Challenges of AlGaN-based deep UV laser diodes

Because of its direct bandgap covering the UV spectral region from 360 nm to 200 nm, the AlGaN material system is a promising candidate for the realization of deep UV laser diodes emitting below 300 nm, being an alternative for bulky and energy consuming excimer or frequency multiplied lasers. Possible applications cover i.e. medical detection, gas sensing, material processing and water purification. However, no AlGaN-based laser diode emitting below 336 nm has been demonstrated so far. Critical aspects about the fabrication of deep UV laser diodes include material quality, hole injection, carrier confinement and optical confinement. We have investigated optically-pumped AlGaN MQW lasers grown on sapphire and bulk AlN substrates. Optically pumped lasing was obtained in a wide spectral range between 237 nm and 293 nm with no significant increase of laser threshold and no change of gain characteristics for emission wavelengths down to 255 nm. The p-type conductivity of AlGaN layers with high aluminum content could be improved by introducing short period superlattices. Current injection of more than 4.5 kA/cm² into a laser diode structureswithp-cladding layers with an average aluminum content of 80% could have been obtained.