TiN

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43370−43376

Effect of Polarization Reversal in Ferroelectric TiN/Hf0.5Zr0.5O2/TiN Devices on Electronic Conditions at Interfaces Studied in Operando by Hard X‑ray Photoemission Spectroscopy Yury Matveyev,*,† Dmitry Negrov,† Anna Chernikova,† Yury Lebedinskii,† Roman Kirtaev,† Sergei Zarubin,† Elena Suvorova,†,§ Andrei Gloskovskii,‡ and Andrei Zenkevich† †

Moscow Institute of Physics and Technology, 9, Institutskiy Lane, Dolgoprudny, Moscow region, 141701, Russia Deutsches Elektronen-Synchrotron, 85 Notkestraße, Hamburg D-22607, Germany § A.V. Shubnikov Institute of Crystallography, Leninsky pr. 59, Moscow 119333, Russia ‡

S Supporting Information *

ABSTRACT: Because of their compatibility with modern Si-based technology, HfO2-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf0.5Zr0.5O2 core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample crosssection in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiOx/TiON layer at both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf0.5Zr0.5O2/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO2-based ferroelectric memory devices under development. KEYWORDS: hafnium oxides, ferroelectric switching, ferroelectric tunnel junctions, ferroelectric field-effect transistors, hard X-ray photoelectron spectroscopy, in operando, electronic band alignment

1. INTRODUCTION

screening lengths, which allows direct electron tunneling. Polarization reversal in the FE barrier alters tunneling electroresistance because of the changes in the asymmetric potential profile including contributions from the varying density of states at the FE/electrode interfaces. Both memory concepts allow a nondestructive readout, and the combination of functional characteristics potentially makes such FE devices promising candidates for “universal” memory combining high speed, nonvolatility, and scalability down to the nanometer range. Indeed, the implementation of both FeFET

The concepts for nonvolatile memory devices utilizing ferroelectric (FE) thin films with electrically switchable spontaneous polarization have an enormous potential owing to high writing speed, very low power consumption, and theoretically unlimited endurance. In particular, in a ferroelectric field-effect transistor (FeFET) memory device, the polarization direction of a FE gate oxide layer affects the threshold voltage controlling the current in the channel of the transistor.1,2 An alternative FE memory concept is based on the polarization-controlled electroresistance effect in the devices termed ferroelectric tunnel junctions (FTJs).3,4 An FTJ incorporates an ultrathin FE barrier sandwiched between two conducting electrodes with different work functions and/or © 2017 American Chemical Society

Received: September 21, 2017 Accepted: November 21, 2017 Published: November 21, 2017 43370

DOI: 10.1021/acsami.7b14369 ACS Appl. Mater. Interfaces 2017, 9, 43370−43376

Research Article

ACS Applied Materials & Interfaces

comprising BaTiO312 or HZO19 layers for a particular direction of the remnant polarization intrinsically adopted by the FE layer, while laboratory XPS was employed to monitor the effect of polarization reversal in bulk BaTiO3 following in situ metal deposition.20 Recently, HAXPES measurements were combined with in situ electrical biasing to perform comprehensive in operando study of polarization- and electric-field-induced effects on the electronic structure at the interface between the metal and bulk FE PMN-PT.21 In this work, we report on the effect of polarization reversal in functional FE TiN/FE-HZO/TiN memory devices on the electronic conditions at the interfaces studied in operando. By combining in situ pulsed switching and measurements of FE remnant polarization in the HZO layer by HAXPES, we monitor the effect of the polarization reversal on the energy position of Ti and Hf core-level lines, from which we derive the quantitative changes in the electronic band line-up at both top and bottom HZO/TiN interfaces. We further employ laboratory XPS and energy dispersive X-ray spectroscopy (EDX) to elucidate the difference in the chemical composition between the top and bottom HZO/TiN interfaces and to correlate it with the observed asymmetry in the electronic conditions following the polarization reversal. The obtained results provide the basis for further optimization of prospective HfO2-based FTJs and FeFET memory devices.

