Excimer laser annealing of boron thin-films to fabricate large-area low-defect p+ Si regions

Boron layers deposited by chemical vapor deposition at 700 °C are exposed to 308 nm excimer laser annealing (ELA). The non-annealed B-deposition is commonly used to form radiation hard p⁺n PureB photodiodes that have nm-shallow junction depths with low dark currents, i.e., the PureB anode region has a high Gummel number. Here a B-layer thickness ranging from a monolayer to 7 nm was studied for deposition in oxide windows to lightly-doped n-Si wafers, and diode contacting was achieved with an Al/1%Si interconnect layer. The focus was placed on a 1-nm-thin B-layer that, after ELA at fluences from 400 to 900 mJ/cm², delivered ideal diodes with anode Gummel numbers at least as high as PureB counterparts. Fluences above 750 mJ/cm² were necessary for melting of the Si to achieve p-region sheet resistance <100 Ω/sq. The maximum active doping level was 2.3 × 10²¹ atoms/cm³ in the melt zone, providing a sheet resistance of 25 Ω/sq at a junction depth of 85 nm for a fluence of 900 mJ/cm². At 600 mJ/cm², about a monolayer of B bonded to Si at the interface remained. This rendered a photodiode with an optimal responsivity of 0.16 A/W at 406 nm and 0.33 A/W at 670 nm wavelengths.