Electron trapping in ferroelectric ${\mathrm{HfO}}_2$

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 ${10}^{19}\text{−}{\mathrm{cm}}^{-3}$ 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 ∼0.2 eV. These electrons are mobile and populate states with thermal ionization energies in the range ∼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.

Figure 1

Schematics of electron transitions during electron tunneling injection from Si into ${\mathrm{SiO}}_2$/${\mathrm{HfO}}_2$ dielectric stack when applying positive charging pulse $V_{\mathrm{charging}}$ to the top gold electrode of the MOS capacitor.

Figure 2

Schematics of electron transition during thermal detrapping in ${\mathrm{SiO}}_2$/${\mathrm{HfO}}_2$ insulating stack when applying positive (a) and negative (b) voltages $V_{\mathrm{hold}}$ to the top gold electrode of the MOS capacitor during temperature ramp up from 77 to 300 K.

Figure 3

$V_{\mathrm{fb}}$ shift vs $V_{\mathrm{charging}}$ as measured at room temperature and at 77 K on (doped) ${\mathrm{HfO}}_2$ samples with and without annealing. The $V_{\mathrm{fb}}$ shift is calculated relative the $V_{\mathrm{fb}}$ value measured before charging at 300 or 77 K respectively. For charging at room temperature, the $V_{\mathrm{fb}}$ shift remaining after ≥48 h relaxation is indicated in the legend and shown by black arrows on the graph. Negative remaining $V_{\mathrm{fb}}$ shift indicates presence of trapped electrons in as-fabricated films.

Figure 4

$V_{\mathrm{fb}}$ shift variation due to optical excitation [panels (a) and (b)] and spectral charge density diagrams [panels (c) and (d)] as measured on as-deposited ${\mathrm{HfO}}_2$ and Al:${\mathrm{HfO}}_2$ samples in the uncharged state (black curves) and after charging pulse of indicated amplitude and ≥48 h relaxation (red curves). Flat-band voltage shift is measured with respect to the initial $V_{\mathrm{fb}}$ value measured at room temperature before charging. Vertical dashed lines mark the energy onset of electron emission from Si valence band over a 7.5-nm-thick ${\mathrm{SiO}}_2$ [17]. Negative SCD values correspond to net electron trapping in the ${\mathrm{SiO}}_2$/${\mathrm{HfO}}_2$ stack.

Figure 5

$V_{\mathrm{fb}}$ shift variation due to optical excitation [panels (a) and (b)] and spectral charge density diagrams [panels (c) and (d)] as measured on ${\mathrm{HfO}}_2$ and Al:${\mathrm{HfO}}_2$ samples annealed for 60 s at 850 °C in the uncharged state (black curves) and after charging pulse of indicated amplitude and ≥48 h relaxation (red curves). Flat-band voltage shift is measured with respect to the initial $V_{\mathrm{fb}}$ value measured at room temperature before charging. Vertical dashed lines mark the energy onset of electron emission from Si valence band over a 7.5-nm-thick ${\mathrm{SiO}}_2$ [17]. Negative SCD values correspond to net electron trapping in the ${\mathrm{SiO}}_2$/${\mathrm{HfO}}_2$ stack.

Figure 6

$V_{\mathrm{fb}}$ shift variation due to optical excitation [panels (a) and (b)] and spectral charge density diagrams [panels (c) and (d)] as measured on ${\mathrm{HfO}}_2$ and Si:${\mathrm{HfO}}_2$ samples annealed for 30 s at 1000 °C in the uncharged state (black curves) and after charging pulse of indicated amplitude and ≥48 h relaxation (red curves). Flat-band voltage shift is measured with respect to the initial $V_{\mathrm{fb}}$ value measured at room temperature before charging. Vertical dashed lines mark the energy onset of electron emission from Si valence band over a 7.5-nm-thick ${\mathrm{SiO}}_2$ [17]. Negative SCD values correspond to net electron trapping in the ${\mathrm{SiO}}_2$/${\mathrm{HfO}}_2$ stack.

Figure 7

(a) Schematics of interface traps discharging in annealed samples during cooling down from room temperature to 77 K (the hatched area represents the density of interface traps that emit electrons into the substrate due to the Fermi level shift); (b) 100 kHz CV curves measured on the annealed ${\mathrm{HfO}}_2$ sample at 300 and 77 K before charging, illustrating $V_{\mathrm{fb}}$ shift corresponding to interface traps discharge; (c) $V_{\mathrm{fb}}$ shift (with respect to the initial $V_{\mathrm{fb}}$ value at room temperature) variation with temperature in annealed ${\mathrm{HfO}}_2$ samples after charge injection at 77 K due to thermal detrapping with (■) and without (□) effect of interface traps recharging, and $V_{\mathrm{fb}}$ shift variation in the uncharged sample representing the effect of interface traps recharging (○).

Figure 8

(a) Schematic distribution of the density of gap states N($t$) in (amorphous) hafnia; (b) an expected detrapping rate variation; and (c) the relation between the thermal detrapping rate and the characteristic range of thermal activation energies of traps.

Figure 9

Flat-band voltage variation rate [panels (a) and (b)] and flat-band voltage shift with respect to the initial $V_{\mathrm{fb}}$ value at room temperature [panels (c) and (d)] versus temperature as measured on as-deposited (undoped and Al-doped) ${\mathrm{HfO}}_2$ samples. Measurements were repeated twice under “holding” voltages of −5 V and at the flat-band condition during sample heating. Additional measurements for uncharged samples were conducted under zero bias during sample heating.

Figure 10

Flat-band voltage variation rate [panels (a) and (b)] and flat-band voltage shift with respect to the initial $V_{\mathrm{fb}}$ value at room temperature [panels (c) and (d)] versus temperature as measured on annealed at 850 °C for 60-s (undoped and Al-doped) ${\mathrm{HfO}}_2$ samples. Measurements were repeated twice under holding voltages of −5 V and at the flat-band condition during sample heating. Additional measurements for uncharged samples were conducted under zero bias during sample heating.

Figure 11

Flat-band voltage variation rate [panels (a) and (b)] and flat-band voltage shift with respect to the initial $V_{\mathrm{fb}}$ value at room temperature [panels (c) and (d)] vs temperature as measured on annealed at 1000 °C for 30-s (undoped and Si-doped) ${\mathrm{HfO}}_2$ samples. Measurements were repeated twice under holding voltages of −5 V and at the flat-band condition during sample heating. Additional measurements for uncharged samples were conducted under zero bias during sample heating.

Figure 12

(a) Shows the top layer of the (−1 1 1) surface of $m\text{−}{\mathrm{HfO}}_2$. Hf ions are colored light blue and O ions are colored red. (b) A close (side on) view of the single electron trap in the monoclinic (−1 1 1) surface. The dark blue surface corresponds to an isosurface ($=0.005$) of the spin density.