MgO epitaxy on GaN (0002) surfaces by molecular beam epitaxy

Abstract

We report on the epitaxial deposition of magnesium oxide films with [111] crystallographic orientation on (0002) GaN by molecular beam epitaxy. Specifically, we use an adsorption controlled growth mechanism to initiate the growth process. Electron diffraction shows a spotty intense pattern without intensity fluctuations during growth and evidence of in-plane twinning. X-ray diffraction reveals the films to be epitaxial with full width at half maximum values of 0.3°, 0.5°, and 1° in 2θ, ϕ, and χ circles, respectively. Wet etching of the GaN surface with a HCl:HF mixture prior to growth is critical for achieving high crystalline quality. Epitaxial growth is observed between room temperature and 650°C, with negligible changes in crystalline quality with increased temperature. Atomic force microscopy analysis shows grainy surfaces with feature sizes near 10nm and rms roughness values of 1.4Å over 1μm2 areas. X-ray diffraction analysis suggests MgO film stability up to 850°C in ex situ air annealing.

Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions

“…Our selection of CaO/GaN is inspired by the need for wide band gap pseudomorphic insulators on Al 1-x Ga x N. Smooth oxide films on GaN provide a parallel platform to LaAlO 3 /SrTiO 3 heterostructres that are of great current interest for nanoelectronic devices based on conducting interfaces. This also is an ideal candidate pair because CaO and other rocksalts exhibit extreme propensity for {001} faceting (from very strong surface energy contrast between the low-index planes) [9][10][11] , whereas the hexagonal face of GaN promotes a 〈111〉 CaO epitaxial growth orientation. With conventional growth techniques, at only several monolayers thickness, rocksalt films collapse into pyramidal islands with {100} faces [9][10][11] .…”

“…This also is an ideal candidate pair because CaO and other rocksalts exhibit extreme propensity for {001} faceting (from very strong surface energy contrast between the low-index planes) [9][10][11] , whereas the hexagonal face of GaN promotes a 〈111〉 CaO epitaxial growth orientation. With conventional growth techniques, at only several monolayers thickness, rocksalt films collapse into pyramidal islands with {100} faces [9][10][11] . This seemingly unavoidable behaviour leads to a grainy morphology and diminished functionality 12,13 .…”

New functionalities in abundant element oxides: ubiquitous element strategy

“…The MgO(111) atomically flat surface realized in the MgO(111)/NiO(111)/YSZ(111) structure is shown in figure 31(c), revealing a single-MgO-layer step (1 MgO layer corresponds to 2.43 Å) and a terrace structure. As MgO films are further deposited on the NiO(111)/YSZ(111) structure, the step and terrace structures gradually disappear, while the root mean square (RMS) surface roughness (R RMS ) was ∼0.2 nm over a 5 × 5 µm 2 area even at 500 MgO layers, which is very small compared with the values for MgO (111) films grown on GaN(0001) [103], 6H-SiC(0001) [104,111] and α-Al 2 O 3 (0001) [111,112]. Considering the strong instability of the MgO(111) polar surface, the realization of such a flat surface for a thick MgO(111) polar film is remarkable.…”

“…It is, however, quite difficult to experimentally form an atomically flat MgO(111) surface. Recently, much effort has been spent to grow atomically flat MgO(111) films on Al 2 O 3 (0001) [101,102], GaN(0001) [103], 6H-SiC(0001) [104], and Ag(111) [105] substrates, and successful results were achieved only for Ag (111). In this section, we review our recent growth studies of flat MgO(111) films on Al 2 O 3 (0001) [106] and on YSZ(111) with a NiO(111) buffer [107].…”

The Controlled Synthesis of Metastable Oxides Utilizing Epitaxy and Epitaxial Stabilization

“…Although the structural quality of films of functional oxides grown on semiconductor substrates is far from the quality of these materials grown on appropriate oxide substrates, significant improvements have been made over the last seven decades (and especially in the last two decades) because oxides were first epitaxially integrated with semiconductors. 296,297 Several routes now exist for the epitaxial integration of functional oxides with semiconductors including (001) Si,196,204,205,210, 226,[313][314][315] Using these routes a multitude of functional oxides, with conducting top and bottom electrodes when desired, have been epitaxially integrated with semiconductor materials. 196,198,204,205,225,226,298,299,301,304,[307][308][309]313,314,[316][317][318][319][320][321][322][323][324][325][326] This capability could play a significant role in future hybrid devices.…”