Suppression of the metal-insulator transition in magnetron sputtered Ti2O3 films

Abstract

We report on the synthesis of titanium suboxide films using a magnetron sputtering setup. The structure of the produced films is characterized using Raman spectroscopy and X-ray diffraction and electrical properties are measured by four-probe method. We demonstrate that depending on the oxidation rate of the growing films it is possible to produce polycrystalline α-TiO, corundum and orthorhombic Ti 2 O 3 or almost amorphous TiO 2 . We focus on the characterization of the two phases of Ti 2 O 3 and show that their structure and electrical properties significantly differ from earlier results obtained for bulk crystals or epitaxial films. Our corundum-type Ti 2 O 3 films have high electrical conductivity (compared to the bulk) and no metal-insulator phase transition: the material is locked in the metallic state. The conductivity of our orthorhombic Ti 2 O 3 is lower compared to epitaxial films; this phase does not demonstrate any phase transitions as well. IntroductionThe first report on titanium sesquioxide (titanium III oxide, or Ti 2 O 3 ) crystal structure was published more than 90 years ago [1]. This material normally crystallizes into corundum-type space group R c with lattice parameters of a = 5.157 Å and c = 13.61 Å. Ti 2 O 3 attracted some interest due to a broad semiconductor to metal phase transition in the temperature range of 150-300 °C [2, 3]. This transition is accompanied by a rapid change of physical properties such as electrical resistivity [4], specific heat [5], elastic constants [6], and magnetic susceptibility [7]. Very recently it was shown that nanoparticle-based Ti 2 O 3 films could be used as an efficient photothermal material promising for seawater desalination and purification [8]. Under high pressures and high temperatures (19 GPa and 1850 K) corundum Ti 2 O 3 phase transforms into golden semiconducting Th 2 S 3 -type structure, which is fully quenchable to ambient conditions (Pnma orthorhombic cell, a = 7.82 Å, b = 2.85 Å, c = 8.12 Å at ambient conditions) [9, 10].Recently, it was shown that Ti 2 O 3 could crystallize into another space group, Immm, with orthorhombic cell parameters of a = 9.39 Å, b = 4.42 Å and c = 2.81 Å. The material was obtained as an epitaxial thin film on (0001) sapphire substrate using pulsed laser deposition and Ti 2 O 3 target at substrate temperatures above 700 °C [11]. Such films are ferromagnetic n-type semiconductors with a very high electron concentration and they also demonstrate a semiconductor to metal transition at about 370 K. The synthesis of the same material by cathodic arc sputtering was reported in our previous article, but that time the phase was not properly identified and characterized [12].In most studies, titanium suboxides are produced by different chemical methods, including carbothermic [13], metallothermic [14] or hydrogen [15] reduction of TiO 2 or by oxidizing different titanium-containing precursors, such as TiCl 3 [16], TiH 2 [17], titanium-tetraisopropoxide [18], and others. These methods usually lead to the formation of the bulk o...

Metallic ground states of undoped Ti2O3 films induced by elongated c-axis lattice constant

“…Figure 1 shows the Raman spectra of the high-temperature-Ti 2 O 3 (HT-Ti 2 O 3 ) and low-temperature-Ti 2 O 3 (LT-Ti 2 O 3 ) films. The Raman spectrum of the HT-Ti 2 O 3 film recorded using a He-Ne laser shows seven peaks, which is consistent with previous reports [11][12][13]24,25 . In corundum-type crystal structures, two A 1g and five E g modes are Raman active following the symmetry considerations 28 .…”

“…12 , 13 (see Supplementary Note 1 ). Our results indicate that both peak structures and resonant Raman effects are good fingerprints to elucidate corundum-type Ti 2 O 3 11 – 13 , 24 , 25 , 29 – 34 .

Figure 1 Investigation of corundum-type crystal structure in the Ti 2 O 3 films.

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“…Therefore, the 1st and 2nd low-frequency modes were assigned to the A 1g and E g modes, respectively, which are consistent with the earlier report by Shin et al 12,13 (see Supplementary Note 1). Our results indicate that both peak structures and resonant Raman effects are good fingerprints to elucidate corundum-type Ti 2 O 3 [11][12][13]24,25,[29][30][31][32][33][34] . Similar Raman spectra were obtained for the LT-Ti 2 O 3 film, as shown in Fig.…”

“…Recently, single-crystalline corundum-type Ti 2 O 3 films were synthesized on isostructural α -Al 2 O 3 (0001) substrates 23 – 25 , wherein their physical properties were found modulated from those of the bulk systems due to the lattice deformations. The MIT temperature was found in the range of ~ 200–300 K for the films, which is ~ 200 K lower than that of the bulk 23 , 24 .…”

Atomic Layer Deposited Ti2O3 Thin Films

“…They exhibit some remarkable properties which are absent in bulk that instigates researchers to explore more in this field. [1] Among them, TiO 2 is one of the most celebrated candidates for photocatalysis, energy conversion etc. To explore sustainable applications, thin films of many titanates and reduced titanates have been investigated.…”