Motokazu Yamada

Characteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting Diodes

Highly efficient light-emitting diodes (LEDs) emitting ultraviolet (UV), blue, green, amber and red light have been obtained through the use of InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 675 nm, whose emission energy was almost equal to the band-gap energy of InN, were fabricated. The dependence of the emission wavelength of the red LED on the current (blue shift) is dominated by both the band-filling effect of the localized energy states and the screening effect of the piezoelectric field. In the red LEDs, a phase separation of the InGaN layer was clearly observed in the emission spectra, in which blue and red emission peaks appeared. In terms of the temperature dependence of the LEDs, InGaN LEDs are superior to the conventional red and amber LEDs due to a large band offset between the active and cladding layers. The localized energy states caused by In composition fluctuation in the InGaN active layer contribute to the high efficiency of the InGaN-based emitting devices, in spite of the large number of threading dislocations and a large effect of the piezoelectric field. The blue and green InGaN-based LEDs had the highest external quantum efficiencies of 18% and 20% at low currents of 0.6 mA and 0.1 mA, respectively.

InGaN-Based Near-Ultraviolet and Blue-Light-Emitting Diodes with High External Quantum Efficiency Using a Patterned Sapphire Substrate and a Mesh Electrode

We markedly improved the extraction efficiency of emission light from the InGaN-based light-emitting diode (LED) chips grown on sapphire substrates. Two new techniques were adopted in the fabrication of these LEDs. One is to grow nitride films on the patterned sapphire substrate (PSS) in order to scatter emission light. Another is to use the Rh mesh electrode for p-GaN contact instead of Ni/Au translucent electrode in order to reduce the optical absorption by the p-contact electrode. We fabricated near-ultraviolet (n-UV) and blue LEDs using the above-mentioned techniques. When the n-UV (400 nm) LED was operated at a forward current of 20 mA at room temperature, the output power and the external quantum efficiency were estimated to be 22.0 mW and 35.5%, respectively. When the blue (460 nm) LED was operated at a forward current of 20 mA at room temperature, the output power and the external quantum efficiency were estimated to be 18.8 mW and 34.9%, respectively.

Phosphor Free High-Luminous-Efficiency White Light-Emitting Diodes Composed of InGaN Multi-Quantum Well.

We fabricated two types of high-efficiency white light-emitting diodes (LEDs) composed of an InGaN multi-quantum well (MQW), which emit light of two or three different colors without phosphors. The Type-1 white LED emits light of two colors (blue and yellow) from the MQW active layer, while the Type-2 LED emits light of three colors (blue, green and red). When the Type-1 white LED was operated at a forward current of 20 mA at room temperature, the color temperature (Tcp), average color rendering (Ra) and luminous efficiency were 7600 K, 42.7 and 11.04 lm/W, respectively. When the Type-2 white LED was operated at a forward current of 20 mA at room temperature, Tcp, Ra and luminous efficiency were 5060 K, 80.2 and 7.94 lm/W, respectively.

Red-Enhanced White-Light-Emitting Diode Using a New Red Phosphor

We fabricated a high-color-rendering, red-enhanced white-light-emitting diode (LED) using a new red phosphor and a short-wavelength YAG phosphor. When the new white-LED was operated at a forward-bias current of 20 mA at room temperature (RT), color temperature (Tcp), the general color rendering index (Ra) and luminous efficiency were 4670 K, 87.7 and 25.5 lm/W, respectively. Most of the color-rendering indexes (CRIs) of the new white-LED were larger than those of current white-LEDs, in which only YAG is used. In particular, the CRI-No.9 value, which shows the color reproduction in the red region, is improved from -2.5 to 62.6.

Current and Temperature Dependences of Electroluminescence of InGaN-Based UV/Blue/Green Light-Emitting Diodes

Current and temperature dependences of the electroluminescence of InGaN UV/blue/green single-quantum-well (SQW)-structure light-emitting diodes (LEDs) were studied. The emission mechanism of InGaN SQW-structure LEDs with emission peak wavelengths longer than 375 nm is dominated by carrier recombination at large localized energy states caused by In composition fluctuation in the InGaN well layer. When the emission peak wavelength becomes shorter than 375 nm, the conventional band-to-band emission mechanism becomes dominant due to poor carrier localization resulting from small In composition fluctuations. In addition, the quantum-confined Stark effect due to the piezoelectric field becomes dominant, which causes a low output power of the UV LEDs.

A Methodological Study of the Best Solution for Generating White Light Using Nitride-Based Light-Emitting Diodes

In search of suitable white-LED for general illumination, we fabricated various types of white-LEDs using different methods. As the first method, we used the multichip method in which multiple emitters were mounted in one package. This type showed a good general color-rendering index (Ra) = 90 by the optimizing the emission wavelength of each LED chip. However, the electric driving circuitry was too complex for use in general illumination. Secondly, we used a monolithic white-LED by using the multicolor emitting multiple-quantum well (MQW) for the active layers, which consisted of quantum wells (QWs) with different In compositions. A high Ra = 80.1 was obtained in the three-color-emitting white-LED but the luminous efficacy (η L ) was only 8.11 1m/W. As the third method, we used the color conversion method using phosphors. We fabricated a white-LED which consisted of a near-UV-LED chip and blue/yellow phosphors in order to improve the luminous efficacy of the white-LED under high forward-bias current. At 100 mA, the luminous flux (I L ) was estimated to be 7.61m. However, this white-LED degraded quickly, because the epoxy resin used for package was the general purpose one and deteriorated under the UV-light from the n-UV-LED. Next, we improved the Ra and η L of a traditional white-LED which consisted of blue-LED chip and yellow phosphor. In order to improve the Ra, we added a newly developed red phosphor. We obtained a Ra = 87.7 at low-color-temperature. Then, in order to improve the efficiency of the white-LED, we improved the extraction efficiency (η EX ) of the blue-LED by using a patterned sapphire substrate and a high reflection Rh-mesh-patterned p-electrode. Then, we obtained a 62.0 1m/W at 20 mA. As a result, we concluded that the color conversion method of using a blue-LED for general illumination has advantages in efficiency, color-rendering, cost and lifetime. It also has simpler electric driving circuitry.

InGaN-based UV/blue/green/amber LEDs

High-efficient light emitting diodes (LEDs) emitting red, amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layers instead of GaN active layers. Red LEDs with an emission wavelength of 680 nm which emission energy was smaller than the band-gap energy of InN were fabricated mainly resulting from the piezoelectric field due to the strain. The localized energy states caused by In composition fluctuation in the InGaN active layer seem to be related to the high efficiency of the InGaN-based emitting devices in spite of having a large number of threading dislocations. InGaN single-quantum-well- structure blue LEDs were grown on epitaxially laterally overgrown GaN and sapphire substrates. The emission spectra showed the similar blue shift with increasing forward currents between both LEDs. The output power of both LEDs was almost the same, as high as 6 mW at a current of 20 mA. These results indicate that the In composition fluctuation is not caused by dislocations, the dislocations are not effective to reduce the efficiency of the emission, and that the dislocations from the leakage current pathway in InGaN.