GAAS-ALGAAS MULTIPLE-QUANTUM-WELL LASERS FOR MONOLITHIC INTEGRATION WITH OPTICAL MODULATORS

Calculations of multiple-quantum-well laser threshold current show that a common minimum current value exists for each number of wells, at an appropriate cavity length. This optimum cavity length decreases rapidly with increasing number of wells, for instance from about 300 to 110-mu-m for one to three wells. Granded-index separate-confinement heterostructure (GRINSCH) lasers with 1-10 quantum wells, grown by molecular beam epitaxy, show consistently low threshold currents that agree well with theoretical predictions. Lasing is achieved at 160 A cm-2 and 4.6 mA for broad-area and ridge wave-guide single-quantum-well devices, respectively. The field-dependent electroabsorption of these devices when operating as wave-guide modulators indicates good modulation properties for one and three quantum-well structures, with on:off ratios above 55 at lasing wavelength. The behavior becomes more complex with increasing number of wells. This systematic study of discrete multiple-quantum-well lasers and modulators demonstrates that GRINSCH structures with 1-3 wells are the most suitable for monolithic integration. Design rules for the laser cavity are also presented for numbers of wells ranging from 1 to 10.