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 tan. 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/cm(2) for 10 mu m wide ridges and 4.7 kA/cm(2) for 1.5 mu m 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/cm(2]). Furthermore we observe a dependency of the COMD level from the pulse width corresponding to filament formation. (c) 2006 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

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 dislocaton 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. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

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 two–dimensional 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. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Electrically degraded GaInN/GaN-based laser ridges were analyzed by optical gain, temperature and power dependent photoluminescence, and time-resolved photoluminescence measurements and compared to nondegraded laser ridges. A decrease of the optical gain and changes in recombination times were observed for the degraded lasers. In the high carrier density regime nonradiative recombination seems to become more efficient in degraded lasers by heat generation, while in the low carrier density regime degraded laser ridges show a higher quantum efficiency than non-degraded laser ridges. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate the epitaxial lateral overgrowth (ELOG) process on silicon carbide (SiC) to obtain defect reduced gallium nitride (GaN). The large wing tilt of about 4 degrees causes bunches of edge dislocations above the edge of the ELOG mask. The PL signal of these regions is diminished and broadened and a defect correlated emission line at about 3.4 eV is detected here exclusively. The GaN above the edge shows higher tensile stress than both the wing and the window regions. In order to reduce the wing tilt and to avoid its detrimental properties, the material of the ELOG mask was changed from SiO 2 to SiN x . With the SiN x mask we were able to reduce the wing tilt by an order of magnitude to values far below 1 degree. The PL signal of the interface regions shows no sign of low quality material above the mask edge.

We study the facet degradation behavior of (Al,In)GaN multiple quantum well laser diodes. Water vapor causes a fast degradation due to facet oxidation of the uncoated facet. Degradation in an inert nitrogen atmosphere is slow and comparable to degradation of GaN laser diodes with coated facets. We also observe a reversible increase in the threshold current density due to a change in absorption caused by surface charges. (C) 2004 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107cm−2 in the wings, compared to 2×109cm−2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (μPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momentum of the μPL spectra as an effective means to measure strain distribution.