Growth and characterization of thin manganese oxide corrosion barrier layers for silicon photoanode protection during water oxidation

https://doi.org/10.1016/j.solmat.2014.12.028Get rights and content

Highlights

  • Manganese oxide was deposited on silicon via physical vapour deposition.

  • It is found to both inhibit oxidation of the silicon and catalyse water oxidation.

  • XPS data confirms that manganese oxide suppresses growth of silicon oxide during water oxidation.

Abstract

In this work the potential of thin manganese oxide layers deposited by physical vapor deposition at moderate vacuum pressure to significantly retard the corrosion of silicon photo-anodes during the oxidation of water is reported.

Results show that manganese oxide layers up to 10 nm thick display sufficient transmission of solar frequency radiation and electrical conductivity to facilitate efficient oxidation of water by the underlying silicon while offering complete protection from oxidation of the silicon substrate for more than 14 h. Data from x-ray photoelectron spectroscopy confirms that the increased lifetime observed when using manganese oxide as a protective layer is a result of the inhibition of the growth of silicon oxide during the reaction.

Introduction

Silicon’s status as the material of choice for semiconductor applications over the past four decades is well documented. In the renewable energy space, silicon has is used to generate power in conventional solar cells both in its amorphous and crystalline phases and the vast majority of commercially purchased solar cells are silicon based. Another area where semiconductors are useful in the generation of renewable fuels is the photo-electrochemical cell (PEC) [1]. Photo-electrochemical cells use sunlight to generate electron–hole pairs in semiconductors that are in direct contact with water. The electron–hole pairs serve to oxidize the water via the reaction:H2O→4H++4e+O2

The generated electrons are carried away via a backside contact where they may either reduce the water at a counter electrode to generate hydrogen, or flow in an external circuit to generate energy by some other means. For water splitting, the difference between the oxidation and reduction half cell potentials must be 1.23 V, meaning the silicon with its band gap of 1.1 eV is not suitable for use in a single-material water splitting cell. It can however be used in conjunction with a counter electrode made from another material where it serves as the oxidizing part of the cell.

Silicon has to date had limited success when used to oxidize water. The reason for this is that the oxidation half-reaction takes place directly at the interface between the silicon and the water. Due to the presence of dangling bonds and the reactivity of oxygen, silicon readily oxidizes when there is a source of oxygen available. As soon as the reaction begins, the substrate starts to oxidize and the water gradually goes from being in contact with a semiconductor whose bandgap is 1.1 eV to an insulator (SiO2) with a bandgap of 9 eV. This effect simultaneously reduces the incident photon flux reaching the silicon, and inhibits the flow of holes from the silicon to the water and ends the reaction.

In order to prevent the oxidation of the silicon anode, thin protective overlayers can be used [2], [3], [4]. Ideally, these overlayers should act as an oxygen diffusion barrier while being transparent to solar radiation and having sufficient conductivity to maintain the desired current density in the cell.

Several works exist in the literature that employ protective layers on top of the silicon in order to curb it’s oxidation, such as nickel [5] which showed 80 h of stability in in 0.65 M K-borate and 0.35 M Li-borate (pH=9.5) and TiO2 [6], an insulating oxide which remains stable for more than 8 h in acidic and basic electrolytes.

Thicker layers where uniformity is less of an issue are desirable. Stranwitz et al. [7] have reported on the stability of manganese oxide protective layers deposited by atomic layer deposition (ALD) while Kainthla [2] studied Mn-oxide prepared in a chemical bath.

In this work, we examine the properties of manganese oxide deposited by physical vapor deposition in vacuum, and use x-ray photoelectron spectroscopy measurements made on samples before and after taking part in water oxidation in order to correlate the observed electrical results with physical changes at the silicon–native oxide and native oxide–manganese oxide interfaces. Section 2 outlines the experimental details including film growth, electrochemical setup, and XPS parameters. In Section 3 we present optical transmission data for the deposited Mn-oxide films, and electrochemical data which shows that the these films provide long term stability to the underlying silicon photo-anode during oxidation, while also catalyzing the reaction. Section 4 presents an XPS study of the samples that took part in water oxidation and shows that the manganese oxide protects the silicon by inhibiting the formation of an SiO2 like layer on top of the native-oxide which grows readily in the absence of the Mn-oxide.

Section snippets

Experimental details

All experiments were performed on silicon (1 0 0) wafers with the n-type samples being phosphorous doped with 100 Ω-cm resistivity while p-type samples were boron doped with a resistivity of.001 Ω-cm. Unless otherwise stated, all samples in this work had a native silicon oxide layer of approximately 1 nm thickness. Some samples had the native oxide stripped by a dip in hydrofluoric acid prior to manganese oxide deposition to evaluate the effect of a pristine silicon surface on the electrochemical

Optical and electrical performance of manganese oxide protected photo-anodes

The first requirement of a protective layer is that it be optically transmitting across the range of the solar spectrum. In order to ascertain to what extent manganese oxide is transparent to radiation in the 300–800 nm range, manganese oxide layers of varying thicknesses were deposited as described in the experimental section on glass slides and the transmission characteristics were evaluated in a spectrophotometer. Losses due to the glass slides have been subtracted from the reported figures.

X-ray photoelectron spectroscopy study

X-ray photoelectron spectroscopy can be used to analyse thin oxide films (<6–7 nm) giving information about the chemical state of the elements in both the substrate and the over-layer. For the purposes of this XPS study, two samples were considered, a silicon substrate with a native oxide surface and the same substrate with a deposited 2 nm film of Mn-oxide. XPS has a sampling depth of 5–10 nm depending on the material in question. As such, a film thickness was targeted that ensured a signal from

Conclusion

Manganese oxide deposited via physical vapor deposition has been shown to be an effective barrier to the oxidation of silicon photo-anodes for water oxidation and water splitting applications while still generating in excess of 500 mV in photo-voltage.

The optimal thickness which strikes a balance between photo-current and protection of the underlying silicon appears to be between 7 and 10 nm.

XPS data confirms that the drop in photocurrent in cells depending on unprotected silicon as the

Acknowledgements

The authors would like to acknowledge Dr. Elaine Spain and the Biomedical Diagnostics Institute at Dublin City University for the use of electrochemical characterization equipment and guidance on experimental setup. We would also like to thank Prof. Robert Forster from the School of Chemistry at Dublin City University for helpful discussions. This work was partly funded by the Irish Research Council New Foundations Scheme.

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