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The authors report the first observation of photoluminescence (PL) from ultrathin InAs/GaAs single quantum wells (SQWs) grown on GaAs A substrates by using molecular beam epitaxy. The surface morphologies of InAs layers grown on GaAs -(2×4) and A-(2×2) reconstructed surfaces are compared with reflection high energy electron diffraction and atomic force microscopy. The quantitative difference between  and A surfaces is quite clear, i.e. the root mean square surface roughness is 50 nm on  surfaces and 0.83 nm on A surfaces after ∼360 monolayers (MLs) growth of InAs. When the thickness of InAs wells in InAs/GaAs SQW samples increases from 1 to 6 ML, the PL peak energy at 10 K gradually decreases from 1.47 to 1.36 eV, the full width at half maximum becomes wider and the intensity abruptly decreases. It has been found that the PL intensity is closely related to the quality of the GaAs cap layer rather than the InAs wells alone.