Electrical Active Defects in the Band-Gap Induced by Ge-Preamorphization of Si-Substrates
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CEMES-CNRS, BP 4347 31055 Toulouse Cedex, FRANCE LEMI, Universit6 de Rouen, FRANCE Complexe scientifique de Rangueil, 31400 Cedex, FRANCE
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ABSTRACT Ultra-shallow p÷-n junctions have been obtained from Ge*-preamorphized and crystalline silicon substrates. B+ and BF 2+ dopants have been used. Boron was implanted at low energy 3 keV / 10"5 cm 2 while an equivalent energy of 15 keV / 101" cm 2 was chosen for BF 2+. Rapid Thermal Annealing (RTA) for 15 s at 950 'C was then used for dopant electrical activation and implantation damage removal. Electrically active defects in these samples were characterized using Deep Level Transient Spectroscopy (DLTS) and isothermal transient capacitance (AC(t, T)). Two electron traps were detected in the upper half of the band gap at Ec - 0.20 eV and Ec - 0.45 eV, respectively. These traps are shown to be induced by the Ge' preamorphization stage. Dopant implantation along with RTA result in the formation of a depth distributed energy continuum for BW and BF 2÷ implants. Each continuum has been ascribed to annealing residual defects. Low energy B' implantation is seen to induce twice as many defects as BF2, implantation. From isothermal transient capacitance (AC(tT)), reliable damage concentration profiles have been obtained, revealing that preamorphization induces not only defects in the regrown silicon layer but also a relatively high concentration of electrically active defects up to 3.5 im into the bulk. INTRODUCTION Among the various approaches to form ultra-shallow p+-n junctions [1-4], preamorphization has been shown to be effective in reducing boron channeling tail. However, its major limitation is the formation of extended defects beneath the amorphous/crystalline (a/c) interface after the solid phase epitaxial regrowth of the amorphous layer. These defects, so called End-Of-Range (EOR) defects, can lead to a large increase of the leakage current [5]. The aim of this work is to investigate the deep levels present in ultra-shallow p'-n junctions obtained from germanium preamorphized Si substrates and subsequent rapid thermal annealing (RTA). Both B' and BF 2 ' dopants were implanted at equivalent energy. The samples were studied by deep level transient spectroscopy (DLTS) to characterize the electrically active defects near the junction as well as deeper in the bulk. In addition to DLTS, an isothermal transient capacitance (AC(t,T)) analysis was undertaken to derive each defect depth distribution. A detailed discussion of each defect origin is also presented.
85 Mat. Res. Soc. Symp. Proc. Vol. 532 © 1998 Materials Research Society
EXPERIMENT Sample description For this study, phosphorus-doped n-type 3-6 i.cm Czochralski grown Si wafers were used. A set of preamorphized substrates was obtained following a Ge' ion implantation performed at a dose of 10'" at.cmr2 , at three different energy levels : 30, 60 and 150 keV. Boron was then implanted : at low energy B' 3 keV / 10"5 cm 2 for a part of the substrates and at an equivalent energy using BF 2' at 15 keV / 10'" cm 2
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