Fabrice Semond

Polarized emission of GaN/AlN quantum dots : single dot spectroscopy and symmetry-based theory

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.

AlGaN photodetectors for applications in the extreme ultraviolet (EUV) wavelength range

We report on the fabrication of Schottky-diode-based Extreme UltraViolet (EUV) photodetectors. The devices were processed on Gallium Nitride (GaN) layers epitaxially grown on 4 inch Silicon (111) substrates by Metal-Organic Chemical Vapor Deposition (MOCVD). Cutoff wavelength was determined together with the spectral responsivity measurements in the Near UltraViolet (NUV) range (200nm to 400nm). Absolute spectral responsivity measurements were performed in the EUV range (5nm to 20nm) with the synchrotron radiation using the facilities of Physikalisch- Technische Bundesanstalt (PTB), located at Berliner Elektronenspeicherring-Gesellschaft fuer Synchrotronstrahlung (BESSY). The described work is done in the framework of the Blind to Optical Light Detectors (BOLD) project supported by the European Space Agency (ESA).

III-nitride on silicon microdisks: electrical injection and bus waveguide side-coupling (Conference Presentation)

Group-III-nitride nanophotonics on silicon is a booming field, from the near-IR to the UV spectral range. The main interest of III-nitride nanophotonic circuits is the integration of active structures and laser sources. Laser sources with a small footprint can be obtained with microresonators formed by photonic crystals or microdisks, exhibiting quality factors up to a few thousand down to the UV-C. So far, single microdisk laser devices have been demonstrated, mostly under optical pumping. Combining microdisk lasers under electrical injection with passive devices represents a major challenge in realizing a viable III-nitride nanophotonic platform on silicon. As a first step to realize this goal, we have separately demonstrated electroluminescence from microdisks and side-coupling of microdisks to bus waveguides with outcoupling gratings in the blue spectral range. We have developed the fabrication of electrically injected microdisks with a circular p-contact on top of the disk that is connected to a larger pad via a mechanically stable metal microbridge. Blue electroluminescence is observed under current injection at room temperature. We also demonstrated high Q factor (Q > 2000) WGMs in the blue spectral range from microdisks side-coupled to bus waveguides, as measured from the luminescence of embedded InGaN quantum wells. The WGM resonances are clearly observed through outcoupling gratings following propagation in partially etched waveguides to remove quantum well absorption. Small gaps between microdisks and bus waveguides of around 100 nm are necessary for efficient coupling in the blue spectral range, which represents a major fabrication challenge. We will discuss the progress brought by these building blocks to generate future III-nitride photonic circuits.