An experimental view on PureB silicon photodiode device physics

PureB silicon photodiode technology is distinguished by enabling nm-shallow junction depths that bring the light-sensitive region right up to the Si surface. Robust light-entrance windows can be made from as little as a layer of 2-nm-thick pure boron while obtaining low dark currents. The understanding that these attractive properties are due to the creation of a layer of fixed negative charge when boron is deposited on silicon is supported by extensive experimental observations some of which will be reviewed in this paper. For example, PureB p⁺n-like diodes with equally attractive I-V characteristics can be fabricated with boron layers deposited in the temperature range from 700°C down to 400°C, at which temperature no doping of the bulk Si can be expected. A number of electrical test structures, specifically developed to study the behavior of as-deposited PureB junctions will be discussed along with experiments designed to investigate the influence of post-processing steps, in particular thermal/laser annealing steps. The experiments show that post-processing can degrade the interface and cause an increase in the otherwise ideal diode saturation current even in situations where the interface is replaced by ultrashallow p⁺-doped bulk Si regions.