(e.g., refs 5−7) and FTJ memory devices has been reported previously (e.g., refs 8−12). However, their commercialization is hindered because of the problems, which mostly arise from the use of classical perovskite FEs, since this class of materials has generally poor compatibility with the Si-based complementary metal-oxide-semiconductor (CMOS) technology. In addition, for FeFETs, one has to use a thick (∼100 nm) FE layer to overcome leakage issues and preserve sufficient remnant polarization. Alternatively, the experimental realization of FTJs has been demonstrated mostly by employing heteroepitaxial FE perovskite films as a tunneling barrier sandwiched between metallic, conductive oxide, or semiconducting electrodes. A few years ago, ferroelectricity has been discovered in doped HfO2-based thin films.13 The advantage of this class of FE materials over perovskites is their perfect compatibility with the modern CMOS technology. Although the true origin of ferroelectricity is still debated, most theoretical14 and experimental15−17 studies assign their FE behavior to the noncentrosymmetric orthorhombic HfO2 phase (Pbc21 symmetry), which crystallizes upon high-temperature annealing and is metastable at room temperature. The stabilization of the FE phase in polycrystalline HfO2 films depends on various parameters, such as dopant species and their concentration, film thickness, conditions of thermal treatment, and the presence of the electrode on top of the FE-HfO2 layer. Following the discovery of FE properties in HfO2, its applicability as a gate oxide in nanoscale FeFET memory devices was successfully demonstrated.6,7 Furthermore, a recent study has shown that ferroelectricity can persist in ultrathin, down to 2.5 nm in thickness, alloyed polycrystalline Hf0.5Zr0.5O2 (HZO) films grown directly on Si substrates.18 It should be noted that the temperature required to crystallize HZO in the FE phase (T ≈ 400 °C) is significantly lower compared to other, doped HfO2 films, and from the back-endof-line device fabrication viewpoint this feature makes the application of HZO films in nonvolatile memory devices very promising. Indeed, the realization of the first CMOScompatible FTJ based on the FE-HZO layer was recently reported.19 However, the functional properties of both HfO2based FeFET and FTJ prototypic memory devices, which have been obtained so far, still need considerable improvement to compete with existing memory technologies. The performance of both types of memory devices may be enhanced by carefully optimizing the electronic properties at the interfaces with the conducting electrodes (or Si substrates). It is therefore of vital importance to obtain quantitative information on the effect of polarization reversal in FE-HfO2 on the electronic band line-up at metal/FE-HfO2 interfaces. Among several analytical techniques which can be used to experimentally determine the electronic band alignment at metal/dielectric interfaces, X-ray photoelectron spectroscopy (XPS) is ideally suited to obtain precise information on both the electronic structure and the chemical properties, provided that its depth sensitivity is sufficient to probe the buried interfaces. This is accomplished here by use of synchrotronbased hard X-ray photoelectron spectroscopy (HAXPES) using a tunable energy range of 5−12 keV, thus enabling to probe the layer(s) down to ∼20 nm beneath the surface. This provides an opportunity for a nondestructive investigation of the interface properties in the functional metal−insulator−metal stacks. Previously, XPS and/or HAXPES was successfully used to derive the electronic band line-up at metal/FE interfaces

2. RESULTS AND DISCUSSION The bottom TiN electrode and 10 nm-thick HZO layer were grown on an Si substrate (dSiO2 ≈ 2 nm) by atomic layer deposition (ALD) in two separate reactors as described in detail previously17,18 (see also the Supporting Information). A TiN overlayer was subsequently deposited by magnetron sputtering with an optimized thickness d ≈ 12 nm to ensure continuous film coverage, sufficient conductivity, and reasonably high yield of photoelectrons from the buried HZO layer. The TiN/HZO/TiN structures were rapidly annealed at T = 400 °C to crystallize a HZO layer in the noncentrosymmetric orthorhombic phase. The functional FE capacitor devices 300 × 300 μm2 were patterned on the Si chip with contact pads to match the X-ray beam size (∼100 × 300 μm2) and to ensure in situ pulsed switching and polarization measurements from the external source-measure unit (details of the device fabrication are described in the Supporting Information, section S1). The schematic view of the samples used for the in operando HAXPES experiments is shown in Figure 1. The prepared FE capacitor devices on the Si chip were subjected to standard electrical characterization prior to HAXPES measurements. To enable precise P−V measure-

Figure 1. Schematic view of TiN/HZO/TiN samples used for the in operando HAXPES experiment. 43371


