Secondary electron doping contrast: Theory based on scanning electron microscope and Kelvin probe force microscopy measurements

Abstract

The secondary electron emission flux in a scanning electron microscope is a powerful tool for delineation of electrically active dopant concentration, built-in potentials, and surface electric fields in semiconductor junctions. In all the secondary electron images of p-n junctions, the p-doped regions appear brighter than n-doped regions. We present a theory for the doping contrast in p-n junctions that is based on the secondary electron emission yield and surface band bending extracted from Kelvin probe force microscopy measurements. We show that the contrast is governed by the secondary electron escape depth, and their escape probability which is related to the secondary electron energy distribution and the effective electron affinity. It is found that the escape depth is the main factor determining the dopant contrast, and the escape probability has a smaller effect. In addition, our theory explains the logarithmic dependence of the measured contrast on the acceptor concentration in silicon reported by many groups.

A quantitative model for doping contrast in the scanning electron microscope using calculated potential distributions and Monte Carlo simulations

“…The simulations are based on a Monte Carlo model, a ray-tracing algorithm for SEs traversing in the electric fields inside and outside the specimen, and a semiconductor-oxide-vacuum finite-element model for calculating these fields. This comprehensive model minimizes the number of unknown parameters in contrast to other functionally different models, e.g., Dapor et al 23 or Volotsenko et al 39 Surface band-bending due to surface states is accounted for, and the effects of patch fields outside the specimen have been calculated. An accurate estimate of the doping contrast can be obtained given the density of surface states.…”

Direct Imaging of p–n Junction in Core–Shell GaN Wires

“…55 In addition, the measured contrast value is influenced by several experimental parameters such as electron dose, ebeam energy, working distance, extractor voltage, and surface treatment as previously reported by several groups. 56 Recently, VC profiles along a p−n junction have been demonstrated not only to map the electrostatic potentials accross a junction with identified doping levels, 54 but even to yield the doping levels in axial GaN nanowire p−n junction where these properties were unknown. 4 In this study, the configuration is the following: an Everhart− Thornley detector with 258 V grid bias, 5 mm typical working distance, ∼1 μm 2 image dimension, and 4−5 keV and 20 pA ebeam energy and current, respectively.…”

Comparison between Energy Straggling Strategy and Continuous Slowing Down Approximation in Monte Carlo Simulation of Secondary Electron Emission of Insulating Materials

“…Secondary electrons are used for imaging in scanning electron microscopes, with applications ranging from secondary electron doping contrast in p-n junctions, [9][10][11][12][13] line-width measurement in critical-dimension scanning electron microscopy, [14][15][16][17][18][19] to the study of biological samples. 20,21) The MC scheme based on the energy straggling strategy takes into account all the single energy losses suffered by each electron in the secondary electron cascade.…”