Dilute nitride alloys

The exponential growth of optical telecommunications and the internet has been underpinned by the development of semiconductor lasers emitting at 1.3 and 1.55 µm, the respective wavelengths at which dispersion is zero and losses minimized, in standard silica optical fibers [1, 2]. Lasers designed to emit at these wavelengths are based primarily on the growth of quaternary InGaAsP or AlInGaAs quantum well (QW) structures on InP substrates. However, despite their widespread applications there are a number of significant limitations associated with incumbent 1.3- and 1.55-µm technologies, several of which are associated with the constraints of growing on InP substrates [3]. This has stimulated significant interest in the development of telecom-wavelength lasers grown on GaAs substrates, which has the potential to deliver a series of key advantages. First, because GaAs is a more robust material, growth can be carried out on larger substrates. Second, growth of reliable semiconductor lasers and related devices on GaAs opens up the possibility of monolithic integration of high-performance photonic components with GaAs-based high-speed electronics. Third, from a fundamental perspective, better optical confinement can be achieved in GaAs-based structures compared to those based on InP, because of the larger refractive index contrast between GaAs and AlGaAs compared to that between InP and the InGaAsP or AlInGaAs quaternary alloys. This 252means that, for example, vertical-cavity surface-emitting lasers (VCSELs) can be grown monolithically on GaAs, but only with great difficulty on InP. Additionally, because AlGaAs has a considerably larger band gap than InP and can be grown lattice-matched on GaAs, it is possible to achieve significantly better carrier confinement in a GaAs-based laser structure, thereby making it possible to overcome the losses associated with carrier leakage from which InP-based devices are known to suffer under high injection currents and at high temperatures.