Structure dependent absorption spectrum changes in IQ.&~~.wN thin films of different thickness (5.5, 17, 34, and 66 nm) are investigated by nondegenerate femtosecond pump and probe spectroscopy at room temperature. These spectrum changes are caused by excitonic absorption quenching and screening of internal piezoelectric fields by photo-induced carriers. Both fast (500 ps) and slow (100 ps) temporal behaviors are observed in differential absorption spectra. Localized carriers in indium-rich regions and/or carriers captured in midgap traps are considered to keep screening the intemal filed and to maintain absorption spectrum changes much longer than the photoluminescence lifetimes.InGaN quantum wells have many interesting optical properties because of carrier localization and existence of large internal electric field [l]. Therefore carrier dynamics in these structures have been studied intensively [2], but details are still unclear. In this paper, we report on structural dependence of transient absorption spectrum in h.loGao.~N thin films. We also report on both fast carrier dynamics studied by nondegenerate femtosecond pump and probe spectroscopy and slow camer dynamics studied by direct observation of photo-induced absorption changes.The investigated samples were grown on (0001) c-plane sapphire substrates by metal-organic chemical vapor deposition. The well/bamer thickness of 4 samples are 5.5 d 5 . 5 nm (12 periods, sample a), 17 d 1 7 nm (4 periods, sample b), 34 t d 3 4 nm (2 periods, sample c), and 66 d 6 6 m (single period, sample d). Each Ino.loGao.90N~,03Gao.97N structure is sandwiched between a 1 . 5~ GaN buffer layer and a 33 nm h~.O3G%.97N cap layer. Nondegenerate femtosecond pump and probe spectroscopy was performed by using a regeneratively amplified femtosecond mode-locked Tisapphire. The photon energy and pulse duration are 1.55 eV and 130 fs, respectively. Frequency doubled pulse was used as the pump pulse and broadband white continuum was used as the probe pulse. All measurements were performed at room temperature. Figure 1 shows differential absorption spectra for 4 samples after pump pulse excitation at the photon energy of 3.1 eV and excitation fluence of 40 pJ/cm*. Differential absorption (Aa(7)) spectrum is defined as the difference of a spectrum at the delay time of '5 compared with that of without pumping. Photoexcited camer density immediately after pumping is approximately 5~1 0 '~c m~~. Excitonic absorption bleaching dominates the A a spectra that exhibit large negative Aa's. This carrier density is enough to screen the internal electric field and this screening causes blueshift of a band edge. Therefore this blueshift of a band edge also changes absorption 0-7803-78881/03/$17.OMQ2003 IEEE 256
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