Anshu Gaur

ABSTRACT Possible mechanisms to achieve high mobility in zinc oxynitride (ZnON) have been investigated by comparison with other thin film semiconductors. Integrated processes to fabricate ZnON TFTs have been developed. Issues and challenges encountered at current stage will be discussed.

IGZO deposition by rotary sputtering targets serves the needs for high-performance, low-manufacturing-cost TFTs. Both, manufacturability of rotary targets and performance of produced TFTs confirm the feasibility and underline the advantages of using rotary technology.

A tungsten plug process is commonly employed in microelectronic interconnect technology. This paper provides a review of the relative advantages of the two variants - blanket and selective tungsten-and subsequently focuses on the former. Specifically, we address the role of thermal gradient that may naturally occur or be intentionally applied during blanket tungsten deposition, whereas most published literature examines the isothermal process. We present a one-dimensional mass transport and deposition model for a cylindrical contact hole/via. In the numerical calculations, a temperature gradient is imposed on a via hole such that its bottom is slightly hotter than the mouth from which the deposition gases enter. We find that the tendency to form a keyhole void diminishes when a temperature gradient is present, as compared to the isothermal process at a temperature corresponding to that either at the mouth or pore bottom. Furthermore, the extent of gain due to a thermal gradient depends on the temperature level; the advantage is greater at higher levels. However, in practical applications, the thermal conductivity of the dielectric layer in which the contact hole is formed should be low in order to achieve the required gradient. This aspect should also be kept in mind when searching for new materials with low dielectric constant.

The metal in the blanket tungsten process for microelectronic interconnects is deposited in via holes or trenches of sub-micron radius/width and depths of the order 2 μm. This process is usually isothermal, based on chemical vapor deposition. However, the resulting concentration gradients along the depth of the via promote formation of a void (keyhole) in the feature, unless processing is at low temperatures and hence, for longer duration. Using process modeling, the feasibility of the two approaches are investigated that rely on a non-isothermal process, which are equally fast and reduces the volume of the keyhole defect. In the first, application of a thermal gradient is examined, which can mitigate the tendency to form keyholes. However, the process conditions require excessive gas flow rates. In another, continuous cooling of the substrate while deposition occurs simultaneously is examined. The method based on continuous cooling appears more feasible in optimizing between keyhole size and process time.

In recent years, single-walled carbon nanotubes (SWCNTs) have become a material of significant interest for future electronic devices. Due to its all-surface-atom structure, properties of carbon nanotubes are highly sensitive to their ambient environment. Understanding carbon nanotubes' inherent properties and how these properties are affected by their environment is of utmost importance in order to realize their potential in devices. Although many studies report useful characteristics of CNT-based devices, underlying mechanism (e.g. changes due to inherent or external factors) often remain unclear or are not well understood. We have explored effects of molecular adsorption, either un-intentional (from ambient environment) or intentional and substrate on resonance Raman scattering measurements on isolated SWCNTs. By correlating Raman scattering measurements with electronic properties of nanotubes, we show that adsorption of O2 from ambient environment leads to charge transfer (and the Fermi level shift below the Dirac point) in metallic nanotubes while no significant effects of O2 adsorption are observed for semiconducting nanotubes from Raman scattering measurements. In metallic nanotubes, adsorption of O 2 is enhanced by the presence of substrate and also leads to significant physical disorder. Variation and complexity in the G-band of metallic nanotube Raman spectrum is explained in terms of Kohn anomalies (KAs) leading to broad and softened ALO1 phonon modes, as well as O2 induced enhancement of double resonance processes. Effects of adsorption of NH3 on Raman scattering measurements are also explored and correlated to charge transfer (Fermi level shift) in carbon nanotubes.

Doping of individual single-walled carbon nanotubes via noncovalent adsorption of polyethylenimine which converts p-type semiconducting nanotubes into n-type is examined by micro-Raman studies. Distinctively different responses are observed in metallic and in semiconducting nanotubes. Very little or no changes in the radial breathing and the disorder modes are observed upon polymer adsorption on semiconducting carbon nanotubes indicating noncovalent nature of this process. Tangential G-band spectral downshift of up to approximately 10 cm(-)(1) without line broadening is observed for semiconducting tubes suggesting similar magnitude of electron transfer as commonly observed in electrochemical doping with alkali metals. Strong diameter dependence is also observed and can be explained by thermal ionization of charge carriers with activation barrier that scales as the energy gap of the semiconducting nanotubes. In contrast, metallic nanotubes exhibit very different behavior with significant line broadening of the G-band and concurrent enhancement of the disorder mode. In certain cases, initially symmetric Lorentzian line shapes of the G-band features with narrow line widths similar to semiconducting tubes are converted to a broad, asymmetric Breit-Wigner-Fano line shape. Implications on the effects of electron injection and the local chemical environment on the intrinsic line shape of isolated carbon nanotubes are discussed.

ABSTRACT Diversity in the Raman G-band phonon modes within individual metallic carbon nanotubes of the same chirality is examined. Comparisons between Raman spectra of as-synthesized nanotubes with those obtained under electrochemical gate potential are made. We show that most of the distribution in line width and peak position of the G-band modes within a single chirality type can be explained by variations in where the Fermi level lies with respect to the band crossing point (i.e., where the nanotube is at zero charge). Varying degree of charge transfer from adsorbed O2 is likely to be the main source of 2 eV or larger range of Fermi level positions. On average, the Fermi level of individual metallic nanotubes lies on the order of 1 eV below the band crossing point. Both charge transfer and physical disorder are evident upon O2 adsorption. Implications of these findings on electron−phonon coupling and charge transfer processes are discussed.

This paper reports on the electrical properties of thin-film transistors (TFTs) that use polymer-coated networks of single-walled carbon nanotubes (SWNTs) as the semiconductor with source and drain electrodes formed by high-resolution printing techniques. P-channel, n-channel, and ambipolar TFTs are demonstrated with bare SWNT networks, networks coated with polyethylene imine and with polyethylene oxide, respectively. Studies of the scaling of properties with channel length and tube density reveal important information about the operation of these devices. Complementary inverters made with n- and p-channel devices show gain larger than one and illustrate the potential use of these types of TFTs for complex logic circuits.

Network behavior in single-walled carbon nanotubes (SWNTs) is examined by polymer electrolyte gating. High gate efficiencies, low voltage operation, and the absence of hysteresis in polymer electrolyte gating lead to a convenient and effective method of analyzing transport in SWNT networks. Furthermore, the ability to control carrier type with chemical groups of the host polymer allows us to examine both electron and hole conduction. Comparison to back gate measurements is made on channel length scaling. Frequency measurements are also made giving an upper limit of approximately 300 Hz switching speed for poly(ethylene oxide)/LiClO(4) gated SWNT thin film transistors.

The role of oxide substrate with respect to O 2 adsorption induced changes in the Raman spectra of individual metallic carbon nanotubes is examined. A chiral metallic nanotube suspended over a trench exhibits a relatively simple two-peak G -band feature with no observable D -band ...

Evolution of G-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all G modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by O2 adsorption causing the Fermi level shift.

Thin film transistors of single-walled carbon nanotubes were operated with polymer electrolyte as gate media. Nearly ideal gate efficiencies allow low-voltage operation with the absence of the common hysteresis problem observed in back-gated carbon nanotube FETs yielding a reliable and simple method for measuring the device characteristics. Furthermore, the conduction type (p/n-type) of the devices can easily be controlled by varying the electrolyte media. Effects such as charge transfer between polymer and nanotubes, and tube-tube interactions in arrays of nanotubes will be discussed.