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Hetero-epitaxial growth and optical properties of LaF3 on CaF2
Thin Solid Films 420 – 421 (2002) 30–37
Hetero-epitaxial growth and optical properties of LaF3 on CaF2 Yusuke Taki*, Kenichi Muramatsu Thin Films R&D Group1, Lens Engineering Development Department, Nikon Corporation, 10-1, Asamizodai 1-Chome, Sagamihara, Kanagawa 228-0828, Japan
Abstract LaF3 films were fabricated on CaF2 (1 1 1) and SiO2 glass substrates at a relatively low temperature of 250 8C by vacuum evaporation. X-ray diffraction revealed that w0 0 lx of the LaF3 film deposited on CaF2 (1 1 1) was parallel with w1 1 1x of CaF2 and the film was also orderly arranged in-plane direction. It has been found out experimentally that LaF3 grows epitaxially on CaF2 (1 1 1). The LaF3 film on CaF2 (1 1 1) had a bulk-like dense structure. On the other hand, SiO2 glass was selected as a typical substrate that did not have any orientation at all. The LaF3 film on SiO2 glass was random poly-crystallites and the film structure was columnar with narrow openings where the air could easily enter and absorb. The insides of LaF3 films on CaF2 (1 1 1) and SiO2 glass had accurately stoichiometric compositions. However, in the films, total areas exposed to the air were hydroxidized and oxidized because of chemical reactions with water and molecular oxygen adsorbed on the exposed areas. Thus, the exposed areas were lack of fluorine. The total area exposed to the air in the columnar LaF3 film on SiO2 glass was much larger than that in the epitaxial LaF3 film on CaF2 (1 1 1). Therefore, the film on SiO2 glass had much more O and OH concentrations than the film on CaF2 (1 1 1). Vacuum ultraviolet (VUV) light was much more absorbed at F-poor areas exposed to the air in LaF3 films than the stoichiometric film insides. It is significant for LaF3 film deposition to make dense film structures and reduce the total exposed areas. Polycrystalline columnar LaF3 films on the materials useless for the epitaxial growth as well as SiO2 glass have much larger optical losses than the epitaxial LaF3 films. The optical transparency of LaF3 films in VUV regions has been improved using hetero-epitaxial growth technique. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Vacuum evaporation; Optical films; Fluoride; Epitaxy; Vacuum ultraviolet (VUV)
1. Introduction Lanthanum fluoride, LaF3, is a primary candidate of high refractive index materials available for optical interference multilayers in deep ultraviolet (DUV) and vacuum ultraviolet (VUV) regions. LaF3 films have been fabricated by many researchers and engineers by using various PVD processes such as vacuum evaporation w1x, molecular beam epitaxy w2x, ion assisted deposition w3x, ion beam sputtering w4x and magnetron sputtering. Vacuum evaporation, currently, is the most appropriate process to prepare LaF3 films with the lowest photo-absorption in VUV regions among all the processes. However, LaF3 films prepared by the vacuum evaporation still have remarkable photo-absorption that must be reduced. Therefore, LaF3 films dominate optical performances of multilayers, which are alternate stacking of LaF3 and MgF2 or AlF3 as a low index material. *Corresponding author. Tel.: q81-42-740-6491; fax: q81-42-7406322. E-mail address: [email protected] (Y. Taki).
LaF3 is also noted as a super ionic conductor and its crystal structure, tysonite, has been minutely examined with X-ray diffraction (XRD) w5x, neutron diffraction w6,7x, 19F NMR w8x, etc. In spite of these examinations, LaF3 lattice structure is still unclear. Nowadays, it is the predominant view that tysonite possesses a trigonal ¯ lattice (P3c1) where fluorine anions occupy three distinct sites. However, a simple hexagonal lattice is not completely denied. Since two fluorine sites of three in the trigonal lattice are structurally and dynamically equivalent w8x, the trigonal lattice may be replaced by the simple hexagonal lattice (P63 ymmc) that has only two fluorine sites. Anyway, both lattices are hexagonaltype. In addition, energetic calculations w9x presume that LaF3 is compatible with CaF2 or CaF2 –YF3 mixture. As for CaF2, single crystals with extremely low photoabsorption are used for VUV–DUV optical lenses. In this study, we have found out experimentally that LaF3 grows hetero-epitaxially onto CaF2 (1 1 1). On the other hand, LaF3 films fabricated so far were random
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poly-crystallites. We have also made it clear that poor crystallinity and porous structures of LaF3 films affect optical transparency in VUV regions. The epitaxial growth of LaF3 is very effective in order to reduce photo-absorption of LaF3 films in VUV regions. 2. Experimental details 2.1. Film preparation CaF2 single crystals and SiO2 glasses polished finely and cleaned ultrasonically were supplied as substrates. The substrate shape was ⭋ 30 mm-disk and the thickness was 3 mm. As for the CaF2 substrates supplied, two kinds of surfaces for deposition, viz., 28- and 158off CaF2 (1 1 1) planes, were prepared. These -off angles were precisely measured with XRD. The SiO2 glasses were used as a typical substrate that did not have any orientations at all. LaF3 films with a thickness of 500 nm were fabricated on both CaF2 and SiO2 substrates under the identical condition by resistively heated vacuum evaporation. LaF3 source was crystal grains with purity of 99.9% and sizes of 2–5 mm3. Background pressure in a vacuum chamber was ;10y5 Pa and the deposition rate was 6 nmymin. The substrates were heated with radiation in advance and substrate temperature was maintained at 250 8C during film deposition. 2.2. Film characterization Crystalline structures of LaF3 films on CaF2 and SiO2 substrates and the hetero-epitaxial growth of LaF3 onto CaF2 were studied with XRD (PHILLIPS, X’PertMRD). Microstructures of LaF3 films on CaF2 and SiO2 substrates were observed with scanning electron microscopy (SEM: HITACHI, S-5000H). Chemical compositions of overall LaF3 films on CaF2 and SiO2 substrates were estimated with electron probe microanalysis (EPMA: JEOL, JXA-8800). A LaF3 crystal grain with a size of approximately 5 mm3 was used as a reference of quantitative calculation with EPMA. Chemical bonding states at the surfaces of LaF3 films on CaF2 and SiO2 substrates were analyzed with high resolution X-ray photoelectron spectroscopy (XPS: Physical Electronics, Quantum 2000). In XPS depth analyses with a sputter-technique by accelerated Arq ions, LaF3 films were selectively sputtered. La-rich surfaces were formed as a result of destruction and defluorination by sputtering. XPS from such damaged surfaces were meaningless. Therefore, in this study, the film surfaces only were analyzed without the sputtertechnique. This selective sputtering phenomenon commonly appears in almost of fluoride and other compounds composed of both large and small atoms, similar to LaF3. Concentration depth profiles of chemical species
Fig. 1. XRD 2u–u patterns from LaF3 films on SiO2 glass and 28-off CaF2 (1 1 1).
