Computational investigation of the phase stability and the electronic properties for Gd-doped HfO<sub>2</sub>

Wang, L. G.; Xiong, Yuhua; Xiao, Wei; Cheng, Lin; Du, Jun; Tu, Hailing; Walle, Axel van de

doi:10.1063/1.4878401

Cited by 24 publications

(22 citation statements)

References 29 publications

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“…4 suggests that while en route to the polar phases (P ca2 1 and P mn2 1 ) of hafnia, the P 4 2 /nmc tetragonal phase has to be stabilized. We note in particular that the P 4 2 /nmc phase may be stabilized by some suitable combination of stress and/or internal/external electric field, e.g., by chemical doping or by fabrication of strainengineered thin film structures [9,46]. In Fig.…”

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confidence: 99%

Pathways towards ferroelectricity in hafnia

et al. 2014

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The question of whether one can systematically identify (previously unknown) ferroelectric phases of a given material is addressed, taking hafnia (HfO2) as an example. Low free energy phases at various pressures and temperatures are identified using a first-principles based structure search algorithm. Ferroelectric phases are then recognized by exploiting group theoretical principles for the symmetry-allowed displacive transitions between non-polar and polar phases. Two orthorhombic polar phases occurring in space groups P ca21 and P mn21 are singled out as the most viable ferroelectric phases of hafnia, as they display low free energies (relative to known non-polar phases), and substantial switchable spontaneous electric polarization. These results provide an explanation for the recently observed surprising ferroelectric behavior of hafnia, and reveal pathways for stabilizing ferroelectric phases of hafnia as well as other compounds.Commonly known structural phases of hafnia (HfO 2 ) are centrosymmetric, and thus, non-polar. Hence, recent observations of ferroelectric behavior of hafnia thin films (when doped with Si, Zr, Y, Al or Gd) [1][2][3][4][5][6][7] are rather surprising as ferroelectricity requires the presence of switchable spontaneous electrical polarization. The emergence of non-polar hafnia-as a linear high dielectric constant (or high-κ) successor to SiO 2 -for use in modern electronic devices (e.g., field-effect transistors) is now well-established [8,9]. If the origins of its unexpected ferroelectricity can be understood and appropriately harnessed, hafnia-based materials may find applications in nonvolatile memories and ferroelectric field effect transistors as well.A broader question that arises within this context, and also the one that will be addressed directly in this contribution, is whether one can systematically identify ferroelectric phases of a given material system. We show that this can indeed be accomplished and ascertained, for the example of hafnia, in two steps. First, a computationbased structure search method, e.g., the minima-hopping method [10][11][12], is used to identify low-energy phases at various pressures and temperatures. Then, ferroelectric phases are singled out by applying the group theoretical symmetry reduction principles, established by Shuvalov for ferroelectricity [13]. These principles allow for the systematic identification of all possible lower symmetry proper ferroelectric phases that can result from highersymmetry non-polar prototype (parent) phases.Using this approach, we find two ferroelectric phases of hafnia, belonging to the P ca2 1 and P mn2 1 orthorhombic space groups, which are close in free energy with the known non-polar equilibrium phases of hafnia over a wide temperature and pressure range. Figure 1(a) displays the computed equilibrium phase diagram of hafnia indicating the regimes at which the known non-polar phases are stable. This includes the low-temperature lowpressure P 2 1 /c monoclinic phase, high-pressure P bca and P nma orthorhombic p...

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Pathways towards ferroelectricity in hafnia

et al. 2014

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“…The other is that trivalent Gd dopants and the oxygen vacancies attract each other to form the dopant-oxygen vacancy complexes, thus removing the oxygen vacancy states from the band gap [14,30,31]. Furthermore, Gd acceptor states could compensate the defect states caused by oxygen vacancies [16].…”

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Interaction of Gd and N incorporation on the band structure and oxygen vacancies of HfO₂ gate dielectric films

Xiong

et al. 2014

Phys. Status Solidi B

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The interaction of Gd and N incorporation on the band gap, band offset, and oxygen vacancies of HfO 2 high-k gate dielectric films has been investigated. The results show that introducing N (9.7% and 17.4%) and Gd into HfO 2 does not dramatically reduce the band gap due to the counteracting effect of Gd doping. Most importantly, Gd and N co-doping increases the conduction band offset of HfO 2 and leads to a much better band offset symmetry compared to undoped or Gd-monodoped HfO 2 . Compared with Gd, N plays a more significant role in improving the band-offset symmetry. The oxygen vacancies are greatly suppressed by Gd and N co-doping under the suitable doping amounts. [8,9]. Thus, incorporating Gd into HfO 2 is expected to increase CBO and improve band offsets symmetry.Particularly, there exist different viewpoints on the effect of trivalent RE doping on oxygen vacancies. It is commonly

