67 research outputs found
Vertical cavity surface emitting laser action of an all monolithic ZnO-based microcavity
We report on room temperature laser action of an all monolithic ZnO-based
vertical cavity surface emitting laser (VCSEL) under optical pumping. The VCSEL
structure consists of a 2{\lambda} microcavity containing 8
ZnO/Zn(0.92)Mg(0.08)O quantum wells embedded in epitaxially grown
Zn(0.92)Mg(0.08)O/Zn(0.65)Mg(0.35)O distributed Bragg reflectors (DBRs). As a
prerequisite, design and growth of high reflectivity DBRs based on ZnO and
(Zn,Mg)O for optical devices operating in the ultraviolet and blue-green
spectral range are discussed.Comment: Copyright (2011) American Institute of Physics. This article may be
downloaded for personal use only. Any other use requires prior permission of
the author and the American Institute of Physics. The following article
appeared in Appl. Phys. Lett. 98, 011101 (2011) and may be found at
http://apl.aip.org/resource/1/applab/v98/i1/p011101_s
Revealing the Competing Contributions of Charge Carriers, Excitons, and Defects to the Non-Equilibrium Optical Properties of ZnO
Due to its wide band gap and high carrier mobility, ZnO is an attractive
material for light-harvesting and optoelectronic applications. Its functional
efficiency, however, is strongly affected by defect-related in-gap states that
open up extrinsic decay channels and modify relaxation timescales. As a
consequence, almost every ZnO sample behaves differently, leading to
irreproducible or even contradicting observations. Here, a complementary set of
time-resolved spectroscopies is applied to two ZnO samples of different defect
density to disentangle the competing contributions of charge carriers,
excitons, and defects to the non-equilibrium dynamics after photoexcitation:
Time-resolved photoluminescence, excited state transmission, and electronic sum
frequency generation. Remarkably, defects affect the transient optical
properties of ZnO across more than eight orders of magnitude in time, starting
with photodepletion of normally occupied defect states on femtosecond
timescales, followed by the competition of free exciton emission and exciton
trapping at defect sites within picoseconds, photoluminescence of defect-bound
and free excitons on nanosecond timescales, and deeply trapped holes with
microsecond lifetimes. These findings do not only provide the first
comprehensive picture of charge and exciton relaxation pathways in ZnO, but
also uncover the microscopic origin of previous conflicting observations in
this challenging material and thereby offer means of overcoming its
difficulties
Recrystallization of MBE‐Grown MoS2 Monolayers Induced by Annealing in a Chemical Vapor Deposition Furnace
A systematic study of MoS2 grown by a combination of physical vapor deposition and post-growth annealing treatment has been conducted. Hereby, MoS2 thin films with thicknesses between 1 and 2 layers are first grown on sapphire by molecular beam epitaxy at different growth temperatures and then transferred to S environment inside a tube furnace for an annealing process. Depending on the growth temperature, the as-grown layers are either amorphous or form a crystalline structure composed of closely packed nanometer-size grains. The annealing process leads to recrystallization of these layers significantly increasing the size of the MoS2 crystalline domains to the range of 50–100 nm. While the originally amorphous layer displays rotational domains after annealing, recrystallization of samples grown at high temperatures yields single crystalline layers. All samples display an increase of the crystallite dimension, which is accompanied by the disappearance of the defect-related peaks in the Raman spectra, sharpening of the excitonic signatures in absorption, and strong enhancement of the photoluminescence yield. The results represent a promising way to combine advantages of physical vapor deposition and a post-growth annealing in a chemical vapor deposition furnace toward fabrication of wafer-scale single crystalline transition metal dichalcogenide mono- and multilayer films on non-van der Waals substrates.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Peer Reviewe
The growth of ZnO crystals from the melt
The peculiar properties of zinc oxide (ZnO) make this material interesting
for very different applications like light emitting diodes, lasers, and
piezoelectric transducers. Most of these applications are based on epitaxial
ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the
thermochemical properties of ZnO make the growth of single crystals difficult:
the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the
melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals
for technical applications can only be grown either by a hydrothermal method or
from "cold crucibles" of solid ZnO. Both methods are known to have significant
drawbacks. Our thermodynamic calculations and crystal growth experiments show,
that in contrast to widely accepted assumptions, ZnO can be molten in metallic
crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result
is believed to offer new perspectives for ZnO crystal growth by established
standard techniques like the Bridgman method.Comment: 6 pages, 6 figures, accepted for J. Crystal Growt
Strong coupling of monolayer WS2 excitons and surface plasmon polaritons in a planar Ag/WS2 hybrid structure
Monolayer (1L) transition metal dichalcogenides (TMDC) are of strong interest
in nanophotonics due to their narrow-band intense excitonic transitions
persisting up to room temperature. When brought into resonance with surface
plasmon polariton (SPP) excitations of a conductive medium opportunities for
studying and engineering strong light-matter coupling arise. Here, we consider
a most simple geometry, namely a planar stack composed of a thin silver film,
an Al2O3 spacer and a monolayer of WS2. We perform total internal reflection
ellipsometry which combines spectroscopic ellipsometry with the
Kretschmann-Raether-type surface plasmon resonance configuration. The combined
amplitude and phase response of the reflected light at varied angle of
incidence proves that despite the atomic thinness of 1L-WS2, the strong
coupling (SC) regime between A excitons and SPPs propagating in the thin Ag
film is reached. The phasor representation of rho corroborates SC as rho
undergoes a topology change indicated by the occurrence of a double point at
the cross over from the weak to the strong coupling regime. Our findings are
validated by both analytical transfer matrix method calculations and numerical
Maxwell simulations. The findings open up new perspectives for applications in
plasmonic modulators and sensors benefitting from the tunability of the optical
properties of 1L-TMDCs by electric fields, electrostatic doping, light and the
chemical environment.Comment: 15 pages, 3 figure
Excited State Charge Transfer Enabling MoS2 Phthalocyanine Photodetectors with Extended Spectral Sensitivity
Monolayer ML transition metal dichalcogenides TMDCs are an attracting new class of two dimensional direct band gap semiconducting materials for optoelectronic device applications. The combination of TMDCs with organic semiconductors holds the promise to further improve device properties with added functionality. Here, we demonstrate that excited state charge transfer from a thin organic absorber layer, i.e., metal free phthalocyanine H2Pc , enhances the photoresponse of ML MoS2 dramatically at the same time also significantly extending it to spectral regions where the TMDC is transparent. The fundamental processes enabling this boost in photodetector performance are unraveled by a combination of photoemission PES , photoluminescence PL , and photocurrent action spectroscopy. Direct and inverse PES reveal a type II energy level alignment at the MoS2 H2Pc interface with a large energy offset of 1 eV, which is sufficient to drive the excited state charge transfer. Time resolved PL measurements evidence highly efficient dissociation of excitons generated in H2Pc when they are in contact with MoS2. Exciton dissociation results in the formation of a charge separated state at the hybrid interface with an energy gap of ca. 1.2 eV, in accordance with PES. This state then dissociates into free carriers and markedly contributes to the current in the photodetector, as demonstrated by photocurrent action spectroscopy. This reveals that the photoconductivity within the MoS2 ML is generated by light directly absorbed in the TMDC and, notably, with comparable efficiency by the absorption by H2Pc. The present demonstration of a highly efficient carrier generation in TMDC organic hybrid structures paves the way for future nanoscale photodetectors with very wide spectral sensitivit
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