Development of III-nitride nanostructures for low threshold lasing and semipolar GaN towards Yellow/Orange lasing

Abstract

Last two decades have seen significant progress in developing III-nitride based laser didoes, in particular, InGaN-based blue laser diodes, which have been commercialised. However, there are a number of significant challenges in growth of longer emission wavelength such as green, yellow and even amber laser diodes. Recently, InGaN/GaN based green laser diodes with around 531 nm have been reported on both free-standing c-plane GaN and semi-polar (20–21) GaN substrates. However, the green laser diodes achieved so far suffer from extremely high thresholds for lasing, typically 1518 KA/cm2. Therefore, it is increasingly interesting to develop III-nitride based lasers with a low threshold for lasing. One of the most promising options to achieve lasing with a low threshold is to adopt an optical micro-cavity, leading to a reduction in cavity mode volume and thus an increase in Purcell factor. Such microdisk lasers would also find a wide applications in fabricating an integration of photonic and electronic circuits on the nanometre/micrometer scale. A low threshold fir lasing is also very important for achieving multi-wavelength lasers, in particular, spanning the full visible spectral region. Such multi-wavelength lasers which could potentially form white lasing find a wide range of applications in fabricating full colour high definition display, next generation vehicle headlighting, etc. Therefore, a microdisk laser is becoming particularly important for III-nitrides, as it leads to a significant reduction in threshold as a result of reduced modal volume. In this study we will demonstrates a number of InGaN/GaN based single microdisk lasers exhibiting low threshold lasing optically pumped in a continuous mode (cw) mode at room temperature. Finally, the paper will provide some latest progress on developing semi-polar GaN materials grown on sapphire. The development of even longer emission wavelength such as yellow and amber laser diodes is severely limited as a result of current growth of GaN along a polar orientation, i.e., on (0001) GaN surface, leading to the well-known "green/yellow" gap. One clear way forward, meeting the fundamental challenge, is to grow along a semi-polar direction. Semi-polar orientations, in particular, (11–22), offer another major advantage, namely, it can also significantly enhance indium incorporation into GaN, which cannot be achieved using c-plane GaN and thus is extremely important for growth of long wavelength emitters. However, the current challenge is due to the crystal quality of semi-polar GaN on sapphire.