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

Abstract

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/cm2. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)