Advanced Atomic Layer Deposition: Ultrathin and Continuous Metal Thin Film Growth and Work Function Control Using the Discrete Feeding Method.

The ultrathin and continuous ruthenium (Ru) film was deposited through an improved atomic layer deposition (ALD) process with a discrete feeding method (DFM), called DF-ALD, employing a cut-in purge step during the precursor feeding. The excess precursor molecules can be physically adsorbed onto the chemisorbed precursors on the substrate during precursor feeding, which screens the reactive sites on the surface. Using DF-ALD, surface coverage of precursors was enhanced because the cut-in purge removes the physisorbed precursors securing the reactive sites beneath them; thus, nucleation density was greatly increased. Therefore, the grain size decreased, which changed the microstructure and increased oxygen impurity concentration. However, a more metallic Ru thin film was formed due to thermodynamic stability and improved physical density. Consequently, DF-ALD enables the deposition of the ultrathin (3 nm) and continuous Ru film with a low resistivity of ∼60 µΩ cm and a high effective work function of ∼4.8 eV.

Black Si Photocathode with a Conformal and Amorphous MoSx Catalytic Layer Grown Using Atomic Layer Deposition for Photoelectrochemical Hydrogen Evolution.

We demonstrated how the photoelectrochemical (PEC) performance was enhanced by conformal deposition of an amorphous molybdenum sulfide (a-MoSx) thin film on a nanostructured surface of black Si using atomic layer deposition (ALD). The a-MoSx is found to predominantly consist of an octahedral structure (S-deficient metallic phase) that exhibits high electrocatalytic activity for the hydrogen evolution reaction with a Tafel slope of 41 mV/dec in an acid electrolyte. The a-MoSx has a smaller work function (4.0 eV) than that of crystalline 2H-MoS2 (4.5 eV), which induces larger energy band bending at the p-Si surface, thereby facilitating interface charge transfer. These features enabled us to achieve an outstanding kinetic overpotential of ∼0.2 V at 10 mA/cm2 and an onset potential of 0.27 V at 1 mA/cm2. Furthermore, the a-MoSx layer provides superior protection against corrosion of the Si surface, enabling long-term PEC operation of more than 50 h while maintaining 87% or more performance. This work highlights the remarkable advantages of the ALD a-MoSx layer and leads to a breakthrough in the architectural design of PEC cells to ensure both high performance and stability.

Tuning Two Interfaces with Fluoroethylene Carbonate Electrolytes for High-Performance Li/LCO Batteries.

Various electrolytes have been reported to enhance the reversibility of Li-metal electrodes. However, for these electrolytes, concurrent and balanced control of Li-metal and positive electrode interfaces is a critical step toward fabrication of high-performance Li-metal batteries. Here, we report the tuning of Li-metal and lithium cobalt oxide (LCO) interfaces with fluoroethylene carbonate (FEC)-containing electrolytes to achieve high cycling stability of Li/LCO batteries. Reversibility of the Li-metal electrode is considerably enhanced for electrolytes with high FEC contents, confirming the positive effect of FEC on the stabilization of the Li-metal electrode. However, for FEC contents of 50 wt % and above, the discharge capacity is significantly reduced because of the formation of a passivation layer on the LCO cathodes. Using balanced tuning of the two interfaces, stable cycling over 350 cycles at 1.5 mA cm-2 is achieved for a Li/LCO cell with the 1 M LiPF6 FEC/DEC = 30/70 electrolyte. The enhanced reversibility of the Li-metal electrode is associated with the formation of LiF and polycarbonate in the FEC-derived solid electrolyte interface (SEI) layer. In addition, electrolytes with high FEC contents lead to lateral Li deposition on the sides of Li deposits and larger dimensions of rodlike Li deposits, suggesting the elastic and ion-conductive nature of the FEC-derived SEI layer.

Sulfur-Enhanced Field-Effect Passivation using (NH4)2S Surface Treatment for Black Si Solar Cells.

We demonstrated surface passivation of a black Si-based solar cell using an (NH4)2S solution to mitigate surface recombination velocity. Incorporated S at the interface between atomic-layer-deposited Al2O3 and black Si by (NH4)2S solution treatment boosted the density of negative fixed charges, S-enhanced field-effect passivation. Furthermore, NH4OH generated during (NH4)2S solution treatment removed the defective Si phase at the black Si surface, the surface cleaning effect. The optimized (NH4)2S solution treatment significantly enhanced the internal quantum efficiency up to ∼17.2% in the short wavelength region, suggesting suppressed surface recombination. As a result, photoconversion efficiency of the cell increased from 11.6 to 13.5%, by 16% compared to the control cells without (NH4)2S solution treatment.

Ultrathin ZnS and ZnO Interfacial Passivation Layers for Atomic-Layer-Deposited HfO2 Films on InP Substrates.

Ultrathin ZnS and ZnO films grown by atomic layer deposition (ALD) were employed as interfacial passivation layers (IPLs) for HfO2 films on InP substrates. The interfacial layer growth during the ALD of the HfO2 film was effectively suppressed by the IPLs, resulting in the decrease of electrical thickness, hysteresis, and interface state density. Compared with the ZnO IPL, the ZnS IPL was more effective in reducing the interface state density near the valence band edge. The leakage current density through the film was considerably lowered by the IPLs because the film crystallization was suppressed. Especially for the film with the ZnS IPL, the leakage current density in the low-voltage region was significantly lower than that observed for the film with the ZnO IPL, because the direct tunneling current was suppressed by the higher conduction band offset of ZnS with the InP substrate.