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Luminescent properties of pulsed laser deposition (PLD) thin films of SrGa2S4:Ce3+
Physica B 439 (2014) 144–148
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescent properties of pulsed laser deposition (PLD) thin films of SrGa2S4:Ce3 þ P.A. Moleme, H.C. Swart n, J.J. Terblans, O.M. Ntwaeaborwa Department of Physics, University of the Free State, P.O Box 339, Bloemfontein ZA9300, South Africa
art ic l e i nf o
a b s t r a c t
Available online 1 December 2013
The influence of different substrate temperatures on the morphological and optical properties of SrGa2S4: Ce3 þ thin films prepared by the pulsed reactive cross laser ablation (PRCLA) technique has been investigated for application in flat panel displays. Auger electron spectroscopy (AES) depth profile analyses on the films were performed in order to measure the stoichiometric distributions of the deposited material as a function of depth. A depletion of S was obtained for the 600 1C substrate temperature sample. AES coupled with a cathodoluminescence (CL) spectroscopy system and X-ray photoelectron spectroscopy (XPS) was employed to measure the CL intensity and the CL intensity degradation as well as the surface chemical changes during and after exposure to 2 keV prolonged electron irradiation. The effect of different pressures on the degradation characteristics of the SrGa2S4: Ce3 þ thin films ablated at 400 1C and 600 1C substrate temperatures was also investigated. The rate of degradation was observed to be slower in vacuum compared to oxygen. CL emission of Ce3 þ was observed for the 400 1C sample and both the emissions of the 600 1C sample showed a red shift of about 32–33 nm, due to a change in the chemical composition and therefore a change in the crystal field. XPS results obtained for the film prepared at 600 1C substrate temperature suggest that a change in elemental bonding occurred during the degradation process. & 2013 Published by Elsevier B.V.
Keywords: SrGa2S4:Ce3 þ FEDs PRCLA CL intensity Electron beam degradation
1. Introduction Most of field emission display (FED) phosphors are powders and it would be advantageous that the phosphor particle size is small due to the fact that it is critical to achieve lower screen loading, high screen resolution and improved phosphor density of the display screen [1,2]. These features are not realized in the conventional route of preparing FED powders due to large grains with irregular morphology. Therefore, a wide variety of synthesis techniques such as combustion, spray pyrolysis and sol–gel have been introduced for preparation of both nano- and sub-micron FED phosphors [3]. It is strongly believed that nanoparticles phosphors could be suitable for low voltage FED applications based on the above reasons. Hence, the use of thin films is considered especially for small displays because thin films are known for better thermal stability, higher lateral resolutions, and better adhesion to the surface as well as less outgassing which makes them ideal for FED device operations [4,5]. Most research has been devoted to the development of such thin film phosphors and high expectations are being placed on the Ce3 þ doped SrGa2S4 because of its good chromaticity, stability, high luminance at low voltage and high current density excitations. The alkaline earth thiogallates, especially of the formula MGa2S4 (M¼ Ca, Sr and Ba), have been
studied as hosts for luminescent activators/dopants such as Ce3þ and Eu2 þ . Upon doping, these thiogallates exhibit high quantum efficiencies, less thermal quenching and greater luminance than other alkaline earth thiogallates of different compositions [6]. Hence Ceand Eu-doped SrGa2S4 have been widely studied as possible blue and green phosphor materials for future FED technology [7–10]. In particular, Ce doped SrGa2S4 as the most promising blue phosphor in the thiogallate family has been investigated for application in full colour FED. In the early stages of the research, most fabrications of the SrGa2S4:Ce3 þ phosphor was in the powder form. Later, thin films were prepared due to better heat sinking properties and lower outgassing rates. However, thin films become less efficient because of lower optical scattering and increased light piping usually observed in them [11]. Several different deposition techniques have been exploited for preparing SrGa2S4:Ce3 þ thin films. In most cases the films exhibit amorphous or poor crystallinity, thus requiring postheating treatment. In the present work, SrGa2S4:Ce3þ thin films were prepared by the pulsed reactive cross laser ablation (PRCLA) technique. The morphology, stoichiometric composition and CL intensity degradation of the SrGa2S4:Ce3 þ thin films are presented
2. Experimental details n
Corresponding author. Tel.: þ 27 514012926; fax: þ27 514013507. E-mail address: [email protected] (H.C. Swart).
