2005 Conference on Lasers & Electro-Optics (CLEO) CMII5 Observations of Exciton Density of State Variations in a ZnO Thin Film with fs Pump-probe Experiments Cheng-Yen Chen, Yen-Cheng Lu, Hsiang-Chen Wang, Fang-Yi Jen, and C. C. Yang Graiduate Institute of Electro-Opticwal Engineering (and Department of Electrical Engineering, National Tatiwan University, 1, Roosevelt Road, Sec. 4, Taiipei, Taiwan, R. 0. C. Bao-ping Zhang and Yusaburo Segawa Photoday,namics Research Center, The Institute of Physical aind Chemicail Research (RIKEN), Sendai, Jalpan Abstract: Variations of exciton density of state around the levels of donor-bound and free excitons in a ZnO thin film are observed with temperature- and photon energy-dependent degenerate fs pump-probe spectroscopy for exploring its exciton dynamics. 2005 Optical Society of America OCIS codes: (320.7150) Ultrafast spectroscopy; (160.6000) Semiconductors, including MQW 1. Introduction The radiative recombination of an electron and a hole in an exciton is more efficient than those of free motion. Therefore, for developing efficient light-emitting devices, compound materials of larger exciton binding energies are more attractive. Because of the large exciton binding energy (60 meV versus 30 meV in GaN) in ZnO, it has attracted quite much attention in crystal growth and optical characterization, including ultrafast carrier dynamics [ 1-4]. In such a compound, excitons exist in several forms, including donor-bound exciton (D°X) and free exciton (FX). Since the high photon emission efficiency of such a compound relies on exciton dynamics, the understandings of exciton density of state distribution and hence ultrafast exciton dynamics are crucially important. In this paper, we observe the variation of exciton density of state and explore the exciton dynamics in a ZnO thin film with fs pump-probe spectroscopy. In particular, we perform temperature- and photon energy-dependent measurements around the D°X and FX levels. Exciton flow among these levels can be evaluated. 2. Sample Preparation and Experimental Methods The sample was grown in an MOCVD reactor on sapphire substrate. The growth temperature was 450 'C for 60 min [5]. The growth pressure was 6 Torr. The in-plane orientation could be controlled with the growth temperature, which could lead to different growth modes at the initial stage of growth. With the aforementioned growth condition, 300 in-plane twist was obtained in the used sample. The second-harmonic generation through a BBO crystal of a fs Ti:sapphire laser with 100 fs in pulse width and 76 MHz in pulse repetition frequency was used for the degenerate pump-probe experiments. The second-harmonic pulse width was about 150 fsec. The pump power was maintained at about 16 mW and the probe power was one tenth of the pump power. 3. Pump-probe Experimental Results Temperature dependent photoluminescence (PL) spectra of the sample are shown in Fig. 1. Here, one can see that the PL peak of increasing width red shifts with temperature. At 10 K, only one major peak of D0X (at 3.365 eV) can be seen. However, at 20 K a small hump around 3.376 eV, corresponding to the emergence of FX, can be observed. This hump becomes more prominent and dominant in spectra beyond 80 K. It red shifts, particularly significantly beyond 80 K, to 3.293 eV at 300 K [6]. Beyond 80 K, two peaks at 3.307 and 3.230 eV (at 80 K), corresponding to the DAP (donor-acceptor pair) and DAP-LO (LO phonon-assisted DAP), respectively, can be identified. It is noted that the D°X feature position does not vary significantly with temperature until it disappears around 100 K. In this temperature range, the FX feature position is almost fixed. The spectral peak energies of D°X and FX at 10 K are used for the pump-probe experiments. Fig. 2 shows the differential transmission profiles at various temperatures when the pump-probe photon energy is 3.366 eV, corresponding to the D°X state at 10 K. Here, one can see that AT/T is significantly strong at 10 and 90 K. Usually the variation magnitude of AT/T represents the band filling effect. Hence, the large magnitudes in differential transmission at 10 and 90 K can be interpreted as high densities of state at the pump-probe photon energies at these two temperatures. As mentioned, this photon energy corresponds to D°X state at 10 K. It approximately coincides with the state of FX at 90 K, as shown in Fig. 1. The relatively higher densities of states at the D°X and FX lead to the stronger pump-probe responses. The two-stage decay time constants of these pump-probe profiles are labeled in the figure. The fast decay in the early stage, ranging from 0.8 to 4.3 ps, describes exciton thermalization through scattering. The relatively slower decay, in the time range of a few tens ps, beyond 100 K 621 I