Shigeya Kimura

Long-life high-reliability Ir-coated dispenser cathode

A long-life high-reliability Ir-coated dispenser cathode has been developed. To study the life characteristics of Ir-coated dispenser cathode, which is provided with a surface alloy layer of εII, extended life tests were conducted over 10,000h. From this life data, this cathode was estimated to have a 200,000h life at 920°Cb with 0.8A/cm2, and to have a 100,000h life at 980°Cb with 2.5A/cm2before emission decreases by 4%. Further, the experiment equation for surface Ir concentration of the cathodes was deduced from the results of surface analysis. Using this equation, the surface alloy layer was evaluated as extremely stable without substantial degradation during cathode life. Thus, the Ir-coated dispenser cathode has been proved to be exceedingly durable and highly reliable.

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.

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.

LED manufacturing issues concerning gallium nitride-on-silicon (GaN-on-Si) technology and wafer scale up challenges

We demonstrated the growth of blue LED structures grown on 4-inch and 8-inch Si (111) substrates. GaN layers that have low threading dislocation density (TDD) of 1.6×108/cm2, which is comparable to state of the art in GaN on sapphire, can be obtained on 4-inch Si substrate by using a new TDD reduction technology using a silicon nitride (SiN) interlayer. A dependence of LED device performance on threading dislocation density was studied by using 4-inch Si substrate. A LED manufacturing technology has been developed by using 8-inch Si towards mass production technology. The light output power representing a median performance exceeded 641 mW at 350 mA, which was comparable to state-of-the-art LED.

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.