Structural transformation and tuning of electronic transitions by W-doping in VO2 thin films

doi:10.1016/j.spmi.2021.106883

Superlattices and Microstructures

Volume 154, June 2021, 106883

https://doi.org/10.1016/j.spmi.2021.106883 Get rights and content

Highlights

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Single-phase and crystalline W-doped VO₂ thin films were deposited using pulsed laser deposition method.
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With just 1.5 at% of doping at V-site, the transition temperature can be decreased from 344 K to 300 K.
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Doping of W at V-site is confirmed by XPS spectra.

Abstract

V_1-xW_xO₂ (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04) thin films have been deposited on single-crystal sapphire substrates using pulsed laser deposition (PLD) technique. The PLD-grown films are pure and crystallographically oriented. The films are found to sustain their crystallinity even at higher percentages of doping. Even a minor inclusion of tungsten (W) starts the process of driving the lattice towards rutile phase, which is confirmed by structural analysis. The doping of W at V-site was confirmed by beamline experiments of X-ray photoemission spectroscopy. As observed from the resistivity curves, initially the transition temperature reduced up to 10 K per 0.5 at % of W doping at V-site, and this decrement is further amplified with an increase in doping. Thus, W doping was successfully employed to alter the structure quite systematically with a significant reduction in the phase transition temperature.

Introduction

In past few years, transition metal oxides have been studied rigorously due to their wide range of properties that are incorporated in various applications like smart devices such as switching [[1], [2], [3], [4]], thermochromic [5,6], electrochromic [[7], [8], [9]], gasochromic [10], photochromic devices [6,11], and sensors [12]. Among these transition metal oxides, vanadium dioxide (VO₂) is one of the promising materials for smart devices. VO₂ is an n-type semiconductor which can undergo a temperature-driven metal-insulator phase transition (MIT) at a temperature of T_c ~340 K [13]. VO₂ exhibits a first-order MIT in which resistivity of the material abruptly changes by 3–5 orders along with a structural transformation from high-temperature rutile (metal) to low temperature monoclinic (insulator) phase [14,15]. The phase transition phenomenon can be explained by the Goodenough model [16]. This model suggests that in the rutile phase, octahedral crystal field splits the d-levels of the V ions into lower-lying triplet t_2g and doublet e_g states. Further, the tetragonal crystal field splits the t_2g into d_|| and π* doublet states. The d_|| orbitals are aligned along the rutile c-axis. In the monoclinic insulating phase, due to the dimerization and tilting of the vanadium atoms, d_|| orbital splits into bonding and antibonding orbitals. The energy difference between bonding d_|| and π*creates the bandgap (Peierls-like) in the insulating state of the VO₂. While this bandgap vanishes due to the overlapping of the d_|| and π* in the metallic state of VO_2.

MIT in VO₂ has applications in many smart devices such as electrical switches [17], memory devices [18], RF microwave switches [19], terahertz metamaterial devices [20], etc. VO₂ thin films can be synthesized using several fabrication techniques such as sol-gel [21], magnetron sputtering [22,23], chemical vapour deposition [24], pulsed laser deposition (PLD) [14,25] etc. PLD provides a highly oriented and single-phase pure thin film. The tendency of vanadium to exist in different valence states with oxygen poses a stiff challenge on the synthesis of a high purity single phase. Hence PLD is used for the deposition of pure stoichiometric single-phase VO₂ thin films. The phase transition temperature of pristine VO₂ thin films can be too high or too low for practical applications. It can be tuned via strain [26], doping [27,28], oxygen content [25] etc., according to the requirement. Doping of an element can be an easy and feasible option for the fine-tuning of the transition temperature. In the past, to increase the transition temperature Al [29], Fe [30], and Cr [31] have been used as dopant element and high-valent transition metals such as Nb [32], Mo [33], and W [34,35] have been used as a dopant to decrease the transition temperature of VO₂ thin films. However, W has been shown to be the most effective dopant. Interestingly the doping of W into the VO₂ matrix affects the transition characteristics. High-valence elements such as W introduce extra electrons into the 3d band of vanadium by charge compensation mechanism. The substitutional doping of the W⁶⁺ in the place of V⁴⁺ shows a remarkable reduction in the phase transition temperature [36].

Recently many groups have been studying the effect of the higher valence atom (W) doping on the electronic and optical properties of VO₂ films [35,37,38]. Still, very limited reports are available on the structural properties of W-doped VO₂ films. In this work, we have studied the changes in structural and electronic properties of the crystallographically oriented pulsed laser deposited VO₂ thin films with inclusion of W doping (V_1-xW_xO₂; x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04). These results can be useful in fine-tuning of structural as well as electronic properties of VO₂ thin films using W doping.

