PACS 78.47.+p, 78.55.Et Time-resolved luminescence spectra of an exciton-exciton scattering process and an electron-hole plasma (EHP) were measured in ZnO epitaxial thin films by the optical Kerr gate method. At moderate excitation density below critical Mott density, stimulated emission of luminescence due to the exciton-exciton scattering process (P band) was observed. The P band shows a red shift and spectral narrowing in the first 1-2 ps after the excitation which reflects a thermal redistribution of excitons in the n = 1 excitonic state. On the other hand, at high excitation density above the critical Mott density, stimulated emission of the luminescence due to the EHP was observed. After thermalization of the EHP, the luminescence intensity of the EHP band decreases with time, and the EHP band converts to the P band after ~10 ps through a reverse Mott transition. 1 Introduction ZnO-based II-VI semiconductors have attracted much interest because of their suitable properties for short wavelength optoelectronic applications. Compared with the other wide gap materials, ZnO has larger exciton binding energy (~60 meV), which assures more efficient excitonic emission at higher temperatures. Recently, stimulated emission and room temperature lasing from an exciton-exciton collision process and an electron-hole plasma (EHP) with a low threshold was observed in ZnO epitaxial thin films [1,2]. Dynamics of such high density states in ZnO epilayers was also studied by pump-probe transient absorption spectroscopy [3], up-conversion technique [4] and optical Kerr gate (OKG) method [5]. The OKG method is considered to be the most powerful method to clarify dynamics of the high density carriers, because time-resolved luminescence spectra are directly observed in femtosecond time regime [5,6]. Here, we report on dynamical behavior of the EHP and an exciton-exciton scattering process and conversion of EHP to high density excitonic state in ZnO epitaxial thin films studied by the OKG method.
