Selective growth of single phase VO2(A, B, and M) polymorph thin films

We demonstrate the growth of high quality single phase films of VO2(A, B, and M) on SrTiO3 substrate by controlling the vanadium arrival rate (laser frequency) and oxidation of the V atoms. A phase diagram has been developed (oxygen pressure versus laser frequency) for various phases of VO2 and their electronic properties are investigated. VO2(A) phase is insulating VO2(B) phase is semi-metallic, and VO2(M) phase exhibits a metal-insulator transition, corroborated by photo-electron spectroscopic studies. The ability to control the growth of various polymorphs opens up the possibility for novel (hetero)structures promising new device functionalities.

We demonstrate the growth of high quality single phase films of VO 2 (A, B, and M) on SrTiO 3 substrate by controlling the vanadium arrival rate (laser frequency) and oxidation of the V atoms.A phase diagram has been developed (oxygen pressure versus laser frequency) for various phases of VO 2 and their electronic properties are investigated.VO 2 (A) phase is insulating VO 2 (B) phase is semi-metallic, and VO 2 (M) phase exhibits a metal-insulator transition, corroborated by photoelectron spectroscopic studies.The ability to control the growth of various polymorphs opens up the possibility for novel (hetero)structures promising new device functionalities.C 2015 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
[http://dx.doi.org/10.1063/1.4906880]Transition metal oxides are used in a wide range of practical applications such as photocatalysts, 1 cathode materials, 2,3 gas sensors, 4 optical switching devices, 5 and intelligent thermo-chromic windows. 6They also exhibit various polymorphic structures, among which many are neither stable in ambient conditions nor can be easily synthesized.These metastable phases have been the subject of considerable interest due to their unique and superior physical and chemical properties.Among those functional complex oxides, vanadium oxides can adopt a wide range of V:O ratios, resulting in different structural motifs with various types of coordination polyhedra. 7In this class, vanadium dioxide (VO 2 ) exhibits a number of polymorphic forms, such as VO 2 (M1), VO 2 (M2), VO 2 (M3), VO 2 (R), VO 2 (A), VO 2 (B), and VO 2 (C).The two layered polymorphs VO 2 (A) and VO 2 (B) are promising materials for science and technology.VO 2 (A) is important for the study of strong electronic correlations resulting from structure and VO 2 (B) is important for its use as electrode materials for batteries.Various preparation techniques have been used and developed to stabilize these phases in bulk and thin film forms. 8,9Using Pulsed Laser Deposition (PLD) technique, thin films of these polymorphs (M1, M2, M3, and R) have been stabilized by substrate engineering 10,11 as well as by adding dopant, such as, Cr. 12 In 1989, Oka et al., 13 reported for the first time the stabilization of polycrystalline powder of VO 2 (A) using hydrothermal synthesis.They observed a weak metal-semiconductor transition at 162.8 • C.They related this behavior to a crystallographic transition between a low temperature phase (LTP, space group P4/ncc, No. 130) and a high temperature phase (HTP, space group I4/m, No. 87).In the following year, Oka et al. 14 reported the VO 2 (B) powder stabilization with a monoclinic structure (space group C2/m, No. 12) which underwent a structural evolution occurring over a temperature range from 180 to 300 K.There are several recent reports on A and B phases in the form of bulk and nano powders where thermal annealing causes them to revert to stable VO 2 (M) phase. 12Furthermore, Raman and differential thermal analysis measurements on nanorods of VO 2 (A) unravel the metastability of the phase. 13On the other hand, VO 2 (B) films showing a single phase by X-ray diffraction were demonstrated 15 but these samples were insulating in contrast to the expected metallic behavior.Despite these studies, a full understanding of electronic and optical properties of VO 2 (A) and VO 2 (B) thin films is still missing.Finding ways to synthesize specific VO 2 polymorphs with defined properties will open up channels for new device designs.
In this study, we grew epitaxial, single phase tetragonal VO 2 (A), and monoclinic VO 2 (B) thin films on (100)SrTiO 3 (STO) substrates using PLD and compare their structural and transport behavior with VO 2 (M) phase films.Cross-sectional transmission electron microscopy (TEM) measurements were done using an aberration-corrected STEM (JEM-ARM200F) facility at JEOL Ltd., Japan.The electrical transport measurements were done in the linear four probe geometry using PPMS Quantum design.The hall voltage measurements on the VO 2 polymorphic thin films were done in the standard van der Pauw arrangement in a DC magnetic field up to 2 T. A commercial vanadium single crystal (100) orientated metal target with 5N purity (from Goodfellow) is used for all the vanadium oxide film growth.The laser energy density and temperature of the substrate were fixed at ∼2 J cm −2 and 500 • C, respectively, during the optimization of film growth parameters.By varying the laser frequency at a constant energy density, we change the arrival rate of atoms to the surface.By tuning the oxygen background pressure, we also change the oxidation of vanadium both in the plume and on the substrate.Thus, one expects a coincident lattice site model to apply here.
