Addition of yttrium into HfO2 films: Microstructure and electrical ...

Addition of yttrium into HfO2 films: Microstructure and electrical properties C. Dubourdieu,a兲 E. Rauwel, and H. Roussel Laboratoire des Matériaux et du Génie Physique, CNRS, Grenoble INP, 3 parvis L. Néel, BP 257, 38016 Grenoble, France

F. Ducroquet Institut de Microélectronique, Electromagnétisme et de Photonique (IMEP-LAHC), CNRS, Grenoble INP, 3 parvis L. Néel, BP 257, 38016 Grenoble, France

B. Holländer Institut für Schichten and Grenzflächen (ISGI) and Center of Nanoelectronic Systems for Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany

M. Rossell and G. Van Tendeloo Electron Microscopy for Material research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

S. Lhostis STMicroelectronics, 850 rue J. Monnet, 38926 Crolles, France

S. Rushworth SAFC Hitech Limited, Bromborough, Wirral CH62 3QF, United Kingdom

共Received 29 September 2008; accepted 3 March 2009; published 13 April 2009兲 The cubic phase of HfO2 was stabilized by addition of yttrium in thin films grown on Si/ SiO2 by metal-organic chemical vapor deposition. The cubic phase was obtained for contents of 6.5 at. % Y or higher at a temperature as low as 470 ° C. The complete compositional range 共from 1.5 to 99.5 at. % Y兲 was investigated. The crystalline structure of HfO2 was determined from x-ray diffraction, electron diffraction, and attenuated total-reflection infrared spectroscopy. For cubic films, the continuous increase in the lattice parameter indicates the formation of a solid-solution HfO2 – Y2O3. As shown by x-ray photoelectron spectroscopy, yttrium silicate is formed at the interface with silicon; the interfacial layer thickness increases with increasing yttrium content and increasing film thickness. The dependence of the intrinsic relative permittivity ␧r as a function of Y content was determined. It exhibits a maximum of ⬃30 for ⬃8.8 at. % Y. The cubic phase is stable upon postdeposition high-temperature annealing at 900 ° C under NH3. © 2009 American Vacuum Society. 关DOI: 10.1116/1.3106627兴

I. INTRODUCTION The first gate oxide containing hafnium has recently been integrated in commercial complementary metal-oxidesemiconductor 共CMOS兲 field-effect transistors for the 45 nm node with the release of the Penryn® microprocessor.1 HfO2 has a medium dielectric permittivity, high band gap, and reasonable band gap offsets with silicon.2–4 The mean static dielectric permittivity ␬ of the monoclinic phase, which is the stable thermodynamic phase at room temperature, is ⬃16. In view of further equivalent oxide thickness reduction, it is desirable to increase the permittivity. Higher ␬ values are also of interest for fabricating highly integrated metalinsulator-metal 共MIM兲 capacitors with high-capacitance values. It has been shown from ab initio calculations that higher-symmetry phases of HfO2 exhibit higher permittivities.5,6 Rignanese calculated values of ␬ ⬃ 26 for the cubic phase and of ␬储 ⬃ 20 and ␬⬜ ⬃ 33 for the tetragonal phase.6 The values for the orthorhombic phase 共with Pnma space group兲 range in between those of the cubic and tetraga兲

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onal phases.7 It is therefore desirable to stabilize these higher-symmetry phases. However, the tetragonal and cubic phases are stable at high temperatures: the monoclinic-totetragonal phase transition occurs at ⬃1700 ° C,8–10 whereas the tetragonal-to-cubic phase transition occurs at ⬃2700 ° C 共Ref. 10兲共for a rising temperature cycle兲. Orthorhombic phases appear under high pressure, with at least two highpressure polymorphs, depending on pressure and temperature conditions.11–15 Similar to the ZrO2 system, the cubic phase of HfO2 is a fluorite-type structure.10,16 Also similar to ZrO2, this cubic structure can be stabilized by adding an appropriate cation, which allows for the formation of a solid solution. Diverse oxides such as CaO, MgO, SiO2, CeO2, and Y2O3 and most of the rare-earth oxides have been studied 共Refs. 8, 17, and 18 and references therein兲. Among these different systems, the hafnia-yttria one has been widely investigated as ceramics or single crystals.17–35 At room temperature, Y2O3 crystallizes in a body-centered-cubic structure 共Tl2O3 type兲, which has some resemblance with that of cubic zirconia or hafnia.36 A pure cubic phase of HfO2 can be stabilized for a minimum content of ⬃5 – 10 mol % Y2O3 depending on the preparation conditions. Caillet et al. found that a single cubic phase 共without the presence of a monoclinic