involved in LaF3 films on CaF2 and SiO2 substrates were investigated with dynamic secondary ion mass spectrometry (SIMS: Physical Electronics, Adept 1010). In SIMS, since species sputtered from the surface of a sample are ionized and counted, selective sputtering phenomena can be neglected. Therefore, SIMS is suitable for depth analyses of chemical species in compounds. Transmission and reflection spectra of the LaF3 films on CaF2 substrates were measured with a DUV–VUV spectrophotometer in the range of 130–250 nm at 1 nm intervals. A deuterium lamp was used as a light source. The samples were introduced inside a chamber which was continuously purged with filtered dry N2 gas to prevent photo-absorption of airborne organic contaminants. The measurement was carried out as follows. At first, nothing was set across a beam pass and the original intensity of light at both transmission and reflection modes was measured as references. Secondary, LaF3coated CaF2 substrates were set across the beam pass. The intensity of the light transmitted through the LaF3coated CaF2 substrates and the light reflected on the substrates were measured. Both transmittance and reflectance were not relative values but absolute ones. Reflection angle at the reflection mode was 108. The optical loss is estimated as follows; L (%)s100ytransmittanceyreflectance.
(1)
3. Results and discussion 3.1. XRD study Fig. 1 shows XRD 2u–u patterns from LaF3 films on SiO2 glass and 28-off CaF2 (1 1 1) substrates. In the pattern from the LaF3 film on the SiO2 glass substrate, 0 0 2, 1 1 1, 3 0 0, 1 1 3, 0 0 4 and 302 reflections of
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Fig. 2. Pole figures of the LaF3 film on 28-off CaF2 (1 1 1), (a) a 0 0 0 2 reflection of LaF3 and (b) reflections of 1 1 2¯ 3 and its family of LaF3.
¯ LaF3 (P3c1) are detected. The pattern is in disagreement with the power diffraction pattern of JCPDS 84-0943 as to the ratio of a 0 0 2 reflection to other lines. Relative intensity of the 0 0 2 reflection from the LaF3 film on the SiO2 glass is much stronger than that of the powder. This result indicates that the LaF3 film on the SiO2 glass is a mixture of c-axis oriented domains and random poly-crystallites. In the pattern from the LaF3 film on the 28-off CaF2 (1 1 1), 0 0 l reflections of only basal ¯ planes in LaF3 (P3c1) and a 1 1 1 reflection of CaF2 (Fm3m) are obtained. This result indicates the c-axis orientation growth of the LaF3 film on the 28-off CaF2 (1 1 1). Moreover, pole figure measurement was carried out in order to acquire reliable information on in-plane directions of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). Pole figures of the LaF3 film on the 28off CaF2 (1 1 1) are shown in Fig. 2. Fig. 2a and b are a 0 0 0 2 reflection of LaF3 and reflections of 1 1 2¯ 3 and its family of LaF3, respectively. Hexagonal sym-
metric peaks can be seen in Fig. 2b. Therefore, the LaF3 film on the 28-off CaF2 (1 1 1) is orderly arranged in-plane direction as well as c-axis direction. Pole figures of the LaF3 film on the 158-off CaF2 (1 1 1) are shown in Fig. 3. Fig. 3a–c are a 1 1 1 reflection of CaF2, a 0 0 0 2 reflection of LaF3 and reflections of 1 1 2¯ 3 and its family of LaF3, respectively. As shown in Fig. 3a and b, the 1 1 1 reflection of CaF2 is located at 158 off the coated surface and the 0 0 0 2 reflection of LaF3 is also located at 158 off the surface. That is, w0 0 lx direction of LaF3 is parallel with w1 1 1x direction of CaF2. Besides, hexagonal symmetric peaks whose center is located at 158 off the coated surface can be observed in Fig. 3c. The LaF3 film on the 158-off CaF2 (1 1 1) is orderly arranged in-plane direction as well as c-axis direction, equally to the LaF3 film on the 28-off CaF2 (1 1 1). Rocking curves of 0 0 0 4 and 1 1 2¯ 1 reflections from the LaF3 films on the 28- and 158-off CaF2 (1 1 1) by 2u-fixed u scan XRD. The results are shown in Table
Fig. 3. Pole figures of the LaF3 film on 158-off CaF2 (1 1 1), (a) a 1 1 1 reflection of CaF2; (b) a 0 0 0 2 reflection of LaF3 and (c) reflections of 1 1 2¯ 3 and its family of LaF3.
