Tungsten doped vanadium dioxide via atomic layer deposition with tailored thermochromic electrical and optical response

Abstract

With the rise of world temperatures due to global warming, the importance of efficient cooling has increased drastically. Radiative coolers provide a route to effectively emit thermal energy into space. In areas with large diurnal ranges or seasonal temperature variations, however, constant cooling is undesired. Self-adaptive radiative coolers can produce large radiated power at high temperatures and reduced radiated power at low temperatures by passively varying their emissivity (ε). These devices rely upon thermochromic materials, which undergo large changes in their material, optical, and electrical response due to a temperature-driven phase transition. These transitions are induced through the addition of thermal energy by absorbing radiation, Joule heating, or conduction. One such material that exhibits these characteristics is vanadium dioxide - VO2 - a solid-solid phase change material that can reliably switch between its low temperature monoclinic and high-temperature rutile phase around 69 °C. By doping, the thermochromic response can be tailored, extending the material uses to areas including smart windows, memristors, and sensing. For these applications, thin-film devices provide a route for low-cost, lightweight structures with enhanced performance through smart metamaterial and multilayer design. The performance of such devices is highly dependent upon film quality and thickness control. By utilising the novel atomic layer deposition (ALD) of VO2 and W-doped VO2 thin films, devices were produced with adjusted thermochromic responses for a variety of optical and electrical applications.
A patterned VO2 metasurface deposited upon an optimised Salisbury screen stack for enhanced infrared IR absorption was fabricated, creating a smart IR emitter with high visual transparency – a smart window. The VO2 thermal emitter demonstrated a tuneable IR emissivity (∆ε) of 0.26, with a 62 % improvement in solar transmittance by the metasurface patterned design compared to its planar counterpart, attributed to the reduced VO2 coverage. The device displays much greater radiated powers under daytime illumination owing to the increased high temperature ε of the device for terrestrial and space applications, displaying the suitability of the design for use in both environments.
Reduction of the transition temperature of VO2 films to room temperature was demonstrated through doping with W. By varying the W/V cycle ratio during ALD deposition, five uniform W-doped VO2 stacks with different W concentrations were fabricated and characterised. Predictable control of the transition temperature was shown to be possible while maintaining material and optical performance. By parameterising their temperature dependent electrical response, a novel model was proposed to predict the thermochromic resistivity response of W:VO2 films based on their W doping at.%. Optimised multilayer W:VO2 device designs were then simulated for use as high-sensitivity microbolometers with extended phase transition responses across chosen operating temperature windows.
To experimentally demonstrate the extended phase transition of multilayer doped W:VO2 stacks, several films were fabricated with thickness and W doping control using ALD. Three W:VO2 devices on high ε SiO2 were fabricated and optically tested, with each displaying a negative thermochromic ε response for optical stealth. Two additional multilayer W:VO2 devices were manufactured as high-sensitivity microbolometers across two operating temperature windows. Depth-resolved XPS verified a W gradient through the as-deposited films of ∆W ≈ 0.7 and 2.0 at.%. Their electrical thermochromic performances were assessed, with both stacks displaying a convoluted bi-layer response indicative of two distinct W:VO2 layers within a single film. This is the first demonstration of an extended phase transition multilayer W:VO2 device deposited by ALD, opening new pathways for VO2 material control.

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Identifiers

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

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

ORCID for Otto Muskens: orcid.org/0000-0003-0693-5504

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