Nanoscale optical absorption spectroscopy by HR-EELS & valence band engineering as a possible route to solve p-doping problem in ZnO

Abstract

The interaction between fast electrons and materials in a TEM can be broadly classified into two categories; elastic and inelastic interaction [1-6]. In elastic interaction there is no exchange in energy but only momentum and for inelastic interaction there is both exchange of energy and momentum between the fast electrons and atomic constituents namely, nucleus and surrounding electrons. In EELS, the elastically scattered electrons form a peak of highest intensity known as the zero loss peak (ZLP) and the inelastically scattered electrons form a spectrum spanning up to 2000 eV accessible through the spectrometer (figure 1.1 (b)). There are various inelastic interactions possible and we will mostly be concerned with two varieties, first type is the low loss signal within few eVs from the ZLP which gives information on the band gap (figure 1.1 (c)) and their types in the material and the second type, core loss spectra e.g., O K edge (figure 1.1 (d)) which provides information on the density of unoccupied states. The low-loss EELS contains intensity due to excitation of weakly bound outer-shell electrons of the atoM.S.. Core-loss EELS contains transitions from the core level of atoM.S. to the unoccupied levels above the Fermi level (EF) of the absorbing atom. The spectrum till ~50 eV from any core loss absorption onset is known as electron energy loss near edge structure (ELNES) and the spectra beyond 50 eV is known as electron energy loss extended fine structure (EXELFS). ELNES provides information on the surrounding electronic structure of the absorbing atoM.S. and EXELFS provides information on the bonding environment and co-ordination.