Imaging the Variation in Band Bending Across a Silicon PN-Junction Surface using Spectromicroscopy


R. J. Phaneuf and H.-C. Kan



In recent years a number of microscopies originally developed for surface studies have been applied to the characterization of semiconductor device structures. Surface/interface effects can provide contrast according to spatial variations in doping, carrier charge density and/or electrostatic potential, however they also complicate the analysis of the images.   For example, for a semiconductor the existence of a large density of surface-states or terminating oxide interface-states within the bulk-forbidden energy gap can result in the formation of a space charge layer and a variation in the electron energy levels with depth beneath the semiconductor surface, or “band-bending”.  There also exists a lateral band-bending or “band line up” across the device (within the surface/interface plane)



Fig. 1.  Composite “Image” of lateral band bending across the two pn junctions

separating a heavily doped (1E18/cm^3) p-type stripe from the surrounding lightly

n-doped (1.6E14/cm^3) Si substrate.   Assembled from 16 individual SPEM

intensity maps  (including those of Fig. 4) whose energies bound the unreacted

Si 2p 3/2 core level (ridge of intensity in this image).



associated with lateral variations in space charge.  The importance of these effects for device performance makes it desirable to record spectroscopically-resolved images, so that the variation in the energy levels across a device can be “imaged” directly.  We have shown that near-threshold photoelectron emission microscopy (PEEM) images of Si pn junctions show contrast according to doping which we interpreted as due to a variation of the near-surface band-bending with doping [1,2].   The lack of an electron energy analyzer, however  prevented a direct quantitative measurement of this effect. More recently we used scanning photoelectron microscopy (SPEM) developed at the




Fig. 2.  Schematic of beamline and scanning xray photoemission microscope at

 Elettra used in these experiments.


 Elettra synchrotron light source in Trieste to characterize the same lateral Si pn-junction arrays imaged previously with PEEM and STM [3].   With the SPEM we measured both energetically-resolved images (see Fig. 4) of the surface of the devices and spatially-resolved spectra (see Fig. 3). Our results show that the energy




Fig. 3. Si 2p core level spectra acquired from within n-type region (red) and

 p-type (blue) region of pn-array, measured through a 11 angstrom thick wet

 chemical oxide..





Fig. 4.  Si 2p Intensity maps showing energy dependent contrast across one

p-stripe (center of each map) of array and surrounding n-type Si substrate

acquired at an xray energy of 496.4 eV and electron kinetic energies of

396.82 eV, 396.98 eV, and 397.22 eV (left-to right). 

bands bend across the space charge region by an amount which varies with oxide preparation. We also are able to identify features in the images and spectra, which are clearly due to anomalous spatial variations in show that the energy bands bend across the space charge region by an amount which varies with oxide preparation.  We are also able to identify features in the images and spectra which are clearly due to anomalous variations in the doping profile across the pn-junction. To our knowledge, this is the first time a pn-junction has been imaged with sufficient energy and spatial resolution to allow the transition-region band bending to be resolved and studied in detail.

[1] M. Giesen, R. J. Phaneuf, E. D. Williams, T. L. Einstein and H. Ibach, Appl. Phys. A 64, 423 (1997).


[2] V. Ballarotto, K. Siegrist, R. J. Phaneuf and E. D. Williams, Surf. Sci. 461 L570 (2000).


[3] R. J. Phaneuf, H. C. Kan, M. Marsi, L. Gregoratti, S. Gunther and L. Gregoratti, J. Appl. Phys. 88, 863 (2000).


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