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Table 1 Results of rocking curve measurement of the LaF3 films on 28- and 158-off CaF2 (1 1 1) Sample
Reflection
FWHM (8)
Spacing (nm)
LaF3 film on 28-off CaF2 (1 1 1)
0004 1 1 2¯ 1 0004 1 1 2¯ 1
0.356 0.715 0.409 0.651
0.1858 0.3233 0.1896 0.3123
LaF3 film on 158-off CaF2 (1 1 1)
1. FWHM values of 0 0 0 4 and 1 1 2¯ 1 peaks represent basal plane’s disorder and hexagon’s twist of the LaF3 films, respectively. In both cases of 28- and 158-off CaF2 (1 1 1), basal plane’s disorder and hexagon’s twist of the LaF3 films are minute enough to be called the hetero-epitaxial growth. As far as literature cited, it is the first experimental confirmation of the fact that LaF3 grows hetero-epitaxially on CaF2 (1 1 1). In addition, the hexagonal lattice of the LaF3 film on the 158off CaF2 (1 1 1) has shorter a-axis and longer c-axis than that of the LaF3 film on the 28-off CaF2 (1 1 1). On the basis of XRD results above, the heteroepitaxial growth of LaF3 onto CaF2 (1 1 1) has been concluded below. CaF2 unit cell, LaF3 unit cell, CaF2 (1 1 1) plane and LaF3 basal plane are shown in Fig. 4. Numerals in parentheses are the number of ions in each unit cell. CaF2 has a cubic lattice named fluorite-type (Fm3m). LaF3, on the other hand, has a trigonal or ¯ or P63 ymmc). In sight simple hexagonal lattice (P3c1 of CaF2 (1 1 1) plane, Ca2q or Fy ions sit on apexes of equilateral triangles. Ca2q triangle layer and Fy triangle layer are alternately stacked. Stacking LaF3
basal plane on the triangle layer, layer mismatch is 6.98%. A side of LaF3 basal plane is nearly twice as long as a side in the triangles of CaF2 (1 1 1). 3.2. SEM observation A sample for SEM observation was prepared as follows. At first, a sample was cut along with a side in the triangles on CaF2 (1 1 1) planes as precisely as possible. The section that appeared after cutting was named cross section A. Then, the sample was cut along with the face that met cross section A at right angles. The section that appeared after cutting was named cross section B. In SEM images of Figs. 5–7, observational directions from cross-section A and cross-section B are called direction A and direction B, respectively. Bird’seye views from direction A and B are also called bird’seye view A and B. Images of cross sections and bird’s-eye views are taken from both direction A and B. Top views above film surfaces were observed as well. Fig. 5 shows SEM images of the LaF3 film on the 28-off CaF2 (1 1 1). Hexagonal texture can be seen. In
Fig. 4. Illustrations of CaF2 unit cell, LaF3 unit cell, CaF2 (1 1 1) plane and LaF3 basal plane.
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Fig. 5. SEM images and illustrations of the LaF3 film on 28-off CaF2 (1 1 1).
the top view, hexagons are spread all over the film surface systematically. In cross section A and bird’s-eye view A from direction A, it can be recognized that a flat face on cross section A is equivalent of hexagons’ sides on the film surface. On the other hand, from direction B, sharp tips on a rugged face of cross section B are recognized to be apexes of hexagons. In conclusion, this film has the dense structure comparing to bulk, as illustrated in Fig. 5. Fig. 6 shows SEM images of the LaF3 film on the 158-off CaF2 (1 1 1). In the top view, terraced field-like texture can be observed. From direction A, sheets are
put step by step on cross section A and one sheet is one step of terraces shown in bird’s-eye view A. On the other hand, from direction B, there are so many stripes inclining at approximately 158 off the vertical line on cross section B and each stripe is equivalent of each step of terraces shown in bird’s-eye view B. Therefore, the film has the structure where inclining sheets are standing orderly, as illustrated in Fig. 6. There are very narrow openings among the inclining sheets and it is considered that the air can enter and absorb there. Fig. 7 shows SEM images of the LaF3 film on the SiO2 glass. At first, cross section A is a section cut by
Fig. 6. SEM images and illustrations of the LaF3 film on 158-off CaF2 (1 1 1).
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Fig. 7. SEM images and illustrations of the LaF3 film on SiO2 glass.
chance because the substrate does not have any orientation. The film consists of long slender columns and narrow openings in which the air can easily enter and absorb. This porous structure is illustrated in Fig. 7. SEM observations in Figs. 5–7 have clarified that total area exposed to the air in a LaF3 film is in order: LaF3 film on the SiO2 glass4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). 3.3. Relationship between chemical bonding states and nanostructures
at the film surfaces. The film surfaces are oxidized and hydroxidized as well as physically adsorbed by H2O. Thus, the film surfaces are lack of fluorine. La–O and La–OH bonds result from chemical reactions with H2O and O2 adsorbed on the film surfaces. The film surface on the SiO2 glass has more oxygen atomic concentration, and is much more oxidized and hydroxidized than those on the offset CaF2 (1 1 1). This result is easily explained from SEM images of top views in Figs. 5–7. The film surface on the SiO2 glass is rough and has much larger area where H2O and O2 are able to adsorb. Moreover, as mentioned above with SEM images, there is possi-
Chemical compositions of LaF3 films estimated with EPMA are indicated in Table 2. Characteristic X-ray signals collected cannot be limited at a surface or any depths. Therefore, each composition means an average value of an overall LaF3 film. These LaF3 films are totally stoichiometric. The oxygen concentration of the LaF3 film on the SiO2 glass is four times higher than those of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). There is no difference between the films on two kinds of offset CaF2 substrates within EPMA sensibility. XPS O 1s spectra from the surfaces of LaF3 films are shown in Fig. 8. These spectra obtained, of course, display bonding conditions of oxygen atoms that exist Table 2 Chemical compositions of LaF3 films on SiO2 glass, and 28- and 158off CaF2 (1 1 1) estimated with EPMA Sample
LayFyO atomic ratio
LaF3 film on 28-off CaF2 (1 1 1) LaF3 film on 158-off CaF2 (1 1 1) LaF3 film on SiO2 glass
1.00y3.05y0.018 1.00y3.01y0.017 1.00y3.06y0.068
Fig. 8. XPS O 1s spectra from the surfaces of LaF3 films on SiO2 glass, and 28- and 158-off CaF2 (1 1 1).
