A microscopic theory is used to analyze optical gain in InGaN∕GaN quantum wells (QW). Experimental data are obtained from Hakki–Paoli measurements on edge-emitting lasers for different carrier densities. The simulations are based on the solution of the quantum kinetic Maxwell–Bloch equations, including many-body effects and a self-consistent treatment of piezoelectric fields. The results confirm the validity of a QW gain description for this material system with a substantial inhomogeneous broadening due to structural variation. They also give an estimate of the nonradiative recombination rate.

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 this work we present degradation studies of GaN based blue‐violet laser diodes grown on different substrates. By replacing the SiC substrate by GaN substrate, we change from hetero to homo epitaxy, while the threading dislocation density (TDD) is reduced by 3 orders of magnitude. A detailed analysis of small signal I –V curves shows an increase of non radiative (NR) recombination centers during aging for laser diodes on SiC substrate. This was not observed for lasers on GaN substrate due to the reduced TDD and therefore reduced number of diffusion channels for Mg‐atoms, acting as NR recombination centers in the active region. With an improved epitaxial structure on GaN substrate, we increased the lifetime of our lasers by a factor of 10. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the electrical and optical characteristics of (AlGaIn)N multiple quantum well light-emitting diodes. Minimizing contact effects by utilizing platinum as p-contact metal, ideality factors as low as 1.1 have been achieved. In agreement with basic semiconductor theory, a correlation between ideality factor and small-current efficiency was found. We were able to emulate the experimental current-voltage characteristic over seven orders of magnitude utilizing a two diode model. This model enables a very good prediction of internal quantum efficiency at moderate current densities out of purely electrically derived parameters.

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We present a process for high quality InGaN laser diodes on free standing GaN substrates with high reproducibility. We demonstrate a self‐aligned process to contact lasers with ridge width <2 μm. The stability of the process allows us to optimize the epitaxial structure on GaN substrates and we achieve cw‐threshold current densities of 2.9 kA/cm2 for 10 μm wide ridges and 4.7 kA/cm2 for 1.5 μm wide ridges. Cw‐slope efficiencies of 1.0 W/A are achieved for both widths. Beside this we notice a reduction of the ideality factor and the forward bias linked to an improvement of the epitaxial interfaces. For maximum optical output we could achieve 3.4 W from one output facet at pulsed operation and a catastrophic optical mirror damage (COMD) level of 67 MW/cm2. Furthermore we observe a dependency of the COMD level from the pulse width corresponding to filament formation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

PACS 42.55.Px, 42.60.Lh, 78.45.+h Height and width of the ridge forming the laser diode waveguide determine threshold current density and lateral mode stability. We measure the optical near-field of the laser mode and simulate the twodimensional mode distribution including waveguide losses and optical gain. The simulations show that weak guiding and not current spreading is the major cause for increased threshold current densities in weakly guided laser diodes. The near-field measurements show fundamental and higher order modes for nominally identical ridge laser diodes. We demonstrate that asymmetric losses in the waveguide bias the lateral mode competition towards higher order modes for an intermediate range of index guiding. This asymmetric damping is specific for (Al,In)GaN laser diodes due to absorption introduced in the p-waveguide by magnesium doping. This competition of lateral modes can also be seen spectrally as two longitudinal mode combs of different optical gain.