Fig. 13: Schematic illustration of polarization-induced p-type doping in graded polar heterostructures.

a Sheets of charge dipoles in every unit cell of the crystal. The net unbalanced polarization charge is shown in (b), which leads to the electric field in (c), and the energyband bending in the valence band in (d) if holes are not ionized. Field ionization of holes results in a steady-state energy-band diagram shown in (e), which highlights the smooth valence-band edge without any potential barriers for hole flow. E_f, is the Fermi level; E_c and E_v are the conduction and valence-band edges, respectively; and E_g is the band gap. Figures reproduced from ref. ¹⁵⁷, © 2010 AAAS