ENGINEERING STRUCTURE AND PHASE VIA TEMPERATURE MANIPULATION IN THIN FILM OXIDES GROWN BY ALD
Piercy, Brandon Deane
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Atomic Layer Deposition (ALD) is a critical thin film deposition technology with applications spanning microelectronics, solar fuels and biological preservation due to its advantages of a wide precursor library for the majority of the periodic table, exquisite control over growth rates, and the ability to deposit at extremely low temperatures. One critical parameter in ALD growth is deposition temperature, which defines the “ALD window”—a temperature range in which ALD growth kinetics hold. Most ALD processes tend to be limited to deposition temperatures of <300 ˚C, causing ALD films to be typically amorphous as-deposited. While amorphous inorganic films are often thought to be structurally equivalent, films grown by ALD can vary significantly in mechnical, optical, thermal, and electronic properties. By changing deposition temperature from room temperature to 150 ˚C, amorphous TiO2 films are shown to exhibit a change in density of 15% and a change in optical polarizability of 10%. These shifts in density correspond to a decrease in the thermal conductivity of TiO2 thin films and a decrease in Ti3+-based electron trap states. High deposition temperatures are usually required to deposit crystalline films, which are typically not compatible with ALD precursors. A method called resistive pulsed-heating ALD (PH-ALD) is introduced as a strategy to circumvent the thermal limitations of ALD precursors by applying a short heat pulse to the film after low-temperature deposition cycles in the ALD process. The PH-ALD technique is used to grow epitaxial ZnO films on c-plane sapphire using the low-temperature diethylzinc-water ALD process. Epitaxial films are grown at pulse temperatures of 500˚C and above, and at deposition cycles:heat pulse ratios of 5:1. Furthermore, the PH-ALD process is used for templating epitaxial growth in ZnO, with films as thin as 5 PH-ALD cycles capable of templating an epitaxial film up to 100 nm thick. Electrical and optical measurements are shown to be comparable to films grown by other physical and chemical vapor deposition techniques, indicating that the PH-ALD method may be a practical approach for the growth of complex crystalline oxide thin films. To support future workers in this field, detailed descriptions of control software, hardware, and reactor designs are provided to enable the development of ALD systems that can incorporate external sensors and controllers for advanced intelligent processing recipes.