Stabilizing the ferroelectric phase in doped hafnium oxide

Abstract

The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a P-r of up to 35 mu C/cm(2). This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO2 films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO2

Physical Mechanisms behind the Field‐Cycling Behavior of HfO2‐Based Ferroelectric Capacitors

“…Besides the more uniform field distribution due to vacancy diffusion, the movement of ions possibly can cause the phase transformation. As reported by Hoffmann et al [50] dopants stabilize the ferroelectric phase, i.e., decrease its formation energy compared to the monoclinic reference phase. This is confirmed also by TEM results that show the transition of the initially tetragonal phase of nonswitching TM-HfO 2 interface to orthorhombic phase (switching) for the woken up device.…”

Ferroelectric HfO2-based materials for next-generation ferroelectric memories

“…41 Experimentally, a larger concentration of oxygen vacancies was observed in the Gd:HfO 2 film grown on the TaN bottom electrode compared with that grown on the TiN bottom electrode. 40 As a result, in the Gd:HfO 2 /TaN film, the fraction of the o-phase became larger and an enhanced P r of up to 35 C/cm 2 was measured. It is noteworthy that despite the positive effect of stabilizing the desired o-phase, the oxygen vacancies may also have some negative effects.…”

“…There are many factors reported hitherto to be responsible for causing the anisotropic stresses and thus stabilizing the ferroelectric o-phase during the film growth, such as doping, 4,5,[23][24][25][26][27][28][29][30][31][32][33][34][35] surface energy effect, [36][37][38] island coalescence, 39 thermal expansion mismatch, 17 capping layer effect, 4,38 and formation of oxygen vacancies. 40 The following of this section will discuss each of these factors in order.…”

“…Although the working mechanisms of the capping layer have not been clearly unraveled, most works have adopted the method of encapsulating HfO 2 -based amorphous thin films with top electrodes prior to annealing. Besides the commonly used TiN, various capping layer materials have been studied, such as Pt, 51 RuO2, 52 TaN, 40 and Ir/IrO x . 53,54 In particular, using Ir/ IrO x as electrodes has successfully solved the long-standing fatigue issue of PZT 55 and is therefore of great interest to be applied to HfO 2 -based devices.…”

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

“…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. However, precise identification of the phase(s) in these films is problematic due to experimental limitations such as the broadness of the thin-film diffraction spectra, unknown film texture and strain fields, and possible presence of multiple phases within a single film.…”