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 406nm 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 −20V reverse bias, whereby, for 4.73μm 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.