Infoscience

Thesis

Growth and characterization of single quantum dot devices for fiber-based quantum communications

Single photon sources have recently attracted significant interest for their potential role in the future applications of quantum cryptography, and in the longer term, quantum computing. Among the different types of single photon emitters, semiconductor quantum dots (QDs) are good candidates as they combine discrete electronic transitions with the possibility of electrical excitation. Self-assembled InAs QDs on GaAs substrate are by far the most studied system as they can easily be grown into a microcavity structure. Embedding these into a microcavity is important as it increases the extraction efficiency, directionality and speeds up the photon emission. Until now, most studies have concentrated on QDs emitting in the λ <1000 nm range. However fiber-based quantum communication applications require an emission wavelength in the 1300 or 1550 nm transmission window where optical losses of silica fiber are minimal. Hence, the purpose of this study is to realize a single photon source from self-assembled InAs/GaAs QDs emitting at 1300 nm. This poses difficulties associated first with the QD epitaxial growth as large and sparse InAs QDs are required, and second with the single photon Detection as single photon detection system in the near-infrared presents a poor quantum efficiency and signal to noise ratio. The first part of this thesis is devoted to optimizing the QD growth. A low area QD density is required in order to obtain the isolation of a single QD. Moreover, the QD size must be large enough to obtain emission wavelengths around 1300 nm at low temperatures (10K), i.e. 1400nm at room temperature. We present here the optimization of a growth procedure that allowed the fabrication of low-density QDs (2·µm-2) emitting at the predicted wavelengths and showing a very good size homogeneity. In addition, we show comparative studies on the optical characteristics of these low-density QDs with higher density QDs. These show that the low area density does not imply a reduction of the carrier capture efficiency of the QDs. The second part of this thesis concentrates on the spectroscopy of single QDs under optical and electrical pumping. By careful optimization of the detection system, single QD emission has been demonstrated. Single photon emission at 1300 nm was also proven by means of an anti-correlation experiment. Preliminary results of low-density QDs inserted into three-dimensional microcavities such as a micropillar and a photonic crystal structures are also presented. Further experiments are under way. Finally, the fabrication and characterization of single QD light emitting diodes (LED) are detailed. Electrical injection poses additional problems, notably in term of device processing and carrier diffusion. Different designs of LED structures have been tested. The results are discussed. Finally, electroluminescence emission from a single QD under electrical injection is reported.

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