0921-4526/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physb.2013.11.048
Silicon (Si) (1 0 0) substrates were first chemically cleaned. A pellet with a 2.4 cm diameter and 6 mm thickness was prepared
P.A. Moleme et al. / Physica B 439 (2014) 144–148
by pressing the SrGa2S4:Ce3 þ powder for 1 h at a pressure of 1.96 107 mbar. It was then annealed for 6 h at 600 1C in vacuum to improve its hardness and was then mounted on a rotating holder lying diagonally across the heater on which Si substrates were mounted. The distance between the target and the substrates was maintained at 4 cm during the deposition of each film. The Lambda Physik EMG 203 MSC 308 nm XeCl excimer laser was used to ablate the target. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 10 5 mbar before backfilling to a pressure of 1.0 10 2 mbar Ar, where Ar was used as pulse gas. The films were deposited at different substrate temperatures ranging from 400 1C to 600 1C using 28,800 pulses. The laser beam was operated at an 8 Hz repetitive rate. The films were prepared using the PRCLA technique in which a synchronized gas pulse crosses the ablation plume close to its origin, increasing the gas phase interaction and the probability of reactive scattering, and also allowing the resulting species to propagate freely away from the localised scattering region thus, giving much brighter plume [12]. Characterisation of the films was carried out with a PHI 700 Auger Nanoprobe unit to obtain SEM micrographs. Images were captured with a 25 kV, 10 nA electron beam and the Auger depth profile analysis was performed by argon (Ar þ ) sputtering with a 2 kV ion beam, current of 2 mA, 1 1 mm2 raster area, and sputter rate of 27 nm/min. The CL intensity degradation of SrGa2S4:Ce3 þ thin films was investigated using Auger electron spectroscopy (AES) coupled with a CL spectrometer. A PHI (model 549) Auger spectrometer and an S2000 Ocean Optics spectrometer were simultaneously used to collect the Auger and CL data respectively, through which surface elemental (chemical) changes were monitored with AES and the light output was measured with a PC2000-UV spectrometer. The primary electron beam current was typically 12 mA. The Auger and CL data were collected in a vacuum chamber with a base pressure in the 9 10 9 mbar range for the films prepared at Tsubstrate ¼400 1C and 600 1C respectively. Afterwards the chamber was backfilled with oxygen to 1.3 10 6 mbar. Throughout the experiment, the Auger and CL data were collected using the same primary electron beam of 2 keV. The decrease of the CL intensities during prolonged electron bombardment of both films was monitored continuously for a period of 6 h at different pressures. The monitored peaks were 442 nm and 485 nm for the film prepared at a substrate temperature of 400 1C and 475 nm and 516 nm for the 600 1C substrate temperature deposited films. The X-ray photoelectron spectroscopy (XPS) data were collected before and after degradation to evaluate the chemical composition and electronic states of the different elements. The data were collected using the PHI 5000 Versa probe-Scanning ESCA microprobe. A low energy Ar þ ion gun and a low energy neutraliser electron gun were used to minimise charging on the surface. Monochromatic Al Kα radiation (hυ¼1486.6 eV) was used as the excitation source. A 25 W, 15 kV electron beam was used to excite the X-ray beam of 100 mm diameter that was used to analyse the Sr 3d, O 1s, Ga 2p and S 2p binding energy peaks (pass energy 11 eV, analyser resolution r0.5 eV). Multipak version 8.2 software [13] was used to analyse the chemical elements and their electronic states using the Gaussian–Lorentz fits.
3. Results and discussions Shown in Fig. 1 are the XRD patterns and the Miller indices of the SrGa2S4:Ce3 þ films prepared at different substrate temperatures ranging from 400 to 600 1C. A SrGa2S4 layer was observed at the growth temperature of 400 1C. From the comparison with the standard powder pattern of SrGa2S4 (JCPDS file no. 77-1189) all
145
Fig. 1. The diffraction patterns of the SrGa2S4:Ce3 þ films deposited at different substrate temperatures.
