Making a semiconductor photodetector

DETAILED DESCRIPTION An object of the present invention is to provide a photodetector, such as a PIN photodiode and an avalanche photodiode, having a GaInAs/InP or GaInAsP/InP system, with a low dark current.
The present invention is based on the finding of the inventors that at the lattice mismatching degree .
DELTA.
a/a between the GaInAs (or GaInAsP) light absorbing layer and the InP buffer layer in the positive (plus) range (especially, from 0 to 0.
2%) the dark current is lower than the negative (minus) range (outside the range of 0 to 0.
2%) at room temperature.
The GaInAs (or GaInAsP) light absorbing layer having an energy bandgap smaller than the InP layer, being a dark current generating region, is under a tension stress at minus .
DELTA.
a/a, but is under a compression stress at plus .
DELTA.
a/a.
Here the present inventors have assumed that the crystal state of the GaInAs (or GaInAsP) layer under a compression stress is more stable than that under a tension stress.
According to the present invention, in a semiconductor photodetector comprising an InP substrate, a first InP layer, a GaInAs (or GaInAsP) layer, and a second InP layer, wherein the layers are successively epitaxially formed on the InP substrate, the lattice constance of the GaInAs (or GaInAsP) layer is larger than that of the first InP layer at room temperature.
All of the layers are sequentially grown by a vapor phase epitaxial process.