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“…This is illustrated schematically in Figure 1a. Since the tetragonal phase of hafnia is more important for technological applications than the monoclinic phase, it is of utmost importance to find a way to stabilize the tetragonal phase at low temperature [1]. Here we observe directly the monoclinic  tetragonal structural phase transformation of an individual hafnia nanorod with atomic resolution, using in situ scanning transmission electron microscopy (STEM).…”

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“…Hafnia is a wide band-gap and high dielectric constant material that is thermally stable on Si substrates, making it an attractive candidate to replace SiO2 as the gate dielectric material in such devices [1]. The tetragonal phase of hafnia, occurring at 1720°C, is desired for device application due to its larger band gap and high permittivity in comparison to the monoclinic phase [1,2]. However, due to the displacive nature of the structural phase transformation, quenching the tetragonal phase to room temperature leads to a reversion back to the monoclinic structure [2].…”

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Direct Observation of Hafnia Structural Phase Transformations

et al. 2017

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The continued miniaturization of electronic device components such as metal oxide field effect transistors has pushed the semiconductor industry to replace silicon dioxide with high-κ dielectrics as the gate material. Hafnia is a wide band-gap and high dielectric constant material that is thermally stable on Si substrates, making it an attractive candidate to replace SiO2 as the gate dielectric material in such devices [1]. The tetragonal phase of hafnia, occurring at 1720°C, is desired for device application due to its larger band gap and high permittivity in comparison to the monoclinic phase [1,2]. However, due to the displacive nature of the structural phase transformation, quenching the tetragonal phase to room temperature leads to a reversion back to the monoclinic structure [2]. It has been shown that the tetragonal phase of ZrO2, an isomorph of HfO2, can be stabilized with respect to the monoclinic phase at room temperature below a critical size of about 15-30 nm. It is estimated that the critical size for stabilization of tetragonal HfO2 is much smaller, at about 4-10 nm, thus making it more difficult to stabilize [3]. This is illustrated schematically in Figure 1a. Since the tetragonal phase of hafnia is more important for technological applications than the monoclinic phase, it is of utmost importance to find a way to stabilize the tetragonal phase at low temperature [1]. Here we observe directly the monoclinic  tetragonal structural phase transformation of an individual hafnia nanorod with atomic resolution, using in situ scanning transmission electron microscopy (STEM).High aspect ratio, monoclinic hafnia nanorods were grown via a nonhydrolytic sol-gel synthesis. The monoclinic nanorods contain multiple twin boundaries on the (100) planes [3]. These twin planes are believed to develop to accommodate strain during the tetragonal to monoclinic phase transformation upon cooling during synthesis. Shown in Figure 1b, when heated in situ in the STEM at 600 C -over 1000 C below the bulk transition temperature -we can observe an atomic-scale structural phase transformation from the monoclinic phase to the tetragonal phase, and upon slow cooling we observe reduction of the nanorods to hafnium metal. The phase transformation nucleates at a twin boundary and propagates one lattice plane at a time via a transformation dislocation. The high-energy twin planes as well as surface energy effects arising from size-confinement combine to depress the transition temperature by more than 1000 C and prevent the tetragonal to monoclinic transformation. Using these in situ techniques, we are able to study the transformation mechanism in real time, providing crucial information necessary to stabilizing tetragonal HfO2 nanocrystals near room temperature. [4] 2092

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

Cited by 24 publications

References 29 publications

Pathways towards ferroelectricity in hafnia

Pathways towards ferroelectricity in hafnia

Interaction of Gd and N incorporation on the band structure and oxygen vacancies of HfO₂ gate dielectric films

Direct Observation of Hafnia Structural Phase Transformations

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

Cited by 24 publications

References 29 publications

Pathways towards ferroelectricity in hafnia

Pathways towards ferroelectricity in hafnia

Interaction of Gd and N incorporation on the band structure and oxygen vacancies of HfO2 gate dielectric films

Direct Observation of Hafnia Structural Phase Transformations

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

Interaction of Gd and N incorporation on the band structure and oxygen vacancies of HfO₂ gate dielectric films