Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their unique electrical, optical, mechanical, and thermal properties not found in their 3D counterparts. They can be obtained by mechanical, chemical, or electrochemical exfoliation. However, these strategies lack uniformity and produce defect-rich samples, making it impossible for large-scale device fabrication. Chemical vapor deposition (CVD) method emerges as the viable candidate to create atomically thin specimens at the technologically relevant scale. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. The spatial inhomogeneity and the associated grain boundaries between randomly oriented domains culminate to the deleterious quality of TMDs, breaking of the long-range crystalline periodicity and introduction of insidious strain. Recent research efforts have therefore dedicated to obtaining the single crystallinity of 2D materials by controlling the orientation and dimensionality to obtain a large-scale and grain boundary-free monolayer films for Si-comparable electron mobility and overcoming the scaling limitation of traditional Si-based microelectronics,. In the first part of this thesis, orientation and dimensionality controlling of TMDs are discussed. To this end, we systematically study the growth of stereotypical molybdenum disulfide (MoS2) monolayer on a c-plane sapphire with CVD to elucidate the factors controlling their orientation. We have arrived at the conclusion that the concentration of precursors- that is, the ratio between sulfur and molybdenum oxide, plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. Later, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer, and single-crystalline MoS2 nanoribbons on β-gallium (iii) oxide (β-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long-range and transport characteristics on par with those seen in exfoliated benchmarks. In the second part, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast as a result of the residual 3-fold astigmatism in modern scanning transmission electron microscopy (STEM) and its unintended impact on violating the power-law dependence of contrast on coordination modes between the transition metal and chalcogenide atoms.

Carbon nanotubes exhibit excellent field emission properties and will likely be prime candidates as electron sources in future vacuum electronic applications. Recent research has focused on enhancing field emission from traditional diode-type emitters by adding a gate electrode between the anode and the cathode. Since the gate to cathode (emitter) distance in this triode-type structure is small relative to the anode to cathode distance, this structure allows relatively small gate voltages to significantly enhance or dampen field emission. The key challenge for this research is: synthesizing vertically aligned carbon nanotube field emitters inside arrays of triode-type devices. The most common "top-down", etch-deposit-synthesis method of synthesizing carbon nanotubes inside gated cavities is discussed here, and a novel "bottom-up" method is presented. This new approach bypasses the lithography and wet chemistry essential to the etch-deposit-synthesis method, instead using a dual-beam focused ion beam (FIB) system to mill cavities into a multi-layered substrate. Here the substrate is designed such that the act of milling a hole simultaneously creates the gate structure and exposes the catalyst from which carbon nanotubes can then be grown. Carbon nanotubes are synthesized using plasma enhanced chemical vapor deposition (PECVD) rather than thermal chemical vapor deposition, due to the superior alignment of the PECVD growth. As dual-beam FIB and PECVD can both be largely computerized, this synthesis method is highly reproducible. The dual-beam FIB also permits a high degree of controllability in gate radius, cavity depth and emitter spacing. The effects of a host of PECVD growth parameters (initial catalyst thickness, gas concentration, growth temperature, temperature ramping rate, chamber pressure, and plasma voltage) were characterized so that the morphology of the carbon nanotube emitters could be controlled as well. This "bottom-up" method is employed to construct functional, large area carbon nanotube field emitter arrays (CNT FEAs). The role of the gate layer in field emission is examined experimentally as well as through theoretical models. Field emission testing revealed that increasing gate voltage by as little as 0.3 V had significant impact on the local electric fields, lowering the turn-on and threshold fields by 3.6 and 3.0 V/µm, respectively, and increasing the field enhancement factor from 149 to 222. A quantum mechanical model of such triode-type field emission indicates that the local electric field generated by a negatively or positively biased gate directly impacts the tunneling barrier thickness and thus the achievable emission current. However, the geometry of triode-type devices (gate height, gate radius, emitter density) can influence the degree to which the gate voltage influences field emission. I demonstrate here an effective method of analytically calculating the effect of various such geometric parameters on the field emission. Results show that gate type (the height of the gate relative the emitter tip) can significantly impact the local electric field and hence the type of applications a device is suitable for. Side gates (gate height < emitter height) induced the highest local electric field, while top gates (gate height > emitter height) provided the greatest controllability. For all gate types, increasing the size of the gate opening increased the local electric field by diminishing the gate-emitter screening effect. However, gate voltages were able to enhance or inhibit the local electric field much more readily with smaller gate radii. Due to the strength of gate-emitter field screening in the triode-type structure, the spacing between emitters had virtually no impact on the local electric field, allowing relatively high emitter densities. These theoretical results, combined with a highly controllable synthesis method, provide valuable information and methodology for those designing and optimizing triode-type devices targeted at specific applications.

