We have reported that a novel quantum structure which we term quantum well island (QWI), a few monolayer thick and sub-micron wide structure, is effective in confining the carriers and enhancing multi-exciton interactions. By embedding InAs-based QWIs in AlGaAs barrier layers, we demonstrated that upconverted photoluminescence (PL) in the visible regime can be obtained by impinging near infrared (IR) photons, which may potentially be applied for intermediate band (IB) solar cells . Further investigation has revealed that the dominant upconversion mechanism is most likely Auger, while two-step excitation may also take place under selected conditions . The upconverted carriers generated by IR irradiation may also be detected as photocurrents. Through a series of studies using this structure, we note the importance of the carrier trapping involved during the upconversion processes. For instance, multiple laser-beam excitation measurements have shown that trapping and re-trapping processes reduce the photocurrents .
However, recently, using a structure that consists of InAs quantum dots embedded in InAs/GaAs multi-quantum wells (MQWs), we find that efficient carrier trapping can enhance upconverted PL . We show the preparation and the control of this structure by molecular beam epitaxy (MBE), and the possible mechanisms of the upconversion. We also discuss how the conversion efficiency may be improved using device structures based on this concept.
 D. M. Tex and I. Kamiya, Phys. Rev. B 83 (2011) 081309.
 D. M. Tex, I. Kamiya, and Y. Kanemitsu, Sci. Rep. 4 (2014) 4125.
 D. M. Tex, T. Ihara, I. Kamiya, and Y. Kanemitsu, to be published.
 Y. Zhang and I. Kamiya, JSAP Spring Meeting, 2016.
We have investigated the terahertz photoresponse of a single semiconductor quantum dot, electrostatically defined by a sharp conducing Atomic Force Microscope tip in contact with a resonant tunneling diode structure. The quantum dot is excited by radiation from a Free Electron Laser in experiments both at room temperature and at cryogenic temperatures. Pronounced resonant tunneling features and classical rectification at frequencies from 0.3 to 3THz are observed in the I-V curves of these devices. These results demonstrate a novel approach to achieving terahertz excitation and studying transport in quantum dots.
Various optical techniques have been developed over the last few years for real-time analysis of surfaces and near-surface regions of semiconductor epitaxy. These techniques are providing insights into microscopic mechanisms of epitaxy and opportunities for sample-driven closed- loop feedback control of the epitaxial growth process. Both aspects are expected to become increasingly important as device complexity increases and tolerances become more stringent. Examples are provided and opportunities discussed.