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B02 group

B02-1 Assessment and control of radiative recombination mechanisms in singularity structures by scanning near-field optical microscopy

  • Principal Investigator / Yoichi Kawakami (Kyoto University / Professor)
  • Co-Investigator / Mitsuru Funato (Kyoto University Associate / Professor)

We strive to clarify the design concept of light emitting devices with the ultimate goal of achieving 100% internal quantum efficiency in the full visible spectrum by focusing on singularity structures such as semipolar micofacets and nanocolumns in (Al,Ga,In)N-based three-dimensional semiconductors. To assess InGaN-based nanostructures and elucidate the mechanism of radiative recombination in singularity structures by visualization of exciton and carrier motions, we employ original laboratory-made SNOM (scanning near-field optical microscopy) systems, including DSNOM (dual-probe SNOM). Moreover, we plan to establish a near-field technique in the DUV (deep ultraviolet) spectra by developing an optical probe material capable of guiding DUV light with a highly efficient transmittance. These efforts should lead to the study of spatially and temporally resolved recombination dynamics in AlGaN-based and InGaN-based nanostructures. Furthermore, we focus on the effect of plasmonics, where the controllability of the emission wavelength and the key for high emission efficiency will be characterized in compositionally modulated singularity structures. These studies should help realize next-generation devices such as phosphor-free tailor-made lighting and highly efficient DUV emitters due to the clear design concept of singularity structures intended to control the optical functionalities.

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B02-2 Studies on the local luminescence dynamics in singularity structures by using spatio-time-resolved cathodoluminescence technique

  • Principal Investigator / Shigefusa F. Chichibu (Tohoku University / Professor)
  • Co-Investigator / Kazunobu Kojima (Tohoku University / Associate Professor)
  • Co-Investigator / Kohei Shima (Tohoku University / Assistant Professor)

Singularity structures including imperfections as well as inhomogeneity contain various types of heterointerfaces within the bulk and/or surfaces. For understanding local carrier and/or emission dynamics in such structures, probing local emission dynamics by using a method that picks off them is mandatory. Our principal task in this singularity project is obtaining the images of multiple mapping of luminescence intensity, energy, and lifetime by using our unique 'Spatio-Time-Resolved Cathodoluminescence (STRCL)' system. This system is basically composed of a cathodoluminescence (CL) measurement system equipped on a scanning electron microscopy (SEM), but its electron beam (e-beam) gun is replaced by a pulsed photoelectron (PE) gun made of Au, which is excited using the frequency-tripled femtosecond pulses of mode-locked Al2O3:Ti laser. Because e-beam pulses are used as an excitation source, STRCL has principally no limitation on the bandgap energy of a material to be measured. We will collaborate with all groups to assess the carrier recombination dynamics and quantum effects in the singularity structures for creating new optoelectronic sciences in semiconductors.

 

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B02-3 Optical properties and functionalities of dense excitonic systems in singularity structures

  • Principal Investigator / Yoichi Yamada (Yamaguchi University / Professor)
  • Co-Investigator / Satoshi Kurai (Yamaguchi University / Assistant Professor)

We maintain a particular focus on the many-body effects of excitons in nitride semiconductors. Owing to the advantages of excitonic nature that are large exciton binding energy and strong oscillator strength, the excitons stably exist at high excitation densities, where the interaction between two or more excitons causes the unique properties that are characterized by the formation of biexcitons and the inelastic scattering of excitons. From the viewpoint of excitonics (exciton engineering), it is important to study the recombination dynamics of dense excitonic phenomena because the contribution of excitonic processes to optical transitions is expected to improve the performance of semiconductor optoelectronic devices, such as light-emitting diodes and laser diodes. The singularity structure we study mainly in the present work is an inhomogeneous system caused by structural imperfection such as alloy fluctuations and quantum-size fluctuations. We study the effect of localization as well as dimensionality on dense excitonic systems by means of a variety of laser spectroscopy: selective and resonant excitation spectroscopy using a wavelength-tunable dye laser, time-resolved spectroscopy using a femtosecond titanium sapphire laser, and space-resolved spectroscopy using a cathodoluminescence mapping system combined with scanning electron microscopy. We clarify the potential of the singularity structure and obtain a guide to the design of high-efficiency excitonic optoelectronic devices.


Localization and radiative recombination of excitons and biexcitons in an inhomogeneous system

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B02-17-1 Theoretical modeling of electronic structures in III-nitride materials with singular structures based on photo-acoustic and photoluminescence simultaneous measurements

  • Principal Investigator / Atsushi A. Yamaguchi (Kanazawa Institute of Technology / Professor)

III-nitride based optical devices such as light-emitting diodes and laser diodes have been already commercialized. However, their active-layer materials, alloy semiconductor InGaN and AlGaN, have singular structures such as alloy compositional fluctuation, spatial strain distribution, and various defects, and their optical properties have not been fully understood. Studies on radiative and nonradiative recombination mechanisms are very important to clarify the electronic structures and carrier dynamics in the III-nitride semiconductors with singular structures. In this study, we will directly observe both the two carrier recombination processes under the same optical excitation condition by photo-acoustic and photoluminescence simultaneous measurements, which we have newly developed, and we will estimate the internal quantum efficiency in the singular structures. Lifetimes of both the radiative and nonradiative recombination processes will be obtained by the combination with photoluminescence decay time measured by time-resolved photoluminescence spectroscopy. Furthermore, we will try to propose a theoretical model to reproduce all the reported experimental results to comprehensively understand the electronic structures and carrier dynamics in the singular structures.

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B02-17-2 Theoretical study of energy-conversion mechanism around structural singularity

  • Principal Investigator / Masato Oda (Wakayama University / Lecturer)

GaN-based light-emitting diodes and laser diodes are representative of the most successful semiconductor devices in recent decades. Although many studies have been carried out on the mechanism of light emission, the mechanism of device degradation remains unclear. To guarantee a sufficient device lifetime, it is inevitable to clear the mechanism of the degradation. The non-radiative dislocations are experimentally observed in the dark region of the degraded devices. It is proposed that a mechanism for the growth of dislocation where point defects (typical structural singularities) in the active layer are gathered via defect reaction to grow the dislocation loops. The defect reaction is occurred via energy conversion between electronic and lattice systems around structural singularities. For example, an enhancement of a phonon mode around Ga vacancy is thought to be an origin of the defect reaction (Figure). In this study, we investigate energy conversion system from electronic to lattice systems using first-principles calculation based on density functional theory.

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B02-17-3 Formation of single photon source in SiC by femtosecond laser processing

  • Principal Investigator / Takuro Tomita (Tokushima University / Associate Professor)

Recently, the formation of point defects on silicon carbide (SiC) after the electron beam irradiation has reported. The spontaneous light emission from these defects confirmed to be emission of single photon. This light shows the features particular to the single photon emission, such as the blinking and and anti-bunching nature. According to this result, in this research, we will try to fabricate single photon source by femtosecond laser irradiation as the tool for fabricating the single photon source, instead of electron beam irradiation. By controlling the irradiation parameters of the laser beam, such as the laser fluence, wavelength, and the repetition rate of the laser beam, we try to control the contribution ratio among several physical processes related to the femtosecond laser ablation. By employing the suitable laser parameters, it is expected that we can realize the formation of point defects by femtosecond laser irradiation by employing the photon picture based femtosecond laser processing.

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