Excitonic effects in ZnO

Abstract: Zinc Oxide (ZnO) is an extensively researched II-VI wide bandgap semiconductor material. As a promising material for future optoelectronic and spintronic applications, it continues to attract enormous amount of interest. Though over the past decades extensive experimental and theoretical work has been done to deepen the understanding of its fundamental material properties, there are still controversial and unexplored areas. The research work summarized in this thesis was aimed at clarifying and exploring some of these areas, as will be introduced below.One of attractive properties of ZnO is a very large binding energy of free excitons (FX), which makes excitonic effects of particular importance in this material. The excitons couple with other elementary excitations inside the material such as longitudinal optical (LO) phonons or photons. The former leads to the intense LO phonon-assisted radiative transitions, while the latter causes formation of the exciton-polariton.The exciton-phonon coupling was suggested to be enhanced in ZnO-based nano- and microstructures. This conclusion was based on the prevalence in these structures at room temperature of LO phonon-assisted FX transitions, which is in contrast with bulk ZnO photoluminescence (PL) where the no-phonon (NP) FX emission dominates. The exact mechanism for this effect, however, was not clear. In paper 1, we have clarified these issues by employing PL and cathodoluminescence (CL) measurements performed for bulk ZnO material and ZnO tetrapods. From spatially resolved CL studies, we have shown that the suppression of the NP FX emission strongly depends on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. The effect is interpreted using a model based on re-absorption due to multiple internal reflections in the vicinity of the FX resonance.As to the exciton-photon coupling, it usually leads to formation of mixed or coupled states of excitons and photons known as exciton-polaritons. The exciton-polariton formation has been demonstrated to lead to slow-down of light in several semiconductor materials such as CdZnTe, GaN, etc. Due to the strong exciton-photon coupling in ZnO, the polariton formation may also affect light velocity in this medium. To explore this effect, we have performed timeof-flight measurement using pulsed laser light. Our studies that are summarized in paper 2 have shown that the group velocity of light in bulk ZnO could be decreased down to 2044km/s and the magnitude of this decrease depends on light polarization. The main physical mechanism responsible for this effect was singled out as being due to the formation of free exciton-polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion was also determined.Excitonic effects in ZnO could also be utilized to investigate fundamental properties of ZnO. For example, previous magneto-optical studies of donor bound excitons allowed to establish ordering of valence band (VB) states and also provided consistent information on the sign and g-factor of holes from the upper A-valence subband. On the other hand, properties of the higher lying B-VB subband were not fully understood. To clarify this issue, we have performed time-resolved and magneto-PL studies for the so-called I6 B and I7 B excitonic transitions which involved a hole from the B-VB subband as summarized in paper 3. From the magneto-PL measurements, values of effective g-factors for conduction band electrons and B valence band holes were determined as ge =1.91, gh =1.79 and gh =0, respectively.