the generation of particles by pulsed laser deposition. The nature of the particles, the energy, the size, the chemistry and microstructure all depend upon the type of material used as the target and the effects of the deposition parameters, e.g., the ambient gas pressure. The effect of inert ambient gas pressure on the nature of particles is most likely related to the increased collisions between the ejected species and the ambient gas as the ambient gas pressure increases. When a laser deposition experiment is done in vacuum, there are virtually no collisions between the ejected species before they reach the substrate. When the ambient gas pressure increases, the vapour species can undergo more collisions that nucleation and growth can occur to form particles before their arrival at the substrate. Since the growth mechanism is by diffusion, the residence time for the particles in the vapour controls the size of the particles. The longer the residence time, as is the case with increased ambient gas pressure, the larger the particle size [8]. It must be mention that some other parameters of growth such as temperature may also influence the evolution of the morphology of the films, not only the ambient gas pressure as mentioned above.

The Y2SiO5:Ce thin films were successfully grown onto the Si(1 0 0) substrates by using the PLD technique with the SnO2 also ablated onto some of the Y2SiO5:Ce phosphor thin films as a coated layer. Characterization (SEM, AFM, EDS and XRD), done on both the Y2SiO5:Ce thin films, uncoated and coated with SnO2, indicated a non-uniform phosphor layer on the Si surface consisting of spherical shaped particles randomly distributed. The coated thin films however resulted in a much lower CL intensity than the uncoated thin films with light emission from the spherical shaped phosphor particles due to a larger critical angle for total internal reflection. Acknowledgements Phosphor Technology LTD for phosphor samples. Dr. Chris Theron for RBS measurements at Ithemba labs, Cape Town, South Africa. Ina Claasens for AFM measurements at Mintek, Johannesburg, South Africa, the financial assistance of the University of the Free State and NML, CSIR, Pretoria. References [1] X.W. Sun, H.S. Kwok, Appl. Phys. A, Mat. Sci. Proc. 69 (1999) 39. [2] H.C. Swart, J.S. Sebastian, T.A. Trottier, S.L. Jones, P.H. Holloway, J. Vac. Sci. Technol. A 14 (3) (1996) 1697. [3] P.H. Holloway, J. Sebastian, T. Trottier, S. Jones, H.C. Swart, R.O. Peterson, Mat. Res. Soc. Symp. Proc. 424 (1997) 425. [4] K.T. Hillie, H.C. Swart, Appl. Surf. Sci. 183 (2001) 304. [5] Q.Y. Zhang, K. Pita, S. Buddhudu, C.H. Kam, J. Phys. D: Appl. Phys. 35 (2002) 3085. [6] M. Ollinger, V. Cracium, S. Nagore, M. Senna, R.K. Singh, ECS Lett. 9 (3) (2006) G80. [7] K.T. Hillie, C. Curren, H.C. Swart, Appl. Surf. Sci 177 (2001) 73. [8] Chen Li-Chyng, Particulates generated by pulsed laser ablation, in: D.B. Chrisey, G.K. Hulber (Eds.), Pulsed Laser Deposition of Thin Films, John Wiley & Sons, Inc, New York, 1994, p. 167. [9] S.H. Shin, J.H. Kang, D.Y. Jeon, D.S. Zang, J. Sol. State Chem. 178 (2005) 2205. [10] J.S. Sebastian, S. L Jones, T. Trottier, H. Swart, P. Holloway, J. SID 3 (4) (1995) 147. [11] J.M. Fitz-Gerald, T.A. Trottier, R.K. Singh, P.H. Holloway, Appl. Phys. Lett. 72 (1998) 1838. [12] E.J. Bosze, G.A. Hirata, J. McKittrick, in: Princ, Mat. Proc, Symp. Mat. Res. Soc. Proc., San Francisco, CA, USA, 558 (1999) 15. [13] E. Coetsee, H.C. Swart, J.J. Terblans, J. Lumin., in press. [14] EDS, [online]. Available from (accessed 24.03.2006). [15] J. Wang, S. Tian, G. Li, F. Liao, X. Jing, J. Electrochem. Soc. 148 (6) (2001) H61. [16] T. Aitasalo, J. Ho¨lsa¨, M. Lastusaari, J. Legendziewicz, J. Niittyykoski, F. Pelle´, Opt. Mater. 26 (2004) 107. [17] X. Quyang, A.H. Kitai, R. Siegele, Thin Solid Films 254 (1995) 268. [18] E.J. Bosze, G.A. Hirata, J. McKittrick, L.E. Shea, in: Mat. Res. Soc. Symp., San Francisco, CA, USA, 508 (1998) 269. [19] P.H. Holloway, T.A. Trottier, B. Abrams, C. Kondoleon, S.L. Jones, J.S. Sebastian, W.J. Thomas, J. Vac. Sci. Technol. B 17 (2) (1999) 758.