Atomic Layer Deposition of Vanadium Dioxide: Synthesis, Doping, and Device Applications

Abstract

Vanadium dioxide (VO₂) is a phase change material whose metal-insulator transition (MIT) has inspired a variety of applications. The MIT makes VO₂ useful for devices with improved switching performance and as the building block for alternative computing architectures such as oscillators and memristors. The pressing issue of integrating VO₂ into the existent semiconductor device fabrication process is the synthesis of VO₂ thin films onto amorphous substrates. This thesis presents the research that aims to synthesize VO₂ thin films by atomic layer deposition (ALD), to dope VO₂ films with germanium (Ge) by ALD, and to demonstrate the device applications of ALD VO₂ films. The VO₂ thin films have been deposited onto amorphous substrates by a process of multiple pulsing ALD. A post-deposition annealing process crystallizes the as-deposited amorphous VO₂ films into polycrystalline films with grains. The sintering process in the films during the annealing is found to be affecting the microstructure including grain size. The characterization has confirmed that the microstructural scaling influences the MIT characteristics, which defines the basic electrical property of the VO₂ films. A Ge-doping into VO₂ films by ALD has been achieved using a mixture of liquid precursors, elevating the transitional temperature of the VO₂ films into the temperature range of circuit applications. The Ge-doping and the film thickness are seen to be modulating the microstructure and the electrical property of the VO₂ films. The ALD VO₂ films have been used in device applications. The microstructural factor is been found to be a prominent factor in the MIT of devices as well. An oscillator neural network has been demonstrated using VO₂ two-terminal resistor in the oscillator unit. The VO₂ films have also been made into selector devices for memristive device array, which forms a neural network. The networks have been used for image recognition. The two applications have proved that the ALD VO₂ films are viable for wafer-scale applications.