Epitaxial Growth of Metal Fluoride Thin Films by Pulsed-Laser Deposition
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Mat. Res. Soc. Symp. Proc. Vol. 397 0 1996 Materials Research Society
target. The substrates were heated to 350-450'C with film growth carried out in either vacuum or 100 mTorr argon. Typical growth parameters yielded a film growth rate of -20 A/min. Substrates used in this effort included (100)-oriented CaF2 and (012) A120 3 (sapphire). DISCUSSION Figure 1 shows x-ray diffraction patterns obtained from a GdLiF 4 thin film deposited on (100) CaF 2 . The film was grown at 450'C in vacuum with a base pressure of 6 x 10-7 Torr. The diffraction pattern indicates strong (002)and (004) GdLiF 4 peaks very close to the (OOP) peaks from the CaF 2 substrate, as well as weaker peaks attributed either to polycrystalline GdLiF 4 or an oxy-fluoride impurity phase. A 4)-scan through the GdLiF 4 (105) indicates that the c-axis oriented material is in-plane aligned with lattice parameters of a=5.21A and c=10.98A. The polycrystalline GdLiF 4 observed in the diffraction pattern appears to originate from a significant density of randomly-oriented GdLiF 4 particulates on the film surface. Particulates are often observed in films grown by on-axis PLD, particularly when the ablated material has a relatively low optical absorption coefficient at the ablation wavelength. Under these conditions, the ablation laser pulse interacts with a relatively large volume of the target material. While some of the ablated material is vaporized in the ablation process, a significant fraction is ejected from the target as particulates. The high density of GdLiF 4 particulates in these films reflects the low optical absorption coefficient of the target at 193 nm. a le. s55 10
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Fig. 1 Four-circle x-ray diffraction patterns from a (a) 0- 2 8 scan and (b) 4)-scan through the GdLiF 4 (105) peak for a GdLiF 4 film grown on (100) CaF2 by pulsed-laser deposition. For waveguide applications, surface roughness is a significant issue, as it leads to optical losses. In an attempt to reduce the density of particulates in these films, we investigated the laser ablation growth of Nd-doped GdLiF 4 films, where Nd was added to increase the optical absorption coefficient of the ablation target at 193 nm and permit more efficient laser ablation using an ArF excimer laser. Nd is also an interesting active rare-earth dopant with a high solubility in GdLiF 4 . Several of the absorption lines of Nd+ 3 ions overlap the emission lines of GaAs-GaAlAs lasers, making Nd-doped thin-film structures suitable for use in GaAs-based optoelectronic systems. 8 Using pulsed-laser deposition, we were able to grow c-axis oriented Gd 0 .9 5Ndo. 05LiF 4 films on a variety of substrates, including (100) CaF 2 and A12 0 3 . Figure 2 shows the x-ray diffraction pattern for a 500 nm-thick, c-axis oriented Gd0 .95Nd 0 .05 LiF4 film grown on single crystal A12 0 3 at 350'C. Similar results were observed on (100)
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