R.F.-sputtered luminescent rare earth and yttrium oxysulphide films

Thin Solid Films, 90 (1982) 181 185 ELECTRONICS AND OPTICS


R.F.-SPUTTERED LUMINESCENT RARE EARTH AND YTTRIUM O X Y S U L P H I D E FILMS* C. SELLA AND J. C. MARTIN Laboratoire de Physique des Mat&iaux, CNRS, Laboratoires de Bellevue, 92190 Meudon (France) Y. CHARREIRE Elkments de transition dans les Solides, Equipe de Recherche associke au C N R S 210, Laboratoires de Bellevue, 92190 Meudon (France) (Received August 31, 1981 ; accepted September 24, 1981)

The rare earth and yttrium oxysulphides form a family of highly luminescent materials and are of practical importance for cathode ray tubes and other visual display devices. The conventional screens are composed of a phosphor powder and have limited brightness, resolution and contrast. The brightness is limited by heating of the phosphor and poor thermal conductivity between the particles and the substrate. The resolution is limited by the particle size. The contrast is reduced by a high diffuse reflectivity of ambient light. Continuous thin films are not subject to these limitations. In previous attempts to produce such films by classical evaporation or sputtering techniques most of the lumipous efficiency of the material was lost. When r.f. sputtering was carried out in standard vacuum equipment (10 6 Torr), a sulphur deficiency in the films was observed owing to the residual oxygen pressure. To overcome these difficulties, we used a new sputtering system designed for high purity deposition and mounted in an ultrahigh vacuum unit. Two methods were used to prepare Y202S and La202S luminescent films activated by Eu 3 + (red) and by Tb 3 + (green): (1) deposition from a pure oxysulphide target with subsequent treatment in H2S between 700 and 850 °C; (2) deposition from a mixed sulphideoxysulphide target with subsequent treatment in argon or an A r - H 2 mixture between 500 and 850 °C. All films were investigated by X-ray diffraction, scanning and transmission electron microscopies, cathodoluminescence and photoluminescence excited with a pulsed laser. The as-deposited films were amorphous and had relatively low luminescence brightness. After the films had been crystallized by an appropriate heat treatment, a maximum luminous efficiency was obtained which approached the theoretical values. On ground glass substrates, the apparent brightness increased by a factor of 4. Finally the use of a high power CO2 laser for annealing allowed the use of ordinary glass substrates instead of quartz or sapphire.

* Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea,Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75

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1. INTRODUCTION Rare earth oxysulphides crystallize in the space group D32d (P3m) and the hexagonal cell contains one R202S unit. The structure is very closely related to the A-type rare earth oxide structure. The main difference is that one of the three oxygen sites is occupied by a sulphur atom; the sulphur atoms form a discrete plane perpendicular to the c axis of the crystal. Between the sulphur planes two layers of the tetrahedral arrangement (RO), "+ have been found 1, giving luminescent properties to this matrix. Rare earth oxysulphides form a family of remarkably stable luminescent compounds. Numerous theoretical investigations 2 6 (on mechanisms of excitation and crystal field theories) and technical applications 7-9 (use of oxysulphides in luminescent tubes, in cathode ray tubes and in other display devices) have been published in the last 10 years. Although the luminous efficiency of rare earth oxysulphides is less than that of conventional sulphides (e.g. ZnS), they are more stable with high power exciting beams. Yocom and Shrader 2 have shown that a power efficiency of 10~o-15~o could be attained for the three fundamental colours with a suitable choice of cations (R and R') and of their concentration. 2. EXPERIMENTAL PROCEDURE

