Power-dependent Micro-photoluminescence Research Articles

The low temperature limit of the excitonic Mott density in GaN: an experimental reassessment

The research on GaN lasers aims for a continuous reduction of the lasing threshold. An approach to achieve it consists in exploiting stimulated polariton scattering. This mechanism, and the associated polariton lasers, requires an in-depth knowledge of the GaN excitonic properties, as polaritons result from the coupling of excitons with photons. Under high excitation intensities, exciton states no longer exist due to the Coulomb screening by free carriers; this phenomenon occurs at the so-called Mott density. The aim of this work is to study the bleaching of excitons under a quasi-continuous optical excitation in a bulk GaN sample of high quality through power dependent micro-photoluminescence and time-resolved experiments at 5 K. Time-resolved photoluminescence allows to measure the carrier lifetime as a function of excitation intensity, which is required for a reliable evaluation of the injected carrier density. The vanishing of excitonic lines together with the red-shift of the main emission evidences the occurrence of the Mott transition for a carrier concentration of (6 ± 3) × 1016 cm−3. This value is more than an order of magnitude smaller than previous determinations published in the literature and is in accordance with many-body calculations.

Enhanced optical properties in a polarization-matched semiconductor plasmonic nanocavity

Plasmonic nanocavity is a novel platform to investigate fundamental light-matter interactions. In such a cavity, the working media are localized or propagating surface plasmons, which are transverse magnetic polarized electromagnetic waves forming at a metal/dielectric interface. Therefore, the carrier dynamics and optical properties of a plasmonic nanocavity are strongly dependent on the polarization matching between the comprising emitter and the plasmonic cavity modes. Here we demonstrate a plasmonic nanocavity with an InGaN/GaN multi-quantum well emitter, fabricated by cost-effective post-growth approaches from a commercially available epiwafer. The polarization state of the emission light from the single nanorod is the same as that of plasmonic cavity mode. As a consequent, significantly enhanced optical properties of the plasmonic nanocavity has been observed, confirmed by time-resolved and power-dependent micro-photoluminescence measurements. This work provides a promising insight into the study on light-matter interaction in a plasmonic nanocavity.

Growth of wurtzite CdTe nanowires on fluorine-doped tin oxide glass substrates and room-temperature bandgap parameter determination

The growth of CdTe nanowires, catalyzed by Sn, was achieved on fluorine-doped tin oxide glass by physical vapor transport. CdTe nanowires grew along the 〈0001〉 direction, with a very rare and phase-pure wurtzite structure, at 290 °C. CdTe nanowires grew under Te-limited conditions by forming SnTe nanostructures in the catalysts and the wurtzite structure was energetically favored. By polarization-dependent and power-dependent micro-photoluminescence measurements of individual nanowires, heavy and light hole-related transitions could be differentiated, and the fundamental bandgap of wurtzite CdTe at room temperature was determined to be 1.562 eV, which was 52 meV higher than that of zinc-blende CdTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands was estimated to be 43 meV.

Wurtzite ZnTe Nanotrees and Nanowires on Fluorine-Doped Tin Oxide Glass Substrates.

ZnTe nanotrees and nanowires were grown on fluorine-doped tin oxide glass by physical vapor transport. Sn from a fluorine-doped tin oxide layer catalyzed the growth at a growth temperature of 320 °C. Both the stem and branch nanowires grew along ⟨0001⟩ in the rarely observed wurtzite structure. SnTe nanostructures were formed in the liquid catalyst and simultaneously ZnTe nanowire grew under Te-limited conditions, which made the formation of the wurtzite structure energetically favorable. Through polarization-dependent and power-dependent microphotoluminescence measurements from individual wurtzite nanowires at room temperature, we could determine the so far unknown fundamental bandgap of wurtzite ZnTe, which was 2.297 eV and thus 37 meV higher than that of zinc-blend ZnTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands is estimated to be 69 meV.

Spin-Related Micro-Photoluminescence in Fe3+ Doped ZnSe Nanoribbons

Spin-related emission properties have important applications in the future information technology; however, they involve microscopic ferromagnetic coupling, antiferromagnetic or ferrimagnetic coupling between transition metal ions and excitons, or d state coupling with phonons is not well understood in these diluted magnetic semiconductors (DMS). Fe3+ doped ZnSe nanoribbons, as a DMS example, have been successfully prepared by a thermal evaporation method. Their power-dependent micro-photoluminescence (PL) spectra and temperature-dependent PL spectra of a single ZnSe:Fe nanoribbon have been obtained and demonstrated that alio-valence ion doping diminishes the exciton magnetic polaron (EMP) effect by introducing exceeded charges. The d-d transition emission peaks of Fe3+ assigned to the 4T2 (G) → 6A1 (S) transition at 553 nm and 4T1 (G) → 6A1 (S) transition at 630 nm in the ZnSe lattice have been observed. The emission lifetimes and their temperature dependences have been obtained, which reflected different spin–phonon interactions. There exists a sharp decrease of PL lifetime at about 60 K, which hints at a magnetic phase transition. These spin–spin and spin–phonon interaction related PL phenomena are applicable in the future spin-related photonic nanodevices.

O-Doping Profile along ZnTe Nanowires Obtained Using Power-Dependent Microphotoluminescence Measurements

We investigated the O-doping profile along ZnTe nanowires using laser power-dependent microphotoluminescence measurements. ZnTe nanowires were grown by physical vapor transport catalyzed by bismuth. The photoluminescence spectra consisted of band-edge green photoluminescence and red photoluminescence due to a highly luminescent intermediate band formed by unintentional O doping. An obvious color-tuning effect of photoluminescence was observed along the nanowire and was suggestive of the competition between two luminescence bands. A significant variation in the intensity of red luminescence from the catalyst tip to the bottom was observed, and the photoluminescence intensity as a function of excitation power at each sampling spot along the nanowire was analyzed using a power law. The exponent value in the power law, from the sampling spots, was determined to be 0.58 ± 0.11, indicating an identical recombination channel for red luminescence. However, the O-doping density shows a superlinear increase from the catalyst tip to the bottom. Besides the direct impingement of doping species on the side wall, the additional incorporation of the doping species surface-migrating from the substrate may explain the nonlinear O-doping profile along the nanowire.

