Optical properties and carrier dynamics in differently strained GaN epilayers grown on Si by MOVPE

Optical properties and carrier dynamics were investigated in a set of samples grown on Si utilizing different layer combinations between the topmost GaN layer and the Si substrate, including AlN/GaN distributed Bragg reflector (DBR) superlattices as well as AlGaN/GaN strain-reducing layers. The use of the AlN/AlGaN superlattice acting both as buffer layer and as DBR opens the way to control the strain in the upper epitaxial layer in GaN-based heterostructures grown on Si. It also can lead to a decrease of the concentration of non-radiative defects in the upper GaN layers, which, in turn, approximately doubles the carrier lifetime and increases the concentration of non-equilibrium carriers. Carrier lifetimes at high excitation energy densities increased from 50-78 ps to 75-200 ps in the layers with DBR. These effects lead to improved PL efficiency and to an increase of the value of optical gain from 300 cm ^-1 up to 4000 cm ^-1. The use of DBR reduces the absorption of light in the Si substrate and may improve the properties of the laser waveguide. All these effects result in a reduction of the laser threshold from 700 to 270 kW/cm ².