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Evidence is reported for the formation of carbon-containing contamination products at the copper-tungsten (Cu-W) interface during the metal organic chemical vapor deposition (MOCVD) of copper on tungsten. Cu(II)bishexafluoroacetylacetonate [Cu(hfac)_2] was physisorbed onto lightly oxidized tungsten (WO_x) at 115K, under ultra-high vacuum conditions, and then annealed sequentially to higher temperatures. Copper reduction was observed by 320K. Carbonaceous and carbidic contamination of the WO_x surface was observed, even after sample warming to 625K in UHV. The results indicate that low temperature MOCVD of Cu may be possible, but interfacial contamination from the organic ligand fragmentation is a major concern.

Chemical vapor deposition (CVD) is an industrially important process to deposit crystalline and amorphous thin films on solid substrates. In situ sensing for CVD is necessary for process monitoring, fault detection, and process control. The challenge of in situ sensing lies in the prohibitive environment of the CVD process. Optical sensors such as the reflectometer and the ellipsometer are the most promising sensors because they can be installed outside of the deposition chamber, and are sensitive and easy to implement. However, the optical sensors do not measure film properties directly. Mathematical methods are needed to extract film properties from indirect optical measurements. Currently the most commonly used method is least squares fitting. In this project, we systematically investigated in situ reflectometry data interpretation based on state estimation theory. Optical models for light reflection on both smooth and rough surfaces were studied. The model validation results indicated that the effective medium model is better than the scalar scattering model when the surface is microscopically rough. The analysis of the observability for the sensor models indicated that the linearized observability does not always guarantee the true observability of a nonlinear system. We studied various state estimators such as batch least squares fitting (BLS), recursive least squares fitting (RLS), extended Kalman filter (EKF), and moving horizon estimation (MHE). It was shown that MHE is the general least-squares-based state estimation and BLS, RLS, and EKF are special cases of MHE. To reduce the computational requirement of MHE, a modified moving horizon estimator (mMHE) was developed which combines the advantage of the computational efficiency in RLS and the a priori estimate in MHE. State estimators were compared in simulated film growth processes, including both process model mismatch and sensor model mismatch, and reflection of both single wavelength and dual wavelength. In the case of process model mismatch and reflection on a smooth surface, there exists an optimum horizon size for both RLS and mMHE, although mMHE is less sensitive to the horizon size and performs better than RLS at all horizon sizes. The estimate with dual wavelength is more accurate than that with single wavelength indicating that estimation improves with more independent measurements. In the case of reflection on a rough surface, RLS failed to give a reasonable estimate due to the strong correlation between roughness and the extinction coefficient. However, mMHE successfully estimated the extinction coefficient and surface roughness by using the a priori estimate. MHE is much more computationally intensive than mMHE and there is no significant improvement on the estimation results. In the case of sensor model mismatch, either state estimator gave a good result, although mMHE consistently gave a better estimate, especially at a shorter horizon size. In order to test the state estimators in a real world environment, we built a cold-wall low-pressure chemical vapor deposition testbed with an in situ emissivity-correcting pyrometer. Fully automatic data-acquisition and instrument-control software was developed for the CVD testbed using LabVIEW. State estimators were compared using two experimental reflectance data sets acquired under different deposition conditions. The estimated film properties are compared with ex situ ellipsometry and AFM characterization results. In all cases, mMHE consistently yielded better estimates for processes under quite different deposition conditions. This indicated that mMHE is a useful and robust state estimator for in situ sensor data interpretation. By using the information from both the process and the sensor model, one can obtain a better estimate. A good feature of mMHE is that it provides such a versatile framework to organize all these useful information and gives a user the opportunity to interact with fitting and make wise decisions in the in situ sensor data interpretation.

