16 research outputs found
Mid-infrared emission and absorption in strained and relaxed direct bandgap GeSn semiconductors
By independently engineering strain and composition, this work demonstrates
and investigates direct band gap emission in the mid-infrared range from GeSn
layers grown on silicon. We extend the room-temperature emission wavelength
above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn
content of 17 at.%. The fundamental mechanisms governing the optical emission
are discussed based on temperature-dependent photoluminescence, absorption
measurements, and theoretical simulations. Regardless of strain and
composition, these analyses confirm that single-peak emission is always
observed in the probed temperature range of 4-300 K, ruling out defect- and
impurity-related emission. Moreover, carrier losses into thermally-activated
non-radiative recombination channels are found to be greatly minimized as a
result of strain relaxation. Absorption measurements validate the direct band
gap absorption in strained and relaxed samples at energies closely matching
photoluminescence data. These results highlight the strong potential of GeSn
semiconductors as versatile building blocks for scalable, compact, and
silicon-compatible mid-infrared photonics and quantum opto-electronics
Unveiling Planar Defects in Hexagonal Group IV Materials
Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I 3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I 3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present
Strong diameter-dependence of nanowire emission coupled to waveguide modes
4 pags., 5 figs.The emission from nanowires can couple to waveguide modes supported by the nanowire geometry, thus governing the far-field angular pattern. To investigate the geometry-induced coupling of the emission to waveguide modes, we acquire Fourier microscopy images of the photoluminescence of nanowires with diameters ranging from 143 to 208 nm. From the investigated diameter range, we conclude that a few nanometers difference in diameter can abruptly change the coupling of the emission to a specific mode. Moreover, we observe a diameter-dependent width of the Gaussian-shaped angular pattern in the far-field emission. This dependence is understood in terms of interference of the guided modes, which emit at the end facets of the nanowire. Our results are important for the design of quantum emitters, solid state lighting, and photovoltaic devices based on nanowires. VC 2016 AIP Publishing LLCThis research was supported by the Dutch technology
foundation STW, which is part of the “Netherlands
Organisation for Scientific Research (NWO),” and partially
funded by the Dutch Ministry of Economic Affairs. This work
was also part of the research program of the “Foundation for
Fundamental Research on Matter (FOM),” which is financially
supported by NWO. J.A.S.G. acknowledges the Spanish
Ministerio de Economía y Competitividad for financial support
through the Grant Nos. NANOPLASþ (FIS2012-31070) and
LENSBEAM (FIS2015-69295-C3-2-P).Peer Reviewe
Efficient water reduction with gallium phosphide nanowires
Photoelectrochemical hydrogen production from solar energy and water offers a clean and sustainable fuel option for the future. Planar III/V material systems have shown the highest efficiencies, but are expensive. By moving to the nanowire regime the demand on material quantity is reduced, and new materials can be uncovered, such as wurtzite gallium phosphide, featuring a direct bandgap. This is one of the few materials combining large solar light absorption and (close to) ideal band-edge positions for full water splitting. Here we report the photoelectrochemical reduction of water, on a p-type wurtzite gallium phosphide nanowire photocathode. By modifying geometry to reduce electrical resistance and enhance optical absorption, and modifying the surface with a multistep platinum deposition, high current densities and open circuit potentials were achieved. Our results demonstrate the capabilities of this material, even when used in such low quantities, as in nanowire
Real-space transfer and current filamentation in AlGaAs/GaAs heterojunctions subjected to high-electric fields
At high electric fields negative differential resistance and oscillatory behavior of the current is observed in 2-dimensional electron gases in modulation doped heterostructures. We develop a model in which the understanding of these phenomena is provided by the ohmic contacts to the 2-dimensional electron gas. The key phenomenon is that at a high electric field, well below the threshold field for real space transfer across the interface between the GaAs and the Al xGa1-xAs, injection of electrons from the contacts into the AlxGa 1-xAs layer opens a conductive channel in the AlxGa1-xAs parallel to the 2-dimensional electron gas in the GaAs layer. We show that avalanche ionization in the AlxGa1-xAs layer leads to current filamentation. We studied this behavior for various experimental conditions by means of a novel technique which we developed for this purpose: the technique of time resolved optical beam induced current
On the origin of the photocurrent of electrochemically passivated p-InP(100) photoelectrodes
III-V semiconductors such as InP are highly efficient light absorbers for photoelectrochemical (PEC) water splitting devices. Yet, their cathodic stability is limited due to photocorrosion and the measured photocurrents do not necessarily originate from H2 evolution only. We evaluated the PEC stability and activation of model p-InP(100) photocathodes upon photoelectrochemical passivation (i.e. repeated surface oxidation/reduction). The electrode was subjected to a sequence of linear potential scans with or without intermittent passivation steps (repeated passivation and continuous reduction, respectively). The evolution of H2 and PH3 gases was monitored by online electrochemical mass spectrometry (OLEMS) and the Faradaic efficiencies of these processes were determined. Repeated passivation led to an increase of the photocurrent in 0.5 M H2SO4, while continuous reduction did not affect the photocurrent of p-InP(100). Neither H2 nor PH3 formation increased to the same extent as the photocurrent during the repeated passivation treatment. Surface analysis of the spent electrodes revealed substantial roughening of the electrode surface by repeated passivation, while continuous reduction left the surface unaltered. On the other hand, photocathodic conditioning performed in 0.5 M HCl led to the expected correlation between photocurrent increase and H2 formation. Ultimately, the H2 evolution rates of the photoelectrodes in H2SO4 and HCl are comparable. The much higher photocurrent in H2SO4 is due to competing side-reactions. The results emphasize the need for a detailed evaluation of the Faradaic efficiencies of all the involved processes using a chemical-specific technique like OLEMS. Photo-OLEMS can be beneficial in the study of photoelectrochemical reactions enabling the instantaneous detection of small amounts of reaction by-products
Epitaxial Ge0.81Sn0.19 Nanowires for Nanoscale Mid-Infrared Emitters
The final publication is available via https://doi.org/10.1021/acsnano.9b02843.Austrian Science Funds (FWF)German Research Foundation (DFG)European Union’s Horizon 202
High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium–Tin–Oxide Mie Scatterers
Photovoltaic cells
based on arrays of semiconductor nanowires promise
efficiencies comparable or even better than their planar counterparts
with much less material. One reason for the high efficiencies is their
large absorption cross section, but until recently the photocurrent
has been limited to less than 70% of the theoretical maximum. Here
we enhance the absorption in indium phosphide (InP) nanowire solar
cells by employing broadband forward scattering of self-aligned nanoparticles
on top of the transparent top contact layer. This results in a nanowire
solar cell with a photovoltaic conversion efficiency of 17.8% and
a short-circuit current of 29.3 mA/cm<sup>2</sup> under 1 sun illumination,
which is the highest reported so far for nanowire solar cells and
among the highest reported for III–V solar cells. We also measure
the angle-dependent photocurrent, using time-reversed Fourier microscopy,
and demonstrate a broadband and omnidirectional absorption enhancement
for unpolarized light up to 60° with a wavelength average of
12% due to Mie scattering. These results unambiguously demonstrate
the potential of semiconductor nanowires as nanostructures for the
next generation of photovoltaic devices