Domain Matching Epitaxial Growth of In 2 O 3 Thin Films on α-Al 2 O 3 (0001)

Oxygen plasma assisted molecular beam epitaxy was used to grow thin films of In 2 O 3 on α-Al 2 O 3 (0001) over a range of substrate temperatures between 300 and 750 °C. The crystal structures and morphologies were examined by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. In all cases, the thermodynamically stable body-centered cubic phase bcc- In 2 O 3 predominates in the films, with an epitaxial relationship In 2 O 3 (111)∥Al 2 O 3 (0001) and In 2 O 3 [11̅10]∥Al 2 O 3 [101̅0] determined by matching between the sublattice oxygen atoms in Al 2 O 3 (0001) and the In atoms in In 2 O 3 (111): this involves a 30° rotation of the epilayer unit cell relative to that of the substrate and a 3:2 coincidence structure. A minority fraction of metastable rhombohedral rh -In 2 O 3 (0001) can be stabilized for substrate temperatures below 550 °C due to the similarity in the bonding symmetries between rh -In 2 O 3 and α-Al 2 O 3 . Despite the large mismatches between In 2 O 3 and Al 2 O 3 for the two epitaxial systems discussed above (−13.2% for bcc -In 2 O 3 and +15.1% for rh -In 2 O 3 ), we show that the epitaxy can be maintained in both cases by matching small but different integral multiples of lattice planes of the In 2 O 3 and the substrate at the interface between the two. Thus, the strain is effectively released by dislocations localized at the interface. This so-called domain matching epitaxial growth mode may open up a new route to fabrication of high-quality crystalline thin films of oxides on highly mismatched substrates.