Emerging Applications of III‐Nitride Nanocrystals

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154944/1/pssa201900885_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154944/2/pssa201900885.pd

Long wavelength quantum-dot lasers selectively populated using tunnel injection

Using measured amplified spontaneous emission data, we have derived and analysed the carrier distribution of a five-layer tunnelling injection quantum-dot structure at temperatures of 300 K and 350 K. The results are consistent with the direct injection of electrons from the injector well into a subset of lower energy dot states. The carrier distribution spectra contain features which suggest that dots of a particular size within the ensemble are preferentially populated leading to a reduced spectral broadening of the emission.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58108/2/sst7_5_018.pd

High-speed quantum dot lasers

The modulation bandwidth of conventional 1.0–1.3 µm self-organized In(Ga)As quantum dot (QD) lasers is limited to ∼6–8 GHz due to hot carrier effects arising from the predominant occupation of wetting layer/barrier states by the electrons injected into the active region at room temperature. Thermal broadening of holes in the valence band of QDs also limits the performance of the lasers. Tunnel injection and p-doping have been proposed as solutions to these problems. In this paper, we describe high-performance In(Ga)As undoped and p-doped tunnel injection self-organized QD lasers emitting at 1.1 and 1.3 µm. Undoped 1.1 µm tunnel injection lasers have ∼22 GHz small-signal modulation bandwidth and a gain compression factor of 8.2 × 10−16 cm3. Higher modulation bandwidth (∼25 GHz) and differential gain (3 × 10−14 cm2) are measured in 1.1 µm p-doped tunnel injection lasers with a characteristic temperature, T0, of 205 K in the temperature range 5–95°C. Temperature invariant threshold current (infinite T0) in the temperature range 5–75°C and 11 GHz modulation bandwidth are observed in 1.3 µm p-doped tunnel injection QD lasers with a differential gain of 8 × 10−15 cm2. The linewidth enhancement factor of the undoped 1.1 µm tunnel injection laser is ∼0.73 at lasing peak and its dynamic chirp is <0.6 ° at various frequencies and ac biases. Both 1.1 and 1.3 µm p-doped tunnel injection QD lasers exhibit zero linewidth enhancement factor (α ∼0) and negligible chirp (< 0.2 °). These dynamic characteristics of QD lasers surpass those of equivalent quantum well lasers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48925/2/d5_13_005.pd

Nitrogen Photofixation over IIIâ Nitride Nanowires Assisted by Ruthenium Clusters of Low Atomicity

In many heterogeneous catalysts, the interaction of supported metal species with a matrix can alter the electronic and morphological properties of the metal and manipulate its catalytic properties. IIIâ nitride semiconductors have a unique ability to stabilize ultraâ small ruthenium (Ru) clusters (ca. 0.8â nm) at a high loading density up to 5â wtâ %. nâ Type IIIâ nitride nanowires decorated with Ru subâ nanoclusters offer controlled surface charge properties and exhibit superior UVâ and visibleâ light photocatalytic activity for ammonia synthesis at ambient temperature. A metal/semiconductor interfacial Schottky junction with a 0.94â eV barrier height can greatly facilitate photogenerated electron transfer from IIIâ nitrides to Ru, rendering Ru an electron sink that promotes Nâ ¡N bond cleavage, and thereby achieving lowâ temperature ammonia synthesis.IIIâ Nitridâ Halbleiter stabilisieren Rutheniumcluster mit Beladungsdichten bis 5â Gew.â %. Der Schottkyâ à bergang an der Grenzfläche zwischen Metall und Halbleiter begünstigt den Transfer von Photoelektronen aus den IIIâ Nitriden auf das Ruthenium, das dadurch die Spaltung der Nâ ¡Nâ Bindung in einer Niedertemperatursynthese von Ammoniak bewirken kann.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137688/1/ange201703301_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137688/2/ange201703301-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137688/3/ange201703301.pd

An AlGaN tunnel junction light emitting diode operating at 255 nm

We report on the demonstration of high-performance tunnel junction deep ultraviolet (UV) light-emitting diodes (LEDs) by using plasma- assisted molecular beam epitaxy. The device heterostructure was grown under slightly Ga-rich conditions to promote the formation of nanoscale clusters in the active region. The device operates at 255 nm with a maximum external quantum efficiency of 7.2% and wall-plug of 4%, which are nearly one to two orders of magnitude higher than those of previously reported tunnel junction devices operating at this wavelength. The devices exhibit highly stable emission, with a nearly constant emission peak with increasing current, due to the strong charge carrier confinement related to the presence of Ga-rich nanoclusters. Efficiency droop, however, is observed at relatively low current densities. Detailed temperature-dependent measurements suggest that the presence of efficiency droop of deep UV LEDs is largely due to electron overflow.http://deepblue.lib.umich.edu/bitstream/2027.42/167201/1/5.0036286.pdfDescription of 5.0036286.pdf : An AlGaN tunnel junction light emitting diode operating at 255 n

Magnetic Field Enhanced Superconductivity in Epitaxial Thin Film WTe2.

In conventional superconductors an external magnetic field generally suppresses superconductivity. This results from a simple thermodynamic competition of the superconducting and magnetic free energies. In this study, we report the unconventional features in the superconducting epitaxial thin film tungsten telluride (WTe2). Measuring the electrical transport properties of Molecular Beam Epitaxy (MBE) grown WTe2 thin films with a high precision rotation stage, we map the upper critical field Hc2 at different temperatures T. We observe the superconducting transition temperature T c is enhanced by in-plane magnetic fields. The upper critical field Hc2 is observed to establish an unconventional non-monotonic dependence on temperature. We suggest that this unconventional feature is due to the lifting of inversion symmetry, which leads to the enhancement of Hc2 in Ising superconductors