Atomic resolution images of hot-tungsten filament chemical-vapor-deposition (CVD) grown epitaxial diamond (100) films obtained in ultrahigh vacuum (UHV) with a scanning tunneling microscope (STM) are reported. A (2x1) dimer surface reconstruction and amorphous atomic regions were observed on the hydrogen terminated (100) surface. The (2x1) unit cell was measured to be 0.51"0.01 x 0.25"0.01 nm2. The amorphous regions were identified as amorphous carbon. After CVD growth, the surface of the epitaxial films was amorphous at the atomic scale. After 2 minutes of exposure to atomic hydrogen at 30 Torr and the sample temperature at 500° C, the surface was observed to consist of amorphous regions and (2x1) dimer reconstructed regions. After 5 minutes of exposure to atomic hydrogen, the surface was observed to consist mostly of (2x1) dimer reconstructed regions. These observations support a recent model for CVD diamond growth that is based on an amorphous carbon layer that is etched or converted to diamond by atomic hydrogen. With further exposure to atomic hydrogen at 500° C, etch pits were observed in the shape of inverted pyramids with {111} oriented sides. The temperature dependence of atomic hydrogen etching of the diamond (100) surface was also investigated using UHV STM, and it was found that it was highly temperature dependent. Etching with a diamond sample temperature of 200° C produced (100) surfaces that are atomically rough with no large pits, indicating that the hydrogen etch was isotropic at 200° C. Atomic hydrogen etching of the surface with a sample temperature of 500° C produced etch-pits and vacancy islands indicating an anisotropic etch at 500° C. With a sample temperature of 1000° C during the hydrogen etch, the (100) surface was atomically smooth with no pits and few single atomic vacancies, but with vacancy rows predominantly in the direction of the dimer rows, indicating that the 1000° C etch was highly anisotropic. Raman spectroscopy was used as a temperature probe, and for determining film quality.

As device dimensions in integrated circuits scale down, there is an increasing need to deposit ultra-thin, smooth, continuous films for use in applications such as the liner in back end processing. The liner must have good adhesion to both Cu and the dielectric, act as a Cu diffusion barrier, and be conductive enough to allow the electroplating of Cu. Ruthenium (Ru) has been considered as a possible material to be implemented into the liner due to its low electrical resistivity, high thermal and chemical stability, and negligible solubility with copper. Chemical vapor deposition (CVD) is an attractive growth technique for Ru films because it allows conformal deposition in high-aspect ratio features. However, there are some limitations that must be overcome in the deposition of Ru films. CVD Ru films suffer from poor nucleation on oxide and nitride substrates. Poor nucleation leads to rough, large-grained polycrystalline columnar films, which may not coalesce into a continuous film until the thickness greatly exceeds the requirements for the liner. This dissertation presents surface chemistry and film growth studies involving Ru CVD and focuses on improving the nucleation and properties of Ru films. In situ surface analysis techniques including X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) were used to study the fundamental adsorption behavior of the Ru precursor, (2,4- dimethylpentadienyl)(ethylcyclopentadienyl)Ru or DER, on polycrystalline Ta, both with and without iodine adsorbed on the Ta. Based upon these results, CVD films were grown using DER/O₂, and it was shown that nucleation and film properties can be improved by the addition of methyl iodide. Ru films grown using DER/O₂ show sparse nucleation, which leads to very rough surface topography and large polycrystalline columnar grains. The addition of methyl iodide during growth significantly improves nucleation and results in smoother, smaller-grained films. Iodine adsorbs on the initially-formed Ru islands and continuously segregates through the film to the surface during the entire deposition. In addition, CVD films grown with Ru₃(CO)₁₂ were studied. Use of the Ru₃(CO)₁₂ precursor results in thin, ultra-smooth films that show little to no columnar grain structure. / text

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