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ments is shown in Figure 2b. The clear FE response is still evident, although the remnant polarization is slightly smaller 2P ∼ 27 μC/cm2. The discrepancy between in situ versus ex situ measured values should be attributed to the worse quality of the transmission line in the ultra-high vacuum chamber. In operando HAXPES measurements were performed at the beamline P09 of PETRA III (DESY) choosing the excitation energy E = 6 keV with an overall energy resolution of about 0.2 eV.24,25 The core-level spectra were acquired in open (short)circuit configurations, when the top TiN electrode is kept floating (grounded) after switching voltage pulses. The evolution of both Ti2p and Hf3d5/2 core-level lines taken in an open-circuit mode following in situ polarization switching by U = ±2.75 V voltage pulses as well as for the nonpoled device is shown in Figure 3. (Note that the spectra were repeatedly acquired upon several switching cycles and showed good reproducibility.) In all cases, the Ti2p line has at least three components, indicating the presence of an oxidized TiO2/ TiON layer. It is worth to note that the Ti2p spectrum can be attributed to the top TiN layer only, because the overall thickness of the top TiN and HZO layers is ∼22−25 nm, resulting in negligible contribution of photoelectrons emitted from the bottom TiN electrode. It is reasonable to suggest, and will be confirmed below, that this oxidized layer is formed at the surface of the deposited top TiN layer upon exposure to air. For open-circuit measurements following in situ polarization switching, the Ti 2p binding energy (BE) shifts ΔBE = +0.33 eV (−0.45 eV) upon polarization switched up (down) with respect to the Ti2p BE for the as-grown nonpoled device. These Ti2p core-level line shifts are defined by a FE charge in the HZO layer. At the same time, the Hf3d5/2 line exhibits both an energy shift and a broadening, pointing toward the potential distribution changes across the HZO layer. To reconstruct the potential profile from the obtained data, we utilized the previously described methodology.26 The Hf3d5/2 peak was modeled as a sum of several components, representing virtual sublayers in HZO, each component having its own energy shift proportional to the potential at a particular depth, and the intensity defined according to the Beer−Lambert law. The line

ments, the technique called PUND (positive-up negativedown22) was used (see Supporting Information, section S2). The ex situ electrical cyclic switching facilitates the so-called “wake-up” effect23 during first ∼90 cycles with the remnant polarization increasing up to 2P ≈ 37 μC/cm2 (see Figure 2a).

Figure 2. (a) Effect of cycling on the value of remnant polarization in the TiN/HZO/TiN capacitor device. (b) Polarization vs voltage curve taken in situ from TiN/HZO/TiN prior to HAXPES measurements.

The same PUND technique was used during in situ measurement of the FE polarization in the HAXPES chamber. The typical P−V curve derived from in situ PUND measure-

Figure 3. Evolution of Hf3d5/2 and Ti 2p core-level line positions upon in situ polarization switching in an open-circuit configuration. 43372


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We note that the CBOs obtained here are significantly smaller than those reported previously at the TiN/HZO interface.18,19 However, in the current work, we used truly FEHZO layers, that is, those subjected to the series of switching cycles, which results in the polarization “wake-up”. Such electrical treatment is known to change both structural and electronic properties of the film.27 The effect of polarization reversal on the band line-up change at the bottom HZO/TiN interface ΔCBObottom = 0.7 eV is significantly larger as compared to the top one (ΔCBOtop = 0.3 eV), which can be related to the different chemical properties at the interfaces. To get deeper insight into possible differences in the chemical structure of the top versus bottom interfaces, we performed the energy dispersive X-ray spectroscopy (EDX) elemental analysis of the TiN/HZO/TiN sample cross-section (see Figure 5, for the detailed description see Supporting

shape of all individual components was taken from the parameters of the Hf3d5/2 peak obtained from the as-grown (not annealed) TiN/HZO/TiN sample, where the HZO layer is non-FE and has no built-in potential across it. To regularize the fits of the position and the shape of Hf3d5/2 line, we performed a simultaneous fit of spectra taken in both open- and short-circuit conditions (see Supporting Information, section S3) and assumed a linear potential drop across the FE layer. The calculated error of the core-level line positions was further added to the original spectrum. The plot of the derived binding energy BEHf3d as a function of HZO depth for both polarization directions is presented in Figure 4. The extracted BEHf3d values at the top and bottom interfaces and for two opposite polarization directions are summarized in Table 1.

Figure 4. Derived BE of the Hf3d5/2 peak position across the FE-HZO layer for two opposite polarization directions. Figure 5. EDX elemental profile across the TiN/HZO/TiN stack. Note oxygen peaks at the interfaces revealed by subtracting “bulk” O component (see details of the EDX analysis in Supporting Information, section S5).

