Optical Studies of Single InGaN/GaN Quantum Dots

• Main Author: Jarjour, Anas F.
• Published: University of Oxford 2007
• Subjects: 535
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491621

Experimental investigation of the optical properties of single InGaN/GaN quantum dots (QDs) is presented. The results are compared to a theoretical model based on an atomistic semi-empirical tightbinding approach. Nonlinear excitation is investigated by micro-photoluminescence (micro-PL), time-resolved PL and PL excitation (PLE) spectroscopy. It is found to produce almost total suppression of the background emission arising from the underlying quantum well. This is explained qualitatively by the enhancement of the nonlinear absorption due to the zero-dimensional confinement in the QDs. PLE spectra show clear evidence for the existence of excited states. We present direct evidence of the control of the oscillator strength of the exciton state in the QD by an applied vertical electric field. This is achieved through the study of the radiative lifetime of the QD embedded in a p-i-ll diode structure. The effect is explained by the increase in the overlap between the electron and the hole wavefunctions due to the partial compensation of the internal piezoelectric field and was found to be accompanied by a large blue shift of the transition energy. The results are found to be in good quantitative agreement with theoretical predictions arising from the theoretical model extended to take into account the effect of the applied field. The induced increase in the overlap between the wavefunctions of the carriers is shown experimentally to enhance the attractive Coulomb interaction leading to the change of the sign of the biexcitonic binding energy. The electroluminescence observed from the structure under forward bias showed clear indication of single QD emission which persists up to ',,85 K. The generation of single photons in the blue spectral region from a single QD is demonstrated. The collection efficiency was enhanced by embedding the QD layer in a low-Q microcavity. The single QD emission is observed to be ',,10 times stronger than typical emission without a cavity. Nonlinear excitation spectroscopy proved essential in these measurements since any background emission would mask the signature of single-photon emission. This result provides a clear evidence of the quantum nature of the emission.