4. Results and discussion
A very well controlled and understood Si-thermal SiO2 material system was chosen to test performance of the modified PCD method in the measurement of carrier lifetime at the semiconductor– oxide interface. For this purpose clean p-type, 150 mm Si wafers were quartered prior to thermal oxidation and then sections originating from the same wafer were oxidized for different times to the oxide thicknesses of 43 nm, 91 nm, and 280 nm. According to the laws of thermal oxidation of silicon the resulting depth of the Si–SiO2 interface was about 20 nm, 42 nm and 129 nm from the wafer surface respectively. Following thermal oxidation, photolithography and etching steps were employed to form contact windows in the SiO2 layer (Fig. 3a). As seen in Fig. 4, the near-surface recombination lifetime increases with oxidation time, i.e. as the Si–SiO2 interface penetrates deeper into silicon, well beyond its initial value of 14.6ls in the case of non-oxidized Si surface (trace A in Fig. 4). The effective carrier lifetime increase, manifesting itself in the increasing decay tail associated with surface recombination [2], reflects decreasing role of surface recombination mostly due to decreasing density of the recombination traps at the Si–SiO2 interface as it is moved deeper intoparts of the wafer unaffectedby surfaceprocessingduring wafer fabrication. A notable change in the shape of PCD curve can be seen when interface depth changes from 20nm (19.4ls, trace B in Fig. 4) to 42 nm (24.0ls, trace C in Fig. 4) indicating penetration of thermal SiO2 beyond the most damaged near-surface regions of Si wafer in the latter case. Thesameexperimentwasperformedusingp-typemulticrystalline (m-c) Si wafers, commonly used in solar cell fabrication, in which either front or back surfaces were polished. The results shown by trace A in Fig. 5 (seff = 4.8ls) indicate heavily damaged by sawing Si surface affecting the minority carrier lifetime
4. ผล และการสนทนาA very well controlled and understood Si-thermal SiO2 material system was chosen to test performance of the modified PCD method in the measurement of carrier lifetime at the semiconductor– oxide interface. For this purpose clean p-type, 150 mm Si wafers were quartered prior to thermal oxidation and then sections originating from the same wafer were oxidized for different times to the oxide thicknesses of 43 nm, 91 nm, and 280 nm. According to the laws of thermal oxidation of silicon the resulting depth of the Si–SiO2 interface was about 20 nm, 42 nm and 129 nm from the wafer surface respectively. Following thermal oxidation, photolithography and etching steps were employed to form contact windows in the SiO2 layer (Fig. 3a). As seen in Fig. 4, the near-surface recombination lifetime increases with oxidation time, i.e. as the Si–SiO2 interface penetrates deeper into silicon, well beyond its initial value of 14.6ls in the case of non-oxidized Si surface (trace A in Fig. 4). The effective carrier lifetime increase, manifesting itself in the increasing decay tail associated with surface recombination [2], reflects decreasing role of surface recombination mostly due to decreasing density of the recombination traps at the Si–SiO2 interface as it is moved deeper intoparts of the wafer unaffectedby surfaceprocessingduring wafer fabrication. A notable change in the shape of PCD curve can be seen when interface depth changes from 20nm (19.4ls, trace B in Fig. 4) to 42 nm (24.0ls, trace C in Fig. 4) indicating penetration of thermal SiO2 beyond the most damaged near-surface regions of Si wafer in the latter case. Thesameexperimentwasperformedusingp-typemulticrystalline (m-c) Si wafers, commonly used in solar cell fabrication, in which either front or back surfaces were polished. The results shown by trace A in Fig. 5 (seff = 4.8ls) indicate heavily damaged by sawing Si surface affecting the minority carrier lifetime
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