AVS2012 Session TF-MoM: ALD Enabled Applications

In this work we investigate atomic layer deposition (ALD) to create transparent oxide diffusion barrier coatings to reduce the rate of tarnishing for silver objects in museum collections. Elevated heating and H₂S pressure tests determined the effect of various thicknesses of Al₂O₃ ALD thin films, ranging from 5 to 100nm thick, and the effect of annealing temperature on the thickness of the tarnish layer (Ag₂S) created at the interface of the ALD coating and the silver substrate. Reflectance spectroscopy and an integrated sphere spectrophotometer were used to measure the thickness of the tarnish layer and indicate the effectiveness of the Al₂O₃ ALD thin films in reducing the tarnishing rate of silver while minimally affecting the visual appearance of the silver. X-ray photoelectric spectroscopy (XPS), and time of flight secondary ion mass spectroscopy (TOF-SIMS) analysis determined the concentration profile of sulfur through the ALD oxide film. A model for predicting the tarnishing kinetics of sulfur diffusion through ALD oxide films is established, influenced by the composition of the alloy, phase of sulfide at the interface, and non-uniformity of diffusion due to pinhole and concentration dependence. Evidence was found for the slow diffusion of sulfur through the bulk of the films and faster diffusion through pinholes in the ALD oxide films.

Due to its unique resistive switching capability, Metal-Insulator-Metal (MIM) diodes have attracted substantial interests ever since 1960’s. The early generation of MIM device (point-contact diode) is composed of a sharp metal tip placed right on top of a planar metal electrode coated with an ultra-thin layer of insulator. It has been envisioned that MIM diodes hold great promise for detecting and mixing high frequency signals up to Terahertz (THz) range. Particularly, several prior works have suggested for operation at THz frequencies, the MIM diodes are anticipated to outperform heterojunction diodes, which have limited cutoff frequency (<3THz). Hence, the MIM devices are well-suited for a wide range of applications in security, imaging and energy scavenging. In lieu of this, microfabricated MIM diodes with low zero-bias impedance have been actively pursued in this work for a strategically-designed antenna-coupled detector with high responsivity.

With introduction of ultra-thin tunneling layer sandwiched between two planar metal electrodes, the microfabricated MIM diodes are amenable for direct integration with ICs. In particular, we have devoted most of our efforts on developing techniques to improve the overall diode characteristics through tuning its tunneling properties. The effective junction capacitance and resistance limit the operation frequency range of tunneling diode. The key factors that affect the frequency range of tunneling diodes are the junction area, defect density, permittivity and thickness of the tunneling layer. For the MIM diode, a small deviation in tunneling barrier thickness can cause a significant change in the junction resistance and its I-V responses. Hence, atomic layer deposition (ALD) process was employed which offer superb uniformity, low defect density, and precise thickness control of the tunneling layer. In this work, MIM diodes with a variety of junction areas ranging from 3µm´3µm to 100µm´100µ have been fabricated with different ALD thin films (e.g., TiO₂ and HfO₂) of varied thicknesses. By systematic investigation of the measured I-V characteristics for MIM devices with a variety of junction materials, junction thickness and junction area, it is evident that the performance of the MIM tunneling diodes can be greatly enhanced through optimizing junction properties (material, thickness, area, etc.). As compared to similar devices reported previously, we have successfully demonstrated high-yield MIM diodes with 1.5nm-thick tunneling junction with unprecedented low junction resistance in the range of 500Ω or even less, thus resulting in greatly enhanced responsivity.

Nanoparticle modification of inorganic thin films, such as TiO₂ or Al₂O₃, represents a method to enhance or alter the catalytic, antimicrobial, or responsive behaviors of a material. The use of nanoparticle inks for application is challenging, as the nanoparticles tend to be stabilized using surfactants or other molecules during the synthesis process, and these ligands can interfere with or prevent nanoparticle adsorption onto the oxide. This work shows the efficacy to attach ligand-free Ag nanoparticles onto TiO₂ surfaces formed by atomic layer deposition (ALD). Sonochemical synthesis is used to create Ag NPs of 14-20 nm diameter in an aqueous solution at room temperature without the need for additives such as stabilizing agents. After ALD of TiO₂ thin films onto polymer substrates, a facile dip coating process at ambient conditions results in the uniform decoration of the ligand free Ag nanoparticles on the TiO₂ surface. The efficiency of nanoparticle adsorption is examined by TEM and EDS. Finally, the photocatalytic degradation of methylene blue is used to assess the photocatalytic efficiency of the fiber based composite.