Growth of NbO2 by Molecular-Beam Epitaxy and Characterization of its Metal-Insulator Transition

Abstract

Thin films of NbO 2 are synthesized by oxide molecular-beam epitaxy on (001) MgF 2 substrates, which are isostructural (rutile structure) with NbO 2 . Two growth parameters are systematically varied in order to identify appropriate growth conditions: growth temperature and the partial pressure of O 2 during film growth. θ-2θ X-ray diffraction measurements identify two dominant phases in this system at background oxygen pressures in the (0.2-6)×10 -7 Torr range: rutile NbO 2 is favored at higher growth temperature, while Nb 2 O 5 forms at lower growth temperature. Electrical resistivity measurements were made between 350 K and 675 K on three epitaxial NbO 2 films in a nitrogen ambient. These measurements show that NbO 2 films grown in higher partial pressures of molecular oxygen have larger temperature-dependent changes in electrical resistivity and higher resistivity at room temperature. INTRODUCTIONThough rare, metal-to-insulator transitions (MITs) occur in some materials and can be triggered by an external stimulus, e.g., a change in temperature, pressure, or the application of light [1]. Figure 1(a) shows the temperature-dependent metal-insulator transitions of singlecrystal MIT materials gleaned from the literature [2-27]. The resistivity data shown was selected from the literature as exemplary due to its large and sudden change in resistivity. The data in Fig. 1 is limited to materials where the MIT is an intrinsic property of the material, i.e., not due to the formation of filaments, or chemical reactions, or recrystallization. For this reason, the data is restricted to high-quality single crystals, where intrinsic behavior is most likely to be exhibited.Changes in electrical resistivities due to MITs can be several orders of magnitude in size and very sharp as quantified in Fig. 1(b), where the change in the electrical resistivity and the temperature of the transition are plotted for each material. The transition region (between the start and end of the MIT, each denoted by an "x") was calculated by finding the second derivative of a 5-point moving smooth utilizing a cubic least-squares best-fit function to the data

Approaching the high intrinsic electrical resistivity of NbO2 in epitaxially grown films

“…5,[13][14][15] For lateral resistive switching devices, NbO2 layers with an in-plane [001] direction have been demonstrated. 14,16 However, low room-temperature resistivity values of only several Ωcm [17][18][19][20] have been reported for thin-films so far compared to the intrinsic resistivity value of 10 kΩcm measured for bulk crystals. 21,22 To improve the resistivity ratio at the MIT for thin-film devices, thus, higher room-temperature resistivity is desirable.…”

Thermal Management in Neuromorphic Materials, Devices, and Networks

“…Thermal and electric conductivity data were extracted from: brain tissue elements, [133] VO 2 , [220] V 2 O 3 , [152,227] NbO 2 . [228,229] The thermal and electrical conductivity of PCMs in the crystalline phase differs from their respective values in the amorphous phase. We consider representative oxides such as, amorphous (a-HfO x ) and crystalline (c-HfO x ) hafnium oxide at room temperature; [147,[230][231][232] amorphous (a-TaO x ) and crystalline (c-TaO x ) tantalum oxide; [145,233] and amorphous (a-TiO x ) and crystalline (c-TiO x ) titanium oxide.…”

Resistive Avalanches in La1–xSrxCoO3−δ (x = 0, 0.3) Thin Films and Their Reversible Evolution by Tuning Lattice Oxygen Vacancies (δ)

“…LaCoO 3 may also exist in the oxygen-deficient stoichiometry of LaCoO 3−δ , where 0 < δ < 0.5 (see Figure ). Hitherto, in the LaCoO 3 and La 0.7 Sr 0.3 CoO 3 films, the MIT is a well-observed phenomenon. − The studies reported so far state that in LaCoO 3 , the ground state consists of only Co 3+ ions specifically in the 3d 6 spin configuration. At low temperatures (below 50 K), it remains in the LS state, i.e., S = 0 (t 2g 6 e g 0 ).…”