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Quantitative Statistical and Strain Mapping Analysis of Grain Anisotropy in VO2 Thin Films Grown by Rf-Biased Reactive Sputtering

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Abstract:

Vanadium dioxide (VO2) is a model correlated oxide whose insulator–metal transition (IMT) can be exploited in a wide range of switching, sensing, and adaptive optical devices. Precise control of its microstructure and stress state is therefore essential for optimizing functionality. In this work, the morphological evolution of VO2 films on Al2O3 (001) substrates deposited by rf sputtering with substrate biasing of 20, 30, and 40 W was investigated using a combined imaging and statistical approach. FE-SEM images revealed a transition from isotropic grain growth at 20 W to the strongly elongated grains at 40 W. Otsu's method was used for grain segmentation, followed by ellipse fitting, which enabled the extraction of aspect ratio (AR) and circularity. Analysis of variance (ANOVA), Welch's ANOVA, and post-hoc comparisons methods confirmed that only film deposited at 40 W showed a significant increase in AR, with the median shifting from ~0.52 to ~0.60. Violin and box plots highlighted wider and multimodal distributions on films deposited with high biasing power. At the same time, histograms confirmed the widest spread at 30 W and a mixed grain size distribution at 40 W. Strain mapping based on AR deviations showed uniform stress at 20 W, localized anisotropy at 30 W, and significant heterogeneous compressive stress at 40 W. A weak but statistically significant Spearman correlation was observed between AR and circularity (ρ = 0.252, p = 0.0000), linking grain elongation to circularity loss, consistent with stress-driven anisotropy in the film deposited with high biasing power. These results demonstrate that substrate biasing resulted in the introduction of grain anisotropy, stress distribution, and microstructural disorder, and thus affects the IMT properties of the VO2 thin films. This combined methodology of images and statistical analysis provides further understanding of both the microstructure and functional performance in correlated oxide films.

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Journal of Nano Research Vol. 93 View Preview

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Periodical:

Journal of Nano Research (Volume 93)

Pages:

41-50

DOI:

https://doi.org/10.4028/p-rAYm6z

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Online since:

July 2026

Authors:

Nurul Hanis Azhan*, Nor Azuana Ramli, Kunio Okimura, Abdul Manaf Hashim

Keywords:

Grain Anisotropy, Morphological Analysis, Rf Bias Sputtering, Strain Mapping, VO2 Thin Films

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© 2026 Trans Tech Publications Ltd. All Rights Reserved

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