Conformal growth of GaAs on Si consists of the confined lateral selective epitaxy of GaAs from GaAs oriented seeds on silicon, the vertical growth being stopped by an overhanging dielectric mask. Low defect GaAs films are obtained due to the absence of direct nucleation of the conformal GaAs epilayers on Si, and to the geometrical hindrance of the propagation of dislocations into the growing layer by the capping surface and by the substrate. GaAs conformal layers grown by hydride vapour phase epitaxy (HVPE) were characterised by microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman. The GaAs conformal layers were found of superior quality since their luminescence emission was enhanced by several orders of magnitude with respect to the seeds directly grown on the Si substrate. CL and MPL images revealed in plane modulation of the luminescence emission. This modulation was associated with residual stress. MicroRaman measurements revealed stress distribution and eventually local symmetry breakdown.