Conversion of an electron–hole plasma into a high density excitonic state in ZnO epitaxial thin films

Abstract

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.

Dynamics of high-density excitons and electron–hole plasma in ZnO epitaxial thin films

“…The peak energy and FWHM at $10 ps after excitation become close to constant values of 3.18 eV and 30 meV, respectively, which are the values observed in the steady-state experiment [11]. Except for the observed time delay of the EHP or P emission, the dynamics of highly excited ZnO seems to be essentially the same as that observed under the excitonic resonant excitation [6]. Fig.…”

“…Recently, we first measured time-resolved luminescence spectra due to both the EHP and [5,6]. At higher excitation density above the critical Mott density, luminescence due to the EHP instantaneously appears.…”

“…The EHP emission becomes replaced by the P emission after several picoseconds through a reverse Mott transition. At lower excitation density below the critical Mott density, on the other hand, the P emission emerges instantaneously and shows a red shift with time, implying that a thermal equilibrium process of excitons toward the bottleneck region takes place [6].…”

Electron energy relaxation in wurtzite ZnO and GaN

“…The most recent time-resolved pump-probe optical measurements with the given photogenerated carrier density report slightly shorter relaxation times at higher densities: 0.77 ps [15] and 0.85 ps-0.7 ps [13] (Fig.3, open square and triangle). Other optical time-resolved measurements yielded the following relaxation times: 1ps [7], [8], 0.98 ps [9], 0.86 ps [10], 1 ps [11], 0.8 ps [12], 1.75ps and 0.2 ps [3], <3.5ps [24], <4.5ps [25] and <30fs [6].…”

Enhanced broadband ultrafast detection of ultraviolet emission using optical Kerr gating

“…Here, excitation pulses of about 70 times less than previous studies on epitaxial thin films. 21,22 Since ZnO is known to display a very complex spectral dynamics within its first few picoseconds, we now evaluate the various Kerr media in resolving such dynamics. The various Kerr media are gated with minimal optical fluence in order not to generate emission or white light supercontinuum, thus not obscuring the time resolve emission of ZnO.…”