EFFECTS OF SUBSTRATE HEATING ON THE SPATIAL UNIFORMITY OF THRESHOLD CURRENT AND EMISSION WAVELENGTH IN GAAS AND INGAAS GRADED-INDEX SEPARATE-CONFINEMENT HETEROSTRUCTURE QUANTUM-WELL LASERS GROWN BY MOLECULAR-BEAM EPITAXY

We show that if the active region of a quantum-well (QW) laser is grown at a high temperature such that the sticking coefficients of group III species are less than unity, the emission wavelength lambda and threshold current density J(th) are sensitive to the spatial variations in substrate growth temperature T(s) despite the time average flux variations of less than 1% across the surface of a rotating 7.5 cm diam substrate in our molecular-beam epitaxy system. InGaAs strained QW lasers, in which the active region is grown at T(s) < 580-degrees-C at which In and Ga have unity sticking coefficients, have uniform J(th) (212 +/- 4 A cm-2) and lambda (989 +/- 1 nm) on a 5 cm diam substrate. These lasers were grown with In mounting. In contrast, GaAs QW lasers, which are grown at approximately 700-degrees-C with In mounting, have less uniform J(th) (395 +/- 20 A cm-2) and lambda (850 +/- 9 nm). This is the result of nonuniform substrate heating which is partly due to voids in the In back coating. This causes variations in GaAs growth rate because of the high temperature dependence of the Ga sticking coefficient at approximately 700-degrees-C. Uniformity of GaAs QW lasers deteriorates further if direct radiation heating of the substrate with In free mounting is used. With In free mounting, the J(th) is minimum (245 A cm-2) in the center and increases monotonically to maximum (365 A cm-2) towards the edge of the wafer, suggesting the degradation of material quality towards the edge. Variation of lambda in GaAs QW lasers is random or systematic depending upon whether they are grown with In mounting or In free mounting, respectively.