Van der Waals semiconductors provide a platform for creating two-dimensional crystals layer-by-layer and engineering excitonic states therein with exceptional properties. We discuss here a few interesting opportunities enabled by interlayer excitons in bilayer transitional metal dichalcogenides (TMDs), including high valley polarizations, lasing in 2D cavities, and tunable interlayer excitons in homo-bilayers.
Monolayer TMDs feature spin-valley locking, enabling valleytronic phenomena and applications. However, strong inter-valley scattering due to electron hole exchange interactions leads to rapid valley depolarization in picoseconds, making it difficult to achieve a high degree of valley polarization. The electron-hole exchange interaction becomes suppressed for interlayer excitons in heterobilayers with type II band alignment. We show highly polarized interlayer excitons with a long valley lifetimein in both spin singlet and brightened triplet states in hetero-bilayers.
TMDs have also garnered intense interest as an active medium, for they feature very strong exciton-photon interactions in a monolayer. However, the rapid radiative decay makes it challenging to establish population inversion. Interlayer excitons decay much more slowly, comparable to excitons in quantum wells of conventional semiconductors. When the interlayer exciton are integrated on a cavity, lasing was established at the cavity resonance accompanied by increased temporal and spatial coherence.
Lastly, due to the strong intra-layer localization of the carriers in 2D materials, interlayer excitons co-exist with intra-layer excitons as meta-stable states even in homo-bilayers without artificial interfaces. These interlayer excitons also feature oscillator strengths between intra-layer excitons and inter-layer ones in hetero-structures, offering a potentially highly tunable system for 2D optoelectronics.
Raman scattering is a powerful probe of local bonding, strain, temperature, and other properties of materials via their influence on vibrational modes or optical phonons. Tip-enhanced Raman spectroscopy (TERS), in which plasmonic modes are excited at the apex of a metal-coated scanning probe tip, enables Raman scattering signals to be detected from nanoscale volumes with precise positional control. We discuss the application of TERS to characterize a variety of semiconductor nanostructures. In studies of Ge-SiGe core-shell nanowires, we measure spatially resolved Raman spectra along the length of a tapered nanowire to demonstrate the ability to measure local strain distributions with nanoscale spatial resolution. In tip-induced resonant Raman spectroscopy of monolayer and bilayer MoS2, we observe large enhancements in Raman signal levels measured for MoS2 associated with excitation of plasmonic gap modes between an Au-coated probe tip and Au substrate surface onto which MoS2 has been transferred. Transitions in B exciton photoluminescence intensity between monolayer and bilayer regions of MoS2 are observed and discussed. Significant differences in nanoscale Raman spectra between monolayer and bilayer MoS2 are also observed. The origins of specific resonant Raman peaks, their dependence on MoS2 layer thickness, and spatial resolution associated with the transition in Raman spectra between monolayer and bilayer regions are described.
In this paper, we present the design guidelines, fabrication challenges and device evaluation results of a surface-normal
photonic crystal waveguide array for high-density optical interconnects. We utilize the slow light effect of photonic
crystals to increase the effective interaction length between photons and medium, which in turn can be used to decrease
the physical length and make compact devices. The effect of the structural parameters variations on the guided mode are
studied in order to provide a guideline for fabrication. Photonic crystal waveguides are vertically implemented in a
silicon-on insulator substrate. Our structure possesses advantages such as universal design, CMOS compatibility, and
simple fabrication process, suitable for high dense on-chip applications. Transmission results show increase of power
near 1.67 μm wavelength, which agrees with our simulation results.
KEYWORDS: Germanium, Silicon, Field effect transistors, Heterojunctions, Transistors, Data modeling, Scanning electron microscopy, Chemical vapor deposition, Transmission electron microscopy, Nanowires
We report the growth and characterization of Ge-SixGe1-x core-shell nanowires. Using a combination of vapor-liquid-solid
nanowire growth and ultra-high-vacuum chemical vapor deposition conformal growth, we demonstrate the
realization of epitaxial Ge-SixGe1-x core-shell nanowire heterostructures with tunable shell content. We investigate the
intrinsic electronic properties of Ge-SixGe1-x core-shell nanowires using back-gate dependent two- and four-terminal
resistance measurements, and demonstrate high performance Ge-SixGe1-x core-shell nanowire field-effect transistors with
highly doped source and drain.
Conference Committee Involvement (10)
Low-Dimensional Materials and Devices 2019
11 August 2019 | San Diego, California, United States
Low-Dimensional Materials and Devices 2018
22 August 2018 | San Diego, California, United States
Low-Dimensional Materials and Devices 2017
9 August 2017 | San Diego, California, United States
Low-Dimensional Materials and Devices 2016
30 August 2016 | San Diego, California, United States
Low-Dimensional Materials and Devices
12 August 2015 | San Diego, California, United States
Nanoepitaxy: Materials and Devices VI
19 August 2014 | San Diego, California, United States
Nanoepitaxy: Materials and Devices V
25 August 2013 | San Diego, California, United States
Nanoepitaxy: Materials and Devices IV
15 August 2012 | San Diego, California, United States
Nanoepitaxy: Materials and Devices III
24 August 2011 | San Diego, California, United States
Nanoepitaxy: Homo- and Heterogeneous Synthesis, Characterization, and Device Integration of Nanomaterials II
1 August 2010 | San Diego, California, United States
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