Deionization of Dopants in Silicon Nanofilms even with Donor Concentration of Greater than 10¹⁹ cm^-3

Understanding the dopant properties in heavily doped nanoscale semiconductors is essential to design nanoscale devices. We report the deionization or finite ionization energy of dopants in silicon (Si) nanofilms with dopant concentration (N_D) of greater than 10¹⁹ cm^-3, which is in contrast to the zero ionization energy (E_D) in bulk Si at the same N_D. From the comparison of experimentally observed and theoretically calculated E_D, we attribute the deionization to the suppression of metal-insulator transition in highly doped nanoscale semiconductors in addition to the quantum confinement and the dielectric mismatch, which greatly increase E_D in low-doped nanoscale semiconductors. Thus, for nanoscale transistors, N_D should be higher than that estimated from bulk Si dopant properties in order to reduce their resistivity by the metal-insulator transition.