Trends in device miniaturization have driven the adoption of new materials that, in turn, have enabled significant advancements in the field of process engineering and integration for semiconductor technology. Continued progress for device scaling is necessary and can be enabled by advances in lithographic techniques and deposition schemes. Thin-film deposition for spacers and etch stop layers has become a mainstay to enable and extend traditional 2D scaling into the 3D realm for fabricating advanced semiconductor devices. For processing 3D structures, controlled film deposition with subnanometer resolution in high aspect ratio features is desired. Area selective deposition (ASD) can be a powerful response to such a challenge. ASD is a type of thin-film deposition technique scheme that can be used to eliminate the need for several expensive and time-consuming lithography steps with fewer performance penalties. In this work, we show ASD of ruthenium (Ru) on 3D molybdenum (Mo)–silicon oxide (SiO2) stacks by utilizing the inherent substrate preference of the Ru precursor to a H-terminated surface. In the best selectivity condition, our results show growth of ∼5 nm Ru on Mo, with no film growth on SiO2. Changes in Ru growth kinetics were observed after dilute hydrofluoric acid (DHF) treatment for both surfaces. Post-DHF treatment, the Ru growth rate on Mo was reduced by 5%. However, on SiO2 (after incubation delay), the growth rate was reduced by 94% compared to untreated surfaces. This translates to a very high difference in the growth rate of Ru on Mo vs SiO2, even after considering the incubation delay. Finally, by using 3D topologies with high aspect ratio holes, we have highlighted that it is important to deconvolute the effects of precursor depletion and selectivity. To the best of our knowledge, this is the first demonstration of ASD of Ru on 3D structures without the use of any blocking layers. Therefore, these results demonstrate a new paradigm for ASD in 3D features.

In this article, we provide an accurate method to determine the interlayer resistivity of 2-D layered systems by directly measuring the resistance at a mono- to bi-layer step and feeding the measurement to a distributed resistance model. We take CVD-grown few-layer graphene (up to four layers) with different twist angles ranging from AB-stacked to totally decoupled graphene as an example. Our results show that the interlayer resistivity of AB-stacked CVD grown bilayer graphene (BLG) is in the range of 50-140 which is two to five orders of magnitude greater than the previously reported values for AB-stacked graphite. On the other hand, twisted BLG shows an interlayer resistivity as low as and it decreases monotonically with increasing the twist angle, suggesting that interlayer conduction is not limited by phonon scattering, as previously reported. Furthermore, the total resistance of twisted BLG was found to be about one order of magnitude lower than its AB-stacked counterpart, which might lead to lower delay and energy-delay product in twisted graphene interconnects. In addition to that, the universality of our approach allows for accurate determination of interlayer resistivity of other 2-D layered systems such as metal dichalcogenides.

The potential performance of 2D NAND flash with a graphene floating gate (FG) layer is presented. The field enhancement factor for patterned CVD graphene sheets atop a tunneling dielectric is experimentally extracted and used to drive higher-level circuit simulations on 64-bit NAND strings. The average field enhancement factor at a barrier height of 3.1 eV was found to be similar to 2.85 with a maximum value of 4. Our modest extracted beta value explains the contradiction in prior experiments that reported a field enhancement factor of few thousands but only 30%-40% improvement in the write voltage of FG memory devices. Design and operational tradeoffs are benchmarked based on these experimental values and it is shown that 2D NAND programming time and/or programming voltage can be suppressed to 10 ns and 5 V, respectively, based on a 65 nm process node. The onset of read disturbs from the more efficient FG layer are identified and shown to be easily mitigated through error correction code.

We report improved control over the quality and uniformity of CVD graphene devices through a novel fabrication technique. An overlying HSQ pinning dielectric is used to physically anchor and protect the graphene sheet, resulting in electron and hole motilities of 25 600 and 23 700 cm(2)/Vs, respectively, record-breaking values for CVD graphene devices. Transitioning from traditional 3-D bulk materials to loosely adhered 2-D graphene sheets, the presented process is expected to bring new focus to post-transfer passivation as a means of fabricating graphene devices closer to their theoretical limits. Based on the extracted mean-free-path (MFP) from the experimental data, the graphene interconnect is benchmarked against the copper interconnect at various widths and edge roughness. Results demonstrate the importance of edge smoothness and MFP, which dictate the potential benefits of graphene interconnects at a narrow dimension.

Vertically oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta2O5 and WO3, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed 100-fold improvement in the measured photocurrent compared to pristine Ta2O5 upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport and the red shift in absorption spectrum with increase of the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and, hence, minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta2O5 nanotubes even after annealing.

Stochastic wiring distribution models are used to predict the improvement in energy obtained by replacing a few or all copper metal levels with graphene nanoribbons (GNRs) in a low-power digital circuit. The models developed here also estimate the degradation in the performance by replacing a few or all copper metal levels with GNRs. Replacing a few local copper interconnect levels with GNRs is expected to reduce the energy consumed by local interconnects, without severely degrading the performance of longer global interconnects. The hybrid GNR+copper interconnect is shown to perform worse compared to the all GNR interconnect, if the length of the GNR segment is greater than a critical value. For a logic circuit with 30k gates, it is shown that the hybrid interconnect offers a 30 to 40% decrease in energy and a 4× decrease in maximum frequency, whereas the all GNR interconnect offers a 50 to 60% decrease in energy and a 7× decrease in maximum frequency. Further, the impact of edge doping on the resistance per unit length of graphene is analyzed.

