Elsevier

Surfaces and Interfaces

Volume 20, September 2020, 100590
Surfaces and Interfaces

Structural and electrical properties of Mg-doped vanadium dioxide thin films via room-temperature ion implantation

https://doi.org/10.1016/j.surfin.2020.100590Get rights and content

Highlights

  • Successful deposition of (011) preferential phase monoclinic structure of VO2 thin films by PLD.

  • Mg ion implantation at higher fluence above 1 × 1017 ions/cm2 result in the degradation of the defined VO2 structure.

  • At higher fluence MgO phase is prevalent at the surface of VO2 resulting in the loss of thermochromism.

  • Mg content tend to increase the carrier concentration, whilst increasing the work function parameter due to the displacement damage and defects induced.

  • Dose implants affect the abrupt nature of VO2 SMT and stabilizing the monoclinic phase as the temperature increases.

Abstract

We report on the structural and electrical properties of Mg ion implanted semiconductor-metal transition of vanadium dioxide thin films with three different ion fluences. The effect of introducing Mg implants showed interchange, rearrangement, and modification of atoms in the lattice structure of VO2. The (011) monoclinic signature of VO2 is significantly affected; (i) peak broadening and shift of the peak position to lower angles at 1 × 1015 ions/cm2, (ii) revert and shift towards high angles at 1 × 1016 ions/cm2 and (iii) ceasing to exist at 1 × 1017 ions/cm2 suggesting a complete devolution of the VO2 thin films at higher fluence. Composition and profiling of the V-O matrix revealed a corresponding decrease of the vanadium edge peak at 1 × 1017 ions/cm2 and a decrease of the oxygen content in implanted films. Whilst the carrier concentration is increased, the magnitude of the semiconductor-metal transition of implanted VO2 thin films is significantly affected with a decrease of career mobility. Whilst increasing ion fluence above 1 × 1017 ions/cm2 at a depth of 40 nm, the Mg implants move relatively towards the surface of VO2 resulting in the loss of the characteristic thermochromism of VO2 films.

Introduction

Materials with first-order phase transition are well studied and have attracted a great of attention in optoelectronic applications. One of the most fascinating oxides among others is vanadium dioxide (VO2) due to its ultrafast reversible phase transition (monoclinic semiconductor – rutile metal phase) at temperature relatively close to room temperature (Ttr=68 °C) [1], [2], [3], [4]. The temperature-driven phase transition is also associated with a rapid resistivity change and this characteristic made VO2 a potential material for resistive switching devices [5,6]. However, the material has major drawbacks which hinder real applications, such as high transition temperature (Ttr), low luminous transparency (Tlum) at temperatures above Ttr, wide hysteresis width, the unfavourable brown-yellowish colour of the film and lastly the alteration of the film's properties with time since it is not thermodynamically stable oxide [5,7].

In order to overcome these drawbacks, proposed mechanisms such as deposition methods, modification, design, and understanding of the physics behind the ultrafast phase transition of VO2, have advanced for the development of potential applications. Maaza et. al, reported on the successfully synthesized pulsed laser VO2 nano-coatings as optical limiters for Nd-Yag sources [8]. Further reports emerged demonstrating the possibility to engineer reversible and tunable VO2 nano-scaled thin films by pulsed laser deposition (PLD) for potential femtosecond tunable optoelectronic nano-gating [9]. High quality monoclinic rutile-type vanadium dioxide (VO2) (M1) nano-particles were synthesized by PLD without post-annealing and on a glass substrate. The films demonstrated a reversible metal-to-insulator transition at ~43 °C, without any doping, paving the way to switchable transparency in optical materials at room temperature [10]. Introducing dopants has proven to be the most effective way to alter and modify the characteristic properties of pure VO2 thin films.

