Broadband PureGaB Ge-on-Si photodiodes responsive in the ultraviolet to near-infrared range

Abstract

Optical characterization of PureGaB germanium-on-silicon (Ge-on-Si) photodiodes was performed for wavelengths between 255 nm and 1550 nm. In PureGaB technology, chemical vapor deposition is used to grow germanium islands in oxide windows to the silicon substrate and then cap them in-situ with nm-thin layers of first gallium and then boron, thus forming nm-shallow p + n diodes. These PureGaB Ge-on-Si photodiodes are CMOS compatible and characterized by low leakage currents. Here they are shown to have high responsivity in the whole ultraviolet (UV) to near infrared (NIR) wavelength range. Particularly, two sets of diodes were studied with respect to possible detrimental effects of the Al metallization/alloying process steps on the responsivity. Al-mediated transport of the Ge and underlying Si was observed if the PureGaB layer, which forms a barrier to metal layers, did not cover all surfaces of the Ge islands. A simulation study was performed confirming that the presence of acceptor traps at the Ge/Si interface could decrease the otherwise high theoretically attainable responsivity of PureGaB Ge-on-Si photodiodes in the whole UV to NIR range. A modification of the device structure is proposed where the PureGaB layer covers not only the top surface of the Ge-islands, but also the sidewalls. It was found that to mitigate premature breakdown, it would be necessary to add p-doped guard rings in Si around the perimeter of Ge islands, but this PureGaB-all-around structure would not compromise the optical performance.

Optoelectrical Operation Stability of Broadband PureGaB Ge-on-Si Photodiodes with Anomalous Al-Mediated Sidewall Contacting

“…As was demonstrated via simulations in Ref. 5 , this can be due to defects at the Ge-Si interface that trap the light-generated carriers. Combined with the fact that the Ge-island thickness is about half the absorption length of the near-infrared (NIR) wavelengths, this can account for the low responsivity.…”

“…Some Ge doping with Ga is expected during the 700°C 10-min-long B-deposition but the resulting profile is not known. Due to the capping layer of B and the previously measured optical response for wavelengths as low as 255 nm, 5 we suspect that the Ga doping is shallow and without roll-off at the Ge surface. Just as has been shown for PureB Si diodes, 7 the reduction in responsivity can be quite low because the whole p-doping gradient effectively repels electrons from the surface.…”

“…4 Despite the unwanted Al-Ge perimeter interaction in the PureGaB photodiodes studied here, the devices distinguished themselves by being responsive at far ultraviolet (FUV) wavelengths as low as 255 nm, as well as having the short wavelength infrared (SWIR) sensitivity characteristic of Ge photodiodes, as already reported in. 5 This broadband nature is due to the damage-free method with which the PureGaB anode is fabricated that gives a shallow, graded p + -doping profile without role-off at the Ge surface. 6,7 This enables low dark currents and photosensitivity close to the Ge surface.…”

Modeling and Simulation Study of Electrical Properties of Ge-on-Si Diodes with Nanometer-thin PureGaB Layer

“…About two decades ago, epitaxial growth of thin Ge films on top of the Si wafers arose as an idea for monolithic integration of a narrow bandgap material with CMOS technology. Methods for epitaxial growth of Ge-on-Si have been developed and high-quality crystalline (c-Ge) layers on Si wafer have been reported [1,3].…”

“…1(a). Low values of reverse current (<10 -14 A/μm 2 ), ideality factor close to unity (n 1.1), low contact resistance, and high responsivity down to 255 nm wavelength has been demonstrated [3]. The latter requires high sensitivity within a few nanometers of the Ge surface.…”