Photoluminescence of electron beam evaporated CaS:Bi thin films

Transcription

1 Journal of Luminescence 104 (2003) Photoluminescence of electron beam evaporated CaS:Bi thin films P.F. Smet*, J. Van Gheluwe, D. Poelman, R.L. Van Meirhaeghe Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, Gent B-9000, Belgium Received 27 May 2002; received in revised form 3 January 2003; accepted 6 January 2003 Abstract For the first time, the photoluminescence (PL) of electron beam evaporated CaS:Bi thin films is reported. Luminescent CaS:Bi powder prepared out of aqueous solutions was used as source material. The influence of substrate temperature on the PL and the morphology of thin films is discussed, and an optimum is determined. Substrate temperatures between 200 C and 300 C lead to good quality thin films with sufficient PL intensity. As-deposited thin films show two emission bands, peaking at 450 and 530 nm. Upon annealing the emission intensity increases, and annealing at 800 C is sufficient to obtain a homogeneously blue emitting thin film (CIE colour coordinates (0.17; 0.12)), thanks to a single remaining emission band at 450 nm. The influence of ambient temperature on the PL of CaS:Bi powder and thin films was also investigated and it was found that CaS:Bi thin films show a favourable thermal quenching behaviour near room temperature. r 2003 Elsevier Science B.V. All rights reserved. PACS: 78.55; Keywords: Thin films; Photoluminescence; CaS:Bi 1. Introduction CaS:Bi powder is a long-known blue emitting phosphor which looks also very promising for thin film electroluminescence (EL), thanks to an emission peak at 450 nm [1 4]. For the first time, the photoluminescent (PL) and morphological properties of CaS:Bi thin films are reported. In this paper, several techniques are discussed for the production of the thin films. When luminescent CaS:Bi *Corresponding author. Tel.: ; fax: address: philippe.smet@rug.ac.be (P.F. Smet). powder, obtained by synthesis out of aqueous solutions, was used as the source material for electron beam evaporation, homogeneously emitting thin films were obtained. The influence of evaporation parameters and annealing on optical and morphological properties is discussed. 2. Experimental For the production of Bi-doped CaSthin films, several techniques were tried. Pressed pellets of cold mixed CaS(CERAC; 99.9%) and Bi 2 S 3 (CERAC; %) did not yield luminescent thin /03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)