As shown by Mapple and Buchanan s, r.f. sputtering from an La202S target with argon as the sputtering medium led to a sulphur deficiency. The resulting films exhibited a luminescence spectrum corresponding mainly to that of La203. To overcome the sulphur deficiency, these investigators used an Ar-H2S mixture as the sputtering gas. This method requires very strict conditions: precise HES partial pressures, a sputtering unit resistant to chemical corrosion and a post-deposition heat treatment of the films in an H2-SO 2 atmosphere at 1000 °C. Because of this heat treatment, the use of sapphire substrates is necessary, glass substrates being impractical. Our films were prepared using an ultrahigh vacuum system (employing turbomolecular and titanium sublimation pumps with a cryogenic trap) attaining 10 8 Torr. As previously reported 8 films sputtered from a pure Y202 S target in a high purity argon (99.9995~o) atmosphere showed a sulphur deficiency. They consist of a mixture of Y202 S and Y203 . The sulphur deficiency increased with the residual oxygen pressure. A reduction in the oxide content can be obtained by sputtering in an Ar-H2 (H2 content between 5~o and 10~o) mixture. Films deposited in this way are amorphous. A post-deposition heat treatment in a sulphurcontaining atmosphere (H2S or CS2) at 850 °C give quite pure oxysulphide films, which are perfectly crystallized, show no trace of oxide and exhibit a strong luminescence. The treatment temperature must be optimized for each rare earth matrix which is monitored by X-ray diffraction and by cathodoluminescence or photoluminescence. Figure 1 shows an increase in the luminosity and a disappearance of the characteristic oxide spectral lines when the heat treatment temperature is increased. Higher temperatures lead to sulphide formation by decomposition of the oxysulphide. In our experiments, temperatures of 850 °C for yttrium and 650 °C for lanthanum matrices gave the best results. To avoid the treatment in H2S at high temperatures we have proposed another







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Fig. 1. Emission spectra of Y202S:1~ Eu films: curve a, as deposited; curve b, after heat treatment at 650 °C for 5 h; curve c, after heat treatment at 850 °C for 5 h.

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method 1o: to overcome the sulphur deficiency we use a target prepared by pressing a mixture of Y202S and X~o Y253 powders. The value o f x is optimized by monitoring the film properties with cathodoluminescence or photoluminescence (Fig. 2). The post-deposition treatments are carried out at 550-650 °C in a neutral atmosphere (argon, N 2 or Ar-H2). At these temperatures, glass substrates (Pyrex, Vycor or Corning 7059) can be used. Figure 3 shows the laser-excited fluorescence spectrum of a film prepared from a Y202S:Tb-6.7~Y2S3 target. The thin film spectrum exhibits the characteristic lines of pure Y202S:Tb. 3.


All films were characterized by X-ray diffraction, electron microscopy and luminescence excited by an electron gun (20 kV; 0.1-1 W cm -2) or by a pulsed N 2 laser. The as-deposited films are amorphous and show very weak fluorescence (Fig. l(a)). The spectrum of these films sputtered from a pure oxysulphide Y202S:Eu target is characteristic of a mixture of oxide and oxysulphide. After heat treatment, high energy electron diffraction and electron microscopy studies (Fig. 4) show a texture with the (001) planes parallel to the substrate. The surface structure is homogeneous with an average crystallite size of 1 lam. On polished substrates the optical power efficiency is about 9~o; this value is very close to the theoretical value (11~o). The loss of 89~o of the light by internal trapping is due to the difference between the refractive indices of the film and of air. To recover some of this light, Mapple and Buchanan 8 used films which had been segmented by a photolithography etching technique. In our studies the brightness of the films was enhanced more simply by using ground glass substrates (Fig. 5). At the optimized roughness the brightness was increased by a factor of 4. After the samples have been coated with a 1000 ~ metallic film as a reflector the brightness is doubled. For industrial applications the use of glass substrates (Pyrex or Corning 7059) limits the heat treatment temperature to 600-700 °C. In this case only La202S gives a good quantum efficiency. However, we also obtain good results using a CO2 laser for annealing in an argon atmosphere. In this way a good crystallization of all

Fig. 4. Scanning electron micrograph of a Y202S:Eu film on polished vitreous quartz (after heat treatment at 850 °C). Fig. 5. Scanning electron micrograph of a Y202S : Eu film on ground glass (after laser annealing).



oxysulphide films is achieved w i t h o u t a n y d a m a g e to the glass substrates. The brightness of these films is higher by a factor of 3 t h a n that of samples subjected to classical heat treatment. This is clearly seen in Fig. 6 which shows the luminescence intensity of two areas exposed to a low power a n d a high power laser beam. S c a n n i n g electron m i c r o s c o p y studies (Fig. 5) show that little or n o change results from the laser annealing.


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Fig. 6. Effect of laser annealing on the luminescencespectra of Y202S:Eu films on glass: curve a, low power laser beam; curve b, high power laser beam. REFERENCES 1 2 3 4 5 6 7 8 9

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