Seznam abstraktů 2020: SEKCE A - NANOMATERIÁLY PRO ELEKTRONICKÉ, MAGNETICKÉ A OPTICKÉ APLIKACE. UHLÍKOVÉ NANOSTRUKTURY, KVANTOVÉ TEČKY

This contribution focuses only on quantum dots (QDs) embedded inside a semiconductor structure. Freestanding QDs (semiconductor, colloidal, metal etc.) with a much larger application field, which can be prepared from the same material, are not subject of this talk. A research report about the whole family of QDs market can be found at (paid) [1].
QDs are small pieces of semiconductor nanocrystals (mainly) with sizes in the range from ones to tens of nanometers. The first idea of QDs (freestanding), having properties different from that of the bulk material, was published by Ekimov and Onushchenko in 1981 [2]. During the early 1990s, embedded QDs were prepared by molecular beam epitaxy (MBE) [3-4] in several laboratories, as described in [5].
MBE QDs seems to be more suitable for attaining an exact layer thickness down to fractions of a monolayer. An attempt to realize industrial MOVPE-prepared QD-based semiconductor lasers for fiber optic telecommunications appeared during the first years of this century (EU project DOTCOM 2002-05), but without a real impact on industrial production.
QDs exhibit unique electronic and optical properties, intermediate between those of bulk semiconductors and discrete atoms or molecules. The reason for this is that the size of QDs is comparable with the de Broglie wavelength of an electron in a crystal. Electrons (and holes) inside QDs behave differently from that found in a “bulk” material. The most important difference is that inside the QDs, electrons (and holes) can occupy only discrete energy levels due to strong localization; however, they also can be found outside QDs with nonzero probability. The position of energy levels depends mainly on the smallest dimension of a QD. Thus, electron-hole recombination energy (and, consequently, the wavelength of the emitted radiation) depends on this size, often more than on the bandgap energy of the material, which is determining for the bulk material. A current review of all groups of semiconductor QD applications including the growth and parameters can be found in [6]: “Applications include QD lasers for integrated silicon photonics and quantum computing. QD LEDs for highly efficient solid-state lighting. QDs for intermediate band solar cells and multi-exciton generation. QDs for bio-medical labeling, imaging, targeted drug delivery, sensing, and therapy.”
References: [1] Press release from: WiseGuyResearch Consultants Pvt Ltd Project Portfolio Quantum Dots 2017 Global Market Expected to Grow at CAGR 11.39% and Forecast to 2019, Quantum Dots Market Research Report 2017.; [2] Ekimov, A.I. and Onushchenko, A.A. (1981) Quantum size effect in three-dimensional microscopic semiconductor crystals. JETP Letters, 34, 345-349.; [3] Leonard, D., Krishnamurthy, M., Reaves, C.M., et al. (1993) Direct formation of quantum‐sized dots from uniform coherent islands of InGaAs on GaAs surfaces. Applied Physics Letters, 63, 3203-3205.; [4] Grundmann, M., Christen, J., Ledentsov, N.N., et al. (1995) Ultranarrow Luminescence Lines from Single Quantum Dots. Physical Review Letters, 74, 4043-4046.; [5] Petroff, P.M. and DenBaars, S.P. (1994) MBE and MOCVD growth and properties of self-assembling quantum dot arrays in III-V semiconductor structures. Superlattices and Microstructures, 15, 15-21.; [6] Zhou, W.D. and Coleman, J.J. (2016) Semiconductor quantum dots. Current Opinion in Solid State and Material Science, 20, 352-360.

A series of thin epoxy resin samples were pepared. Short recycled carbon micro fibers in concentrations of 0-33% by weight were used as composite reinforcement. The geometry and structure of the fillers were characterized using SEM and Raman DXR microscopy images. It has been found that the dielectric properties of composites strongly depend on the CFR content and show an increase in values at and above the percolation threshold. Below the percolation threshold, the distance between the fibers is large enough so that there is no interaction of the local fields created by the adjacent fibers and the dielectric constant in this region increases slowly. As the filler concentration increases, the Maxwell Wagner effect increases and leads to a sharp increase in the real permittivity values. Furthermore, it was found in this work that the permittivity increases with increasing concentration of particles even after reaching the percolation threshold over the entire frequency band.

Epitaxial growth of diamond is critically important for the fabrication of diamond based electronic devices. It requires a careful control of dopants with a minimum concentration of unwanted defects. The p-type epitaxial boron-doped diamond shows higher carrier mobility and lower compensation on (100) oriented substrates in comparison to the one grown on (111) substrates. But (100) epitaxial layers suffers from the formation of pyramidal hillocks or non-epitaxial crystallites defects. The emerging study of the epitaxial diamond growth on the (113) vicinal surfaces evidences high growth rates and low structural defects concentrations with both p and n type doping. In this work, we compare of the morphology and dopant concentration incorporation of heavily boron-doped (113) epitaxial diamond layers with (100) and (111) epitaxial layers. Epitaxial layers were grown using a 1.5 kW resonance cavity Microwave Plasma Enhanced Chemical Vapor Deposition (MWPECVD) system (AX5010, Seki Diamond Systems). The surface morphology of epitaxial layers was studied by scanning electron microscopy and atomic force microscopy whereas the boron incorporation homogeneity is determined by Raman spectroscopy mapping. The boron concentration was determined using the recent published analysis method based on the zone-center phonon diamond line broadening. Heavily boron-doped (113) epitaxial diamond layers can be grown at high growth rate with smooth surface, without pyramidal hillocks or non-epitaxial crystallite defects, and homogeneous boron concentration. These results confirm that epitaxial diamond growth on (113) vicinal surfaces is a promising solution for the development and the fabrication of diamond based electronic devices.