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.