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...

The surface chemistry and the interface formation during the initial stages of the atomic layer deposition (ALD) of Al2O3 from trimethylaluminum (TMA) and H2O on InP(100) were studied by synchrotron radiation photoemission spectroscopy and scanning tunneling microscopy. The effect of the ex situ surface cleaning by either H2SO4 or (NH4)2S was examined. It is shown that the native oxide on the InP surface consisted mainly of indium hydrogen phosphates with a P enrichment at the interface with InP. After a (NH4)2S treatment, S was present on the surface as a sulfide in both surface and subsurface sites. Exposure to TMA led to the formation of a thin AlPO4 layer, irrespective of the surface cleaning. The surface Fermi level of p-type InP was found to be pinned close to midgap after H2SO4 cleaning and moved only slightly further toward the conduction band edge upon TMA exposure, indicating that the AlPO4/InP interface was rather defective. (NH4)2S passivation led to a Fermi level position of p-type InP close to the conduction band edge. Hence, the InP surface was weakly inverted, which can be attributed to surface doping by S donors. TMA exposure was found to remove surface S, which was accompanied by a shift of the Fermi level to midgap, consistent with the removal of (part of) the S donors in combination with a defective AlPO4/InP interface. Further TMA/H2O ALD did not lead to any detectable changes of the AlPO4/InP interface and suggested simple overgrowth with Al2O3.

The influence of different wet chemical treatments (HCl, H 2 SO 4 , NH 4 OH) on the composition of InP surfaces is studied by using synchrotron radiation photoemission spectroscopy (SRPES). It is shown that a significant amount of oxide remains present after immersion in a NH 4 OH solution which is ascribed to the insolubility of In 3+ at higher pH values. Acidic treatments efficiently remove the native oxide, although components like P 0 , In 0 and P (2± )+ suboxides are observed. Alternatively, the influence of a passivation step in (NH 4 ) 2 S solution on the surface composition was investigated. The InP surface after immersion into (NH 4 ) 2 S results in fewer surface components, without detection of P 0 and P (2± )+ suboxides. Finally, slight etching of InP surfaces in HCl/H 2 O 2 solution followed by a native oxide removal step, showed no significant effect on the surface composition.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-08 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-08 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-08 to IP