Room temperature single photon emission from planar GaN/AlN quantum dot samples grown by MBE

We present an overview on the growth and the optical properties of GaN quantum dots (QDs) embedded in planar AlN grown on the following three different types of substrates: single AlN crystal, AlN on sapphire, and AIN on Si(111). QD density control over three orders of magnitude from 10 8 to 10 11 cm -2 is demonstrated by changing the GaN growth rate. After having established a phase diagram for the QD formation and the critical thickness for the 2D-3D transition, we focus on a comparative study on the optical fingerprint of individual QDs. We identify numerous excitonic complexes in individual GaN QDs and analyze their suitability for single-photon emission at non-cryogenic temperatures as attested by the corresponding g (2) -traces derived from photon correlation measurements. Clear antibunching with g (2) (0)=0.17±0.03 is observed at 300 K, which originates from the decay of single excitons in an individual GaN QD. Finally, an excitation power dependent analysis of the photon statistics reveals the limiting factors for the purity of our single photon sources, provides insight into the physical mechanism of spectral diffusion, and a general pathway towards future optimization, which aims for electrically driven single photon sources operating at 300 K.

Optical characterisation of single quantum dots emitting at 1300nm

Carl Zinoni

This thesis deals with the optical characterization of single quantum dot devices emitting at 1300nm. Thanks to the development and optimization of the growth technique we were able to achieve at the same time emission at 1300nm and ultra low QD densities. Our single QD devices present clear and reproducible spectral signatures in which we can identify exciton, biexciton and charged exciton transitions. Quasi-resonant excitation at 70K demonstrates background free single exciton transitions, which is very promising for the realization of a single photon device operating at temperatures in easy reach of thermoelectric coolers. A time-correlated single photon counting setup was built and used to measure the radiative lifetimes of single exciton transitions. These measurements also present new evidence on a background emission superposed to the narrow spectral transitions. Demonstration of single photon emission at these wavelengths required building a setup to measure the correlations between fiber-coupled single photons in a 300ps time window, emitted from a nano-device in free space at cryogenic temperatures, and with the capability of maintaining the optical alignment on a micrometer scale for several hours. With such a setup we have demonstrated that our QDs can generate single photon states at 1300nm. We used this single photon source to characterize novel detectors based on superconducting nanowires and measured for the first time the intensity correlation function at 1300nm on single photons from a QD. These detectors show at least 2 orders of magnitude improvement on the signal to noise ratio as compared to InGaAs APDs; this is very important since, for a QKD system, the detector noise, amongst others, determines the maximum distance over which a secure key can be exchanged. It should be noted that, due to the di±culties in these measurements, to date there has been only one other clear demonstration of single photon emission at 1300nm.

EPFL2007

Tailored-Potential Pyramidal Quantum Dot Heterostructures

Valentina Troncale

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

EPFL2010

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