2 146 P.F. Smet et al. / Journal of Luminescence 104 (2003) films, probably due to the preferential evaporation of the Bi 2 S 3 grains, well before the sublimation temperature for CaSwas reached. Later, e-beam evaporation of undoped CaSand thermal coevaporation of Bi 2 S 3 was tried. However, the corresponding thin films showed no PL emission, probably caused by fluctuations in the necessarily very low evaporation rate of Bi 2 S 3 compared to CaS. Similar problems have been reported for CaS:Pb, Pb also having a low melting point [5]. This situation changed drastically when pellets of luminescent CaS:Bi were used; in this case bright blue emitting CaS:Bi thin films could be obtained. Several techniques are described in literature for the production of doped or undoped sulphides [6], although carbonates are used most of the time. In contrast to this dry method, aqueous solutions were used in the present study, in order to obtain an optimal distribution of the dopants. The luminescent CaS:Bi powder was produced as follows. An aqueous solution of calcium nitrate (Baker s; >99%) was mixed with metallic bismuth (Johnson Matthey) dissolved in nitric acid, the ratio of bismuth to calcium being 1 mol%. This solution was mixed with an aqueous solution of ammonium sulphate (FERAK; >99%) in a Teflon beaker. The resulting solution was dried in a furnace (at approximately 150 C) and a white powder was obtained. After grinding and drying at 500 C in a nitrogen ( %) atmosphere, anhydrous CaSO 4 was obtained according to the X-ray diffraction pattern. After grinding, the powder was sintered at 1000 C in H 2 Sfor 2 h. Sulphurization took place, and a single CaS phase was observed in X-ray diffraction (ASTM PDF File no ). Working with solutions allows a precise concentration and a homogeneous distribution of the dopant. This is in contrast to evaporation out of a cold pressed pellet of CaS and a (obviously small amount of) bismuth compound. In the latter case, the distribution of the dopant over the pellet is not very homogeneous, as only a physical mixing of the powder grains occurs. Thin films with a thickness of approximately 900 nm were produced out of CaS:Bi pellets with e- beam evaporation (ESV6 electron gun in a Leybold Univex 450 vacuum system), at a growth rate of 1.5 nm/s, on vycor (corning 7913) substrates. To reduce sulphur deficiency of the thin films, H 2 Swas added with a partial pressure of Pa during deposition [7]. The influence of substrate temperature was studied in the range from 100 Cto500 C. The evaporated thin films were annealed in nitrogen at different temperatures (from 600 C to 1000 C) during 2 min, using a rapid thermal process (AST Super Heat System 1000). Emission spectra were recorded using a microchannel plate intensified optical multichannel analyser (EG&G OMA III). Powders and thin films were excited using a pulsed nitrogen laser at a wavelength of nm (PRA LN 1000) or a highpressure Xenon lamp (150 W) with a UV prism monochromator (Carl Zeiss M4 QIII). 3. Results and discussion 3.1. CaS:Bi luminescent powder The synthesized CaS:Bi powder shows a very bright, deep blue and homogeneous PL emission, peaking at 450 nm (CIE x ¼ 0:16; y ¼ 0:03), due to the 6s6p ( 3 P 1 ) 6s 2 ( 1 S 0 ) transition [4]. Upon cooling to 80 K, the emission maximum shifts to 437 nm. With decreasing temperature, the peak intensity increases and the emission band sharpens (Fig. 1a and b). Although bismuth is homogeneously distributed over the CaSO 4, only a certain part is incorporated in the CaSduring the sulphurization process (bismuth compounds were seen to condense in the cooler parts of the furnace). Therefore, the bismuth concentration in the synthesized powder will be lower than the concentration in the solution. The solid solubility of Bi 3+ in CaScan be estimated by comparing to the case of La 3+,an ion with the same valence state and similar ionic radius. Choi et al. found a very low solid solubility for La 3+ in CaSprepared under H 2 S, when no codoping with monovalent ions was applied [8]. Codoping with Na + led to an increased solubility of La 3+ in CaS, due to an ionic radius almost

3 P.F. Smet et al. / Journal of Luminescence 104 (2003) CaS:Bi thin films Fig. 1. (a) Influence of ambient temperature on the emission spectrum of CaS:Bi powder: a; 80 K, b; 200 K, c; 291 K. (b) Influence of ambient temperature on the peak PL intensity of CaS:Bi powder. identical to Ca 2+. Choi et al. suggested that the size effect is as important as the charge effect in the replacement of Ca 2+, since codoping with potassium (K + having a large ionic radius) prevents a good solid solubility of La 3+ in CaS. In this study however, no monovalent codoping was added during the synthesis of the CaS:Bi powder, as codoping can influence the emission characteristics of CaS:Bi. The addition of Na + for instance can lead to several emission bands at longer wavelengths and can cause a strong concentration quenching of the emission band at 450 nm [4]. To avoid this complication, the effects of codoping were not investigated in this work Influence of substrate temperature The as-deposited layers show bluish green PL emission, with the emission intensity varying with the substrate temperature (Fig. 2). Upon nitrogen laser excitation (337.1 nm), the emission consists of two broad emission bands, situated at 450 and 530 nm (Fig. 3). Upon band gap excitation (lo265 nm), only the green emission is present. Therefore, the emission spectrum indeed consists of two distinct peaks, and thin film interference effects can be ruled out. The emission intensity of the as-deposited thin films decreases with increasing substrate temperature (Fig. 2), and almost no emission is visible when thin films are evaporated at a substrate temperature of 500 C. This can be easily related to the low sticking coefficient of Bi, as Bi has a very low melting point (271 C). Therefore, the Bi concentration in thin films will always be lower than in the CaS:Bi pellets. Preliminary X-ray photoelectron spectroscopy (XPS) measurements on thin films did not yield any detectable bismuth signal. This was probably due to the overlap between bismuth peaks and the stronger calcium and sulphur peaks. Additional concentration measurements will be performed in the near future Effects of annealing on the luminescent properties Annealing at 600 C (2 min) leads to a considerable increase in emission intensity (Fig. 3), due to an enhancement of both the blue and the green emission band. Annealing at 800 C makes the blue emission dominate over the green emission, leading to a blue emitting thin film, with CIE colour coordinates of (0.17; 0.12). Fig. 3 shows the effect of different annealing temperatures on a typical thin film, evaporated at 200 C. An increase in annealing temperature from 800 C to 1000 C does not alter the emission spectrum, but leads to a slight decrease in the overall PL intensity. This is probably caused by out-diffusion of Bi at these temperatures. Thin films evaporated below 200 C did not withstand annealing at 1000 C, as the layer started to peel off, probably due to excessive thermal stresses.