peaks as indicated in Fig. 1(a) were found to belong to the orthorhombic SrGa2S4 crystal structure. Note that the peaks labelled x were the only peaks that did not match the diffraction of SrGa2S4. A preferential growth along the orientation (0 6 2) was observed when the substrate temperature was increased to 500 1C and 600 1C. In addition, small signatures of the (3 9 1) and (3 7 5) peaks which also belong to the orthorhombic SrGa2S4 crystal structure were detected. Similar results as the once observed at 400 1C and 500 1C substrate temperatures were reported by Heikkinen et al. [14]. The XRD patterns in Fig. 1 show that the film growth is sensitive to the substrate temperature. The intensity of the (0 6 2) peak was observed to increase with an increase in the substrate temperature from 400 1C to 600 1C. These results also indicate that crystallinity can be achieved even without postdeposition annealing. Tanaka et al. [15] investigated the crystallinity of the SrGa2S4:Ce3 þ thin films grown on quartz glass substrates with the MBE technique. The films were grown at substrate temperatures of 400–600 1C. They reported a gradual decrease in the XRD peak intensity at higher temperatures than 600 1C and lower temperatures than 500 1C and found the best substrate temperature for SrGa2S4 film growth to be around 560 1C. Fig. 2 shows SEM images of the surfaces of the SrGa2S4:Ce3 þ thin films ablated in an Ar environment at different substrate temperatures ranging from 400 1C to 600 1C. The field of view of the images is 10 mm. The SEM images of all the films exhibited surfaces with non-uniformly distributed spherical particles and clearly visible cracks. Zhao et al. [16] reported similar results when examining thin films cracks on Pt/Ti/Si (1 0 0) thin films after annealing at 650 1C. They pointed out that when the film thickness was larger than the critical thickness (0.78 mm in their case), the crack density, which is defined as the reciprocal of the crack spacing, increased rapidly with an increasing film thickness. Increase in substrate temperature to 600 1C, resulted in a more rough film surface with bigger crack openings and an increased number of particles of not well defined boundaries. Christoulaskis et al. [17] noted that films grain boundaries gain mobility at high substrate temperatures. This movement of grain boundaries has influence on grain growth as well as recrystallisation and surface diffusion. Fig. 3(a) and (b) shows AES depth profile analysis of the films deposited at substrate temperatures of 500 1C and 600 1C, respectively. The main elements, Sr, Ga and S were present in all depth
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Fig. 2. SEM images of SrGa2S4:Ce3 þ thin films deposited in an Ar environment at (a) 400 1C, (b) 500 1C and (c) 600 1C.
Fig. 3. AES depth profiles of SrGa2S4:Ce3 þ thin films deposited in an Ar environment at (a) 500 1C and (b) 600 1C substrate temperatures.
profiles performed on the films, with the detection of the presence of atmospheric C and O as well as Si from the substrate. The activator (Ce3 þ ) could not be detected due to the low concentration thereof and the limitation in the sensitivity of AES. It must be pointed out that the S concentration in the 600 1C film is much lower than in the rest of the films as seen in Fig. 3(a) and (b), if compared with each other. There is also a positive gradient in the S concentration moving from the surface of the layer towards the interface of both the thin films, indicating that some of the S atoms were lost during the deposition process, at these substrate temperatures. The concentration of Ga was also observed to be low in both films. This behaviour might be attributed to the sticking coefficient of Ga which is below unity for the growth substrate temperature in the range of 500–600 1C and the very low melting point of Ga (29.76 1C). The large oxygen concentration suggests the formation of oxides during the ablation process of the SrGa2S4:Ce3 þ thin films and/or that the obtained particles were all oxidised at the outside of the particles. The depth profiles were measured with an electron spot size larger than the average particle sizes in the thin films. The outside of these particles will therefore also contribute to the obtained elemental concentration of the thin films. It is however, clear that the 600 1C contain less S atoms in the thin film. The CL intensities as a function of wavelength spectra for measurements taken before and after degradation at a pressure of 1.3 10 6 mbar O2 and substrate temperatures as indicated are shown in Fig. 4. The broad emission peaks around 443 nm and 485 nm for the film deposited at 400 1C and peaks around 475 nm and 518 nm for the 600 1C ablated film are all attributed to Ce3 þ transitions. That is, the two well-known emission peaks associated with the 5d (T2g)-4f (2F5/2) and from the 5d (T2g)-4f (2F7/2) radiative transitions of Ce3 þ [18]. A clear red-shift of about 32 nm
Fig. 4. CL intensity as a function of wavelength spectra of the SrGa2S4:Ce3 þ film deposited at 400 1C and 600 1C substrate temperatures for the degradation performed at 1.3 10 6 mbar O2.
in the emission peaks was observed for the film deposited at the higher substrate temperature. The Ce3 þ emission in SrGa2S4 lies at a shorter wavelength than for example SrS [19]. The splitting of the energy levels (4f and 5d) (crystal field splitting) occurs because of the interactions of the ions and atoms onto each other in the crystal field. This effect differs from crystal field to crystal field, depending on the type of materials and crystal structures. The fact that a depletion of S was measured for the 600 1C sample clearly indicates a difference in chemical environment (different crystal fields) for the two films, which were responsible for the red shift. A significant decrease in the CL intensity was observed during the electron degradation study of both films. The intensity dropped to
P.A. Moleme et al. / Physica B 439 (2014) 144–148
147
Fig. 5. Normalised CL intensity as a function of electron dose of the SrGa2S4:Ce3 þ thin films degraded at an oxygen pressure of (a) 1.3 10 6 mbar and base pressure of (b) 9 10 9 mbar.
