Abstract: A semiconductor structure and methods for making it, for use in opto-electronic devices, employs only MOVPE growth steps. The structure is based on a mesa having substantially non-reentrant sides. To make it, an initial semiconductor structure is produced which comprises a substrate with a mesa thereon, the mesa having a self-aligned, central stripe of metal organic vapour phase growth suppressing material on its uppermost surface. Burying layers are then grown by MOVPE at either side of the mesa, the stripe removed, and covering layers grown over the mesa and adjoining regions of the burying layers. To make an opto-electronic device, a silica window can be formed on the uppermost surface of the covering layers and contacts provided through the window and to the remote face of the substrate. Two methods of making the initial semiconductor structure are described. Devices such as optical detectors and waveguides can be made using methods according to the invention.