Ultraviolet (UV) Irradiation Effect on Minority-Carrier Recombination Lifetime in Silicon Wafers with Oxide and Nitride
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hereafter denote the sample with CVD oxide as OXC, thermal oxide as OXT, native oxide as OXN, and CVD nitride as NIc. Samples were irradiated at room temperature with low pressure Hg-Ar discharge lamps which emit light with the primary band at a wavelength of 2537A (4.9 eV). The exposure time was set to be 20 minutes for each experiment. The minority carrier lifetime is measured with the LIFETECH-88 system[5]. In this system, electron and hole pairs are generated by laser beam (X=9100A or 8300A), and the minority-carrier density is monitored with a microwave reflection power through a waveguide, from which a decay curve is finally obtained and recorded in a computer. EXPERIMENTAL RESULTS AND DISCUSSION Thermal oxide samples, OXT The most striking feature of the UV effect is the dramatic decrease of the effective lifetime after irradiation. Shown in Fig. 1 are typical decay curves recorded before and after 20min UV irradiation for a p-type wafer with 211A thermal oxide. The effective lifetime is reduced almost one decade, from 150pts down to 20gs. Such an effect is observed in all samples regardless of the conductivity type and resistivity of the substrate, the oxidation temperature, and the oxide thickness. The lifetime is decreased almost linearly with the duration of UV irradiation and is stabilized after about 15min, depending on the oxide thickness. The decreased lifetime is fairly stable and a thermal annealing up to 200'C brings about little change.
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Time (js) Fig. 1 Decay curves recorded at room temperature for a silicon wafer with 211i thermal oxide after 20-min UV irradiation The decrease in lifetime can be exclusively ascribed to the interface traps creation by the UV irradiation, which is really unusual because it seems hard to explain according to either of the two traditional models, i.e., stress field induced electron heating [6] and ionization radiation generated hole-trapping [71. Hole-trapping is one of the major effects of an ionization irradiation, which can generate electron-hole pairs in the oxide. A sub-bandgap irradiation such as the UV photons used in this work is energetically not able to generate any electron-hole pairs directly in the oxide. On the other hand, electron heating accounts for the interface-trap creation during high field stress and internal photoemission [8] as well. In this work, however, no electric field is applied on the samples at all. We propose [91 that photoemission of electrons and holes from silicon into silicon oxide takes place simultaneously during the 4.9 eV UV irradiation, resulting in the interface traps creation. Based upon early works on internal photoemission experiment, it is thermodynamically possible that both electrons from Ev(Si) and holes from Ec(Si) can be
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elevated by 4.9eV UV photon to Ec(Si02) and Ev(Si02) at the same time, respectively. Without electric field applied, the excited electrons and holes may diffuse across the interface into the oxide simultaneously due to the
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