UV to NIR detection with p⁺n-type PureB Ge-on-Si photodiodes fabricated on lightly-doped n-Si substrates

Broadband PureB Ge-on-Si photodiodes were fabricated by chemical-vapor deposition (CVD) of crystalline Ge-islands in either 1-µm- or 3.5-μm-thick thermal oxide windows patterned on lightly-doped n-Si substrates. To form the anode, the Ge was capped in-situ with a 10-nm-thick boron layer at 700 °C. An ultrashallow junction was formed by the B-Ge interfacial bonding, giving optoelectrical characteristics similar to those of PureB Si photodiodes that have been commercialized for photon/electron beams with a few nanometer penetration-depths. This chemically formed junction is damage free, resulting in a low density of interface traps and low dark currents. The high density of misfit dislocations at the Ge-Si interface coupled with the lightly-doped Si substrate led to the formation of a p-type interfacial potential barrier. The responsivity of 406 nm light detected at the Ge surface was compared to longer wavelength light detected deeper in the Ge for different post-deposition processing. Combined with simulations, a negative impact of the Ge-Si interfacial barrier on photogenerated hole collection at the anode was identified. Exposure to H2 during thermal processing was also found to degrade the responsivity. Optoelectrical characterization was performed showing that the barrier could be reduced by applying up to -20 V reverse bias, whereby, for 4.73 μm thick Ge islands, responsivities of 0.13, 0.26, 0.50, 0.44 A/W at wavelengths 406, 670, 1310 and 1550 nm, respectively, were achieved with vertical illumination.