Fig. 6. XPS deconvoluted scans (high resolutions) and fitting results for regions before (a) and (c) and after degradation (b) and (d) of the 600 1C SrGa2S4:Ce3 þ phosphor thin film with degradation carried out at 1.3 10 6 mbar O2.
less than 30% and 20% of the original intensity due to the prolonged electron bombardment of the respective 400 and 600 1C thin films. The effects of different pressures on the degradation characteristics of the films deposited at 400 1C and 600 1C substrate temperatures are shown in Fig. 5. The results show a rapid decrease of the CL intensity for all the degradation studies. In general, the rate of CL intensity degradation was faster when the chamber was backfilled with oxygen. A faster degradation for the 600 1C sample was observed for both pressures. Swart and Holloway [1,5,20,22] have shown that degradation of phosphors are
normally due to electron stimulated surface chemical reactions (ESSCR). It was shown that the electron beam stimulates the molecular gas species into atomic species, which react with phosphor to form other non-luminescent layers with the consequent loss of CL intensity. The CL degradation cannot always be explained by the formation of the non-luminescent layers alone and a defect theory has been proposed to reconcile the difference [20–22]. The degraded spot was clearly visible on the thin film samples. XPS survey spectra (not shown) for the degraded spot and the undegraded part of the film prepared at 600 1C confirmed the
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presence of the major elements and changes in the XPS peak positions and shapes. Fig. 6(a) and (b) shows the Sr 3d fitted peaks for the undegraded and degraded spot for the Sr 3d5/2 and 3d3/2 peaks. It is clear that the peak shape has changed due to the electron degradation process and that extra chemical species have formed on the surface. The peaks correspond to the Sr–S (132.5 and 134.3 eV) and Sr–Sr (134.3 and 135.6 eV) bonds in the modified SrGa2S4 for the undegraded spot. The origin of the metallic bond is described elsewhere [23]. Additional peaks on both sides of the Sr–S peaks were detected on the degraded spot which might signify the formation of Sr–O and Sr–S–O containing species, and were identified as SrO and SrSO4 peaks as shown in Fig. 6(b). The formation of SrSO4 was confirmed with the increase in the S 2p peak (Fig. 6(c) and (d)) at 168.8 (2p3/2) and 170.0 eV (2p1/2) associated with SrSO4. The formation of Ga2O3 species was also confirmed with Ga 2p and O 1s (not shown) peaks. It is therefore clear that the ESSCR was also responsible for the degradation in the CL intensity as in the case of other phosphors. 4. Conclusion Commercial SrGa2S4:Ce3 þ was deposited on the Si (1 0 0) wafers by the PRCLA technique. The films were prepared at different substrate temperatures. SEM images of all the films indicate that the film consists of non-uniform spherical particles with cracks in the films. Depth profiles performed showed the presence of all the major elements with atmospheric O and C. A larger O signal as expected was observed. Depletion of S occurred at 600 1C. The CL degradation under prolonged electron irradiation occurred due to the formation of SrSO4, SrO and Ga2O3 species on the surface due to ESSCR. The degradation rate for phosphor is fast and therefore phosphor will not be suitable for FED applications.
Acknowledgements Authors are thankful to the University of the Free State and the South African National Research Foundation (NRF) for the financial
