Plasma Physics in Pulsed Laser Deposition of Hydrogen-free Diamond-like Carbon Films and Nanocomposites.

pulsed laser deposition;diamond-like carbon;laser ablation;laser plasma;magnetic field;nanoparticles

Haverkamp, Jason Dirk ; Dr. J. Gilligan, Committee Member,Dr. G. Duscher, Committee Member,Dr. M.A. Bourham, Committee Co-Chair,Dr. J. Narayan, Committee Co-Chair,Haverkamp, Jason Dirk ; Dr. J. Gilligan ; Committee Member ; Dr. G. Duscher ; Committee Member ; Dr. M.A. Bourham ; Committee Co-Chair ; Dr. J. Narayan ; Committee Co-Chair

University:North Carolina State University

关键词: pulsed laser deposition;    diamond-like carbon;    laser ablation;    laser plasma;    magnetic field;    nanoparticles;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/3919/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

This dissertation focuses on the analysis of the plasma plume created in pulsed laser ablation thin film deposition of diamond-like carbon and the correlation of the characteristics of the plume to thin film properties.Diamond-like carbon films were deposited on silicon substrates by pulsed laser deposition at different laser energy densities.Important plasma parameters, such as ion kinetic energy, ion density, and electron temperature are altered by changing the laser energy density.These plasma properties determine the coordination states of carbon atoms within the deposited film.The diamond-like and graphite-like coordination states of carbon, termed sp³ and sp², respectively, determine film properties such as hardness, optical properties, and electronic properties.The sp³ fraction of the diamond-like carbon was directly determined through electron energy loss spectroscopy.The microstructure of the sp² coordinated carbon was determined with visible Raman spectroscopy.Plasma properties were analyzed by quadruple Langmuir probes and mass loss measurements.Langmuir probe measurements indicate that ion density, ion flow speed, and electron temperature increase with laser energy density.Mass loss measurements show that the plume has an ionization fraction between 5 and 10 percent.Therefore, neutral particles have a significant role in film growth.Current models for the growth of diamond-like carbon films are reviewed.A deposition model based on electronic excitation is proposed.The probability of surpassing the energy barrier between sp² and sp³ coordination is increased via an effective reduction of the activation barrier due to electronic excitation.The energy for electronic excitation is supplied by electron and photon interactions with ions and neutrals in the plume, as well as recombination of ions at the surface.To investigate the effect of magnetic fields on plasma properties and film growth, a strong magnetic field was placed perpindicular to the direction of plasma flow.Plasma flow speed, electron temperature, and ion density were studied with quadruple Langmuir probes.Magnetic probes investigated the interaction between the flowing plasma and the external magnetic field.A correction for the influence of the magnetic field on the collection of electrons by the quadruple Langmuir probe was derived.Plasma flow speed was reduced due to interaction with the magnetic field.The kinetic β was ≈ 1 * 10⁻⁴, indicating that magnetic field energy density is significantly greater than the flow energy density.Magnetic probe data shows a field deflection of ≈ 10 Gauss, consistent with small kinetic β.Field line diffusion occurs on a faster time scale than predicted by electron-ion resistivity.The contribution of neutrals to the resistivity is necessary to explain the observed field diffusion rate.Electron temperature increased by a factor of 3 to 4 over the unmagnetized electron temperatures.Electron heating by field line diffusion is eliminated due to the weak field line deflection.Plume slowing is a mechanism by which the electron temperature may increase.This processes requires an anamolously high electron-ion collision frequency.Ion density results show an instability at the ion cyclotron frequency and higher frequency noise.A classical Rayleigh-Taylor instability can be eliminated due to unfavorable field geometry and slow growth time.The observed instability satisfies a transverse ion-cyclotron instability. Noise at higher frequencies may be a result of the Kelvin-Helmholtz instability.A diamond-like carbon film was deposited in the presence of the external magnetic field.Its sp³ fraction was analyzed with electron energy loss spectroscopy.The sp³ fraction was less than the sp³ fraction for a diamond-like carbon film deposited at the same laser energy.However, in the magnetized deposition case, the ions have a smaller flow speed than in the unmagnetized case.The observed sp³ fraction in the deposition with the magnetic field is larger than expected for the ion kinetic energy.This is due to the presence of neutrals whose flow speeds are unaffected by the magnetic field.No evidence of significant plume deflection was observed.Large compressive stresses within as-deposited diamond-like carbon films prevent the growth of thick films.Diamond-like carbon films were doped with carbide and non-carbide forming elements, titanium and copper, respectively, to reduce the internal stress.The films were analyzed with electron energy loss spectroscopy to determine the sp³ fraction in the doped diamond-like carbon films.Visible Raman spectroscopy was used to quantify stress reduction.Nanohardness measurements were performed to investigate the change in hardness with dopant concentration.High resolution transmission electron microscopy and Z-constrast imaging were used to determine the ordering of the dopants within the diamond-like carbon.High resolution transmission electron microscopy revealed the formation of nanocrystals of titanium and copper.Z-constrast imaging reveals that non-carbide forming elements form self assembled arrays of nanoparticles, whereas carbide forming elements form layers.At the dopant concentrations used, the dopants form nanocrystals, rather than occupying substitutional or interstitial sites within the diamond-like carbon film.Raman spectroscopy indicates that the addition of copper is more effective than titanium at reducing internal stresses.Hardness measurements indicate that the addition of dopants decreases the hardness of diamond-like carbon films.Electron energy loss spectroscopy showed an apparent reduction in sp³ fraction with the addition of dopants.The use of electron energy loss spectroscopy to determine sp³ fraction in doped diamond-like carbon may not be reliable.