Ultra High Voltage Electron Microscopy Study of {113}-Defect Generation in Si Nanowires
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Ultra High Voltage Electron Microscopy Study of {113}-Defect Generation in Si Nanowires J. Vanhellemont1,2, S. Anada1, T. Nagase1, H. Yasuda1, H. Bender3, R. Rooyackers3, and A. Vandooren3 1 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Japan. 2 On leave from Department of Solid State Sciences, Ghent University, Belgium. 3 IMEC, Leuven, Belgium. ABSTRACT Results are presented of a study of {113}-defect formation in Si nanowires with diameters ranging from 50 to 500 nm. The Si nanowires, used for the processing of tunnel-FET's, are etched into a moderately doped epitaxial Si layer on a heavily doped n-type Si substrate. {113}defects are created in situ by 2 MeV e-irradiation at temperatures between room temperature and 375 ºC in an ultra high voltage electron microscope. The observations are discussed in the frame of intrinsic point defect out-diffusion and interaction with dopant atoms. INTRODUCTION Si nanowire-based tunnel-FET characteristics are intensively studied as a function of nanowire diameter and doping [1]. A significant reduction of ion implanted B diffusion inside small diameter nanowires and a non-uniform diffusion depth in larger diameter nanowires are observed [2]. Using process simulations, this behavior was attributed to a reduced transient enhanced diffusion close to the nanowire sidewall caused by the recombination of excess selfinterstitials. The shallower profile in narrower nanowires was assumed to be related to an enhanced self-interstitial annihilation. High fluxes of high energy electrons are known to create {113}-defects in thin Si foils and it is well-understood now that these defects are self-interstitial clusters [3,4]. {113}-defects are also formed during low temperature ion implantation and during prolonged anneals of Czochralskigrown Si in the temperature range between 500 and 650 ºC. During subsequent thermal anneals at higher temperature, the{113}-defects dissolve and act as a source of self-interstitials. This can lead, for example, in the case of dopant ion implanted Si to (unwanted) transient enhanced diffusion of the dopant. Electrically active dopants like boron and phosphorus, interfaces and local stress fields have an influence on the {113}-defect formation kinetics and stability [4-7]. Doping with not electrically active impurities such as oxygen or germanium also has a clear impact on the {113}-defect formation [8]. In an Ultra High Voltage Electron Microscope (UHVEM), the formation of intrinsic point defect clusters can be studied in situ while varying in a controlled way experimental parameters such as e-beam intensity, irradiation temperature, impurity concentration, capping layers on the sample and even vacuum in the specimen chamber. In a UHVEM, electron dose rates up to 1025 m-2s-1 can be achieved which is at least 5 orders of magnitude higher than what can be obtained in MeV electron accelerators. In situ e-irradiation thus offers a unique possibility to investigate intrinsic point defect interactions at high concentrations with variou
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