Droplet epitaxy nanostructures for device applications

Droplet epitaxy (DE) is a non-conventional growth technique based on molecular beam epitaxy. This method allows for the fabrication of lattice-matched and strain-free self-assembled III-V nanostructures, reducing the size dispersion between 5 and 20%. Thanks to the versatility of the DE, a fine tuning of the dot shape and density is possible by changing the parameters that control the Ga atom diffusion on the surface. The density can span between 10 7 and 1011 cm-2. The control on the shape allows to select the aspect ratio of the dots, thus permitting to change the quantum confinement and to tailor the optical and electronic property of the dots to fit the needs of different applications. In this talk the main aspects of the growth and characterization of DE nanostructures will be discussed, together with different applications recently developed with DE. In particular, I will show the results of the insertion of a layer of quantum dots grown by DE in a solar cell. By tuning the size of the QDs it is possible to change the position of the intermediate band, and by tuning their aspect ratio the high energy states of the QDs can also be tuned in order to have a small electron-phonon coupling with the barrier. Moreover, the lack of defect and wetting layer states can greatly reduce thermal escape of carriers from the IB, leaving photon-induced transitions the dominant ones, as requested by intermediate band theory [1]. I will also show the fabrication of GaAs single photon emitters integrated on Si substrates demonstrating how DE makes also possible the growth of bright III-V quantum emitters on silicon substrates thus paving the route to the integration of optically efficient III-V nanostructures on CMOS technology [2]. The last application I will describe is the growth of ultra high density QDs for the fabrication of quantum dot infrared photodetectors (QDIP) active in the range between 2 and 8 μm, as a first step toward the the fabrication of a III-V based QDIP integrated on Si [3]. [1] Scaccabarozzi, Adorno, Bietti, Acciarri, Sanguinetti, Physica Status Solidi (RRL) - Rapid Research Letters 3, 173, (2013) [2] Cavigli, Bietti, Accanto et al., Applied Physics Letters, 100, 231112, 2012. [3] Frigerio, Isella, Bietti, Sanguinetti, SPIE Newsroom, 2013