consisting of two or more alternating self-limiting surface reactions. These selflimiting surface reactions enable thin-film deposition with thickness uniformity over large-area substrates, (sub-) monolayer thickness control, and conformal deposition in 3D structures. [1] During the first ALD cycles the precursors mainly react with the substrate rather than with the ALD-grown material. The surface termination of the substrate can therefore strongly affect the growth behavior during this initial period. [1][2][3] Depending on the nature of the ALD-grown material, the substrate, and the process conditions, ALD can lead to different growth regimes, resulting, for instance, in the deposition of ultrathin continuous films, deposition of highly dispersed nanoparticles, or area-selective deposition. [4] Continuous thin films have a wide variety of applications including nanoelectronics, coatings, and optical components, and their deposition requires either 2D growth or high particle density to achieve fast film closure. Nanoparticles dispersed on a surface are desired for heterogeneous catalysis, and their production requires island-type deposition with a well-defined particle size and particle density. Area-selective deposition can enable nanoscale bottom-up patterning, which allows accurate self-alignment between small features which is difficult to achieve in conventional top-down patterning. [5] To enable area-selective deposition, the growth behavior should be surface-dependent such that the deposition is at the same time favored on designated areas of the substrate and inhibited on others. For each of the aforementioned applications, an understanding of the surface dependence of the initial stages of growth can inform the tailoring of the ALD process to the desired application. [6] ALD of noble metals has received considerable attention because of its potential in applications such as catalysis [7] and nanoelectronic devices. [8] Ruthenium is considered an ideal candidate for novel nanoscale catalysts [9,10] as well as for replacing copper as a conductor in future low-level nano-interconnect structures for integrated circuits. [8] ALD of Ru however presents application-specific challenges. On one hand, nanoparticles of a specific size are desired for high catalytic activity. [10] On Understanding the growth mechanisms during the early stages of atomic layer deposition (ALD) is of interest for several applications including thin film deposition, catalysis, and area-selective deposition. The surface dependence and growth mechanism of (ethylbenzyl)(1-ethyl-1,4-cyclohexadienyl) ruthenium and O 2 ALD at 325 °C on HfO 2 , Al 2 O 3 , OH, and SiOSi terminated SiO 2 , and organosilicate glass (OSG) are investigated. The experimental results show that precursor adsorption is strongly affected by the surface termination of the dielectric, and proceeds most rapidly on OH terminated dielectrics, followed by SiOSi and finally SiCH 3 terminated dielectrics. The initial stages of growth are characterized by the formation a...
Transition metal compounds showing a metal-insulator transition (MIT) show complex behavior due to strongly correlated electron effects and offer attractive properties for nano-electronics applications, which cannot be obtained with regular semiconductors. MIT based nano-electronics, however, remains unproven, and MIT devices are poorly understood. We point out and single out one of the major hurdles preventing MIT-electronics: obtaining a high Off resistance and high On-Off resistance ratio in an MIT switch. We show a path toward an MIT switch fulfilling strict Off and On resistance criteria by: (1) Obtaining understanding of the VO2-interface, a protoypical MIT material interface. (2) Introducing a MIT tunnel junction concept to tune switch resistances. In this junction, the metal or insulating phase of the MIT material controls how much current flows through. Adapting the junction's parameters allows tuning the MIT switch's Off and On resistance. (3) Providing proof of principle of the junction and its switch resistance tuning capability, experimentally in two forms. (4) Showing theoretically how stringent Off and On resistance specifications can be fulfilled. The prototypical VO2 MIT results in an abrupt change in bulk electrical resistivity at ∼68 °C. We show that the VO2 MIT manifests itself in an abrupt interfacial transition of current across a VO2-barrier interface forming a tunnel junction. In a first tunnel junction form, a two orders of magnitude abrupt change in contact resistivity induced by the bulk MIT is shown in VO2-metal contact structures. VO2-metal contact properties are discussed in detail, and the work function of VO2 is found to be 5.2eV(25 °C)−5.3eV(90 °C). In a second junction form, an abrupt change in tunneling current of up to an order of magnitude caused by the bulk MIT is shown to be present in VO2-insulator-metal capacitor structures with atomic layers deposition (ALD) Al2O3 and HfO2 barrier layers. The capacitors show the feasibility of using the MIT to switch a component to a high Off resistance state. Current and capacitance-voltage characteristics of the capacitors are analyzed as well as voltage or field dependent MITs at VO2 interfaces. The abrupt change in current across the VO2 interface is shown to be driven by the change in free carriers in bulk VO2 across the MIT.
Growing nanometer-thin HfO 2 films by atomic layer deposition ͑ALD͒ for implementation in advanced transistor structures is controlled by the density of reactive OH sites on the surface. The impact of thin SiO 2 starting surfaces, grown by wet chemical processes and by wetting a thermal oxide, on the nucleation and growth of ALD HfO 2 has therefore been evaluated. Our results demonstrate that both surface pretreatments display the same dependence of the initial HfO 2 growth on the interfacial layer thickness. This correlation is first characterized by a linear increase, which can be interpreted in terms of increasing OH surface concentration. Once an ellipsometric oxide thickness of approximately 0.8 nm is reached, saturation of the HfO 2 deposition occurs. Maximal OH coverage of the surface or steric hindrance of the adsorbed precursor molecules could explain this observation. However, the increased growth-per-cycle at lower deposition temperatures can be attributed to an improved hydroxylation of the surface, excluding steric hindrance as the primary factor causing saturation. Furthermore, electrical characterization revealed that both interfacial oxides show identical leakage scaling behavior down to an equivalent oxide thickness of 0.8 nm.As gate dielectric thicknesses in complementary metal oxide semiconductor ͑CMOS͒ devices are scaled to the subnanometer regime, the conventional thermal SiO 2 or SiON are no longer acceptable due to the high direct tunneling leakage current. High-materials like HfO 2 are attractive replacement candidates because of their high dielectric constant ͑ Ϸ 20͒, wide bandgap, and thermal stability on silicon at elevated temperatures. 1 In atomic layer deposition ͑ALD͒, a dielectric layer is grown by alternately pulsating a metal precursor and an oxidizing agent to the substrate, separated by an inert gas ͑e.g., N 2 ͒ purge. For ALD HfO 2 , HfCl 4 is the preferred metal precursor while H 2 O has commonly been used as the oxidizing agent. By separating these precursor pulses, HfO 2 growth only occurs through gas-substrate reactions and is limited to the deposition of one monolayer per ALD cycle. However, a lack of available surface sites, initially present and afterwards formed after each reaction cycle, and/or steric hindrance of the adsorbed precursor molecules could cause saturation of these substrate reactions and subsequently limit the HfO 2 deposition to less than a monolayer. 2,3 An increase in the initial amount of reactive sites on the silicon surface therefore improves growth of the dielectric. This amount can be optimized by a silicon surface pretreatment, resulting in a hydroxylterminated silicon oxide interface. However, care should be taken to control the thickness of the oxide, as it contributes to the capacitance of the dielectric stack.Here, we report a study to determine the efficiency of various SiO 2 -based starting surfaces for the nucleation and growth of ALD HfO 2 films. Hydrous chemical oxides have already been adapted as promising because they are thought to c...
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