Amorphous Diamond-like Carbon Films Research Articles

Raman Spectra of Diamondlike Amorphous Carbon Films

Raman Spectra of Diamondlike Amorphous Carbon Films

Chemical structure and physical properties of diamond-like amorphous carbon films prepared by magnetron sputtering

Thin films of amorphous carbon (a–C) and amorphous hydrogenated carbon (a–C:H) were prepared using magnetron sputtering of a graphite target. The chemical structures of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical band gap, and electrical resistivity were determined, and their relation to the film's chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical band gap, mass density, and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp2-bonded clusters in the a–C films and changes in the termination of the graphitic clusters and sp3-bonded networks by hydrogen in the a–C:H films.

Electron cyclotron resonance deposition of diamond-like films

Electron cyclotron resonance (ECR) microwave plasma CVD has been developed at low pressures (10 −4–10 −2 torr) and at ambient and high substrate temperatures (up to 750°C), to achieve large-area (>4 in. diameter) depositions of diamond-like amorphous carbon (a- C:H) films. The application of a rf bias to the substrate stage, which induces a negative self-bias voltage, is found to play a critical role in determining carbon bonding configurations and in modifying the film morphology. There are two distinct types of ECR-deposited, diamond-like films. One type of diamond-like film exhibits a Raman spectrum consisting of broad and overlapping, graphitiD (1360 cm −1, line width = 280 cm −1) and G (1590 cm −1, line width 140 cm −1) lines, and the other type has a broad Raman peak centered at approximately 1500 cm −1. Examination of plasma species by optimal emission spectroscopy shows no correlation between the CH ∗ emission intensity and the deposition rate of diamond-like films.

Electron spectroscopy amd electron microscopy of a-C:H films

Some of the techniques used in electron Spectroscopy and electron microscopy may be combined to give a full characterization of new materials. This requires a correlation of results from structural, microstructural and analytical techniques. Hydrogenated amorphous carbon (a-C:H), diamond and diamond-like carbon films were examined using XPS, XAES, TEM and SEM.

Growth mechanism of diamond-like amorphous carbon films

In situ optical emission spectra (OES) have proven to be an important diagnostic tool for monitoring the deposition process of diamond-like hydrogenated amorphous carbon (DL a-C:H) films from rf discharge decompsoition of CH 4 + Ar. The film deposition rate depends on the processing parameters, especially the self-bias between the electrodes. For our system, iamond-like hydrogenated amorphous carbon film growth is governed by a balance between the decomposition and etching process. The implications of the results for the plasma and surface chemistry are discussed.

Optical-absorption edge and disorder effects in hydrogenated amorphous diamondlike carbon films.

Optical absorption (\ensuremath{\sim}1--5 eV) in rf-deposited hydrogenated (20--40 at. %) amorphous ``diamondlike'' carbon films has been studied as a function of heat treatment. For E\ensuremath{\gtrsim}2.5 eV a Tauc behavior is exhibited. Optical band gaps ${E}_{g}$ are \ensuremath{\lesssim}2.4 eV. At lower energies, band tailing with an Urbach focus is found. ${E}_{0}$, the width of the tail, is quantitatively explained in terms of the annealing temperature ${T}_{H}$. Increasing ${T}_{H}$ (<700 K) reduces the hydrogen content, lowers ${E}_{g}$, and appears to harden the films. ${E}_{g}$ is determined to be a linear function of ${E}_{0}$. Additional analysis was performed in terms of a divergence temperature, ${T}_{0}$; the results of this analysis are comparable to those observed in a-Si:H.

Formation of crystalline diamond from amorphous diamond-like carbon films by pulsed laser irradiation

The formation of crystalline diamond films from amorphous diamond-like carbon films by pulsed laser irradiation with a 300 μs non-Q-switched Nd:YAG laser has been established by a combined study of transmission electron microscopy, x-ray photoelectron spectroscopy, and electrical resistivity. The films have been prepared by glow discharge decomposition of a mixture of propane, n-butane, and hydrogen in a rf plasma operating at a frequency of 13.56 MHz. Prior to laser irradiation, the films have been found to be amorphous by transmission electron microscope studies. After irradiation, the electron diffraction patterns clearly point out the formation of cubic diamond structure with a lattice spacing of 3.555 Å. However, the close similarity between diamond and graphite electron diffraction patterns could sometimes be misleading regarding the formation of a diamond structure, and hence, x-ray photoelectron spectroscopic studies have been carried out to confirm the results. A chemical shift in the C 1s core level binding energies towards higher values, viz., from 286.5 to 287.8 eV after laser irradiation, and a high electrical resistivity >1013 Ω cm are consistent with the growth of diamond structure. This novel ‘‘low-temperature, low-pressure’’ synthesis of diamond films offers enormous potential in terms of device compatibility with other solid-state devices.

