Andrew G. Scheuermann

Effects of catalyst material and atomic layer deposited TiO2 oxide thickness on the water oxidation performance of metal–insulator–silicon anodes

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO2 thickness and (2) the catalyst material in catalyst/TiO2/SiO2/Si anodes. Uniform films of atomic layer deposited TiO2 are prepared in the thickness range ∼1–12 nm on degenerately-doped p+-Si, yielding water oxidation overpotentials at 1 mA cm−2 of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO2 thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to ∼21 mV of added overpotential at 1 mA cm−2 per nanometer of TiO2 for TiO2 thicknesses greater than ∼2 nm. For thinner TiO2 layers, an approximately thickness-independent overpotential is observed. The linear behavior for anodes with thicker TiO2 layers is consistent with the predicted effect of bulk TiO2-limited electronic conduction on the voltage required to sustain the current density across the TiO2/SiO2 insulator stack. Eight different oxygen evolution catalysts of thickness 1–3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned.

Atomic Layer Deposited Corrosion Protection: A Path to Stable and Efficient Photoelectrochemical Cells

A fundamental challenge in developing photoelectrochemical cells for the renewable production of solar chemicals and fuels is the simultaneous requirement of efficient light absorption and robust stability under corrosive conditions. Schemes for corrosion protection of semiconductor photoelectrodes such as silicon using deposited layers were proposed and attempted for several decades, but increased operational lifetimes were either insufficient or the resulting penalties for device efficiency were prohibitive. In recent years, advances in atomic layer deposition (ALD) of thin coatings have made novel materials engineering possible, leading to substantial and simultaneous improvements in stability and efficiency of photoelectrochemical cells. The self-limiting, layer-by-layer growth of ALD makes thin films with low pinhole densities possible and may also provide a path to defect control that can generalize this protection technology to a large set of materials necessary to fully realize photoelectrochemical cell technology for artificial photosynthesis.

Conductance and capacitance of bilayer protective oxides for silicon water splitting anodes

State-of-the-art silicon water splitting photoelectrochemical cells employ oxide protection layers that exhibit electrical conductance in between that of dielectric insulators and electronic conductors, optimizing both built-in field and conductivity. The SiO2-like layer interposed between a deposited protective oxide film and its Si substrate plays a key role as a tunnel oxide that can dominate the total device impedance. In this report, we investigate the effects of changes in interfacial SiO2 resistance and capacitance in the oxide bilayer through both solid state leakage current and capacitance–voltage measurements and through electrochemical methods applied to water splitting cells. Modelling is performed to describe both types of data in a simple and intuitive way, allowing for insights to be developed into the connections among both the dielectric (charge storage) and conductive (charge transport) properties of bilayer protective oxides and their effects on oxygen evolution performance. Finally, atomic layer deposited (ALD) Al2O3 is studied as an insulator layer with conductivity intermediate between that of tunnel oxide SiO2 and the more conductive ALD-TiO2, to further generalize this understanding.

Titanium Oxide Crystallization and Interface Defect Passivation for High Performance Insulator-Protected Schottky Junction MIS Photoanodes

Atomic layer deposited (ALD) TiO2 protection layers may allow for the development of both highly efficient and stable photoanodes for solar fuel synthesis; however, the very different conductivities and photovoltages reported for TiO2-protected silicon anodes prepared using similar ALD conditions indicate that mechanisms that set these key properties are, as yet, poorly understood. In this report, we study hydrogen-containing annealing treatments and find that postcatalyst-deposition anneals at intermediate temperatures reproducibly yield decreased oxide/silicon interface trap densities and high photovoltage. A previously reported insulator thickness-dependent photovoltage loss in metal-insulator-semiconductor Schottky junction photoanodes is suppressed. This occurs simultaneously with TiO2 crystallization and an increase in its dielectric constant. At small insulator thickness, a record for a Schottky junction photoanode of 623 mV photovoltage is achieved, yielding a photocurrent turn-on at 0.92 V vs NHE or -0.303 V with respect to the thermodynamic potential for water oxidation.

Engineering Interfacial Silicon Dioxide for Improved Metal–Insulator–Semiconductor Silicon Photoanode Water Splitting Performance

Silicon photoanodes protected by atomic layer deposited (ALD) TiO2 show promise as components of water splitting devices that may enable the large-scale production of solar fuels and chemicals. Minimizing the resistance of the oxide corrosion protection layer is essential for fabricating efficient devices with good fill factor. Recent literature reports have shown that the interfacial SiO2 layer, interposed between the protective ALD-TiO2 and the Si anode, acts as a tunnel oxide that limits hole conduction from the photoabsorbing substrate to the surface oxygen evolution catalyst. Herein, we report a significant reduction of bilayer resistance, achieved by forming stable, ultrathin (<1.3 nm) SiO2 layers, allowing fabrication of water splitting photoanodes with hole conductances near the maximum achievable with the given catalyst and Si substrate. Three methods for controlling the SiO2 interlayer thickness on the Si(100) surface for ALD-TiO2 protected anodes were employed: (1) TiO2 deposition directly on an HF-etched Si(100) surface, (2) TiO2 deposition after SiO2 atomic layer deposition on an HF-etched Si(100) surface, and (3) oxygen scavenging, post-TiO2 deposition to decompose the SiO2 layer using a Ti overlayer. Each of these methods provides a progressively superior means of reliably thinning the interfacial SiO2 layer, enabling the fabrication of efficient and stable water oxidation silicon anodes.

Characterization of the photocurrents generated by the laser of atomic force microscopes

The conductive atomic force microscope (CAFM) has become an essential tool for the nanoscale electronic characterization of many materials and devices. When studying photoactive samples, the laser used by the CAFM to detect the deflection of the cantilever can generate photocurrents that perturb the current signals collected, leading to unreliable characterization. In metal-coated semiconductor samples, this problem is further aggravated, and large currents above the nanometer range can be observed even without the application of any bias. Here we present the first characterization of the photocurrents introduced by the laser of the CAFM, and we quantify the amount of light arriving to the surface of the sample. The mechanisms for current collection when placing the CAFM tip on metal-coated photoactive samples are also analyzed in-depth. Finally, we successfully avoided the laser-induced perturbations using a two pass technique: the first scan collects the topography (laser ON) and the second collects the current (laser OFF). We also demonstrate that CAFMs without a laser (using a tuning fork for detecting the deflection of the tip) do not have this problem.

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2–RuO2 Alloy Schottky Contacts for Silicon Photoanodes

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions ≥ 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

Series resistance and gate leakage correction for improved border trap analysis of Al2O3/InGaAs gate stacks

As the size of electronic devices scales down, series resistance (RS) and gate leakage effects are commonly observed in electrical measurement of metal-oxide-semiconductor gate stacks. As a result of their effects on device characteristics, these phenomena complicate the analysis of border trap density (Nbt) in the gate insulator using capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. In this work, we develop methods to correct for the effects of RS and gate leakage in Al2O3/InGaAs gate stacks to enable reliable fitting of C-V and G-V data to determine Nbt. When tested using data from Pd/Al2O3/InGaAs gate stacks, the RS correction method successfully removes the RS-induced high frequency dispersion in the accumulation region of the C-V curves and provides an accurate extraction of RS and Nbt. The gate leakage correction method is tested on gate stacks with high gate leakage current of ∼25 μA at 2 V bias, and is found to effectively fit capacitance and conductance data, to achieve consi...