Authored by Pergament A
In the work, we explore the modification of the structure of vanadium dioxide films, as well as the metal-semiconductor phase transition in them, when hydrated by the method of plasma-immersion ion implantation. Based on a detailed X-ray analysis and the Raman spectra of the initial and hydrated films, it is shown that the plasma-ion modification of the transition and the metallization of vanadium dioxide (at a hydrogen concentration above 10at. %) are most likely associated with electron-correlation effects amplified by hydrogen implantation and an increase in the free charge carrier density in the material. Meanwhile, the structural changes at the transition practically do not manifest themselves: the hydrated metallic vanadium dioxide remains in the monoclinic phase. The results of plasma-immersion implantation are compared with experiments where other methods of hydration of vanadium dioxide are applied.
Keywords: Vanadium dioxide; Metal-Insulator transition; Crystal structure; Doping; Plasma-immersion ion implantation.
Opinion
The metal-insulator transition (MIT) in VO2, occurring at Тt = 340K, is of great importance both from the viewpoint of technical applications and in fundamental terms of understanding the physics of MIT [1,2]. One of the effective ways to change the transition parameters and study its physical mechanism is doping with various elements, including hydrogen [3-10]. Previously we have reported [11,12] on the change in the properties of vanadium dioxide films and in the MIT parameters at hydrogenation by the method of plasma-immersion ion implantation (PIII). It has been found that the introduction of hydrogen by the PIII method into vanadium dioxide films with a relatively low hydrogen concentration (less than 10 at.%) leads to a decrease in the conductivity jump, i.e. to the suppression of MIT, and above 10 at.% – to the complete metallization of the VO2 film. As a result of the implantation of hydrogen ions, which play the role of electron donor centers, it is possible to create a metastable VO2 phase with metallic conductivity at a temperature below the temperature of the equilibrium phase transition for the undoped material. In addition, by varying the implantation dose, one can change the conduction jump and the phase transition temperature, which is important for practical applications [11,12].
One of the most important and still unresolved problems in the VO2 metallization with hydrogen is the question of whether such metallic vanadium dioxide remains in the monoclinic phase characteristic of the crystalline state of the material at T<Тt, or the doping with hydrogen simultaneously leads to the structural phase transition into the rutile tetragonal phase corresponding to metallic pure (undoped) VO2 at T > Тt.
In some works (see, for example, [3]), it is argued that during the metallization with hydrogen, VO2 does not change the structure, which is recognized as an argument in favor of the correlation-induced Mott mechanism as the main driving force in the development of the MIT in VO2. In other papers [4,6], it is reported that, on the contrary, the introduction of hydrogen results in a structural transition to the rutile phase, and this, in turn, drastically changes the view on the driving force of the MIT and highlights the Peierls structural transition. It should be noted that such discrepancies are only natural, if we take into account the complexity of conducting an ideally correct experiment, since there exists a possibility of the hydrogen gradual release from the film during the measurement process.
In the works [3,4], the hydration of VO2 films has been carried out by holding the samples in heated glycerol. The penetration of hydrogen into the vanadium dioxide films occurs from thermally decomposing glycerol. The temperature dependences of the conductivity and reflection spectra of pure and hydrogenated samples, as well as the Raman spectra, have been studied. According to [3], for a hydrogenated film, the Raman spectrum characterizing the monoclinic phase does not depend in any way on the degree of doping, i.e. no structural changes in the crystal lattice occur during hydrogenation, despite the strong changes in the optical constants and electrical conductivity of the film.
On the other hand, the main conclusions reached by the authors of [4] are that, as the degree of hydrogenation increases, the MIT in HxVO2 is suppressed and may completely disappear at x ~0.04. The instability of hydrated vanadium dioxide does not unequivocally confirm or disprove this assumption. Yet the authors believe that the hydrated VO2 possesses a tetragonal structure.
