Kenjiro Uesugi

Performance enhancement of blue light-emitting diodes with InGaN/GaN multi-quantum wells grown on Si substrates by inserting thin AlGaN interlayers

We have grown blue light-emitting diodes (LEDs) having InGaN/GaN multi-quantum wells (MQWs) with thin AlyGa1−yN (0 < y < 0.3) interlayers on Si(111) substrates. It was found by high-resolution transmission electron microscopy observations and three-dimensional atom probe analysis that 1-nm-thick interlayers with an AlN mole fraction of less than y = 0.3 were continuously formed between GaN barriers and InGaN wells, and that the AlN mole fraction up to y = 0.15 could be consistently controlled. The external quantum efficiency of the blue LED was enhanced in the low-current-density region (≤45 A/cm2) but reduced in the high-current-density region by the insertion of the thin Al0.15Ga0.85N interlayers in the MQWs. We also found that reductions in both forward voltage and wavelength shift with current were achieved by inserting the interlayers even though the inserted AlGaN layers had potential higher than that of the GaN barriers. The obtained peak wall-plug efficiency was 83% at room temperature. We suggest...

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.

High-efficiency blue LEDs with thin AlGaN interlayers in InGaN/GaN MQWs grown on Si (111) substrates

We demonstrate high-efficiency blue light-emitting diodes (LEDs) with thin AlGaN interlayers in InGaN/GaN multiquantum wells (MQWs) grown on Si (111) substrates. The peak external quantum efficiency (EQE) ηEQE of 82% at room temperature and the hot/cold factor (HCF) of 94% have been obtained by using the functional thin AlGaN interlayers in the MQWs in addition to reducing threading dislocation densities (TDDs) in the blue LEDs. An HCF is defined as ηEQE(85°C)/ηEQE(25°C). The blue LED structures were grown by metal-organic chemical vapor deposition on Si (111) substrates. The MQWs applied as an active layer have 8- pairs of InGaN/AlyGa1-yN/GaN (0≤y≤1) heterostructures. Thinfilm LEDs were fabricated by removing the Si (111) substrates from the grown layers. It is observed by high-resolution transmission electron microscopy and three-dimensional atom probe analysis that the 1 nm-thick AlyGa1-yN interlayers, whose Al content is y=0.3 or less, are continuously formed. EQE and the HCFs of the LEDs with thin Al0.15Ga0.85N interlayers are enhanced compared with those of the samples without the interlayers in the low-current-density region. We consider that the enhancement is due to both the reduction of the nonradiative recombination centers and the increase of the radiative recombination rate mediated by the strain-induced hole carriers indicated by the simulation of the energy band diagram.

Demonstration of novel high-efficiency blue LEDs on silicon substrates

High-efficiency gallium-nitride-based LEDs have become widely popular as these devices have found several applications, e.g., as white LEDs in general lighting.1 Many of these LEDs are grown on aluminum oxide (Al2O3), silicon carbide (SiC), or gallium nitride (GaN) substrates.2 In addition, GaN-based LEDs grown on silicon (Si) substrates have attracted much attention because of their low fabrication cost. These GaN-based LEDs, however, tend to include high densities of dislocations and cracks because of the large mismatch of lattice constants and thermal expansion coefficients between the GaN-based layers and the Si wafers.3 Many groups have attempted to tackle this issue and have reported improvements in the manufacture of GaN-based LEDs.4, 5 For example, gas flow conditions and precise control of temperature in a reactor have been optimized.5 In addition, we have previously developed LEDs with low threading dislocation densities (TDDs), i.e., of less than 2 108cm2 on Si (111 crystallographic plane) substrates.6, 7 Those TDD values were almost the same as those of LEDs grown on Al2O3 (0001) substrates. We achieved this reduction of TDDs by using multiple modulations of dislocations during the formation of GaN islands on a silicon nitride (SiN) interlayer, and from the growth of the SiN cap layers on the GaN island surfaces. In the past, we have also confirmed that the external quantum efficiency (EQE) and the hot/cold factors (HCFs) of LEDs grown on Si (111) substrates increase with decreasing edge-type dislocations in the low-current-density region.8 We also showed that the EQE of a sample with a low TDD is almost as high as that of an LED grown on an Al2O3 (0001) substrate at room temperature.8 Figure 1. Schematic diagrams of the multi-quantum well structures investigated in this study. (a) A reference structure that consists of indium gallium nitride (InGaN) well layers and gallium nitride (GaN) barrier layers. (b) A structure with thin aluminum gallium nitride interlayers that partly replace the GaN barrier layers.