Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO

Abstract

ii Cover figure (front): 5K time-resolved photoluminescence (TRPL) of ZnO bulk single crystal from Tokyo Denpa Co. (top-right). The TRPL image was measured in a forward-transmission geometry i.e. laser excitation at back side and PL detection from front side. Also shown is an illustrative two-dimensional hexagonal lattice (bottom-left) with typical crystal defects: substitutional impurity, interstitial and vacancy.Cover figure (back): 5K three-dimensional TRPL image of ZnO bulk single crystal from Tokyo Denpa Co.. The TRPL images were recorded in two geometries: (1) back-scattering as marked by arrow 'a' and (2) forwardtransmission as marked by arrow 'b'.Copyright ©2014 Shula Chen, unless otherwise stated. Excitonic Effects and Energy Upconversion in Bulk and Nanostructured Linköping Studies in Science and Technology Dissertation No. 1560 Printed by LiU-Tryck, Linköping, Sweden, 2014 iii Abstract Zinc Oxide (ZnO), a II-VI wurtzite semiconductor, has been drawing enormous research interest for decades as an electronic material for numerous applications. It has a wide and direct band gap of 3.37eV and a large exciton binding energy of 60 meV that leads to intense free exciton (FX) emission at room temperature. As a result, ZnO is currently considered among the key materials for UV light emitting devices with tailored dimensionality and solid-state white lighting. Full exploration of ZnO for various applications requires detailed knowledge of its fundamental and material-related properties, which remains incomplete. The research work summarized in this thesis addresses a selection of open issues on optical properties of ZnO based on (but not limited to) detailed time-resolved photoluminescence (PL) and magneto-optical studies of various excitonic transitions as specified below.Paper 1 and 2 analyze recombination dynamics of FX and donor bound excitons (DX) in bulk and tetrapod ZnO with the aim to evaluate contributions of radiative and non-radiative carrier recombination processes in the total carrier lifetime. We show that changes in relative contributions of these processes in "bulk" and near-surface areas are responsible for bi-exponential exciton decays typically observed in these materials. The radiative FX lifetime is found to be relatively long, i.e. >1 ns at 77 K and >14 ns at room temperature. In the case of DX, the radiative lifetime depends on exciton localization. Radiative recombination is concluded to dominate the exciton dynamics in "bulk regions" of high-quality materials. It leads to appearance of a slow component in the decays of no-phonon (NP) FX and DX lines, which also determines the dynamics of the longitudinal optical (LO) phonon-assisted and two-electronsatellite DX transitions. On the other hand, the fast component of the exciton decays is argued to be a result of surface recombination.Paper 3 evaluates exciton-phonon coupling in bulk and tetrapod ZnO. It is found that, in contrast to bulk ZnO, the NP FX emission in ZnO tetrapods is weak as compared with the LO-phonon-assisted transi...

Effect of Gold Loading on Time-Resolved ps Photoluminescence of ZnO

“…The exact meaning of this is still under study and each may not be attributed to a unique physical property. While some authors gave evidence that the two-decay (or life) time t 1 and t 2 are related to bound and free excitons (e-h), , the exciton has a dissociation energy of about 60 meV, others have attributed them to nonradiative (quench of the luminescence due to phonon interaction) and radiative (CB to VB e-h recombination) decay, respectively . Still others have linked the decay to surface-mediated e-h recombination events .…”