Wiring quantum dots

Today, most modern technology relies mainly on materials with reduced dimensionalities, such as thin films (2D), nanowires (1D), and quantum dots (0D). An extreme success in synthesis, characterization, and application of the respective - but separated - material classes has been achieved within the past 30 years. In this research proposal we merge: (a) three young and important research groups out of Switzerland, Austria and Germany, as well as (b) two of the most important nanostructures for future applications: quantum dots and nanowires, which will result into nano-materials with superior functionality. The synthesis techniques will be based on the use of phase separation as a vehicle for creating inhomogeneities within particular nanowire materials. Three promising approaches will be investigated within our research: (i) synthesis of stoichiometrically unstable compounds during growth, (ii) subsequent ion implantation beyond solubility limits, and (iii) controlled phase conversion by diffusion. The resulting "wired quantum dots" will need a comprehensive structural characterization using e.g. electron microscopy, X-Ray diffraction and Raman scattering so that the optimum synthesis parameters can be identified. The main focus of this project is the investigation of the correlation between the structure and functionality of the wired quantum dots, in order to enable novel electronic and photonic devices. A long term goal of the project includes the realization of a proto-type light emitting diode device and an electrical memory transistor with high- performance properties.

Fundamental investigations of electronic and optical properties of nanowire heterostructures have triggered tremendous research on novel device concepts. Particularly, integrating photonic devices into the Si platform is a challenge that requires the development of elaborate device designs. A hybrid platform, integrating IIIV nanocrystals or quantum dots into Si nanostructures, would give access to a wide range of high-speed electronic and high-performance photonic applications. Within the austrian sub-project we successfully demonstrated the feasibility of two approaches for such wired quantum dot formation namely ion implantation beyond the solubility limit, and subsequent controlled phase conversion by millisecond flash lamp annealing, and a thermal induced reaction between metallic pads and the vapor-liquid-solid grown nanowires leading to controlled phase conversion. Thereby we demonstrated the integration of IIIV compound semiconductors and even metallic segments into crystalline Si nanowires.Using the first approach we achieved for the first time the formation of InAs, InP, GaAs and InGaAs quantum dots monolithically integrated in vapor-liquid-solid grown Si nanowires via millisecond range liquid-phase epitaxy regrowth. Scanning as well as transmission electron microscopy and Raman spectroscopy were utilized to prove the crystallinity of the quantum dots within the nanowires. Using -photoluminescence spectroscopy a distinct emission was observed for these InAs, InP as well as GaAs quantum dots and very recently we realized a proto-type light emitting device based on a Ga-GaAs-Si nanowire heterostructure. Further electro-optical characterization via scanning photocurrent spectroscopy was performed revealing enhanced light absorption of the nanowire heterostructures beneficial for sensor and photovoltaic applications.Further, in very close collaboration with the German partner of this project, with respect to process optimization we did an extensive study of the Si nanowire response to high fluence ion implantation. Thereby we have shown that the geometrical constrains of nanostructures directly influence sputtering with good agreement to independently published predictions. The irradiation of Si nanowires revealed a previously unknown plastic deformation which has not been observed in bulk silicon at such low ion energies. Most remarkably with the alternative i.e. thermal induced phase diffusion approach very recently we achieved an axial metal/semiconductor/metal nanowire heterostructure with atomically sharp interfaces. Aside of the remarkable formation process of e.g. Al/Ge/Al axial nanowire heterostructures, which is currently investigated with great emphasis in cooperation with Martien den Hertog from the Inst. NEEL CNRS in Grenoble, we demonstrated that the Al-Ge-Al nanowire FET exhibited both negative differential resistance and impact ionization even at room temperature.