We investigated optical properties of GaAs/GaAs1-xPx(x≃0.2) quantum wells (QWs) with each well width (Lw) of 2.7–10.0 nm grown on (100) and (n11)A GaAs substrates (n=3, 4, 5) by molecular beam epitaxy with the use of P2 and As4 molecular beams generated from polycrystalline GaP chunk and solid As sources. It was found that the P contents (x) in GaAs1-xPx layers strongly depend on both the substrate orientation and substrate temperature (Ts). In the whole range of the Ts (535–640 °C), the largest P contents (x=0.19–0.23) were observed in the (411)A GaAs1-xPx layers, and the (100) GaAs1-xPx layers showed the smallest x (0.08–0.18). The full width at half maximum of photoluminescence (PL) peak (λ=758 nm) from the (411)A GaAs/GaAsP QW with Lw=2.7 nm was as small as 5 meV which is only 66% of that of the (100) QW probably due to improved interface flatness of the (411)A QW. Furthermore, the PL intensities of the (411)A and (311)A QWs were more than one order of magnitude larger in intensity than that of the (100) QWs. © 1999 American Vacuum Society.