Frank J. Crowne

Strain and structure heterogeneity in MoS2 atomic layers grown by chemical vapour deposition

Monolayer molybdenum disulfide (MoS2) 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 MoS2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS2. Recently, large-size monolayer MoS2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS2 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 MoS2.

Temperature-dependent phonon shifts in monolayer MoS2

We present a combined experimental and computational study of two-dimensional molybdenum disulfide and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g mode than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system.

Vertical 2D/3D Semiconductor Heterostructures Based on Epitaxial Molybdenum Disulfide and Gallium Nitride

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·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

Dyakonov–Shur plasma excitations in the channel of a real high-electron mobility transistor

The arguments leading to the prediction of instability of plasma excitations in a drifting 2D electron gas given by Dyakonov and Shur are made more rigorous by treating a real high-electron mobility transistor (HEMT) channel biased in its ohmic region (i.e., below pinchoff), including nonuniformity of the channel charge density. Expressions are derived for the changes in resonant frequency and gain of the Dyakonov–Shur modes arising from finite channel mobility and density nonuniformity to first order in the momentum relaxation time. It is found that channel nonuniformity (constriction) weakens the instability, in keeping with the recent results of Cheremisin and Samsonidze [Semiconductors 33, 578 (1999)], possibly explaining why it has not been observed in real HEMTs.

Nonlinear response of two-dimensional electron plasmas in the conduction channels of field effect transistor structures

The response of an electron gas in a quasitwo-dimensional conduction channel depends on a characteristic frequency ω0, defined as the ratio of the plasma wave velocity to the channel length. For a short-gate high electron mobility transistor ω0 can be in the terahertz range. We have used a self-consistent hydrodynamic model of the confined electron plasma to show that significant nonlinear effects are present in its response to harmonic signals at microwave frequencies much lower than ω0. We obtain the oscillatory time dependence of the terminal currents and study the interior dynamics of the electron plasma. We find that in certain device parameter ranges the essential nonlinearity in the microwave response may lead to nonlinear hydrodynamic effects, such as shock wave propagation in the conduction channel. When the boundary conditions at the source and drain terminals are asymmetric, the nonlinear plasma oscillations result in a nonzero dc component of the terminal current, which could be measured by us...

Blueshift of the A-exciton peak in folded monolayer 1H-MoS2

The large family of layered transition-metal dichalcogenides is widely believed to constitute a second family of two-dimensional (2D) semiconducting materials that can be used to create novel devices that complement those based on graphene. In many cases these materials have shown a transition from an indirect bandgap in the bulk to a direct bandgap in monolayer systems. In this work we experimentally show that folding a 1H molybdenum disulphide (MoS2) layer results in a turbostratic stack with enhanced photoluminescence quantum yield and a significant shift to the blue by 90 meV. This is in contrast to the expected 2H-MoS2 band structure characteristics, which include an indirect gap and quenched photoluminescence. We present a theoretical explanation to the origin of this behavior in terms of exciton screening.

Microwave response of a high electron mobility transistor in the presence of a Dyakonov–Shur instability

The plasma-wave response of the two-dimensional electron gas that forms the active layer of a high-electron mobility transistor (HEMTs) makes a contribution to the high-frequency behavior of these devices that is distinct from their adiabatic response, i.e., unrelated to low-frequency parameters such as dc transconductance, capacitances, and channel resistance, which are usually derived from dc IV curves. Since the plasma-wave response has the potential to make the HEMT active at very high (terahertz) frequencies, it is important to frame its description within the standard language of microwave device engineering, i.e., as admittance or S-parameters of the device. In this paper a full set of microwave admittance parameters is derived for a real HEMT, based on recent work by the author [J. Appl. Phys. 87, 8056–8064 (2000)].

Microfields Induced by Random Compensated Charge Pairs in Ferroelectric Materials

The dc and microwave responses of the BaxSr1-x (X,Y)yTi1-yO3 family of ferroelectric compounds with various substitutional additives X 3+ , Y 5+ are analyzed by combining the random-field technique with the mean-field (Landau-Devonshire) theory of ferroelectricity, along with a self-consistent computation of the dielectric constant of the host material in the presence of the impurity fields. The fields in the material are assumed to arise from charge compensation at the Ti 4+ sites, leading to permanent dipoles made up of the resulting positive and negative ions separated by a few lattice constants. It is shown that whereas completely random placement of positive and negative ions generates a Holtsmark distribution of electric field, with infinite second moment and hence extremely large fluctuations in field strength, the association of ionized impurities into permanent dipoles leads to much lower fluctuations in field and a distribution with finite second moment, which makes a self-consistent dielectric constant meaningful.

Dielectric and X-ray Diffraction Analysis of Ba(Ga,Ta)0.05Ti0.90O3

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

Electrical and Structural Properties of Ba(Y3+, Sb5+)0.05Ti0.90O3

The microwave ceramic Ba(Y,Sb)0.05Ti0.90O3, based upon random B-site dipole-like substitutions is structurally and electrically characterized over the temperature range −55°C to 120°C. X-ray diffraction information coupled together with scanning electron microscopy reveals that the material is single phase and has submicron grain sizes. Results show the tetragonal phase ranges from −30°C to 200°C, which is a broader temperature range than that of the parent material BaTiO3, 0°C to 120°C. The material Ba(Y,Sb)0.05Ti0.90O3 also has a diffuse relative high dielectric constant of about 10,000 that peaks at the orthorhombic-tetragonal structural phase transition around −30°C.