Dimitri Dini

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.

Recent results of blue and green InGaN laser diodes for laser projection

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E0=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C.

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%.

Gain analysis of blue nitride-based lasers by small signal modulation

With a small signal frequency-modulation of the driving current, the resonance frequency and the damping factor of the optical output power response of blue nitride-based ridge lasers grown on [0001]-plane gallium-nitride substrates were investigated with a network analyzer setup. From the linear dependence of the squared resonance frequency on the driving current, the gain coefficients of the logarithmic gain model could be extracted being 7680 cm−1 for blue nitride-based lasers. For this purpose, additional parameters such as the carrier density and the confinement factor were assigned by carrier lifetime and quantum efficiency measurements and one dimensional transfer matrix simulations, respectively.

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.

An industry perspective on the optimization of InGaN lasers and LEDs via modeling

In this paper, we illustrate the role of modeling in the development of commercial nitride-based lasers and LEDs. Aside from optimizing device performance, joint analysis of simulations and experimental results can shed light into the intrinsic properties of the InGaN/GaN material system.