Photo-excited Carrier Dynamics In Representative Metal Oxide Materials

Metal oxide materials have been widely used in diverse fields such as solar cell, photocatalysis, UV laser, integrated optical waveguide, spintronics, thin-film transistor and piezoelectric sensor owing to their excellent structu re flexibility, remarkable stoichiometric tunability and intriguing correlations. The macroscopic properties of metal oxides rely on the complex interactions between various microscopic excitations. It is of paramount importance to explore the photo-excited carrier dynamics in metal oxide not only for understanding the vital information such as electron-phonon coupling or defect trapping, but also for characterizing the photo-physical parameters such as carrier absorption cross-section. This dissertation investigate the photo-excited carrier dynamics in four representative metal oxide materials from weak correlated binary semiconducting oxides to strong correlated perovskite oxide using time-resolved pump-probe spectroscopy. The main works are listed below:(1) We investigate the two-photon excited carrier decay dynamics in zinc oxide single crystal using time-resolved pump-probe with phase object technique with 20 ps laser pulses at 532 nm. The result demonstrates that the carrier decay process in Zn O is related to the excitation intensity. We find the Zn vacancies inside Zn O could be thermally ionized when input laser intensity is larger than 0.4 GW/cm2, which lead to a carrier decay pathway with related decay lifetime about 200 ps. We propose a two-decay-pathway model to explain the observed data and evaluate the carrier-related and defect-related physical parameters.(2) We investigate the ultrafast carrier dynamics in stoichiometry modified tin oxide Sn Ox(1≤x≤2) thin films using femtosecond transient absorption measurements. We find the increment of oxygen concentration leads to a blue-shift of the band gap of tin oxide. And the mechanism of carrier relaxation is found to be different in stoichiometric Sn O2, Sn O and oxygen deficient Sn Ox films. The decay dynamics of all three films are composed of an ultrafast, a medium and a slow component with time constants of 1 ps, 10 ps and several ns, respectively. We analyze the photo-physical parameters and discuss th e decay mechanisms of tin oxide films.(3) We investigate the carrier dynamics in strontium titanate crystal using time-resolved pump-probe with phase object technique with 20 ps laser pulses at 532 nm. We find the photo-excited carriers in strontium titanate are not free carriers, but large polarons influenced by electron-phonon coupling. We also investigate the ultrafast dynamics of strontium titanate using femtosecond transient absorption measurements. The results demonstrate the electron-electron coupling and electron-phonon coupling are all ultrafast processes which are shorter than 300 fs.(4) We investigate the ultrafast carrier dynamics in strained and unstrained europium titanate thin films using wavelength-tunable femtosecond transient absorption measurements. We find the p-d interband transition is independent of the strain, however, strain has a remarkable effect on the f-d interband transition in europium titanate. We determine the carrier lifetime of p-d and f-d transitions in strained and unstrained europium titanate films. And we find a 1.5 ps ultrafast decay process in both films, which we attribute to the carrier-phonon coupling in europium titanate.The results of this dissertation show the influence of various intrinsic and extrinsic parameters on photo-excited carrier decay dynamics in metal oxides, which could be useful for a deeper understanding of the underlying photo-physical mechanism. Our results also revealed the photo-physical parameters in some representative metal oxides, which may serve as an importance benchmark for future development of oxide optoelectronic devices.