Temperature dependency of the emission properties from positioned In(Ga)As/GaAs quantum dots

In this letter we study the influence of temperature and excitation power on the emission linewidth from site-controlled InGaAs/GaAs quantum dots grown on nanoholes defined by electron beam lithography and wet chemical etching. We identify thermal electron activation as well as direct exciton loss as the dominant intensity quenching channels. Additionally, we carefully analyze the effects of optical and acoustic phonons as well as close-by defects on the emission linewidth by means of temperature and power dependent micro-photoluminescence on single quantum dots with large pitches.

Lasing properties of non-polar GaN quantum dots in cubic aluminum nitride microdisk cavities

We demonstrate laser emission from optically pumped non-polar cubic GaN quantum dots embedded in cubic aluminum nitride microdisks. Power dependent micro-photoluminescence studies at low temperature (∼10 K) revealed S-shaped curves of the integral mode intensity. We observed whispering gallery modes with quality factors up to 5000 at the high energy side (4 eV, i.e., ∼310 nm wavelength) in photoluminescence spectra of microdisks with a diameter of 2.5 μm. Furthermore, we have determined the spontaneous emission coupling factors to β = 0.12 and β = 0.42 for resonator modes of different radial orders.

Optical and microstructural study of a single layer of InGaN quantum dots

Two typical kinds of InGaN quantum dots (QDs) have been grown on sapphires under different conditions through modifying the NH3 flow rate using metal-organic chemical vapor deposition: small spherical dots with a high dot density and large truncated pyramidal dots with a low dot density. The small dots have been found typically coherent and defect-free, while a strain relaxation has often been observed in the large dots. Consequently, this leads to a massive difference in optical properties between them. The optical properties have been investigated by means of temperature-dependent and excitation power-dependent microphotoluminescence measurements. It has been found that the small spherical QDs show higher optical quantum efficiency and much weaker piezoelectric field induced quantum-confined Stark effect than the large truncated QDs. Based on the energy balance between the strain and surface energy, the influence of V/III ratio on the transition from two-dimensional to three-dimensional growth mode during the QD growth has been discussed.

Single-Photon Emission from a Single InAs Quantum Dot

Excitation power-dependent micro-photoluminescence spectra and photon-correlation measurement are used to study the optical properties and photon statistics of single InAs quantum dots. Exciton and biexciton emissions, whose photoluminescence intensities have linear and quadratic excitation power dependences, respectively, are identified. Under pulsed laser excitation, the zero time delay peak of second order correlation function corresponding to exciton emission is well suppressed, which is a clear evidence of single photon emission.

Catalyst-free GaN nanorods grown by metalorganic molecular beam epitaxy

High-density GaN nanorods with outstanding crystal quality were grown on c-sapphire substrates by radio-frequency plasma-assisted metalorganic molecular beam epitaxy under catalyst- and template-free growth condition. Morphological and structural characterization of the GaN nanorods was employed by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy (HRTEM). These results indicate that the rod number density can reach 1/spl times/10/sup 10/ cm/sup -2/ and the nanorods are well-aligned with preferentially oriented in the c-axis direction. Meanwhile, no metallic (Ga) droplet was observed at the end of the rods, which is the intrinsic feature of vapor-liquid-solid method. Nanorods with no traces of any extended defects, as confirmed by TEM, were obtained as well. In addition, optical investigation was carried out by temperature- and power-dependent micro-photoluminescence (/spl mu/-PL). The PL peak energies are red-shifted with increasing excitation power, which is attributed to many-body effects of free carriers under high excitation intensity. The growth mechanism is discussed on the basis of the experimental results. Catalyst-free GaN nanorods presented here might have a high potential for applications in nanoscale photonic devices.

Excited states and multi-exciton complexes in single CdTe quantum dots

We present an optical study of excited states in single CdTe quantum dot (QDs). Using micro-photoluminescence excitation spectroscopy, absorption up to two confined excited levels have been observed in some dots. Power-dependent micro-photoluminescence is then used to study the occupation of excited states. The emission pattern is characteristic of the increase of the exciton number in the QD (shell-filling). A clear identification of the different multi-exciton complexes has been obtained in a highly symmetric dot. The evolution of the different multi-exciton intensities can then be reproduced by solving the rate equations for multi-exciton state occupancy and the fit by this simple model provides an estimate of the radiative lifetime of the different multi-exciton complexes.

Optical properties of InAs quantum dots formed on GaAs pyramids

Optical properties of InAs quantum dots on GaAs pyramidal structures formed by selective area metal organic vapor phase epitaxy have been studied with low-temperature excitation powerdependent microphotoluminescence (μ-PL) and microphotoluminescence excitation (μ-PLE) spectroscopy. The power-dependent μ-PL spectra show well-defined ground and excited states whose interlevel energy spacings are estimated to be about 21 meV. These experimental results demonstrate the discrete nature of zero-dimensional density of states in position and number controlled InAs quantum dots (QDs) selectively grown on GaAs pyramids. We have also observed relative strong PLE peaks which are ascribed to the multiple longitudinal optical phonon resonance. Excited states and carrier relaxation of InAs QDs formed on GaAs pyramids are discussed.