Showing papers by "Jun Kang published in 2013"

Band offsets and heterostructures of two-dimensional semiconductors

Abstract: The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) are investigated from first principles calculations. The band alignments between different MX2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering.

Electronic structural Moiré pattern effects on MoS2/MoSe2 2D heterostructures.

Abstract: The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moire pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moire pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other el...

Monolayer semiconducting transition metal dichalcogenide alloys: Stability and band bowing

Abstract: expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8eV to 1.0eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics. V C 2013 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4799126]

Mechanical and Electronic Properties of Graphyne and Its Family under Elastic Strain: Theoretical Predictions

Abstract: Using the first-principles calculations, we investigate the mechanical and electronic properties of graphyne and its family under strain. It is found that the in-plane stiffness decreases with increasing the number of acetylenic linkages, which can be characterized by a simple scaling law. The band gap of the graphyne family is found to be modified by applying strain through various approaches. While homogeneous tensile strain leads to an increase in the band gap, the homogeneous compressive strain as well as uniaxial tensile and compressive strains within the imposed range induce a reduction in it. Both graphyne and graphyne-3 under different tensile strains possess direct gaps at either M or S point of Brillouin zone, whereas the band gaps of graphdiyne and graphyne-4 are always direct and located at the Γ point, irrespective of strain types. Our study suggests a potential direction for fabrication of novel strain-tunable nanoelectronic and optoelectronic devices.

Well controlled multiple resistive switching states in the Al local doped HfO2 resistive random access memory device

Abstract: The resistive switching behaviors in the sandwiched Ti/HfO2/Pt devices with different doping condition were systematically investigated We show that, comparing with the undoped and the Al layer doped HfO2 devices, significant improvement of switching characteristics is achieved in the Al local doped HfO2 device, including uniformity, reliability, and operation current As a result, well controlled multiple switching states are obtained in the local doping device by modulating the set current compliance or the maximal reset voltage, respectively Our results suggest that the switching characteristics of HfO2 device are very closely related to the inducement and controlling of conductive filaments’ growth in the dielectric layer, which can be considered in the optimization of resistive random access memory device design

Metal to semiconductor transition in metallic transition metal dichalcogenides

Abstract: We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX2 (M = Nb, Ta; X = S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ∼13 A and ∼7 A for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.

Design of Shallow Acceptors in GaN through Zinc--Magnium Codoping: First-Principles Calculation

Abstract: In this work, we propose a novel approach to reduce the ionization energy of acceptors in GaN through Zn–Mg codoping. The characteristics of the defect states and the valence-band maximum (VBM) were investigated via first-principles calculation. Our results indicated that the original VBM of the host GaN could be altered by Zn–Mg codoping, thus improving the p-type dopability. We show that the calculated ionization energy e(0/-) of the Zn–Mg acceptor is only 117 meV, which is about 90 meV shallower than that of the isolated Mg acceptor.