The contribution of reduction of threading dislocation densities (TDDs) to optical properties is investigated for
InGaN/GaN light-emitting diodes (LEDs) grown on sapphire substrate. The external quantum efficiency (EQE) curves
depending on the TDDs are discussed both theoretically and experimentally. At the current density of <20 A/cm2, the
EQE increases with decreasing the edge-type TDD from 5 e8/cm2 to 2 e8/cm2. The current density at the maximum EQE
shifts to lower value as the edge-type TDD decreases, whereas the EQE presents no remarkable difference in the highercurrent
density range irrespective of the TDD. According to the rate equation (ABC) model, the peak shift reflects the
Shockley-Read-Hall non-radiative process (A coefficient). Analysis of the photoluminescence (PL) decay and the
dependence of integrated PL intensity on excitation power reveals that the threading dislocations act as non-radiative
recombination centers in the multiple quantum well active region. The TDD of <2 e8/cm2 is required for highly efficient
blue LEDs operating at current density of around 15 A/cm2, whereas the TDD of <5 e8/cm2 in required for the LEDs
operating at around 50 A/cm2.
We have directly observed that InGaN quantum well layers were incoherently grown on 5-nm-thick GaN barrier layers
in an InGaN/GaN multiple quantum well (MQW) system of a blue light-emitting diode by using a lattice image obtained
by high-resolution transmission electron microscopy and fast Fourier transform mapping (FFTM) analysis of the lattice
image. The lattice disorder was observed in the middle of the InGaN well layer by using high-angle annular dark field
(HAADF) scanning transmission electron microscopy (STEM). In contrast, FFTM of the InGaN well layers with 10-nm-thick
barrier layers showed the intervals of the (01-10) lattice planes were homogeneous, and the lattice disorder was not
observed in the HAADF-STEM image. These results indicate that the excess stain in the InGaN/GaN MQW having
thinner GaN barrier layers induces the lattice disorder in the InGaN well layers. Indium composition fluctuation in the
InGaN well layer was also observed by using three-dimensional atom probe analysis. It indicates that the incorporation
of indium atoms is affected by the imperfect structural properties of the MQW system with thinner GaN barrier layers.
The intensity of electroluminescence from the sample with 10-nm-thick barrier layers in the MQWs was higher than that
from the sample with 5-nm-thick barrier layers.
The optical properties of InGaN quantum wells on misoriented GaN (0001) substrates were investigated. The fluctuation
of peak wavelength and full width at half maximum of micro-photoluminescence from InGaN quantum wells was large
when the misorientation angle was 0.0o. The micro-photoluminescence showed narrow-width spectra, with full width at
half maximum below 60 meV, of InGaN quantum wells grown on GaN (0001) substrates with a misorientation angle of
around 0.28o toward [11-00] direction. These results indicate that InGaN quantum wells have high crystalline quality when
InGaN quantum wells are grown with misorientation angle between 0.2o and 0.3o toward [11-00] direction.
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