Time Correlated Single Photon Spectroscopy on Pyramidal Quantum Dots

Abstract

Generation of non-classical light is both of fundamental interest and a common condition for quantum information applications (QIA). One feasible type of single photon emitter for QIA is based on semiconductor quantum dots (QDs), due to their atomic-like energy structure and their possibility to be integrated with other semiconductor devices on the same chip. Sitecontrolled QDs with highly linear polarized emission are a prerequisite for certain QIA and a close to room temperature operation is demanded for widespread applications.III-nitride QD can have the deep confinement potentials needed for high temperature operation, and the demonstration of single photon emission at room temperature was recently reported for a GaN QD [Nano Lett. 14, 982 (2014)]. Asymmetric III-nitride QD emits light with a high degree of linear polarization. To make site-controlled nitride-based QDs a promising approach is to deposit a thin layer of InGaN on top of hexagonal GaN micropyramids. QDs formed on the apex of the pyramids grown with this approach have been shown to exhibit single and sharp InGaN related emission lines with a high degree of linear polarization [Nano Lett. 11, 2415. A simple elongation of the pyramid base gives control of the polarization direction [Light: Sci. Appl. 3, e139 (2014)].The work presented in this thesis deals with time correlation measurements, to measure, for the first time, the single photon properties of these pyramidal QDs.A time correlated single photon spectroscopy (TCSPS) setup was assembled, tested and used to perform measurements on these pyramidal QDs. The TCSPS apparatus measures the time differences τ between subsequent photons emitted from the sample. In the spectrally filtered light of one emission line in the emission spectra, e.g. exciton emission, of a QD two or more photons cannot be emitted simultaneously, i.e. the photons are sent out one by one. A histogram of the ensemble of measured time differences (∼ 10 6 events) will then for the ideal case have no events for τ = 0, and very few for τ close to zero. This histogram, when normalized, is under certain conditions equal to the second order coherence function g (2) (τ ). In reality, however, there are photons coming from other sources close to the QD, i.e. background emission, that reach the detector and reduce the dip in the correlation histogram for small τ . There is also an statistical uncertainty in the measured time differences and finally the finite bin width used in the histogram that i deteriorate the measured correlation function. To understand the influence on g (2) (τ ) from background emission, instrument response function and the bin width, on the measurement on excitonic emission, simulations and calculations were made. The crucial variables were, for our samples and setup, the level of the background emission and the instrument response function.A post growth process was developed to cover the lower parts of the pyramid sides as well as the area between the pyramids with a metal film, to reduce the backgrou...