Effect of deposition conditions and buffer layers on amorphous or polytype phase formation in Al2O3 thin films by chemical vapor deposition using tri-methyl aluminum
Al2O3 thin films were deposited on (001)Si substrate through Cr2O3/yttria-stabilized-zirconia (YSZ) buffer layer by cold-wall type chemical vapor deposition method with tri-methyl aluminum as a raw material. By changing the deposition temperature, different polytypes of Al2O3 thin films were formed. At lower temperatures (1123–1173 K), η-Al2O3 and amorphous Al2O3 were found in mixture. With increasing the deposition temperature, the series of Al2O3 polytypes (γ, κ and α) appeared in the order of decreasing the unit cell volume per Al atom. At 1323 K, single-phase α-Al2O3 thin film was obtained in success. On (00l)Cr2O3/YSZ/Si substrate, epitaxial (00l)α-Al2O3 thin-film was grown, however, on (00l)YSZ/Si substrate, epitaxial (00l)κ-Al2O3 thin film was formed. It shows that buffer layer also has much influence on polytype of Al2O3 thin film. On the other hand, there exist the polycrystalline α-Al2O3 and Cr2O3 have the same Miller index (h k l), therefore, polycrystalline α-Al2O3 thin film was deposited on the polycrystalline Cr2O3 buffer layer in which each Al2O3 and Cr2O3 grain has an epitaxial relation. This epitaxial growth can be explained by both the similarity in crystal structure between Al2O3 film and Cr2O3 buffer layer, and also their moderate lattice mismatch.
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