Hiroaki Okagawa

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.

Internal quantum efficiency of highly-efficient InxGa1−xN-based near-ultraviolet light-emitting diodes

The internal quantum efficiency (IQE) of highly-efficient near-UV light-emitting diodes, which shows an external quantum efficiency of 43% at 406 nm, has been measured by excitation power and temperature-dependent photoluminescence (PL). Assuming peak PL quantum efficiency at 8 K is 100%, peak IQE at 300 K was measured to be as high as 63%. At the injected carrier density, which corresponds to 20 mA current injection, IQE and light extraction efficiency were estimated to be about 54% and 80%, respectively.

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.

A UV light-emitting diode incorporating GaN quantum dots

The fabrication and evaluation of a UV light-emitting diode (LED) incorporating GaN quantum dots as the active layer is demonstrated. The GaN quantum dots were fabricated on an AlxGa1-xN (x ~0.1) surface using Si as an antisurfactant. Exposing the AlxGa1-xN surface to the Si antisurfactant prior to GaN growth enabled the formation of quantum dots on a surface where growth by the Stranski-Krastanov mode would not be possible. A fairly high density of dots (1010–1011 cm-2) with controllable dot sizes was achieved. Room temperature luminescence at 360 nm was clearly observed during current injection (cw) into an LED structure including the GaN quantum dots. The origin of the electroluminescence is discussed by comparing it to photoluminescence measurements.

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.

Temperature dependence of Stokes shift in InxGa1−xN epitaxial layers

Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was characteristically common to all of the samples used in the present work, and was observed to be more prominent for the samples with higher indium compositions.Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was charact...

Band discontinuity at AlxGa1−xP/GaP heterointerfaces studied by capacitance measurements

The band discontinuity at AlxGa1-xP/GaP heterointerfaces for varying Al content has been studied by capacitance-voltage measurements and self-consistent numerical analysis of the apparent carrier concentration profile by taking account of the true heterointerface position and donor deionization. By extrapolating the results for the AlxGa1-xP/GaP system, it is shown that the band lineup of the AlP/GaP heterojunction is of type II with a conduction-band discontinuity of 0.23 eV. Discussion is presented of comparison with the previously reported values.

Selective Area Growth of GaN by MOVPE and HVPE

Recent successful results on the selective area growth (SAG) of GaN that has been done by MOVPE and HVPE are shown. The SAG were carried out on MOVPE-grown GaN (0001) / sapphire substrates with lined or dotted SiO 2 masks. Sub-micron GaN dot and line structures are fabricated by the SAG in MOVPE, so that smoothly overgrown GaN layers are successfully realized using the epitaxially lateral overgrowth (ELO) technique. The ELO structures are confirmed to be good quality GaN single crystal with a smooth surface, no grain boundaries, and low-dislocation densities. In addition, thick GaN bulk single crystals without any cracks are grown by the SAG in HVPE. Crystalline and optical properties of the GaN bulk are much improved. The reduction in the thermal strain due to the growth on the limited area as well as the ELO are found to be effective to reduce crystalline defects of the GaN bulk single crystals.

Hydrogen and Nitrogen Ambient Effects on Epitaxial Lateral Overgrowth (ELO) of GaN Via Metalorganic Vapor-Phase Epitaxy (MOVPE)

Ambient gas effect on the epitaxial lateral overgrowth (ELO) of GaN via metalorganic vapor-phase epitaxy (MOVPE) on a MOVPE-grown GaN (0001) / sapphire (0001) substrate with a SiO 2 stripe mask has been studied by means of field-emission scanning electron microscopy (SEM) and high-resolution X-ray diffraction (XRD) analysis. Different ambient gases of nitrogen, hydrogen and their mixture (mixture ratio, hydrogen: nitrogen = 1: 1) affect the lateral overgrowth rate, the surface morphology and the crystalline tilting of ELO-GaN layers. XRD revealed that the ELO-GaN layer on the SiO 2 mask aligned along the 1 00> direction exhibited anisotropic crystalline tilting toward 2 0>. For ELO-GaN growth in nitrogen ambient, the growth rate of the (0001) facet decreases, the lateral overgrowth rate increases and the tilting of the ELO-GaN layer increases, while no smooth surface is obtained, in comparison with ELO-GaN growth in hydrogen ambient. For the mixture ambient, a smooth surface with a fast lateral overgrowth rate is achieved and the dislocation density is not more than 10 7 cm -2 , which is comparable to that in hydrogen ambient.