Progress in quantum well lasers: application of strain

Quantum well (QW) lasers are now being made routinely due to the remarkably well-developed OMVPE, MBE, and related epitaxial growth techniques. Because of the quantization, with consequent modification of the density of states of the injected electrons and holes in an extremely thin active layer, this type of lasers has been demonstrated to be superior to conventional double heterostructure (DH) lasers e.g. in threshold current, differential gain, characteristic temperature, maximum operating temperature, and power conversion efficiency. Recently, strained-layer structures with the QW layer composed of a semiconductor having a significantly different lattice parameter to the substrate material, have been introduced. The built-in strain resulting from the tetragonal distortion of the QW layer, splits the degeneracy of the heavy hole (HH) and light hole (LH) bands at the zone centre facilitating a new range of band structures for further enhanced device performance [1,2]. To date, strained-layer QW lasers showing enhanced performance over lattice matched QW and DH lasers emitting at visible (Al/sub x/Ga/sub y/ln/sub 1-x-y/P/GaAs), near infrared (AI/sub xGa/sub y/ln/sub 1-x-y/As/GaAs, employing AlGaAs or InGaP cladding layers) and long wavelengths (ln/sub x/Ga1.,As,P,_,/lnP and AI,Ga/sub y/ln/sub 1-x-y/As/InP) have been reported. This paper reviews the present state of the art in lattice matched and strained-layer MQW lasers in the above mentioned materials systems.