V. Kümmler

Facet degradation of GaN heterostructure laser diodes

We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation.

Optical gain, carrier-induced phase shift, and linewidth enhancement factor in InGaN quantum well lasers

Adapting the Hakki Paoli method to group III nitrides, we measure gain, differential gain, carrier-induced change of refractive index, carrier-induced phase shift, and the antiguiding factor. Our measurements also cover the low-carrier-density regime, in which spontaneous and piezoelectric fields and Coulomb interaction are only partially screened. This regime is most interesting as a comparison with existing theoretical simulations, including many-body effects.

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.

Degradation Analysis of InGaN Laser Diodes

The current status of InGaN-MQW-laser diodes developed at Osram OS is presented. These lasers are grown on n-conducting SiC, enabling a vertical current path, cleaved facets and excellent heat spreading. The temperature rise during cw operation is measured for different mountings. A p-side up mounted diode with thermal resistance of 18 K/W showed 143 h of cw lasing at 1 mW optical power (T = 25 °C). DC and pulsed aging shows current as main degradation reason compared to heat for InGaN-LDs. Photoluminescence spectra of the quantum wells are being compared before and after degradation caused by current.

Excitonic signature in gain and carrier induced change of refractive index spectra of (In,Al)GaN quantum well lasers

The exciton is observed in (In,Al)GaN laser diodes as resonance in the optical gain spectra and in the spectra of the carrier induced change of the refractive index. The observed instability of the exciton with respect to the free electron-hole plasma with increasing carrier densities is accompanied by a blueshift of the exciton resonance due to the quantum confined Stark shift. The experiments confirm central points of many-body simulations of InGaN∕GaN quantum wells. The exciton becomes unstable near threshold and so lasing occurs from the electron–hole plasma.

Gradual facet degradation of (Al,In)GaN quantum well lasers

In our study, III-nitride laser diodes with uncoated facets obtained by cleavage show a much faster degradation than coated ones. An increase in threshold current and drop of slope efficiency suggest increased absorption losses. Degradation experiments in different atmospheres prove the influence of the respective atmosphere and indicate the growth of an oxide film leading to increased absorption. Because the observed degradation is insensitive to the photon density we suggest nonradiative centers, which are saturated at low photon densities, to be at the origin of degradation. No evidence for photon enhanced degradation of coated laser diodes was found. A dielectric coating efficiently protects the facets.

Influence of the transverse and lateral waveguide on the far field pattern in GaN based laser structures

The waveguiding properties of nitride laser diodes are investigated by calculating the optical field inside and by measuring the far field pattern behind the structures. The layer design of the diodes leads to a zeroth order TE mode in transverse direction with an optical confinement factor of Γ = 0.027. The far field pattern behind the samples is rather streaky. Higher-order modes guide the intensity due to the fact that the lateral waveguide is too wide. The interdependence between the density of stripes in the far field pattern and the ridge width is analysed.

Laser Diode Facet Degradation Study

We study the degradation behaviour of GaN gain guided laser diodes (LDs) on SiC substrates with cleaved facets and reflective coatings on none, one, or both facets. This allows us to demonstrate that in addition to volume effects there is a contribution of the laser facets to laser degradation. We observe that for the uncoated LDs the threshold current density is increasing considerably faster compared to LDs with mirror coatings. Degradation is observed during operation but not during storage at ambient conditions and thus expected to be photon or current induced. Operation of the uncoated laser in a nitrogen atmosphere reduces the degradation rate with respect to operation in air.