Until recently most semiconductor device structures grown by MBE have used As4. In such structures during the growth of (Al,Ga)As alloys a characteristic roughness is observed under some growth conditions. The threshold current of lasers grown using As4 is also strongly influenced by the choice of substrate temperature. It is possible therefore that these two factors are related. We have compared the morphology of thick films of Al0.5Ga0.5As grown using As2 and As4 over the temperature range 610 to 710°C. For films grown with As4 the characteristic roughness in such structures is observed but over the whole temperature range films grown with As2 have a specular appearance. We have grown, using As2, a series of graded refractive index separate confinement heterostructure single quantum well (57 Å) lasers with and without pre-layers at temperatures from 610 to 710°C. In this series of samples the Al and Ga fluxes were kept constant, the nominal Al fractions for the outer confinement and barrier regions were 50% and 25%, respectively. A similar set of stepped separate confinement structures with prelayers was grown for comparison. In a fourth series of structures the separate confinement and part of the outer cladding regions were replaced by short period superlattices. Contrary to previous reports for structures grown using As4, no strong dependence of threshold current on growth temperature was observed. Results for four sets of lasers will be discussed in detail. An additional benefit of using As2 is the reduced loss of Ga at high temperatures due to a reduction of free metallic group III element present on the surface during growth. © 1991.