Responsivity and impact ionization coefficients of Si/sub 1-x/Ge/sub x/ photodiodes

The spectral response and impact ionization coefficient ratio of Si/sub 1-x/Ge/sub x/ have been determined. Measurements were made on p/sup +/-i-n/sup +/ diodes grown by solid/gas source molecular beam epitaxy. The diodes are characterized by reverse breakdown voltages of 4-12 V and dark currents of 20-170 pA//spl mu/m/sup 2/. The long wavelength cut-off of the diodes increases from 1.2 /spl mu/m to 1.6 /spl mu/m as x increases from 0.08 to 1.0 with a maximum responsivity of 0.5 A/W in all the diodes tested. The ratio /spl alpha///spl beta/ varies from 3.3 to 0.3 in the same composition range, with /spl alpha///spl beta/=1 at x/spl cong/0.45. These results have important implications in the use of this material system in various photodetection applications.