support. We also thank the University of Stellenbosch – Physics Department for the PLD system used to prepare the thin films. References [1] P.H. Holloway, S.L. Jones, T.A. Trottier, J.S. Sebastian, B. Abrams, C. Kondoleon, W.J. Thomes, H.C. Swart, J. Vac. Sci. Technol. B 17 (1999) 758. [2] G. Wakefield, E. Holland, P.J. Dobson, J.L. Hutchison, Adv. Mater. 13 (2001) 1557. [3] H.L. Li, Z.L. Wang and J.H. Hao, IOP Conf. Ser.: Mater. Sci. Eng. 1 2009, 012010. [4] A. Vecht, C. Gibboson, D. Davies, X. Jing, P. Marsh, T. Ireland, J. Silver, A. Newport, D. Barber, J. Vac. Sci. Technol. B 17 (2) (1999) 750. [5] K.T. Hillie, H.C. Swart, Appl. Surf. Sci. 183 (3–4) (2001) 304. [6] J.T. Anderson, Ph.D. dissertation, Oregon State University, ProQuest Information and Learning Company, 2008. [7] F.E. Zhang, S. Yang, . Stoffers, J. Penczek, P.N. Yocom, D. Zaremba, B.K. Wagner, C.J. Summers, Appl. Phys. Lett. 72 (1998) 2226. [8] O.N. Dyazorski, T. Mikami, K. Ohmi, S. Tanaka, H. Kobayashi, J. Electrochem. Soc. 146 (1999) 1215. [9] P.D. Rack, T.A. O’Brien, M.C. Zerner, P.H. Holloway, J. Appl. Phys. 86 (1999) 2377. [10] P. Benalloul, C. Barthou, J. Benoit, L. Eichenauer, A. Zeinert, Appl. Phys. Lett. 63 (1993) 1954. [11] W. Zhu, Vacuum Microelectronics, Bell Lab. – Lucent Technologies, NJ, USA, 2001. [12] Paul Scherrer Institute, Material Group, 5232 Villigen PSI, Schweiz. 〈http:// material.web.psi.ch/Research/Thin_Films/Methods/PLD.htm〉. [13] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI, Inc., 370 Enzo, Chigasaki 253-8522, Japan, 1995. [14] H. Heikkinen, L.S. Johansson, E. Nykänen, L. Niinistö, Appl. Surf. Sci. 133 (1998) 205. [15] K. Tanaka, Y. Inoue, S. Okamoto, K. Kobayashi, J. Cryst. Growth 150 (1995) 1211. [16] M.-H. Zhao, R. Fu, D. Lu, T.-Y. Zhang, Acta Mater. 50 (2002) 4241. [17] S. Christoulakis, M. Suchea, N. Katsarakis, E. Koudoumas, Appl. Surf. Sci. 253 (2007) 8169. [18] E. Coetsee, J.J. Terblans, O.M. Ntwaeaborwa, H.C. Swart, Phys. B: Phys. Condens. Matter 404 (22) (2009) 4426. [19] K. Tanaka, Y. Inoue, S. Okamoto, K. Kobayashi, K. Takizawa, Jpn. J. Appl. Phys 36 (1997) 3517. [20] H.C. Swart, J.S. Sebastian, T.A. Trottier, S.L. Jones, P.H. Holloway, J. Vac. Sci. Technol. A 14 (3) (1996) 1697. [21] H.C. Swart, L. Oosthuizen, P.H. Holloway, G.L.P. Berning, Surf. Interface Anal. 26 (5) (1998) 337. [22] Paul H. Holloway, O Martin Ntwaeaborwa, Hendrik C. Swart, J. Vac. Sci. Technol. A 31 (5) (2013), 050808-1, Article number (050808). [23] P.A. Moleme, S.S. Pitale, H.C. Swart, O.M. Ntwaeaborwa, Phys. B: Phys. Condens. Matter 407 (2012) 1645.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescent properties of pulsed laser deposition (PLD) thin films of SrGa2S4:Ce3 þ P.A. Moleme, H.C. Swart n, J.J. Terblans, O.M. Ntwaeaborwa Department of Physics, University of the Free State, P.O Box 339, Bloemfontein ZA9300, South Africa
art ic l e i nf o
a b s t r a c t
Available online 1 December 2013
The influence of different substrate temperatures on the morphological and optical properties of SrGa2S4: Ce3 þ thin films prepared by the pulsed reactive cross laser ablation (PRCLA) technique has been investigated for application in flat panel displays. Auger electron spectroscopy (AES) depth profile analyses on the films were performed in order to measure the stoichiometric distributions of the deposited material as a function of depth. A depletion of S was obtained for the 600 1C substrate temperature sample. AES coupled with a cathodoluminescence (CL) spectroscopy system and X-ray photoelectron spectroscopy (XPS) was employed to measure the CL intensity and the CL intensity degradation as well as the surface chemical changes during and after exposure to 2 keV prolonged electron irradiation. The effect of different pressures on the degradation characteristics of the SrGa2S4: Ce3 þ thin films ablated at 400 1C and 600 1C substrate temperatures was also investigated. The rate of degradation was observed to be slower in vacuum compared to oxygen. CL emission of Ce3 þ was observed for the 400 1C sample and both the emissions of the 600 1C sample showed a red shift of about 32–33 nm, due to a change in the chemical composition and therefore a change in the crystal field. XPS results obtained for the film prepared at 600 1C substrate temperature suggest that a change in elemental bonding occurred during the degradation process. & 2013 Published by Elsevier B.V.
