Accuracy of scanning capacitance microscopy for the delineation of electrical junctions

Abstract

In this work, we investigate the theoretical and the experimental accuracy limits of the delineation of pn junctions by scanning capacitance microscopy. We compare three different techniques basing both on experimental data and on one-and two-dimensional simulations of scanning capacitance measurements of epitaxial and shallow junctions.

Scanning capacitance spectroscopy on n+-p asymmetrical junctions in multicrystalline Si solar cells

“…Within the depletion region, asymmetrical spectra result from the uneven charge flow from the two sides of the junction, due to the respective charge screening lengths and carrier overlapping. 4,8 At the EJ location, the spectrum is fit well by a sum of the typical n-and p-Gaussian like curves, with equal magnitudes. The influence of the p-signature persists well within the n-region, 125 nm beyond the n-depletion edge.…”

“…These spectra can be fit to a Gaussian like curve centered around V cp ¼ À0.56 V and with half width r p ¼ 0.9 V, as shown by the solid line in the figure. 4,5,7,8 As the tip nears the EJ location and moves into the n region, the spectra change significantly. At the EJ (#48), the dC/dV spectrum contains strong negative and positive contributions.…”

“…The spectra in this transition are complicated; similar spectra have not been reported in the literature. 1,4,5,7,8 The transit first exhibits the two local dC/dV maxima and minima and the W-shaped C-V s curves in #51-#58 (x ¼ 433-355 nm), then the single negative dC/dV valleys and the U-shaped C-V s curves in #58-#64 (x ¼ 355-285 nm). However, the spectra in this location range (x ¼ 355-285 nm) is different from the typical ndoping, showing a larger negative dC/dV and a deeper C-V s valley with a larger n-doping, which is opposite to the typical n-doping trend.…”

“…However, because the value of V s yielding a flat-band condition is, in general, different between the p-and n-regions of the device, and because the capacitance response near the junction is complicated by the low carrier density in the junction depletion region, the apparent EJ location, as determined by dC/dV ¼ 0, depends on the value of V s and has an error of 6W, where W is the junction depletion width. [1][2][3][4] For a narrow junction in a field-effect transistor, W is similar to the SCM probe size, a few tens of nm. However, for low-doped n þ -p junctions, such as those in crystalline Si (c-Si) solar cells, W is typically a few hundred nm, and a more complete understanding of the SCM signal in the vicinity of the junction is desirable for characterizing these devices.…”

“…6 Scanning capacitance spectroscopy (SCS), a measurement of SCM dC/dV as a function of V s at designated locations in the SCM image, provides a richer characterization of the sample response. 4,5,7,8 In unipolar regions, the sign of dC/dV does not depend on V s and is positive for p-type material and negative for n-type. The dC/dV-V s spectra show a single peak at the flat-band voltage.…”