The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Preparation of diamond like carbon thin films above room temperature and their properties

Diamond like carbon (DLC) thin films were deposited on p-type silicon (p-Si), quartz and ITO substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at different substrate temperatures (RT ∼ 300 °C). Argon (Ar: 200 sccm) was used as carrier gas while acetylene (C₂H₂: 20 sccm) and nitrogen (N: 5 sccm) were used as plasma source. Analytical methods such as X-ray photoelectron spectroscopy (XPS), FT-IR and UV–visible spectroscopy were employed to investigate the structural and optical properties of the DLC thin films respectively. FT-IR spectra show the structural modification of the DLC thin films with substrate temperatures showing the distinct peak around 3350 cm^− 1 wave number; which may corresponds to the sp² C–H bond. Tauc optical gap and film thickness both decreased with increasing substrate temperature. The peaks of XPS core level C 1 s spectra of the DLC thin films shifted towards lower binding energy with substrate temperature. We also got the small photoconductivity action of the film deposited at 300 °C on ITO substrate.     

Characterization of amorphous carbon films deposited by surface wave plasma CVD

Many dangling bonds in hydrogenated amorphous carbon (a-C:H) films are usually generated by bombardments of high-energy ion precursors in typical chemical vapor deposition (CVD). To generate low dangling bonds, a-C:H films should be deposited from low-energy radical species. Surface wave plasma (SWP) generates low-energy and high-density radicals. We prepare a-C:H films using SWP and investigate the relationship between the plasma characteristics and structures of a-C:H films. The microwave of the TM01 mode was introduced through the dielectric window and SWP generate under the dielectric window. An Ar and C₂H₂ plasma mixture mainly consists of neutral radical species, and the electron temperature is as low as 1 eV. Electron density significantly decreases with increasing distance from the dielectric window. The a-C:H films are prepared from these hydrocarbon and carbon low-energy radicals as main precursors. The sp² bonded network cluster size in a-C:H films increase with electron density in SWP. This structure change is the influence of the termination structure of clusters changing to CH from CH₃ and CH₂.     

Optoelectronic properties of nitrogenated amorphous carbon films synthesized by microwave surface wave plasma chemical vapor deposition system

The n-type nitrogen doped amorphous carbon (a-C:N) thin films have been grown by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) system on silicon, quartz and ITO substrates at different nitrogen flow rates (1 to 4 sccm). The effects of nitrogen doping on chemical, optical, structural and electrical properties were studied through X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, Raman spectroscopy and solar simulator measurements. Argon, acetylene and nitrogen are used as plasma sources. Optical band gap decreased and nitrogen atomic concentration (%) increased with increasing nitrogen flow rate as a dopant. The a-C:N/p-Si based device exhibits photovoltaic behavior under illumination (AM 1.5, 100 mW/cm²), with a maximum open-circuit voltage (V_oc), short-circuit current (J_sc) and fill factor of 4.2 mV, 7.4 μA/cm² and 0.25 respectively.     

Nitrogen incorporated diamond-like carbon films by microwave surface wave plasma CVD

Nitrogen incorporated diamond like carbon films have been deposited by microwave surface wave plasma chemical vapor deposition (MW-SWP-CVD), using methane (CH₄) as the source of carbon and with different nitrogen flow rates (N₂ / CH₄ flow ratios between 0 and 3). The influence of the nitrogen incorporation on the optical, structural properties and surface morphology of the carbon films were investigated using different spectroscopic techniques. The nitrogen has been incorporated into DLC:N films which was confirmed by the X-ray photoelectron spectroscopy (XPS) measurement. Moreover, the nitrogen incorporation was accompanied by a variation in the optical gap, which was attributed to the removal or creation of band tail states.     

