Noncontact Characterization for Ultraviolet Light Irradiation Effect on Si-SiO 2 Interface
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NONCONTACT CHARACTERIZATION IRRADIATION EFFECT ON
FOR ULTRAVIOLET LIGHT Si-Si02 INTERFACE
SHIMURA K. KATAYAMA* AND F. North Carolina State University, Dept. of Mat. Sci.& Eng., Raleigh, NC 27695-7916 USA *On leave from Showa Denko Silicon K.K., 1505 Shimokagemori, Chichibu, Saitama 369-18 Japan ABSTRACT The effect of ultraviolet (UV) irradiation on the minoritycarrier surface recombination lifetime (Ts) in silicon wafers with native or thermal oxide was studied with a noncontact laser/microwave photoconductance (LM-PC) technique. The Ts greatly increases in samples with native oxide after the irradiation. The dominant factor for the Ts change can be negative charges created by photo-injected electrons in the surface area. On the other hand, the irradiation decreases Ts in silicon with thermal oxide. The Ts decrease is due to the generation of carrier recombination centers with an energy level around 0.2eV at the Si-SiO2 interface. §1.
INTRODUCTION
Recently, an ultraviolet (UV) light irradiation effect on effective minority-carrier recombination lifetime (Teff), 1 / T eff = 1 / Ts + 1 / Tb where Ts is surface component and t b is bulk component, in silicon wafers was investigated by Katayama et al. using a noncontact laser/microwave photoconductance (LM-PC) technique (1]. It has been shown that the UV irradiation greatly increases Teff from several gs to several hundred ps in both n- and p-type samples with native oxide. The increase in Teff can be attributed to an increase in Es [1]). It has been also reported by Gruenbaum et al. (2,3] that UV irradiation decreases the efficiency of single-crystal silicon point-contact solar cells. The degradation mechanism has been attributed to the generation of interface traps, which decrease Ts, due to UV light pohotoinjected electrons from the silicon conduction- and valence-bands into thermal oxide. The irradiation effect on Ts is contrary between samples with native and thermal oxides. In this study, the effect in both the samples were characterized with the LM-PC technique, and a generated trap level in these samples was further investigated.
§2.
EXPERIMENTAL
PROCEDURE
Samples used in this study were both n- and p-type (100) Czochralski (CZ) silicon wafers, and were cleaned with standard RCA cleaning processes (4]. No specific treatment was applied to the sample with native oxide. To investigate the UV irradiation effect on samples with thermal oxide, the samples were oxidized in a dry oxygen ambient at temperatures from 800 to 1100°C for 15 to 120min so that the oxide thickness was changed from 5 to 200nm. Effective lifetime, T eff, was investigated with the noncontact LM-PC method using the LIFETECH-88® measurement system (SEMITEX Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
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Co., Ltd.) (5] . In the technique, electron-hole pairs are generated by excitation with a laser beam (pulsewidth = 150ns, T wavelength = 830nm), and eff was measured by analyzing the time decay of microwave (9.6GHz) reflecting power. Deep level transient spectr
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