Yanhong Liu

SPM investigation of diamond-like carbon and carbon nitride films

Scanning probe microscopy was used to evaluate and compare the surface roughness, mechanical and tribological properties of hydrogenated (a-C:H) and tetrahedral (ta-C) diamond-like carbon (DLC) and amorphous carbon nitride (a-C:N) films. Compared to the a-C:H and a-C:N films, the ta-C films exhibit the lowest surface roughness. The soft surface layers of DLC and a-C:N films were revealed by nanowear tests and their thickness varies over the range of 0.2 to 4.1 nm. The nanoscale friction coefficient measurements from lateral force microscopy shows that these films have obviously different friction coefficients. The lower friction coefficients of ta-C and a-C:N films can be attributed to the existence of soft graphite-like surface structure. We proposed the deposition processes of DLC and a-C:N films, where their surface roughness, structure and mechanical properties were associated with the vapor plasma particle energy distribution.

Deposition of diamond-like carbon films by barrier discharge plasma with 1.4 and 20 kHz power sources

Hydrogenated diamond-like carbon (DLC) films were deposited on Si substrates at 1.4 and 20 kHz a.c. power frequencies by dielectric barrier discharge (DBD) technology. Atomic force microscope analysis showed that the roughness parameter Rq changed from 0.27 to 0.107 nm with decreasing the deposition pressure from 2.48 to 1.65 Torr. The charge-coupled device camera images and measurements of high-voltage and current waveforms of the DBD plasma indicate that the low-pressure DBD consists of spatially uniform glow-like single breakdowns that contribute to the formation of smooth surfaces of the deposited films. Fourier transform infrared spectroscopy analysis and hardness measurement results showed that the properties of the films deposited at 20 kHz power frequency changed from those of graphite-like to diamond-like to polymer-like with increasing the deposition pressure from 1.12 to 2.48 Torr. Increasing the power frequency from 1.4 to 20 kHz may cause an increase of the deposition rate from 4.5 to 26 nm/min. The higher breakdown voltage at 20 kHz power frequency may produce hydrocarbon ions with higher energies in the cathode sheath near the substrates, which would have much effect on hardness of the deposited DLC films.

Filtered pulsed carbon cathodic arc: Plasma and amorphous carbon properties

The carbon plasma ion energies produced by the filtered pulsed cathodic arc discharge method were measured as a function of filter inductance. The energy determination is based on the electro-optical time-of-flight method. The average ion energies of the pulsed ion beams were found to depend upon the rise time and duration of pulsed arc currents, which suggests that a gain of ion kinetic energy mainly arises from the electric plasma field from the ambipolar expansion of both electrons and ions, and an electron drag force because of the high expansion velocity of the electrons. The tetrahedral amorphous carbon (ta-C) films with a sp3 fraction of ∼70% were deposited on silicon substrates at the average ion energies of >6 eV in the highly ionized plasmas. The ta-C films were found to be covered with a few graphitelike atomic layers. The surface properties of ultrathin carbon films, such as nanoscale friction coefficients, surface layer thickness, and silicon contents were strongly dependent on the ion energi...

Surface and structural properties of ultrathin diamond-like carbon coatings

Abstract Nanoscale wear resistance, friction, and electrical conduction tests using atomic force microscope (AFM) have been conducted on ultrathin diamond-like carbon (DLC) coatings, including tetrahedral amorphous carbon (ta-C) deposited using pulsed cathodic arc (PCA) and filtered-PCA, and hydrogenated amorphous carbon (a-C:H) deposited using electron cyclotron resonance—chemical vapor deposition (ECR-CVD). The low-resistant layers at the surfaces of these thin DLC coatings were revealed by AFM-based nanowear tests. Their thickness is mainly determined by the deposition methods and does not show an obvious variation with the coating thickness decreasing from tens of nm to a few nm. The ∼3 nm ta-C coatings from PCA and filtered-PCA deposition were found to have the stable bulk structure beneath the thin (0.3–0.95 nm) surface layers. The ∼3 nm a-C:H coating from ECR-CVD had the extremely low load-carrying capacity and exhibited the evidence of coating delamination, which can be related to the thicker (1.5±0.1 nm) soft surface layers of a-C:H coatings. The results from conducting-AFM measurements indicate that a-C:H coatings have H and sp3 C enrichment surface layers while the soft surface layers of ta-C coatings have graphite-like structure. The nanoscale friction coefficients of these thin ta-C and a-C:H coatings were compared by AFM-based lateral force microscope. The lower friction coefficient of ta-C coatings can be attributed to the existence of graphite-like surface structure.

Percolation network in resistive switching devices with the structure of silver/amorphous silicon/p-type silicon

Conducting pathway of percolation network was identified in resistive switching devices (RSDs) with the structure of silver/amorphous silicon/p-type silicon (Ag/a-Si/p-Si) based on its gradual RESET-process and the stochastic complex impedance spectroscopy characteristics (CIS). The formation of the percolation network is attributed to amounts of nanocrystalline Si particles as well as defect sites embedded in a-Si layer, in which the defect sites supply positions for Ag ions to nucleate and grow. The similar percolation network has been only observed in Ag-Ge-Se based RSD before. This report provides a better understanding for electric properties of RSD based on the percolation network.

Growth processes and surface properties of diamondlike carbon films

In this study, we compare the deposition processes and surface properties of tetrahedral amorphous carbon (ta-C) films from filtered pulsed cathodic arc discharge (PCAD) and hydrogenated amorphous carbon (a-C:H) films from electron cyclotron resonance (ECR)-plasma source ion implantation. The ion energy distributions (IEDs) of filtered-PCAD at various filter inductances and Ar gas pressures were measured using an ion energy analyzer. The IEDs of the carbon species in the absence of background gas and at low gas pressures are well fitted by shifted Maxwellian distributions. Film hardness and surface properties show a clear dependence on the IEDs. ta-C films with surface roughness at an atomic level and thin (0.3–0.9 nm) graphitelike layers at the film surfaces were deposited at various filter inductances in the highly ionized plasmas with the full width at half maximum ion energy distributions of 9–16 eV. The a-C:H films deposited at higher H∕C ratios of reactive gases were covered with hydrogen and sp3 bo...

Percolation mechanism through trapping/de-trapping process at defect states for resistive switching devices with structure of Ag/SixC1−x/p-Si

Pure SixC1−x (x > 0.5) and B-containing SixC1−x (x > 0.5) based resistive switching devices (RSD) with the structure of Ag/SixC1−x/p-Si were fabricated and their switching characteristics and mechanism were investigated systematically. Percolation mechanism through trapping/ de-trapping at defect states was suggested for the switching process. Through the introduction of B atoms into SixC1−x, the density of defect states was reduced, then, the SET and RESET voltages were also decreased. Based on the percolation theory, the dependence of SET/RESET voltage on the density of defect states was analyzed. These results supply a deep understanding for the SiC-based RSD, which have a potential application in extreme ambient conditions.