We further determined the electronic band line-up at both interfaces by employing the well-known XPS methodology.12,18 The CBO at the TiN/HZO interface can be calculated according to the formula18

Information, section S5) and additional laboratory XPS analysis (the data shown in Supporting Information, section S4). Both techniques have clearly revealed the presence of oxidized TiN layers at both interfaces, but the important difference is that the thickness of the top Ti oxide layer dtop ≈ 1.5 nm is significantly larger compared to the one at the bottom (dbottom ≈ 0.5 nm). The oxidized layer at the interface should affect the effective screening length λeff of polarization charges in the TiN electrode.21,28 However, in contrast to the simple model describing larger CBO changes in terms of weaker screening of polarization charges in case of a poor metal, we observe bigger changes in CBO at the bottom interface, where the thicknesses of TiON/TiO2 layer is smaller. The discrepancy can be explained by the different origin of TiOxNy layers at the interfaces. Whereas the bottom interfacial oxide forms upon the exposure of TiN layer to the atmospheric air (thereby prior to HZO growth by ALD), the top TiOxNy layer is believed to form at a later stage, that is, during the annealing of the as-

CBO = Eg + (BE Hf 3d5/2 − VBM)HZO − (BE Ti 2p − E F)TiN − (BE Hf 3d5/2 − BE Ti 2p)TiN/HZO

where VBM is the valence band maximum for bulk HZO, EF is the Fermi level of the TiN electrode and Egthe band gap of HZO. The energy difference (ETi2p − EF)TiN = 455.94 ± 0.05 eV was measured on the same sample with the grounded top electrode. (EHf3d5/2 − VBM)HZO = 1659.11 ± 0.05 eV was measured on a TiN/HZO structure upon plasmochemical removal of the top TiN electrode. The band gap for the FEHZO layer has been previously measured by reflection electron energy loss spectroscopy and yielded Eg ≈ 5.0 eV.18 The resulting CBO at the top TiN/HZO interface following “up” to “down” polarization switching changes from 0.7 to 1.0 eV, whereas at the bottom HZO/TiN interface the corresponding change is from 1.7 to 1.0 eV (see Table 1).

Table 1. Hf3d5/2 Core-Level Line BE and Derived Conduction Band Offsets (CBO) at the Top and Bottom Interfaces of TiN/ HZO/TiN Devices for Two Opposite Polarization Directions top interface

bottom interface

polarization

BE Hf3d5/2, eV

CBOTiN/HZO, eV

BE Hf3d5/2, eV

CBOTiN/HZO, eV

up down

1663.37 ± 0.05 1663.13 ± 0.08

0.7 ± 0.1 1.0 ± 0.1

1662.37 ± 0.07 1663.08 ± 0.16

1.7 ± 0.1 1.0 ± 0.2

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Figure 6. Reconstructed electronic band diagrams across TiN/HZO/TiN devices for both polarization directions.

significantly lower than the vacuum work function of TiN WFvac ≈ 4.6 eV, indicating a strong Fermi level pinning at both interfaces. Such pinning should be attributed to the large density of interface states in the HZO layer.

grown TiN/HZO/TiN structure. Such annealing presumably induces the redox reaction at the top interface, yielding the oxidation of TiN close to the interface and generation of oxygen vacancies in HZO, which was previously suggested to be the reason for “pinned” or even “killed” polarization in FEs.29 The large concentration of mobile charged oxygen vacancies in this defective HZO sublayer screens the polarization charges and leads to the weak response to the polarization switching, eventually resulting in much smaller charge redistribution at the top interface. By contrast, because TiOxNy at the bottom interface was formed prior to the deposition of HZO layer, no or minimal redox reaction occurs during postdeposition annealing, leaving the HZO layer close to the bottom interface fully functional. The reconstructed electronic band structure of the TiN/FEHZO/TiN capacitor in the short-circuit configuration for both polarization directions is summarized in Figure 6. It may be of interest to compare the observed changes of CBO at the bottom interface (which essentially is the Schottky barrier height) with those derived from simple theoretical models. The general relation for the spatial distribution of the electrostatic potential across a polarized FE layer sandwiched between two electrodes has been derived previously.21 For the modeling, we make similar assumptions as in ref 21: there are no free charges in the interfacial TiO2/TiON layer and the thickness of the FE-HZO layer is much larger compared to that of interfacial oxide. Then, ΔV across the interface upon polarization reversal can be estimated as: ΔV =

