Takashi Tsunekawa

High Output Power InGaN Ultraviolet Light-Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy

Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. In this study, the PSS with parallel grooves along the sapphire direction was fabricated by standard photolithography and subsequent reactive ion etching (RIE). The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5×108 cm-2. The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-UV-LED was operated at a forward-bias current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-bias current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.

High Output Power InGaN Ultraviolet Light‐Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy

Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5 x 10 8 cm -2 . The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the UV-LED was operated at a forward-biased current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-biased current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.

High-output power near-ultraviolet and violet light-emitting diodes fabricated on patterned sapphire substrates using metalorganic vapor phase epitaxy

The external quantum efficiency (EQE, ηe) of conventional near-ultraviolet (NUV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure is limited by high dislocation density and by the narrow escape cone due to total internal reflection at the GaN/air or sapphire/air interface. We have fabricated the NUV and violet InGaN-MQW-LEDs with the high EQE on the patterned-sapphire substrate (PSS) using a single growth process by metal-organic vapor phase epitaxy (MOVPE). The PSS with parallel grooves along the <11-20>GaN direction or the <1-100>GaN direction was fabricated by a standard photolithography and subsequent reactive ion etching (RIE). In this study, fabricated the LED on the PSS with parallel grooves along the <11-20>GaN direction. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has dislocation density of 1.5x108 cm-2. The LEPS-NUV (or violet)-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-NUV-LED (the emission peak wavelength λp: 382 nm) was operated at a forward-bias current of 20 mA at room temperature (RT), the output power (Po) and the EQE were 15.6 mW and 24%, respectively. When the LEPS-violet-LED (λp: 405 nm) was operated at a forward-bias current of 20 mA at RT, the Po and the EQE were 26.3 mW and 43%, respectively. Furthermore, we obtained the Po of approximately 61 mW at 50 mA and 111 mW at 100 mA, respectively. It was revealed that the PSS is very effective in reducing the dislocation density and for increasing the extraction efficiency due to the multiple scattering of the emission light at the GaN/patterned sapphire interface.

Intense Ultraviolet Electroluminescence Properties of the High-Power InGaN-Based Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates.

The electroluminescence and photoluminescence characteristics of high-efficient InGaN multi-quantum-well ultraviolet light-emitting diodes have been investigated. There appeared a single emission band in the electroluminescence spectra at about 3.235 eV with a band width of 90 meV at room temperature under direct current. With increasing forward current, the luminescence intensity was not saturated, and increased linearly with increasing injection current up to 50 mA. Under pulsed current conditions at room temperature, the luminescence intensity increased linearly with increasing injection current up to 1000 mA, and a shift of the electroluminescence peak position was not observed. These results indicated that the injected carriers were confined efficiently in the active layer, and also suggested the possibility of realizing ultraviolet laser diodes. It was revealed that the forward-biased electroluminescence spectrum at 4 K reflected the distribution of hot electrons injected into the active layer. The maximum temperature of hot electrons was estimated to be about 350 K under a forward-biased pulsed current of about 500 mA, which was much higher than the lattice temperature.

Optical Parametric Oscillator on 1-mm-Thick Periodically Poled LiNbO3 with 29 mm Interaction Length

A 1-mm-thick periodically poled LiNbO3 (PPLN) crystal with a domain-inverted period of 29.8 µm and an interaction length of 29 mm has been successfully fabricated by an electric field poling process. The average ratio of the domain-inverted width to the domain-inverted period (duty cycle) on a +Z surface and a -Z surface were 54.01% and 57.10%, respectively. To investigate the uniformity of domain-inverted structures in the Y-Z plane, the distribution of the optical parametric oscillator (OPO) idler output energy was measured, and an almost constant OPO idler output energy was obtained at all positions on the Y-Z plane. Therefore, the 1-mm-thick periodically domain-inverted structure can be uniformly obtained over the entire aperture. Finally, the OPO using this PPLN crystal with a large aperture of 1 mm×1 mm was demonstrated.