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

Abstract

Oxygen plasma assisted molecular beam epitaxy was used to grow thin films of In2O3 on α-Al2O3(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-In2O3 predominates in the films, with an epitaxial relationship In2O3(111)∥Al2O3(0001) and In2O3 [11̅10]∥Al2O3[101̅0] determined by matching between the sublattice oxygen atoms in Al2O3(0001) and the In atoms in In2O3(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-In2O3(0001) can be stabilized for substrate temperatures below 550 °C due to the similarity in the bonding symmetries between rh-In2O3 and α-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.

Epitaxial undoped indium oxide thin films: Structural and physical properties

“…For T≤300 1C, the epitaxial relationships correspond to the "hexagon on hexagon" growth, which is not the most likely in terms of energy. Indeed, in this configuration as noted previously the In ions from the films and O ions of the substrate do not coincide optimally [5]. On the contrary, in the case of the other epitaxial relationship (301 rotation of the film hexagon):…”

“…5, is shown the Tauc plot for film grown at 500 1C. The direct optical band gap values are varying slightly with substrate temperature, being lower than value of 3.75 eV, typically reported in previous studies [1,2,5,8]. The value of In 2 O 3 direct electronic band gap was recently revised to be ≤3 eV [29] and the low-energy tail which is seen below the absorption edge (Fig.…”

“…With the respective in-plane parameters of hexagon and square unit of the two lattices (0.476 nm for the sapphire and 1.0118 nm for In 2 O 3 ), a very significant lattice mismatch is deduced. However, the specific orientations of the films, can be explained in the frame of the domain matching epitaxy or extended atomic distance mismatch approach [5,25], in which "m" lattice units of the film match with "p" lattice units of the substrate. The values of m and p are defined as the minimum integers which satisfy the following relation: md f ≈pd s or ½d f =d s ≈½p=m d f and d s being the respective atomic distances in the film and substrate parallel directions.…”

“…The In ions are into a good coincidence with the O ions of the substrates, leading to a lower interfacial energy which would favour this configuration [5]. However, with the first epitaxial relationship (hexagon on hexagon), the minimum size of the epitaxial domain, which is deduced from the lattice matching, is rather limited (1.4038 nm as deduced from Table 2), while the corresponding minimum size of the domain in the other epitaxial relationship is much higher, 5.7232 nm.…”

“…The formation of In 2 O 3 thin films has been studied by various methods such as molecular organic chemical vapour deposition [5][6][7][8], plasma assisted molecular beam epitaxy [9,10], sputtering [11], atomic layer deposition [12] or pulsed laser deposition [13]. The epitaxial growth of In 2 O 3 on Y-stabilized ZrO 2 , c-cut sapphire, InAs or MgO single crystal substrates has been already reported [4,5,13]. Whatever the growth methods, the epitaxial film formation was generally carried out at relatively high substrate temperature, i.e.…”