Carbon nanotubes: opportunities and challenges

Current issues and problems in the chemical vapor deposition (CVD) of diamond are those which relate to its characterization, its nucleation on foreign surfaces, the question of its formation in preference to the other phases of solid carbon (for example, graphite, chaoite, or lonsdaleite), why different morphologies and crystallographic orientations (textures) are seen in different experiments or with different parameters in the same experiment, and finally whether well-crystallized metastable phases can be obtained by CVD in other material systems or are only a peculiarity of carbon chemistry. Whether a given carbon coating is justly described as diamond has been such an issue, and coatings should clearly show evidence for diamond by x-ray diffraction and Raman spectroscopy before the claim of diamond is made. Experimental results have not been consistent in many cases, and much work remains to be done before an accurate assessment can be made of the technological impact of the development.

https://science.sciencemag.org/content/sci/247/4943/688.full.pdf

Current Issues and Problems in the Chemical Vapor Deposition of Diamond

On etudie le depot chimique en phase vapeur et la croissance cristalline de revetement de diamant

/pdf/diamond-ceramic-coating-of-the-future-31m184yw0t.pdf

Diamond—Ceramic Coating of the Future

Hydrogen-terminated diamond surfaces and interfaces

Progress and prospects of group-III nitride semiconductors

Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by Auger electron spectroscopy, secondary ion mass spectrometry, electron and x-ray diffraction, electron energy loss spectroscopy, and electron microscopy. They were determined to be similar to natural diamond in terms of composition, structure, and bonding. The surface morphology of the diamond films was a function of position on the sample surface and the methane concentration in the feedgas. Well-faceted diamond crystals were observed near the center of the sample whereas a less faceted, cauliflower texture was observed near the edge of the sample, presumably due to variations in temperature across the surface of the sample. Regarding methane concentration effects, threefold {111} faceted diamond crystals were predominant on a film grown at 0.3% CH4 in H2 while fourfold {100} facets were observed on films grown in 1.0% and 2.0% CH4 in H2. Transmission electron microscopy of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In addition, cross-sectional TEM has revealed a 50 A epitaxial layer of β3–SiC at the diamond-silicon interface of a film grown with 0.3% CH4 in H2 while no such layer was observed on a diamond film grown in 2.0% CH4 in H2.

Characterization of diamond thin films: Diamond phase identification, surface morphology, and defect structures

The extreme thermal and electronic properties of diamond and of silicon carbide, and the direct band gap of gallium nitride, provide multiplicative combinations of attributes which lead to the highest figures of merit for any semiconductor materials for possible use in high power, high speed, high temperature and high frequency applications. The deposition of monocrystalline diamond, at or below 1 atm total pressure and at a temperature T , has been achieved on diamond substrates; the deposited film has been polycrystalline on all other substrates but the achievement is no less significant. For electronic applications, heteroepitaxy of single-crystal films of diamond, an understanding of mechanisms of nucleation and growth, methods of impurity introduction and activation, and further device development must be achieved. Stoichiometric gallium nitride free of nitrogen vacancies has apparently not been obtained. Thus, knowledge of the defect chemistry of this material, the growth of semiconducting films on foreign substrates, and the development of insulating layers and of their low temperature deposition as well as device fabrication procedures must be achieved. By contrast, all of these problems have already been solved for silicon carbide, including the operation of a MOSFET at 923 K — the highest operating temperature ever reported for a field-effect device. However, considerable research remains to be done regarding the development of large silicon carbide substrates, of ohmic and rectifying contacts, of new types of devices, and of low temperature techniques for the deposition of insulating layers. Fugitive donor and acceptor species in unintentionally doped samples must also be identified and controlled.

Critical evaluation of the status of the areas for future research regarding the wide band gap semiconductors diamond, gallium nitride and silicon carbide

The I-V characteristics of titanium contacts on polycrystalline diamond have been correlated with x-ray-photoelectron-spectroscopy (XPS) and Auger-electron-spectroscopy (AES) characterizations of the interface. As-deposited titanium contacts were rectifying in nature because of minimal interaction between as-deposited titanium and diamond as confirmed via XPS and AES. Once annealed, however, these contacts became Ohmic. The change was related to the formation of a carbide at the interface as observed by XPS. The Schottky-barrier height of the titanium contacts, which was determined by valence-band XPS, decreased from 1.3 to 0.8 eV as a result of the postdeposition annealing. It is believed that the carbide formation at the interface creates a diamond surface layer rich in electrically active defects which lower the barrier height of the metal and increase the leakage current. The interface between titanium and an argon-sputtered diamond surface was also characterized. Titanium formed as-deposited Ohmic contacts on the sputtered surface. A high density of ion-radiation-induced defects and a formation of a carbide during deposition both contributed to the Ohmic-contact formation. These contacts remained Ohmic after postdeposition annealing despite the fact that the annealing did not increase the carbide formed at the interface. It is believed that the carbide formed by the deposition of titanium behaved as a diffusion barrier to prevent the damaged layer from being annealed out into the titanium overlayer. It is concluded that most materials will yield rectifying contacts on a clean diamond surface. Ohmic contacts can be obtained by modifying the interface in some way (i.e., carbide formation, sputtering, etc.).

Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface. I. Gold contacts: A non-carbide-forming metal.

Recent advances in the chemical vapor deposition (CVD) and characterization of diamond films on nondiamond substrates are reviewed. Major growth techniques, including hot filament CVD; microwave, RF, or DC plasma enhanced CVD; and combustion flame growth; as well as a number of hybrid and novel approaches, are described and analyzed. Results from the major categories of diamond film characterization, including diamond phase identification, nucleation and interfacial phenomena, morphology, and defects, as well as their correlations with electrical properties, are examined and discussed. Although most of the information presented is equally applicable to protective and wear-resistant coating application, emphasis is placed in the areas most pertinent to microelectronics. >

Growth and characterization of diamond films on nondiamond substrates for electronic applications

Laser reflection interferometry (LRI) has been shown to be a useful in situ technique for measuring growth rate of diamond during microwave plasma chemical vapor deposition (MPCVD). Current alternatives to LRI usually involve ex situ analysis such as cross-sectional SEM or profilometry. The ability to measure the growth rate in ‘real-time’ has allowed the variation of processing parameters during a single deposition and thus the extraction of much more information in a fraction of the time. In situ monitoring of growth processes also makes it possible to perform closed loop process control with better reproducibility and quality control. Unfortunately, LRI requires a relatively smooth surface to avoid surface scattering and the commensurate drop in reflected intensity. This problem was remedied by greatly enhancing the diamond particle nucleation via the deposition of an intermediate carbon layer using substrate biasing. When an unscratched silicon wafer is pretreated by biasing negatively relative to ground while in a methane-hydrogen plasma, nucleation densities much higher than those achieved on scratched silicon wafers are obtained. The enhanced nucleation allows a complete film composed of small grains to form in a relatively short time, resulting in a much smoother surface than is obtained from a film grown at lower nucleation densities.

B.E. Williams

Papers

Characterization of diamond thin films: Diamond phase identification, surface morphology, and defect structures

Critical evaluation of the status of the areas for future research regarding the wide band gap semiconductors diamond, gallium nitride and silicon carbide

Correlation of the electrical properties of metal contacts on diamond films with the chemical nature of the metal-diamond interface. I. Gold contacts: A non-carbide-forming metal.

Growth and characterization of diamond films on nondiamond substrates for electronic applications

In situ growth rate measurement and nucleation enhancement for microwave plasma CVD of diamond