3. CONCLUSIONS In this work, we have studied the effect of FE polarization reversal on the electronic band line-up in TiN/Hf0.5Zr0.5O2/ TiN devices in operando by HAXPES. By combining in situ pulsed switching and accurate monitoring of the core-level line shifts within the floating top TiN electrode with respect to Hf0.5Zr0.5O2, we obtain the polarization-dependent CBO at the interfaces ΔCBObottom = 0.7 eV and ΔCBOtop = 0.3 eV. The difference in the electronic properties is ascribed to the varying chemical structure of the interfaces originating from the fabrication procedure. A strong pinning of the TiN Fermi level in contact with FE Hf0.5Zr0.5O2 is derived from these results. The experimental methodology presented here and the first quantitative results contribute to the understanding of metal/FE interfaces in realistic devices and help to facilitate the implementation of FE HfO2-based nonvolatile memory devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14369. Additional information and figures related to the growth and characterization of the FE TiN/Hf0.5Zr0.5O2/TiN devices as well as the full description of the HAXPES data fitting procedure (PDF)

2P ΔCBO = HZO e c TiOx Ny

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where PHZOthe remnant polarization of the HZO layer, cTiOxNythe capacitance of the TiOxNy layer. Assuming a specific capacitance CTiOxNy = εε0/d and putting d = 0.45 nm, εTiOxNy = 50, PHZO = 0.2 C/m2, we get ΔV = 0.4 V. This value is in reasonable agreement with the experimentally derived ΔCBO = 0.7 eV. The deviation is attributed to variations in the dielectric permittivity of the TiO2/TiON layer. Finally, we discuss the effective work function (WFeff) of TiN in contact with the HZO layer, which can be extracted from our data by summing CBO and electron affinity of FE-HZO. Because no report could be found on the electron affinity of the alloyed Hf0.5Zr0.5O2 film, we estimate it between the values for the binary compounds Ea ≈ 2 eV and Ea ≈ 1.6 eV for HfO2 and ZrO2, respectively.30 Then, WFeff of TiN in contact with FEHZO falls in the range WFeff ≈ 2.3 ÷ 3 eV and ∼2.6 ÷ 3.7 eV at the bottom and top interfaces, respectively. These values are

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yury Matveyev: 0000-0001-7661-8462 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed using equipment of the MIPT Shared Facilities Center and with financial support from the Ministry of Education and Science of the Russian Federation (grant no. 43374


