679wileyonlinelibrary.com attractive for potential applications, such as nanoelectronic switches, [3][4][5] transistors, [ 6 ] optical devices, [ 7,8 ] and micromechanical devices. [ 9 ] VO 2 thin fi lms have been synthesized by a variety of deposition techniques, such as pulsed laser deposition (PLD), [10][11][12][13][14] molecular beam epitaxy (MBE), [ 15 ] reactive sputtering, [ 16,17 ] sol-gel processing, [ 18 ] chemical vapor deposition (CVD), [ 19 ] thermal oxidation, [ 20 ] and ion beam deposition. [ 21,22 ] Most of the studies report on VO 2 fi lms with thicknesses in the range of 40-200 nm. Sub-10 nm continuous fi lms of ≈2 nm thickness have been deposited both by PLD [ 23 ] and MBE [ 15 ] on monocrystalline TiO 2 and MITs with resistivity changes of ≈500 × and ≈25 × , respectively. However, the use of TiO 2 monocrystals as a substrate is unfavorable for practical nanoelectronic applications and PLD and MBE are not deposition techniques that are well suited for device manufacturing. By contrast, VO 2 fi lms deposited by techniques suitable for manufacturing, including atomic layer deposition (ALD), have typically been noncontinuous and have shown a strongly degraded MIT when the fi lm thicknesses were below 40-50 nm. [24][25][26] In recent years, ALD [ 27,28 ] has become the reference technique for the deposition of dielectric [ 29 ] and metallic [ 30 ] thin fi lms for nanoelectronic applications. [31][32][33] ALD is characterized by self-limiting surface reactions, which enables a precise control over fi lm thickness and stoichiometry. In addition, the high conformality allows deposition onto three-dimensional (3D) structures, as increasingly required for advanced nanoelectronic applications. However, the ALD growth of thin high quality VO 2 is not established yet. VO 2 ALD has been reported using vanadyl acetonate and O 2 [ 34 ] or VOCl 3 . [ 35 ] X-ray diffraction (XRD) indicated the presence of VO 2 and signs of MITs have been observed. Yet, these processes have not been able to achieve thin, continuous, and phase-pure fi lms. By contrast, the ALD from tetrakis(ethylmethylamino) vanadium (TEMAV) and O 3 has led to continuous smooth fi lms that show an MIT down to a thickness of ≈40 nm. [36][37][38][39] ALD VO 2 from TEMAV with H 2 O as oxygen source has been reported to lead to mixed valence VO x fi lms which can be converted to VO 2 by postannealing. [ 40 ] Nevertheless, no continuous ALD VO 2 fi lms featuring an MIT Nanoscale morphology of vanadium dioxide (VO 2 ) fi lms can be controlled to realize smooth ultrathin (<10 nm) crystalline fi lms or nanoparticles with atomic layer deposition, opening doors to practical VO 2 metal-insulator transition (MIT) nanoelectronics. The precursor combination, the valence of V, and the density for as-deposited VO 2 fi lms, as well as the postdeposition crystallization annealing conditions determine whether a continuous thin fi lm or nanoparticle morphology is obtained. It is demonstrated that the fi lms and particles possess both a structural and an electronic t...
Vanadium oxide (VO2) thin films were prepared by atomic layer deposition using TEMAV (tetrakis[ethylmethylamido]vanadium) precursor and ozone as the reactant gas. Study on the precursor as well as oxidizer doses and temperature dependence showed none of them exhibited the characteristics of ideal ALD. The VO2 phase formation pathways, its process window, and surface roughness are found to be sensitive to the anneal conditions applied and the substrate used. The VO2 morphology on Al2O3 was found to be island-like whereas on Si/SiO2 either a nano particle formation or a continuous film was obtained. GIXRD demonstrated the VO2 crystallization window to be very narrow on Al2O3 and thick SiO2 while a relatively broad window is obtained on 1 nm SiO2. A reversible change in sheet resistance was measured with more than three orders of magnitude for a 30 nm film.
NiO thin films are deposited by atomic layer deposition (ALD) from the Ni(dmamb) 2 (dmamb ¼ 1-dimethylamino-2-methyl-2-butanolate) precursor using O 3 as the oxidizer. The films are analyzed for wafer uniformity, structure, composition, morphology, microstructure, and homogeneity. The Ni(dmamb) 2 half-cycle shows an initial rapid partial saturation followed by slower further adsorption. By contrast, the O 3 half-cycle shows good saturation behavior. In the studied deposition temperature range for ALD, the films are polycrystalline with negligible amounts of carbon in the films. Furthermore, the films are homogeneous in thickness and composition, demonstrating that high-quality NiO films can be deposited by ALD from Ni(dmamb) 2 .
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