The assessment of emission mechanism in GaN-based nano-structures by scanning near-field optical microscopy

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The purpose of this project is to assess and/or control the radiative recombination processes in InGaN/GaN-based quantum wells by employing photoluminescence (PL) mapping with scanning near field optical microscopy (SNOM), by which dramatic improvement of emission efficiency as well as the extension of operatable wavelength can be achieved in emitters such as light emitting diodes (LEDs), laser diodes (LDs) and new phosphors. Our approach is to fabricate micro/nano structures for multi-wavelength lighting, to elucidate fundamental optical properties, and to make a positive feedback to future device structures. Main results are summarized as shown below.(1)We have developed the experimental technique by combining the SNOM-PL with atomic force microscopy (AFM), where the correlation was clearly revealed between dislocations and weak emission regions. Moreover, time-resolved PL measurements with this method showed the dynamical behavior of photo-generated carriers and/or excitons in potential fluctuations. It was found that the pathway to nonradiative recombination centers is effectively hindered by the compositional modulation around the dislocations, so that the carrier diffusion is spatially unisotropic process.(2)We have proposed the re-growth technique, where well-established c-oriented GaN is used as a seed. It has been demonstrated that the growth on c-oriented GaN patterned with a stripe along the [11^^-00] direction forms (0001), <112^^-2>, and <112^^-0> facets and that the QW on the <112^^-2> facet involves weaker electric fields and has a higher PL efficiency, compared with conventional c-oriented InGaN/GaN QWs. Moreover, rainbow color PL covering the entire visible range has been achieved from InGaN/GaN QWs on the <112^^-2> facets with a few micron-width.