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Ultrathin Films of VO on R-Cut Sapphire Achieved by Post-Deposition Etching Tony Yamin, Shai Wissberg, Hagai Cohen, Gili Cohen-Taguri, and Amos Sharoni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02859 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 21, 2016
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ACS Applied Materials & Interfaces
Ultrathin Films of VO2 on R-Cut Sapphire Achieved by Post-Deposition Etching
Tony Yamin1,2, Shai Wissberg1,2, Hagai Cohen3, Gili Cohen-Taguri2 and Amos Sharoni1,2,* 1
Department of Physics, Bar Ilan University, Ramat-Gan, Israel IL-5290002
2
Bar-Ilan Institute of Nanotechnology & Advanced Materials, Ramat-Gan, Israel IL-5290002
3
Department of Chemical Research Support, Weizmann Institute of Science, Rehovot IL-
76100
ABSTRACT: The metal-insulator transition (MIT) properties of correlated oxides thin-films, such as VO2, are dramatically effected by strain induced at the interface with the substrate, which usually changes with deposition thickness. For VO2 grown on r-cut sapphire, there is a minimum deposition thickness required for a significant MIT to appear, around 60 nm. We show that in these thicker films an interface layer develops, which accompanies the relaxation of film strain and enhanced electronic transition. If these interface dislocations are stable at room temperature, we conjectured, a new route opens to control thickness of VO2 films by postdeposition thinning of relaxed films, overcoming the need for thickness dependent strainengineered substrates. This is possible only if thinning does not alter the films’ electronic properties. We find that wet etching in a dilute NaOH solution can effectively thin the VO2 films, which continue to show a significant MIT, even when etched to 10 nm, for which directly deposited films show nearly no transition. The structural and chemical composition were not modified by the etching, but the grain size and film roughness were, which modified the hysteresis width and magnitude of the MIT resistance change.
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1. INTRODUCTION Strongly correlated transition metal oxides exhibit rich and complex phase transitions,1-2 including metal insulator transitions (MIT) with intriguing electronical properties3-5 and potential for applications such as switches or sensors in the emerging field of oxide electronics.6-7 VO2 is a prototypical MIT material with a sharp first-order MIT near room temperature (~ 340K), has been extensively studied, both to gain insight of the physics governing the MIT3, 8-11 and for novel device applications such as Mott-transistors,12-15 optical switches16 and memristors for neuromorphic computation.17-18 In VO2 thin films, and many other thin films of correlated oxides with MITs, the thickness of the deposited films can strongly modify their MIT properties (e.g. transition magnitude, critical temperature, hysteresis width),19-24 which may also depend on additional deposition parameters (deposition method, temperature, oxidation pressure, lattice mismatch and strain, etc.)25-30 further complicating the picture. Thus growth dynamics and dislocation formation become a major hurdle that may slow down research and application realization. In general, the interface between the film and the substrate and the consequent lattice mismatch may result in strained films with different properties than of a relaxed single crystal. 20, 31-34 The thickness increases the strain energy of the film, culminating in strain relaxation through dislocation formation in the film during growth at elevated temperatures or while cooling.35-37 Thus, in many cases a minimum thickness is needed for full development of the electronic MIT, and it is nearly impossible to grow very thin layers of VO2 directly on functional substrates such as sapphire or silicon.19, 38-39 It is possible, however, to attain very thin VO2 layers (~10 nm), by choice of specific substrates, but these also result in strong changes of the transition temperature with thickness, as demonstrated for VO2 on TiO2 substrates40-41 or in the nickelate NdNiO3 for a variety of substrates.42 In these cases there is usually a maximum allowed thickness before strain relaxation degrades the film properties. The increasing knowledge and control of interface properties has driven research to explore the possibilities of preparing materials that do not appear naturally, by “strain-engineering”, with envisioned “tunable” properties.43-46 We highlight a different route to gain additional control on film thickness and control of film properties. As pointed above, the reason for a critical minimum thickness required for high quality films is that a process of strain relaxation is initiated if the energy of the strained film is larger than that needed to form a dislocation. Since the films are deposited at elevated temperatures, the dislocations are usually frozen at lower temperatures. So, it could be possible to modify the film thickness post-deposition by thinning a thick film after cooling. A similar approach was shown to be effective for thinning commercial semiconductor-on2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
insulating substrates to the desired thickness, down to a few nanometers, with minimal harm to mobility and roughness.47-48 In this paper we prove the validity of this approach for correlated-oxide VO2 thin films deposited on r-cut sapphire. We show that above a critical thickness (around 60 nm for our deposition conditions) the film is characterized by high-quality electronic MIT and unstrained structural properties, which coincides with the appearance of a disordered layer, ~1-2 nm thick, at the interface between the sapphire and VO2 - most likely responsible for the relaxation. We tested different thinning methods for post-deposition thinning, and present a process in which the 60 nm VO2 film can be wet-etched by dilute NaOH solution49 to ~10 nm, and below. The etching results in only some degradation in the transport properties, but a much pronounced MIT compared to a film deposited to the same thickness. Further analysis of the etched films indicated that the crystallographic structure, oxygen stoichiometry and vanadium valance were not modified by the etching process, however film roughness and grain size distribution increased.
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2. EXPERIMENTAL SECTION 2.1 Fabrication. We deposited high-quality VO2 thin films on R-cut sapphire by RFmagnetron sputtering of a V2O3 target, as described previously.49 Deposition times were determined to reach the different thicknesses, of 60 nm, 30 nm, and 13 nm, used in this study, and thickness was further verified for each sample, see below. Thinning of the films was performed by three different methods, illustrated in Figure 1: (a) wet-etching in a NaOH solution, (b) dry-etching in an inductively coupled plasma reactiveion-etching (ICP-RIE) system (Plasma-Therm, Versaline), and (c) low-energy ion milling (Copra GT, CCR Tech.). Previous attempts to use high energy Ar-ion milling for electrode fabrication were unsuccessful.49. For wet-etching we immersed the sample in a NaOH solution consisting of 200 μM NaOH in 100 ml H2O and at 363 K. These conditions resulted in an etch rate of approximately 6 nm/hour. In order to increase uniformity we placed the sample on a Teflon holder and continuously stirred the solution. The ICP-RIE recipe was: a pressure of 0.66 Pa SF6 with RF bias 100 W resulting in an etching rate of 44 nm/min. For the milling we used a low energy Ar plasma beam source energy estimated to be
