We investigated correlations between nanoscopic optical and structural properties in violet-emitting, blueemitting, and green-emitting In x Ga 1−x N / GaN quantum wells ͑QWs͒ by means of scanning near-field optical microscopy ͑SNOM͒ and atomic force microscopy. Only in the blue-emitting QW, threading dislocations were not major nonradiative recombination centers ͑NRCs͒. SNOM data indicated that NRCs in the blue-emitting QW are surrounded by energy levels higher than those for radiative recombination. Such potential distributions realize "antilocalization" of carriers to NRCs, which is the cause of high emission quantum efficiencies in blue emitters.
Strain relaxation effect by nanotexturing InGaN/GaN multiple quantum wellEffects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells grown by metalorganic chemical vapor depositionThe optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 m, 1 m, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence ͑PL͒ 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.
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 piezoelectric polarization field is suppressed in InGaN∕GaN nanocolumns.
Spatial distribution of photoluminescence ͑PL͒ with spectral, spatial, and/or time resolution has been assessed in an In x Ga 1Ϫx N single-quantum-well ͑SQW͒ structure using scanning near-field optical microscope ͑SNOM͒ 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.
Spatially resolved photoluminescence ͑PL͒ of InGaN / GaN / AlGaN-based quantum-well-structured light-emitting diodes ͑LEDs͒ with a yellow-green light ͑530 nm͒ and an amber light ͑600 nm͒ 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.
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