Decrease in Minority Carrier Lifetime of Crystalline Silicon Caused by Rapid Heating
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Decrease in Minority Carrier Lifetime of Crystalline Silicon Caused by Rapid Heating Toshiyuki Sameshima, Kochi Betsuin, and Shinya Yoshidomi Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ABSTRACT Change in the light-induced minority carrier effective lifetime Weff of crystalline silicon caused by rapid laser heating is reported. The top surface of n- and p-type silicon substrates with thicknesses of 520 Pm coated with thermally grown SiO2 layers were heated by a 940 nm semiconductor laser for 4 ms. Weff was measured by a method of microwave absorption caused by carriers induced by 620 nm light illumination at 1.5 mW/cm2. Weff for light illumination of the top surfaces was decreased to 1.0x10-5 and 4.8x10-6 s by laser heating at 5.0x104 W/cm2 for n- and ptype 520-Pm-thick silicon substrates, respectively. The decrease in Weff resulted from the generation of defect states associated with the carrier recombination velocity at the top surface region, Stop. Laser heating increased Stop to 6000 and 10000 cm/s for n- and p-type silicon samples, respectively. Heat treatment at 400oC for 4h markedly decreased Stop to 21 and 120 cm/s, respectively, for n- and p-type silicon samples heated at 5.0x104 W/cm2. Laser heating at 4.0x104 W/cm2 for 4 ms was also applied to samples treated with Ar plasma irradiation at 50 W for 60 s, which decreased Weff (top) to 2.0x10-5 s and 3.9x10-6 s for n- and p-type silicon samples, respectively. Laser heating successfully increased Weff (top) to 2.8x10-3 and 4.1x10-4 s for n- and p-type samples, respectively. Laser irradiation at 4x104 W/cm2 played a role of curing recombination defect sites. INTRODUCTION Rapid laser heating is an attractive method for activating silicon implanted with dopant atoms. Many studies have proved the advantages of rapid laser heating [1-6]. Dopant atoms are completely activated by laser heating at a very high temperature because they are easily moved and incorporated into silicon lattice sites during heating. No substrate heating is necessary. It is important to reduce the thermal budget for fabricating semiconductor devices at a low cost. High activation ratio and no marked impurity diffusion are also important for fabricating a shallow pn junction on the order of 10 nm in 30-nm node metal-oxide-semiconductor MOS transistor devices [7, 8]. Laser-induced rapid heating to a very high temperature in the solid phase has the advantage of maintaining implanted-impurity profiles. A continuous wave CW infrared semiconductor laser is attractive for this purpose because it can stably emit light at a high power of ~10 kW with a high conversion efficiency of ~50%. However, movement of impurities to lattice sites also means that silicon atoms forming the lattice structure have a possibility of moving out to interstitial places and forming vacancies. Vacancies can disappear when silicon is gradually cooled down to the initial temperature because interstitial silicon atoms come back to the lattice sites in the lowest energy state. On the other ha
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