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and XPS that the exposed areas in the LaF3 films are also oxidized and hydroxidized similar to the film surfaces. Relationship between chemical bonding states and nanostructures of the LaF3 films is concluded as follows. The insides of the LaF3 films are stoichiometric. The total exposed areas including the film surfaces are oxidized and hydroxidized, in other words, they run out of fluorine owing to La–O and La–OH. This is a material feature of LaF3 and it is impossible to prevent total exposed area of LaF3 from oxidization and hydroxidization. In brief, total OqOH concentrations in a LaF3 film are in proportion to total area exposed to the air in the film, and it is in order: LaF3 film on the SiO2 glass 4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). 3.4. Effect of nanostructures on VUV transparency
Fig. 9. SIMS concentration depth profiles of (a) O and (b) OH species involved in LaF3 films on SiO2 glass, and 28- and 158-off CaF2 (1 1 1).
bility that the air enter narrow openings in these three types of LaF3 films and adsorb on the exposed areas. Therefore, it is presumed that such exposed areas in the films are also oxidized and hydroxidized similar to the film surfaces. Fig. 9 shows extremely small quantities of (a) O and (b) OH concentration depth profiles of LaF3 films investigated with SIMS. In Fig. 9, sputtering time of the horizontal line represents the depths from the film surfaces. The sputtering time of 0 s means the film surfaces, which were analyzed with XPS. The interfaces between the LaF3 films and the substrates lie at approximately 900–1000 s. O and OH concentrations of the LaF3 film on the SiO2 glass are uniformly high through the depth direction. On the contrary, in the LaF3 films on the offset CaF2 (1 1 1), O and OH species involved are low levels except the regions near the film surfaces. In the depth regions approximately 300–800 s, the O and OH concentrations of the LaF3 film on the SiO2 glass are approximately 10 times and 40 times as high as those on the offset CaF2 (1 1 1), respectively. In addition, SIMS investigation revealed the difference between the LaF3 films on two kinds of offset CaF2 (1 1 1) substrates, which could not be recognized with EPMA. In case of the LaF3 film on the 28-off CaF2 (1 1 1), both O and OH species locate at just the film surface. On the other hand, the LaF3 film on the 158off CaF2 (1 1 1) involves much O and OH species at the region of 0–300 s near the film surface. In summary, SIMS results has proved the presumption from SEM
Fig. 10 shows transmission and reflection spectra of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). The LaF3 film on the 158-off CaF2 (1 1 1) has less transmittance than the LaF3 film on the 28-off CaF2 (1 1 1) in the VUV range under 185 nm because of photo-absorption by La–O and La–OH bonds. The difference in transmittance is increasing at shorter wavelengths. The film on the 28-off CaF2 (1 1 1) has higher reflectance than the film on the 158-off CaF2 (1 1 1) because the film on the 28-off CaF2 (1 1 1) has a denser structure comparing to bulk and a higher refractive index. Fig. 11 shows optical losses of the LaF3 films on the 28- and 158-off CaF2 (1 1 1) at 130, 140, 150, 157 (F2 laser), 170, 180 and 193 (ArF laser) nm calculated by using Eq. (1). At 193 nm of ArF laser in the DUV region, optical losses of both films are approximately equal to zero. However, in the VUV region, remarkable
Fig. 10. Transmission and reflection spectra of the LaF3 films on 28and 158-off CaF2 (1 1 1).
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LaF3 films which have low photo-absorption in VUV regions. 4. Conclusions
Fig. 11. Optical losses of the LaF3 films on 28- and 158-off CaF2 (1 1 1) at 130, 140, 150, 157, 170, 180 and 193 nm calculated by using Eq. (1).
optical losses exist. The LaF3 film on the 158-off CaF2 (1 1 1) has larger optical losses than the LaF3 film on the 28-off CaF2 (1 1 1). The difference in optical loss is increasing at shorter wavelengths. Consequently, optical losses of a LaF3 film in VUV regions are in proportion to total OqOH concentrations in the film, and it is in order: LaF3 film on the SiO2 glass 4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). The SiO2 glass substrates used in this study is a representative of the materials which are not effective in growing LaF3 epitaxially. Polycrystalline LaF3 films are deposited on all kinds of materials useless for the epitaxial growth, similar to the case of SiO2 glass, and the polycrystalline films must lose much more VUV light than epitaxial LaF3 films. VUV light is much more absorbed at oxidized, hydroxidized and F-poor exposed areas in LaF3 films than the stoichiometric film insides. It is significant for LaF3 film deposition to make dense film structures and reduce the total areas exposed to the air. The heteroepitaxial growth of LaF3 films on CaF2 (1 1 1) is one of the most important ways to fabricate bulk-like dense
In this study, it has been revealed that LaF3 films grow hetero-epitaxially on CaF2 (1 1 1) at 250 8C and the films have dense structures, ranking with LaF3 bulk. On the other hand, conventional LaF3 films deposited on the materials useless for the epitaxial growth consist of random poly-crystallized columns and narrow openings in which the air can easily enter and absorb. Total exposed areas to the air in LaF3 films are oxidized and hydroxidized even if the insides of the films are precisely stoichiometric. This is a material feature of LaF3 and it is impossible to prevent the total exposed areas in LaF3 from oxidization and hydroxidization. La–O and La–OH absorb VUV light under 185 nm. Conventional columnar LaF3 films have serious photo-absorptions in VUV regions. Therefore, to reduce the total exposed areas in LaF3 films is very significant for VUV applications. The hetero-epitaxial growth of LaF3 films on CaF2 (1 1 1) is one of the most important ways to fabricate bulk-like dense LaF3 films which have little exposed areas at relatively low temperatures. The optical transparency of LaF3 films in VUV regions has been improved using the hetero-epitaxial growth technique. References w1x S. Niisaka, T. Saito, J. Saito, A. Tanaka, A. Matsumoto, M. Otani, R. Biro, C. Ouchi, M. Hasegawa, Y. Suzuki, K. Sone, Appl. Opt. 41 (2002) 3242. w2x P.W. Sullivan, T.I. Cox, R.F.C. Farrow, G.R. Jones, D.B. Gasson, C.S. Smith, J. Vac. Sci. Technol. 20 (1982) 731. w3x J.D. Targove, L.J. Lingg, J.P. Lehan, H.A. Macleod, Appl. Optics 27 (1988) 213. w4x J. Kolbe, H. Schink, SPIE Thin Films Optical Syst. 1782 (1992) 435. w5x W. Korczak, P. Mikolajczak, J. Crystal Growth 61 (1983) 601. w6x D. Gregson, C.R.A. Catlow, A.V. Chadwick, G.H. Lander, A.N. Cormack, B.E.F. Fender, Acta Crystal B39 (1983) 687. w7x A. Zalkin, D.H. Templeton, Acta Crystal B41 (1985) 91. w8x A.F. Privalov, H.M. Vieth, I.V. Murin, J. Phys. Chem. Solids 50 (1989) 395. w9x K. Schubert, J. Less-Common Metals 154 (1989) 39.
