Multi-photon Absorption Spectroscopy for Three-dimensional Nanoscale Imaging of GaN

由于半导体中的应变和缺陷直接关系到器件的性能，因此目前对其影响的研究非常广泛。一般来说，根据衬底、结构和生长条件的不同，存在从原子到微观尺度的各种类型的缺陷。此外，由于晶格常数和缺陷/杂质的不匹配引起的局部应变场对在外源衬底上生长的薄膜也很重要。因此，高空间分辨率的三维（3D）分析对于精确表征是必要的。三维光学表征的一种方法是使用非线性过程，其中远低于带隙的激发允许在任何深度进行横截面分析。此外，非线性光谱学的使用往往能显著提高信号的灵敏度。在这项工作中，我们展示了i)多光子诱导吸收成像和ii)四波混频（FWM）光谱…更多的是氮化镓（GaN）在应变和缺陷方面的精确表征。这里开发的两种技术不仅允许对GaN进行非破坏性和非侵入性的光学表征，而且还允许对包括纳米结构在内的其他半导体材料进行非破坏性和非侵入性的光学表征，这将有助于诊断复杂的器件结构。在双光子吸收（TPA）成像中，我们成功地以高空间分辨率绘制出缺陷的三维分布。此外，利用瞬态TPA测量，我们能够获得精确的TPA系数，这是评估光学损伤阈值的有用参数之一。在GaN薄膜中，我们发现局部区域存在显著较高的TPA系数，这可能是由于微观尺度上缺陷态的浓度。在另一种基于简并FWM技术的非线性光谱中，我们成功地非常精确地绘制了局部应变场。由于在FWM中衍射信号随着激子振荡器强度的四次方而增加，激子光谱表现出由样品中单轴应变引起的强极化。高偏振光谱使我们能够精确地确定局部应变和缺陷所施加的分裂能。我们测量了各种样品，包括各向同性衬底上的GaN薄膜，并检查了可检测的单轴应变下限，其分辨率与传统的x射线衍射分析相当。由于在FWM中衍射信号随着激子振荡器强度的四次方而增加，激子光谱表现出由样品中单轴应变引起的强极化。高偏振光谱使我们能够精确地确定局部应变和缺陷所施加的分裂能。我们测量了各种样品，包括各向同性衬底上的GaN薄膜，并检查了可检测的单轴应变下限，其分辨率与传统的x射线衍射分析相当。少

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 … More 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