The electronic structure change with Gd doping of HfO2 on silicon

Abstract

Gd-doped HfO2 films deposited on silicon substrates undergo a crystallographic change from monoclinic to fluorite (cubic) phase with increasing Gd concentrations. The crystallographic phase change is accompanied by a small increase in the valence bandwidth and in the apparent band offset in the surface region. Electrical measurements show pronounced rectification properties for lightly doped Gd:HfO2 films on p-Si and for heavily-doped Gd:HfO2 films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level.

Comparison of n-type Gd2O3and Gd-doped HfO2

“…Of course f shell delocalization in metallic systems can also occur, but is more expected for the 5f levels, and Joyce and Durakiewicz, and their colleagues [5][6][7][8][9] and others [10] also found clear evidence of dispersion in the uranium 5f levels in uranium and uranium compounds. In the Gd 3+ and mixed valence systems, Gd 4f hybridization with nearest neighbor atoms is expected [11,12] and observed. In ErAs(100), strong 4f hybridization is implicated, and a direct confirmation of an occupied 4f band structure might be inferred from the data but could not established [13,14].…”

“…We have compared the occupied density of states for Gd-doped HfO 2 for various levels of Gd doping, observing changes that could be attributed to either increasing Gd concentration or the monoclinic to cubic structural phase change that also accompanies increased doping levels [12]. To better assess the Gd 4f interaction with the oxygen nearest neighbors, it is essential to compare the n-type Gd-doped HfO 2 with n-type Gd 2 O 3 , as both are in the monoclinic phase when grown on Si(100) by pulsed laser deposition.…”

Computational investigation of the phase stability and the electronic properties for Gd-doped HfO2

“…This Gd doping concentration is consistent with the experimental observations. 4,6 The Gd doping concentration we predict at which the phase transition occurs is close to the value of 13% estimated based on the structural energy difference at the Gd concentration of 3.125% (i.e., one doping Gd atom in the 96-atom supercell). 8 The authors 8 also predicted that the cubic phase is more stable than the tetragonal phase when the Gd doping concentration greater than 6.7%.…”

“…4 The RE doping also enlarges the band gap, increases the permittivity, and suppresses the formation of oxygen vacancies. 12 Recent experimental studies 4,6 found when the Gd doping is beyond a certain concentration around 15%-20% the doped HfO 2 is completely transformed to the tetragonal phase. The density functional theory (DFT) calculations were also used to investigate the phase stability, point defects and defect complexes in Gd-doped HfO 2 .…”

“…At ambient condition, it has a monoclinic (m) phase and undergoes a transition to the tetragonal (t) phase at 2000 K and to the cubic (c) phase at 2900 K. The higher symmetry tetragonal phase is desired for device applications because it has a larger band gap 2 and a higher permittivity [3][4][5] compared to the lower symmetry monoclinic phase. There exist considerable experimental and theoretical studies [4][5][6][7][8][9][10][11] on the stabilization of the tetragonal phase HfO 2 by various doping elements. Rare earth (RE) doped HfO 2 is especially interesting because the doping does not produce gap states.…”