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phase兲 is stabilized from 8 mol % Y2O3 at 1800 ° C.20 Isupova et al. reported the lower limit for the pure fluorite phase at 5 mol % Y2O3 after calcinations at 1900 ° C and at 10 mol % Y2O3 after calcinations at 1300 ° C.21 Schieltz et al. reported a phase boundary between 6 and 8 mol % Y2O3 at 1500 ° C.23 The cubic solid solution is stable over a wide compositional range. Caillet et al. reported an upper limit of 40%.20 Stacy and Wilder found the upper limit to be 53 mol % at 1600 ° C and to increase up to 55 mol % at 2200 ° C.25 Portnoi et al. reported a value up to 60%.24 A phase diagram of the hafnia-yttria system has been proposed by several groups 共Refs. 21, 24, 25, 27, and 28 and references therein兲. In the form of thin films, few studies have been devoted to this solid-solution system, and only limited selected compositions were investigated.37–44 In this study, the addition of yttrium in HfO2 was studied in a wide compositional range, from 1.5 to 99.5 at. % Y content. The films were grown by metal-organic chemical vapor deposition 共MOCVD兲 on Si/ SiO2 共t ⬃ 0.8– 1.0 nm兲. The crystalline structure and the microstructure of the films, as well as the nature of the interface with silicon, were investigated by means of several techniques. We show that the cubic phase of HfO2 can be stabilized for 6.5 at. % Y at a temperature as low as 470 ° C. The formation of a solid solution is discussed, and the interface of the film with the substrate is analyzed. Finally, the electrical properties of the films are discussed, with a special emphasis in the range of 0 – 10 at. % Y. II. EXPERIMENT Hf–Y–O thin films were grown on p-type Si 共100兲 / SiO2 共⬃0.8– 1.0 nm兲 by MOCVD. A liquid-injection delivery scheme was used, which is based on the injection of microamounts of a precursor solution using microvalves such as those used in the automotive industry.45 Flash volatilization occurs upon injection into the evaporator, and the reactive gaseous species are transported toward the deposition chamber using a carrier gas. This technique has been used for the growth of HfO2 films.46,47 For addition of yttrium into HfO2, hafnium and yttrium precursor solutions were mixed in a single cocktail in different ratios to vary the Y content in the films. Octane solutions with Hf共OtBu兲2共mmp兲2 and Y共tmhd兲3 共both 0.05M兲 were used as liquid precursors. The evaporator temperature was set to 160– 200 ° C using a positive gradient from the injection zone toward the deposition chamber. The injection frequency was either 0.33 or 1 Hz, and the injected volume was typically of the order of microliters. Note that, from the different analyses, the injection frequency had no effect on the film’s microstructure. The deposition temperature was varied in the range 330– 600 ° C, and the working pressure was fixed at 133 Pa. Ar was used as a carrier gas 关100 sccm 共sccm denotes cubic centimeter per minute at STP兲兴 and dioxygen as an oxidizing agent 共250 sccm兲. In this study, different film thicknesses were prepared for each composition, ranging from 2 to 97 nm 共thick films of 40– 97 nm were prepared for x-ray diffraction studies兲. The film thickness is controlled in our process via the J. Vac. Sci. Technol. A, Vol. 27, No. 3, May/Jun 2009

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volume of injected liquid, while the precursor solution concentration, injection frequency, opening-time duration, and pressure in the evaporator are set. The thickness can be precisely controlled by controlling the number of injected droplets, and the reproducibility is given by the reproducibility of the injection volume, which is said to be ⬃3% by the constructor. With this method, nanometer-scale control of the growth can be achieved, down to one unit cell.48 The average growth rate for pure HfO2 depends on the volume of each injected droplet, on the injection frequency, and on the growth temperature. In the present process conditions, it is typically of ⬃21.2 nm/ ml at 500 ° C 共⬃0.077 nm/ s兲. For the growth of Hf–Y–O films, the growth rate strongly depends on the yttrium content, as will be discussed later. The cationic composition of the films was determined from Rutherford backscattering spectrometry 共RBS兲. The crystalline structure was studied by x-ray diffraction, electron diffraction, and attenuated total-reflection Fouriertransform infrared spectroscopy 共ATR-FTIR兲. X-ray diffraction 共␪ / 2␪ scans兲 was performed using Cu K␣1 radiation 共␭ = 1.54056 Å兲. ATR measurements were carried out using a homemade system with a Ge prism.49 The infrared spectra were acquired in the range of 600– 4500 cm−1 and referenced to the spectrum when no sample was coupled to the prism. The density and thickness of the films were determined from the fitting of x-ray reflectometry curves using a two-layer model. Transmission electron microscopy 共TEM兲 was performed on selected samples in cross-section views. Finally, the bonding arrangements in the films and at the interface between the film and substrate were analyzed by x-ray photoelectron spectroscopy 共XPS兲. XPS spectra were collected with an Al K␣ x-ray source, and a hemispherical electron analyzer was used in fixed-pass energy mode 共20 eV兲. The photoelectron take-off angle was fixed at 45°. No Ar sputtering was performed prior to measurements to prevent film or interface degradation. Energy-shift calibration was performed by positioning the Si 2p core level from the Si substrate at 99.3 eV. Electrical measurements were performed on MOS structures. The top gold electrodes were deposited by evaporation through a hard mask. Four different electrode areas were prepared, ranging from ⬃50⫻ 50 to ⬃ 150⫻ 150 ␮m2. Capacitance-voltage 共C-V兲 curves were recorded using an HP4284A precision LCR meter. Interface state densities 共Dit兲 were extracted from the “high-low frequency” method. Current-voltage 共I-V兲 curves were measured using an HP4156B parameter analyzer.

