Domain Matching Epitaxial Growth of In2O3 Thin Films on α-Al2O3(0001)

Oxygen plasma assisted molecular beam epitaxy was used to grow thin films of In2O3 on alpha-Al2O3(0001) over a range of substrate temperatures between 300 and 750 degrees 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-In2O3 predominates in the films, with an epitaxial relationship In2O3(111)parallel to Al2O3(0001) and In2O3 [1 (1) over bar 10]parallel to Al2O3[10 (1) over bar0] determined by matching between the sublattice oxygen atoms in Al2O3(0001) and the In atoms in In2O3(111): this involves a 300 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-In2O3(0001) can be stabilized for substrate temperatures below 550 degrees C due to the similarity in the bonding symmetries between rh-In2O3 and alpha-Al2O3. Despite the large mismatches between In2O3 and Al2O3 for the two epitaxial systems discussed above (-13.2% for bcc-In2O3 and +15.1% for rh-In2O3), we show that the epitaxy can be maintained in both cases by matching small but different integral multiples of lattice planes of the In2O3 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.