NANOSCALE ELECTRIC AND OPTICAL PHENOMENA IN SEMICONDUCTORS AND PIEZOELECTRIC MATERIALS INVESTIGATED BY SCANNING PROBE MICROSCOPY
| Autor: | Wei-Sheng Su, 蘇偉盛 |
|---|---|
| Rok vydání: | 2007 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 95 In this thesis, we have applied the powerful technique called scanning probe microscopy to investigate several newly developed semiconductors as well as piezoelectric materials. A variety of interesting phenomena have been discovered and the related physical mechanisms have also been proposed. The main features of our results are as follow: (1) Electrostatic force Spectroscopy: Application to local electronic transitions in InN epifilms. A technique based on electrostatic force microscopy in which light is used to change the charge states of the local region in a solid is introduced and demonstrated. This technique provides a unique feature that it can be used to probe local electronic transitions of a solid in a sub-micron scale. As an illustration, it has been applied to study local electronic structure in InN epifilms. Combining with atomic force microscopy, it is found that surface state density in the dale region is larger than that of the pinnacle region, and an electron accumulation layer does exist on the surface. In addition, the magnitude of the surface band bending obtained for the regions with different surface states is consistent with the result measured by other techniques. We point out that light-induced scanning electrostatic force spectroscopy is a very useful tool to probe the local electronic transitions of a solid in a sub-micron scale with high sensitivity. (2) Generation of electricity in GaN nanorods induced by piezoelectric effect. Conversion of mechanical energy into electric energy has been demonstrated in GaN nanorods. The measurement was achieved by deflecting GaN nanorods with a conductive atomic force microscope PtIr tip in contact. The mechanism relies on the coupling between piezoelectric and semiconducting properties in GaN nanorod, which creates a strain field and drives the charge flow across the nanorod. The result shown here opens up a new opportunity for harvesting electricity from wasted mechanical energies in the ambient environment, which may lead to the realization of self-powered nanodevices. (3) Built-in surface electric field, piezoelectricity and photoelastic effect in GaN nanorods for nano-photonic device. Novel behaviors arise from the coupling between built-in surface electric field, piezoelectricity, electron-hole pairs, and external light beam have been observed in GaN nanorods. When optical excitation density was increased, a blueshift in the photoluminescence spectra and a redshift in the frequency of GaN A1(LO) phonon were observed. The underlying mechanism was attributed to the screening of built-in surface electric field by photoexcited carriers, and through the converse piezoelectric effect, the internal strain was reduced. The existence of built-in surface electric field in GaN nanorods has been confirmed by scanning Kelvin probe microscopy. Our results firmly establish that the photoelastic effect does exist in GaN nanorods. This finding reveals novel properties arising from the inherent large surface-to-volume ratio of nanostructures, it thus is applicable to many other nanomaterials. It also underpinns the principle for applications in nano-photonic devices. (4) Piezoelectric Enhancement and Domain Switching in PMN-PT Polycrystalline Sheets by Piezoresponse Force Microscopy. Recently it was found that the piezoelectric response of substrate-free polycrystalline (Pb(Mg1/3Nb2/3)O3)0.63(PbTiO3)0.37 (PMN-37PT) sheets increased by several fold at applied electric fields E ≧ 2 kV/cm. Here we provide direct evidence of polarization switching occurring at such electric fields through piezoresponse force microscopy on a 40 |
| Databáze: | Networked Digital Library of Theses & Dissertations |
| Externí odkaz: |
|