4 148 P.F. Smet et al. / Journal of Luminescence 104 (2003) Fig. 2. Influence of substrate temperature and annealing temperature on the PL intensity at 450 nm. Fig. 3. Influence of annealing temperature on the emission spectrum of a CaS:Bi thin film evaporated at 200 C: (a) asdeposited; (b) anneal at 600 C; (c) anneal at 800 C. l exc ¼ 337 nm. Nevertheless, annealing at 800 C is sufficient to obtain homogeneous and bright blue emission. This is a considerable advantage over CaS:Pb, where the need for annealing at very high temperatures was reported [5]. The effects of Bidiffusion during annealing should be taken into account when EL devices are studied, as diffusion of dopant ions to the semiconductor insulator interface and to the insulating layers can detoriate the electrical properties of the device. Some discussion remains over the origin of a green emission band in CaS:Bi luminescent powders. Yu et al. assigned an emission peak at 520 nm in Na + codoped CaS:Bi powder to the defect centre formed by Bi 3+ Na + pairs [9]. Kim et al. suggested that an emission band at 515 nm originated from the combination of the Bi 3+ ion residing at a Ca 2+ place (combined with a sulphur vacancy) with a Ca 2+ vacancy [4]. In the present experiments, no sodium was added as a codopant on purpose; furthermore, all production steps are thought to be sodium free. In our powder phosphors, no green emission was seen next to the blue emission band at 450 nm. On the other hand, an emission band at 530 nm is present in our as-deposited CaS:Bi thin films. This emission band seems not related to bismuth, as undoped CaSthin films also showed a strong green emission band peaking at 530 nm upon band gap excitation. These undoped thin films were grown both from CaSpowder prepared out of aqueous solutions and out of commercially available CaS(CERAC; 99.98%). In both cases annealing at 800 C reduces this emission band, and only little green emission remains. Therefore, this emission band, which disappears at higher substrate and/or upon higher annealing temperature, can be related to self-activated emission of CaS, associated with structural defects in the host lattice. Thus, this emission band at 530 nm is probably not correlated with bismuth, nor with sodium. Further research will be dedicated to understand this green emission band in powders and thin films.

5 P.F. Smet et al. / Journal of Luminescence 104 (2003) Effects of annealing on the morphological properties To evaluate morphological properties of the thin films, several techniques were used: X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). XRD spectra clearly show an increase in crystallinity (evaluated by the diffraction peak height) when thin films are evaporated at higher substrate temperatures and/or upon higher annealing temperatures. Although the thin film evaporated at 100 C shows the highest PL intensity after annealing, this substrate temperature is too low to grow wellformed thin films. Upon annealing, thermally induced stress leads to cracks, which cause the layer to peel off. Higher substrate temperature reduces this effect. Therefore, we have to find an optimum between high quality thin films (which require higher substrate temperatures) and the emission intensity (which is related to the concentration of Bi in the thin film; as stated before, this concentration decreases with increasing substrate temperature). A substrate temperature between 200 C and 300 C seems to be a good compromise. AFM measurements on as-deposited thin films revealed a reduction in surface roughness with higher substrate temperatures (from an average surface roughness R a ¼ 31 nm at a substrate temperature of 200 C to R a ¼ 4 nm at 500 C). Upon annealing, no significant grain growth was visible with AFM. The addition of codopants, which can act as a flux, may be necessary to improve crystalline properties of the thin films. For instance, the addition of Ag to CaS:Cu thin films was found to lead to a considerable increase in grain growth and crystallinity [10]. A monovalent codopant could also compensate for the charge mismatch between the trivalent bismuth ion and the divalent host anions Thermal quenching behaviour The temperature dependence of the PL emission of the CaS:Bi thin films was studied in the range from 80 to 325 K. At 80 K, the emission peaks at 438 nm. Higher temperature results in a broadening of the blue emission peak and a decrease of Fig. 4. Influence of ambient temperature on the peak PL intensity of a CaS:Bi thin film evaporated at 200 C and annealed at 800 C. the total intensity. At room temperature the blue emission is slightly shifted towards longer wavelengths, and peaks at 450 nm. The PL intensity of the green emission is slightly dependent on temperature, with maximum intensity near room temperature. The total emission intensity is only slightly dependent on temperature in the range K (Fig. 4). This favours CaS:Bi thin films over CaS:Cu,Ag and SrS:Cu,Ag thin films, where the blue emission shows a strong thermal quenching near room temperature [11]. The shift of the blue emission peak and the thermal quenching behaviour are in good correspondence with the results on luminescent CaS:Bi powder (Fig. 1a and b). As stated before, a remarkable difference is the green emission band in thin films, which is absent in the powder. 4. Conclusions and perspectives A production technique based on aqueous solutions for the starting material allowed the electron beam evaporation of highly luminescent CaS:Bi thin films. As-deposited layers show a bluish green PL emission, with decreasing intensity at higher substrate temperatures. Besides the blue emission band assigned to bismuth, a strong green emission band is present upon band gap excitation, probably due to self-activated emission in

