The effect of dopant diffusion vapor pressure on the properties of sulfur and selenium doped silicon infrared detectors

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Abstract
The n-type Si:S and Si:Se photoconductors with peak infrared response at about 5.5 and 3.5 μm and wavelength cutoffs at 6.8 and 4.1 μm, respectively, were prepared by diffusing sulfur and selenium into high-purity boron doped silicon wafers in closed tubes. The diffusions were performed at 1200 °C and the diffusion vapor pressures were varied by controlling the mass of volatile diffusants. The measured thermal impurity activation energies, the spectral responses, and the detector performance with temperature were found to depend on diffusion pressure. Optimum Si:S detectivity at ∼5.5 μm was obtained by using a diffusion pressure of 1 atm. Optimum Si:Se detectivity of 2.2×1011 cm Hz1/2 W−1 at 3.5 μm out to about 120 K was obtained using a diffusion pressure of 2 atm. This represents the highest operating temperature of any 3–5 μm silicon detector. For the higher pressures, increased spectral response contributions were found at peak wavelengths of 11 and 5.5 μm with cutoffs at about 12.0 and 6.2 μm for the Si:S and Si:Se, respectively. This response is identified with the contribution from impurity molecules whose population increases with diffusion pressure and which have lower ionization energies than the isolated impurities.