Section snippets

Experimental

Bulk samples of V_1-xW_xO₂ (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04) were prepared using solid-state reaction method. For pellets making, high purity powders of V₂O₃ (>99.99% pure) and WO₃ (>99.99% pure) were mixed in appropriate molar ratios. The mixture of powders was grinded for 4 h in order to make the homogeneous mixture then palletization was done with the hydraulic press. All the pellets were sintered at 1000 °C in Ar atmosphere for 24 h simultaneously. Thin films were deposited on

Structure and morphology

Fig. 1(a) shows the XRD patterns of VO₂ (~40 nm) [39] and W-doped VO₂ thin films grown on sapphire substrates. Here only (020) peaks of VO₂ thin films for doped and undoped films are detected [35]. There is no peak observed related to other crystalline phases of vanadium oxide and no sign of diffraction peak corresponding to the tungsten oxide. Thus, W has been doped at V-site, and these thin films are highly pure and crystalline in nature. The lattice parameter ‘b’ was calculated using 2θ

Conclusions

A series of crystallographically oriented thin films V_1-xW_xO₂ (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04) were deposited on single-crystal sapphire substrate. With the inclusion of W doping, a gradual structural transformation from monoclinic to rutile has been observed without the appearance of any phase anomalies and losing the crystallinity of the films. The XPS spectra show successful doping of W in the VO₂ films and the effect of doping on the valence state of vanadium. With the

Author statement

Komal Mulchandani: Methodology, Investigation, Data curation, Formal analysis, Writing - Original Draft, Writing - Review & Editing, Ankit Soni: Validation, Review and editing, Komal Pathy: Review and editing, Krushna R. Mavani: Conceptualization, Validation, Supervision, Funding acquisition, Review and editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the financial support by SERB, New Delhi, in the form of a project (EMR/2017/001821 of KRM), and BRNS, India (project no. 37(3)/14/28/2017-BNRS/37225 of KRM). KM acknowledges DST, for INSPIRE fellowship (IF170085). Authors are thankful to RRCAT, Indore for providing XPS facility. The facility of Raman Spectrometer under DST-FIST project (SR/FST/PSI-225/2016) of Discipline of Physics, IIT Indore, is acknowledged. The experimental facilities were provided by Sophisticated

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Structural transformation and tuning of electronic transitions by W-doping in VO2 thin films

Highlights

Abstract

Introduction

Section snippets

Experimental

Structure and morphology

Conclusions

Author statement

Declaration of competing interest

Acknowledgements

J. Cryst. Growth

Thin Solid Films

J. Solid State Chem.

Thin Solid Films

Sol. Energy Mater. Sol. Cells

Mater. Res. Bull.

Mater. Res. Bull.

Thin Solid Films

Thin Solid Films

Phys. B Condens. Matter

J. Alloys Compd.

Appl. Surf. Sci.

Ceram. Int.

Electrical switching dynamics and broadband microwave characteristics of VO2 radio frequency devices

J. Appl. Phys.

Multilayer metal-oxide memristive device with stabilized resistive switching

Adv. Mater. Technol.

Bipolar resistive switching with unidirectional selector function in nitride/oxide heterostructures

J. Phys. D Appl. Phys.

Properties of resistive structures based on gallium oxide polymorphic phases

Tech. Phys. Lett.

VO2 thermochromic metamaterial-based smart optical solar reflector

ACS Photonics

VO2-based composite films with exemplary thermochromic and photochromic performance

J. Appl. Phys.

Handbook of Inorganic Electrochromic Materials

Infrared-sensitive electrochromic device based on VO2

Appl. Phys. Lett.

Advances on tungsten oxide based photochromic materials: strategies to improve their photochromic properties

J. Mater. Chem. C.

Oxide electronics utilizing ultrafast metal-insulator transitions

Annu. Rev. Mater. Res.

Oxides which show a metal-to-insulator transition at the neel temperature

Phys. Rev. Lett.

Structural transformation and tuning of electronic transitions by W-doping in VO₂ thin films

Electrical switching dynamics and broadband microwave characteristics of VO₂ radio frequency devices

VO₂ thermochromic metamaterial-based smart optical solar reflector

VO₂-based composite films with exemplary thermochromic and photochromic performance

Infrared-sensitive electrochromic device based on VO₂