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  4. Nano-Acoustic Wave Synthesizing and 2D Nano-Imaging
 
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Nano-Acoustic Wave Synthesizing and 2D Nano-Imaging

Date Issued
2005
Date
2005
Author(s)
Yu, Cheng-Ta
DOI
en-US
URI
http://ntur.lib.ntu.edu.tw//handle/246246/50654
Abstract
We have successfully developed a system to generate arbitrary-waveform nano-acoustic-waves (NAWs) with a piezoelectric InGaN/GaN single quantum well (SQW). Based on an optical coherent control technique, the piezoelectric SQW is regarded as an acoustic waveform synthesizer and acoustic frequency tunability in the sub-terahertz range is realized within only one fixed sample. By utilizing the transient transmission measurement, the detection of generated acoustic frequency varying from 0.1 THz to 0.88 THz has been demonstrated. It is also possible to manipulate the generated acoustic waveform by engineering the optical excitation intensities. This flexible system enabled further studies in nano-ultrasonics. With NAWs composed of acoustic pulses generated from the piezoelectric SQW, a saturation phenomenon of acoustic generation was observed. The acoustic generation mechanisms, especially Coulomb screening of the piezoelectric field in SQW, were discussed with optical excitation power dependency. At high photo-excited carrier concentrations, the acoustic generation induced by screening of built-in piezoelectric field tends to be saturated. To analyze this phenomenon, a simplified charged-parallel-plate model was introduced and a 1.1 MV/cm built-in piezoelectric field can also be roughly estimated. By means of transient reflectivity measurement, lifetime of acoustic waves in piezoelectric semiconductors can be estimated. With the generated NAWs propagating along the c-axis of a GaN thin film, the lifetime of the 500 GHz coherent longitudinal-acoustic phonons in GaN was measured to be >420 ps, corresponding to a penetration depth more than 3.4 micron. In this thesis, we also performed 2-D nano-ultrasonic measurement with NAWs. We adopted a 3-period InGaN/GaN multiple-quantum-well (MQW) as our NAW source for a preliminary ultrasonic study with fixed single acoustic frequency of ~500 GHz. Based on 1-D nano-scan performed by acoustic generation and echo detection, we built a 2D image system for nano-ultrasonics. We took a GaN thin-layer etched with a striped pattern on the top surface for nano-ultrasonic demonstration. A 2D surface profile was clearly measured and an etching depth of ~35 nm can be resolved, which was in great agreement with the measurement of a commercial AFM. To make sure of the lateral resolution restricted by the spot size of NAWs, a resolving distance between two acoustic spots was measured to be ~190 nm in our system.
Subjects
奈米音波
奈米超音波影像
nano-acoustic waves
nano-ultrasonics
Type
thesis

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