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IMID 2009 DIGEST •
Abstract
Comparative study of the dependence of the firing voltage of MgSrO and MgCaO protective layers on chemical composition was carried out. It was observed that the firing voltage increases when CaO/MgO or SrO/MgO ratio increases.
1. Introduction
There were several publications that CaSrO protective layers have a higher coefficient γ for Xe
and allows to lower the firing voltage when used in PDPs [1-2]. Unfortunately, this material is extremely sensitive to H2O and CO2, forming hydroxides and carbonates of Ca and Sr. It is necessary to avoid exposure of deposited CaSrO protective layers to atmosphere by processing PDP front glasses with CaSrO films in inert atmosphere or vacuum. This makes manufacturing of PDP panels more complex and more expansive.
In an attempt to reduce sensitivity of a protective layer to contamination with H2O and CO2, we tested MgCaO and MgSrO protective layers. MgO is substantially less sensitive to water vapors and carbon dioxide. Many research groups used doping of MgO, including doping with CaO or SrO, to improve properties of MgO protective layers [3-4]. Our goal was different. We wanted to obtain protective layers of MgCaO or MgSrO solid solutions. In this case the lattice constants and the band structure of a prepared solid solution is essentially different from the lattice constants and the band structure of initial compounds.
Unlike CaO and SrO, MgO does not form stable solid solutions with CaO or SrO. To prepare metastable MgCaO or MgSrO protective layers, we used Pulsed Laser Deposition (PLD). This deposition method allows to obtain energies of laser-evaporated atoms and ion that strongly exceed their thermal energies in a target. These higher energies of
deposited atomic particles increase their surface mobility on a substrate and promote formation of a metastable MgCaO or MgSrO layers.
By changing composition of MgCaO and MgSrO solid solutions, the dependence of the firing voltage on CaO/MgO or SrO/MgO ratio was determined in our experiments.
2. Experimental
MgCaO and MgSrO protective layers were deposited by PLD using a KrF excimer laser (wavelength 248 nm). Laser pulse energy was 200 mJ and laser pulse frequency was 30 Hz. Energy density of a laser beam on a PLD target was approximately 3 J/cm2. A disk-type cells, described elsewhere [5], were used as substrates for the protective layers. 2” Mg disk with an attached piece of Ca or Sr as well as a mixture of Mg and Ca powders were used as a target for PLD. Deposition was carried out at substrate temperature of 4500 C in 2.5 sccm O
2 gas flow to obtain oxide films. Oxygen pressure was around 2.5x10-5 Torr during deposition. The thickness of films was in a range of 200-500 nm. CaO/MgO or SrO/MgO ratio was varied by changing area of the Ca or Sr piece exposed to a laser beam or by changing Ca/Mg ratio for mixed metal powders.
3. Results and discussion
The deposited protective layers were analyzed by XRD. Results of XRD measurements are presented in Fig. 1 a, b. Positions of (200) peaks for different MgSrO films and (220) peaks for different MgCaO films were evaluated. The choice of these XRD peaks was based on their well-defined character for all films that were analyzed. For both MgCaO and MgSrO, XRD peak position is shifted to the left when content
Study of MgSrO and MgCaO thin films as protective
layers for PDPs.
Y. T. Matulevich*, Sung-Hwan Moon, Jong-Seo Choi, Dong-Sik Zang
Material Laboratory, Corporate R&D center, Samsung SDI, 428-5, Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-577, Korea
TEL:+82-31-2884616, e-mail:[email protected] Keywords : protective layer, MgCaO, MgSrO
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• IMID 2009 DIGEST
of CaO or SrO increases. It means that the lattice constant is changed (increased), indicating formation of a solid solution in the deposited films. MgO/CaO and MgO/SrO ratios, presented in Figure 1, were calculated from the shift of XRD peaks. Assuming Vegard’s law for the lattice constant of a solid solution, d[MgXO] = x·d[MgO] + (1 –x)·d[XO], MgO concentration x in the solid solution was calculated.
XRD peaks for MgCaO and MgSrO films are also broader if compared to MgO. Full width at half maximum (FWHM) for XRD peaks of MgO is below 0.60 while for MgCaO FWHM is above 0.70. The broadening is related to an increased stress in the deposited films and to a smaller grain size. This broadening is especially big for MgSrO films, with XRD peak FWHM above 10. This is easy to explain if we take into account, that ionic radius of Sr is much bigger than ionic radius of Mg.
