Christoph Eichler

True Green Laser Diodes at 524 nm with 50 mW Continuous Wave Output Power on c-Plane GaN

We pushed direct green laser diodes towards longer wavelengths at 524–532 nm based on improvements of epitaxial design and material quality on c-plane GaN substrate. Mounted ridge laser diodes show significant performance improvement in cw operation. For 524 nm laser, wall plug efficiency up to 2.3% at 50 mW optical output power is achieved. In pulse mode operation we demonstrate broad-area test lasers with an emission wavelength of 531.7 nm. Nonpolar and polar substrates are compared with respect to indium content in InGaN quantum wells. The limiting factors for achieving longer wavelengths and better performance of green lasers are discussed from this viewpoint.

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.

Recent results of blue and green InGaN laser diodes for laser projection

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E0=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C.

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.

Near-field and far-field dynamics of (Al,In)GaN laser diodes

Near- and far-field dynamics of edge-emitting (Al,In)GaN laser diodes are measured simultaneously with a 100 nm spatial and a 5 ns temporal resolution using a scanning near-field microscope. We reconstruct the phase distribution at the laser diode facet. Beam steering and near-field mode dynamics are interpreted in terms of thermal and carrier induced change of refractive index in the waveguide.

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.

Blue Superluminescent Light-Emitting Diodes with Output Power above 100 mW for Picoprojection

We present a blue InGaN research and development superluminescent light-emitting diode (SLED) that is suitable for picoprojection. The SLED reaches an output power of >100 mW with a peak wavelength of 443 nm and a spectral bandwidth of >2.6 nm as well as a single-mode far-field driven in cw mode at 25 °C. In order to figure out an optimized waveguide design, which enables such a high output power at lowest operation current, we compare the performance of diodes with curved and tilted shaped ridges in detail, using the lasing threshold current as a criterion for lasing or superluminescence, respectively.

Cyan Superluminescent Light-Emitting Diode Based on InGaN Quantum Wells

An InGaN superluminescent light-emitting diode (SLED) with emission as long as 500 nm is presented. Up to now, an SLED with such a long wavelength was hindered, because the gain of indium-rich layers was not sufficient for operating in superluminescence mode. We used an epitaxial structure with high material gain as well as a special chip design of curved waveguide to solve this problem. The SLED reached an output power of >4 mW with a spectral bandwidth of 4.4 nm driven in pulsed mode.

Absorption and light scattering in InGaN-on-sapphire- and AlGaInP-based light-emitting diodes

Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm/sup -1/ for the scattering model and 4 cm/sup -1/ for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness (d=75 nm) and its absorption coefficient (/spl alpha/ /spl ap/ 3900 cm/sup -1/). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of /spl alpha/=30 cm/sup -1/ includes a contribution from the absorbing substrate.