Study on nonradiative recombination mechanism in widegap semiconductors

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The purpose of this work is to (1) characterize quantitatively the dynamics of capture processes of excitons and/or carriers injected to active layers in widegap semiconductors, (2) clarify the atomic nature of nonradiative recombination centers and correlate with their electronic states, and (3) develop an atomically controlled growth technology by making positive feedback to the growth conditions taking into account the data obtained at (1) and (2). Many of photo-generated and/or electrically injected excitons/carriers looses their energy by the nonradiative recombination processes. Therefore, the elimination as well as the assessment of such mechanism would lead to the improvement of emission efficiency.In this research project, we have succeeded in the observation of radiation-less processes related to carrier diffusion, and heat generation and diffusion due to nonradiative recombination, by employing transient grating spectroscopy based on third order optical nonlinearity. So far, such observation was difficult in spite of its importance.Moreover, we have developed microscopic heat-detecting spectroscopy by combining transient lens spectroscopy with optical microscopy, by which spatial and temporal measurement of the photo-thermal process was assessed at GaN epilayers with low dislocation density, and generation and diffusion of the heat originating from nonradiative recombination could be estimated quantitatively with a spatial resolution of 3 μm. As a result, it was confirmed that threading dislocations act as nonradiative recombination centers of excitons just after photo-generation.Furthermore, we have set up a scanning near-field optical microscopy, by using an apparatus which is capable of detecting both radiative and nonradiative recombination processes complimentary. We expect the future development of the research because of the improvement of spatial resolution down to from sum-micron to nanoscopic level.