This paper proposes the use of a monolithically integrated back scattering layer to improve the efficiency of ultra-thin GaAs single-junction solar cells. The device consists of a double-heterostructure GaAs/InGaP single-junction solar cell integrated with a lattice-matched ZnSe layer etched as the back scattering layer coated with a highly reflective Au film and flip-chip bonded on a carrier substrate. ZnSe surface was roughened to form different textures by wet etching. Without anti-reflective coating, 300×300 μm2 devices with a 300 nm GaAs active region and a reflective ZnSe/Au back scattering layer, demonstrate a short-circuit current density of 17.2 mA/cm 2, an open-circuit voltage of 0.915 V, and a power conversion efficiency of 11.8% under 1 sun solar radiation (AM 1.5G, 0.1 W/cm2). External quantum efficiency (EQE) measurements show EQE over 65% at 400 nm and 30% at 850 nm.