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Proceedings Paper

Dynamics of intervalley transitions and propagation of coherent acoustic phonons in GaN single crystals studied by femtosecond pump-probe spectroscopy
Author(s): Shuai Wu; Jie Zhang; A. Belousov; J. Karpinski; Roman Sobolewski
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Paper 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.

Paper Details

Date Published: 28 February 2008
PDF: 13 pages
Proc. SPIE 6894, Gallium Nitride Materials and Devices III, 68940K (28 February 2008); doi: 10.1117/12.769522
Show Author Affiliations
Shuai Wu, Univ. of Rochester (United States)
Jie Zhang, Univ. of Rochester (United States)
A. Belousov, ETH Zurich (Switzerland)
J. Karpinski, ETH Zurich (Switzerland)
Roman Sobolewski, Univ. of Rochester (United States)

Published in SPIE Proceedings Vol. 6894:
Gallium Nitride Materials and Devices III
Hadis Morkoç; Cole W. Litton; Jen-Inn Chyi; Yasushi Nanishi; Euijoon Yoon, Editor(s)

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