Restricted-access al-mediated material transport in al contacting of pureGaB Ge-on-Si p+n diodes

The effectiveness of using nanometer-thin boron (PureB) layers as interdiffusion barrier to aluminum (Al) is studied for a contacting scheme specifically developed for fabricating germanium-on-silicon (Ge-on-Si) p + n photodiodes with an oxide-covered light entrance window. Contacting is achieved at the perimeter of the Ge-island anode directly to an Al interconnect metallization. The Ge is grown in oxide windows to the Si wafer and covered by a B and gallium (Ga) layer stack (PureGaB) composed of about a nanometer of Ga for forming the p + Ge region and 10 nm of B as an interdiffusion barrier to the Al. To form contact windows, the side-wall oxide is etched away, exposing a small tip of the Ge perimeter to Al that from this point travels about 5 μm into the bulk Ge crystal. In this process, Ge and Si materials are displaced, forming Ge-filled V-grooves at the Si surface. The Al coalesces in grains. This process is studied here by high-resolution cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy that confirm the purities of the Ge and Al grains. Diodes are fabricated with different geometries and statistical current–voltage characterization reveals a spread that can be related to across-the-wafer variations in the contact processing. The I–V behavior is characterized by low dark current, low contact resistance, and breakdown voltages that are suitable for operation in avalanching modes. The restricted access to the Ge of the Al inducing the Ge and Si material transport does not destroy the very good electrical characteristics typical of PureGaB Ge-on-Si diodes

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

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.</p

Nanolayer boron-semiconductor interfaces and their device applications

Nanolayers of pure boron (PureB) deposited on Si form p+-type regions at deposition temperatures from 50 °C to 700 °C. At 700 °C, commercial PureB photodiodes are produced for advanced detection systems including those in extreme-ultraviolet (EUV) lithography systems. In addition, potent MEMS applications of B-nanolayers have been demonstrated. Attractive diode characteristics were also found for devices where Ga wetting-layers were applied to the Si surface, both with/without an additional B capping-layer. The resulting “PureGa” and “PureGaB” diodes are assessed here in the light of investigations focused on the B-Si interface properties in PureB diodes. A very high density of acceptor states at the Si interface appears to be related to the p-dopant property of both B and Ga, even though diffusion into the Si is not expected for the applied processing temperatures from 50 °C to ~ 450 °C

Non-linear behavior of Al-contacted pure amorphous boron (PureB) devices at low temperatures

Deposition of pure amorphous boron (PureB) layers on n-type Si results in p+n-like devices even in cases where B in-diffusion during the deposition is not expected. It is suspected that such behavior is due to the formation of an interfacial hole layer (IHL) between the PureB and Si. To further investigate physical mechanisms governing conduction of holes across the PureB/Si interface and through the IHL, electrical measurements were performed from room temperature down to cryogenic temperatures as low as 100 K. In this paper, current-voltage (I-V) measurements are made on structures where the PureB connects to p-type Si regions. One set of devices comprises ring-shaped structures designed for measuring the conductance through the IHL. In these structures, the PureB layer is deposited in rings that are contacted at the inner and outer perimeter with Al. Another set of samples includes devices where the PureB layer was deposited on p-type bulk Si. At room temperature, a close to linear change of current with voltage was seen irrespective of the PureB layer thickness and post-deposition processing. Lowering the operating temperature led to an increasingly non-linear I-V characteristics. Plausible explanations for the non-linear behavior are considered and discussed in the paper