Hetero-epitaxial growth and optical properties of LaF3 on CaF2 Yusuke Taki*, Kenichi Muramatsu Thin Films R&D Group1, Lens Engineering Development Department, Nikon Corporation, 10-1, Asamizodai 1-Chome, Sagamihara, Kanagawa 228-0828, Japan
Abstract LaF3 films were fabricated on CaF2 (1 1 1) and SiO2 glass substrates at a relatively low temperature of 250 8C by vacuum evaporation. X-ray diffraction revealed that w0 0 lx of the LaF3 film deposited on CaF2 (1 1 1) was parallel with w1 1 1x of CaF2 and the film was also orderly arranged in-plane direction. It has been found out experimentally that LaF3 grows epitaxially on CaF2 (1 1 1). The LaF3 film on CaF2 (1 1 1) had a bulk-like dense structure. On the other hand, SiO2 glass was selected as a typical substrate that did not have any orientation at all. The LaF3 film on SiO2 glass was random poly-crystallites and the film structure was columnar with narrow openings where the air could easily enter and absorb. The insides of LaF3 films on CaF2 (1 1 1) and SiO2 glass had accurately stoichiometric compositions. However, in the films, total areas exposed to the air were hydroxidized and oxidized because of chemical reactions with water and molecular oxygen adsorbed on the exposed areas. Thus, the exposed areas were lack of fluorine. The total area exposed to the air in the columnar LaF3 film on SiO2 glass was much larger than that in the epitaxial LaF3 film on CaF2 (1 1 1). Therefore, the film on SiO2 glass had much more O and OH concentrations than the film on CaF2 (1 1 1). Vacuum ultraviolet (VUV) light was much more absorbed at F-poor areas exposed to the air in LaF3 films than the stoichiometric film insides. It is significant for LaF3 film deposition to make dense film structures and reduce the total exposed areas. Polycrystalline columnar LaF3 films on the materials useless for the epitaxial growth as well as SiO2 glass have much larger optical losses than the epitaxial LaF3 films. The optical transparency of LaF3 films in VUV regions has been improved using hetero-epitaxial growth technique. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Vacuum evaporation; Optical films; Fluoride; Epitaxy; Vacuum ultraviolet (VUV)
1. Introduction Lanthanum fluoride, LaF3, is a primary candidate of high refractive index materials available for optical interference multilayers in deep ultraviolet (DUV) and vacuum ultraviolet (VUV) regions. LaF3 films have been fabricated by many researchers and engineers by using various PVD processes such as vacuum evaporation w1x, molecular beam epitaxy w2x, ion assisted deposition w3x, ion beam sputtering w4x and magnetron sputtering. Vacuum evaporation, currently, is the most appropriate process to prepare LaF3 films with the lowest photo-absorption in VUV regions among all the processes. However, LaF3 films prepared by the vacuum evaporation still have remarkable photo-absorption that must be reduced. Therefore, LaF3 films dominate optical performances of multilayers, which are alternate stacking of LaF3 and MgF2 or AlF3 as a low index material. *Corresponding author. Tel.: q81-42-740-6491; fax: q81-42-7406322. E-mail address: [email protected] (Y. Taki).
LaF3 is also noted as a super ionic conductor and its crystal structure, tysonite, has been minutely examined with X-ray diffraction (XRD) w5x, neutron diffraction w6,7x, 19F NMR w8x, etc. In spite of these examinations, LaF3 lattice structure is still unclear. Nowadays, it is the predominant view that tysonite possesses a trigonal ¯ lattice (P3c1) where fluorine anions occupy three distinct sites. However, a simple hexagonal lattice is not completely denied. Since two fluorine sites of three in the trigonal lattice are structurally and dynamically equivalent w8x, the trigonal lattice may be replaced by the simple hexagonal lattice (P63 ymmc) that has only two fluorine sites. Anyway, both lattices are hexagonaltype. In addition, energetic calculations w9x presume that LaF3 is compatible with CaF2 or CaF2 –YF3 mixture. As for CaF2, single crystals with extremely low photoabsorption are used for VUV–DUV optical lenses. In this study, we have found out experimentally that LaF3 grows hetero-epitaxially onto CaF2 (1 1 1). On the other hand, LaF3 films fabricated so far were random
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poly-crystallites. We have also made it clear that poor crystallinity and porous structures of LaF3 films affect optical transparency in VUV regions. The epitaxial growth of LaF3 is very effective in order to reduce photo-absorption of LaF3 films in VUV regions. 2. Experimental details 2.1. Film preparation CaF2 single crystals and SiO2 glasses polished finely and cleaned ultrasonically were supplied as substrates. The substrate shape was ⭋ 30 mm-disk and the thickness was 3 mm. As for the CaF2 substrates supplied, two kinds of surfaces for deposition, viz., 28- and 158off CaF2 (1 1 1) planes, were prepared. These -off angles were precisely measured with XRD. The SiO2 glasses were used as a typical substrate that did not have any orientations at all. LaF3 films with a thickness of 500 nm were fabricated on both CaF2 and SiO2 substrates under the identical condition by resistively heated vacuum evaporation. LaF3 source was crystal grains with purity of 99.9% and sizes of 2–5 mm3. Background pressure in a vacuum chamber was ;10y5 Pa and the deposition rate was 6 nmymin. The substrates were heated with radiation in advance and substrate temperature was maintained at 250 8C during film deposition. 2.2. Film characterization Crystalline structures of LaF3 films on CaF2 and SiO2 substrates and the hetero-epitaxial growth of LaF3 onto CaF2 were studied with XRD (PHILLIPS, X’PertMRD). Microstructures of LaF3 films on CaF2 and SiO2 substrates were observed with scanning electron microscopy (SEM: HITACHI, S-5000H). Chemical compositions of overall LaF3 films on CaF2 and SiO2 substrates were estimated with electron probe microanalysis (EPMA: JEOL, JXA-8800). A LaF3 crystal grain with a size of approximately 5 mm3 was used as a reference of quantitative calculation with EPMA. Chemical bonding states at the surfaces of LaF3 films on CaF2 and SiO2 substrates were analyzed with high resolution X-ray photoelectron spectroscopy (XPS: Physical Electronics, Quantum 2000). In XPS depth analyses with a sputter-technique by accelerated Arq ions, LaF3 films were selectively sputtered. La-rich surfaces were formed as a result of destruction and defluorination by sputtering. XPS from such damaged surfaces were meaningless. Therefore, in this study, the film surfaces only were analyzed without the sputtertechnique. This selective sputtering phenomenon commonly appears in almost of fluoride and other compounds composed of both large and small atoms, similar to LaF3. Concentration depth profiles of chemical species
Fig. 1. XRD 2u–u patterns from LaF3 films on SiO2 glass and 28-off CaF2 (1 1 1).
