Unique prospects of graphene-based THz modulators

The modulation depth of 2-D electron gas (2DEG) based THz modulators using AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%. The metal gate not only attenuates the THz signal (> 90%) but also severely degrades the modulation depth. The metal losses can be significantly reduced with an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high mobility of holes that is comparable to electron mobility in conventional semiconductors. The hole conductivity in graphene can be electrostatically tuned in the graphene-2DEG parallel capacitor configuration, thus more efficiently tuning the THz transmission. In this work, we show that it is possible to achieve a modulation depth of > 90% while simultaneously minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac point in graphene.Comment: 15 pages, 3 figures, 1 tabl

Graphene as Transparent Electrode for Direct Observation of Hole Photoemission from Silicon to Oxide

The outstanding electrical and optical properties of graphene make it an excellent alternative as a transparent electrode. Here we demonstrate the application of graphene as collector material in internal photoemission (IPE) spectroscopy; enabling the direct observation of both electron and hole injections at a Si/Al2O3 interface and successfully overcoming the long-standing difficulty of detecting holes injected from a semiconductor emitter in IPE measurements. The observed electron and hole barrier heights are 3.5 eV and 4.1 eV, respectively. Thus the bandgap of Al2O3 can be further deduced to be 6.5 eV, in close agreement with the valued obtained by vacuum ultraviolet spectroscopic ellipsometry analysis. The detailed optical modeling of a graphene/Al2O3/Si stack reveals that by using graphene in IPE measurements the carrier injection from the emitter is significantly enhanced and the contribution of carrier injection from the collector electrode is minimal. The method can be readily extended to various IPE test structures for a complete band alignment analysis and interface characterization.Comment: 15 pages, 5 figure

Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2

Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si3N4 substrate-supported monolayer and few-layer MoS2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS2. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS2 along with controlling their dimensions

Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E-2g(1) and the out-of-plane A(1g) modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity kappa = (34.5 +/- 4) W/mK at room temperature agrees well with the first-principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical

Exploring Novel Electronic and Optoelectronic Devices based on Layered Materials

The past decade has witnessed an enormous rise of two-dimensional layered materials (2DLMs) in the scientific community, including graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), and black phosphorous (bP) etc. The nature of weak van der Waals (vdW) interactions between the atomically thin layers makes it possible to isolation them into few-layer or monolayer form without surface dangling bonds. Remarkable and unique properties appear at the atomically thin limit, and thus opening new opportunities to explore ultra-thin electronic and optoelectronic applications, such as quantum tunneling based devices, ultrafast photodetectors, broadband modulators, solar cells etc. The focuses of this work are developed along two streams: property characterizations of 2DLMs and their novel device explorations. On the characterization front, spectroscopic techniques are utilized for the first time to determine the band offsets of graphene on oxide structures and simultaneously extracting the work function of graphene; using Raman spectroscopy coupled with comprehensive thermal transport modeling, in-plane thermal conductivity in atomically thin TMD monolayers is characterized for the first time. On the device aspect, graphene based terahertz (THz) modulators are invented, promising superior performance; various types of vdW heterojunctions (HJs) are built and tested. For example, vdW Esaki tunneling diodes are demonstrated for the first time, which are subsequently used to build a proof-of-concept radio frequency (RF) oscillator