Ferroelectricity in undoped hafnium oxide

Abstract

We report the observation of ferroelectric characteristics in undoped hafnium oxide thin films in a thickness range of 4–20 nm. The undoped films were fabricated using atomic layer deposition (ALD) and embedded into titanium nitride based metal-insulator-metal (MIM) capacitors for electrical evaluation. Structural as well as electrical evidence for the appearance of a ferroelectric phase in pure hafnium oxide was collected with respect to film thickness and thermal budget applied during titanium nitride electrode formation. Using grazing incidence X-Ray diffraction (GIXRD) analysis, we observed an enhanced suppression of the monoclinic phase fraction in favor of an orthorhombic, potentially, ferroelectric phase with decreasing thickness/grain size and for a titanium nitride electrode formation below crystallization temperature. The electrical presence of ferroelectricity was confirmed using polarization measurements. A remanent polarization Pr of up to 10 μC cm−2 as well as a read/write endurance of 1.6 × 105 cycles was measured for the pure oxide. The experimental results reported here strongly support the intrinsic nature of the ferroelectric phase in hafnium oxide and expand its applicability beyond the doped systems

Polarization switching behavior of Hf–Zr–O ferroelectric ultrathin films studied through coercive field characteristics

“…12,[21][22][23] It is widely recognized that the ferroelectricity in HfO 2 is attainable in a particular range of dopant concentrations, and its spontaneous polarization is induced by the displacement of oxygen ions in the noncenter symmetric orthorhombic phase crystal. 1,4,5) The ferroelectricity of HfO 2 films is maximized in ultrathin films of around 10 nm thickness.…”

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

“…In recent years, hafnium dioxide (HfO 2 ) based insulators have emerged as the primary contenders to replace traditional SiO 2 in a variety of nano-electronic devices ranging from deep-scaled transistors to DRAM 1,2 and non-volatile memory cells. 3,4 Furthermore, thanks to the ferroelectricity property of both doped 5,6 and pure 7 HfO 2 , novel applications including memories 8 and high sub-threshold slope transistors 9 are envisioned. However, the reliability of the dielectric associated with electron trapping appears to be the "show stopper": It has already been shown that the positive bias-temperature instability (PBTI) limits the gate oxide scaling in metal-HfO 2 -Si transistors [10][11][12][13] while in flash cells, electron trapping in the intergate HfO 2 insulator degrades the program/erase window, retention, and endurance.…”

Prediction of new metastable $$\hbox {HfO}_{2}$$ HfO 2 phases: toward understanding ferro- and antiferroelectric films

“…Additional relaxations in atomic positions and cell shape follow in order to relieve strain, to the extent allowed by the symmetry of the ordered oxygen ion/vacancy arrangement, yet those additional relaxations are much smaller than the initial oxygen ion displacements to the sites of the Pbcm lattice. 2 The resulting hierarchy of structures is illustrated in Fig. 2d.…”

“…The interest in metastable hafnia (HfO 2 ) and zirconia (ZrO 2 ) phases has been intensified with the recent reports of ferroelectric and antiferroelectric responses in "doped" (alloyed) and some pure (Zr,Hf)O 2 films [1,2]. This discovery is of great interest to the semiconductor industry and has led to intensive experimental [3][4][5][6][7][8][9][10][11][12][13][14] and theoretical [9,[15][16][17][18][19][20][21][22] research, because these materials are believed to avoid the problems typical for the traditional ferroelectric materials (such as lead zirconate titanate) during integration into microelectronic devices.…”

“…On the other hand, it appears unlikely that very similar displacements lead to multiple equilibrium positions, e.g., there may not be multiple energy minima for an oxygen ion displaced by a different amount in a particular direction. 4 For our systematic search for metastable phases, we therefore make the following assumptions: (i) All the low-energy structures are represented by (relatively small) distortions of the fluorite structure; (ii) all these structures can be obtained by relaxing atomic positions from an appropriate initial position, such that all the initial positions can be enumerated; (iii) specifi- 2 For example, in monoclinic HfO 2 , the "initial" oxygen ion displacements are nearly 1Å, whereas the additional cell internal relaxations are ∼0.3Å for Hf atoms and ∼0.1Å for the remaining oxygen ions. 3 Specifically, the oxygen ions are displaced along the tetragonal axis, traditionally denoted z; we denote it x for consistency with the rest of the structures in Fig.…”