Near-far IR photoconductivity damping in hyperdoped Si at low temperatures

Silicon p-n junction photoelement fabricated on a p-doped wafer by sulfur-based n-doping of its sub-micron thick surface layer, exhibits at liquid-helium temperatures impurity-based near-far IR (2–21 μm) photoconductivity spectra in the form of well-resolved separate bands of neutral and ionized atomic-like and cluster-like sulfur centers. Temperature variation in the range of 5–105 K demonstrates first at lower temperatures < 35 K strong damping of IR photoconductivity related to cluster-like sulfur centers with ultralow activation energy ≈ 4 meV, corresponding to excitation of the lowest energy of Raman-active phonon in orthorhombic crystalline sulfur lattice. Further increase in temperature results in the next damping step above 85 K for all spectral bands above 1800 cm-1 with higher activation energy ≈ 20 meV, representing the lowest energy of Raman-active vibration of octagon molecules in the crystalline sulfur. Broad near-far IR photosensitivity of the hyperdoped material, provided by the concentration-driven sulfur aggregation and quantum-level temperature control of its photoconductivity, paves the way for silicon photonics in far-IR and, possibly in the future, even THz spectral regions.