In the later part of the paper dealing with the TEM cross section results indeed, this is seen to be true.The films of VO 2 M, A, and B under investigation are of thicknesses 60 nm, 60 nm, 40 nm, respectively, confirmed by Rutherford back scattering (RBS) and TEM.The 2D XRD patterns are recorded for all samples of the series (see supplementary material for X-Ray reciprocal space map using 2D detector, Fig. S1 17 ).The 1D XRD patterns shown in Fig. 1(c) are extracted from 2D XRD patterns by integrating the intensity along the χ direction.We observed that the VO 2 (M) phase reflections are not aligned in χ with the substrate, while for VO 2 (A) and VO 2 (B), reflections are perfectly aligned with the substrate (χ = 0).At low pressure (1 × 10 −4 Torr), M phase is stabilized with the off-chi peaks at 2θ = 27.89• , assigned to (011) crystallographic plane of VO 2 (M).By increasing the pressure to 7.5 × 10 −4 Torr, we produced a mixture of M and A phases where the peak at 2θ = 29.62 • is assigned to (220) plane of VO 2 (A) phase, 16 while a single phase VO 2 (A) film is stabilized at and above a pressure of 5 × 10 −3 Torr.Intriguingly, at the same oxygen pressure, reduction of the laser frequency to 2 Hz gave rise to a single phase VO 2 (B) film.A peak at 2θ = 29.05• in the XRD pattern is assigned to (002) plane of VO 2 (B) phase. 15The effect of oxygen partial pressure and laser frequency on the stability of these polymorphs were explored and a phase diagram is established (Fig. 1(d)) for different phases of VO 2 .The rocking curves were recorded on each sample in order to check the crystallinity (see supplementary material for rocking curve, Fig. S2 17 ).VO 2 (A) shows a high crystalline quality (FWHM = 0.1 • ), while the crystal quality decreases for VO 2 (B) (FWHM = 0.6 • ) and VO 2 (M) (FWHM = 0.9 • ) phases (see supplementary material for comparison of the rocking curves and the calculated d spacings, Table S1 17 ).We have further verified the stoichiometry of VO 2 (A) and VO 2 (B) films using oxygen resonance Rutherford back scattering technique which confirms the composition to be VO 2±0.02 (see supplementary material for oxygen resonance Rutherford backscattering spectra, Fig. S3 17 ).
If we look at the various phases of VO 2 , they all have different V-V inter-atomic distances. 18ne can empirically understand the type of bond length formed between the V-V due to V-V and V-O-V atomic interactions and its dependence on the influence of the surrounding oxygen.At lower oxygen growth pressures, the atomic interactions are likely to favor shorter V-V distances.At low oxygen pressures, VO 2 (R) phase is preferred at the growth temperature which subsequently results in the M phase at room temperature due to the metal-insulator transition (MIT).When the oxygen pressure is increased, the A phase with a longer average V-V distance 19 may be favored.Further, by decreasing the V arrival rate (i.e., lowering the laser frequency), the B phase with the longest V-V distance is stabilized. 14Moreover, the symmetry of the phases could also be an important factor for their stabilization.The relatively more symmetric structure of VO 2 (A) over VO 2 (B) suggests VO 2 (A) to be more stable over a wide range of laser frequency.Our explanation is rather qualitative.The actual process can be much more complicated which might involve type of connectivity of the anion sublattice, e.g., number of corner, edge, or face sharing oxygen octahedra.Also, the high degree of structural imperfections suggests that defect formation at the interface (as seen in the TEM images in Fig. 2) might play a critical role as well.
Figs. 2(a)-2(e) and Figs.2(h)-2(l) present the processed atomic resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images of VO 2 (A) and VO 2 (B) thin films, respectively.The brighter spots represent V atoms, while the position of oxygen rows between the V rows is not resolved.For VO 2 (A), the V atoms in 001 domains (Figs. 2(a confirms the respective tetragonal and monoclinic symmetry of the films.From these HAADF-STEM image analyses, we can conclude that the VO 2 (A) and VO 2 (B) thin films present a structure expected from bulk studies. 14,16The thin film microstructure studies underline the presence of 90 • -oriented crystallographic domains in both samples.The epitaxial relationship for VO 2 (A) and VO 2 (B) can be written as The evaluated β angle (∼107.3(1)• ) is very close to the bulk reported value 14 106.97 • .In the TEM, the films do look coherent with respect to the substrate which may be a consequence of the coincident lattice but we noticed in both the A and B phases a ∼1 nm buffer layer different from that of the A or B phase which surprisingly preserve the coherence of the over layer with respect to the substrate.Further analysis is under investigation to characterize precisely this buffer layer which is beyond the scope of this paper.