Characteristics of nitrogen doped diamond-like carbon thin films grown by microwave surface-wave plasma CVD

Nitrogen doped diamond-like carbon (DLC:N) thin films were deposited on p-type silicon (p-Si) and quartz substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at low temperature (< 100 °C). For films deposition, argon (Ar: 200 sccm), acetylene (C₂H₂:10 sccm) and nitrogen (N: 5 sccm) were used as carrier, source and doping gases respectively. DLC:N thin films were deposited at 1000 W microwave power where as gas composition pressures were ranged from 110 Pa to 50 Pa. Analytical methods such as X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, FTIR and Raman spectroscopy were employed to investigate the chemical, optical and structural properties of the DLC:N films respectively. The lowest optical gap of the film was found to be 1.6 eV at 50 Pa gas composition pressure.     

Effects of iodine doping on optoelectronic properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

We report the effects of iodine (I) doping on the electrical and optical properties of diamond-like carbon (DLC) thin films grown on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100 °C). For film deposition, we used argon gas with methane or camphor dissolved with ethyl alcohol composition as plasma source. The optical gap and photoconductivity measurements of the samples were carried out before and after the iodine doping. The results show that optical gap dropped from 3.4 to 0.9 eV corresponding to nondoping to iodine-doping conditions, respectively. The photovoltaic measurements show that the open-circuit voltage (V_oc) and short-circuit current density (J_sc) of I-doped DLC film deposited on n-type silicon substrate under light illumination (AM1.5, 100 mW/cm²) were approximately 177 mV and 1.15 μA, respectively, and the fill factor was found to be 0.217.     

Effect of substrate bias voltage on the properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

Diamond-like carbon (DLC) thin films were deposited on silicon and ITO substrates with applying different negative bias voltage by microwave surface wave plasma chemical vapor deposition (MW SWP-CVD) system. The influence of negative bias voltage on optical and structural properties of the DLC film were investigated using X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. Optical band gap of the films decreased from 2.4 to 1.7 with increasing negative bias voltage (0 to − 200 V). The absorption peaks of sp³ CH and sp² CH bonding structure were observed in FT-IR spectra, showing that the sp²/sp³ ration increases with increasing negative bias voltage. The analysis of Raman spectra corresponds that the films were DLC in nature.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Cathodoluminescence of fluorine doped amorphous carbon nanoparticles deposited by a filtered cathodic arc plasma system

The fluorine doped amorphous carbon nanoparticles (a-C:F NPs) films with sizes 50-100 nm were deposited on polyethylene terephthalate in an atmosphere of CF₄ by a 90°-bend magnetic filtered cathodic arc plasma system. The surface morphology of a-C:F NPs films was observed by field emission scanning electron microscope and atomic force microscope. The microstructure and chemical bonding nature of the a-C:F NPs films were investigated by Raman, X-ray diffraction and X-ray photoelectron spectroscopy. This work presents cathodoluminescence (CL) spectra of a-C:F NPs films obtained at 1.9-2.4 eV and verifies luminescence from a-C:F NPs films in the visible region. The incorporation of fluorine into the carbon network results in orange emission (∼2.03 eV) due to the transitions between fluorine-related electron levels and σ* states, and the red emission (∼1.97 eV) results from the recombination of carriers in the valence π and conduction π* states. The peak at ∼2.10 eV may result from the defects of the structures in a-C:F NPs films.     

Unipolar resistive switching behavior of amorphous SrMoO4 thin films deposited at room temperature

Amorphous SrMoO₄ (SMO) thin films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature by pulsed laser deposition and the resistive switching (RS) behavior of the Au/SMO/Pt devices was investigated. The Au/SMO/Pt devices exhibit typical unipolar RS behavior with excellent switching parameters as follows: high resistance ratio (~10⁵) between the low resistance state (LRS) and high resistance state (HRS), non-overlapping switching voltages, and good endurance and retention characteristics. Detailed analysis of their current-voltage characteristics reveals that the conduction mechanisms are Ohmic conduction in the LRS and lower voltage region of HRS, and Poole-Frenkel emission in the higher voltage region of the HRS. Temperature dependent resistance measurements, combined with x-ray photoelectron spectroscopy and model analysis indicate that the unipolar RS behavior of the Au/SMO/Pt devices could be understood by a conical conducting filaments (CFs) model in which the conical CFs are composed of oxygen vacancies. The conical CFs extend from the cathode to anode during the forming process and the observed RS behavior occurs in the localized region near the anode. These results suggest that the room-temperature- deposited amorphous SMO thin films could find potential application in nonvolatile RS memory.     

