Thermal activation mechanism of sulfur impurities in sulfur-hyperdoped silicon films

Abstract

Thermal annealing has been widely used in the fabrication of microelectronic devices. However, the sub-bandgap infrared (wavelength λ = 1100–2500 nm) absorption of deep-level impurity hyperdoped Si wafer material usually decreases after thermal annealing treatment. This restricts the application of hyperdoped Si in room-temperature infrared photodetectors and broad-spectrum solar cells. Using a newly developed fabrication method, hyperdoping of Si with S was achieved by the nanosecond laser melting of Si/S composite films. Then, a thermal annealing treatment of S-hyperdoped Si (Si:S) films was carried out in a temperate range of T = 473–1273 K and a time range of t = 60–600 s under nitrogen ambient condition. The results show that at T = 1223 or 1273 K and t = 300 or 600 s, the sub-bandgap infrared and above-bandgap (λ = 200–1100 nm) absorbance were several times higher than those of unannealed Si:S films. Furthermore, the product of sheet electron concentration and electron mobility (NS×μ) indicates that these annealing conditions also showed better electrical properties than those of unannealed Si:S films. Combined with the microstructure analysis, the thermal activation mechanism can be proposed as follows: The thermal annealing treatment increased the crystallinity of Si:S films and the concentration of electrically active S impurities in the films. Then, the active S impurity atoms increased the ability of chemical bonding with Si atoms and forming SiS2. This effect induced by thermal annealing led to a high potential of optoelectronic devices fabricated with such Si:S films.