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1026-C04-02

The Effect of Surface States on Secondary Electron (SE) Dopant Contrast from Silicon p-n Junctions Augustus Kuan Way Chee1, Conny Rodenburg2, and Colin John Humphreys1 1 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom 2 Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom ABSTRACT Detailed computer modelling using finite-element analysis was performed for Si p-n junctions to investigate the effects of surface states and doping concentrations on surface bandbending, surface junction potentials and external patch fields. The density of surface states was determined for our Si specimens with a native oxide layer. Our calculations show that for a typical density of surface states for a Si specimen with a native oxide layer, the effects of external patch fields are negligible and the SE doping contrast is due to the built-in voltage across the p-n junction modified by surface band-bending. There is a good agreement between the experimental doping contrast and the calculated junction potential just below the surface, taking into account surface states, for a wide range of doping concentrations.

INTRODUCTION Two-dimensional dopant mapping using secondary electrons (SEs) in a FEG-SEM is a promising technique for studying dopant profiles in semiconducting materials at high spatial resolution. It has been experimentally observed that the SE contrast correlates with the logarithm of the doping concentration in a semiconductor p-n junction [1-3]. The determination of dopant distributions directly from SE contrast imaging is in principle a non-destructive and rapid in-situ characterisation method for semiconductor devices, but it has not yet found widespread application due to the lack of an accurate and robust quantification procedure. Although it is known that the SE contrast produced by a p-n junction is a function of the built-in potential across the junction, modified by surface band-bending and external local fields (patch fields) above the specimen [3-5], a lack of understanding of the respective contributions from these factors has hampered accurate quantification. To analyse the problem, we have performed detailed computer modelling using two-dimensional finite-element calculations to investigate the effects of surface charges and doping concentrations on surface band-bending, surface junction potentials and external patch fields. The results are compared with experimental data from silicon specimens containing p-n junctions for a series of boron (p-type) concentrations.

SE dopant profiling and surface junction potential determination Previous work [3] examined the SE contrast obtained from a series of voltage biasing experiments on a Si p-n junction specimen with a native oxide layer, and determined the applied bias for which there was zero doping contrast across a p-n junction with donor and acceptor concentrations of 5 × 1018 cm-3. This applied bias was about 0.7 V for