Marc Schillgalies

500 nm electrically driven InGaN based laser diodes

Based on recent improvements of growth of In-rich InGaN quantum wells with low defect density, we demonstrate current driven InGaN laser diodes at wavelengths as long as 500 nm. The laser structures are grown on c-plane GaN substrate and are processed as broad oxide-insulated stripe laser diodes. We discuss the impact of the piezoelectric field on the emission energy of long wavelength laser diodes for this growth orientation. The combination of low threshold current density of 8.2 kA/cm2 with high slope efficiency of 650 mW/A enables high output powers up to several tens of milliwatts.

Substrate Modes of (Al,In)GaN Semiconductor Laser Diodes on SiC and GaN Substrates

In semiconductor laser diodes layers with high refractive index can act as parasitic waveguides and cause severe losses to the optical mode propagating in the longitudinal direction. For (Al,In)GaN laser diodes, the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers, hence the name substrate modes. A set of four different experiments shows the effect of substrate modes in the near-field (the most direct evidence of substrate modes), as side lobes in far-field, oscillations of the optical gain spectra, and as dependency of threshold current on n-cladding thickness. We derive several basic properties of the substrate modes by simple estimates. For a quantitative analysis we employ a 2-D finite element electromagnetic simulation tool. We simulate periodic variations in the cavity gain spectrum that explain the measurements in terms of absolute value and oscillation amplitude. We show that it is necessary to include the refractive index dispersion in order to get the correct period of the gain oscillations. Furthermore, we use the simulations to optimize the laser diode design with respect to substrate mode losses within the constraints given, e.g., by growth conditions

True blue InGaN laser for pico size projectors

Red, green and blue semiconductor lasers are of great interest for full color laser projection. Mobile applications require low power consumption and very small laser devices. InGaN lasers are the best choice for the blue color in applications with output power requirements below 100mW: (1) they have much higher wall plug efficiencies than conventional blue frequency doubled diode pumped solid state lasers and (2) they are more compact than semiconductor IR lasers with subsequent second harmonic generation. We present blue InGaN lasers with high efficiency at a power consumption of several 100mW. Excellent epitaxial quality permits low internal losses. Threshold current densities and slope efficiencies are further optimized by improving the facet coating. The laser threshold current is as low as 25mA and the slope efficiency reaches 1W/A. We present a wall plug efficiency of 15% at output power levels of 60mW.

Spectral dynamics of 405 nm (Al,In)GaN laser diodes grown on GaN and SiC substrate

We investigate the spectral properties of violet 405 nm (Al,In)GaN laser diodes (LDs). Depending on the substrate the LDs are grown on, the lasing spectra show significant differences. LDs grown on low dislocation GaN substrate have a broad spectrum with several longitudinal modes, while LDs grown on SiC substrate are lasing on a single longitudinal mode.With increasing current, the laser emission of LDs grown on SiC substrate jumps from one longitudinal mode to another (mode hopping), whereas GaN substrate LDs show a smooth but asymmetric mode comb. The different envelopes of these spectra can be understood by assuming a variation of the gain for each individual longitudinal mode. With a high spectral resolution setup, we measure the gain of each longitudinal mode, employing the Hakki-Paoli method. Measurements show a slightly fluctuating gain for the modes of GaN substrate LDs, but much larger fluctuations for LDs on SiC substrate. We carry out simulations of the longitudinal mode spectrum of (Al,In)GaN laser diodes using a rate equation model with nonlinear gain (self saturation, symmetric and asymmetric cross saturation) and including gain fluctuations. With a set of parameters which is largely identical for LDs on either substrate, the simulated spectra truly resemble those typical for LDs on GaN or SiC substrate.

Burn-in mechanism of 450nm InGaN ridge laser test structures

We investigated the short term stability of the optical output power of 450nm InGaN test lasers. The short term degradation strongly depended on ridge width. It was mainly caused by an increase in threshold current. From measurements of subthreshold wave-length blueshift, carrier lifetime, and output power, we found a decrease in carrier density after 15h of aging. We show a direct correlation of the short term aging with current spreading effects.

Progress of blue and green InGaN laser diodes

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and wall plug efficiency. We report improvements of the performance of true blue single mode InGaN laser at 450nm with output power of more than 200mW in cw operation for temperatures between 20°C and 60°C. We succeeded in temperature independent high wall plug efficiency of 15-18% for stable output power levels from 100 to 200mW with estimated life times >4000h in cw operation. Furthermore, we present pioneering studies on long green InGaN laser diodes. The technological challenge is to achieve InGaN-quantum wells with sufficiently high material quality for lasing. We investigate the density of non radiative defects by electro-optical measurements confirming that low defect densities are essential for stimulated emission. Laser operation at 516nm with more than 50mW output power in cw operation is demonstrated in combination with a high wall plug efficiency of 2.7%.

Measurement and simulation of the lateral mode profile of broad ridge 405 nm (Al,In)GaN laser diodes

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.

Time-resolved scanning near-field microscopy of InGaN laser diode dynamics

We combine a scanning near-field microscope (SNOM) with a time-resolved detection scheme to measure the mode dynamics of InGaN laser diodes emitting at 405 nm. Observed phenomena are filaments, mode competition, near-field phase dynamics, near-field to far-field propagation, and substrate modes. In this article we describe in detail the self-built SNOM, specialized for these studies. We also provide our recipe for SNOM tip preparation using tube etching. Then we compare the mode dynamics for a 3 μm narrow and a 10 μm wide ridge waveguide laser diode.

Spectral measurements and simulations of 405 nm (Al, In)GaN test laser structures grown on SiC and GaN substrate

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.

Analytical methods to study loss mechanisms and lifetime investigations of blue InGaN laser diodes

We present analytical methods to study the physical mechanisms of the optical output stability of blue InGaN laser diodes in short and long-term stress experiments. Lasers of three ridge widths were stressed under constant current condition at a constant current density. The output power of a 1.8μm ridge laser decreased down to 40% within 15h mainly due to an increase in threshold current. Broader ridges showed a more stable output power. The decline in output power was related to changes in quantum efficiency and longer carrier lifetimes after stress. A simple recombination model was fitted to the measurements indicating no increase in non-radiative recombination centers. Instead the longer carrier lifetimes could be well explained with a decrease in carrier density due to an additional current spreading. These results were confirmed by changes in the sub-threshold wavelength shift before and after aging.