Structural twinning-induced insulating phase in CrN (111) films

Qiao Jin, Zhiwen Wang, Qinghua Zhang, Jiali Zhao, Hu Cheng, Shan Lin, Shengru Chen, Shuang Chen, Haizhong Guo, Meng He, Chen Ge, Can Wang, Jia-Ou Wang, Lin Gu, Shanmin Wang, Hongxin Yang, Kui-juan Jin, and Er-Jia Guo

Phys. Rev. Materials 5, 023604 – Published 22 February 2021

Abstract

Electronic states of a correlated material can be effectively modified by structural variations delivered from a single-crystal substrate. In this paper, we show that the CrN films grown on MgO (001) substrates have a (001) orientation, whereas the CrN films on $α − {Al}_{2} O_{3}$ (0001) substrates are oriented along the (111) direction parallel to the surface normal. Transport properties of CrN films are remarkably different depending on crystallographic orientations. The critical thickness for the metal-insulator transition in CrN 111 films is significantly larger than that of CrN 001 films. In contrast to CrN 001 films without apparent defects, scanning transmission electron microscopy results reveal that CrN 111 films exhibit strain-induced structural defects, e.g., the periodic horizontal twinning domains, resulting in an increased electron scattering facilitating an insulating state. Understanding the key parameters that determine the electronic properties of ultrathin conductive layers is highly desirable for future technological applications.

Received 24 October 2020
Accepted 9 February 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.023604

Physics Subject Headings (PhySH)

Domains Electrical conductivity Microstructure Phase transitions Transition temperature

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Qiao Jin^1,2,*, Zhiwen Wang^3,*, Qinghua Zhang¹, Jiali Zhao¹, Hu Cheng⁴, Shan Lin^1,2, Shengru Chen^1,2, Shuang Chen⁵, Haizhong Guo⁵, Meng He¹, Chen Ge¹, Can Wang^1,2,6, Jia-Ou Wang ⁷, Lin Gu ^1,2,6, Shanmin Wang⁴, Hongxin Yang^3,†, Kui-juan Jin^1,2,6,‡, and Er-Jia Guo ^1,6,8,§

¹Beijing National Laboratory for Condensed Matter Physics and Institute of Physics,Chinese Academy of Sciences, Beijing 100190, China
²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
³Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China
⁴Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
⁵School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450001, China
⁶Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
⁷Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁸Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

^*These authors contributed equally to this work.
^†Correspondening author: hongxin.yang@nimte.ac.cn
^‡kjjin@iphy.ac.cn
^§ejguo@iphy.ac.cn

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Vol. 5, Iss. 2 — February 2021

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Images

Figure 1
Structural and electronic state characterization for (001)- and (111)-oriented CrN films. (a) Crystal structure of CrN (space group Fm-3 $m$ ). Blue and orange spheres represent the Cr and N atoms, respectively. (b), (c) Planar views of the CrN and ${Al}_{2} O_{3}$ crystal structure for (111) and (0001) orientations, respectively. (d) XRD θ-2θ scans of 20-nm-thick CrN films grown on ${Al}_{2} O_{3}$ (0001) and MgO (001) substrates. The diffraction peaks from the substrates are indicated by asterisk (*). The curves are offset for clarity. Insets show the high-magnified scans around 111 and 001 peaks of the CrN film grown on ${Al}_{2} O_{3}$ and MgO substrates. The corresponding rocking curves for (e) CrN 111 and (f) CrN 001 peaks, respectively. The FWHMs of both rocking curves are ∼0.04 °. (g) XAS at N K and Cr L edges for the (001)- and (111)-oriented 12-nm-thick CrN films. The reference data from Ref. [14] is shown for direct comparison.
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Figure 2
Transport properties of CrN films. (a) ρ-T curves of the (001)- and (111)-oriented CrN films with the thickness of 20 nm. Inset: the linear fit of $\ln (ρ) - {(T)}^{- 1}$ curve of a 20-nm-thick CrN 111 film. (b) ρ-T curves of the CrN 111 films with the thickness ranging from 2.4 to 120 nm. (c) $T_{N}$ and (d) ρ of (001)-oriented CrN film as a function of film thickness. (e) ρ of (111)-oriented CrN film as a function of film thickness. Previous studies have shown that the critical thickness for metal-to-insulator transition (MIT) in the CrN 001 films is approximately 12 nm [30], whereas the critical thickness of MIT in the CrN 111 films increases to 50 nm.
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Figure 3
Microstructural characterizations of CrN films. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images of (a) (001)- and (b) (111)-oriented CrN films. The HAADF-STEM images of CrN 001 and CrN 111 films are acquired along the [100] and [−1100] zone axis, respectively. (c), (d) High-magnified STEM images of CrN 001 film bulk and CrN-MgO interface, respectively. The layer-resolved integrated red line profile is plotted in (g). (e), (f) High-magnified STEM images of CrN 111 film bulk and CrN- ${Al}_{2} O_{3}$ interface, respectively. The yellow dashed lines indicate the positions of structural twins. The layer-resolved integrated green line profile is plotted in (h).
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Figure 4
Orientation-dependent band structure of CrN films. (a), (b) Band structures of CrN films with the fixed thickness of 10 u.c. The band gap in CrN 001 films is −13 meV, which suggests a metallic state. In contrast, the band gap in CrN 111 films with an identical thickness is 66 meV, which demonstrates an insulating state.
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