Electron trapping in ferroelectric HfO2

Charge trapping study at 300 and 77 K in ferroelectric (annealed Al- or Si-doped) and nonferroelectric (unannealed and/or undoped) ${\mathrm{HfO}}_{2}$ films grown by atomic layer deposition reveals the presence of ``deep'' and ``shallow'' electron traps with volume concentrations in the ${10}^{19}\text{\ensuremath{-}}{\mathrm{cm}}^{\ensuremath{-}3}$ range. The concentration of deep traps responsible for electron trapping at 300 K is virtually insensitive to the oxide doping by Al or Si but slightly decreases in films crystallized by high-temperature annealing in oxygen-free ambient. This behavior indicates that the trapping sites are intrinsic and probably related to disorder in ${\mathrm{HfO}}_{2}$ rather than to the oxygen deficiency of the film. Electron injection at 77 K allowed us to fill shallow electron traps energetically distributed at \ensuremath{\sim}0.2 eV. These electrons are mobile and populate states with thermal ionization energies in the range \ensuremath{\sim}0.6--0.7 eV below the ${\mathrm{HfO}}_{2}$ conduction band (CB). The trap energy depth and marginal sensitivity of their concentration to crystallization annealing or film doping with Si or Al suggests that these traps are associated with boundaries between crystalline grains and interfaces between crystalline and amorphous regions in ${\mathrm{HfO}}_{2}$ films. This hypothesis is supported by density functional theory calculations of electron trapping at surfaces of monoclinic, tetragonal, and orthorhombic phases of ${\mathrm{HfO}}_{2}$. The calculated trap states are consistent with the observed thermal ionization (0.7--1.0 eV below the ${\mathrm{HfO}}_{2}$ CB) and photoionization energies (in the range of 2.0--3.5 eV below the ${\mathrm{HfO}}_{2}$ CB) and support their intrinsic polaronic nature.