55 60 65 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 In te ns ity , a rb . u ni ts 2θ, degree MgO:CaO =80/20 MgO:CaO =86/14 MgO:CaO =90/10 MgO
a
)
38 39 40 41 42 43 44 45 46 47 48 0 1000 2000 3000 4000 5000b
)
In te ns ity , a rb . u ni ts 2θ, degree MgO MgO:SrO =89/11 MgO:SrO =87/13 MgO:SrO =85/15 MgO:SrO =74/26Fig. 1. XRD for MgCaO (a) and MgSrO (b) protective layers deposited by PLD. For clarity, the XRD curves were shifted vertically to reduce overlapping.
In Fig. 2 a, b examples of SEM data for the deposited protective layers are presented. Grains of irregular shape are observed on a surface of an
MgCaO film. The in-plane size of the grains is up to 100nm. For an MgSrO film, grains are hardly visible. Instead, a network of small cracks is present. These cracks are caused by a stress induced in MgSrO films by elevated substrate temperature during PLD deposition and by big difference in ionic radius of Sr and Mg.
Fig. 2. SEM for MgO/CaO = 86/14 (a) and MgO/SrO = 89/11 (b) protective layers deposited by PLD.
The firing voltage of the prepared protective layers was measured in a Ne-He-Xe gas discharge using a disk-type cell described in [5]. Content of Xe in the gas mixture was 15%. Gas pressure was 350 Torr and a gas-filled discharge gap was 120 µm. The results of measurements of the firing voltage for MgCaO and MgSrO layers are presented in Fig. 3. The Von_EBD line corresponds to the firing voltage of an MgO protective layer deposited by Electron Beam Deposition (EBD). EBD is a conventional method of MgO film deposition in PDP mass production. So it is reasonably to use the firing voltage for an EBD deposited MgO film as a reference. From Fig.3, for both MgCaO and MgSrO films the firing voltage
b)
a)
14-4 / Y.T. Matulevich
IMID 2009 DIGEST •
increases when content of CaO or SrO in the films increases. In case of MgSrO films the dependence of the firing voltage on SrO content is almost linear. MgCaO films demonstrate similar behavior except for the film with 20% CaO content. It is interesting to notice that this deviation is related to a target used for PLD deposition of this film. It was 2” Mg disk with an attached piece of Ca metal while for other MgCaO films mixtures of Mg and Ca powders pressed in pellets were used as PLD targets. For deposition of all MgSrO films 2” Mg disk with an attached piece of Sr metal was used. Character of a PLD target has an affect on the firing voltage of deposited films. Nevertheless, an increased content of CaO or SrO in a film leads to an increase in the firing voltage.
0 10 20 30 40 100 110 120 130 140 150 160 170 180 190 V _o n, v ol ts
SrO or CaO content, % MgSrO PLD films MgCaO PLD films
Von_EBD
Fig. 3. The firing voltage data for the protective layers deposited by PLD.
It was also observed that stability of a gas discharge was substantially reduced for films with a high content of CaO or SrO. It may be related to a reduced ability of such films to keep the wall charge. From Fig. 3, the firing voltage of an MgO protective layer deposited by PLD is lower by approximately 35 volts if compared to an MgO layer deposited by EBD. It indicates that PLD films of MgO have a higher quality (e.g. crystallinity, density) if compared to EBD films. This is related to a higher energy of incident atoms and ions forming an MgO film during PLD.
4. Summary
MgCaO and MgSrO protective layers of different composition were prepared by PLD. The firing voltage for these films depends on film composition. The firing voltage for the films increases, when CaO or SrO content in the films increases. In case of MgSrO films the dependence of the firing voltage on amount of SrO was almost linear. It is not clear at the moment what is a mechanism behind this dependence. All the films where exposed to air prior to the firing voltage measurements so this dependence may be related to an increasing sensitivity of the films to water vapors and carbon dioxide. It may be also related to an increasing stress in the films when content of CaO or SrO increases. Additional studies are needed.
5. References
1. Y. Motoyama, T.Kurauchi, SID 06 Digest, 1384 (2006).
2. Y.T. Matulevich et al, IMID 07 Digest, 213 (2007).
3. Ishimoto, R. A. Baragiola, and T. Shinoda, “Proceedings of the7th International Display Workshop”, SID, Kobe, Japan, (2000).
4. Patent of Mitsubishi Materials Corporation N2000-0048076 from 25.07.2000.
5. M.-S. Lee, Y. Matulevich, J.-H. Kim, H.-Y. Chu, S.-S. Suh, S.-S.-K. Kim, J.-S.-S. Choi and D.-S.-S. Zang, SID 06 Digest 37, p.1388 (2006).
