Quantum Dot Nanoprecipitates in Semiconductor Structures

Epitaxially controlled semiconductor quantum dots are of tremendous interest for advanced optoelectronic devices due to their zero dimensional electronic density of states and wide the tunability of their optical transitions by size, shape and chemical composition. In this research project, a novel synthesis method will be developed based on phase separation between immiscible semiconductor materials and the unique structural and electronic properties of the resulting quantum dot nanoprecipitates will be investigated and evaluated. In contrast to conventional methods for quantum dot fabrication, by our new synthesis method highly symmetric quantum dots with extraordinary structural and optical properties are formed. This includes atomically sharp interfaces, no intermixing between dots and matrix material, nearly spherical shape as well as intense room temperature photoluminescence emission. Moreover, the coherent epitaxial embedding in high quality semiconductor matrix materials allows their monolithic integration into semiconductor devices. The goal of this project is to advance the novel synthesis method for application to a wide range of material systems and to evaluate in detail the structural and electronic properties of the synthesized quantum dot nanoprecipitates for potential device applications. The focus will be on heterostructures of materials with wide miscibility gap resulting from a lattice-type mismatch of the combined materials. This applies in particular for the combination of zinc-blende II-VI and rock-salt IV-VI semiconductors. Growth studies will be performed using advanced molecular beam epitaxy methods, including in-situ electron diffraction and scanning tunneling microscopy. This will yield efficient schemes for controlling the size, shape and composition of the dots. To develop a detailed understanding of the kinetics and energetics of the formation process, the role of the interface energy and lattice-mismatch strains will be investigated, complemented by in-situ and ex-situ annealing experiments. High resolution transmission electron microscopy and x-ray diffraction will be employed for structure determination, and the atomic configuration of the interfaces will be investigated in detail. Optical and electronic properties will be determined by photoluminescence and excitation spectroscopy and the results compared to band structure calculations. By developing a wide range of material combinations, a wide wavelength coverage of emission and absorption will be achieved, which opens many pathways for actual device applications.

Epitaxially controlled semiconductor quantum dots are of tremendous interest for advanced optoelectronic devices due to their zero dimensional electronic density of states and wide the tunability of their optical transitions by size, shape and chemical composition of the quantum dots. For this purpose, in this project a novel synthesis method was developed based on phase separation between immiscible semiconductor materials. This leads to a unique topological transformation of nanometer thick two-dimensional films into zero-dimensional nanoprecipitates upon post growth thermal annealing, resulting in highly symmetric quantum dots with extraordinary structural and optical properties such as atomically sharp interfaces, no intermixing between dots and matrix material, nearly spherical shape as well as intense room temperature photoluminescence emission. In addition, the coherent epitaxial incorporation in high quality semiconductor matrix materials allows their monolithic integration into semiconductor devices.In this project, this novel synthesis method was applied to a variety of different material systems comprising of narrow band gaps IV-VI semiconductors such a PbTe and PbSe and wide band gap II-VI semiconductors such as CdTe and ZnTe grown by molecular beam epitaxy. By optimizing the growth and annealing conditions, excellent control of the quantum dot size and shape was demonstrated as well as efficient luminescence emission over a wide wavelength region from 1.3 to 6 m, which is important for mid-infrared device applications. By detailed in situ transmission electron microscopy investigations combined with theoretical simulation based on the Monte Carlo method as well as the Cahn-Hilliard continuum approach, the dynamics of the formation process as well as kinetic parameters were clarified. Thus, a comprehensive fundamental understanding of the process was obtained. Detailed information on the optical and electronic properties was obtained by photoluminescence investigations, indicating a favorable type I band alignment in particular for the PbTe CdTe model system. Based on the strong luminescence of the resulting quantum dots, mid-infrared light emitting devices and lasers were developed with emission in the 2 5 m wavelength region operating up to room temperature. This may open a pathway for practical device applications in molecular gas analysis for greenhouse gas sensing, pollution monitoring, medical diagnostic as well as industrial process control.