Akio Kaneta

Origin of high oscillator strength in green-emitting InGaN∕GaN nanocolumns

Optical characterization has been performed on an InGaN∕GaN nanocolumn structure grown by nitrogen plasma assisted molecular beam epitaxy not only in macroscopic configuration but also in a microscopic one that can be assessed to a single nanocolumn. The photoluminescence (PL) decay monitored at 500nm is fitted with a double exponential curve, which has lifetimes of 0.67 and 4.33ns at 13K. These values are two orders of magnitude smaller than those taken at the same wavelength in conventional InGaN∕GaN quantum wells (QWs) grown toward the C orientation. PL detection of each single nanocolumn was achieved using a mechanical lift-off technique. The results indicate that the very broad, macroscopically observed PL spectrum is due to the sum of the sharp PL spectrum from each nanocolumn, the peak energy of which fluctuates. Moreover, unlike conventional QWs, the blueshift of a single nanocolumn is negligibly small under higher photoexcitation. These findings suggest that carrier localization as well as the pi...

Optical properties of InGaN/GaN nanopillars fabricated by postgrowth chemically assisted ion beam etching

The optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 μm, 1 μm, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence (PL) and time-resolved PL measurements suggest inhomogeneous relaxation of the lattice-mismatch induced strain in the InGaN layers. By comparing to a strain distribution simulation, we found that partial stain relaxation occurs at the free side wall, but strain remains in the middle of the pillar structures. The strain relaxation leads to an enhanced radiative recombination rate by a factor of 4–8. On the other hand, nonradiative recombination processes are not strongly affected, even by postgrowth etching. Those characteristics are clearly reflected in the doughnut-shape emission patterns observed by optical microscopy.

Spatial and temporal luminescence dynamics in an InxGa1-xN single quantum well probed by near-field optical microscopy

Spatial distribution of photoluminescence (PL) with spectral, spatial, and/or time resolution has been assessed in an InxGa1−xN single-quantum-well (SQW) structure using scanning near-field optical microscope (SNOM) under illumination-collection mode at 18 K. The near-field PL images revealed the variation of both intensity and peak energy in PL spectra according to the probing location with the scale less than a few hundredths of a nanometer. PL linewidth, the value of which was about 60 meV in macroscopic PL, was as small as 11.6 meV if the aperture size was reduced to 30 nm. Clear spatial correlation was observed between PL intensity and peak wavelength, where the regions of strong PL intensity correspond to those of long wavelength. Time-resolved SNOM–PL study showed the critical evidence that supports the model of diffusion of carriers to potential minima.

Radiative and nonradiative recombination processes in GaN-based semiconductors

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

Confocal microphotoluminescence of InGaN-based light-emitting diodes

Spatially resolved photoluminescence (PL) of InGaN/GaN/AlGaN-based quantum-well-structured light-emitting diodes (LEDs) with a yellow-green light (530 nm) and an amber light (600 nm) was measured by using confocal microscopy. Submicron-scale spatial inhomogeneities of both PL intensities and spectra were found in confocal micro-PL images. We also found clear correlations between PL intensities and peak wavelength for both LEDs. Such correlations for yellow-green and amber LEDs were different from the reported correlations for blue or green LEDs. This discrepancy should be due to different diffusion, localization, and recombination dynamics of electron-hole pairs generated in InGaN active layers, and should be a very important property for influencing the optical properties of LEDs. In order to explain the results, we proposed a possible carrier dynamics model based on the carrier localization and partial reduction of the quantum confinement Stark effect depending on an indium composition in InGaN active layers. By using this model, we also considered the origin of the reduction of the emission efficiencies with a longer emission wavelength of InGaN LEDs with high indium composition.

Discrimination of local radiative and nonradiative recombination processes in an InGaN/GaN single-quantum-well structure by a time-resolved multimode scanning near-field optical microscopy

Precise identification of recombination dynamics based on local, radiative, and nonradiative recombination has been achieved at room temperature in a blue-light-emitting InxGa1−xN/GaN single-quantum-well structure by comparing the photoluminescence (PL) spectra taken by illumination-collection mode (I-C mode) and those by illumination mode (I-mode) in scanning near-field microscopy. The PL data mapped with PL lifetimes, as well as with PL spectra, revealed that the probed area could be classified into four different regions whose dominating processes are (1) radiative recombination within a probing aperture, (2) nonradiative recombination within an aperture, (3) diffusion of photogenerated excitons/carriers out of an aperture resulting in localized luminescence, and (4) the same diffusion process as (3), but resulting in nonradiative recombination.

Weak Carrier/Exciton Localization in InGaN Quantum Wells for Green Laser Diodes Fabricated on Semi-Polar {2021} GaN Substrates

Carrier/exciton localization in InGaN quantum wells (QWs) for green laser diodes fabricated on semi-polar {2021} GaN substrates is assessed using time-resolved photoluminescence (TRPL) spectroscopy. The estimated characteristic energy, which represents the localization depth in a {2021} InGaN QW, is 15.1 meV. This value is much smaller than that reported for c-plane green InGaN QWs, indicating a high compositional homogeneity of {2021} InGaN QWs and consequently suggesting that the GaN semi-polar {2021} plane is suitable for fabricating green laser diodes.

In inhomogeneity and emission characteristics of InGaN

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N ultraviolet LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature (RT). It was found that addition of a small amount of In results in the reduction of nonradiative recombination centres originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N blue LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behaviour, indicating an important role of exciton localization. The lasing mechanism was investigated for In0.1Ga0.9N near ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) and In0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed from the nearly delocalized states (the lowest quantized levels (LQLs) within quantum wells) in the violet LD, while it was from highly localized levels with respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV for the blue LD. It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the timescale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photo-excitation.

Visualization of the Local Carrier Dynamics in an InGaN Quantum Well Using Dual-Probe Scanning Near-Field Optical Microscopy

We have developed dual-probe scanning near-field optical microscopy (SNOM) to visualize detailed carrier diffusion/recombination processes and applied it to the assessment of the local carrier dynamics in an InGaN single quantum well. It is clearly demonstrated that the carrier motion is strongly affected by the potential distribution within InGaN; potential ridges prevent carriers from diffusing outside them, whereas potential peaks cause carriers to travel a roundabout route around them. As a consequence, carriers anisotropically diffuse for several hundred nanometers along a specific direction toward a strong-photoluminescence domain. Thus, the dual-probe SNOM technique is a powerful nanoscopic tool, and may be versatile for characterizing photonic materials.

Efficient green emission from (112¯2) InGaN∕GaN quantum wells on GaN microfacets probed by scanning near field optical microscopy

Nanoscopic optical characterization using scanning near field optical microscopy was performed on a (112¯2) microfacet quantum well (QW). It was revealed that the carrier diffusion length in the (112¯2) QW is less than the probing fiber aperture of 160nm, which is shorter than that of the (0001) QWs and is attributed to much faster radiative recombination processes in the (112¯2) QW due to a reduced internal electric field. Owing to this short diffusion length, the correlation between the internal quantum efficiency (IQE) and emission wavelength is elucidated. The highest IQE is ∼50% at 520nm, which is about 50nm longer than in (0001) QWs, suggesting that the (112¯2) QW is a suitable green emitter.