Hajime Nago

Optical properties of InGaN/GaN MQW LEDs grown on Si (111) substrates with low threading dislocation densities

We have grown blue light-emitting diodes (LEDs) with low threading dislocation densities (TDDs) by using SiN interlayers on Si (111) substrates. Our growth technique using SiN layers makes it possible to decrease twist components (edge-type threading dislocation components). The edge-type TDDs are almost the same values as those of LEDs grown on Al2O3 (0001) substrates. EQE of LEDs grown on Si (111) substrates increases with decreasing edge-type dislocation in the low-current-density region, and the EQE of the sample with low TDD is almost as high as that of the LED grown on an Al2O3 (0001) substrate at room temperature. It is found that the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region, but are less than those of an LED grown on an Al2O3 (0001) substrate. Time-resolved photoluminescence (TRPL) shows that the dominant origin of the thermal quenching is edge-type dislocations in our samples, but other defects such as screw-type dislocations also contribute to it. We also found the fluctuated emission patterns consisting of bright and dark areas originated from the difference of Shockley–Read–Hall (SRH) type defect densities in the multi-quantum wells (MQWs) grown on Si (111) substrates. The bright areas spread, and the configurations of the bright areas change into ring-like patterns with reducing edge-type TDDs. We suggest that the internal quantum efficiency (IQE) of dark areas should be promoted to improve the performance of the MQWs grown on Si (111) substrates.

Effect of dislocation density on efficiency curves in InGaN/GaN multiple quantum well light-emitting diodes

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.

Direct observation of lattice constant variations depending on layer structures in an InGaN/GaN MQW LED

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.

Narrow-width photoluminescence spectra of InGaN quantum wells grown on GaN (0001) substrates with misorientation toward [1100] direction

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.