Design and Characterization of 1.3-1.6 µm Metamorphic Materials and Lasers on GaAs

Licentiatavhandling, 2006

The development of fiber-optical networks for broad-band access is expected to create a huge market for laser transmitters in the 1.3-1.55 µm wavelength range. The existing InP-based lasers have poor temperature stability. GaAs-based lasers can have better thermal properties due to larger conduction band offset in the quantum well (QW) and as a further advantage they are compatible with the highly reflective AlAs/GaAs distributed Bragg reflectors (DBRs) used in vertical-cavity surface-emitting lasers (VCSELs). However, in order to achieve lasing wavelengths longer than 1.2-1.25 µm on GaAs traditionally either highly strained GaInNAsQWs or InAs quantum dots (QDs) have been used. In this work we investigate a different scheme to realize 1-3-1.55 µm emission on GaAs, the metamorphic technique. Metamorphic growth is a method for producing semiconductor heterostructures on a virtual substrate with a desired lattice constant.We use an alloy graded InGaAs buffer layer to bridge the gap between the GaAs substrate and the virtual substrate, and focus on optimization of buffer layer design and epitaxial growth parameters. The graded buffer layers show typical cross-hatch surface morphologies with minimum root-mean-square roughness as low as 1.1 nm for In[0.25]Ga[0.75]As, providing smooth, relaxed templates for growth of devices. We demonstrate that it is possible to achieve strong light emission from InGaAs QWs on GaAs wafers in the broad wavelength range of 1.25-1.61 µm. We successfully demonstrate the first 4µmwide ridge waveguide lasers operating at 1.25-1.285 µm wavelength under pulsed condition. The minimum threshold current is 60 mA for a 1.5 mm long cavity and maximum slope efficiency is 18 %. Pulsed lasing can be sustained for a maximum duty-cycle of 80 %.