Optical semiconductor device and method of manufacturing the same

A method of manufacturing an optical semiconductor device, comprising:

• forming a doping layer, an active layer including at least a quantum dot, and a contact layer of an opposite conduction type to the doping layer on one surface of a semiconductor substrate in this order;
• forming a mask layer on the contact layer;
• forming a resist in island shape on the mask layer;
• making the mask layer into a hard mask by etching the mask layer using the resist as a mask;
• receding a side surface of the hard mask to expose a contact region of the contact layer and to make the resist overhang beyond the upper surface of the hard mask;
• forming an insulating layer on the resist and on the contact layer outside the contact region;
• forming a conductive layer on the insulating layer and on the contact layer within the contact region;
• lifting-off the insulating layer and the conductive layer on the resist by removing the resist, so that the insulating layer, which has a first opening whose size is such that the contact region is included in the first opening, is left on the doping layer, and the conductive layer, which has a second opening whose size is such that the second opening is included in the first opening, is left on the doping layer as a first electrode layer;
• and
• forming a second electrode layer on the other surface of the semiconductor substrate.