The electrochemical instability of semiconductors in aqueous electrolytes has impeded the development of robust sunlight-driven water-splitting systems. We review the use of protective thin films to improve the electrochemical stability of otherwise unstable semiconductor photoelectrodes (e.g., Si and GaAs). We first discuss the origins of instability and various strategies for achieving stable and functional photoelectrosynthetic interfaces. We then focus specifically on the use of thin protective films on photoanodes and photocathodes for photosynthetic reactions that include oxygen evolution, halide oxidation, and hydrogen evolution. Finally, we provide an outlook for the future development of thin-layer protection strategies to enable semiconductor-based solar-driven fuel production.

Aluminum oxide thin films fabricated via atomic layer deposition are introduced as passivating tunneling interlayers between hole-selective molybdenum oxide contacts and silicon absorbers. Surface recombination velocity and specific contact resistivity are reported as a function of Al ₂ O ₃ thickness. The effects of substrate chemical pretreatment, the thermal history of the Al ₂ O ₃ layers prior to MoO _x deposition, and the thermal history of the completed Al ₂ O ₃ /MoO _x stacks were also investigated. When an SiO _x /Al ₂ O ₃ passivating stack was incorporated and the completed test structure was annealed at 200 °C, the observed recombination velocities were reduced from ∼10 000 cm/s for an unpassivated (initially hydrogen-terminated) Si/MoO _x direct contact to ∼500 cm/s, while maintaining a contact resistivity at or below 0.1 Ω·cm ² . The data demonstrate the capability of ultrathin Al ₂ O ₃ to improve Si/MoO _x contact properties and may be of interest in the design of future Si heterojunctions.

Thin films of niobium nitride are useful for their physical, chemical, and electrical properties. NbN superconducting properties have been utilized in a wide range of applications. Plasma-enhanced atomic layer deposition (PEALD) of NbN with (t-butylimido) tris(diethylamido) niobium(V) and remote H2/N2 plasmas has been investigated. Deposited film properties have been studied as a function of substrate temperature (100–300 °C), plasma power (150–300 W), and H2 flow rate (10–80 sccm). PEALD NbN films were characterized with spectroscopic ellipsometry (thickness, optical properties), four point probe (resistivity), x-ray photoelectron spectroscopy (composition), x-ray reflectivity (density and thickness), x-ray diffraction (crystallinity), and superconductivity measurements. Film composition varied with deposition conditions, but larger cubic NbN crystallites and increased film density at higher substrate temperatures and H2 flow rates lead to room temperature resistivity values as low as 173 μΩ cm and superconductivity critical temperatures as high as 13.7 K.

A better understanding of amorphous aluminum oxide’s structure and electronic properties is obtained through combined experimental and computational approaches. Grazing incidence X-ray scattering measurements were carried out on aluminum oxide thin films grown using thermal atomic layer deposition. The corresponding pair distribution functions (PDFs) showed structures similar to previously reported PDFs of solid-state amorphous alumina and molten alumina. Structural models based on crystalline alumina polymorphs (PDFgui) and amorphous alumina (molecular dynamics, MD) were examined for structural comparisons to the experimental PDF data. Smaller MD models were optimized and verified against larger models to allow for quantum chemical electronic structure calculations. The electronic structure of the amorphous alumina models yields additional insight into the band structure and electronic defects present in amorphous alumina that are not present in crystalline samples.

A suite of four volatile aminosilanes, cyclic azasilanes, was used to deposit silicon dioxide (SiO₂) films by atomic layer deposition (ALD) over the temperature range 100–300 °C by reaction with O₃.

Post deposition annealing of molecular layer-deposited (MLD) hafnicone films was examined and compared to that of hafnium oxide atomic layer-deposited (ALD) films. Hafnicone films were deposited using tetrakis(dimethylamido)hafnium (TDMAH), and ethylene glycol and hafnia films were deposited using TDMAH and water at 120 °C. The changes in the properties of the as-deposited hafnicone films with annealing were probed by various techniques and then compared to the as-deposited and annealed ALD hafnia films. In situ X-ray reflectivity indicated a 70% decrease in thickness and ∼100% increase in density upon heating to 400 °C yet the density remained lower than that of hafnia control samples. The largest decreases in thickness of the hafnicone films were observed from 150 to 350 °C. In situ X-ray diffraction indicated an increase in the temperature required for crystallization in the hafnicone films (600 °C) relative to the hafnia films (350 °C). The changes in chemistry of the hafnicone films annealed with and without UV exposure were probed using Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy with no significant differences attributed to the UV exposure. The hafnicone films exhibited lower dielectric constants than hafnia control samples over the entire temperature range examined. The CF₄/O₂ etch rate of the hafnicone films was comparable to the etch rate of hafnia films after annealing at 350 °C. The thermal conductivity of the hafnicone films initially decreased with thermal processing (up to 250 °C) and then increased (350 °C), likely due to porosity generation and subsequent densification, respectively. This work demonstrates that annealing MLD films is a promising strategy for generating thin films with a low density and relative permittivity.