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(13) Böscke, T. S.; Müller, J.; Bräuhaus, D.; Schröder, U.; Böttger, U. Ferroelectricity in hafnium oxide thin films. Appl. Phys. Lett. 2011, 99, 102903. (14) Clima, S.; Wouters, D. J.; Adelmann, C.; Schenk, T.; Schroeder, U.; Jurczak, M.; Pourtois, G. Identification of the Ferroelectric Switching Process and Dopant-Dependent Switching Properties in Orthorhombic HfO2: A first principles insight. Appl. Phys. Lett. 2014, 104, 092906. (15) Zhou, D.; Müller, J.; Xu, J.; Knebel, S.; Bräuhaus, D.; Schröder, U. Insights into Electrical Characteristics of Silicon Doped Hafnium Oxide Ferroelectric Thin Films. Appl. Phys. Lett. 2012, 100, 082905. (16) Sang, X.; Grimley, E. D.; Schenk, T.; Schroeder, U.; LeBeau, J. M. On the Structural Origins of Ferroelectricity in HfO2 Thin Films. Appl. Phys. Lett. 2015, 106, 162905. (17) Zarubin, S.; Suvorova, E.; Spiridonov, M.; Negrov, D.; Chernikova, A.; Markeev, A.; Zenkevich, A. Fully ALD-grown TiN/ Hf0.5Zr0.5O2/TiN stacks: Ferroelectric and structural properties. Appl. Phys. Lett. 2016, 109, 192903. (18) Chernikova, A.; Kozodaev, M.; Markeev, A.; Negrov, D.; Spiridonov, M.; Zarubin, S.; Bak, O.; Buragohain, P.; Lu, H.; Suvorova, E.; Gruverman, A.; Zenkevich, A. Ultrathin Hf0.5Zr0.5O2 Ferroelectric Films on Si. ACS Appl. Mater. Interfaces 2016, 8, 7232−7237. (19) Ambriz-Vargas, F.; Kolhatkar, G.; Broyer, M.; Hadj-Youssef, A.; Nouar, R.; Sarkissian, A.; Thomas, R.; Gomez-Yáñez, C.; Gauthier, M. A.; Ruediger, A. A Complementary Metal Oxide Semiconductor Process-Compatible Ferroelectric Tunnel Junction. ACS Appl. Mater. Interfaces 2017, 9, 13262−13268. (20) Chen, F.; Klein, A. Polarization dependence of Schottky barrier heights at interfaces of ferroelectrics determined by photoelectron spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 094105. (21) Kröger, E.; Petraru, A.; Quer, A.; Soni, R.; Kalläne, M.; Pertsev, N. A.; Kohlstedt, H.; Rossnagel, K. In situ hard x-ray photoemission spectroscopy of barrier-height control at metal/PMN-PT interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 235415. (22) Scott, J. F.; Kammerdiner, L.; Parris, M.; Traynor, S.; Ottenbacher, V.; Shawabkeh, A.; Oliver, W. F. Switching Kinetics of Lead Zirconate Titanate Submicron Thin-Film Memories. J. Appl. Phys. 1988, 64, 787−792. (23) Zhou, D.; Xu, J.; Li, Q.; Guan, Y.; Cao, F.; Dong, X.; Müller, J.; Schenk, T.; Schröder, U. Wake-up effects in Si-doped hafnium oxide ferroelectric thin films. Appl. Phys. Lett. 2013, 103, 192904. (24) Strempfer, J.; Francoual, S.; Reuther, D.; Shukla, D. K.; Skaugen, A.; Schulte-Schrepping, H.; Kracht, T.; Franz, H. Resonant scattering and diffraction beamline P09 at PETRA III. J. Synchrotron Radiat. 2013, 20, 541−549. (25) Gloskovskii, A.; Stryganyuk, G.; Fecher, G. H.; Felser, C.; Thiess, S.; Schulz-Ritter, H.; Drube, W.; Berner, G.; Sing, M.; Claessen, R.; Yamamoto, M. Magnetometry of buried layersLinear magnetic dichroism and spin detection in angular resolved hard X-ray photoelectron spectroscopy. J. Electron Spectrosc. Relat. Phenom. 2012, 185, 47−52. (26) Matveyev, Y. A.; Markeev, A. M.; Lebedinskii, Y. Y.; Chouprik, A. A.; Egorov, K. V.; Drube, W.; Zenkevich, A. V. Resistive switching effect in HfxAl1‑xOy with a graded Al depth profile studied by hard Xray photoelectron spectroscopy. Thin Solid Films 2014, 563, 20−23. (27) Martin, D.; Müller, J.; Schenk, T.; Arruda, T. M.; Kumar, A.; Strelcov, E.; Yurchuk, E.; Müller, S.; Pohl, D.; Schröder, U.; Kalinin, S. V.; Mikolajick, T. Ferroelectricity in Si-Doped HfO2 Revealed: A Binary Lead-Free Ferroelectric. Adv. Mater. 2014, 26, 8198−8202. (28) Stengel, M.; Aguado-Puente, P.; Spaldin, N. A.; Junquera, J. Band alignment at metal/ferroelectric interfaces: Insights and artifacts from first principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 235112. (29) Pešić, M.; Fengler, F. P. G.; Larcher, L.; Padovani, A.; Schenk, T.; Grimley, E. D.; Sang, X.; LeBeau, J. M.; Slesazeck, S.; Schroeder, U.; Mikolajick, T. Physical Mechanisms behind the Field-Cycling Behavior of HfO2-Based Ferroelectric Capacitors. Adv. Funct. Mater. 2016, 26, 4601−4612.

RFMEFI59417X0014). Parts of the research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (Helmholtz-Gemeinschaft Deutscher Forschungszentren). Funding for the HAXPES instrument beamline P09 by the Federal Ministry of Education and Research (BMBF) under contracts 05KS7UM1 and 05K10UMA with Universität Mainz and 05KS7WW3, 05K10WW1, and 05K13WW1 with Universität Würzburg is gratefully acknowledged. We acknowledge the help from P.A. Buffat (EPFL) in analyzing EDX data.

