Controlling Growth of Metals and Semiconductors for Optoelectronic and Photonic Device Application

Abstract

For past decades, research on semiconductor/metal thin films and nanostructures have earned a great interest from both scientific and engineering community due to its impact on optoelectronic device application. Ability to precisely control the growth and morphology of these metal/semiconductor thin films and nanostructures can play a key role in enhancing the performance of such device. Various methods like physical vapor depositions (PVD), chemical vapor deposition (CVD), or molecular beam epitaxy (MBE) have been extensively studied to obtain desired semiconductor / thin films and nanostructures. Regardless of the methods, it is difficult to precisely control the growth of nanostructure and thin film as one intended due to various reasons like large surface energy difference or lattice mismatch between the substrate and the film. The aim of this paper is to explore various growth regimes by controlling the growth mode and utilize metal/semiconductor thin films and nanostructures to solve challenging problems in optoelectronic and photonic applications.

The first part of the thesis focuses on controlling the growth of silver (Ag) film by inhibiting de-wetting property to make the film continuous down to extremely thin regime (< 5nm). By controlling the nucleation sites, extremely thin and smooth Ag film is obtained which can be used as a transparent conductor for optoelectronic devices. Detailed study of governing electron transport mechanism in extremely Ag film is discussed and its association with optical properties are discussed. Then, this film is demonstrated as a transparent anode to solve challenging problems in light emitting devices. Also, methods to further enhance the optical property and environmental stability of Ag film as a transparent conductor for its commercialization is detailed. Additionally, the transparent conductor film is integrated with amorphous silicon thin-film (via CVD) photodiode array to show its potential for in-display optical fingerprint sensor.

The second part of the thesis focuses on taking advantage of de-wetting property of Ag adatom on oxide substrate to create metal-nanocomposite layer having fine-sized Ag nanoparticles embedded into silicon nitride dielectric. This can be done in one-step process by using PVD, a method widely practiced in industry for thin film deposition. This nanocomposite layer is used to enhance the angle-robustness of colored low-emissive coating for window application.

The last part of the thesis focuses on layer-by-layer growth of highly ordered single crystalline InGaN nanostructure with high indium (In) composition by using molecular beam epitaxy (MBE). With the aid of nanopatterning lithography, lattice mismatch between InGaN ternary and GaN binary structure can be efficiently relaxed thereby enabling the growth of single crystalline InGaN semiconductor with high In composition. This semiconductor is used as a photoanode to convert solar energy into chemical energy by splitting water molecules. Detailed study of charge transport from this single-crystalline semiconductor to liquid junction is studied using electrochemical impedance spectroscopy.