Photoconductivity and Photoluminescence of Amorphous Carbon Nitride a-CN x Films Prepared by the Layer-by-Layer Method
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discharge of molecule hydrogen H 2 of 99.995 %. This process r Time .... __ Sample # #59 1#60 #61 a by was controlled (a) Sputtering time (s) _600- 36 0 300 Typical microcomputer. (b) First evacuating time (s) 30 4-30 conditions of sputtering were 30 (c) Etching time (s) 60 L 60 13.56 MHz rf-power of 85 W, -100__ (d) Second evacuating time (s) 30 30130 N2 sputtering gas of 0.12 Torr and the substrate temperature (a)-(d) Total time of Icycle (s) 760 1480 420 Ts=200 VC.For the etching of a# of the layer-by-layer process 80 153 CNx, atomic hydrogens are (Results the glow prepared with #61 #59 #60 discharge of H2 of 0.50 Torr at 1.63 1.73 1.57 RPefractive index the same temperature and rfFilm thickness (nm) .1100 1100 41300 power as the sputtering. The 0.68 LO.6- _0.69 LN/C ratio for conditions preparation 1.8-1 2.0 1 20-2 samples #59-61 are shown in Qjauc gap (eV) table 1. LLa-CNx films were designed to become film thickness about 1100 nm by changing the deposition, the etching and the evacuating conditions. Growth and etching rates of a-CNx film are about 30-35 and 60-90 A/min respectively. Photoconductivity was measured by using monochromatic light sources made from W-halogen or xenon lamp with a chopping frequency of 3.5 Hz. Excitation light intensity is monitored by using pyroelectric. element Hamamatsu Photonics P2613-6. The vacuum deposited Al planar gap electrodes on LLa-CNx films were used for photoconductivity measurement with a gap width of 50 uim and an electrode width of 5 mm. Voltage applied to electrodes was dc 20 V and measurement was done at room temperature. Photoluminescence spectra were obtained by using the excitation with an argon-ion laser of 488 nm at 10 K and with a helium-cadmium (He-Cd) laser at room temperature. Table.1
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EXPERIMENTAL RESULTS The properties of LLa-CNx shown in table I do not change so much with the deposition parameters. Typical LLa-CNx films are transparent brown to yellow in color. The optical energy gap E0 obtained from Tauc's plot and nitrogen-carbon ratio N/C for LLa-CNx are about 2.0 eV and 0.7 respectively. Fig. I shows the dependence of photoconductivity on the excitation power at a fixed photon energy. Photocurrent Ip of LLa-CNx films depend on the excitation photon power 1. For example, photocurrent at 4 eV is larger than 3 eV at the same excitation power. Thus photocurrents of LLa-CNx films depend on excitation power and photon energy. At 3 and 4 eV, photocurrent Ip at 3.5 Hz depend on light intensity I as I 0.86 and 1 1.13, respectively. These intensity dependence measured at 3.5 Hz can be explained by the monomolecular process with the effect of localized electronic states. The slight decrease of photocurrent is observed at the excitation of 4 eV which is explained tentatively as a degradation of photocurrent by the increase of surface defect density. Fig.2 shows the dependence of photocurrent in LLa-CNX films on photon energy measured at a fixed photon number F. Photocurrent of LLa-CNx films increase from 2 to 6.2 eV. In detail,' 500
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