Diane S. Knight

Characterization of diamond films by Raman spectroscopy

As the technology for diamond film preparation by plasma-assisted CVD and related procedures has advanced, Raman spectroscopy has emerged as one of the principal characterization tools for diamond materials. Cubic diamond has a single Raman-active first order phonon mode at the center of the Brillouin zone. The presence of sharp Raman lines allows cubic diamond to be recognized against a background of graphitic carbon and also to characterize the graphitic carbon. Small shifts in the band wavenumber have been related to the stress state of deposited films. The effect is most noticeable in diamond films deposited on hard substrates such as alumina or carbides. The Raman line width varies with mode of preparation of the diamond and has been related to degree of structural order. The Raman spectrum of hexagonal diamond (lonsdaleite) is distinct from that of the cubic diamond and allows it to be recognized.

The effect of oxygen in diamond deposition by microwave plasma enhanced chemical vapor deposition

High quality diamond thin films were deposited on different substrates at temperatures from 300 to 1000 °C by the microwave plasma enhanced chemical vapor deposition (MPCVD) system. The quality of deposited diamond films was improved by adding oxygen in the gas mixtures. Different ratios of methane to oxygen concentration in hydrogen at different temperatures have been studied. At high temperatures (800–1000 °C), the addition of oxygen will not only enhance the growth rate of deposited films but also extend the region of diamond formation. At low temperatures ( 900 °C) were either graphitic or diamond containing a large amount of graphitic or amorphous carbon and at low temperatures (

Effect of oxygen in diamond deposition at low substrate temperatures

Thin diamond films were deposited on different substrates at temperatures below 500 °C by a microwave plasma‐enhanced chemical vapor deposition system. The deposited films were amorphous carbon or diamond films depending on the different gas mixtures used. The addition of oxygen to the gas mixtures was found to be critical for diamond growth at low temperatures. Without oxygen, the deposited films were white soots and easily scratched off. Increasing the oxygen input improved the quality of the Raman peaks and increased the film transparency. The diamond films were also characterized by scanning electron microscopy.

Surface-enhanced Raman spectroscopy of chemical vapor deposited diamond films

By using surface‐enhanced Raman spectroscopy it was possible to clearly identify very thin diamond and amorphous carbon coatings which were not detectable by normal Raman spectroscopy. A very small amount of silver was sputtered onto the surface of thin diamond depositions. Raman spectra measured through the silver layer exhibited the 1332 cm−1 diamond line and broadbands due to other forms of carbon. Raman scattering measured through silver coatings directly on the silicon substrate revealed extremely thin layers of amorphous carbon.

Ferroelectricity in the ReBa2Cu3O7−δ superconductors

Abstract Several independent pieces of physical evidence lead us to propose that the recently discovered high-temperature superconductors transform to a ferroelectric state prior to the onset of superconductivity.

Cryogenic cathodoluminescence of plasma‐deposited polycrystalline diamond coatings

The cathodoluminescence spectra of microwave plasma‐deposited polycrystalline diamond films have been measured at liquid‐nitrogen temperatures over the spectral region of 230–800 nm. The diamond coatings had been deposited under several different deposition temperatures and reactant compositions. Measurements on natural type‐IIB diamond crystals were made for comparison. The intrinsic exciton emission bands which fall in the UV just below the band edge were observed, as well as several defect and impurity bands which extend throughout the visible part of the spectrum. SEM micrographs and Raman spectra were obtained for the same set of samples used for the cathodoluminescence measurements. It was found that the diamond‐related cathodoluminescence features were most intense in samples whose Raman spectra exhibited the most intense cubic diamond line at 1332 cm−1 and the least intense graphitic band at about 1500 cm−1.

Raman spectra of gel-prepared titania-silica glasses

Abstract Ultralow expansion TiO2SiO2 glasses can be prepared by a variety of sol-gel routes. The Raman spectra are similar to the spectra of glasses prepared by high-temperature melt and vapor deposition techniques. The Raman spectra resemble the spectra of silica glass with the addition of a polarized band at 1106 cm−1and a depolarized band at 933 cm−1which varied in intensity with titanium concentration. The spectra are explained in terms of titanium-rich clusters within a matrix of silica-rich glass. The onset of crystallization in heat-treated glasses is seen in the Raman spectra as a series of sharp bands characteristic of the anatase form of TiO2.

Raman Spectroscopic Study of the Rare Earth Sesquisulfides

Abstract Raman spectra have been measured on carefully synthesized and characterized sesquisulfides of the rare earth ions plus yttrium and scandium. Six structure-types are represented. The Raman spectra are diagnostic of the structure-types. Raman line widths indicate structural disorder only in the defect gamma-type structure. High wavenumber bands shift with ionic radius of the rare earths and only slightly with cation coordination number.

Raman And Fluorescence Spectroscopic Characterization Of Diamonds And CVD Diamond Films

The Raman spectrum is a characteristic signature that identifies diamond carbon, diamond-like carbon, graphitic carbon in various structural states, and possibly another structural family of carbon. The single sharp Raman line at 1332 cm-1 allows cubic diamond to be recognized against a background of other types of carbon. Small shifts in band wavenumber have been related to stress state in deposited films and line width relates to structural order. Impurities in diamond act as luminescence centers. Some of these can be excited by the laser line used for Raman spectroscopy and the Raman spectrometer used as a luminescence spectrometer. Four major centers and several minor ones have been identified in CVD diamond films.

Low Temperature Diamond Deposition On Glass

Diamond films by chemical vapor deposition have good potential for optical application only if transparent films can be produced. Thin diamond films were deposited on silicon, lead glass, MgO, fused silica and soda-lime silica at low temperatures (<500°C) by microwave plasma enhanced chemical vapor deposition. Low temperatures were achieved either by lowering microwave powers and pressures or by remoting the plasma. The deposited films were either white and transluscent or highly transparent depending on the different gas mixtures being used. The effect of gas composition on diamond formation will be discussed. Raman peak shifts of 2 to 8 cm-1 were observed due to the strain in the film. The exact temperature of the growth surface is uncertain as measurements were made only of the substrate bulk. Estimated temperatures were reported by careful calibration.