Proc. SPIE. 6997, Semiconductor Lasers and Laser Dynamics III
KEYWORDS: Superposition, Refractive index, Waveguides, Semiconductor lasers, Near field scanning optical microscopy, Thermal effects, Optical simulations, Modes of laser operation, Temperature metrology, Near field optics
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.
The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell
heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting
single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are
responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by
growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light
emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane
grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE
measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the
internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature
and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously.
Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly
influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the
radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency.
The measurement of the bias and temperature dependent photoluminescence, photocurrent and their decay times allows
to deduce important physical properties such as barrier height, electron-hole overlap and the magnitude of the
piezoelectric field in InGaN quantum wells. However the analysis of these experiments demands for a detailed physical
model based on a realistic device structure which is able to predict the measured quantities. In this work a selfconsistent
model is presented based on a realistic description of the alloy and doping profile of a green InGaN single
quantum well light emitting diode. The model succeeds in the quantitative prediction of the quantum confined Stark
shift and the associated change in the electron-hole overlap measured via the change in the bimolecular decay rate using
literature parameters for the piezoelectric constants. The blue shift of the emission under forward current conditions can
be attributed to the carrier induced screening of the piezoelectric charges as predicted by the model. The photocurrent is
calculated via thermionic tunneling through the barriers using a WKB-approximation and the calculated potential profile
for the tunneling barrier. From the fact that the bias and temperature dependence of the experimentally observed
photocurrent cannot be described by the thermionic tunneling model even though the theoretical potential profile fits
excellent to the luminescence data, we conclude that the carrier escape is dominated by a different mechanism such as
defect- or phonon-assisted tunneling.