A single-etch-step process for the fabrication of high-efficiency diffractive optical elements is presented. The technique uses subwavelength surface relief structures to create a material with an effective index of refraction determined by the fill factor of the binary pattern. Fabrication is performed using electron beam lithography and reactive-ion-beam etching on bulk GaAs, but the process is applicable to any material for which well-controlled etches exist. In this work, we designed and fabricated a blazed transmission grating for operation at 975 nm. The blazed grating exhibits a diffraction efficiency into the first order of 85% of the transmitted power.