Structural and surface characterizations of 2D ß-In2Se3/3D ß-Ga2O3 heterostructures grown on c-Sapphire substrates by molecular beam epitaxy.

Integrating two-dimensional (2D) layered materials with wide bandgap ß-Ga2O3 has unveiled impressive opportunities for exploring novel physics and device concepts. This study presents the epitaxial growth of 2D ß-In2Se3/3D ß-Ga2O3 heterostructures on c-Sapphire substrates by plasma-assisted molecular beam epitaxy. Firstly, we employed a temperature-dependent two-step growth process to deposit Ga2O3 and obtained a phase-pure ( 2 ¯ 01 ) ß-Ga2O3 film on c-Sapphire. Interestingly, the in-situ reflective high-energy electron diffraction (RHEED) patterns observed from this heterostructure revealed the in-plane 'b' lattice constant of ß-Ga2O3 ~ 3.038Å. In the next stage, for the first time, 2D In2Se3 layers were epitaxially realized on 3D ß-Ga2O3 under varying substrate temperatures (Tsub) and Se/In flux ratios (RVI/III). The deposited layers exhibited (00l) oriented ß-In2Se3 on ( 2 ¯ 01 ) ß-Ga2O3/c-Sapphire with the epitaxial relationship of [ 11 2 ¯ 0 ] ß-In2Se3 || [010] ß-Ga2O3 and [ 10 1 ¯ 0 ] ß-In2Se3 || [102] ß-Ga2O3 as observed from the RHEED patterns. Also, the in-plane 'a' lattice constant of ß-In2Se3 was determined to be ~ 4.027Å. The single-phase ß-In2Se3 layers with improved structural and surface quality were achieved at a Tsub ~ 280 °C and RVI/III ~ 18. The microstructural and detailed elemental analysis further confirmed the epitaxy of 2D layered ß-In2Se3 on 3D ß-Ga2O3, a consequence of the quasi-van der Waals epitaxy. Furthermore, the ß-Ga2O3 with an optical bandgap (Eg) of ~ 5.04 eV (deep ultraviolet) when integrated with 2D ß-In2Se3, Eg ~ 1.43eV (near infra-red) can reveal potential applications in the optoelectronic field.

Pressure induced structural phase crossover of a GaSe epilayer grown under screw dislocation driven mode and its phase recovery.

Hydrostatically pressurized studies using diamond anvil cells on the structural phase transition of the free-standing screw-dislocation-driven (SDD) GaSe thin film synthesized by molecular beam epitaxy have been demonstrated via in-situ angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The early pressure-driven hexagonal-to-rock salt transition at approximately ~ 20 GPa as well as the outstandingly structural-phase memory after depressurization in the SDD-GaSe film was recognized, attributed to the screw dislocation-assisted mechanism. Note that, the reversible pressure-induced structural transition was not evidenced from the GaSe bulk, which has a layer-by-layer stacking structure. In addition, a remarkable 1.7 times higher in bulk modulus of the SDD-GaSe film in comparison to bulk counterpart was observed, which was mainly contributed by its four times higher in the incompressibility along c-axis. This is well-correlated to the slower shifting slopes of out-of-plane phonon-vibration modes in the SDD-GaSe film, especially at low-pressure range (< 5 GPa). As a final point, we recommend that the intense density of screw dislocation cores in the SDD-GaSe lattice structure plays a crucial role in these novel phenomena.

Substrate-induced strain in 2D layered GaSe materials grown by molecular beam epitaxy.

Two-dimensional (2D) layered GaSe films were grown on GaAs (001), GaN/Sapphire, and Mica substrates by molecular beam epitaxy (MBE). The in situ reflective high-energy electron diffraction monitoring reveals randomly in-plane orientations of nucleated GaSe layers grown on hexagonal GaN/Sapphire and Mica substrates, whereas single-orientation GaSe domain is predominant in the GaSe/GaAs (001) sample. Strong red-shifts in the frequency of in-plane [Formula: see text] vibration modes and bound exciton emissions observed from Raman scattering and photoluminescence spectra in all samples are attributed to the unintentionally biaxial in-plane tensile strains, induced by the dissimilarity of symmetrical surface structure between the 2D-GaSe layers and the substrates during the epitaxial growth. The results in this study provide an important understanding of the MBE-growth process of 2D-GaSe on 2D/3D hybrid-heterostructures and pave the way in strain engineering and optical manipulation of 2D layered GaSe materials for novel optoelectronic integrated technologies.

Screw-Dislocation-Driven Growth Mode in Two Dimensional GaSe on GaAs(001) Substrates Grown by Molecular Beam Epitaxy.

Regardless of the dissimilarity in the crystal symmetry, the two-dimensional GaSe materials grown on GaAs(001) substrates by molecular beam epitaxy reveal a screw-dislocation-driven growth mechanism. The spiral-pyramidal structure of GaSe multi-layers was typically observed with the majority in ε-phase. Comprehensive investigations on temperature-dependent photoluminescence, Raman scattering, and X-ray diffraction indicated that the structure has been suffered an amount of strain, resulted from the screw-dislocation-driven growth mechanism as well as the stacking disorders between monolayer at the boundaries of the GaSe nanoflakes. In addition, Raman spectra under various wavelength laser excitations explored that the common ε-phase of 2D GaSe grown directly on GaAs can be transformed into the ß-phase by introducing a Se-pretreatment period at the initial growth process. This work provides an understanding of molecular beam epitaxy growth of 2D materials on three-dimensional substrates and paves the way to realize future electronic and optoelectronic heterogeneous integrated technology as well as second harmonic generation applications.