Electrical transport has been measured on the various phases of VO 2 as shown in Fig. 3.The resistivity vs temperature behavior of VO 2 (M) is in good agreement with the literature. 20,21In the high temperature metallic phase, the resistivity is relatively temperature-independent and attains 3 × 10 −4 Ω cm at 400 K.In the semiconducting state, the resistivity reaches to 1 × 10 −1 Ω cm at 300 K.The transition temperature during the heating cycle is ∼340 K which matches with the literature 20 while it shifts to 330 K during cooling cycle with a hysteresis of 10 K (see supplementary material for dlog(R)/dT versus temperature plot, Fig. S4 17 ).The VO 2 (A) film shows a typical semiconducting behavior with a resistivity of 17 Ω cm at room temperature which drops to 0.3 Ω cm at 550 K. VO 2 (B) films exhibit a change of 4 orders (from 4 m Ω cm to 22 Ω cm) in resistivity while cooling from 400 K to 150 K accompanied by a persistent hysteresis during the thermal cycle which is consistent with reports in bulk VO 2 (B). 22These measurements confirm that VO 2 (A) is insulating, VO 2 (M) is semiconducting, while VO 2 (B) is semimetal at room temperature.Figs.4(a)-4(c) summarize the carrier densities and respective mobilities for the three films measured at various temperatures.The carrier densities and mobilities for the M phase as well as for its high temperature metallic R phase are comparable to the literature values. 20VO 2 (B) film shows a ∼6 orders of decrease in carrier density from 5 × 10 22 cm −3 (comparable to that of metallic R phase) to 1 × 10 17 cm −3 accompanied by an increase of 2 orders of magnitude in mobility from 0.03 to 2.15 cm 2 V −1 s −1 measured at 300 K and C was observed and this is not surprising as even in the bulk material, the observed transition was quite weak. 13On the contrary, the transport measurement of VO 2 (B) films supports the previous low-temperature X-ray studies on VO 2 (B) which revealed an evolution from a high temperature monoclinic metallic phase to another monoclinic insulating phase at low temperature. 14ig. 5 shows the valence band photo emission spectra of A, B, and M phases of VO 2 measured by hard X-ray photoelectron spectroscopy (HAXPES).The excitation energy and the Fermi level of VO 2 is calibrated using Au Fermi level spectra.Before measuring X-ray photo electron spectra of each of the samples, we have performed measurements on Au Fermi level and used this value of energy to calibrate the excitation energy.In Fig. 5(a), the broad and higher intensity feature between 2 eV and 9 eV binding energies arises primarily from O 2p states whereas the relatively lower intensity peaks nearer to the Fermi energy (E F ) are mainly contributed by V 3d-like states.We focus on the V 3d related states in the vicinity of the E F , shown in Fig. 5(b), that control the low energy excitations in these systems.Spectra for both VO 2 (M) and VO 2 (A) phases exhibit a single peak structure with no spectral intensity at the Fermi energy, consistent with the insulating state of both these compounds.These spectral features correspond to the lower Hubbard band of these strongly correlated systems and often have been termed the incoherent feature.The band gaps estimated from these spectra are approximately 0.32 eV and 0.72 eV for the VO 2 (M) and VO 2 (A) phases, respectively.Band gaps are calculated based on the assumption that the conduction band is pinned close to the Fermi energy (i.e., the energy gap between the conduction band and the Fermi level is assumed to be very small.).In contrast, VO 2 (B) phase V 3d spectral feature is relatively broad in nature.The broad spectral feature suggests two partially overlapping peaks in this case, with one peak position at about 1.6 eV, while the other at about 0.6 eV binding energy.The feature at the 1.6 eV binding energies appears between the spectral features of the lower Hubbard band in VO 2 (M) and VO 2 (A) phases, and can, therefore, be attributed to the incoherent spectral feature of the lower Hubbard band in VO 2 (B) phase.The remaining feature, much closer to and with a finite intensity at the Fermi energy, clearly establishes the metallic nature of VO 2 (B) phase at the room temperature; this low energy spectral feature is called the coherent peak in this case.Thus, the photoemission spectra for all three phases are consistent with the observed transport properties of VO 2 in these three phases.
In conclusion, we have shown that the three polymorphs of VO 2 (A, B, and M) thin films can be prepared in single phase by controlling the vanadium arrival rate and the oxygen pressure both on SrTiO 3 .The phase diagram for the growth of the various phases indicates that the VO 2 (M/R) is the most stable phase, next is VO 2 (A) followed by VO 2 (B) as the least stable phase.The transport studies indicate that B is semi-metallic, A is insulating, while M is semiconducting which is corroborated by the HAXPES measurements.Thus, control of cationic and anionic atomic arrival rates is a powerful processing step for the growth of novel functional polymorphic materials.