Antonio Llopis

Carrier Dynamics in UV InGaN Multiple Quantum Well Inverted Hexagonal Pits

The emission and recombination characteristics of UV or blue light emission from InGaN/GaN quantum well (QW) structures influenced by V-shaped pits have been investigated by near-field and time-resolved photoluminescence measurements. Localization of charge carriers due to the potential barriers caused by the V-shaped pit formation is observed to be modified by thermal excitation. Temperature dependence of recombination dynamics shows evidence of a more complex potential barrier produced by the inverted hexagonal pits embedded within the multiple QWs. The emission from the narrow V-shaped pit QWs shows anomalous temperature dependence behavior that is significantly different from the emission from c-plane QWs. The carrier recombination process in c-plane QWs is significantly longer ~ 5 ns compared to the ~ 1.5 ns in V-shaped pit QWs at low temperatures due to the larger piezoelectric fields in wider wells. At room temperature, the recombination lifetimes are comparable due to increased carrier separation and delocalization within the V-shaped pit QWs.

Comparison of electrostatic and localized plasmon induced light enhancement in hybrid InGaN/GaN quantum wells

The light enhancement phenomena in InGaN/GaN multi-quantum wells (MQWs) infiltrated with metal nanoparticles (NPs) are studied using resonant and off-resonant localized plasmon interactions. The emission and recombination characteristics of carriers in InGaN/GaN MQW structures with inverted hexagonal pits (IHPs) are modified distinctly depending on the nature of their interaction with the metal NPs and with the pumping and emitted photons. It is observed that the emission intensity of light is significantly enhanced when the emission energy is off-resonant to the localized plasmon frequency of the metal nanoparticles. This results in enhanced emission from MQW due to Au nanoparticles and from IHPs due to Ag nanoparticles. At resonant-plasmon frequency of the Ag NPs, the emission from MQWs is quenched due to the re-absorption of the emitted photons, or due to the drift carriers from c-plane MQWs towards the NPs because of the Coulomb forces induced by the image charge effect.

Metallic Nanodroplet Induced Coulomb Catalysis for Off-Resonant Plasmonic Enhancement of Photoemission in Semiconductors

The enhancement of light from semiconductors due to surface plasmons coupled resonantly to its emission is limited because of dissipation in the metal and is also restricted by the dielectric characteristics and homogeneity of the metal–semiconductor interface. We report a new mechanism based on electrostatic interactions of carriers and their image charges in metals to generate more photons from optical sources at frequencies that are off-resonant to the localized plasmon frequency. Coulomb catalysis of carrier accumulation resulting from the inhomogeneity of metal nanodroplets on a semiconductor’s surface can result in an enhancement of light that is nondissipative and does not require resonant coupling of plasmons to the emission wavelength. The enhancement occurs because of an increase in the ratio of radiative to nonradiative recombination in the vicinity of metal nanoparticles. It is equally effective with any type of metal and enhances radiation at any frequency, a property that is of principal importance for the realization of widely tunable semiconductor emitters. This fundamental mechanism provides a new perspective for improving the efficiency of light emitters and controlling carrier concentration on the nanoscale. The structural characteristics of the hybrid metal–semiconductor emitters are studied using electron microscopy and atomic force microscopy. We demonstrate the electrostatic mechanism by studying steady-state and transient photoluminescence from two-dimensional semiconductors, such as GaAs/AlGAs quantum wells, and bulk semiconductors, such as ZnO thin films, emitting in the near-IR and UV wavelength regimes, respectively.

Electrostatic enhancement of light emitted by semiconductor quantum well

Carrier dynamics in metal-semiconductor structures is driven by electrodynamic coupling of carriers to the evanescent field of surface plasmons. Useful modifications in electron and hole dynamics due to presence of metallic inclusions show promise for applications from light emitters to communications. However, this picture does not include contributions from electrostatics. We propose here an electrostatic mechanism for enhancement of light radiated from semiconductor emitter which is comparable in effect to plasmonic mechanism. Arising from Coulomb attraction of e-h pairs to their electrostatic images in metallic nanoparticles, this mechanism produces large carrier concentrations near the nanoparticle. A strong inhomogeneity in the carrier distribution and an increase in the internal quantum efficiency are predicted. In our experiments, this manifests as emission enhancement in InGaN quantum well (QW) radiating in the near-UV region. This fundamental mechanism provides a new perspective for improving the efficiency of broadband light emitters.

Plasmonic modification of electron-longitudinal-optical phonon coupling in Ag-nanoparticle embedded InGaN/GaN quantum wells

Surface plasmon enhanced GaN and InGaN quantum wells (QWs) show promise for use as room-temperature light emitters. The effectiveness of the plasmon enhancement, however, is limited by the strong electron/hole and longitudinal optical phonon coupling found in the III-V nitrides. The electron-phonon coupling within semiconductor QWs has been modified using silver nanoparticles embedded within the QWs. Direct evidence is provided for this change via confocal Raman spectroscopy of the samples. This evidence is augmented by Angle-dependent photoluminescence experiments which show the alteration of the electron-phonon coupling strength through measurement of the emitted phonon replicas. Together these demonstrate a direct modification of carrier-phonon interactions within the system, opening up the possibility of controlling the coupling strength to produce high-efficiency room-temperature light emitters.

Nano-scale strain mapping using near-field spectroscopy

A technique is presented for mapping the strain in light-emitting nano- and hetero structures using near-field spectroscopy. This technique makes use of theoretical calculations to extract the strain from near-field data.

Near-field optical spectroscopy of Ga nanoparticles for plasmonics

Near-field optical spectroscopy of nanoscale Ga droplets on GaAs exhibits quenching of photoluminescence emission due to coupling with surface plasmon. Ga droplets exhibit antenna like behavior associated with a red-shift in the near-field photoluminescence emission.