Carbon-Rich a-SiC:H Alloys As Dielectric Materials: Injection Mechanisms in MIM-Type Structures
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CARBON-RICH a-SiC:H ALLOYS AS DIELECTRIC MATERIALS: INJECTION MECHANISMS IN MIM-TYPE STRUCTURES R. VINCENZONI, G. MASINI, G. LEO, G. GUATTARI, F. GALLUZZI Dept. of Electronic Engineering, University of Rome, Via Eudossiana 18, 00184 Rome, Italy.
ABSTRACT Electronic properties of C-rich a-Sil-xCx:H alloys are investigated through the characterization of MIM-type structures, by means of current-voltage measurements carried out at different temperatures. These measurements provide some new insight in both the dielectric behavior of the material and the physics of the meta!/a-Si I-xCx:H junction. INTRODUCTION RF glow-discharge deposited a-Sil-xCx:H thin films show promising characteristics for optoelectronic applications in solar cells, photodetectors, and light-emitting devices [1]. Besides the thorough investigation upon thin films of silicon-rich alloys, an increasing interest has recently been devoted to Si-C alloys containing large carbon fractions x [2]-[5]. In particular, the following properties have been demonstrated for films with x>0.5, deposited from highly hydrogen-diluted methane/silane gas mixtures: 1) high optical gap, related to the absence of graphitic phases; 2) high photo-conductivity, reflecting the relative lack of defects in the material. The low dark-conductivity values also reported for these films suggest their possible use as dielectric materials. The present work is meant to investigate the electronic behavior of TCO/a-Sil-xCx:H/Au structures [6,7], as high electric fields are applied. EXPERIMENTAL Undoped a-SilxCx:H films with thickness of 0.8 to 1.2 g±m were deposited onto Corning glass and TCO-coated glass by RF glow-discharge decomposition of hydrogen, methane and silane at a total pressure of 1 Torr, RF power of 0.3 W/cm 2 , substrate temperature of 300'C. The hydrogen flow was kept constant (90 sccm) while methane and silane were mixed at variable concentrations, with a total constant flux of 10 sccm (see Tab. 1). The carbon content x was detected by Electron Microprobe Analysis, and optical transmission measurements in both UV-VIS and IR range were performed, in order to determine the optical gap and the refractive index of the films. SiMCH 4 4 (%) 4 + CH
80 85 90 95
X(%) Eopt(eV)
52 56 69 73
2.13 2.14 2.27 2.52
n 3.3 2.5 2.2 2.1
Tab.l: Carbon content in both gas and solid phases, and optical properties (optical energy gap and refractive index) of the silicon-carbon alloys used in the MIM-type structures. The electrical characterization of single films was carried out by performing conductivity measurements both in dark conditions and under 100mW/cm 2 white light illumination. The electrical characterization of TCO/a-Sil-xCx:H/Au devices was performed by currentvoltage measurements, carried out in the voltage range ±100 Volt (electric field lower than 106 V/cm), at several temperatures between 20 and 200'C. Capacitance measurements were also Mat. Res. Soc. Symp. Proc. Vol. 297. 01993 Materials Research Society
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performed as the probe signal frequency varied in the range 1
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