J. H. Chae

Measurement of junction temperature in GaN-based laser diodes using voltage-temperature characteristics

We present a method to determine junction temperature in GaN-based laser diodes (LDs) for simple, fast, and reliable characterization of thermal properties. The large change of forward operation voltage with temperature in GaN laser diodes is advantageously used to measure junction temperature. Using this method, we compare junction temperature of LD structures with different substrates and chip mounting methods. It is found that the junction temperature can be reduced considerably by employing GaN substrates or epi-down bonding. For epi-down bonded LDs, as much as two-fold reduction in junction temperature is achieved compared to epi-up bonded ones and junction temperature rise in this case is only about 13 degrees for more than 100 mW-output power.

Highly stable temperature characteristics of InGaN blue laser diodes

We report stable temperature characteristics of threshold current and output power in InGaN blue laser diodes emitting around 450nm. The threshold current is changed by <3mA in operation temperature range from 20to80°C, and even negative characteristic temperature is observed in a certain temperature range. This peculiar temperature characteristic is attributed to originate from unique carrier transport properties of InGaN quantum wells with high In composition, which is deduced from the simulation of carrier density and optical gain. In addition, slope efficiency is also maintained well and wall plug efficiency is even improved as temperature increases.

Evaluation of radiative efficiency in InGaN blue-violet laser-diode structures using electroluminescence characteristics

The authors analyzed radiative efficiency of InGaN laser diodes (LDs) emitting at 405nm. Based on semiconductor rate equations, the radiative efficiency is unambiguously determined by the analysis of electroluminescence characteristics. The radiative efficiency exceeds 70% even far below threshold of ∼3mA at a high temperature of 80°C. This highly radiative characteristic is attributed to reduced contribution of nonradiative recombination in LDs with low-dislocation-density active material. It is also found that the radiative efficiency is almost independent of threshold current, indicating that nonradiative recombination is not a major factor which determines lasing threshold in 405nm emitting InGaN LDs having low dislocation density.

Single-mode blue-violet laser diodes with low beam divergence and high COD level

We demonstrate GaN-based high-power single transverse-mode laser diodes (LDs) emitting at 405 nm. LD structures are designed to exhibit a high level of catastrophic optical damage and small beam divergence angle. By the control of refractive index profiles, we achieved a vertical beam divergence angle of as low as 17.5/spl deg/ and maximum output power of as high as 470 mW under continuous-wave operation condition. In addition, nearly fundamental transverse-mode operation is demonstrated up to 500-mW pulsed output power by far-field investigation.

High power AlInGaN-based blue-violet laser diodes

High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.

Phosphor concentration and geometry for high power white light emitting diode

The most common method of making white light emitting diode (LED) is to mix the blue light from the LED die and the wavelength converted yellow light from the phosphor layer. The color conversion efficiency depends on the geometry and concentration of the phosphor layer including phosphor material. Thus the optimization of the phosphor geometry and concentration make increase the luminous efficiency of the white LED. In this paper, the remote phosphor scheme is optimized focusing on increasing the luminous efficiency in high power. The phosphor layer is separated by the silicone resin from the LED die. The silicone resin covers the LED die with dome shape to increase the extraction efficiency. The phosphor layer has very large volume with dilute concentration. The separation of phosphor layer from LED die and very large volumetric dilute phosphor layer were great important role in increasing the luminous flux. The improved luminous flux was 15% for 1mm2 LED die at 700mA.

Recent progress of high-power InGaN blue-violet laser diodes

We report on the development of GaN-based violet laser diodes (LDs) for the high-capacity optical storage application and blue LDs for the laser projection display application. InGaN LDs with emission wavelength of ~405 nm are already being adopted for next-generation optical-storage systems. We present results on >400 mW single-mode output power under pulsed operation which can be employed in 100 Gbyte multi-layer BD systems. We designed LD layer structures to exhibit high level of catastrophic optical damage (COD) and small beam divergence. In addition, GaN-based blue LDs with emission wavelength of ~450 nm have also been developed for the application to the blue light sources of laser display systems. We demonstrate single-mode blue InGaN LDs with >100 mW CW output power. Interestingly, we observed anomalous temperature characteristics from the blue InGaN LDs, which has shown highly-stable temperature dependence of output power or even negative characteristic temperature (T0) in a certain operation temperature range. This unusual temperature characteristic is attributed to originate from unique carrier transport properties of InGaN QWs with high In composition, which is deduced from the simulation of carrier density and optical gain.

Increase of output power and lifetime by improving the heat dissipation of GaN-based laser diodes

The enhanced output power with improved lifetime is required for the GaN-based blue-violet laser diode (LD) as a light source for Blu-ray Disc or HD-DVD. In this paper, the output power levels and aging behaviors in GaN-based LDs grown on sapphire substrates were compared in epi-up and epi-down bonding. At low current level, the two bondings show little differences in L-I characteristics. At high current level, however, the epi-up bonding shows a rapidly decreased slope efficiency in L-I characteristics with increasing current injection. On the contrary, the slope efficiency in epi-down bonding is not so much deteriorating as that in epi-up bonding. The differences in junction temperature between epi-up and epi-down bonding are large at higher current levels. The junction temperature of epi-up bonding is about two times higher than that of epi-down bonding, implying efficient heat dissipation in epi-down bonding. At aging test, the epi-down bonding LD shows lower degradation rate at the aging slope than that of epi-up bonding LD. The degradation rate is accelerated by poor heat dissipation in epi-up bonding. Thus, for the higher power and longer lifetime, it is necessary to employ efficient heat dissipation structures such as epi-down bonding for the GaN-based LD on sapphire substrate.

Waveguide structural effect on ripples of far-field pattern in 405-nm GaN-based laser diodes

We investigated the dependency of waveguide structures on ripples of far-field patterns in 405nm GaN-based laser diodes theoretically and experimentally. As the n-type cladding layer thickness decreases, the passive waveguide modes strongly interact with an active layer mode. This suggests that the thicknesses of n-AlGaN/GaN superlattice clad and n-GaN waveguide layers have significant influences on FFP ripples. We successfully obtained very smooth far-field patterns perpendicular to the junction plane by optimizing both n-AlGaN/GaN clad layer thickness and n-GaN waveguide layer thickness.

High-power blue-violet laser diodes with improved beam divergence and high COD level

GaN-based laser diodes for 405 nm high-power applications are demonstrated. By decreasing the Al concentration of n-cladding layers, the vertical divergence angle was reduced to <18/spl deg/ and the average COD level was increase to >300 mW.