Fabrication of quantum dot and cavity nanostructures

Abstract

EXTENDED ABSTRACTIn this tutorial, I'll discuss fabrication aspects of cavities that contain quantum dots as active emitters. As far as the quantum dots are concerned, the most established fabrication technique is self-assembled Stranski-Krastanov growth [1]. This mechanism relies on the supply of a material with a lattice constant larger than that of the substrate, and quantum dots will form spontaneously after a certain planar layer thickness is exceeded. While self-assembled quantum dot growth has been reported for a number of material systems, the InGaAs/GaAs material combination remains the workhorse for QD cavity structures due to the excellent optical quality of the dots and the mature technology that is available for the GaAs material system.Optical confinement in resonators is achieved by total internal reflection at the semiconductor -air interface, the formation of an optical bandgap in structures that feature a periodic modulation of the refractive index, or a combination of both mechanisms. The different available resonator types [2] (photonic crystal [3], whispering gallery mode [4], micropillar [5]) have their pros and cons with respect to the ratio of the quality factor and the mode volume, the coupling efficiency to in-plane and out-of-plane modes, and the possibility to tune the resonance. A major issue in the fabrication of quantum dot -cavity structures is the spectral and spatial resonance of the dot with the cavity. The commonly used approach defines several hundred or thousand structures on a chip, followed by a screening process that identifies good devices. This technique is perfectly suitable for the fabrication of single structures, and several breakthrough experiments in the field have been performed using devices made this way. However, fabrication of devices that rely on several coupled QDresonators is not possible.Several techniques have been reported to address this issue. One possibility is to locate the quantum dots before the fabrication of the cavities using e.g. photoluminescence. The position of the dot with respect to reference markers on the sample can then be used to define the resonator by electron beam lithography of the resonator pattern after aligning to the very same markers [6]. Alternatively, a second laser beam that is collinear with the PL-excitation laser can be used to expose a layer of photoresist on the sample. This technique has even been used to define coupled resonators with modes that match the emission of the exciton and biexciton [7]. The advantage of this 'fabrication of the cavity around the quantum dot' approach is that no compromise has to be made with respect to the epitaxy of the dots. However, fabrication of devices that contain dots at defined positions is not possible, since the placement of the resonators is determined by the (random) location of the quantum dots. This can be circumvented by the use of site-controlled quantum dots [8]. In this case, a patterned surface directs the nucleation of the dots at specific positions. However, the dots ...