Structure and Photoluminescence of Hydrogenated Amorphous Carbon Films Produced by Using Aromatic Hydrocarbon Source
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ABSTRACT The organic carbon source -xylene(C 6 H6-C2H 4), was used to fabricate a-C:H films in plasma enhanced chemical vapor deposition system. The structures and bonding configurations were characterized and compared with that of conventional methane-based samples by using Fourier Transform Infrared spectroscopy and Raman scattering technique. Strong room temperature luminescence in blue-green light range was found and the PL feature was quite different from that of methane-based a-C:H films.
INTRODUCTION Hydrogenated amorphous carbon (a-C:H) films and its-based materials are becoming increasingly important for their applications in various devices. The structures and optical properties are quite different depending on the fabrication techniques and conditions[1,2]. It
has been reported that, by using aromatic hydrocarbon source - xylene (C8 H 10), an intense blue light emission can be observed from hydrogenated amorphous silicon-carbon films at room temperature[3,4]. In this report, a series of a-C:H films were produced in PECVD system by using xylene as a carbon source. The deposited samples were characterized by using Fourier Transform Infrared (FT-IR) spectroscopy, Raman and photoluminescence techniques to investigate film structures and luminescence properties. EXPERIMENT The samples of a-C:H films were deposited in conventional plasma enhanced chemical vapor deposition (PECVD) system. When using xylene as a carbon source, the liquid xylene source was kept at a constant temperature of 100'C and its vapor was introduced into the deposition chamber by using Ar gas with flow rate of 5 sccm. During the deposition, the rf power and the reaction pressure is 30W and 80mtorr, respectively. For comparison, methane source was also used to prepare a-C:H film at rf power of 50W with CH 4 gas flow rate of 10scem. The substrate temperature is 100°C and the total thickness are around 250nm for all the samples. 353
Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
RESULTS It was found that the visible luminescence can be observed from xylene-based a-C:H films and from methane-based samples, respectively. Figure 1 gives the room temperature photoluminescence spectra excited by using Xe lamp with wavelength of 300nm. For sample deposited by using methane source, a broad PL band centered at 500 nm can be found, which has no sharp features. With increasing the excitation light wavelength, the luminescence intensity is gradually decreased and the PL peak energy is slightly red-shifted. These results are consistence with the other report[2]. For sample deposited by using xylene source, the PL spectrum contains three peaks (3.4eV, 3.0eV and 2.6eV), which is located in the blue-green light range. It is worth noting that the luminescent intensity for xylene-based a-C:H film is much stronger than that for methane-based samples. With a change in the wavelength of excitation light, the peak energies are do not change with errors of measurement but their relative intensities are changed. As shown in Fig.2, the P
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