Clifford J. Robinson

Large‐area mosaic diamond films approaching single‐crystal quality

The seeding for large‐area mosaic diamond films approaching single‐crystal quality is described. The technique includes patterned etching of relief structures in Si substrates, deposition from a slurry and orientation of macroscopic diamond seed crystals in the structures, and chemical vapor deposition overgrowth of the diamond seeds to form a continuous film. The film comprises ∼100 μm single crystals, which are separated by low‐angle grain boundaries of a few degrees or less. We believe that these low‐angle grain boundaries will not affect the electrical properties of majority‐carrier devices.

Nuclear magnetic resonance and infrared absorption studies of hydrogen incorporation in polycrystalline diamond

Hydrogen incorporated into polycrystalline diamond was found to correlate with the optical absorption in the 8 to 10 μm wavelength IR region, where transparency is desired. In this first detailed study of diamond films by 1H NMR, average concentrations were found to be <0.25 at. % H. However, segregation produces regions of extremely high local hydrogen density as evidenced by a broad (50–70 kHz) Gaussian NMR component. The majority of this clustered hydrogen is rigidly held, while a fraction undergoes motional narrowing at room temperature, most likely due to methyl group rotation. Sites at grain boundaries are capable of accounting for a significant fraction of this hydrogen, as are defects and voids within the crystal or a heavily hydrogenated phase stable at the deposition temperature.

Quantitative correlation of infrared absorption with nuclear magnetic resonance measurements of hydrogen content in diamond films

Hydrogen concentrations in polycrystalline diamond films were found to correlate well with the integrated intensity of the CH‐stretch region in the infrared (IR) spectrum for films with a total hydrogen content <0.10 at. %. Comparison of IR to nuclear magnetic resonance (NMR) measurements yielded an effective absorption coefficient for the CH‐stretch region of 4.3±0.8×103 l mol−1 cm−2, similar to literature values for condensed‐phase hydrocarbons. In several films, the dominant stretch modes occur at ∼2850 and 2920 cm−1, typical of CH2 groups. The presence of these modes is accompanied by an increase in the Gaussian linewidth in the NMR spectrum, indicating a decrease in the interproton spacings within the film. In films with higher total hydrogen contents, a simple linear relationship between hydrogen content and intensity in the CH‐stretch region is no longer applicable.

Phonon‐defect scattering in high thermal conductivity diamond films

We have investigated the thermal conductivity of large diamond samples grown by both hot filament and microwave plasma assisted chemical vapor deposition in order to study in detail the processes limiting heat conduction in this system. For samples containing nearly 100% diamond material and no apparent defects, the thermal conductivity is consistent with that expected for polycrystalline diamond with a given crystallite size. In films prepared by the hot filament technique, we observe an additional scattering of phonons near 60 K, which we attribute to either a resonant phonon‐defect interaction, or a crossover from geometrical to Rayleigh phonon‐defect scattering.

Transmission of phonons through grain boundaries in diamond films

With the advent of new growth techniques, research on the properties of diamond has undergone a rebirth in recent years. The capability of producing very high quality polycrystalline films has greatly enhanced the prospects of exploiting the extreme physical properties of this substance. One area of great interest has been the thermal conductivity K of synthetically produced diamond. Vapordeposited material is now made routinely with room temperature thermal conductivities up to 85% that of bulk single crystals. It is not yet clear, however, what processes limit the conductivity to values below that of single crystal material. Some time ago, it was recognized”2 that the polycrystalline nature of the film morphology could provide some limitation to the thermal conductivity at and even above room temperature. More recently, detailed studies of the anisotropy of the thermal conductivity3 as well as a through-the-thickness gradient in K’,~ have provided some indirect evidence of the influence of grain boundary scattering. On the other hand, it has also been shown6 that very low levels of defects within the grains, most likely associated with small concentrations of nondiamond carbon, are capable of strongly degrading the thermal conductivity over the temperature range 20-300 K. Separation of the influence of grain boundaries and other defects is complicated by the fact that samples with smaller grains usually have more defects since the latter tend to be concentrated at grain boundaries7 One can, in principle, differentiate to some extent between the effects of intragrain defects and grain boundaries on the heat conduction by measuring K

Diamond for high heat flux applications

In polycrystalline CVD diamond of useful macroscopic dimensions, which may be considered for high heat flux applications, thermal conductivity parameters are largely determined by grain size resulting from growth morphology, defects and impurities in the material. Thermal conductivity has been measured in a number of state-of-the-art diamond samples, by the steady state technique, over the temperature range 6 to 400 K. The results are presented, and discussed in terms of microstructural differences between samples. At approximately 30 K, a departure from normal Debye type behavior is observed as a lowering of the predicted conductivity. At higher temperatures, this departure becomes less significant so that above approximately 350 K, where only Umklapp processes contribute to phonon scattering, the measured thermal conductivity is close to that predicted by the model and in good agreement with reference data for natural type IIa single crystal diamond. To account for the observed temperature dependence of conductivity, an additional phonon scattering term is used which may be described as Rayleigh scattering at low temperature by defects of 0.7 to 1.3 nm in size.

Polycrystalline Diamond for Infrared Optical Applications Prepared by the Microwave Plasma and Hot Filament Chemical Vapor Deposition Techniques

Abstract Absorption in the 8-12 μm spectral region in diamond films is dominated by single phonon effects due to departures from perfect crystallinity in the deposits. Conditions which maximize crystalline perfection must therefore be chosen to deposit diamond having the highest infrared (IR) transparency. Diamond having promising IR transmission has been deposited by both the hot filament and microwave plasma techniques. Quantitative measurement of IR absorption in polycrystalline diamond requires improvements in surface preparation techniques to reduce scatter and an understanding of the magnitude of surface absorption effects.

Critical-point phonons of diamond

The phono-dispersion curves derived from neutron-scattering experiments performed on diamond are not accurate enough to yield the exact frequencies of critical-point (CP) phonons and, thus, to provide a satisfactory interpretation of second-order optical spectra. A self- consistent analysis of such spectra proved to be difficult because it is not a straightforward task to assign second-order absorption and scatter features to specific two-phonon summations. A more effective method for obtaining accurate CP-phonon frequencies involves investigating defect-activated one-phonon absorptions; in this paper, the authors take advantage of the availability of chemically vapor-deposited (CVD) diamond for the purpose of locating and assigning infrared (IR) absorption features in the one-phonon region to CP phonons at the Brillouin-zone boundary. Fourier-transform IR absorbance spectra of CVD diamond exhibit a complex structure at wavenumbers below the 1333-cm-1 band-mode cutoff, which is induced by nitrogen-associated defect centers and yields the precise positions of sixteen zone-edge CP phonons. In conjunction with the triply-degenerate zone-center mode, this set of phonons then provides the basis for predicting the positions of second-order optical features through simple summations. Taking the selection rules into account, the procedure yields excellent results, not only in terms of CVD-diamond IR spectra, but also in regard to earlier measurements of the intrinsic two-phonon absorption coefficient of type-IIa natural diamond and the second-order polarization-dependent Raman-scatter characteristics recorded by Solin and Ramdas.

Exafs Study of the Stability of Amorphous TbFe Thin Films

EXAFS is used to measure the local atomic structure of Fe in Au doped Tb-Fe thin film alloys. The as deposited sample shows structural fcatures which are essentially identical to those of the undoped films. Au additions stabilizes the amorphous structure against recrystallization, however, the loss of magnetic anisotropy under thermal annealing is not reduced. This demonstrates that magnetic relaxation in these alloys does not involve crystallization of the amorphous structure.