Synthesis And Optical Properties Of Germanium-riched Oxide Nanoparticles

At room temperature, bulk Si and bulk Ge have extremely low photoluminescence efficiency, so III-V compound semiconductors were preferred for the applications in the optoelectronics during the past several decades.However, when the size of semiconductor materials decreases down to nanometers, quantum confinement effect will tremendously change the electronic bands of the material, making it possible to manipulate the physical properties. Since the observation of room temperature visible photoluminescence from both porous silicon and Ge nanocrystals embedded in SiO2 glassy matrices which were formed by magnetron co-sputtering in 1990 and 1991 respectively, the optical properties and their mechanisms intrigued great interests all over the world. Meanwhile, there is a promising prospect of applying germanium nanostructures on optoelectronic devices, solar cells and sensors. In this thesis, germanium-riched oxide nanoparticles were synthesized through cluster beam deposition technique and visible room temperature photoluminescence was observed and studied. The main results of this thesis are summarized as follows:1. Mixing buffer gas with a small fraction of water vapor was found to be an effective approach to synthesize germanium-riched oxide nanoparticles using gas phase cluster beam deposition. An apparatus, which can control the partial pressure of water vapor and promote gas aggregation cluster growth procedure, was designed and assembled. By controlling the flow rate of the water vapor, germanium nanoparticles with different oxidation degree were successfully formed. Compared with other synthesis approach, this one has two merits:First, the germanium oxide nanoparticles can be deposited on different kinds of substrates. Second, the synthesized germanium nanoparticles are well-defined so that the photoluminescence mechanism will be investigated clearly.2. We found that the increasing partial pressure of water vapor would determine the microstructures of the germanium oxide nanoparticles.Two kinds of microstructures related to different water vapor partial pressures were observed. When the water vapor partial pressure was relatively low, core-shell microstructure with single-crystal Ge as the core and GeOx as the shell were formed and the thickness of the GeOx shell increased when the water vapor partial pressure was raised up. When the water vapor partial pressure was relatively high, germanium-riched oxide nanoparticles were formed, with small Ge nanocrystals (with size of 3-4nm) dispersed uniformly in the GeOx matrix.3. The optical gap of the germanium oxide nanoparticle film experienced a blue shift compared with bulk germanium. It is also found that as the oxidation degree increased, the blue shift became more prominent and the maximum blue shift was found to be 1.83eV, which can be attributed to the quantum confinement effect in Ge nanoctrystals. When excited with a 405 laser, germanium-riched oxide nanoparticle with uniformly dispersed small Ge nanocrystals was found to emit visible light at room temperature and the broad PL band ranges from 470nm to 600nm. Through temperature-variation PL and PL decay measurements, we conclude that the broad PL band around 500nm is not consistent with the electron-hole radiative recombination process from Ge nanocrystals, but originates from the Ge-related oxygen defects in the interfaces and GeOx layers and conforms to quantum confinement/luminescence center model.4. The coupling between the localized field of Ag nanoparticles’ surface plasmon and optical process inside germanium-riched oxide nanoparticles was investigated. The experimental results show that under the influences of localized field of Ag nanoparticles’surface plasmon, the photoelectrons which were generated from Ge nanocrystals and excited into the conduction band, were trapped in the interfaces between Ge nanocrystals and GeOx shells and then transferred to Ag nanoparticles, resulting in a quenching of the emission.