In recent years, large investments into the research of semiconducting two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have elucidated interesting device related physical phenomena such as valleytronics [1], 2D superconductivity [2], 2D excitonic effects [3] and vertical tunneling [4]. TMDs offer layer-dependent chemical tunability of electronic and optoelectronic properties governed by interlayer van der Waals (vdW) forces [5]. Because of their layered nature, these low-dimensional materials can be combined to form multifunctional heterostructure materials exhibiting entirely new physical systems offering new degrees of flexibility in designing electronics, optoelectronics and other novel devices [6], [7]. In the last couple of years, the focus in the 2D materials research have shifted from exploration of proof-of-concept devices using mechanically exfoliated materials to more advanced device processing using high-quality large-scale growth based on advanced scalable vdW-epitaxy techniques such as powder vapor deposition (PVD) and chemical vapor deposition (CVD).

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer MoS2 (1L-MoS2) inside a Au/p-GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial 1L-MoS2 in the form of triangle domains are grown by powder vaporization on a p-doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that 1L-MoS2 remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the 1L-MoS2 is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical Au/1L-MoS2/p-GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, 1L-MoS2 appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a pn-junction. The I–V analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the Au/1L-MoS2/GaN junction.

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm2 and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

The unintentional background electron population and associated interface and surface conductivity in a heterostructure of InAs0.58Sb0.42 with a bandgap of 0.144 eV and AlInSb was studied with multi-carrier Hall-effect analysis. A free electron bulk concentration at 77 K was found with a density of 2.4 × 1015 cm−3and mobility of 140 000 cm2 V−1 s−1. A surface electron accumulation layer was observed with a density of 5.5 × 1011 cm−2 and mobility of 4500 cm2 V−1 s−1 that is consistent with predictions of surface Fermi level pinning. Another accumulation layer was identified at the interface with the AlInSb of 4 × 1011 cm−2 with a mobility of 37 000 cm2 V−1 s−1. The origin of the defects and the implications for device structures are discussed.

Ba(Ga,Ta)(0.05)Ti0.90O3, a B-site dipole-like substituted material is investigated for structural phase transitions over the temperature range 30 to 900 degrees C using x-ray diffraction. Rietveld refinement of the data suggests the material to be Pm[GRAPHICS]m (cubic) from 200 to 900 degrees C, P4/mmm (tetragonal) from 30 to 200 degrees C with phases similar to those of BaTiO3 [12]. Average grain size determined using scanning electron microscopy is 650nm. Measurements of dielectric properties from -50 to 120 degrees C and over a frequency range of 10Hz to 2MHz show a relatively flat dielectric constant that is electric-field tunable indicating that Ba(Ga,Ta)(0.05)Ti0.90O3 is a reasonable candidate for frequency agile components.

Monolayer molybdenum disulfide (MoS 2 ) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS 2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS 2 . Recently, large-size monolayer MoS 2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS 2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS 2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS 2 by three-dimensional finite element analysis. Furthermore, our work demonstrates that photoluminescence mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS 2 .

Thermal and hysteresis effects are studied for the first time in Al2O3 top-gated, CVD grown monolayer MoS2 field effect transistors (FETs). Stressing with an applied bias reversed the hysteresis rotation in the high temperature I-ds-V-gs transfer characteristics and this behavior, indicative of a multilevel trap model, was explained by charge carriers interacting with traps possibly at the MoS2/dielectric interface and within the CVD grown MoS2. High temperature FET characteristics measured up to 125 degrees C demonstrated pinch-off degradation and the influence of trapping/detrapping rates in both the top and bottom gate dielectric. This indicates the importance of maintaining oxide and interface quality for good FET performance. Published by Elsevier Ltd.