A. Gupta

Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

We have made a comparative study of epitaxial growth of VO2 thin films on c-cut (0001) and r-cut (11¯02) sapphire substrates, and the semiconductor to metal transition (SMT) characteristics of these films have been correlated with their structural details. On c-sapphire, VO2 grows epitaxially in (002) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained on the basis of similarity of atomic arrangements in (200) and (2¯11) planes. The thermal hysteresis (ΔH), sharpness of the transition (ΔT), and the transition temperature for VO2 films on c-sapphire were found to be 4.8, 8.5, and 72.6u2009°C, respectively, which were higher than the corresponding values of 3.3, 5.4, and 60.3u2009°C for films on r-sapphire. The SMT temperature for VO2 films on c-sapphire was close to the bulk value of 68.0u2009°C....

Effect of Li doping in NiO thin films on its transparent and conducting properties and its application in heteroepitaxial p-n junctions

Li doped NiO (LixNi1−xO) thin films were epitaxially grown along [111] orientation on c-sapphire by pulsed laser deposition. The structural, electrical, and optical properties of the films were investigated using x-ray diffraction, four probe technique, and UV-visible spectra, respectively. The epitaxial growth of [111] Li doped NiO on [0001] sapphire was determined by using high resolution x-ray Φ scan. Effects of the deposition condition and Li doping concentration variations on the electrical and optical properties of Li doped NiO films were also investigated. The analysis of the resistivity data show that doped Li ions occupy the substitutional sites in the films, enhancing the p-type conductivity. The minimum resistivity of 0.15 Ωu2009cm was obtained for Li0.07Ni0.93O film. The activation energy of Li doped NiO films were estimated to be in the range of 0.11–0.14 eV. Based upon these values, a possible electrical transport mechanism is discussed. A p-n heterojunction has also been fabricated for the opti...

Semiconductor to metal transition characteristics of VO2 thin films grown epitaxially on Si (001)

We report semiconductor to metal transition (SMT) characteristics of vanadium dioxide (VO2) grown epitaxially on Si (001) at 500u2009°C. The epitaxial integration with Si (001) was achieved by using epitaxial tetragonal yttria-stabilized zirconia (YSZ) as an intermediate buffer layer, which was grown in situ. From x-ray (θ-2θ and ϕ-scan) and electron diffraction studies, we established that VO2 and YSZ grow in (020) and (001) orientations, respectively, on Si (001) substrate and epitaxial relationship was established to be “VO2[001] or VO2[100]”//YSZ[110]//Si [100] and VO2(010)//YSZ(001)//Si(001). VO2/YSZ/Si(001) heterostructures showed approximately three orders of magnitude reversible change in resistivity and hysteresis of ∼6u2002K upon traversing the transition temperature. A 10u2009°C increase in the SMT temperature of these VO2 films, compared to the value reported for bulk VO2, has been explained on the basis of uniaxial stress along the c-axis, which can stabilize the covalent monoclinic phase up to higher te...

Near bulk semiconductor to metal transition in epitaxial VO2 thin films

We have been able to achieve semiconductor-to-metal transition (SMT) temperature in VO2 thin films close to the values reported for bulk VO2 single crystals. This was achieved by complete relaxation of misfit strain, which leads to a negligible tension/compression along VO2 [001], upon introduction of NiO buffer layer on c-plane sapphire substrate. In this paper, we discuss the mechanism behind complete relaxation of misfit strain which occurs under the paradigm of domain-matching epitaxy, where integral multiples of planes match across the interface. NiO buffer layers were grown in situ, prior to the VO2 deposition, using pulsed-laser deposition technique. X-ray θ-2θ, φ, and pole figure scans were performed for structural characterization of the VO2/NiO/Al2O3 (0001) heterostructure. All the constituent layers of the heterostructure were found to be epitaxial with orientation relationship: (020)VO2∥(111)NiO∥(0001)Al2O3 and ⟨100⟩VO2∥⟨110⟩NiO∥⟨101¯0⟩Al2O3. Parameters related to SMT, such as hysteresis and t...

Ab initio analysis of silyl precursor physisorption and hydrogen abstraction during low temperature silicon deposition

Abstract The energetics of silyl (SiH 3 ) precursor surface adsorption and hydrogen abstraction on a monohydride terminated silicon surface are described. The abstraction of surface hydrogen by H radicals is more exothermic, and proceeds with a smaller kinetic barrier than H abstraction by silyl. Surface adsorption and abstraction were analyzed using both multi-parent configuration interaction (CI) and several density functional approaches using the Si 4 H 10 cluster representing a monohydride terminated silicon (1xa01xa01) surfaces, and results from the two techniques are critically compared and evaluated. Hydrogen abstraction by H is found to proceed through a kinetic barrier that is between 0 kcal/mol predicted by DFT and 7.2 kcal/mol determined from CI, consistent with experimental values of ∼2 kcal/mol. The barrier height for H abstraction by silyl (without zero point and H tunneling corrections) is determined to be between 4.1 kcal/mol calculated using DFT, and 14.2 kcal/mol determined from the multi-parent CI. These calculations indicate that during typical low temperature silicon deposition processes, H abstraction by impinging hydrogen atoms dominates H abstraction by SiH 3 and plays an important role in creation of surface dangling bonds. None of the Si–H/silyl potential energy surfaces obtained from CI and DFT methods show evidence for stable physisorbed three-center ue606Si–H–(SiH 3 ) p bond, which is commonly presumed in several models of silicon thin film deposition. We discuss these results in relation to experimental analysis of surface diffusion kinetics in film deposition, and suggest alternate growth models, including H-mediated Si–Si bond breaking and/or direct silyl insertion, to describe activated low temperature silicon-based film deposition.

