Heteroepitaxial growth of MgO(111) thin films on Al2O3(0001): Evidence of a wurtzite to rocksalt transformation

Abstract

We report on a growth study of MgO films deposited on Al 2 O 3 (0001) substrates by magnetron sputtering. The films exhibited a preferred rocksalt MgO(111) orientation. Surprisingly, depending on the O 2 gas flow ratio, a structure of graphiticlike wurtzite MgO(0001) has been revealed. The observed Mg-O perpendicular bond length reduction is accompanied by an atomically flat surface morphology for the development of MgO(111) films; the transition to the bulk rocksalt structure occurs in the 3-6 nm coverage range. Previously, relaxation of the electrostatic instability of MgO (111) Nanoscale alkaline earth metal oxides, in particular, MgO, are very promising materials for applications as adsorbents for decontaminating wastewater due to their favorable electrostatic attraction mechanism, and the simplicity of their production from abundant natural minerals.1 Furthermore, the MgO(111) facet, which not only presents the largest number of dangling bonds per atom, but also alternating layers of O 2− and Mg 2+ , and thus, a strong electrostatic field, is obviously anticipated to be more reactive, compared to nonpolar surfaces. In this regard, recent studies on MgO(111) have suggested that the polar oxide surface may be of interest for the catalytic splitting of water and for hydrogen storage. 2On the other hand, there is an increasing interest in the growth of high quality MgO films because of their unique physical properties which allow the fabrication of buffered epitaxial layers of ferroelectric materials and superconductors, as well as wide-band-gap semiconductors for many optoelectronic applications.3 MgO(111) layers are also interesting for the exploitation of spintronic devices. 4 Therefore, it remains a great challenge to develop general methods for the controllable growth of unusual (111) exposed MgO crystal surfaces. Such a surface has attracted a great deal of attention from both experimental and theoretical studies. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In spite of the large number of studies made, many features of this process are not yet understood. Therefore, the investigations described herein are aimed at better understanding the microstructural details of MgO(111) films. To this end, MgO thin films were grown on Al 2 O 3 (0001) ("c-plane sapphire") substrates by sputtering deposition. While bulk MgO has a sixcoordinated rocksalt (B1) crystal structure with a lattice constant a = 0.421 nm, the Al 2 O 3 is rhombohedral (a = 0.476 nm, c = 1.299 nm). As a result, the epitaxy of MgO films on sapphire is not straightforward. There are several experiments showing that (111)-oriented MgO can be accommodated onto sapphire(0001), since it offers the lowest substrate-to-film inplane lattice mismatch (about 8%). One possible mechanism is that growth proceeds by matching domains with a filmto-substrate lattice ratio of 4:5. 20 However, these (111) polar films are intrinsically unstable. Several mechanisms, including vacancy formation, hydroxylation, surface reconstruction, and metallization a...

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