Shanshan Su

Effect of intervalley interaction on band topology of commensurate graphene/EuO heterostructures

In a heterostructure of graphene and the ferromagnetic insulator EuO, the Eu atoms induce proximity exchange and inter-valley interactions in the graphene layer. Constrained by the lattice symmetries, and guided by ab initio calculations, a model Hamiltonian is constructed that describes the low-energy bands. Band parameters such as proximity induced exchange splitting, spin orbit coupling, and inter-valley interaction are determined. Calculations of the Chern number identify the conditions under which the hetero-structures exhibit topologically non-trivial bands that give rise to the quantum anomalous Hall effect with a Hall conductivity of σxy = 2e /h.

Hot carrier-enhanced interlayer electron–hole pair multiplication in 2D semiconductor heterostructure photocells

Strong electronic interactions can result in novel particle-antiparticle (electron-hole, e-h) pair generation effects, which may be exploited to enhance the photoresponse of nanoscale optoelectronic devices. Highly efficient e-h pair multiplication has been demonstrated in several important nanoscale systems, including nanocrystal quantum dots, carbon nanotubes and graphene. The small Fermi velocity and nonlocal nature of the effective dielectric screening in ultrathin layers of transition-metal dichalcogenides (TMDs) indicates that e-h interactions are very strong, so high-efficiency generation of e-h pairs from hot electrons is expected. However, such e-h pair multiplication has not been observed in 2D TMD devices. Here, we report the highly efficient multiplication of interlayer e-h pairs in 2D semiconductor heterostructure photocells. Electronic transport measurements of the interlayer I-VSD characteristics indicate that layer-indirect e-h pairs are generated by hot-electron impact excitation at temperatures near T = 300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrate near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels. Our findings, which demonstrate efficient carrier multiplication in TMD-based optoelectronic devices, make 2D semiconductor heterostructures viable for a new class of ultra-efficient photodetectors based on layer-indirect e-h excitations.

Interlayer resistance of misoriented MoS2

Interlayer misorientation in transition metal dichalcogenides alters their interlayer distance, total energy, electronic band structure, and vibrational modes, but its effect on the interlayer resistance is not known. This study analyzes the interlayer resistance of misoriented bilayer MoS2 as a function of the misorientation angle, and it shows that interlayer misorientation exponentially increases the electron resistivity while leaving the hole resistivity almost unchanged. The physics, determined by the wave functions at the high symmetry points, are generic among the popular semiconducting transition metal dichalcogenides (TMDs). The asymmetrical effect of misorientation on the electron and hole transport may be exploited in the design and optimization of vertical transport devices such as a bipolar transistor. Density functional theory provides the interlayer coupling elements used for the resistivity calculations.

Graphene contacts to a HfSe2/SnS2 heterostructure

Two-dimensional (2D) heterostructures and all-2D contacts are of high interest for electronic device applications, and the SnS2/HfSe2 bilayer heterostructure with graphene contacts has some unique, advantageous properties. The SnS2/HfSe2 heterostructure is interesting because of the strong intermixing of the two conduction bands and the large work function of the SnS2. The band lineup of the well separated materials indicates a type II heterostructure, but the conduction band minimum of the SnS2/HfSe2 bilayer is a coherent superposition of the orbitals from the two layers with a spectral weight of 60% on the SnS2 and 40% on the HfSe2 for AA stacking. These relative weights can be either increased or reversed by an applied vertical field. A 3×3 supercell of graphene and a 2×2 supercell of SnS2/HfSe2 have a lattice mismatch of 0.1% and both the SnS2/HfSe2 conduction band at M and the graphene Dirac point at K are zone-folded to Γ. Placing graphene on the SnS2/HfSe2 bilayer results in large n-type charge transfer doping of the SnS2/HfSe2 bilayer, on the order of 1013/cm2, and the charge transfer is accompanied by a negative Schottky barrier contact for electron injection from the graphene into the SnS2/HfSe2 bilayer conduction band. Binding energies and the anti-crossing gaps of the graphene and the SnS2/HfSe2 electronic bands both show that the coupling of graphene to the HfSe2 layer is significantly larger than its coupling to the SnS2 layer. A tunneling Hamiltonian estimate of the contact resistance of the graphene to the SnS2/HfSe2 heterostructure predicts an excellent low-resistance contact.

Chemical vapor deposition and phase stability of pyrite on SiO2

Semiconducting pyrite (cubic-FeS2) is of great interest for photovoltaics, energy-storage and catalysis applications due its remarkable optical, electrochemical and catalytic properties in combination with its high abundance, low raw material cost and environmental benignancy. In addition, recent theoretical studies indicate that it is possible to synthesize two-dimensional (2D) FeS2 with atomic thickness, and 2D FeS2 possesses highly tunable electronic and magnetic properties that do not exist in its bulk form, enabling its application in nanoelectronics. Herein, we report the first growth of single-phase FeS2 on SiO2 substrates at temperatures between 300 °C and 600 °C by atmospheric pressure chemical vapor deposition (CVD). The temperature-dependent growth studies suggest that air-stable FeS2 crystals with 2D morphologies grow at 450 °C and above while smaller irregular-shaped FeS2 with low crystallinity and poor stability form at lower temperatures. We also demonstrate the patterned growth of 2D hexagonal crystals on SiO2 substrates using graphene as a template at 600 °C. Raman spectroscopy measurements in conjunction with ab initio density functional theory (DFT) calculations confirm that the growth up to 600 °C does not include any other phase than FeS2. Moreover, we show that laser-induced local phase transformations from FeS2 (pyrite phase) and FeS (troilite phase) can be monitored in-situ by the changes in Raman spectra. Our method paves the way toward scalable synthesis of phase-pure FeS2 crystals on SiO2 substrates, which is fully compatible with semiconductor processing. This method can be also further developed and adopted for the synthesis of atomically thin 2D FeS2 layers and their heterostructures with graphene that may bring enhanced or novel properties.

Strain Gated Bilayer Molybdenum Disulfide Field Effect Transistor with Edge Contacts

Silicon nitride stress capping layer is an industry proven technique for increasing electron mobility and drive currents in n-channel silicon MOSFETs. Herein, the strain induced by silicon nitride is firstly characterized through the changes in photoluminescence and Raman spectra of a bare bilayer MoS2 (Molybdenum disulfide). To make an analogy of the strain-gated silicon MOSFET, strain is exerted to a bilayer MoS2 field effect transistor (FET) through deposition of a silicon nitride stress liner that warps both the gate and the source-drain area. Helium plasma etched MoS2 layers for edge contacts. Current on/off ratio and other performance metrics are measured and compared as the FETs evolve from back-gated, to top-gated and finally, to strain-gated configurations. While the indirect band gap of bilayer MoS2 at 0% strain is 1.25 eV, the band gap decreases as the tensile strain increases on an average of ~100 meV per 1% tensile strain, and the decrease in band gap is mainly due to lowering the conduction band at K point. Comparing top- and strain-gated structures, we find a 58% increase in electron mobility and 46% increase in on-current magnitude, signalling a benign effect of tensile strain on the carrier transport properties of MoS2.