Influence of Nanostructure Geometry on Electronic Properties

Abstract

Recently, new quantum features have been studied in the area of nanostructured layers. It emerges that properties of nanostructures depend not only on their size but also on their geometry. Particularly, a nanograting (NG) on the surface of the thin layer imposes additional boundary conditions on electron wave function and forbids some quantum states. Density of quantum states reduces. Unlike conventional quantum well, state density per volume, is reduced in the case of NG layer. This leads to changes in electronic properties. Electrons, rejected from forbidden quantum states, have to occupy states with higher energy. In the case of semiconductor layers, electrons rejected from the valence band have to occupy empty quantum states in the conduction band. Such increase in conduction band electron concentration can be termed as geometry-induced doping or G-doping. G-doping is equivalent to donor doping from the point of view of the increase in electron concentration. However, there are no ionized impurities. This preserves charge carrier scattering to the intrinsic semiconductor level and increases carrier mobility with respect to the donor-doped layer. As rejected electrons occupy quantum states with the higher energy, the chemical potential of NG layer increases and becomes NG size dependent. We regard a system composed of NG layer and an additional layer on the top of the NG forming periodic series of p-n junctions. In such system, charge depletion region develops inside the top of NG and its effective height reduces, becoming a rather strong function of temperature T. Consequently, T-dependence of chemical potential magnifies and Seebeck coefficient S increases. Calculations show one order of magnitude increase in the thermoelectric figure of merit ZT relative to bulk material. In the case of metal layers, electrons rejected from forbidden quantum states below Fermi energy, occupy quantum states above Fermi energy. Fermi energy moves up on energy scale and work function (WF) reduces. WF reduction was observed in thin amorphous Au films grown on thermally oxidized Si substrates. WF was measured using Kelvin Probe and PEEM microscope. WF reduction depended on film internal structure and NG sizes. Maximum reduction in WF of 0.56 eV has been obtained.