Keywords: SrGa2S4:Ce3 þ FEDs PRCLA CL intensity Electron beam degradation
1. Introduction Most of field emission display (FED) phosphors are powders and it would be advantageous that the phosphor particle size is small due to the fact that it is critical to achieve lower screen loading, high screen resolution and improved phosphor density of the display screen [1,2]. These features are not realized in the conventional route of preparing FED powders due to large grains with irregular morphology. Therefore, a wide variety of synthesis techniques such as combustion, spray pyrolysis and sol–gel have been introduced for preparation of both nano- and sub-micron FED phosphors [3]. It is strongly believed that nanoparticles phosphors could be suitable for low voltage FED applications based on the above reasons. Hence, the use of thin films is considered especially for small displays because thin films are known for better thermal stability, higher lateral resolutions, and better adhesion to the surface as well as less outgassing which makes them ideal for FED device operations [4,5]. Most research has been devoted to the development of such thin film phosphors and high expectations are being placed on the Ce3 þ doped SrGa2S4 because of its good chromaticity, stability, high luminance at low voltage and high current density excitations. The alkaline earth thiogallates, especially of the formula MGa2S4 (M¼ Ca, Sr and Ba), have been
studied as hosts for luminescent activators/dopants such as Ce3þ and Eu2 þ . Upon doping, these thiogallates exhibit high quantum efficiencies, less thermal quenching and greater luminance than other alkaline earth thiogallates of different compositions [6]. Hence Ceand Eu-doped SrGa2S4 have been widely studied as possible blue and green phosphor materials for future FED technology [7–10]. In particular, Ce doped SrGa2S4 as the most promising blue phosphor in the thiogallate family has been investigated for application in full colour FED. In the early stages of the research, most fabrications of the SrGa2S4:Ce3 þ phosphor was in the powder form. Later, thin films were prepared due to better heat sinking properties and lower outgassing rates. However, thin films become less efficient because of lower optical scattering and increased light piping usually observed in them [11]. Several different deposition techniques have been exploited for preparing SrGa2S4:Ce3 þ thin films. In most cases the films exhibit amorphous or poor crystallinity, thus requiring postheating treatment. In the present work, SrGa2S4:Ce3þ thin films were prepared by the pulsed reactive cross laser ablation (PRCLA) technique. The morphology, stoichiometric composition and CL intensity degradation of the SrGa2S4:Ce3 þ thin films are presented
2. Experimental details n
Corresponding author. Tel.: þ 27 514012926; fax: þ27 514013507. E-mail address: [email protected] (H.C. Swart).
0921-4526/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physb.2013.11.048
Silicon (Si) (1 0 0) substrates were first chemically cleaned. A pellet with a 2.4 cm diameter and 6 mm thickness was prepared
P.A. Moleme et al. / Physica B 439 (2014) 144–148
by pressing the SrGa2S4:Ce3 þ powder for 1 h at a pressure of 1.96 107 mbar. It was then annealed for 6 h at 600 1C in vacuum to improve its hardness and was then mounted on a rotating holder lying diagonally across the heater on which Si substrates were mounted. The distance between the target and the substrates was maintained at 4 cm during the deposition of each film. The Lambda Physik EMG 203 MSC 308 nm XeCl excimer laser was used to ablate the target. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 10 5 mbar before backfilling to a pressure of 1.0 10 2 mbar Ar, where Ar was used as pulse gas. The films were deposited at different substrate temperatures ranging from 400 1C to 600 1C using 28,800 pulses. The laser beam was operated at an 8 Hz repetitive rate. The films were prepared using the PRCLA technique in which a synchronized gas pulse crosses the ablation plume close to its origin, increasing the gas phase interaction and the probability of reactive scattering, and also allowing the resulting species to propagate freely away from the localised scattering region thus, giving much brighter plume [12]. Characterisation of the films was carried out with a PHI 700 Auger Nanoprobe unit to obtain SEM micrographs. Images were captured with a 25 kV, 10 nA electron beam and the Auger depth profile analysis was performed by argon (Ar þ ) sputtering with a 2 kV ion beam, current of 2 mA, 1 1 mm2 raster area, and sputter rate of 27 nm/min. The CL intensity degradation of SrGa2S4:Ce3 þ thin films was investigated using Auger electron spectroscopy (AES) coupled with a CL spectrometer. A PHI (model 549) Auger spectrometer and an S2000 Ocean Optics spectrometer were simultaneously used to collect the Auger and CL data respectively, through which surface elemental (chemical) changes were monitored with AES and the light output was measured with a PC2000-UV spectrometer. The primary electron beam current was typically 12 mA. The Auger and CL data were collected in a vacuum chamber with a base pressure in the 9 10 9 mbar range for the films prepared at Tsubstrate ¼400 1C and 600 1C respectively. Afterwards the chamber was backfilled with oxygen to 1.3 10 6 mbar. Throughout the experiment, the Auger and CL data were collected using the same primary electron beam of 2 keV. The decrease of the CL intensities during prolonged electron bombardment of both films was monitored continuously for a period of 6 h at different pressures. The monitored peaks were 442 nm and 485 nm for the film prepared at a substrate temperature of 400 1C and 475 nm and 516 nm for the 600 1C substrate temperature deposited films. The X-ray photoelectron spectroscopy (XPS) data were collected before and after degradation to evaluate the chemical composition and electronic states of the different elements. The data were collected using the PHI 5000 Versa probe-Scanning ESCA microprobe. A low energy Ar þ ion gun and a low energy neutraliser electron gun were used to minimise charging on the surface. Monochromatic Al Kα radiation (hυ¼1486.6 eV) was used as the excitation source. A 25 W, 15 kV electron beam was used to excite the X-ray beam of 100 mm diameter that was used to analyse the Sr 3d, O 1s, Ga 2p and S 2p binding energy peaks (pass energy 11 eV, analyser resolution r0.5 eV). Multipak version 8.2 software [13] was used to analyse the chemical elements and their electronic states using the Gaussian–Lorentz fits.
