2006 Fiscal Year Final Research Report Summary
Multi-photon Absorption Spectroscopy for Three-dimensional Nanoscale Imaging of GaN
| Project/Area Number |
16310076
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Nanomaterials/Nanobioscience
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| Research Institution | HOKKAIDO UNIVERSITY |
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Principal Investigator |
TODA Yasunori Hokkaido Univ., Grad. School of Eng., Asso.Prof., 大学院工学研究科, 助教授 (00313106)
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| Co-Investigator(Kenkyū-buntansha) |
ADACHI Satoru Hokkaido Univ., Grad. School of Eng., Asso.Prof., 大学院工学研究科, 助教授 (10221722)
ARAKAWA Yasuhiko Univ. of Tokyo, Institute of Industrial Science, Prof., 生産技術研究所, 教授 (30134638)
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| Project Period (FY) |
2004 – 2006
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| Keywords | Gallium Nitride / Nonlinear spectroscopy / Two-photon absorption / Exciton / Time-resolved spectroscopy / Four-wave mixing |
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| Research Abstract |
Since strain and defects in semiconductors directly connect with the device performance, the influence has been studied extensively so far. Generally, there are various types of defects ranging from an atomic to a microscopic scale depending on the substrate, structure and growth conditions. In addition, the local strain fields due to mismatches in the lattice constants and the defects/impurities are also important for the thin films grown on foreign substrates. Therefore, three-dimensional (3D) analysis with a high spatial resolution is necessary for the precise characterization.
One approach to the optical characterization in 3D is the use of nonlinear processes, in which excitation well below the band-gap allows for cross-sectional analysis at any depth. Furthermore, the use of nonlinear spectroscopy often provides significant enhancement of the signal sensitivity. In this work, we demonstrated i) multiphoton induced absorption imaging and ii) four-wave mixing (FWM) spectroscopy for
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the precise characterization of Gallium Nitride (GaN) in terms of strain and defects. Both techniques developed here allow for nondestructive and noninvasive optical characterization not only for GaN but also for other semiconductor materials including nanostructures, which would be useful for diagnosing complex device structures.
In the two-photon absorption (TPA) imaging, we successfully mapped out the 3D distribution of defects with high spatial resolution. Moreover, using transient TPA measurements, we were able to obtain precise TPA coefficient that is one of the useful parameters for evaluating the optical damage threshold. In the GaN films, we showed that localized regions exist with significantly higher TPA coefficients, possibly due to the concentrations of defect states on a microscopic scale. In another nonlinear spectroscopy based on degenerate FWM technique, we successfully mapped out the local strain-field very precisely. Since the diffraction signal in FWM increases as the fourth power of the exciton oscillator strength, the exciton spectra show strong polarizations resulting from the uniaxial strain in the sample. The highly polarized spectra allow us to determine the precise splitting energy imposed by the local strain and defects. We measured various samples including GaN films on isotropic substrates, and examined the lower limit of detectable uniaxial strain, which is comparable with the resolution of conventional X-ray diffraction analysis.
Since the diffraction signal in FWM increases as the fourth power of the exciton oscillator strength, the exciton spectra show strong polarizations resulting from the uniaxial strain in the sample. The highly polarized spectra allow us to determine the precise splitting energy imposed by the local strain and defects. We measured various samples including GaN films on isotropic substrates, and examined the lower limit of detectable uniaxial strain, which is comparable with the resolution of conventional X-ray diffraction analysis. Less
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