Previous studies have exceedingly reported on the modification of the semiconductor-metal transition properties of VO2 using dopants such as Tin (Sn)[11], Aluminium (Al) [12,13], Tungsten (W) [13,17,[20], [21], [22], [23]], Molybdenum (Mo) [16,24], Niobium (Nb) [25], Magnesium (Mg) [26, 27], Titanium (Ti) [16,19], Fluorine (F) [15,20], Silicon (Si) [18] and Cesium (Ce0 [14]. Overall conclusions that Tungsten (W) is the most effective dopant for reducing the transition temperature (Ttr) much closer to room temperature [27], [28], [29] whilst Mg has also been well reported to effectively reduce Ttr and further increase the transmission properties of VO2 (Tlum) [27, 30]. Most of these reports used solution-based and chemical methods such as spin-coating and hydrothermal synthesis for doping of VO2 films. Bulk and surface property modification of VO2 thin films are significant for a wide range of optoelectronic applications.

In this study; we use standard microelectronics technique of ion implantion to dope the VO2 thin films. Ion implantation is particularly of interest in this work since it has better advantages compared to diffusion methods. Dopants can be introduced in a semiconductor in a more controlled way as compared to chemical doping techniques and can be easily repeated [31,32]. Synthesis of the VO2 (M)/VO2 (R) requires a controlled process that provides the necessary oxygen stoichiometry and correct crystalline structure because vanadium has different many oxidation states such as V2O5, V2O3, and VO. In this study, pristine VO2 thin film was deposited by the PVD 3000 fully automated laser ablation system. Compared with other conventional laser ablation deposition and physical deposition methods, the PVD 3000 scanning options make it possible to obtain a very good homogeneity and uniformity of thin metallic layers on surfaces with very good control on the deposition temperature [33]. Magnesium was chosen as the doping material via ion beam implantation technology without conducting any post-treatment for the recrystallization of the implanted film, recovery and improvement of thermochromism. Nanothermochromics of Mg-doped VO2 Mg has been reported to effectively reduce Ttr of VO2 and additional benefits for practical arrangement and design of smart window technology such as a less yellowish appearance [27]. Panagopoulou et.al reported on the thermochromic performance of Mg-doped VO2 thin films on functional substrates for glazing applications. They recorded the lowest transition temperature for Mg-doped VO2 at Ttr = 35 °C using reactive magnetron sputtering on deposited on glass coated with SnO2 buffer layer and, ZnO/glass substrates [34]. The effect of modification and property change of Mg-doped VO2 thin films studied by XRD, RBS, Hall-Effect and Four-point probe measurements.

Section snippets

Experimental details: synthesis and ion implantation of VO2

100 nm thick VO2 thin film deposited on a diameter Si (100) substrate was used in the study and these samples were fabricated by reactive pulsed laser deposition (RDLP) technique, equipped with KrF laser wavelength of 248 nm and a vacuum base pressure of 10−6 Torr. The target (vanadium)-substrate distance was set at around 6.5 cm for a good uniformity of the thickness film, with substrate temperature kept at 500 °C in a pure oxygen environment. 1 × 1 cm deposited thin films of VO2 were

Structural analysis of VO2: Mg

Prior to implantation, DYNAMIC-TRIM [36] simulations were performed to calculate Mg depth profiles for the 35 keV implantation with fluences between 1 × 1015 ions cm−2 and 1 × 1017 ions cm−2. The profiles are shown in Fig. 2. The calculations predict a mean projected range of 40 nm coupled with a maximum implantation depth of around 80 nm. For these fluences, the Mg peak concentration varies between 0.2 and 17 at%. At fluences larger than 1 × 1016 Mg cm−2, the Mg profile intersects with the

Conclusion

This work is based on the investigation of the effect of implanted Mg ions on the structure and electrical properties of VO2 thin films. VO2 thin films with a prevalent monoclinic crystal phase were successfully deposited by PLD and the desired Mg ions were successfully implanted at three different fluences 1 × 1015, 1 × 1016 and 1 × 1017 ions/cm2 with no foreign atom detected. The experimental data measured structurally and electrically revealed the following important results; at lower doses

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.

Acknowledgment

The author wishes to appreciate the financial support from the National Research Foundation of South Africa. Also, to thank the GNS-Science at New Zealand, University of South Africa, University of The Western Cape, University of Pretoria and iThemba LABS (material research department) for providing the facilities required for the ion implantation and characterization facilities to analyse our samples.

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