Recent technology development of logic devices based on 2-D semiconductors such as MoS<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula>, WS<inline-formula> ...<tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula>, and WSe<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula> has triggered great excitement, paving the way to practical applications. Making low-resistance p-type contacts to 2-D semiconductors remains a critical challenge. The key to addressing this challenge is to find high-work function metallic materials which also introduce minimal metal-induced gap states (MIGSs) at the metal/semiconductor interface. In this work, we perform a systematic computational screening of novel metallic materials and their heterojunctions with monolayer WSe<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula> based on ab initio density functional theory and quantum device simulations. Two contact strategies, van der Waals (vdW) metallic contact and bulk semimetallic contact, are identified as promising solutions to achieving Schottky-barrier-free and low-contact-resistance p-type contacts for WSe<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula> p-type field-effect transistor (pFETs). Good candidates of p-type contact materials are found based on our screening criteria, including 1H-NbS<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula>, 1H-TaS<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula>, and 1T-TiS<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula> in the vdW metal category, as well as Co<inline-formula> <tex-math notation="LaTeX">_{\text{3}}</tex-math> </inline-formula>Sn<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula>S<inline-formula> <tex-math notation="LaTeX">_{\text{2}}</tex-math> </inline-formula> and TaP in the bulk semimetal category. Simulations of these new p-type contact materials suggest reduced MIGS, less Fermi-level pinning effect, negligible Schottky barrier height and small contact resistance (down to 20 <inline-formula> <tex-math notation="LaTeX">\Omega \mu </tex-math> </inline-formula>m).
Finding an effective and controllable way to create a sizable energy gap in graphene-based systems has been a challenging topic of intensive research. We propose that the hybrid of boron nitride and ...graphene (h-BNC) at low BN doping serves as an ideal platform for band-gap engineering and valleytronic applications. We report a systematic first-principles study of the atomic configurations and band gap opening for energetically favorable BN patches embedded in graphene. Based on first-principles calculations, we construct a tight-binding model to simulate general doping configurations in large supercells. Unexpectedly, the calculations find a linear dependence of the band gap on the effective BN concentration at low doping, arising from an induced effective on-site energy difference at the two C sublattices as they are substituted by B and N dopants alternately. The significant and tunable band gap of a few hundred meVs, with preserved topological properties of graphene and feasible sample preparation in the laboratory, presents great opportunities to realize valley physics applications in graphene systems at room temperature.
Palladium diselenide (PdSe
), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device ...applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe
promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe
is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe
flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe
enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe
in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe
also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe
for device applications.
We optically create an enhanced ferroelectric polarization in ferroelectric/ferromagnet heterostructures that remains stable for over one day, as detected using second harmonic generation. This opens ...up the possibility of non-contact optically-controlled data storage.
Low-resistance p-type contacts to two-dimensional (2D) semiconductors remains a critical challenge towards the industrial application of 2D channel materials in advanced logic technology. To address ...this challenge, we computationally screen and identify designs for ultralow-resistance p-type contacts to 2D semiconductors such as WSe 2 by combining ab initio density-functional-theory (DFT) and quantum device simulations. Two new contact strategies, van der Waals metallic contact (such as 1H-NbS 2 ), and bulk semimetallic contact (such as Co 3 Sn 2 S 2 ), are identified as realistic pathways to achieving Schottky-barrier-free and low-contact-resistance p-type contacts for 2D semiconductor pFETs. Simulations of these new strategies suggest reduced metal-induced gap states, negligible Schottky barrier height and small contact resistance (down to ~20 Ω·μm). Preliminary experimental results in developing Co 3 Sn 2 S 2 as a new semimetal contact material are also demonstrated.
