Solid-phase epitaxy of ion-implanted LiNbO 3 for optical waveguide fabrication
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I. INTRODUCTION This article reports on the fabrication of planar optical waveguides in LiNbO 3 by Ti implantation. The results contain two important implications: First, in the field of material science, the results show that nonequilibrium processing and solid-phase epitaxy can be successfully applied to a very complex four-constituent system. The regenerated LiNb 1 _ x Ti x O 3 crystal, which is grown from an unstable amorphous solid solution, cannot be grown by other means, for instance, by equilibrium diffusion. Second, for the technology of electro-optics, it is shown that planar optical waveguides of excellent quality can be fabricated by this process and that this process provides a higher Ti concentration in a shallower surface layer, thus achieving a better confinement of light in a smaller volume. Although not discussed in this article, recent results have demonstrated that it is possible to use this versatile approach together with a new masking technique for the fabrication of active devices, such as switches and modulators that will run on a lower drive power than the current layouts. It is well known that ion implantation is a very powerful tool in the fields of metal processing and semiconductor device fabrication. Very frequently it results in unique surface alterations and materials properties not achievable by equilibrium processing methods. However, radiation damage to crystalline samples is often an unwanted consequence that can leave the implanted surface heavily defected or even amorphous. In such instances, a successful thermal treatment must be devised to remove the undesirable damage and restore crystallinity. Successful annealing techniques are of paramount importance in semiconductor processing and range from furnace annealing to pulsed laser proAlso at the Institut fur Festkorperforschung, Kernforschungsanlage, D-5170 Jiilich, W. Germany. b) Permanent address: U. S. Army Missile Laboratory, Redstone Arsenal, Alabama 35898.
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J. Mater. Res. 2 (2), Mar/Apr 1987
http://journals.cambridge.org
cessing. Both the ion-induced damage and the recrystallization are much more complex in LiNbO 3 than in Si. A major accomplishment of the present study was to understand the damage and devise a successful annealing process for this complex electro-optical insulating crystal. To our knowledge, this is the first time that ion implantation has been applied successfully to these materials. The LiNbO 3 crystals have a complex physical and chemical nature. If these crystals are damaged by ion bombardment, they show a very strong tendency to decompose and to display segregation and precipitation. A careful study of the ion-induced segregation and the annealing characteristics of LiNbO 3 has been performed in this laboratory and published recently.1>2 We have been able to suppress the chemical instability problem of the implanted amorphous solid solution by cooling the substrates to liquid nitrogen temperature and thereby reducing the mobility of the constituents sufficiently during implantation to av
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