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Title: Study of diamond materials with different morphologies
Other Titles: Duo zhong biao mian xing mao jin gang shi cai liao de yan jiu
Authors: Wu, Yu (吳宇)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Diamond thin films
Notes: CityU Call Number: TP873.5.D5 W8 2004
Includes bibliographical references (leaves 100-113)
Thesis (M.Phil.)--City University of Hong Kong, 2004
v, v, v, 120 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: This work steams from one of the most attractive and challenging goals in synthesis of heteroepitaxial diamond materials and is one of the milestones towards the growth of heteroepitaxial single crystal diamond films. The works herein were based on the synthesis of various diamond structures with different morphologies, studying of their epitaxial relationships with substrates and relevant physical properties. It demonstrates a high level of controlling the deposition parameters for synthesizing different diamond structures. More importantly, the originality of this work is in pioneering of novel conical diamond arrays and elucidating the Raman signature of nanodiamond materials. In this work, microcrystalline and nanocrystalline diamond films were synthesized by using a commercial 1.5 kW microwave plasma enhanced chemical vapor deposition (CVD) system. Both types of materials were prepared by employing two steps deposition, i.e., nucleation and growth processes. The bias enhanced diamond nucleation and diamond seeding/scratching methods were adopted as the first step in the diamond synthesis. The nucleation was followed with selective textural growth to obtain the diamond films with desired morphologies and promote heteroepitaxial relationship between the films and substrates. Such processes were carried out by carefully tuning the deposition parameters including substrate temperature, composition of CVD gas mixture and microwave power supplied into plasma. A part of this work aims at heteroepitaxial growth of diamond films which was performed on two types of substrates, silicon(001) and Ir(111)/CaF2 (111)/Si(111) layered substrates employing either biased enhanced nucleation or nucleation induced from ultrasonic scratching and seeding. In the growth step, the alpha parameter associated with substrate temperature and concentration of hydrocarbon precursor in CVD gas phase controlled the textural growth and coalescence of neighboring crystallites. The diamond synthesis at such conditions led to local heteroepitaxial relationships between the grown (001) diamond films and (001) silicon substrates as revealed by high-resolution transmission electron microscopy and no transition layer was detected at the diamond/silicon interface. High quality (111) textured diamond films with possibly partial epitaxial relationship respect to the iridium (111) substrate were deposited via using ultrasonic scratching/seeding nucleation method and were observed by scanning electron microscopy. In our best knowledge, the diamond synthesis on Ir(111)/CaF2(111)/Si(111) layered substrates yielding textural growth with potential local heteroepitaxial relationship is the first work ever reported. Far more exciting output of this work is in the development of conical arrays made of either single crystal diamond cones or cones composed of nanodiamond. These two different conical structures are prepared from the similar hydrogen reactive ion etching (RIE) process applied to structurally dissimilar materials - polycrystalline diamond and nanodiamond. The polycrystalline diamond films were prepared in hydrogen/methane CVD environment, which had abundance of CH3 radicals. The best single crystalline diamond cones were produced by RIE on pyramidal textured diamond (001) films, which was prepared with alpha parameter close to the value of three. In contrast, nanodiamond cones resulted from hydrogen reactive ion etching of nanodiamond films, in which nanodiamond grains clustered into columns penetrating through the entire film. In general, nanodiamond films grew in CVD environments was dominated by C2 dimers. The high aspect ratio, smoothness of the single crystal diamond cone surface, cone rigidity and possibility of doping indicates that these materials are the best choice of constructing the probes used for scanning probe microscopy (SPM) including atomic force microscopy (AFM), scanning tunneling microscopy (STM) and others. The investigation of the field electron emission properties of the developed nanodiamond conical arrays showed a better performance and approach to the level of turn on field that is close to carbon nanotubes without optimizing their cone density. It is believed that the further improvement of field electron emission property of nanodiamond by optimizing the cone density will make it to be the potential materials for producing the field electron emission devices. Another very important output with a scientific value reported here is the contribution of revealing the Raman spectral signature of nanodiamond structures. The extended analysis of nanodiamond films formed the base for elucidating bizarre peaks at 1140 and 1480cm-1, which often appeared in nanodiamond film Raman spectra. These peaks were incorrectly ascribed to nanodiamond structures. Such interpretation was accepted by scientific community and used for several years. However, our unique experiment in which deuterium substituted hydrogen and comparative work of Ferrari group in Cambridge University proved that the peak of our concern are associated with C-H (C-D) bonded sp2 vibration modes of trans-polyacetylenes confined in the nanocrystalline diamond film surface but not with C-C bonded sp3 nanocrystalline diamond crystals. This is well demonstrated by shifting the peaks down to 860 and 1430 cm-1 when deuterium substitutes hydrogen in CVD environment of nanodiamond synthesis.
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