6 150 P.F. Smet et al. / Journal of Luminescence 104 (2003) CaSassociated with structural defects. Annealing at 800 C is sufficient to obtain bright, blue emission at 450 nm and to eliminate the green emission band. The rather low process temperatures during evaporation and annealing, combined with the favourable thermal quenching behaviour, make CaS:Bi a promising candidate as a blue emitter for AC thin film EL. In the field of thin film EL, there is still need for an efficient blue emitter, compatible with reasonably low processing temperatures [12]. If CaS:Bi is indeed found to exhibit bright and stable blue EL emission, this could be a further step towards low cost full colour thin film EL displays. The effects of codoping with monovalent ions on luminescence and morphology of both powder and thin films will be investigated in the near future. Further research will be conducted on the incorporation and mobility of the bismuth ions in powder and thin films. Acknowledgements The authors are grateful to C. Fouassier and F. Guillen at the Institut de la Chimie de la Mati"ere Condens!ee in Bordeaux (France), for the necessary information needed for the development of the powder production method out of aqueous solutions at our laboratory. The authors wish to thank O. Janssens for XRD and SEM measurements, and L. Van Meirhaeghe for AFM measurements. This research is supported by FWO-Vlaanderen (Fund for scientific research- Flanders). References [1] P. Lenard, F. Schmidt, R. Tomaschek, Phosphoreszenz und Fluoreszenz, Akademische Verlagsgesellschaft, Leipzig, 1928, p [2] W. Lehmann, J. Luminescence 6 (1973) 455. [3] N. Yamashita, S. Asano, J. Phys. Soc. Jpn. 41 (1976) 536. [4] C.H. Kim, C.H. Pyun, H. Choi, S.J. Kim, Bull. Korean Chem. Soc. 20 (1999) [5] J. Versluys, D. Poelman, D. Wauters, R.L. Van Meirhaeghe, J. Phys.: Condens. Matter 13 (2001) [6] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, Boca Raton, 1999, pp [7] D. Poelman, R. Vercaemst, R.L. Van Meirhaeghe, W.H. Lafl!ere, F. Cardon, J. Luminescence 75 (1997) [8] H. Choi, C.H. Kim, C.H. Pyun, S.J. Kim, J Solid State Chem. 131 (1997) [9] I. Yu, H.L. Park, H.K. Kim, S.K. Chang, C.H. Chung, Phys. Status Solidi (b) 153 (1989) K [10] D. Wauters, D. Poelman, R.L. Van Meirhaeghe, F. Cardon, J. Phys.: Condens. Matter 12 (2000) [11] P. Smet, D. Wauters, D. Poelman, R.L. Van Meirhaeghe, Solid State Commun. 118 (2001) [12] W. Tong, Y.B. Xin, B.K. Wagner, W. Park, C.J. Summers, Proceedings of the 10th International Workshop on Inorganic and Organic Electroluminescence, Hamamatsu, Japan, 2000, pp