28 February 2008 Dynamics of intervalley transitions and propagation of coherent acoustic phonons in GaN single crystals studied by femtosecond pump-probe spectroscopy
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Proceedings Volume 6894, Gallium Nitride Materials and Devices III; 68940K (2008); doi: 10.1117/12.769522
Event: Integrated Optoelectronic Devices 2008, 2008, San Jose, California, United States
Abstract
We report our experimental and theoretical studies on the time-resolved dynamics of conduction electrons and coherent acoustic phonons (CAPs) in high quality GaN single crystals. Our measurements were performed using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopy by scanning the transient differential reflectivity (ΔR/R) signal of the probe beam. We have found that the threshold for the intervalley transition of electrons between the conduction band Γ and L valleys appears at the energy of 4.51 eV at 300 K and it increases to 4.57 eV at 100 K. We have also numerically simulated intervalley scattering dynamics and found that the characteristic scattering time constants were temperature independent and fitted extremely well our experimental data. The electron scattering time from the Γ to L valley was limited by the 150-fs width of our pump pulses, while the return process of electrons from L to Γ was characterized by the scattering time of 1 ps, and the total depopulation time of the L valley was ~20 ps. On the exponentially decaying part of the ΔR/R transient, we have observed pronounced CAP oscillations with the intrinsic lifetime of at least 100 ns. The CAP amplitude was only dependent on the pump photon energy, while the oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope (velocity of the acoustic phonon propagation) determined by the speed of sound in GaN. The CAP characteristics agreed well with a developed theoretical model.
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Shuai Wu, Jie Zhang, A. Belousov, J. Karpinski, Roman Sobolewski, "Dynamics of intervalley transitions and propagation of coherent acoustic phonons in GaN single crystals studied by femtosecond pump-probe spectroscopy", Proc. SPIE 6894, Gallium Nitride Materials and Devices III, 68940K (28 February 2008); doi: 10.1117/12.769522; https://doi.org/10.1117/12.769522
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KEYWORDS
Gallium nitride

Electrons

Scattering

Crystals

Phonons

Picosecond phenomena

Acoustics

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