The effect of metal ion doping on electronic band structure and charge carriers' effective mass of Ta2O5 were studied using hybrid functionals in density functional theory. PBE0 hybrid functional proved to be very efficient in predicting the band structure with less than 5% error compared to experimental data. The bandgap decreases monotonically as the percentage of the dopant increases. Furthermore, the indirect bandgap behavior of Ta2O5 was found to initially increase with doping before it decreases again to its original value of pristine Ta2O5. We found that high doping content or even mixing with another metal is required in order to modify the band structure of Ta2O5. The effect of doping on the crystal structure was also studied. XRD measurements show that the crystal lattice tends to expand upon doping with metals with larger atomic radius than Ta and this effect is more pronounced as the dopant content increases.

The band structure and bandgap of Ta2O5 are extremely controversial issues. Herein, the use of a hybrid functional reduces the error in bandgap estimation from 95% to 5% resulting in a bandgap of 3.7 eV. This is expected to help controlling the electronic and structural properties of the material.

The electronic properties of Ta-W-O system have been studied using density functional theory with PBE0 hybrid functional. The bandgap calculations showed a large and composition-dependent bowing parameter. While the large bowing parameter at low W content (<25%) is related to the charge exchange between the localized W 5d state and the extended states in the conduction band, the volume change is considered the main reason for the large bowing parameter at high W content (>25%). The electron excitation from the Ta2O5 valence band to WO3 conduction band at high W content may contribute to the pronounced decrease in the conduction band energy. The staggered bandgap type between Ta2O5 and WO3, as revealed from the energy band diagram, resulted in efficient charge carriers' separation. The minimum hole effective mass occurs along the y-direction and decreases monotonically with increasing W content. Combining the wide tunability of bandgap with the small effective mass, it is suggested that Ta-W-O alloys can replace the various III-V materials for high efficiency solar cells as well as other optoelectronic devices. (C) 2013 AIP Publishing LLC.

This article reviews the use of Density Functional Theory (DFT) to study the electronic and optical properties of solar-active materials and dyes used in solar energy conversion applications (dye-sensitized solar cells and water splitting). We first give a brief overview of the DFT, its development, advantages over ab-initio methods, and the most commonly used functionals and the differences between them. We then discuss the use of DFT to design optimized dyes for dye-sensitized solar cells and compare between the accuracy of different functionals in determining the excitation energy of the dyes. Finally, we examine the application of DFT in understanding the performance of different photoanodes and how it could be used to screen different candidate materials for use in photocatalysis in general and water splitting in particular.

This work would not have been possible without the support and valuable inputs of many people to whom I am really grateful. First of all, I would like to express my deepest gratitude to my parents and my sister for their continuous encouragement and support. They bore a lot especially in my hard moments. It is also a great honor for me to thank Dr Nageh Allam, my co-supervisor, who guided me throughout the development of this thesis and who taught me how to carry out cutting edge research and be part of the research community. He has just been an amazing supervisor. Not forgetting Dr Yehea Ismail, my supervisor, who provided for me a great research environment and didn’t hesitate to provide me with all the materials I needed. Additionally, I can’t express my thanks to Dr. Mostafa El Sayed, from Georgia Institute of Technology, who hosted me in his lab to conduct the experimental part of this work. Also I would like to thank Dr Faisal Alamgir and Dr Hong Li, from Georgia Institute of Technology, for their fruitful conversations and discussions and their insightful suggestions. Special thanks shall also go to Dr Walid Hassan who helped me understand Density Functional Theory and guided me throughout the whole computational section. And last but my no means least, I would like to thank all the group members, whether at AUC or at Georgia Institute of Technology, for being such a great company and giving me help whenever I need it.

The Effect of introducing CuInSe2 (CIS) Nanocrystals with the electrolyte on the photocurrent response of DSSCs was investigated showing great enhancement in optical switching and electrical stability of DSSCs by improving the electrolyte’s reduction rate.

The effect of CuInSe2 (CIS) nanocrystal shape, incorporated with the electrolyte solution, was investigated. CIS nanoflowers give the best performance when both conversion efficiency and photocurrent response are taken into account.

An efficient technique for loading search area in block-based motion estimation is proposed. The main advantage of the proposed technique is to reduce number of search area loading cycles by taking advantage of the vertical and horizontal overlap between adjacent search areas and thus improve the overall performance of the motion search and frame encoding. The proposed technique was implemented in FPGA as part of the flexible triangle search (FTS) motion estimation algorithm. However, the proposed technique can be used in similar search algorithm. The FTS is a fast block-matching algorithm for motion estimation proposed in previous work. The FTS can be used for block-based motion estimation where it can locate the best matching blocks between two frames using a search triangle that changes its direction and its size through a set of operations. These operations provide the triangle with the large flexibility to locate the best matching block in less number of search iterations. Simulation results indicates that the proposed technique reduces number of cycles required for motion estimation by around 8% and thus enable the video encoder to support higher frequencies or larger resolutions. The proposed design was implemented, simulated, and tested using VHDL and synthesized using Xilinx ISE for the Xilinx Spartan3 device. The results obtained were compared to an FPGA implementation of the FTS algorithm published in previous work.