Uwe Strauß

500 nm electrically driven InGaN based laser diodes

Based on recent improvements of growth of In-rich InGaN quantum wells with low defect density, we demonstrate current driven InGaN laser diodes at wavelengths as long as 500 nm. The laser structures are grown on c-plane GaN substrate and are processed as broad oxide-insulated stripe laser diodes. We discuss the impact of the piezoelectric field on the emission energy of long wavelength laser diodes for this growth orientation. The combination of low threshold current density of 8.2 kA/cm2 with high slope efficiency of 650 mW/A enables high output powers up to several tens of milliwatts.

InGaN laser diodes with 50 mW output power emitting at 515 nm

We demonstrate direct green laser operation from InGaN based diodes at wavelengths as long as 515.9 nm with 50 mW output power in pulse operation. A factor of ∼10 defect reduction for the In-rich InGaN quantum wells based on improvements of the epitaxial growth process and design of the active layers on c-plane GaN-substrates makes it possible to demonstrate laser operation at room temperature. Micrometer-scale photoluminescence mappings and electro-optical measurements confirm the reduction of nonradiative defects in the emitting layers. The 11 μm broad-area gain-guided laser structures were driven in pulse operation to minimize thermal effects and to accurately measure the laser temperature dependence. The threshold current density was ∼9 kA/cm2 and the fitted slope efficiency had a value of ∼130 mW/A for an optical output up to 50 mW.

Lateral and longitudinal mode pattern of broad ridge 405nm (Al, In)GaN laser diodes

The lateral mode profile of pulsed broad ridge 405nm (Al, In)GaN laser diodes grown on GaN and SiC substrates, respectively, is investigated by temporal and spectral resolved scanning near-field optical microscopy. During the first microsecond of the pulse, we observe changes both in the spatial mode profile and in the spectral regime caused by thermal and carrier induced modification of the waveguide refractive index, before stable filaments build up. In quasi-cw operation, a correlation between the lateral mode profile and the corresponding spatial resolved longitudinal mode pattern can be found. The results show that different filaments have different effective refractive indices and thus build up separate longitudinal mode combs.The lateral mode profile of pulsed broad ridge 405nm (Al, In)GaN laser diodes grown on GaN and SiC substrates, respectively, is investigated by temporal and spectral resolved scanning near-field optical microscopy. During the first microsecond of the pulse, we observe changes both in the spatial mode profile and in the spectral regime caused by thermal and carrier induced modification of the waveguide refractive index, before stable filaments build up. In quasi-cw operation, a correlation between the lateral mode profile and the corresponding spatial resolved longitudinal mode pattern can be found. The results show that different filaments have different effective refractive indices and thus build up separate longitudinal mode combs.

Failure mechanisms of gallium nitride LEDs related with passivation

This paper analyzes the thermally-activated failure mechanisms of GaN LEDs under thermal overstress related with the presence of a PECVD SiN passivation layer. It is shown that the properties of the passivation layer can remarkably affect devices reliability during high-temperature stress: degradation mechanisms identified consist in emission crowding and series resistance increase, attributed to the thermally-activated indiffusion of hydrogen from the passivation to the p-layer, and subsequent p-doping compensation

Scaling brilliance of high power laser diodes

New direct diode laser systems and fiber lasers require brilliant fiber coupled laser diodes for efficient operation. In the German funded project HEMILAS different laser bar designs are investigated with tailored beam parameter products adapted for efficient fiber coupling. In this paper we demonstrate results on 9xx and 1020nm bars suitable for coupling into 200μm fibers. With special facet technology and optimised epitaxial structure COD-free laser bars were fabricated with maximum efficiency above 66%. For short bars consisting of five 100μm wide emitters 75W CW maximum output power was reached. In QCW-mode up to 140W are demonstrated. The 10% fill factor bars with 4mm cavity are mounted with hard solder. Lifetime tests in long pulse mode with 35W output power exceed 5000 hours of testing without degradation or spontaneous failures. Slow axis divergence stays below 7° up to power levels of 40W and is suitable for simple fiber coupling into 200μm NA 0.22 fibers with SAC and FAC lenses. For fiber coupling based on beam rearrangement with step mirrors, bars with higher fill factor of 50% were fabricated and tested. The 4mm cavity short bars reach efficiencies above 60%. Lifetime tests at accelerated powers were performed. Finally fiber coupling results with output powers of up to 2.4 kW and beam quality of 30 mm mrad are demonstrated.

