Quantum dot display | EPFL Graph Search

A quantum dot display is a display device that uses quantum dots (QD), semiconductor nanocrystals which can produce pure monochromatic red, green, and blue light. Photo-emissive quantum dot particles are used in LCD backlights or display color filters. Quantum dots are excited by the blue light from the display panel to emit pure basic colors, which reduces light losses and color crosstalk in color filters, improving display brightness and color gamut. Light travels through QD layer film and traditional RGB filters made from color pigments, or through QD filters with red/green QD color converters and blue passthrough. Although the QD color filter technology is primarily used in LED-backlit LCDs, it is applicable to other display technologies which use color filters, such as blue/UV active-matrix organic light-emitting diode (AMOLED) or QNED/MicroLED display panels. LED-backlit LCDs are the main application of photo-emissive quantum dots, though blue OLED panels with QD color filters

GaN Quantum Dots for Room Temperature Excitonic Physics

Sebastian Pascal Tamariz Kaufmann

Quantum dots (QDs) are the result of quantum confinement in the three spatial directions, as such they exhibit remarkable properties, of which the most important is perhaps the absence of a continuum of states. The allowed energetic levels are discrete, similarly to the energy dispersion in single atoms. As a result, for an individual QD, single photon emission can be achieved. In particular, GaN QDs display an enhanced thermal robustness in comparison to conventional III-V QDs and remain bright even at room temperature. Furthermore, as opposed to other thermally stable QD systems such as colloidal QDs, GaN QDs are epitaxially grown and hence can be easily integrated into photonic devices. However, III-nitride technology is comparatively less mature than traditional III-V QDs and several challenges remain regarding material quality, particularly in terms of structural defects, point defects, and doping of high Al content AlGaN. With the aim of integrating GaN QDs into photonic nanostructures such as waveguides and cavities based on membrane photonic crystals (PhC), the thesis is oriented towards the development of GaN QDs on thin AlN layers on Si(111). III-nitrides enjoy a high etching selectivity with Si, which allows the fabrication of membrane PhC. The thesis begins by developing high quality AlN on Si(111) and describing the growth of AlN by NH3-molecular beam epitaxy (MBE) at temperatures above 1000 °C. At the limit between layer by layer and step flow growth, very smooth AlN can be achieved even for thin layers on Si(111). These layers, together with the knowledge acquired in the growth of AlN serve as a stepping stone for the growth of GaN QDs. GaN QDs have been produced by NH3-MBE since the first demonstration in 1999, by a modified Stranski-Krastanov (m-SK) growth that relies on a growth interruption. Here we develop the growth of GaN on AlN so that a spontaneous transition into a SK growth occurs. This is analogous to the seminal InAs/GaAs system and as such, the same principles for QD size and density control hold. III-nitrides are often grown heteroepitaxially and the resulting layers have a large threading dislocations density (TDD) (10^9 - 10^10 1/cm²). Dislocations offer nucleation centers due to a strain dipole that limits the control over the size and density of the QDs. Then by employing, AlN single crystal substrates with TDD in the order of 10^3 1/cm² this limitation is overcome. In this manner, we demonstrate size and density control by varying the amount of deposited GaN and the diffusion length. QD densities ranging from 10^8 to 10^11 1/cm² are achieved. Finally, an approach capable of yielding low QD densities even on highly dislocated substrates based on ripening and evaporation is presented. The optical properties of GaN/AlN QDs were investigated. They display high internal quantum efficiencies at 300 K in the order of 40-60 %. The control over the QD size and density is demonstrated by PL measurements. Single QDs are inquired thanks to the possibility of low QD densities. A sample consisting of GaN QDs in AlN on Si(111) was studied in depth. These QDs exhibit multiexcitonic complexes and remain bright even when increasing the temperature to 300 K. At this temperature, single photon emission is revealed by second-order autocorrelation measurements g(2) (¿) with g(2) (0) = 0.17 ± 0.03. Finally, GaN QDs are demonstrated to be bright at room temperature with absolute counts in the order of 10^6 1/s.

EPFL2019

Optically active quantum dots in bottom-up nanowires

Yannik Fontana

This thesis is dedicated to the discovery and progressive study of quantum emitters embedded in the shell of coaxial gallium arsenide/ aluminum gallium arsenide nanowires. The bottom-up core/shell nanowires were grown in a molecular beam epitaxy machine. During the shell growth, diffusion-driven phenomena lead to segregation effects. Gallium-rich regions are formed at the nanoscopic scale. The observation has been made that the reduced dimensionality of these regions provides true tridimensional confinement for the carriers. The recombination spectra of the electrons with the holes in what was coined shell quantum dots (shell-QDs) thus appear as sets of narrow, intense peaks. The formation of shell quantum dots is taking place on a large range of growth temperatures and nominal alloy fractions, giving freedom to engineer the growth process. The shell thickness plays an important role in the quantum dot density and total ensemble spectrum. In addition, the adjunction of an aluminum arsenide predeposition layer increasing the local curvature has been seen to foster the quantum dots formation. Single emitter spectroscopy reveals the few-particles electronic structure of quantum dots, with systematic signatures for the different degrees of occupation of the quantum dot. The shape anisotropy of the quantum dots leads to observable spin-spin interactions, which lift the degeneracy of the exciton level (one hole and one electron). Generally undesirable, this effect allows here to find that the orientation of the quantum dots in the nanowire is not hard-wired to the growth direction or to the nanowire long axis. This observation is confirmed by magneto-photoluminescence experiments. The energetic splitting and shift of the spin sublevels when an external magnetic field is applied also confirms the small size of the quantum dots. It is found that for GaAs in the strong confinement regime, the Landé coefficients of the electron and hole take opposite signs and are dependent on the angle at which the field is applied. These effects allow to tune the exciton composite Landé coefficient and could be used to reduce the splitting between the exciton spin sublevels or create optically degenerate coupled systems. Finally, the sub-nanosecond dynamics happening in the quantum dots are probed with time-correlated photon counting. It is shown that the carriers in the shell are quickly captured by the quantum dots. In addition, it is proposed that the electron population is reduced due to diffusion-assisted mechanisms or through electron-donor recombination.

EPFL2015

Toward Bright and Pure Single Photon Emitters at 300 K Based on GaN Quantum Dots on Silicon

Raphaël Butté, Gordon Jens Callsen, Nicolas Grandjean, Kanako Shojiki, Johann Nicolaï Stachurski, Sebastian Pascal Tamariz Kaufmann

Quantum dots (QDs) based on III-nitride semi-conductors are promising for single photon emission at noncryogenic temperatures due to their large exciton binding energies. Here, we demonstrate GaN QD single photon emitters operating at 300 K with g((2)) (0) = 0.17 +/- 0.08. At this temperature, single photon emission rates up to 10 6 s(-1) are reached, while g((2)) (0)

AMER CHEMICAL SOC2020