involved in LaF3 films on CaF2 and SiO2 substrates were investigated with dynamic secondary ion mass spectrometry (SIMS: Physical Electronics, Adept 1010). In SIMS, since species sputtered from the surface of a sample are ionized and counted, selective sputtering phenomena can be neglected. Therefore, SIMS is suitable for depth analyses of chemical species in compounds. Transmission and reflection spectra of the LaF3 films on CaF2 substrates were measured with a DUV–VUV spectrophotometer in the range of 130–250 nm at 1 nm intervals. A deuterium lamp was used as a light source. The samples were introduced inside a chamber which was continuously purged with filtered dry N2 gas to prevent photo-absorption of airborne organic contaminants. The measurement was carried out as follows. At first, nothing was set across a beam pass and the original intensity of light at both transmission and reflection modes was measured as references. Secondary, LaF3coated CaF2 substrates were set across the beam pass. The intensity of the light transmitted through the LaF3coated CaF2 substrates and the light reflected on the substrates were measured. Both transmittance and reflectance were not relative values but absolute ones. Reflection angle at the reflection mode was 108. The optical loss is estimated as follows; L (%)s100ytransmittanceyreflectance.
(1)
3. Results and discussion 3.1. XRD study Fig. 1 shows XRD 2u–u patterns from LaF3 films on SiO2 glass and 28-off CaF2 (1 1 1) substrates. In the pattern from the LaF3 film on the SiO2 glass substrate, 0 0 2, 1 1 1, 3 0 0, 1 1 3, 0 0 4 and 302 reflections of
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Fig. 2. Pole figures of the LaF3 film on 28-off CaF2 (1 1 1), (a) a 0 0 0 2 reflection of LaF3 and (b) reflections of 1 1 2¯ 3 and its family of LaF3.
¯ LaF3 (P3c1) are detected. The pattern is in disagreement with the power diffraction pattern of JCPDS 84-0943 as to the ratio of a 0 0 2 reflection to other lines. Relative intensity of the 0 0 2 reflection from the LaF3 film on the SiO2 glass is much stronger than that of the powder. This result indicates that the LaF3 film on the SiO2 glass is a mixture of c-axis oriented domains and random poly-crystallites. In the pattern from the LaF3 film on the 28-off CaF2 (1 1 1), 0 0 l reflections of only basal ¯ planes in LaF3 (P3c1) and a 1 1 1 reflection of CaF2 (Fm3m) are obtained. This result indicates the c-axis orientation growth of the LaF3 film on the 28-off CaF2 (1 1 1). Moreover, pole figure measurement was carried out in order to acquire reliable information on in-plane directions of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). Pole figures of the LaF3 film on the 28off CaF2 (1 1 1) are shown in Fig. 2. Fig. 2a and b are a 0 0 0 2 reflection of LaF3 and reflections of 1 1 2¯ 3 and its family of LaF3, respectively. Hexagonal sym-
metric peaks can be seen in Fig. 2b. Therefore, the LaF3 film on the 28-off CaF2 (1 1 1) is orderly arranged in-plane direction as well as c-axis direction. Pole figures of the LaF3 film on the 158-off CaF2 (1 1 1) are shown in Fig. 3. Fig. 3a–c are a 1 1 1 reflection of CaF2, a 0 0 0 2 reflection of LaF3 and reflections of 1 1 2¯ 3 and its family of LaF3, respectively. As shown in Fig. 3a and b, the 1 1 1 reflection of CaF2 is located at 158 off the coated surface and the 0 0 0 2 reflection of LaF3 is also located at 158 off the surface. That is, w0 0 lx direction of LaF3 is parallel with w1 1 1x direction of CaF2. Besides, hexagonal symmetric peaks whose center is located at 158 off the coated surface can be observed in Fig. 3c. The LaF3 film on the 158-off CaF2 (1 1 1) is orderly arranged in-plane direction as well as c-axis direction, equally to the LaF3 film on the 28-off CaF2 (1 1 1). Rocking curves of 0 0 0 4 and 1 1 2¯ 1 reflections from the LaF3 films on the 28- and 158-off CaF2 (1 1 1) by 2u-fixed u scan XRD. The results are shown in Table
Fig. 3. Pole figures of the LaF3 film on 158-off CaF2 (1 1 1), (a) a 1 1 1 reflection of CaF2; (b) a 0 0 0 2 reflection of LaF3 and (c) reflections of 1 1 2¯ 3 and its family of LaF3.