In the work [6], hydrogen stabilization of metallic vanadium dioxide in single-crystal nanobeams has been studied. Raman and conventional optical microscopy, electron diffraction and transmission electron microscopy provide evidence that the structure of the metallic post-hydrogenation state is similar to that of the rutile state. First-principles electronic structure calculations confirm that a distorted rutile structure is energetically favored following hydrogenation, and also that such doping favors metallicity from both the Mott and Peierls perspectives.
Note that the main methods of hydration of vanadium dioxide are based on thermal diffusion of atomic [7] or molecular [6,8] hydrogen. For example, in [6], the samples were annealed in a tube furnace in the hydrogen flow at T = 180 °C for 12 hours. Some other methods, such as cathode polarization in an electrolyte [9], the above-mentioned exposure to hot glycerol [3,4], and even exposure to acid solution (using metal particles to prevent dissolution) [10], have also been applied.
The method of hydration proposed in this work for VO2 using PIII [11-13] is fundamentally new as compared to the above. The PIII method [14], first used in the present work (and in our previous works [11-13]) for doping vanadium oxide thin films, possesses a number of advantages. At the PIII treatment, the sample is placed directly into the low-temperature plasma, and the implantation is carried out immediately over the entire surface of the sample. This leads to a uniform implantation over a large area with a rapid gaining the necessary dose (and, hence, the dopant concentration), while the treatment process is low-temperature. The doping homogeneity depends only on the plasma uniformity above the sample surface, and the energy of the implanted ions can be varied from unities to hundreds of keV [12].
One of the advantages of the PIII method, exactly for vanadium dioxide, is that, as shown in [13], hydrogen, when a sample is heated or stored after doping, leaves the VO2 films much slower than at doping in hot glycerol. In addition, we note that in a recent paper [15], it has been reported that ion implantation (in this case, in a noble gas plasma) can be used to modify the parameters of the MIT in vanadium dioxide by creating defects.
The aim of this work is to investigate the regularities of modification of the crystal structure and physical properties of vanadium dioxide thin films when they are doped with hydrogen using the method of plasma-immersion ion implantation.
Experimental Methods
Thin film samples of 200-nm-thick polycrystalline vanadium dioxide on glass-ceramic and single-crystal silicon substrates were fabricated using an AJA ORION 5 setup by reactive magnetron sputtering in a mixture of argon and oxygen (the partial pressures of Ar and O2 were 4.3 and 0.7 Torr, and the DC generator power was 200 W) with subsequent annealing at a temperature of 500°C in oxygen.
The implantation of hydrogen into the VO2 films was carried out in the PIII setup described in [12] under the following conditions the discharge current was 9.5 A, discharge voltage – 65.9 V, cathode heating current – 65 A, pressure – 4 Pa, and the gas flow was 0.0018 m3Pa/s. The amplitude of the voltage pulse Vimp applied to the sample was 2 kV, Iimp – 20 мА; the pulse duration was Δ = 10 μs, the time of sample treatment in plasma varied in the range of 1 to 5 minutes, and the pulse repetition rate was 1.5 kHz.
The samples were characterized by X-ray diffraction (XRD) analysis and dispersive Raman spectrometry. The XRD analysis was carried out using a DRON-3 diffractometer with CuKα radiation (wavelength 1.5418 Å), and a LiF crystal monochromator was installed in the reflected beam. The measurements were conducted over the angular range 2ϴ = 10° to 70° with the step of 0.1°, and the time of shooting of each point was 25 seconds. Also, we used an ARL X’TRA diffractometer with an HTK 2000 high-temperature chamber to measure XRD patterns in the temperature range 20 to 100 °C. The CuKα radiation was applied in this case too, and the scattering angular range 2ϴ was taken from 24° to 32° which corresponded to the VO2 (011) diffraction line. The shooting step was 0.1°, and the shooting time of each point was 2 seconds.
The Raman analysis of the samples, both initial and after hydrogen implantation, was made using a Nicolet Almega XR Dispersive Raman spectrometer with the reference beam of 532- nm in the Raman shift spectral range of 100-1500cm-1.
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