Effects of spatial confinement on conduction electrons in semiconductor nanostructures

Abstract

Semiconductor nanostructures show electrical and optical properties which can be very different from bulk semiconductors. The various effects that occur due to the spatial confinement of electrons in such structures are of scientific importance. In addition, semiconductor nanostructures are very promising for a number of applications in the opto-electronic industry. For example semiconductor components like transistors are made still smaller, essentially to make them faster and more efficient. In the first chapter of this thesis, we introduce the physical concepts of the energy levels in bulk semiconductors and semiconducting nanostructures. In chapter 2 we discuss an experimental method for the contactless measurement of the dielectric constant of a non-conductive sample at microwave frequencies (1010Hz). The dielectric constant is obtained from the reflection spectrum of a microwave cavity loaded with the sample. This method is very suited to measure small changes in the complex dielectric constant (=10-6), with a time resolution of 10 nanoseconds. In chapter 3, we use this method to determine the dielectric constant of porous n-doped GaP (structure sizes of about 150 nm) and their changes under constant illumination and illumination with a laser pulse. The results can be quantitatively understood from a model which describes the porous material as a collection of conductive spheres surrounded by a depletion layer. The polarizability of such a composed sphere with a total radius of 75nm, is calculated with an accurate hydrodynamic model which also describes diffusion of the electrons due to a concentration gradient. The change in the dielectric constant under illumination can be described with this model as a change in the width of the depletion layer. We find that the electron mobility inside the conductive sphere is 40 times smaller than in bulk GaP, and up to five orders of magnitude larger than the long-range mobility in porous GaP. The transient changes in the dielectric constant upon excitation with a laser pulse and with constant illumination reflect the dynamics of electron-hole photogeneration and bulk and surface recombination in the porous GaP. In the last chapter, we report on the optical transitions between the discrete conduction levels in few-electron artificial atoms strongly confined in ZnO nanocrystals with diameter between 3 and 6 nm. The artificial atoms are prepared by two methods. The first method uses an assembly of weakly coupled ZnO nanocrystals in which electrons are injected electrochemically; the average electron number is obtained from the injected charge and the number of quantum dots in the assembly. In the second method acolloidal solution of ZnO nanocrystals is used; few-electron artificial atoms are obtained by photogeneration of electron-hole pairs and subsequent holescavenging.The charged ZnO nanocrystals show broad spectra in the near IR; the shape and total absorption intensity being determined only by the average electron number. The spectra can be explained by taking into account the allowed electric dipole transitions between the atom-like orbitals of the ZnO nanocrystals and the size distribution of the nanocrystals in the sample.