3,981 research outputs found
Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures
Vertically stacked van der Waals heterostructures constitute a promising
platform for providing tailored band alignment with enhanced excitonic systems.
Here we report observations of neutral and charged interlayer excitons in
trilayer WSe2-MoSe2-WSe2 van der Waals heterostructures and their dynamics. The
addition of a WSe2 layer in the trilayer leads to significantly higher
photoluminescence quantum yields and tunable spectral resonance compared to its
bilayer heterostructures at cryogenic temperatures. The observed enhancement in
the photoluminescence quantum yield is due to significantly larger
electron-hole overlap and higher light absorbance in the trilayer
heterostructure, supported via first-principle pseudopotential calculations
based on spin-polarized density functional theory. We further uncover the
temperature- and power-dependence, as well as time-resolved photoluminescence
of the trilayer heterostructure interlayer neutral excitons and trions. Our
study elucidates the prospects of manipulating light emission from interlayer
excitons and designing atomic heterostructures from first-principles for
optoelectronics.Comment: 25 pages, 5 figures(Maintext). 9 pages, 7 figures(Supplementary
Information). - Accepted for publication in npg: 2D materials and
applications and reformatted to its standard. - Updated co-authors and
references. - Title and abstract are modified for clarity. - Errors have been
corrected, npg: 2D materials and applications (2018
Absence of quantum-confined Stark effect in GaN quantum disks embedded in (Al,Ga)N nanowires grown by molecular beam epitaxy
Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light
emitting diodes could potentially be overcome by utilizing nanowire
heterostructures, exhibiting high structural perfection and improved light
extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire
ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The
nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire
segments essential for efficient light extraction. These quantum disks are
found to exhibit intense emission at unexpectedly high energies, namely,
significantly above the GaN bandgap, and almost independent of the disk
thickness. An in-depth investigation of the actual structure and composition of
the nanowires reveals a spontaneously formed Al gradient both along and across
the nanowire, resulting in a complex core/shell structure with an Al deficient
core and an Al rich shell with continuously varying Al content along the entire
length of the (Al,Ga)N segment. This compositional change along the nanowire
growth axis induces a polarization doping of the shell that results in a
degenerate electron gas in the disk, thus screening the built-in electric
fields. The high carrier density not only results in the unexpectedly high
transition energies, but also in radiative lifetimes depending only weakly on
temperature, leading to a comparatively high internal quantum efficiency of the
GaN quantum disks up to room temperature.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters (2019),
copyright (C) American Chemical Society after peer review. To access the
final edited and published work see
https://doi.org/10.1021/acs.nanolett.9b01521, the supporting information is
available (free of charge) under the same lin
Surface recombination measurements on III–V candidate materials for nanostructure light-emitting diodes
Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes
Superinjection of holes in homojunction diodes based on wide-bandgap semiconductors
Electrically driven light sources are essential in a wide range of
applications, from indication and display technologies to high-speed data
communication and quantum information processing. Wide-bandgap semiconductors
promise to advance solid-state lighting by delivering novel light sources.
However, electrical pumping of these devices is still a challenging problem.
Many wide-bandgap semiconductor materials, such as SiC, GaN, AlN, ZnS, and
Ga2O3, can be easily doped n-type, but their efficient p-type doping is
extremely difficult. The lack of holes due to the high activation energy of
acceptors greatly limits the performance and practical applicability of
wide-bandgap semiconductor devices. Here, we study a novel effect which allows
homojunction semiconductors devices, such as p-i-n diodes, to operate well
above the limit imposed by doping of the p-type material. Using a rigorous
numerical approach, we show that the density of injected holes can exceed the
density of holes in the p-type injection layer by up to three orders of
magnitude, which gives the possibility to significantly overcome the doping
problem. We present a clear physical explanation of this unexpected feature of
wide-bandgap semiconductor p-i-n diodes and closely examine it in 4H-SiC,
3C-SiC, AlN and ZnS structures. The predicted effect can be exploited to
develop bright light emitting devices, especially electrically driven
non-classical light sources based on color centers in SiC, AlN, ZnO and other
wide-bandgap semiconductors.Comment: 6 figure
Photoluminescence from silicon dioxide photonic crystal cavities with embedded silicon nanocrystals
One dimensional nanobeam photonic crystal cavities are fabricated in silicon
dioxide with silicon nanocrystals. Quality factors of over 9 x 10^3 are found
in experiment, matching theoretical predictions, with mode volumes of
1.5(lambda/n)^3 . Photoluminescence from the cavity modes is observed in the
visible wavelength range 600-820 nm. Studies of the lossy characteristics of
the cavities are conducted at varying temperatures and pump powers. Free
carrier absorption effects are found to be significant at pump powers as low as
a few hundred nanowatts.Comment: 13 pages 9 figure
Red-Emitting III-Nitride Self-Assembled Quantum Dot Lasers.
Visible and ultra-violet light sources have numerous applications in the fields of solid state lighting, optical data storage, plastic fiber communications, heads-up displays in automobiles, and in quantum cryptography and communications. Most research and development into such sources is being done using III-nitride materials where the emission can be tuned from the deep UV in AlN to the near infrared in InN. However due to material limitations including large strain, piezoelectric polarization, and the unavailability of cheap native substrates, most visible devices are restricted to emission near GaN at 365nm up to around 530nm. These dots are formed by the relaxation of strain, and it has been shown both theoretically and experimentally that the piezoelectric field and the resultant quantum confined stark effect are significantly lower than those values reported in comparable QWs. As a result, the radiative carrier lifetimes in such dots are typically around 10-100 times smaller than those in equivalent QWs. Furthermore, the quasi-three dimensional confinement of carriers in the InGaN islands that form the dots can reduce carrier migration to (and therefore recombination at) dislocations and other defects.
In the present study, molecular beam epitaxial growth and the properties of InGaN/GaN self-assembled quantum dots have been investigated in detail. The quantum dots, emitting at 630nm, have been studied optically through temperature-dependent, excitation-dependent, and time-resolved photoluminescence. A radiative lifetime of ~2ns has been measured in these samples. Samples with varying number of dot layers were grown and characterized structurally by atomic force microscopy. The growth conditions of the dots have been optimized including the InGaN and GaN thickness and the nitrogen interruption time. The optimized dots have been incorporated into edge-emitting laser heterostructures. Other optimizations including the novel use of an all In0.18Al0.82N cladding are incorporated into the laser heterostructure to optimize the output power and reduce loss.The first red emitting quantum dot lasers, emitting at up to 630nm have been realized in the present study. These lasers show good performance compared with other material systems, including InGaAlP/GaAs and AlGaAs based red lasers.The maximum measured output power is 30mW, making them suitable for the applications discussed above.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120878/1/tfrost_1.pd
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