Controlling the surface topology and hence the hydrophobicity of amorphous carbon thin films

Amorphous carbon films with different surface topologies were synthesized on a Si substrate by plasma enhanced chemical vapor deposition at various pressures using acetylene as a carbon precursor. The samples were characterized by scanning electron microscopy, atomic force microscopy and Fourier transformed infrared spectroscopy. The contact angles for water with the as-prepared carbon films were measured and found to vary in the range 40-145^o. The surface energies of the as-prepared carbon films have been calculated using the Owens method in the hydrophilic region for the two liquids namely water and glycerol. It was found that with the decrease of polar component of the surface energy the surface gradually became hydrophobic. The contact angle was also found to be independent of the pH of water from extremely acidic to extremely basic.     

Synthesis of nitrogen incorporated diamond-like carbon thin films using microwave surface-wave plasma CVD

Nitrogenated diamond-like (DLC:N) carbon thin films have been deposited by microwave surface wave plasma chemical vapor deposition on silicon and quartz substrates, using argon gas, camphor dissolved in ethyl alcohol composition and nitrogen as plasma source. The deposited DLC:N films were characterized for their chemical, optical, structural and electrical properties through X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Raman spectroscopy, atomic force microscope and current–voltage characteristics. Optical band gap decreased (2.7 to 2.4 eV) with increasing Ar gas flow rate. The photovoltaic measurements of DLC:N / p-Si structure show that the open-circuit voltage (V_oc) of 168.8 mV and a short-circuit current density (J_sc) of 8.4 μA/cm² under light illumination (AM 1.5 100 mW/cm²). The energy conversion efficiency and fill factor were found to be 3.4 × 10^{− 4}% and 0.238 respectively.     

Boron-doped hydrogenated amorphous carbon films grown by surface-wave mode microwave plasma chemical vapor deposition

We report the effects of boron (B) doping on optical and structural properties of the hydrogenated amorphous carbon thin films grown by surface-wave mode microwave plasma (SW-MWP) chemical vapor deposition (CVD) on n-type silicon and quartz substrates at room temperature. Argon and acetylene were used as a carrier and carbon source gases respectively. Analytical methods such as X-ray photoelectron spectroscopy (XPS), Nanopics 2100/NPX200 surface profiler, JASCO V-570 UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy were employed to investigate the properties of the films. Low atomic concentration of B (0.08 at.%) was found in the doped film. The optical band gap of the undoped film was 2.6 eV and it decreased to 1.9 eV for the B-doped film. Structural property shows the crystalline structure of the film and it has changed after incorporating B as a dopant. The structural modifications of the films leading to being more graphite in nature were confirmed by the Raman and FT-IR characterization.     

Surface morphology and evolution of amorphous carbon thin films

The surface morphology of disordered carbon films grown by nanosecond pulsed laser ablation of graphite is reviewed. It is shown that the presence of a background gas can have a profound effect on the plume of material ejected during ablation. At low pressures smooth films are produced but at higher pressures rough films with an evolution from a nodular morphology to a large area cluster-assembled morphology occurs. The surface morphology changes with increasing background pressure as a result of collisions, which reduce the kinetic energy of the ejected material and allow for cluster formation within the plume. It is shown that the energy of some of the carbon ablated species in vacuum can exceed 100 eV. The nature of the species present in the plume is discussed in terms of electron–ion recombination and impact ionisation/excitation. The cluster-assembled films are shown to be useful as a scaffold for supporting metal nanoparticles to produce substrates for surface enhanced Raman spectroscopy.     

