Step engineering for nucleation and domain orientation control in WSe2 epitaxy on c-plane sapphire.

Epitaxial growth of two-dimensional transition metal dichalcogenides on sapphire has emerged as a promising route to wafer-scale single-crystal films. Steps on the sapphire act as sites for transition metal dichalcogenide nucleation and can impart a preferred domain orientation, resulting in a substantial reduction in mirror twins. Here we demonstrate control of both the nucleation site and unidirectional growth direction of WSe2 on c-plane sapphire by metal-organic chemical vapour deposition. The unidirectional orientation is found to be intimately tied to growth conditions via changes in the sapphire surface chemistry that control the step edge location of WSe2 nucleation, imparting either a 0° or 60° orientation relative to the underlying sapphire lattice. The results provide insight into the role of surface chemistry on transition metal dichalcogenide nucleation and domain alignment and demonstrate the ability to engineer domain orientation over wafer-scale substrates.

Alloy-Free Band Gap Tuning across the Visible Spectrum.

We present evidence, from theory and experiment, that ZnSnN_{2} and MgSnN_{2} can be used to match the band gap of InGaN without alloying-by exploiting cation disorder in a controlled fashion. We base this on the determination of S, the long-range order parameter of the cation sublattice, for a series of epitaxial thin films of ZnSnN_{2} and MgSnN_{2} using three different techniques: x-ray diffraction, Raman spectroscopy, and in situ electron diffraction. We observe a linear relationship between S^{2} and the optical band gap of both ZnSnN_{2} (1.12-1.98 eV) and MgSnN_{2} (1.87-3.43 eV). The results clearly demonstrate the correlation between controlled heterovalent cation ordering and the optical band gap, which applies to a broad group of emerging ternary heterovalent compounds and has implications for similar trends in other material properties besides the band gap.

The presence of a (1 × 1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces.

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 × 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.

Nanobeam electron diffraction and high resolution imaging analysis of InN films grown on sapphire.

A JEOL JEM-3000F field emission, analytical, high-resolution transmission electron microscope (HRTEM) was used to study InN films grown on sapphire substrates. It was found that, while the InN films maintained the hexagonal (wurtzite) structure, InN nanodomains with a cubic (zincblende) structure were also formed in the films. Nanobeam electron diffraction techniques were applied for identification of the cubic phase. The identification of the cubic InN was confirmed by HRTEM structural imaging. The cubic InN nanodomains are 3-10 nm in diameter, and are orientated in two different orientations with their [110](cubic) and [110](cubic) axes parallel to each other and their (111)(cubic) planes parallel to the (0001)(hex) plane of the hexagonal InN.