Engineering Desired Grain Orientation and Boundaries Separating Adjacent Grains in Polycrystalline Thin Films
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ABSTRACT A simple process, involving a controlled temperature gradient on the surface of the substrate during the deposition of a thin film is described. This technique is similar to zone-refining but produces chains of impurity free grains. Simultaneously, dopants and other defects are confined to the grain boundaries separating two such chains. For a small temperature gradient only preferred orientation is obtained within a chain. For medium values of temperature gradient the grain boundaries within a chain are engineered to be either on a low energy coincidence site lattice (CSL) or a twin depending on the magnitude of gradient. For high magnitudes of temperature gradient, the chain is predicted to be a single crystal strip. INTRODUCTION In semiconducting materials doping is usually used to generate free electrons. However, the dopant atoms, even if they replace the host atoms substitutionally, introduce strain due to difference in ionic radius as well as being a charge center in the host lattice. Free electron waves no longer see a perfect periodic lattice and are scattered reducing the free electron mobility. Thus even if the material is a single crystal, the mobility is generally lower in a doped material as compared to an undoped one. One solution to the problem may be the production of alternating doped and undoped single crystal multilayer structures so that electrons are generated in the doped layer while being mobile in the undoped one. However, production of such structures is difficult and very expensive and for many materials it is difficult to prevent interlayer diffusion. Polycrystalline materials on the other hand are cost effective and easy to fabricate and thus are very attractive for numerous applications. However, there are several other factors, including those For example, in mentioned above, affecting the physical properties of these materials. orthorhombic YBa 2Cu 30 7 (YBCO) high Tc superconductors, the current density is higher by a factor of 5-10 in the basal plane than in the c-direction [1]. The critical current density at the grain boundary also decreases with increasing misorientation angle in such materials [2]. In polysilicon low angle and high angle grain boundaries have shown strong electrical activity as recombination centers [3]. However coherent twin and other low energy CSL boundaries are not at all or very weakly active. This paper describes a simple inexpensive method to achieve high electron mobilities in polycrystalline thin film materials by: (i) arranging the impurities in the film such that zones of heavily doped regions are formed to provide free electrons, but are separated by relatively impurity free zones to conduct electrons. (ii) by producing such an orientation of grains in the impurity free zones which provides maximum electron mobility within' each grain. (iii) and by producing twin and CSL grain boundaries in the impurity free zones so that mobile electrons suffer minimal scattering while going from one grain to the other. 45 Mat. Res. Soc. Symp. Proc. Vol. 4
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