Field emission from tetrahedral amorphous carbon films with various surface morphologies

Field emission properties of tetrahedral amorphous carbon films prepared by filtered cathodic vacuum arc technique have been compared with different surface morphologies. With fewer cycles of conditioning, field emission from relatively rough granular ta-C films on nickel-coated silicon substrates was routinely improved, due to a local field enhancement resulting from both a ‘protrusion-on-protrusion’ geometry and a relatively high sp² content in the film. A 2-MeV ion implantation machine was also employed to intentionally produce local graphitic channels in smooth ta-C films with a high fraction of sp³ content on bare silicon. A relatively low threshold field was obtained from the ta-C film implanted at a dose of 10¹² cm⁻², which still remained an extremely smooth surface. However, for the highly graphitic sample implanted with a higher dose of over 4×10¹³ cm⁻², no electron field emission was observed, even under a very high electric field of 40 V μm⁻¹. Therefore, a suitable sp² content in an sp³ matrix, resulting in graphitic conductive channels in amorphous carbon films to produce a local field enhancement, may be the main factor in obtaining low threshold fields. Furthermore, protrusive structures could further increase the field enhancement factor, due to a ‘protrusion-on-protrusion’ geometry.     

Boron doped amorphous carbon thin films grown by r.f. PECVD under different partial pressure

Boron doped hydrogenated amorphous carbon (a-C) thin films have been deposited by r.f.-plasma CVD with a frequency of 13.56 MHz at room temperature using pure methane as a precursor of carbon source mixed with hydrogen (H₂) as a carrier gas. The films were prepared by varying the r.f. power, different flow rates of CH₄, and partial pressure of mixed gas (CH₄/H₂) using solid boron as a target. The thickness, structural, bonding and optical properties of the as-deposited films were studied by Alpha step surface profiler, Raman, FT-IR, XPS and UV–visible spectroscopy. It was found that changing the deposition pressure in presence of solid boron dopant in the r.f. PECVD process has a profound effect on the properties of the deposited films, as evidenced from their Raman scattering and optical results. The grown p-C: B films were found very smooth and thickness in the range of 240 to 360 nm for 1 h deposition. Films deposited at lower pressure appear brownish color whereas those deposited at higher pressure appear pale yellowish. The as-deposited film is found to be dominated by sp² rather than sp³, which might be due to the formation of small crystallites. The optical band gap is found to be reduced from 2.601.58 eV as the partial pressure of CH₄/H₂ gas is reduced.     

Defect studies and photoelectrical properties of phosphorus doped amorphous carbon films

This paper reports on the successful deposition of n-type phosphorus doped carbon (n-C:P) thin films and fabrication of n-C:P/p-Si cells by pulsed laser deposition (PLD) using graphite target at room temperature. The cell performances have been tested in the dark for the current–voltage (I–V) rectifying curve and I–V working curve under illumination when exposed to AM 1.5 SUN illumination condition (100 mW/cm², 25 °C). The cells fabricated using 7% of phosphorus by weight percentages in the graphite target (Pwt%) show the highest energy conversion efficiency, η=1.14% and fill factor, FF=41%. The quantum efficiency of the cells is observed to improve with Pwt%. The dependence of Pwt% on the optical and physical properties of the deposited films and the photovoltaic characteristic of the n-C:P/p-Si heterojunction cells are discussed.     

Electro- and magneto-transport properties of amorphous carbon films doped with iron

Electro- and magneto-transport properties of amorphous carbon films doped with iron element have been systematically studied. The electro-transport mechanism of the films is dominated by thermal activation at T > 200 K, Mott-type variable range hopping (VRH) at 200 K > T > 60 K and Efros-Shklovskii type (ES-) VRH at T < 60 K. An anomalous giant positive magnetoresistance (MR) 6.40% is found at the ES-VRH range, which is attributed to the spin blockage effect. At high temperatures, an anomalous Hall effect (AHE) is also found with a large AHE coefficiency 49.6 μΩcm/T. Electron energy loss spectroscopy (EELS) reveals that iron atoms chemically bond with carbon matrix. These iron carbides exist as amorphous nanoparticles with a diameter of 6-12 nm, which is regarded as the origin of the MR and AHE. Besides, the films are p-type conductive at high temperature, which might be related with the iron doping. These properties make iron doped amorphous carbon films applicable in carbon-based solar cells, magnetic sensors or some other multifunctional devices.