Atomic Layer Deposition (ALD) Process Development of Nb-doped TiO₂ as a Transparent Conducting Oxide (TCO) and ALD of HfO₂/Nb₂O₅ Bilayers as Insulating Barriers for Metal/Insulator/Insulator/Metal (MIIM) Diodes
Transparent conductive oxides (TCOs), primarily indium doped tin oxide, have been widely used in numerous fields since decades ago, such as solar cells, displays, OLEDs and ‘smart windows’ etc. Owing to the scarcity of indium, there is a great demand for new alternative materials. Metal/insulator/metal (MIM) diodes are a critical component of high speed hot electron transistors as well as optical rectennas for energy harvesting. The performance of the MIM diode is one of the major factors limiting these applications. The objectives of this thesis are to (i) deposit and investigate the electrical and optical properties Nb doped TiO₂ as a TCO and (ii) to deposit and investigate the electrical properties HfO₂ and Nb₂O₅ in MIIM diodes. These films are deposited using atomic layer deposition (ALD). Due to potential nucleation delays, doping and formation of laminates via ALD is not always straightforward.
For Nb-doped TiO₂ films, different doping strategies are investigated along with the impact of post deposition forming gas anneals. It is found that the Nb content and the resistivity of Nb-doped TiO₂ can be precisely controlled by Nb₂O₅ cycle ratios. The lowest resistivity film of approximately 4.2 x 10-3 Ω-cm is achieved with a 0.39 atomic Nb/(Ti+Nb) ratio, and has a transmittance spectra of 62% - 82% in the visible region, and an RMS roughness of 0.18 nm.
ALD is also used to fabricate TaN/HfO₂/Nb₂O₅/Al and TaN/Nb₂O5/HfO₂/Al diodes. Though all as-deposited diodes show a poor asymmetry (η[subscript asym]), η[subscript asym] can be significantly improved by post deposition annealing in air and forming gas.
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