The effects of composition and doping on the response of GeC-Si photodiodes

The spectral responses of a series of heterojunction diodes of p-type Ge_1-yC_y on n-type Si (100) substrates were measured by Fourier transform infrared (IR) spectroscopy. Alloy layers 0.5 μm thick were grown by molecular beam epitaxy at a substrate temperature of 400°C and were doped p-type with different C concentrations. With increasing C content, the diode dark current decreased, and the optical absorption band edge shifted toward higher energy by 70 meV for 0.12 at% of C. The increase in energy was attributed to the composition dependence of the bandgap rather than to strain relaxation, because the GeC layers were nearly relaxed with the same strain. The photoresponsivity was 0.07 A/W at a wavelength of 1.55 μm, and 0.2 A/W at a wavelength of 1.3 μm. These measurements show that GeC photodetectors have good properties and reasonable response at technologically important near-IR wavelengths and can be fabricated by heteroepitaxy for compatibility with Si integrated circuits