Origin of the Electrical Barrier in Electrolessly Deposited Platinum Nanoparticles on p-Si Surfaces

Abstract

Pt deposited by either sputtering or electron-beam (e-beam) evaporation on p-Si forms an ohmic contact, with zero photovoltage and very little photogenerated charge-carrier collection. However, electro- or electroless deposition of Pt onto p-Si produces a rectifying junction that generates a photovoltage of ∼300 mV under simulated 1 sun illumination. To explain these differences, we have characterized junctions formed by electroless or e-beam deposition of Pt onto H-terminated or oxide-coated p-Si substrates using impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical current density vs potential (J–E) characteristics. When Pt was deposited electrolessly, XPS and TEM measurements revealed a thin interfacial SiOx layer of 1–6 nm thickness under the Pt overlayer. Moreover, open-circuit potential measurements under illumination on electrolessly deposited Pt on a p-Si electrode showed that the junction was a function of the Nernstian potential of the contacting electrolyte solution. Creating an analogous junction by e-beam deposition of Pt required oxidation of the Si surface prior to Pt deposition, followed by etching in HF to remove oxide on the exposed Si surface. The resulting structure has both an interfacial SiOx layer under the Pt and a H-terminated Si surface on the bare areas. Additionally, under a H2 atmosphere, Pt can adsorb hydrogen that can diffuse to the SiOx/Pt interface and produce a dipole layer. This information allowed formulation of a model for the charge transfer across p-Si/SiOx/Pt interfaces. When in contact with a solution having a kinetically facile redox couple, the current is carried across the Si/electrolyte interface, and the electrode has the properties of a semiconductor/liquid junction. In contrast, when in contact with a solution with a large kinetic barrier to interfacial charge transfer, such as the hydrogen evolution reaction, the current instead passes predominantly through the SiOx layer to the Pt and then reacts with protons in the solution. In this situation, the junction to the semiconductor is buried and occurs at the Si/SiOx/Pt interface. The Si/SiOx/Pt contact displays an increase in barrier height due to the hydrogen-induced dipoles. Consequently, the barrier height for an electrode made by electroless deposition of Pt onto Si is determined by the pathway that the electrons traverse to reach the solution.