Synthesis and characterization of fluorene-type and hydrogenated amorphous carbon thin films in RF and DC glow discharges

Mansuroğlu, Doğan
Fluorene (C13H10)-type and hydrogenated amorphous carbon (a-C:H) thin films were synthesized under a capacitively coupled single radio frequency (RF) and a direct current (DC) plasma systems. In the first system, the RF plasma reactor, circular parallel plate electrodes were used; the upper electrode was connected to 13.56 MHz RF power while the lower electrode was grounded. To control the positive ion bombardment, a negative bias voltage was applied between the holder of the substrate and the wall of the chamber. For the second system, the DC plasma reactor, consisted of three cylindrical electrodes and two quartz tubes. In both systems, the thin films were deposited on glass discs and silicon wafers. The fluorene-type thin films were produced in the RF plasma reactor under the biphenyl (C12H10)/methane (CH4) and biphenyl/ethylene (C2H4) mixtures, and the methane plasma was used to produce the a-C:H films. In the DC plasma reactor, the vaporization of the powder monomer (fluorene) was used. To improve the electrical and optical properties of the thin films, the generated films were doped by Iodine (I2) and annealed under variable temperatures. The thin films were applied to a crystalline silicon (c-Si) solar cell and an increase around 5 % was observed in the efficiency of the solar cell. The chemical structures and physical properties of the thin films were examined under such plasma parameters as the plasma power, gas flow rate, negative bias voltage, deposition time, and plasma pressure. Diagnostics of the system was done using a double Langmuir probe. To characterize the chemical structural properties of thin films Fourier Transform Infrared (FTIR) and X-ray Photoelectron (XPS) Spectroscopy were used, and the changes in the plasma chemistry during the deposition were measured by Residual Gas Analysis (RGA) Mass Spectrometer and Optical Emission Spectroscopy (OES). According to results, applying negative bias voltage could be considered an effective way in controlling the reactions in plasma depositions, and an increase in the quality of generated films when the plasma power and gas flow rate increased as well as the deposition time was generally observed. Moreover, the morphology of the films was analyzed by X-ray diffraction (XRD) and the nanotopographic properties of the surfaces were examined using an Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM). The surfaces of fluorene-type thin films had remarkable nanostructured characteristics generated by various negative bias voltages, and the rough surfaces made them useful for photovoltaic applications. Furthermore, the optical properties of the films were examined with UV-visible Spectroscopy and the thickness was measured with an ellipsometer and Dektak Profilometer. From the results, it could be suggested that the structures of thin films produced by plasma polymerization technique were different than the ones produced by the conventional techniques. Additionally, the different chains order were observed in the film structure with respect to independent plasma parameters and the conductivity of the films was improved by controlling of the plasma parameters and iodine doping. Finally, for the determination of hydrophobic behavior of the thin films, the contact angle measurement method was used.


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Citation Formats
D. Mansuroğlu, “Synthesis and characterization of fluorene-type and hydrogenated amorphous carbon thin films in RF and DC glow discharges,” Ph.D. - Doctoral Program, Middle East Technical University, 2015.