Application-oriented nitride substrates: The key to long-wavelength nitride lasers beyond 500 nm

We present results based on quantum mechanical estimates of the longest emission wavelength for nitride laser diodes grown on c-plane GaN/sapphire substrates. The results indicate that the absence of polarization-induced electric fields in nonpolar/semipolar GaN substrates does not necessarily guarantee that nitride lasers will operate at the longest possible wavelength for a given set of parameters. Our calculations suggest that the limit on the longest possible wavelength of nitride lasers is constrained by the lattice mismatch rather than by the strength of the polarization-induced electric field. Although it may be possible to develop lasers that approach the green portion of the electromagnetic spectrum (similar to 520 nm) by growing the structures on nonpolar/semipolar GaN substrates, the development of red and near-infrared nitride lasers appears extremely difficult by merely growing the structures on any crystallographic orientation of the GaN substrate. We suggest that efficient lasers emitting at the green, red, and near-infrared wavelengths can be developed by growing the laser structures on a proposed application-oriented nitride substrate (AONS) that is lattice-matched to the epilayers grown on it. The AONSs are bulk InxGa1-xN ternary substrates with Indium compositions chosen to lattice-match the epilayers to be grown on them. The concept of the AONS can be extended deep into the infrared region by increasing the Indium mole fraction of the quantum well layers in the active region of the laser and by choosing the AONS that best matches the specific wavelength desired. We believe it would be possible, by using this concept, to make nitride lasers at the fiber-optic communication windows at 1.3 and 1.55 mu m, thus eliminating the need to use the hazardous arsenides/phosphides materials currently used to make the communications lasers.