Gain Coefficient and Lasing Threshold in GaN-Based Lasers

GaN-based laser diodes have now been shown to be reliable, potentially extremely useful devices [1], These laser diodes have been developed within a very short time span of about two years. Starting with pulsed operation at room temperature [2] mid later CW operation [3], we have experienced an exponentially increasing device lifetime. 

Nevertheless, the threshold current density of GaN-based laser diodes is still almost one order of magnitude higher than in other III-V material systems. Being one of the most basic laser properties, this highlights the present lack of understanding of the lasing mechanism in nitride-based semiconductors. 

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

Unlike atomic or solid-state lasers, the lasing transitions in a semiconductor laser are transitions between continua of extended states rather than between localised states. The inversion criterion [4] then is that the electron and hole quasi-Fermi levels must be separated by more than the bandgap energies. The spectrum of the optical gain g is given by [5,6] 

Essentially, it is determined by the combined density-of-states of the conduction and valence bands and by the difference of the electron and hole distribution functions. In fact, the effect of the band structure is two-fold. On one hand a large combined density-of-states is beneficial for a large optical gain, and on the other hand large densities-of-state correspond to large carrier densities in order to achieve inversion. 

---

C5.3 Gain coefficient and lasing threshold in GaN-based lasers C5.4 Theoretical and experimental results on GaN-based lasers... 

---