3. Results and discussions Shown in Fig. 1 are the XRD patterns and the Miller indices of the SrGa2S4:Ce3 þ films prepared at different substrate temperatures ranging from 400 to 600 1C. A SrGa2S4 layer was observed at the growth temperature of 400 1C. From the comparison with the standard powder pattern of SrGa2S4 (JCPDS file no. 77-1189) all
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Fig. 1. The diffraction patterns of the SrGa2S4:Ce3 þ films deposited at different substrate temperatures.
peaks as indicated in Fig. 1(a) were found to belong to the orthorhombic SrGa2S4 crystal structure. Note that the peaks labelled x were the only peaks that did not match the diffraction of SrGa2S4. A preferential growth along the orientation (0 6 2) was observed when the substrate temperature was increased to 500 1C and 600 1C. In addition, small signatures of the (3 9 1) and (3 7 5) peaks which also belong to the orthorhombic SrGa2S4 crystal structure were detected. Similar results as the once observed at 400 1C and 500 1C substrate temperatures were reported by Heikkinen et al. [14]. The XRD patterns in Fig. 1 show that the film growth is sensitive to the substrate temperature. The intensity of the (0 6 2) peak was observed to increase with an increase in the substrate temperature from 400 1C to 600 1C. These results also indicate that crystallinity can be achieved even without postdeposition annealing. Tanaka et al. [15] investigated the crystallinity of the SrGa2S4:Ce3 þ thin films grown on quartz glass substrates with the MBE technique. The films were grown at substrate temperatures of 400–600 1C. They reported a gradual decrease in the XRD peak intensity at higher temperatures than 600 1C and lower temperatures than 500 1C and found the best substrate temperature for SrGa2S4 film growth to be around 560 1C. Fig. 2 shows SEM images of the surfaces of the SrGa2S4:Ce3 þ thin films ablated in an Ar environment at different substrate temperatures ranging from 400 1C to 600 1C. The field of view of the images is 10 mm. The SEM images of all the films exhibited surfaces with non-uniformly distributed spherical particles and clearly visible cracks. Zhao et al. [16] reported similar results when examining thin films cracks on Pt/Ti/Si (1 0 0) thin films after annealing at 650 1C. They pointed out that when the film thickness was larger than the critical thickness (0.78 mm in their case), the crack density, which is defined as the reciprocal of the crack spacing, increased rapidly with an increasing film thickness. Increase in substrate temperature to 600 1C, resulted in a more rough film surface with bigger crack openings and an increased number of particles of not well defined boundaries. Christoulaskis et al. [17] noted that films grain boundaries gain mobility at high substrate temperatures. This movement of grain boundaries has influence on grain growth as well as recrystallisation and surface diffusion. Fig. 3(a) and (b) shows AES depth profile analysis of the films deposited at substrate temperatures of 500 1C and 600 1C, respectively. The main elements, Sr, Ga and S were present in all depth
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Fig. 2. SEM images of SrGa2S4:Ce3 þ thin films deposited in an Ar environment at (a) 400 1C, (b) 500 1C and (c) 600 1C.
Fig. 3. AES depth profiles of SrGa2S4:Ce3 þ thin films deposited in an Ar environment at (a) 500 1C and (b) 600 1C substrate temperatures.
profiles performed on the films, with the detection of the presence of atmospheric C and O as well as Si from the substrate. The activator (Ce3 þ ) could not be detected due to the low concentration thereof and the limitation in the sensitivity of AES. It must be pointed out that the S concentration in the 600 1C film is much lower than in the rest of the films as seen in Fig. 3(a) and (b), if compared with each other. There is also a positive gradient in the S concentration moving from the surface of the layer towards the interface of both the thin films, indicating that some of the S atoms were lost during the deposition process, at these substrate temperatures. The concentration of Ga was also observed to be low in both films. This behaviour might be attributed to the sticking coefficient of Ga which is below unity for the growth substrate temperature in the range of 500–600 1C and the very low melting point of Ga (29.76 1C). The large oxygen concentration suggests the formation of oxides during the ablation process of the SrGa2S4:Ce3 þ thin films and/or that the obtained particles were all oxidised at the outside of the particles. The depth profiles were measured with an electron spot size larger than the average particle sizes in the thin films. The outside of these particles will therefore also contribute to the obtained elemental concentration of the thin films. It is however, clear that the 600 1C contain less S atoms in the thin film. The CL intensities as a function of wavelength spectra for measurements taken before and after degradation at a pressure of 1.3 10 6 mbar O2 and substrate temperatures as indicated are shown in Fig. 4. The broad emission peaks around 443 nm and 485 nm for the film deposited at 400 1C and peaks around 475 nm and 518 nm for the 600 1C ablated film are all attributed to Ce3 þ transitions. That is, the two well-known emission peaks associated with the 5d (T2g)-4f (2F5/2) and from the 5d (T2g)-4f (2F7/2) radiative transitions of Ce3 þ [18]. A clear red-shift of about 32 nm
Fig. 4. CL intensity as a function of wavelength spectra of the SrGa2S4:Ce3 þ film deposited at 400 1C and 600 1C substrate temperatures for the degradation performed at 1.3 10 6 mbar O2.