Temperature dependence of blue InGaN lasers

True blue lasers with wavelengths of ~450 nm are of great interest for full color laser projection. These kind of applications usually require high output power and, in particular, an excellent wall plug efficiency within a wide temperature range. In this paper we therefore present experimental and theoretical investigations of the temperature behavior of 60mW InGaN lasers in a range of -10 °C to 100 °C. The laser parameters threshold current density, slope efficiency and operating voltage describe the wall plug efficiency of the device. The slope efficiency does not show any significant temperature dependence which is due to an almost temperature independent injection efficiency in the temperature range that is of interest for most commercial applications. In contrast, the laser threshold current density increases with temperature and we determine a characteristic temperature T0 of about 141K for our devices emitting at 445nm. This increasing threshold current density can be explained by lower gain of the quantum wells at higher temperature. Furthermore, Auger recombination influences the threshold as verified by simulations. The second electro-optical parameter is the electrical voltage, which is dominated by electrical barriers. The voltage decreases with increasing temperature and compensates the increasing threshold current resulting in a nearly constant high wall plug efficiency of 13% between -10°C and 100°C.

Temperature and current dependence of the optical intensity and energy shift in blue InGaN-based light-emitting diodes: comparison between electroluminescence and cathodoluminescence

Optical spectra of InGaN-based multiple quantum well test structures have been measured by complementary techniques: electroluminescence (EL) and cathodoluminescence (CL). A strong temperature and current dependence of the peak energy is found, highlighting the existence of a distribution of InGaN localized states, with a broadening parameter ranging between 29 and 18 meV, and the presence of internal fields screened with different efficiencies by means of the external excitations. The effects of the different injection mechanisms, mainly with (EL) or without (CL) an external bias, are underlined also by the evolution of the emission intensity with temperature. In CL, it shows a continuous decay versus T, characterized by estimated activation energies of 26 ± 14 meV and 136 ± 20 meV, due to carrier thermal escape processes. In EL, a non-monotonic temperature dependence at a fixed current is revealed. The existence of an intensity maximum and its temperature position, increasing from 120 to 180 K for currents ranging from 0.05 to 2 mA, are discussed.

Measurement and simulation of the lateral mode profile of broad ridge 405 nm (Al,In)GaN laser diodes

For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the UV and blue spectral range, filaments or higher order lateral modes build p, which influence the far-field beam quality. We investigate the lateral profile of the optical laser mode in the waveguide experimentally by temporal and spectral resolved scanning near-field optical microscopy measurements on electrically pulsed driven laser diodes and compare these results with one-dimensional simulations of the lateral laser mode in the waveguide. We present a model that describes the optical mode profile as a superposition of different lateral modes in a refractive index profile which is modified by carrier- and thermal-induced effects. In this way the mode dynamics on a nanosecond to microsecond time scale can be explained by thermal effects.

Power blue and green laser diodes and their applications

InGaN based green laser diodes with output powers up to 50mW are now well established for variety of applications ranging from leveling to special lighting effects and mobile projection of 12lm brightness. In future the highest market potential for visible single mode profile lasers might be laser projection of 20lm. Therefore direct green single-mode laser diodes with higher power are required. We found that self heating was the limiting factor for higher current operation. We present power-current characteristics of improved R and D samples with up to 200mW in cw-operation. An optical output power of 100mW is reached at 215mA, a current level which is suitable for long term operation. Blue InGaN laser diodes are also the ideal source for phosphor based generation of green light sources of high luminance. We present a light engine based on LARP (Laser Activated Remote Phosphor) which can be used in business projectors of several thousand lumens on screen. We discuss the advantages of a laser based systems in comparison with LED light engines. LARP requires highly efficient blue power laser diodes with output power above 1W. Future market penetration of LARP will require lower costs. Therefore we studied new designs for higher powers levels. R and D chips with power-current characteristics up to 4W in continuous wave operation on C-mount at 25°C are presented.

Recent advances in VECSELs for laser projection applications

Laser projectors integrated in portable devices offer a new platform for media display but put strong demands on the laser sources in terms of efficiency, modulation band width, operating temperature range and device cost. Osram Opto Semiconductors has developed and produces synthetic green lasers for projection applications on which the latest results are reported. Based on vertical external cavity surface emitting laser (VECSEL) technology and second harmonic generation an output power of >75mW has been achieved. The maximum output power is to a large extent limited by the high thermal resistance of the monolithic VECSEL chip used. To overcome the thermal limitations a new thinfilm VECSEL chip design is proposed where the epitaxial layers are transferred to a silicon carrier and processed on wafer level, thus significantly lowering the thermal resistance and improving the maximum output power.