微波吸收法研究ZnO光电子衰减过程

Measurement of Decay Process of Photoelectrons from ZnO by Microwave Absorption Method

微波吸收无接触测量技术可以用于半导体粉体材料、微晶材料等研究光生载流子衰减过程。本文采用微波吸收法在室温下分别测量了ZnO纳米材料和微晶材料的光电子衰减过程。发现在紫外激光短脉冲激发下,两种材料的导带光电子寿命有很大的差异,ZnO微晶粉体材料的光电子寿命为50ns,而ZnO纳米材料的光电子寿命仅为10ns。分析认为纳米ZnO的光电子寿命缩短是由于纳米ZnO晶体的表面积远远大于体材料的表面积,纳米材料的表面形成了大量的缺陷能级,加速了光电子的表面复合,缩短了光电子的寿命。纳米材料内部缺陷增多和量子限域效应同样会缩短光电子的寿命。

Illuminated by ultraviolet light, the electrons in semiconductor transfer from valence band to conduction band with the absorption of photo energy, the formation of free electrons and hole carriers. The electrons in the processes of migration in the conduction band may be affected by different defect energy level, such as being captured by electron traps, recombination with luminescent centers, or recombination with the hole carrier in valence band. The investigation of the decay processes of photoelectrons in conduction band is helpful in achieving an understanding of the energy level structure for semiconductor materials. Contactless measurement of the decay processes of photoelectrons of microcrystal materials and powder semiconductor materials can be carried out using the microwave absorption dielectric spectrum detection technique, with high precision and low interference. ZnO crystals is a kind of semiconductor materials with a band with an energy gap of 337eV at room temperature. The ZnO experimental samples are prepared using hydrothermal synthesis method that is often used in preparation of high purity and fine dispersion ZnO crystals. The decay processes of photoelectrons of nano ZnO materials and ZnO microcrystal materials have been measured respectively in this paper by using microwave absorption method. The photoelectron timeresolved spectrum of nano ZnO and ZnO microcrystals were obtained. The evidence difference of photoelectron lifetime between two kinds of materials was found after excitation with a shortpulse laser. The photoelectron lifetime of nano ZnO was found only to be 10ns and the photoelectron lifetime of ZnO microcrystals was found to be 50ns. It is believed that the photoelectron lifetime of nano ZnO is shorter than that of ZnO microcrystals because the surface area of nano material is larger than that of microcrystal materials. The defect energy levels on the surface area of nano materials also result in the increasing of recombination rate of the photoelectrons.