Vanadium dioxide for nanophotonic memory: device design and low-temperature annealing

Abstract

Optical memory is considered a promising solution to overcome the bottlenecks of the traditional von Neumann architecture, with its core challenge lying in the search for suitable functional materials. Vanadium dioxide (VO₂), as a solid-state phase change material, exhibits great potential in optical circuits, neuromorphic computing, and radiative cooling due to its phase transition properties near room temperature. Currently, VO₂ based memory devices typically rely on the intrinsic hysteresis characteristics of the material, posing significant challenges for large-scale production and design optimisation. Moreover, the practical application of VO₂ in optical memory remains constrained by fabrication techniques. Conventional VO₂ thin film fabrication generally requires high-temperature (>400°C) direct deposition or the deposition of amorphous VO₂ followed by high-temperature annealing to achieve high-quality crystalline phases. However, such high-temperature processes not only limit the compatibility of VO₂ with CMOS integrated circuits, particularly post-metallisation processes, but also make it difficult to deposit VO₂ onto flexible polyimide substrates, restricting its applications in flexible substrates.

To address these challenges, this thesis explores a low-temperature VO₂ thin film fabrication method combining atomic layer deposition (ALD) and low-temperature annealing. Systematic material characterisation demonstrates, for the first time, the successful realisation of high-quality VO₂ thin films on a silicon substrate under a 300°C annealing condition, significantly reducing the thermal budget. Further investigations reveal that lowering the annealing temperature to 250°C results in insufficient VO₂ crystallisation, whereas increasing it to 400°C leads to over-oxidation, forming V₂O₅. Additionally, this method is applied to flexible polyimide substrates and verifies that the high-quality phase change characteristics of VO₂ are preserved. 4 Through Raman spectroscopy and X-ray diffraction (XRD) identified that the annealing process primarily alters crystal strain, which in turn influences the phase transition temperature and refractive index. This insight provides a qualitative understanding of how high-quality VO₂ thin films can be obtained for specific applications. Furthermore, the investigation of the annealing behaviour of W-doped VO₂ found that tungsten doping affects both the annealing and oxidation processes. The results indicate that finely tuned annealing conditions are required to achieve W:VO₂ films with high optical contrast.

This thesis also proposed a device-level bistability approach that leverages the refractive index contrast between the insulating and metallic phases of VO₂. This method exhibits different bistability ranges across various device structures, offering a novel approach to VO₂-based optical memory. Unlike conventional VO₂ optical memory that relies on the material’s hysteresis properties through globally change of the ambient temperature, this approach enhances the robustness of large-scale manufacturing. Furthermore, the bistability characteristics of the microring structure enable it to mimic short-term associative learning. By systematically optimising the structure through finite element simulations, I achieve an optical contrast of 0.59 (ER=8.04 dB). The volatile nature of VO₂ allows it to mimic the learning and forgetting mechanisms of biological neural systems. Additionally, within the bistability range, only a small input stimulus is required to maintain associative memory states.

This work paves the way for low-power on-chip photonic neuromorphic computing based on VO₂, expanding its potential applications in CMOS-compatible optical memory application.

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Identifiers

ORCID for Yuxin Du: orcid.org/0009-0007-7189-803X

ORCID for Xu Fang: orcid.org/0000-0003-1735-2654

ORCID for Kees De Groot: orcid.org/0000-0002-3850-7101

ORCID for Kai Sun: orcid.org/0000-0001-6807-6253

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