Investigation On Fabrication And Luminescence Properties Of 2D ZnO

As II-VI oxide semiconductor with bandgap of 3.3 7 eV and exciton binding energy of 60 meV at room temperature,ZnO has been considered as a promising material for potential application in optoelectronic devices due to its unique optical properties.As active layer,2D quantum wells structure can realize the effective control of carriers,which is the key to improve the luminescence efficiency of ZnO.There are numerous issues in the luminescence behavior of ZnO/ZnMgO quantum wells that worth further investigation,especially the behavior of exciton recombination and its dependence on the polarity.In addition,with the discovery of novel 2D materials like graphene and the invention of related novel optoelectronic devices,the fabrication of free-standing 2D ZnO will bring unique optoelectronic properties and extended applications to ZnO.In this thesis,we mainly focus on 2D ZnO materials,while illuminating recombination dynamics of excitons in 2D ZnO and exploring novel 2D ZnO materials.The details are as follows:1.The behavior of exciton recombination in Si-based ZnO/Zn0.9Mgi.0O multiple quantum wells grown by PLD was investigated detailly via temperature-dependence steady-state and time-resolved photoluminescence.There is a linear increase in the radiative decay time between 19 and 300 K,which is the characteristic feature for pure 2D excitons.The excitons are effectively confined in the quantum wells and do not thermalize to 3D excitons till room temperature.The radiative decay rate at 300 K is 0.87 ns-1,which is higher than those reported in the literatures,indicating the high quality and intrinsic high-efficiency luminescence characteristic of the quantum wells.In addition,the absence of internal electric field leading to QCSE,results in the spatially separated wavefunctions and increasing in PL lifetime.2.The influence of polarity on the exciton recombination in ZnO/ZnMgO multiple quantum wells grown by MBE has been investigated systematically.The dominant processes of non-radiative recombination of excitons in polar and non-polar quantum wells are different.For non-polar ZnO/ZnMgO quantum wells,the non-radiative recombination of excitons is dominated by non-radiative recombination centre defects,without delocalization and decomposition of exciton;the significant broadening and abnormal temperature-dependence characteristic indicate the strong localization of exciton while board distribution of localized states;under high excitation density,the luminescence property of non-polar ZnO/ZnMgO quantum wells is different from that of polar ZnO/ZnMgO quantum wells,while the P-band threshold of non-polar ZnO/ZnMgO quantum wells is lower than that of polar ZnO/ZnMgO quantum wells.3.Free-standing atomically thin ZnO layers were successfully synthesized via oxidizing hydrothermally grown ultrathin zinc chalcogenides layers.The lateral size of ZnO ultrathin layers is several micrometers,and the thickness is determined to be as thin as-2 nm.The thermal treatment process can help to remove the organic groups and to get atomically thin ZnO layers without damageing the structure integrity.The main experimental parameters including hydrothermal growth temperature,hydrothermal growth time and thermal treatment temperature have been investigated systematically to obtain the optimal parameters.4.The recombination behavior of photogenerated carriers in ZnO ultrathin layers was investigated via temperature-dependence steady-state and time-resolved photoluminescence.Some novel properties that different from that of ZnO bulk materials have been found.At low temperature,the LO-replicas of free exciton dominate the NBE emission,in which the dependence of the emission probability on the exciton kinetic energy is much smaller than that of bulk ZnO.The radiative recombination rate of deep-level luminescence has zero-dimensional feature.The negative thermal quenching behavior in the temperature range of 160-250 K indicates a detrapping/delocalization of photogenerated carriers,resulting in high internal quantum efficiency of-30%at room temperature.5.The potential application of ZnO ultrathin layers as UV-excited phosphor has been preminarily explored.By optimizing the composition proportion,the mixing of layered ZnO sample with red phosphor and blue quantum dot can form white emission phosphor.High quality white emission under 390 nm UV LED excitation has been obtained,with CIE coordinates of(0.329,0.332).