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Table 1 Results of rocking curve measurement of the LaF3 films on 28- and 158-off CaF2 (1 1 1) Sample
Reflection
FWHM (8)
Spacing (nm)
LaF3 film on 28-off CaF2 (1 1 1)
0004 1 1 2¯ 1 0004 1 1 2¯ 1
0.356 0.715 0.409 0.651
0.1858 0.3233 0.1896 0.3123
LaF3 film on 158-off CaF2 (1 1 1)
1. FWHM values of 0 0 0 4 and 1 1 2¯ 1 peaks represent basal plane’s disorder and hexagon’s twist of the LaF3 films, respectively. In both cases of 28- and 158-off CaF2 (1 1 1), basal plane’s disorder and hexagon’s twist of the LaF3 films are minute enough to be called the hetero-epitaxial growth. As far as literature cited, it is the first experimental confirmation of the fact that LaF3 grows hetero-epitaxially on CaF2 (1 1 1). In addition, the hexagonal lattice of the LaF3 film on the 158off CaF2 (1 1 1) has shorter a-axis and longer c-axis than that of the LaF3 film on the 28-off CaF2 (1 1 1). On the basis of XRD results above, the heteroepitaxial growth of LaF3 onto CaF2 (1 1 1) has been concluded below. CaF2 unit cell, LaF3 unit cell, CaF2 (1 1 1) plane and LaF3 basal plane are shown in Fig. 4. Numerals in parentheses are the number of ions in each unit cell. CaF2 has a cubic lattice named fluorite-type (Fm3m). LaF3, on the other hand, has a trigonal or ¯ or P63 ymmc). In sight simple hexagonal lattice (P3c1 of CaF2 (1 1 1) plane, Ca2q or Fy ions sit on apexes of equilateral triangles. Ca2q triangle layer and Fy triangle layer are alternately stacked. Stacking LaF3
basal plane on the triangle layer, layer mismatch is 6.98%. A side of LaF3 basal plane is nearly twice as long as a side in the triangles of CaF2 (1 1 1). 3.2. SEM observation A sample for SEM observation was prepared as follows. At first, a sample was cut along with a side in the triangles on CaF2 (1 1 1) planes as precisely as possible. The section that appeared after cutting was named cross section A. Then, the sample was cut along with the face that met cross section A at right angles. The section that appeared after cutting was named cross section B. In SEM images of Figs. 5–7, observational directions from cross-section A and cross-section B are called direction A and direction B, respectively. Bird’seye views from direction A and B are also called bird’seye view A and B. Images of cross sections and bird’s-eye views are taken from both direction A and B. Top views above film surfaces were observed as well. Fig. 5 shows SEM images of the LaF3 film on the 28-off CaF2 (1 1 1). Hexagonal texture can be seen. In
Fig. 4. Illustrations of CaF2 unit cell, LaF3 unit cell, CaF2 (1 1 1) plane and LaF3 basal plane.
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Fig. 5. SEM images and illustrations of the LaF3 film on 28-off CaF2 (1 1 1).
the top view, hexagons are spread all over the film surface systematically. In cross section A and bird’s-eye view A from direction A, it can be recognized that a flat face on cross section A is equivalent of hexagons’ sides on the film surface. On the other hand, from direction B, sharp tips on a rugged face of cross section B are recognized to be apexes of hexagons. In conclusion, this film has the dense structure comparing to bulk, as illustrated in Fig. 5. Fig. 6 shows SEM images of the LaF3 film on the 158-off CaF2 (1 1 1). In the top view, terraced field-like texture can be observed. From direction A, sheets are
put step by step on cross section A and one sheet is one step of terraces shown in bird’s-eye view A. On the other hand, from direction B, there are so many stripes inclining at approximately 158 off the vertical line on cross section B and each stripe is equivalent of each step of terraces shown in bird’s-eye view B. Therefore, the film has the structure where inclining sheets are standing orderly, as illustrated in Fig. 6. There are very narrow openings among the inclining sheets and it is considered that the air can enter and absorb there. Fig. 7 shows SEM images of the LaF3 film on the SiO2 glass. At first, cross section A is a section cut by
Fig. 6. SEM images and illustrations of the LaF3 film on 158-off CaF2 (1 1 1).
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Fig. 7. SEM images and illustrations of the LaF3 film on SiO2 glass.
chance because the substrate does not have any orientation. The film consists of long slender columns and narrow openings in which the air can easily enter and absorb. This porous structure is illustrated in Fig. 7. SEM observations in Figs. 5–7 have clarified that total area exposed to the air in a LaF3 film is in order: LaF3 film on the SiO2 glass4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). 3.3. Relationship between chemical bonding states and nanostructures
at the film surfaces. The film surfaces are oxidized and hydroxidized as well as physically adsorbed by H2O. Thus, the film surfaces are lack of fluorine. La–O and La–OH bonds result from chemical reactions with H2O and O2 adsorbed on the film surfaces. The film surface on the SiO2 glass has more oxygen atomic concentration, and is much more oxidized and hydroxidized than those on the offset CaF2 (1 1 1). This result is easily explained from SEM images of top views in Figs. 5–7. The film surface on the SiO2 glass is rough and has much larger area where H2O and O2 are able to adsorb. Moreover, as mentioned above with SEM images, there is possi-
Chemical compositions of LaF3 films estimated with EPMA are indicated in Table 2. Characteristic X-ray signals collected cannot be limited at a surface or any depths. Therefore, each composition means an average value of an overall LaF3 film. These LaF3 films are totally stoichiometric. The oxygen concentration of the LaF3 film on the SiO2 glass is four times higher than those of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). There is no difference between the films on two kinds of offset CaF2 substrates within EPMA sensibility. XPS O 1s spectra from the surfaces of LaF3 films are shown in Fig. 8. These spectra obtained, of course, display bonding conditions of oxygen atoms that exist Table 2 Chemical compositions of LaF3 films on SiO2 glass, and 28- and 158off CaF2 (1 1 1) estimated with EPMA Sample
LayFyO atomic ratio
LaF3 film on 28-off CaF2 (1 1 1) LaF3 film on 158-off CaF2 (1 1 1) LaF3 film on SiO2 glass
1.00y3.05y0.018 1.00y3.01y0.017 1.00y3.06y0.068
Fig. 8. XPS O 1s spectra from the surfaces of LaF3 films on SiO2 glass, and 28- and 158-off CaF2 (1 1 1).
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and XPS that the exposed areas in the LaF3 films are also oxidized and hydroxidized similar to the film surfaces. Relationship between chemical bonding states and nanostructures of the LaF3 films is concluded as follows. The insides of the LaF3 films are stoichiometric. The total exposed areas including the film surfaces are oxidized and hydroxidized, in other words, they run out of fluorine owing to La–O and La–OH. This is a material feature of LaF3 and it is impossible to prevent total exposed area of LaF3 from oxidization and hydroxidization. In brief, total OqOH concentrations in a LaF3 film are in proportion to total area exposed to the air in the film, and it is in order: LaF3 film on the SiO2 glass 4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). 3.4. Effect of nanostructures on VUV transparency
Fig. 9. SIMS concentration depth profiles of (a) O and (b) OH species involved in LaF3 films on SiO2 glass, and 28- and 158-off CaF2 (1 1 1).