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ABBREVIATIONS FeFET, ferroelectric field-effect transistors; FTJ, ferroelectric tunnel junctions; HAXPES, hard X-ray photoemission spectroscopy; FE, ferroelectric; CMOS, complementary metaloxide-semiconductor; HZO, Hf0.5Zr0.5O2; CBO, conduction band offset; VBM, valence band maximum; ALD, atomic layer deposition

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REFERENCES

(1) Okuyama, M. Ferroelectric-Gate Field Effect Transistor Memories. In Topics in Applied Physics; Park, B. E., Ishiwara, H., Okuyama, M., Sakai, S., Yoon, SM., Eds.; Springer, 2016; Vol. 131, Chapter 1, pp 3−20. (2) Mathews, S.; Ramesh, R.; Venkatesan, T.; Benedetto, J. Ferroelectric field effect transistor based on epitaxial perovskite heterostructures. Science 1997, 276, 238−240. (3) Zhuravlev, M. Y.; Sabirianov, R. F.; Jaswal, S. S.; Tsymbal, E. Y. Giant electroresistance in ferroelectric tunnel junctions. Phys. Rev. Lett. 2005, 94, 246802. (4) Kohlstedt, H.; Pertsev, N. A.; Contreras, J. R.; Waser, R. Theoretical current-voltage characteristics of ferroelectric tunnel junctions. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 125341. (5) Sugibuchi, K.; Kurogi, Y.; Endo, N. Ferroelectric field-effect memory device using Bi4Ti3O12 film. J. Appl. Phys. 1975, 46, 2877. (6) Yurchuk, E.; Müller, J.; Hoffmann, R.; Paul, J.; Martin, D.; Boschke, R.; Schlösser, T.; Müller, S.; Slesazeck, S.; Bentum, R.; Trentzsch, M.; Schröder, U.; Mikolajick, T. HfO2-Based Ferroelectric Field-Effect Transistors with 260 nm Channel Length and Long Data Retention. 4th IEEE International Memory Workshop, 2012; pp 1−4. (7) Mueller, S.; Muller, J.; Hoffmann, R.; Yurchuk, E.; Schlosser, T.; Boschke, R.; Paul, J.; Goldbach, M.; Herrmann, T.; Zaka, A.; Schroder, U.; Mikolajick, T. From MFM Capacitors Toward Ferroelectric Transistors: Endurance and Disturb Characteristics of HfO2-Based FeFET Devices. IEEE Trans. Electron Devices 2013, 60, 4199−4205. (8) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthélémy, A.; Bibes, M. Giant Tunnel Electroresistance for Non-destructive Readout of Ferroelectric States. Nature 2009, 460, 81−84. (9) Gruverman, A.; Wu, D.; Lu, H.; Wang, Y.; Jang, H. W.; Folkman, C. M.; Zhuravlev, M. Y.; Felker, D.; Rzchowski, M.; Eom, C.-B.; Tsymbal, E. Y. Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale. Nano Lett. 2009, 9, 3539−3543. (10) Crassous, A.; Garcia, V.; Bouzehouane, K.; Fusil, S.; Vlooswijk, A. H. G.; Rispens, G.; Noheda, B.; Bibes, M.; Barthélémy, A. Giant Tunnel Electroresistance with PbTiO3 Ferroelectric Tunnel Barriers. Appl. Phys. Lett. 2010, 96, 042901. (11) Yamada, H.; Garcia, V.; Fusil, S.; Boyn, S.; Marinova, M.; Gloter, A.; Xavier, S.; Grollier, J.; Jacquet, E.; Carrétéro, C.; Deranlot, C.; Bibes, M.; Barthélémy, A. Giant Electroresistance of Super-Tetragonal BiFeO3-Based Ferroelectric Tunnel Junctions. ACS Nano 2013, 7, 5385−5390. (12) Zenkevich, A.; Minnekaev, M.; Matveyev, Y.; Lebedinskii, Y.; Bulakh, K.; Chouprik, A.; Baturin, A.; Maksimova, K.; Thiess, S.; Drube, W. Appl. Phys. Lett. 2013, 102, 062907. 43375


Research Article

ACS Applied Materials & Interfaces (30) Zheng, W.; Bowen, K. H.; Li, J.; Da̧bkowska, I.; Gutowski, M. Electronic Structure Differences in ZrO2 vs HfO2. J. Phys. Chem. A 2005, 109, 11521−11525.

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