in the emission peaks was observed for the film deposited at the higher substrate temperature. The Ce3 þ emission in SrGa2S4 lies at a shorter wavelength than for example SrS [19]. The splitting of the energy levels (4f and 5d) (crystal field splitting) occurs because of the interactions of the ions and atoms onto each other in the crystal field. This effect differs from crystal field to crystal field, depending on the type of materials and crystal structures. The fact that a depletion of S was measured for the 600 1C sample clearly indicates a difference in chemical environment (different crystal fields) for the two films, which were responsible for the red shift. A significant decrease in the CL intensity was observed during the electron degradation study of both films. The intensity dropped to
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Fig. 5. Normalised CL intensity as a function of electron dose of the SrGa2S4:Ce3 þ thin films degraded at an oxygen pressure of (a) 1.3 10 6 mbar and base pressure of (b) 9 10 9 mbar.
Fig. 6. XPS deconvoluted scans (high resolutions) and fitting results for regions before (a) and (c) and after degradation (b) and (d) of the 600 1C SrGa2S4:Ce3 þ phosphor thin film with degradation carried out at 1.3 10 6 mbar O2.
less than 30% and 20% of the original intensity due to the prolonged electron bombardment of the respective 400 and 600 1C thin films. The effects of different pressures on the degradation characteristics of the films deposited at 400 1C and 600 1C substrate temperatures are shown in Fig. 5. The results show a rapid decrease of the CL intensity for all the degradation studies. In general, the rate of CL intensity degradation was faster when the chamber was backfilled with oxygen. A faster degradation for the 600 1C sample was observed for both pressures. Swart and Holloway [1,5,20,22] have shown that degradation of phosphors are
normally due to electron stimulated surface chemical reactions (ESSCR). It was shown that the electron beam stimulates the molecular gas species into atomic species, which react with phosphor to form other non-luminescent layers with the consequent loss of CL intensity. The CL degradation cannot always be explained by the formation of the non-luminescent layers alone and a defect theory has been proposed to reconcile the difference [20–22]. The degraded spot was clearly visible on the thin film samples. XPS survey spectra (not shown) for the degraded spot and the undegraded part of the film prepared at 600 1C confirmed the
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presence of the major elements and changes in the XPS peak positions and shapes. Fig. 6(a) and (b) shows the Sr 3d fitted peaks for the undegraded and degraded spot for the Sr 3d5/2 and 3d3/2 peaks. It is clear that the peak shape has changed due to the electron degradation process and that extra chemical species have formed on the surface. The peaks correspond to the Sr–S (132.5 and 134.3 eV) and Sr–Sr (134.3 and 135.6 eV) bonds in the modified SrGa2S4 for the undegraded spot. The origin of the metallic bond is described elsewhere [23]. Additional peaks on both sides of the Sr–S peaks were detected on the degraded spot which might signify the formation of Sr–O and Sr–S–O containing species, and were identified as SrO and SrSO4 peaks as shown in Fig. 6(b). The formation of SrSO4 was confirmed with the increase in the S 2p peak (Fig. 6(c) and (d)) at 168.8 (2p3/2) and 170.0 eV (2p1/2) associated with SrSO4. The formation of Ga2O3 species was also confirmed with Ga 2p and O 1s (not shown) peaks. It is therefore clear that the ESSCR was also responsible for the degradation in the CL intensity as in the case of other phosphors. 4. Conclusion Commercial SrGa2S4:Ce3 þ was deposited on the Si (1 0 0) wafers by the PRCLA technique. The films were prepared at different substrate temperatures. SEM images of all the films indicate that the film consists of non-uniform spherical particles with cracks in the films. Depth profiles performed showed the presence of all the major elements with atmospheric O and C. A larger O signal as expected was observed. Depletion of S occurred at 600 1C. The CL degradation under prolonged electron irradiation occurred due to the formation of SrSO4, SrO and Ga2O3 species on the surface due to ESSCR. The degradation rate for phosphor is fast and therefore phosphor will not be suitable for FED applications.
Acknowledgements Authors are thankful to the University of the Free State and the South African National Research Foundation (NRF) for the financial
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