A semiconductor optical element includes: a first conductivity type semiconductor substrate; and a laminated body disposed on the first conductivity type semiconductor substrate. The laminated body includes, in the following order from a side of the first conductivity type semiconductor substrate: a first conductivity type semiconductor layer; an active layer; a second conductivity type semiconductor layer; and a second conductivity type contact layer. The second conductivity type semiconductor layer includes: a carbon-doped semiconductor layer in which carbon is doped as a dopant in a compound semiconductor; and a group 2 element-doped semiconductor layer in which a group 2 element is doped as a dopant in a compound semiconductor. The carbon-doped semiconductor layer is disposed at a position closer to the active layer than the group 2 element-doped semiconductor layer.

A method of fabricating a gain medium includes growing a p-type layer doped with zinc on a substrate, growing an undoped layer including one or both of InP or InGaAsP on the p-type layer, growing a region that includes multiple quantum wells (MQWs) on the undoped layer, and growing an n-type layer on the region. The undoped layer has a thickness that is sufficient to prevent Zn diffusion from the p-type layer into the region during subsequent growth or wafer fabrication steps.

A light emitting device includes: a first n-type semiconductor layer disposed on a substrate; a tunnel junction layer disposed on a part of the first n-type semiconductor layer; a p-type semiconductor layer disposed on the first n-type semiconductor layer and covering the tunnel junction layer; an active layer disposed on the p-type semiconductor layer; and a second n-type semiconductor layer disposed on the active layer.

A method of fabricating a gain medium includes growing a p-type layer doped with zinc on a substrate, growing an undoped layer including one or both of InP or InGaAsP on the p-type layer, growing a region that includes multiple quantum wells (MQWs) on the undoped layer, and growing an n-type layer on the region. The undoped layer has a thickness that is sufficient to prevent Zn diffusion from the p-type layer into the region during subsequent growth or wafer fabrication steps.

An AlGaInPAs-based semiconductor laser device includes a substrate, an n-type clad layer, an n-type guide layer, an active layer, a p-type guide layer composed of AlGaInP containing Mg as a dopant, a p-type clad layer composed of AlInP containing Mg as a dopant, and a p-type cap layer composed of GaAs. Further, the semiconductor laser device has, between the p-type guide layer and the p-type clad layer, a Mg-atomic concentration peak which suppresses inflow of electrons, moving from the n-type clad layer to the active layer, into the p-type guide layer or the p-type clad layer.

A vertical cavity surface emitting laser includes: an active layer including a quantum well structure including one or more well layers including a III-V compound semiconductor containing indium as a group III constituent element; an upper laminated region containing a carbon dopant; and a substrate for mounting a post including the active layer and the upper laminated region, in which the active layer is provided between the upper laminated region and the substrate, the quantum well structure has a carbon concentration of 210.sup.16 cm.sup.3 or less, and the upper laminated region includes a pile-up layer of indium at a position away from the active layer.

A light emitting device includes: a first n-type semiconductor layer disposed on a substrate; a tunnel junction layer disposed on a part of the first n-type semiconductor layer; a p-type semiconductor layer disposed on the first n-type semiconductor layer and covering the tunnel junction layer; an active layer disposed on the p-type semiconductor layer; and a second n-type semiconductor layer disposed on the active layer.

An optical semiconductor device includes an InP substrate; an active layer disposed above the InP substrate; a n-type semiconductor layer disposed below the active layer; and a p-type clad layer disposed above the active layer, wherein the p-type clad layer includes one or more p-type In.sub.1-xAl.sub.xP layers, the Al composition x of each of the one or more p-type In.sub.1-xAl.sub.xP layers is equal to or greater than a value corresponding to the doping concentration of a p-type dopant, and the absolute value of the average strain amount of the whole of the p-type clad layer is equal to or less than the absolute value of a critical strain amount obtained by Matthews' relational expression, using the entire layer thickness of the whole of the p-type clad layer as a critical layer thickness.

A vertical cavity surface emitting laser includes: an active layer; a first laminate for a first distributed Bragg reflector; and a first intermediate layer disposed between the active layer and the first laminate. The first intermediate layer has first and second portions. The first laminate, the first and second portions of the first intermediate layer, and the active layer are arranged along a direction of a first axis. The first laminate and the first portion of the first intermediate layer each include a first dopant. The active layer has a first-dopant concentration of less than 110.sup.16 cm.sup.3. The first portion of the first intermediate layer has a first-dopant concentration smaller than that of the first laminate. The second portion of the first intermediate layer has a first-dopant concentration smaller than that of the first portion of the first intermediate layer.

An optical semiconductor device includes an InP substrate; an active layer disposed above the InP substrate; a n-type semiconductor layer disposed below the active layer; and a p-type clad layer disposed above the active layer, wherein the p-type clad layer includes one or more p-type In.sub.1-xAl.sub.xP layers, the Al composition x of each of the one or more p-type In.sub.1-xAl.sub.xP layers is equal to or greater than a value corresponding to the doping concentration of a p-type dopant, and the absolute value of the average strain amount of the whole of the p-type clad layer is equal to or less than the absolute value of a critical strain amount obtained by Matthews' relational expression, using the entire layer thickness of the whole of the p-type clad layer as a critical layer thickness.