Light emitting diodes (LEDs) are set for a brilliant future: it is forecasted that they will one day replace all other lighting devices because they consume less power. For LEDs emitting in the near UV, the blue and the green, the active material is gallium nitride (GaN). One could increase the efficiency of these LEDs by choosing an original growth geometry. This is the path followed by researchers at our laboratory.

The Ga-N bond is strongly ionic. Some facets of the GaN crystal are thus more or less polar. The usual growth orientation of GaN is a polar direction. When grown this way, quantum wells – which are thin semiconductor layers (here GaN) embedded in another semiconductor (here AlN) – are subject to a large internal electric field. One consequence of this is a large reduction in the radiating recombination rate, which can limit device performances. These effects are attenuated if the growth follows a non-polar direction. This is a simple idea, but it is not that easy to produce: in that case, the grown layers contain a large quantity of defects. We have chosen an intermediate solution: growth along semi-polar crystallographic directions.

In order to eliminate parasitic crystal phases, we had to opt for a growth mode that modulates the metal/nitrogen ratio during the layer deposition. In this way, we could grow multi-quantum wells with very good optical properties. One success criterion is the very weak red -shift of the emission when the quantum well thickness is increased. This is the signature of a weak internal electric field and of a faster radiating recombination, which can also be directly observed on the figure.