Optical semiconductor device, optical subassembly, and optical module

An optical semiconductor device, comprising:

• an InP substrate;
• an active layer disposed above the InP substrate;
• a semiconductor layer of a first conductivity type disposed below the active layer;
• and
• a clad layer of a second conductivity type disposed above the active layer, wherein
• the clad layer of the second conductivity type includes one or more In1-xAlxP layers of the second conductivity type,
• an Al composition x of each of the one or more In1-xAlxP layers of the second conductivity type is equal to or greater than a value corresponding to a doping concentration of a dopant of the second conductivity type, and
• an absolute value of an average strain amount of a whole of the clad layer of the second conductivity type is equal to or less than an absolute value of a critical strain amount obtained by Matthews' relational expression, using an entire layer thickness of the whole of the clad layer of the second conductivity type as a critical layer thickness.