Size control of GaAs quantum dots grown by droplet epitaxy for device applications

Droplet epitaxy (DE) is a growth technique based on molecular beam epitaxy proposed in 1991 by Dr. N. Koguchi [1]. This method allows for the fabrication of wetting layer free, lattice-matched and strain-free III-V nanostructures self-assembled, reducing the size dispersion between 5 and 20%. Thanks to the separation of the group III and group V atom irradiation on the substrate, it is possible to obtain a fine tuning of the quantum dot (QD) shape and density by simply changing the parameters that control the group III atom diffusion on the surface. It is possible to select a density between 107 and 1011 cm-2 and the aspect ratio of the dots, thus allowing 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 presentation we discuss the main aspects of the growth and characterization of DE nanostructures, together with the advantage of shape control for different applications recently developed. The first application is related to the fabrication of intermediate band solar cells, introducing a layer of quantum dots grown by DE in a standard AlGaAs solar cell. By tuning the size of the QDs it is possible to change the position of the intermediate band, and by tuning the aspect ratio of the QDs 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 intermediate band, leaving photon-induced transitions the dominant ones, as requested by intermediate band theory [2]. The second application is related to the fabrication of GaAs single photon emitters integrated on Si substrates. Hanbury-Brown-Twiss measurements demonstrates how the single nanostructure is a single photon emitter up to temperature of liquid nitrogen, thus demonstrating how DE makes also possible the growth of bright III-V quantum emitters on silicon substrates and paving the route to the integration of optically efficient III-V nanostructures on CMOS technology [3]. The last application 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 [4].[1] N.Koguchi, S.Takahashi, T.Chikyow, Journal of Crystal Growth 111, 688 (1991) [2] A.Scaccabarozzi, S.Adorno, S.Bietti, M.Acciarri, S.Sanguinetti, Physica Status Solidi (RRL) - Rapid Research Letters 3, 173, (2013) [3] L.Cavigli, S.Bietti, M.Accanto et al., Applied Physics Letters, 100, 231112, 2012. [4] J.Frigerio, G.Isella, S.Bietti, S.Sanguinetti, SPIE Newsroom, 2013