bility that the air enter narrow openings in these three types of LaF3 films and adsorb on the exposed areas. Therefore, it is presumed that such exposed areas in the films are also oxidized and hydroxidized similar to the film surfaces. Fig. 9 shows extremely small quantities of (a) O and (b) OH concentration depth profiles of LaF3 films investigated with SIMS. In Fig. 9, sputtering time of the horizontal line represents the depths from the film surfaces. The sputtering time of 0 s means the film surfaces, which were analyzed with XPS. The interfaces between the LaF3 films and the substrates lie at approximately 900–1000 s. O and OH concentrations of the LaF3 film on the SiO2 glass are uniformly high through the depth direction. On the contrary, in the LaF3 films on the offset CaF2 (1 1 1), O and OH species involved are low levels except the regions near the film surfaces. In the depth regions approximately 300–800 s, the O and OH concentrations of the LaF3 film on the SiO2 glass are approximately 10 times and 40 times as high as those on the offset CaF2 (1 1 1), respectively. In addition, SIMS investigation revealed the difference between the LaF3 films on two kinds of offset CaF2 (1 1 1) substrates, which could not be recognized with EPMA. In case of the LaF3 film on the 28-off CaF2 (1 1 1), both O and OH species locate at just the film surface. On the other hand, the LaF3 film on the 158off CaF2 (1 1 1) involves much O and OH species at the region of 0–300 s near the film surface. In summary, SIMS results has proved the presumption from SEM
Fig. 10 shows transmission and reflection spectra of the LaF3 films on the 28- and 158-off CaF2 (1 1 1). The LaF3 film on the 158-off CaF2 (1 1 1) has less transmittance than the LaF3 film on the 28-off CaF2 (1 1 1) in the VUV range under 185 nm because of photo-absorption by La–O and La–OH bonds. The difference in transmittance is increasing at shorter wavelengths. The film on the 28-off CaF2 (1 1 1) has higher reflectance than the film on the 158-off CaF2 (1 1 1) because the film on the 28-off CaF2 (1 1 1) has a denser structure comparing to bulk and a higher refractive index. Fig. 11 shows optical losses of the LaF3 films on the 28- and 158-off CaF2 (1 1 1) at 130, 140, 150, 157 (F2 laser), 170, 180 and 193 (ArF laser) nm calculated by using Eq. (1). At 193 nm of ArF laser in the DUV region, optical losses of both films are approximately equal to zero. However, in the VUV region, remarkable
Fig. 10. Transmission and reflection spectra of the LaF3 films on 28and 158-off CaF2 (1 1 1).
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LaF3 films which have low photo-absorption in VUV regions. 4. Conclusions
Fig. 11. Optical losses of the LaF3 films on 28- and 158-off CaF2 (1 1 1) at 130, 140, 150, 157, 170, 180 and 193 nm calculated by using Eq. (1).
optical losses exist. The LaF3 film on the 158-off CaF2 (1 1 1) has larger optical losses than the LaF3 film on the 28-off CaF2 (1 1 1). The difference in optical loss is increasing at shorter wavelengths. Consequently, optical losses of a LaF3 film in VUV regions are in proportion to total OqOH concentrations in the film, and it is in order: LaF3 film on the SiO2 glass 4LaF3 film on the 158-off CaF2 (1 1 1))LaF3 film on the 28-off CaF2 (1 1 1). The SiO2 glass substrates used in this study is a representative of the materials which are not effective in growing LaF3 epitaxially. Polycrystalline LaF3 films are deposited on all kinds of materials useless for the epitaxial growth, similar to the case of SiO2 glass, and the polycrystalline films must lose much more VUV light than epitaxial LaF3 films. VUV light is much more absorbed at oxidized, hydroxidized and F-poor exposed areas in LaF3 films than the stoichiometric film insides. It is significant for LaF3 film deposition to make dense film structures and reduce the total areas exposed to the air. The heteroepitaxial growth of LaF3 films on CaF2 (1 1 1) is one of the most important ways to fabricate bulk-like dense
In this study, it has been revealed that LaF3 films grow hetero-epitaxially on CaF2 (1 1 1) at 250 8C and the films have dense structures, ranking with LaF3 bulk. On the other hand, conventional LaF3 films deposited on the materials useless for the epitaxial growth consist of random poly-crystallized columns and narrow openings in which the air can easily enter and absorb. Total exposed areas to the air in LaF3 films are oxidized and hydroxidized even if the insides of the films are precisely stoichiometric. This is a material feature of LaF3 and it is impossible to prevent the total exposed areas in LaF3 from oxidization and hydroxidization. La–O and La–OH absorb VUV light under 185 nm. Conventional columnar LaF3 films have serious photo-absorptions in VUV regions. Therefore, to reduce the total exposed areas in LaF3 films is very significant for VUV applications. The hetero-epitaxial growth of LaF3 films on CaF2 (1 1 1) is one of the most important ways to fabricate bulk-like dense LaF3 films which have little exposed areas at relatively low temperatures. The optical transparency of LaF3 films in VUV regions has been improved using the hetero-epitaxial growth technique. References w1x S. Niisaka, T. Saito, J. Saito, A. Tanaka, A. Matsumoto, M. Otani, R. Biro, C. Ouchi, M. Hasegawa, Y. Suzuki, K. Sone, Appl. Opt. 41 (2002) 3242. w2x P.W. Sullivan, T.I. Cox, R.F.C. Farrow, G.R. Jones, D.B. Gasson, C.S. Smith, J. Vac. Sci. Technol. 20 (1982) 731. w3x J.D. Targove, L.J. Lingg, J.P. Lehan, H.A. Macleod, Appl. Optics 27 (1988) 213. w4x J. Kolbe, H. Schink, SPIE Thin Films Optical Syst. 1782 (1992) 435. w5x W. Korczak, P. Mikolajczak, J. Crystal Growth 61 (1983) 601. w6x D. Gregson, C.R.A. Catlow, A.V. Chadwick, G.H. Lander, A.N. Cormack, B.E.F. Fender, Acta Crystal B39 (1983) 687. w7x A. Zalkin, D.H. Templeton, Acta Crystal B41 (1985) 91. w8x A.F. Privalov, H.M. Vieth, I.V. Murin, J. Phys. Chem. Solids 50 (1989) 395. w9x K. Schubert, J. Less-Common Metals 154 (1989) 39.