TaN-TiN binary alloys and superlattices as diffusion barriers for copper interconnects

Binary alloys and superlattices of TaN-TiN thin films were grown on Si(100) substrates with a TiN buffer layer using pulsed laser deposition. A special target assembly was used to manipulate the concentrations of these binary component films. The 60% TaN resulted in a TaN (3 nm)/TiN (2 nm) superlattice, while 30% and 70% TaN generated uniform TaxTi1−xN alloys. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) confirmed the single-crystalline nature of these films. Four-point probe resistivity measurements suggest that these alloy and superlattice films have a lower resistivity than pure single-crystalline TaN films. The Cu-diffusion characteristic studies showed that these materials would have the potential as high-temperature diffusion barriers for Cu in ultra-large-scale integration technology.

High work function (p-type NiO1+x)/Zn0.95Ga0.05O heterostructures for transparent conducting oxides

We report the growth and properties of heterostructure thin films consisting of a thin overlayer of p-NiO1+x on Zn0.95Ga0.05O (GZO) by pulsed laser deposition for transparent electrode applications. The GZO films with a thin p-type NiO1+x overlayer exhibited a higher work function. It is envisaged to facilitate hole injection across the heterojunction in a solid state device resulting in improved device efficiency. The crystalline quality of the bilayer films was investigated by x-ray diffraction. NiO1+x overlayers showed the preferred orientation along the [1 1 1] direction on Zn0.95Ga0.05O (0 0 0 1) films deposited on a glass substrate while they were epitaxial when the substrate used was sapphire. The effects of the NiO1+x overlayer thickness variation and Li doping on the electrical and optical properties of NiO1+x/Zn0.95Ga0.05O bilayer films were also investigated. The bilayer films with an optimized overlayer thickness showed good optical transparency (≥85%) and low resistivity of ~10−4 Ω cm up to temperatures as low as 100 K. Using x-ray photoelectron spectroscopy it has been established that nickel in NiO1+x exists in multiple oxidation states of Ni2+ and Ni3+. The presence of Ni3+ gives rise to p-type conductivity in non-stoichiometric NiO1+x. Additionally, ultraviolet photoelectron spectroscopy studies showed that the bilayer films have high work function values ranging from 5.2 to 5.3 eV. A correlation between the surface work function and Ni3+/Ni2+ ratio has also been established.

Growth and characteristics of TaN/TiN superlattice structures

Epitaxial B1 NaCl-structured TaN(3 nm)/TiN(2 nm) superlattice structures were grown on Si(100) substrates with a TiN buffer layer, using pulsed-laser deposition. A special target assembly was used to manipulate the thickness of each layer. X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy (Z contrast) studies confirmed the single-crystalline nature of the superlattice with a uniform layer structure. Nanoindentation results suggest the high hardness of these superlattice structures. Four-point-probe resistivity measurements show low resistivity of the heterostructures and a Cu diffusion characteristic study proved this superlattice system can be a promising diffusion barrier and can withstand 700u200a°C annealing for 30 min.

Effect of hydrogen on adsorbed precursor diffusion kinetics during hydrogenated amorphous silicon deposition

Fractal analysis of the surface topography is used to study the effects of hydrogen dilution on the surface transport kinetics during the plasma deposition of hydrogenated amorphous silicon. Images obtained from atomic force microscopy are examined using dimensional fractal analysis, and surface diffusion lengths of growth precursors are estimated from the measured correlation lengths. The addition of small amounts of hydrogen (H2/SiH4 ratios <10/1) during deposition leads to a decrease in the diffusion length, but larger hydrogen dilutions result in increased diffusion length. Moreover, the measured surface diffusion activation barrier is reduced from 0.20 eV for deposition from pure SiH4 to 0.13 eV with high hydrogen dilution. Results are consistent with recent models for precursor surface transport during low-temperature deposition, and give insight into critical processes for low-temperature silicon crystallization.

The effect of matrix type on self-assembly of nanoparticles for mechanical and magnetic applications

The controlled synthesis of nanostructured particles with uniform size, shape, composition, and preferred orientation is a formidable task. Some conventional techniques have been demonstrated with limited success; however, reproducible processing schemes for heterogeneous multifunctional materials are still not satisfactory. To realize the advantageous technical applications of nanostructured materials, self-assembly or self-organizing methods are currently under investigation. Our results show that the matrix or template used for directed growth of nanoparticles (Fe, Ni) significantly affects the resultant mechanical properties of the multilayered structure. For example, a crystalline material like TiN, which grows epitaxially on silicon, results in embedded epitaxial nanoparticles. Conversely, an amorphous template like Al2 O3 results in polycrystalline magnetic particles. In this work, we will discuss the effect of matrix type, specifically yttria stabilized zirconia (YSZ), on the orientation of nanoparticle assemblies. The resultant mechanical and magnetic properties of the multilayer structures will be discussed in the oral presentation.© 2005 ASME