InGaN/GaN quantum dot lasers

Low threshold visible lasers are required for a number of applications including full-color mobile projectors, heads-up displays, optical information processing systems and a host of medical applications. Lasers emitting at λ= 420 nm (blue) and 520 nm (green), have been realized with GaN-based InGaN/GaN multi-quantum wells (MQWs) as the gain media. With one exception, these laser heterostructures have been grown by metal organic chemical vapor deposition (MOCVD). The large In composition and associated mismatch strain in the ternary InGaN quantum wells lead to clustering effects and a large piezoelectric polarization field, especially for c-plane heterostructures, both of which are detrimental for laser performance. The threshold current density of these lasers is generally very large due to reduced electron-hole (e-h) overlap in the quantum wells. Additionally, a large blue shift of the emission peak with injection is observed due to the quantum confined Stark effect (QCSE) associated with the polarization field. It is known that material inhomogeneities and the piezoelectric field increase in InGaN/GaN quantum wells with increasing In content. Furthermore, a wider well width that is needed for emission at longer wavelengths is not an option since the band bending due to a strong polarization field reduces e-h wavefunction overlap significantly. While shorter wavelength lasers have been realized, red-emitting lasers incorporating InGaN/GaN MQWs have not yet been demonstrated.