Responsivity and impact ionization coefficients of Si1-xGex photodiodes

The spectral response and impact ionization coefficient ratio of Si_1-xGe_x have been determined. Measurements were made on p⁺-i-n⁺ 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/μm² . The long wavelength cut-off of the diodes increases from 1.2 μm to 1.6 μm 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 α/β varies from 3.3 to 0.3 in the same composition range, with α/β=1 at x≅0.45. These results have important implications in the use of this material system in various photodetection applications