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Conductance Imaging of the Depletion Region of Biased Silicon PN Junction Device Jeong Young Park 1,3, R. J. Phaneuf 2,3, and E. D. Williams1,3 1 Department of Physics, University of Maryland College Park, Maryland 20742 2 Department of Materials Science and Engineering, University of Maryland College Park, Maryland 20742 3 Laboratory for Physical Sciences, College Park, Maryland 20740 ABSTRACT Simultaneous conductance imaging and constant current mode STM imaging have been used to delineate Si pn junction arrays over a range of reverse bias conditions. Conductance has been obtained by adding a modulation signal to voltages applied in the p and n regions of a model device, and by measuring the modulation signal of the tunneling current with a lock-in amplifier. Both constant current and conductance imaging of the electrically different regions (n, p, and depletion zone) show a pronounced dependence on applied pn junction bias. The conductance contrast is mainly due to electrically different behaviors of metal-gapsemiconductor junction which are determined by the tip-induced band bending of the oxidepassivated silicon surface.

INTRODUCTION Semiconductor devices have been widely studied with scanning tunneling microscope (STM) which can probe the surface topographical and electrical structure with high lateral resolution [1-7]. On clean Si surfaces, dopant-dependent contrast is not observed in STM due to surface Fermi-level pinning caused by large density of surface states [8]. However STM images measured on H-terminated or oxide-covered Si devices show variation with dopant type and concentration due to a partial passivation of surface states [9,10]. Recently, Hildner et al.[1] and Yu et al. [2] have used STM and scanning tunneling spectroscopy (STS) to image a depletion zone related feature at a passivated Si pn junction surface. However, the incorporation of both electronic and topographical information in STM constant-current images make it difficult to interpret STM images unambiguously. In this study, we used simultaneous acquisition of STM topographical images and differential conductance images to characterize the electronic features of a lateral Si pn junction. Tunneling spectra were measured across a pn junction to give a qualitative explanation for the variation of conductance.

EXPERIMENTAL DETAILS The device studied in this experiment has been described elsewhere [1,11,12]. It consists of an array of stripes of p-type doping (NA=1018 /cm3), within a lightly n-type doped (ND = 1.6 x 1014 /cm3) Si(100) substrate. The surface is terminated by wet chemical oxide, prepared using the Shiraki procedure [13], and placed into the ultrahigh vacuum chamber using a load-lock system within 30 minutes of etching. An STM (a Park Scientific Autoprobe VP) in a UHV chamber with the base pressure of 8x10-11 torr was used in this study. J2.3.1

In conductance mapping, the sample bias (U) and the tunneling current (I) are given by following equation (1) and (2), U = U o + U 1 cos(ω 0 t ) I (U ) = I (U 0 ) + (

(1)

∂I 1  ∂