Raman spectra of diamondlike amorphous carbon films

Raman spectra of diamondlike amorphous carbon (a-C) films prepared under atmosphere with various hydrogen gas content have been measured as a function of excitation wavelength. The Raman spectral profiles vary with excitation wavelength depending on electronic absorption spectra associated with π-π* electronic transitions. Dependence of Raman spectra on excitation wavelength is interpreted in terms of π-π* resonant Raman scattering from aromatic rings with various sizes rather than polyene chains. The relative intensity of a 1400 cm−1 band against a 1530 cm−1 band is found to decrease with an increase of sp3 content in a-C films. It is shown that the relative intensity can be used as a parameter for sp3 content.

Raman spectra of diamondlike amorphous carbon films

Raman spectra of the diamondlike amorphous carbon (a-C) films prepared by the plasma CVD method have been measured as a function of the excitation wavelength. A Raman band centered at about 1530 cm-1 and a broad shoulder band at around 1400cm-1 are observed in the spectra excited by the 5145 A line. The spectral profile is confirmed to depend on the excitation wavelength. A comparison of the spectral profiles between the tetrahedrally (sp3) and trigonally (sp2) bonded materials suggests that the Raman bands in the a-C films originate from carbon clusters with the sp2 configuration.

EELS analysis of vacuum arc-deposited diamond-like films

Abstract Electron energy-loss spectroscopy measurements have been made on amorphous diamond-like carbon films produced by condensing the plasma stream from a filtered vacuum arc. The results are compared with spectra from diamond, graphitized carbon and amorphous carbon. Although the energy-loss spectra reveal the presence of a small quantity of sp2-bonded material in the diamond-like film, the high plasmon frequency and shape of the K-edge show that the material is essentially an amorphous form of diamond. The fraction of sp2- bonded carbon was quantified and found to be of the order of 15%. It was not possible to determine if the sp2 carbon was on the surface or throughout the bulk.

Resonant Raman scattering of diamondlike amorphous carbon films

Raman spectra of diamondlike amorphous carbon (a-C) films prepared by the sputtering method have been measured as a function of the excitation wavelength. The Raman spectral profiles vary sensitively, reflecting the change of absorption spectra associated with π-π* electronic transitions with the hydrogen content of the films. From a comparison between Raman and electronic absorption spectra, it is confirmed that the Raman spectral variation with the excitation wavelength is due to the π-π* resonant Raman scattering from π-bonded (sp2) carbon clusters.

Graphitization of amorphous diamondlike carbon films by ion bombardment

Amorphous hydrogenated carbon films prepared by glow discharge decomposition of CH4 at a power density of 0.4 W cm−2 were bombarded with hydrogen and nitrogen ions of energies between 100 and 600 keV. The structural changes due to various ion doses were analyzed using Raman, photoluminescence, and infrared spectroscopy. Nitrogen bombardment produces essentially a hydrogen depletion in the whole dose interval as shown by the IR spectra; as a consequence there is a drop of several orders of magnitude in the integrated photoluminescence intensity, and carbon–carbon double bonds, characteristic of a graphiticlike structure, appear in the Raman spectra. The same general effects are produced under proton bombardment, but their intensities change more gradually with the ion dose, in agreement with a vacancy production mechanism.

Photoluminescence properties of diamond-like amorphous carbon films

Photoluminescence in diamond-like amorphous carbon films shows broad, featureless, weak temperature dependent and wide range variable spectral properties. The PL peak position is also excitation energy dependent. Compared with optical band gap measurement, large self-traped energy for carriers are deduced.

InP MIS structures with diamondlike amorphous carbon films deposited by ion-beam sputtering and from plasma : Jae E. Oh, Joel D. Lamb, Paul G. Snyder, John A. Woollam and David Liu. Solid-St. Electron.29(9), 933 (1986)

InP MIS structures with diamondlike amorphous carbon films deposited by ion-beam sputtering and from plasma : Jae E. Oh, Joel D. Lamb, Paul G. Snyder, John A. Woollam and David Liu. Solid-St. Electron.29(9), 933 (1986)

InP MIS structures with diamondlike amorphous carbon films deposited by ion-beam sputtering and from plasma

The interfacial electronic properties of diamondlike carbon on both n-InP and p-InP have been studied. These measurements include conductance and capacitance versus frequency or bias voltage for metal-insulator-semiconductor structures. Carbon was deposited by both ion-beam sputtering, and from an rf plasma. We find that interface electronic state densities range from 10 11 to 10 13 states/eV cm 2, depending on sample and preparation procedures. Normalized conductance versus frequency data show an unusual behavior, namely, two loss peaks having greatly differing voltage dependencies. We speculate on the physical origins of the two peaks.

