Synthesis of VO2 Thin Films by Atomic Layer Deposition with TEMAV as Precursor

Among the vanadium oxides, VO2 has attracted a lot of attention due to its remarkable metal-insulator transition (MIT) or semiconductor-metal transition (SMT) behavior. At the critical transition temperature near 68C a drastic reversible variation in its electrical and optical properties is occurring [1]. Electrically, the resistivity change can be as large as four or five orders of magnitude; optically, VO2 thin films are transparent with the insulating phase and highly reflective in the metallic state. All these properties result from a transformation in crystallographic structure, which transitions from the monoclinic crystal structure below the transition temperature to the tetragonal phase above the transition temperature. In microelectronics VO2 constitutes an interesting potential material for applications in nonvolatile resistive memories, and optical sensors. In addition VO2 is a thermochromic material with applications in smart windows and green buildings. In the past VO2 films have been fabricated by using techniques as varied as Reactive Bias Target Ion Beam Deposition (RBTIBD), magnetron sputtering, metal organic chemical deposition (MO-CVD), pulsed laser deposition (PLD) and sol-gel spin coating. Recently, atomic layer deposition (ALD) has been extensively investigated for the deposition of ultra thin films of semiconductors, metals, alloys and oxides. This technique exhibits self-limiting surface reactions, which means accidental overdosing of precursors does not result in increased film deposition on the substrate surface. This self-terminating character is able to accurately control thickness and composition of ultra thin films and achieves uniformity and exceptional conformality of complex nanostructures. It is necessary to notice that up to the present time there were only a few chemical precursor available, such as bis[2,4-pentanedionato] vanadyl(II) [VO(acac)2], to be used for the synthesis of VO2 thin films by ALD. However, these materials can only be vaporized at higher temperature for ALD processes [2]. In this work, a novel metal-organic ALD precursor, Tetrakis[ethylmethylamino] vanadium {V(NEtMe)4} [TEMAV], was employed as vanadium precursor source to develop an ALD process for the synthesis of stoichiometric VO2 films. During this investigation, VO2 thin films were grown on Si and sapphire substrates using the Savannah 100 ALD system from Cambridge Nanotech. TEMAV and H2O were employed as vanadium precursor and oxidizing agent. Generally 20 sccm N2 was used as a carrier gas for the precursors. The growth temperature was set at 150C. However, VO2 thin films obtained by ALD are amorphous, since the growth temperature is lower than the crystallizing temperature. Therefore, post ALD deposition thermal heat treatment has to be used to produce the technologically important polycrystalline stoichiometric VO2 structure. The as-grown thin films were annealed with Ar or N2 plus O2 with different ratios in the furnace. From XRD measurements we conclude that the amorphous phase of as-grown thin films was changed into polycrystalline phase after 450C annealing. The crystal structure and phase purity of VO2 thin films was characterized by X-ray diffraction (XRD). Film surface morphology was inspected using a field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) as shown in Figure 1. The composition and stoichiometry were analyzed by energy dispersive X-ray spectroscopy analysis (EDS) and X-ray photoelectron spectroscopy (XPS). The mechanical properties of the film were measured by nanoindentation.