Threshold and Temperature Characteristics of InGa(N)As-GaAs Multiple Quantum Well Lasers

Licentiate thesis, 2008

Semiconductor lasers emitting in the 1.3 μm regime are of interest for applications in access-networks like fiber-to-the-home and radio-over-fiber systems. Such fiber optical networks are expected to replace the copper-based access-networks currently in use due to a continuously increasing demand on user bandwidth. To facilitate a widespread implementation of such networks, low-cost semiconductor lasers emitting at 1.3 μm are needed. A significant improvement in cost efficiency is obtained with lasers capable of un-cooled operation. For this reason much research has been devoted to GaAs-based lasers which offer an inherent improved temperature stability compared to the temperature-sensitive InP-based lasers traditionally used. The work presented in this thesis deals with InGa(N)As multiple quantum-well (QW) lasers grown on GaAs, with the aim of improving and understanding their temperature characteristics. A performance comparison between InGaNAs/GaAs lasers and other GaAs and InP-based lasers is presented. The epitaxial material is grown by molecular beam epitaxy (MBE). By optimizing MBE growth conditions we have obtained record low values of threshold current density of 107 and 133 A/cm2/QW for triple QW 1.2 μm InGaAs and 1.3 μm InGaNAs lasers, respectively. A thorough investigation of the temperature dependence of the threshold current (Ith) for ridge wave guide InGaNAs double QW lasers is presented. The good temperature stability of such lasers is usually ttributed to large amounts of defect recombination as well as a large conduction band offset. This work, however, reveals that their good temperature stability also to a large extent arises from a significant and weakly temperature dependent lateral diffusion current, which is not an effect intrinsic to InGaNAs but rather related to the geometry of the laser cavity. By impeding lateral diffusion it should be possible to reduce Ith and still obtain a relatively good temperature performance.