Optical constants and associated functions of metastable diamondlike amorphous carbon films in the energy range 0.5–7.3 eV

The complex refractive index N(ω)=n+ik and the complex dielectric constant ε(ω)=ε1+iε2 are presented for diamondlike amorphous carbon (a-C) films in the photon energy range 0.5–7.3 eV. The effective number of valence electrons neff per carbon atom, the static dielectric constant ε0,eff, and the energy loss function Im[−1/ε(ω)] are deduced via the use of sum rules and are used to interpret the optical data. The a-C films were deposited using an unbalanced magnetron gun to sputter a graphite target (effective sputtering area of 20 cm2) in ultrapure argon gas. The magnetron is characterized by a high deposition flux of condensing atoms (1.5×1014–1.2×1016 cm−2 s−1) and a concomitant high ion flux (6×1014–2.5×1016 cm−2 s−1). A series of films were prepared by sputtering at different power levels in the range 5–500 W. Insulating substrates were used which allowed the films to self-bias negatively with respect to the plasma, so that the films were bombarded during their growth by Ar+ ions of energy 16–13 eV at an Ar+/C arrival rate ratio varying from about 4 to 2. A transition in the optical properties, physical properties (density, conductivity, microhardness), and microstructure is observed with the most rapid changes occurring at low sputtering power. The optical data are discussed in terms of interband transitions appropriate to amorphous semiconductors, and by comparison with the optical constants and the band structure of crystalline graphite and diamond. We find that films possess a metastable bonding configuration of a mixture of sp3 (tetrahedral) and sp2 (trigonal) bonds with the average coordination of the carbon atoms varying from 3.76 to 3.44. This fourfold-to-threefold transition in bonding is attributed to ion-induced structural modification of the amorphous carbon matrix. Weak plasma peaks at about 5 eV and the trends in neff and ε0,eff indicate that the π electron is localized leading to a hopping conductivity and a large optical gap, E0=0.40–0.74 eV.

Amorphous diamond-like carbon films: Effect of deposition rate on optical and mechanical properties

Optical and mechanical measurements were carried out on samples of amorphous hydrogenated carbon prepared by cathodic plasma deposition. The effects of deposition rate and hydrogen content were examined separately. The results were compared with those of amorphous hydrogenated silicon films.

Diamondlike amorphous carbon films prepared by magnetron sputtering of graphite

Amorphous carbon films, a-C, were prepared by employing dc magnetron sputtering of a graphite target in an argon plasma. A series of a-C films were deposited as a function of sputtering power. The power was varied in the range 5–500 W and the target effective sputtering area was about 20 cm2. The physical and optical properties of the a-C films show a dependence on the sputtering power. The films, although unhydrogenated, possess diamondlike properties such as high hardness, HV=1200–2400 kgf mm−2, and are transparent in the infrared (IR) region with optical gap Eopt =0.40–0.74 eV. We observe a systematic variation of film properties with increasing sputtering power which suggest a transition from fourfold (diamondlike) to threefold (graphitic) coordination of the carbon atoms. For films prepared at low sputtering power about 3/4 of the carbon atoms have tetrahedral coordination and 1/4 have trigonal coordination. At the highest sputtering power the a-C films consist of equal mixtures of tetrahedrally and trigonally bonded carbon atoms.

Fourfold to threefold transition in diamondlike amorphous carbon films: A study of optical and electrical properties

Hard, diamond-like amorphous carbon films (a-C) were prepared as a function of sputtering power density by employing dc planar magnetron sputtering of a graphite target in pure argon. Films deposited at 300 K and at increasing sputtering power density in the range 0.25–25 W cm−2 show a transition of optical and electrical properties, with the room temperature electrical conductivity increasing from 5×10−4 to 5 Ω−1 cm−1 and the optical gap decreasing from 0.74 to 0.40 eV. The imaginary part ε2 of the complex dielectric function is determined in the photon energy range 0.5–7 eV and shows a clear dependence on sputtering power density. The ε2 spectra for a a-C films possess features typical of amorphous semiconductors. neff, the effective number of valence electrons per carbon atom taking part in optical transitions, is determined via a sum rule operation on ε2. Comparisons of neff for each a-C film with that for crystalline graphite allow the average coordination of the carbon atom to be determined. We observe a systematic four-fold to three-fold transition, with the ratio of carbon atoms having four-fold sp3 configuration (diamond-like) to carbon atoms with three-fold sp2 configuration (graphite) varying from 3:1 to 1:1.

Hydrogen evolution from a-C:H films

Diamond-like amorphous carbon films with high hardness, high electrical resistivity, good optical transparency into the infra-red, and good adherence to their substrates may be produced by many methods, most of which result in the incorporation of 20–40 at% hydrogen. These a-C:H films are similar to a-Si:H films and the present work contributes additional evidence for this similarity. a-C:H films were deposited on glass or NaCl-coated glass in a capacitively coupled rf discharge in CH 4. The nature of the films depends upon the specific power density (SPD): polymeric for SPD < 10W cm −2 torr −1, diamond-like for SPD between 10 and 20, and graphitic for SPD>25. The amount of chemisorbed hydrogen in the films, as determined by differential scanned calorimetry (DSC), also depends on the SPD (and concomitant negative self-bias of the substrate), decreasing by a factor of six as the SPD increases from 10 to 20 and the negative self-bias increases form 400–650V. Additionally, the exothermic DSC peak sometimes consists of two or more partially resolved overlapping peaks.