Georg Brüderl

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.

True blue InGaN laser for pico size projectors

Red, green and blue semiconductor lasers are of great interest for full color laser projection. Mobile applications require low power consumption and very small laser devices. InGaN lasers are the best choice for the blue color in applications with output power requirements below 100mW: (1) they have much higher wall plug efficiencies than conventional blue frequency doubled diode pumped solid state lasers and (2) they are more compact than semiconductor IR lasers with subsequent second harmonic generation. We present blue InGaN lasers with high efficiency at a power consumption of several 100mW. Excellent epitaxial quality permits low internal losses. Threshold current densities and slope efficiencies are further optimized by improving the facet coating. The laser threshold current is as low as 25mA and the slope efficiency reaches 1W/A. We present a wall plug efficiency of 15% at output power levels of 60mW.

Formation of nanovoids in high-dose hydrogen implanted GaN

The formation of nanovoids upon high-dose hydrogen implantation and subsequent annealing in GaN is investigated by transmission electron microscopy. The epitaxial GaN layers on sapphire were implanted at room temperature with H2+ ions at 100keV with a dose of 13×1016cm−2. Cross section transmission electron microscopy investigations revealed that nanovoids about 2nm in diameter had formed during hydrogen implantation at room temperature while large microcracks (∼150–200nm long) occurred upon annealing (1h at 700°C) leading to surface blistering. The nanovoids serve as precursors to the microcrack formation and are essential for the blistering process.

Beyond blue pico laser: development of high power blue and low power direct green

There is a big need on R&D concerning visible lasers for projection applications. The pico-size mobile projection on the one hand awaits the direct green lasers with sufficiently long lifetimes at optical powers above 50mW. In this paper we demonstrate R&D-samples emitting at 519nm with lifetimes up to 10.000 hours. The business projection on the other hand requires high power operation and already uses blue lasers and phosphor conversion, but there is a strong demand for higher power levels. We investigate the power limits of R&D laser structures. In continuous wave operation, the power is limited by thermal roll-over. With an excellent power conversion efficiency of up to 29% the thermal roll-over is as high as 2.5W for a single emitter in TO56 can. We do not observe significant leakage at high currents. Driven in short pulse operation to prevent the laser from self heating, linear laser characteristics of optical power versus electrical current are observed up to almost 8W of optical power.

Burn-in mechanism of 450nm InGaN ridge laser test structures

We investigated the short term stability of the optical output power of 450nm InGaN test lasers. The short term degradation strongly depended on ridge width. It was mainly caused by an increase in threshold current. From measurements of subthreshold wave-length blueshift, carrier lifetime, and output power, we found a decrease in carrier density after 15h of aging. We show a direct correlation of the short term aging with current spreading effects.

Longitudinal mode competition and mode clustering in (Al,In)GaN laser diodes.

Longitudinal mode competition in (Al,In)GaN laser diodes at λ = 445nm and 515 nm with mode competition frequencies from 10 MHz to 150 MHz is observed. Up to two dozen lasing modes oscillate with the lasing mode rolling from the short wavelength edge to the long wavelength edge of the gain profile. The experimental results can be described very well with a set of multi-mode rate equations including self-, symmetric and asymmetric cross gain saturation. By tuning essential parameters of the gain saturation terms, mode competition disappears and single mode operation as well as mode clustering is found. This proves that the mechanisms of gain saturation have not only a profound impact on the complex temporal-spectral behavior but also explains mode clustering in (Al,In)GaN laser diodes, both in pulsed and continuous wave (cw) operation as a natural nonlinear effect without the necessity to add noise.

Investigation of low-resistance metal contacts on p-type GaN using the linear and circular transmission line method

In this work we investigated the specific contact resistances of the different metallizations Pt, Pd, and Ni on p-type GaN. Those materials were deposited both by thermal and electron beam evaporation on LED wafer material grown on SiC by MOCVD after using a standard surface treatment. Realizing various annealing steps we were able to achieve results in the low 10 -3 Ω cm 2 range. To determine those values, TLM (transmission line method) patterns were made by photolithography technique. To proof the usability of the TLM measurements on LED wafer material a comparison of the results obtained by linear and circular test structures with different geometries is given. Furthermore, the Pt, Pd and Ni contacts were examined by temperature dependent TLM measurements to get information concerning the current transport mechanism at the p-GaN-metal interface. The experiments showed only a weak temperature dependence of the contact resistances which indicates that mainly the field emission determines the contact resistance.

Recent advances in c-plane GaN visible lasers

Blue and green InGaN-based R&D laser structures on c-plane GaN substrates are investigated. We analyzed carrier injection efficiencies as well as internal quantum efficiencies up to laser threshold. The injection efficiency of the blue laser structure is measured to be 78%. The internal quantum efficiency of spontaneous emission reaches 50% at 30A/cm2 and 32% at laser threshold. For the green laser structure we found an injection efficiency of 71%, a maximum of internal efficiency of 36% and, at laser threshold. a value of 28%. Both, recombination on defects as well as Auger effect are identified as relevant loss processes up to the laser threshold. An improved 515nm R&D single mode laser in TO56 can is presented. The optical output power of the green single mode laser reaches 250mW in continuous wave operation underneath thermal roll-over. Wall plug efficiency is as high as 9%. In the next step we investigate high power multimode lasers. The new power green R&D laser reaches maximum power of 1.25W at thermal roll-over. The currentoutput characteristic is nearly linear up to 0.9A and 0.6W. At higher currents thermal bending is observed. We measured a maximum wall plug efficiency of the green multimode laser of 13%. The power blue R&D laser in TO90 metal can reaches 5.5W prior to roll-over having the wall plug efficiency of 32% at 3.5W.

Gain analysis of blue nitride-based lasers by small signal modulation

With a small signal frequency-modulation of the driving current, the resonance frequency and the damping factor of the optical output power response of blue nitride-based ridge lasers grown on [0001]-plane gallium-nitride substrates were investigated with a network analyzer setup. From the linear dependence of the squared resonance frequency on the driving current, the gain coefficients of the logarithmic gain model could be extracted being 7680 cm−1 for blue nitride-based lasers. For this purpose, additional parameters such as the carrier density and the confinement factor were assigned by carrier lifetime and quantum efficiency measurements and one dimensional transfer matrix simulations, respectively.

Optical gain and saturation in nitride-based laser structures

We have performed systematic studies of the optical gain and its saturation in (In, Ga)N/GaN/(Al, Ga)N laser structures that depend on the excitation density and number of quantum wells. The unsaturated gain factor which was obtained by the variable stripe-length method increases with excitation power, i.e., increasing modal gain. The gain factor also increases with a decreasing number of quantum wells, as is shown by the investigation of a series of laser structures with 3, 4, 5, and 10 quantum wells for fixed modal gain. Values up to 40 dB at 300 K were measured. Thermal activation energies obtained by temperature dependent photoluminescence measurements yield information on the influence of nonradiative recombination processes on optical gain saturation.