3. Results and discussion
Fig. 1(a) and (b) show the dependence of GPC and corresponding nr of PEALD SnO2thin films on the SnCl4 and O2 plasma pulse time at a fixed plasma power of 180 W. The GPC of SnO2 thin films was saturated at about 0.072 nm/cycle when the SnCl4 and O2plasma pulse time exceeded 1 sec and 5 sec, respectively. Thus, the precursor and the plasma pulse dependency on GPC exhibited the typical ALD self-saturation characteristics. However, plasma power lower than 180 W resulted in a lower GPC of SnO2 thin films at fixed SnCl4 (1 sec) and O2 plasma (5 sec) pulse times, as shown in Fig. 1(c). This result occurred because a longer plasma pulse time than 5 sec was required to complete the reaction with a lower plasma power, due to lower energy availability for the reaction. When the plasma pulse time increased to 10 sec for 100W, the GPC of the film was also increased to 0.072 nm/cycle. Meanwhile, no significant change of the nr (about 1.95), which corresponded to that of bulk SnO2 of 2.006, was observed, regardless of growth conditions. Finally, the thickness of the film increased linearly with increasing deposition cycles as shown in Fig. 1(d), which represented the unique characteristics of precise thickness control in PEALD process. Therefore, the value of the slope (about 0.072 nm/cycle), calculated by the linear regression, could be considered as the GPC of SnO2 in this study [2] and [10], which was consistent with the GPC from Fig. 1(a–c). The extrapolation of the film thickness reveals that the nucleation delay for PEALD-SnO2 is very short, which means that O2 plasma apparently promotes the nucleation of SnO2 thin films by providing reactive oxygen atoms from the gas phase. It was noted that the saturated GPC (0.072 nm/cycle) was about two times higher than that (0.04 nm/cycle) synthesized by the same SnCl4 precursor and an H2O reactant, as reported earlier [9]. It was reported that higher GPCs were exhibited for ALD TiO2 thin films using ozone and PEALD-TiO2 using O2 plasma due to the formation of highly reactive atomic O, which acted as a more efficient adsorption site for TiCl4 than hydroxyl groups [13] and [14]. They suggested that the excess of atomic O created by the O2 plasma at the film surface could, similar to hydroxyl groups, enable the formation of oxychlorides as surface intermediate species [14]. Similarly, the high GPC of PEALD-SnO2, as compared with ALD-SnO2, was considered to be contributed by the highly reactive excess atomic O generated from the decomposition of O2 gas during the plasma process, which could acted as a more efficient adsorption site for SnCl4 than hydroxyl groups.
3. ผลลัพธ์ และสนทนาFig. 1(a) and (b) show the dependence of GPC and corresponding nr of PEALD SnO2thin films on the SnCl4 and O2 plasma pulse time at a fixed plasma power of 180 W. The GPC of SnO2 thin films was saturated at about 0.072 nm/cycle when the SnCl4 and O2plasma pulse time exceeded 1 sec and 5 sec, respectively. Thus, the precursor and the plasma pulse dependency on GPC exhibited the typical ALD self-saturation characteristics. However, plasma power lower than 180 W resulted in a lower GPC of SnO2 thin films at fixed SnCl4 (1 sec) and O2 plasma (5 sec) pulse times, as shown in Fig. 1(c). This result occurred because a longer plasma pulse time than 5 sec was required to complete the reaction with a lower plasma power, due to lower energy availability for the reaction. When the plasma pulse time increased to 10 sec for 100W, the GPC of the film was also increased to 0.072 nm/cycle. Meanwhile, no significant change of the nr (about 1.95), which corresponded to that of bulk SnO2 of 2.006, was observed, regardless of growth conditions. Finally, the thickness of the film increased linearly with increasing deposition cycles as shown in Fig. 1(d), which represented the unique characteristics of precise thickness control in PEALD process. Therefore, the value of the slope (about 0.072 nm/cycle), calculated by the linear regression, could be considered as the GPC of SnO2 in this study [2] and [10], which was consistent with the GPC from Fig. 1(a–c). The extrapolation of the film thickness reveals that the nucleation delay for PEALD-SnO2 is very short, which means that O2 plasma apparently promotes the nucleation of SnO2 thin films by providing reactive oxygen atoms from the gas phase. It was noted that the saturated GPC (0.072 nm/cycle) was about two times higher than that (0.04 nm/cycle) synthesized by the same SnCl4 precursor and an H2O reactant, as reported earlier [9]. It was reported that higher GPCs were exhibited for ALD TiO2 thin films using ozone and PEALD-TiO2 using O2 plasma due to the formation of highly reactive atomic O, which acted as a more efficient adsorption site for TiCl4 than hydroxyl groups [13] and [14]. They suggested that the excess of atomic O created by the O2 plasma at the film surface could, similar to hydroxyl groups, enable the formation of oxychlorides as surface intermediate species [14]. Similarly, the high GPC of PEALD-SnO2, as compared with ALD-SnO2, was considered to be contributed by the highly reactive excess atomic O generated from the decomposition of O2 gas during the plasma process, which could acted as a more efficient adsorption site for SnCl4 than hydroxyl groups.
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