Niobium prepared by rf sputtering has been found to nucleate and grow differently on the basal plane of low dislocation density (chemical vapor deposited) and high dislocation density (Czochralski‐grown) sapphire ( α ‐ Al 2 O 3 ) wafers. The low dislocation α ‐ Al 2 O 3 showed three different orientations related to each other by a threefold axis: ( 110 ) Nb / / ( 0001 ) α ‐ Al 2 O 3 and [ 110 ] Nb / / a ⃗ 1 , a ⃗ 2 , a ⃗ 3 α ‐ Al 2 O 3 , while those films grown on more imperfect, high dislocation substrates, generally showed only one orientation: ( 111 ) Nb ‖ ( 0001 ) α ‐ Al 2 O 3 and [ 110 ] Nb ‖ [ 1120 ] α ‐ Al 2 O 3 .
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Cuomo et al. (1973) studied this question.