  A key issue in the development of high-performance semiconductor devices is
the ability to properly measure active dopants at the nanometer scale. 4D
scanning transmission electron microscopy and off-axis electron holography have
opened up the possibility of studying the electrostatic properties of a p-n
junction with nm-scale spatial resolution. The complete description of a p-n
junction must take into account the precise evolution of the concentration of
dopants around the junction, since the sharpness of the dopant transition
directly influences the built-in potential and the maximum electric field.
Here, a contacted silicon p-n junction is studied through in-situ biased
4D-STEM. Measurements of electric field, built-in voltage, depletion region
width and charge density in the space charge region are combined with
analytical equations as well as finite-element simulations in order to evaluate
the quality of the junction interface. The nominally-symmetric, highly doped
($N_A = N_D = 9\space x \space10^{18} cm^{-3}$) junction presents an electric
field and built-in voltage much lower than expected for an abrupt junction.
These experimental results are consistent with electron holography data. All
measured junction parameters are compatible with the presence of an
intermediate region with a graded profile of the dopants at the p-n interface.
This hypothesis is also consistent with the evolution of the electric field
with bias. These results demonstrate that in-situ biased 4D-STEM enables a
better understanding of the electrical properties of semiconductor p-n
junctions with nm-scale resolution.
