Materials issues towards green laser diodes

For the past 15 years, group-III nitrides have shown an unprecedented development. Driven by applications in lighting and optical data storage quantum well heterostructures with high efficiency in the violet-blue spectral region have been realized. For green wavelengths, however, the efficiency of light-emitting diodes encounters the green gap, laser diodes are still limited to a greenish blue. Among the problems responsible for that are the huge piezoelectric fields as well as the large strain arising from the large Indium mole fraction required to reach the green region. The diminishing refractive index contrast between GaN and AlGaN represents an additional challenge for laser waveguide structures. In order to reduce the internal fields we are studying semipolar and nonpolar growth planes instead of the polar c-plane. Careful low-temperature growth of GaInN turns out to be the key to achieve homogeneous high In incorporation. Additional care needs to be applied to the growth of cladding layers in order to avoid thermal damage to those high-In layers. We use high-resolution XRD to control relaxation as well as transmission electron microscopy to study defects in those layers. Important information regarding homogeneity comes from micro-photoluminescence as well as from cathodoluminescence studies. Optical gain measurements reveal key laser properties such as peak gain, carrier losses, and optical losses.