Transition-metal dichalcogenide (TMDC) homo- and heterostacks hold tantalizing prospects for being integrated as active components in future van der Waals (vdW) electronics and optoelectronics. ...However, most TMDC homo- and heterostacks are created by onerous mechanical exfoliation, followed by a mixing-and-matching process. While versatile enough for pilot demonstrations, these strategies are clearly not scalable for practical technologies and widespread implementations. Here, we report a two-step epitaxy strategy that promotes the growth of second-layer TMDCs on the basal plane of the first TMDCs epilayer. The first-layer TMDCs are grown on substrates where the tensile strength can be tuned by the control of chemical environments. The succeeding epilayers then prefer to grow layer-by-layer on the highly tensile-strained first layers. The result is the growth of high-density TMDC homo (WSe2) bilayers and hetero (WSe2–MoS2) bilayers with an exceedingly high yield (>99% bilayers) and uniformity. A density functional theory simulation further sheds light on how strain engineering shifts the subsequent layer growth preference. Second-harmonic generation and high-angle annular dark-field scanning transmission electron microscopy collectively attest to the AB and AA′ stacking between the TMDC epi- and overlayers. The proposed strategy could be a versatile platform for synthesizing diverse arrays of vdW homo- and heterostacks, thus providing prospects for realizing large-scale and layer-controllable two-dimensional electronics.
A novel wafer-scale semi-automated dry transfer process for monolayer (1L) CVD WS 2 was developed utilizing the weakly coupled interface between semimetal (Bi) and two-dimensional (2D) semiconductor ...(WS 2 ). Bi semimetal serves as a gently adhesive transfer template for 2D materials, introducing minimal additional defects during the transfer process. Based on 2D materials processed using this new transfer method, semimetal-contacted (Bi and Sb) monolayer CVD WS 2 nFETs were further demonstrated at wafer scale. Our CVD 1L WS 2 nFETs fabricated using semimetal-assisted transfer with semimetal (Bi and Sb) contacts show record high on-current of 250 µA/µm and 243 µA/µm at V DS = 1 V, and record low contact resistance of 0.63 kΩ*µm and 0.73 kΩ*µm, respectively.
The electronic and optical response of Bernal stacked bilayer graphene with geometry modulation and gate voltage are studied. The broken symmetry in sublattices, one dimensional periodicity ...perpendicular to the domain wall and out-of-plane axis introduces substantial changes of wavefunctions, such as gapless topological protected states, standing waves with bonding and anti-bonding characteristics, rich structures in density of states and optical spectra. The wavefunctions present well-behaved standing waves in pure system and complicated node structures in geometry-modulated system. The optical absorption spectra show forbidden optical excitation channels, prominent asymmetric absorption peaks, and dramatic variations in absorption structures. These results provide that the geometry-modulated structure with tunable gate voltage could be used for electronic and optical manipulation in future graphene-based devices.
We demonstrate single crystal growth of wafer-scale hexagonal boron nitride (hBN), an insulating atomic thin monolayer, on high-symmetry index surface plane Cu(111). The unidirectional epitaxial ...growth is guaranteed by large binding energy difference, ~0.23 eV, between A- and B-steps edges on Cu(111) docking with B6N7 clusters, confirmed by density functional theory calculations.
This dissertation describes the theoretic studies of magnetic moment and spinorbit interaction in vacuum (Dirac wavepacket) and solid state systems, such as semiconductors. The semiclassical approach ...developed here provides a simple and intuitive picture for the origin of spin and spin-orbit coupling. In the Dirac model, the spin magnetic moment is originated from the self-rotating Dirac wavepacket with a correct g-value. The spin-orbit interaction is related to Berry connection (gauge potential) and the model is generalized to solid state systems. The Rashba effect caused by the spin-orbit coupling in a crystal with asymmetric potential in heterostructure quantum well is calculated by semiclassical spindependent scattering. The exact treatment of interface phase accumulation provides a justification of spin-dependent boundary condition at interface derived in previous treatment using Löwdin decomposition. Other spin-orbit coupling related phenomena in solid state system are also discussed in this thesis.
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