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.

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.

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.

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.

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.

The carrier-induced refractive index change and the linewidth enhancement factor a are investigated by measuring the optical gain in an InGaN/GaN laser structure. Using the Kramers-Kronig relation we determined the maximum refractive index change to -0.0044 for a carrier density four times as large as the transparency carrier density. Though the change is higher than in conventional III-V structures, its influence on the waveguiding properties of the laser structure is small. The alpha factor varies between 2.1 and 6.3 in the investigated range of the carrier density and is thus comparable to that known from InGaAs/GaAs quantum well structures. (C) 2003 WILEY-VCH V Hag GmbH & Co. KGaA, Weinheim

Adapting the Hakki-Paoli method for blue laser diodes we measure gain spectra for (In/Al)GaN on SiC substrate laser diodes in the low carrier density regime. From the measured longitudinal mode spacing we calculate the effective refractive index and can exclude a mode spacing 'anomaly.' We demonstrate that the substrate has a considerable influence on the modal gain. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

The waveguiding properties of nitride laser diodes are investigated for different Al concentrations in the cladding layers by calculating the optical field inside and by measuring the far field pattern behind the structures. The thicknesses of the p-cladding layer and of the electron blocking layer are also taken into account. The best optical confinement is reached for 10% and 12% Al in the n- and p-cladding layer, respectively. For these Al concentrations the full width at half maximum of the intensity distribution in the far field is increased by 15%. To prevent the leaking of the optical mode into the p-GaN cap layer the p-cladding layer should not be thinner than 270 nm.

Optical gain and absorption of different laser-structures were measured and calculated in order to clarify the influence of internal fields on these properties. For the calculations a model is used that is based on the assumption of hand-to-band transitions and includes the effect of piezoelectric fields. The experimentally found energy shifts between absorption and the onset of gain could be explained only through the presence of an internal field. The minimal threshold current densities for a double quantum well laser and for a well thickness of 2 nm were confirmed.

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.