Browsing by Author "Wen, Yu-Chieh"

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Acoustic Velocity and Optical Index Birefringence in A-Plane ZnO Thin Films
(2011-02-01)
Chen, Yi-Hsin
;
Wen, Yu-Chieh
;
Liu, Wei-Rein
;
Hsieh, Wen-Feng
;
Sun, Chi-Kuang
The longitudinal acoustic velocity in the direction normal to the c-axis and the birefringence of refractive index of a-plane zinc oxide (ZnO) thin films were measured by ultrafast transient reflection measurement. Through Fourier analysis, the experimental data show that the acoustic velocity is about 6000 m/s and the measured refractive index near the band gap, which is higher than the previously reported results by 20 similar to 30%.
Photogeneration of coherent shear phonons in orientated wurtzite semiconductors by piezoelectric coupling
(2009-11-01)
Wen, Yu-Chieh
;
Ko, Tsung-Shine
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Chyi, Jen-Inn
Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.