We report on blue and green light-emitting-diodes (LEDs) grown on (11-22)-GaN templates. The templates were created
by overgrowth on structured r-plane sapphire substrates. Low defect density, 100 mm diameter GaN templates were
obtained by metal organic vapour phase epitaxy (VPE) and hydride VPE techniques. Chemical-mechanical polishing
was used to obtain smooth surfaces for the subsequent growth of LED structures. Ohmic contacts to the p-type GaN
were obtained despite the lower activated acceptor levels. The LEDs show excellent output power and fast carrier
dynamics. Freestanding LEDs have been obtained by use of laser-lift-off. The work is the result of collaboration under
the European Union funded ALIGHT project.
KEYWORDS: Waveguides, Refractive index, Semiconductor lasers, Temperature metrology, Modes of laser operation, Near field optics, Near field scanning optical microscopy, Optical simulations, Thermal effects, Superposition
For broad ridge (Al,In)GaN laser diodes, which are inevitable for high output power applications in the UV
and blue spectral range, filaments or higher order lateral modes build p, which influence the far-field beam
quality. We investigate the lateral profile of the optical laser mode in the waveguide experimentally by temporal
and spectral resolved scanning near-field optical microscopy measurements on electrically pulsed driven laser
diodes and compare these results with one-dimensional simulations of the lateral laser mode in the waveguide.
We present a model that describes the optical mode profile as a superposition of different lateral modes in a
refractive index profile which is modified by carrier- and
thermal-induced effects. In this way the mode dynamics on a nanosecond to microsecond time scale can be explained by thermal effects.
We investigate two types of 405 nm (In, Al)GaN test laser structures (TLSs), one of them grown on SiC substrates,
the other grown on low dislocation density freestanding GaN substrates. Measuring the lasing spectra of these
structures, we observe an individual behavior depending on the substrate. TLSs on GaN substrates show a
broad longitudinal mode spectrum above threshold, whereas TLSs on SiC are lasing only on one mode with
various jumps of the laser emission at certain currents. Estimating the gain of each longitudinal mode with the
Hakki-Paoli method, we find minute variations of the gain for TLSs on GaN substrate. In contrary, TLSs on
SiC substrate show much larger fluctuations of the gain for individual longitudinal modes. Using a rate equation
model with nonlinear gain effects, we simulate the longitudinal mode spectrum of both types of TLSs. Once we
modify the gain of each longitudinal mode as observed in the gain measurements, the simulated spectra resemble
the SiC or GaN substrate TLS spectra.
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