III. RESULTS AND DISCUSSION A. Stabilization of cubic phase

In Fig. 1, the Y content in the films grown at 600 ° C is plotted as a function of the Y content in the precursor solution. These percentages are defined as the number of Y atoms divided by the total number of Y and Hf atoms in the film and in the precursor solution, respectively. The cationic composition in the films was determined from RBS 共with two

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FIG. 1. Film composition 共as measured by RBS兲 as a function of precursor solution composition for Hf–Y–O films grown at 600 ° C on Si/ SiO2 共the thickness is in the range of 40– 97 nm兲. The inset shows the density of the same films 共determined from XRR兲 as a function of film composition.

distinct peaks for Hf and Y兲. As shown in Fig. 1, the yttrium content in the film is lower than in solution. This behavior is attributed, in part, to the different decomposition temperatures of the two precursors: the Y共tmhd兲3 molecule requires a higher temperature to fully decompose, as compared to Hf共OtBu兲2共mmp兲2. The composition of the films deposited at 470 and 500 ° C was also measured. We did not find significant differences in Y contents. The inset in Fig. 1 shows the density of the films as a function of the Y content. The density was determined from x-ray reflectometry measurements 共XRR兲. The experimental curves were fitted using a twolayer model for the interface 共yttrium silicate兲 and the film 共Hf–Y–O兲. The density gradually decreases with the increase in yttrium content, which is expected owing to the much lower density of bulk Y2O3 共5.03 g / cm3兲 as compared to bulk HfO2 共cubic phase: 10.56 g / cm3兲.50,51 For 99.5 at. % Y, only yttrium silicate is formed, which explains the rather low density of 4.05 g / cm3. There are no large differences in the film densities between 410 and 600 ° C below 40 at. % Y in the films. However, the crystallinity of the films is different. Figures 2共a兲 and 2共b兲 show the ␪ / 2␪ scans performed on thick films 共⬃72– 97 nm兲 grown at different temperatures and containing 2.5 and 9.9 at. % Y, respectively. The crystallization starts for temperatures of 400 ° C. Independent of composition, larger diffraction peaks are observed below 500 ° C, which is associated with a smaller grain size. For temperatures of 450 ° C and higher, the diffraction peaks for the films with 2.5 at. % Y indicate the presence of a mixture of monoclinic HfO2 and of a higher HfO2 symmetry phase. The higher-symmetry phase is evidenced by the peak at 2␪ = 30.35° 共⫾0.05兲, which does not belong to the monoclinic phase. The film with 9.9 at. % Y clearly crystallizes in a single cubic HfO2 phase 共diffraction peaks at 30.38°, 35.22°, 50.65°, 60.15°, and 74.42°兲.51 No monoclinic phase is detected within the sensitivity of x-ray diffraction. The stabilization of a purely cubic phase happens in between 5.7 and 9.9 at. % for thick films 共艌70 nm兲. In Fig. 2共c兲, we compare the x-ray diagrams at 600 ° C for both compositions. The 200 peak for the cubic phase is not present at 5.7 at. % Y. We will JVST A - Vacuum, Surfaces, and Films

FIG. 2. ␪ / 2␪ x-ray diffraction patterns for films grown at different temperatures with 共a兲 2.5 at. % Y and 共b兲 9.9 at. % Y. 共c兲 ␪ / 2␪ x-ray diffraction patterns for films with 5.7 and 9.9 at. % and growing temperature of 600 ° C. All films have a thickness larger than 72 nm, except for the film grown at 330 ° C, which has a thickness of 42 nm.

show later that the peak at ⬃35.45° is attributed instead to an orthorhombic phase 共or to the monoclinic phase兲. We used ATR-FTIR to precisely determine the threshold amount of yttrium needed to induce a crystalline change in thin films 共t ⬍ 15 nm兲. We show in Fig. 3共a兲 the ATR spectra for three films with increasing amounts of yttrium 共the thickness is in the range of 11.5– 14.4 nm兲. For pure HfO2 films, we observe a doublet at 690 and 760 cm−1. The peak at 760 cm−1 is characteristic of the monoclinic structure.46,52 This peak disappears for 6.5 at. % 共and higher兲, and a single peak is observed at 690 cm−1. We have not found referenced

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FIG. 4. SIMS profiles on a Hf–Y–O film grown at 470 ° C with 6.5 at. % Y and thickness of 15.2 nm.

FIG. 3. 共a兲 Attenuated total-reflection-FTIR spectra of films grown at 470 ° C with different Y contents 共the thickness ranges between 11.5 and 14.4 nm兲 and compared to a pure HfO2 crystalline film 共9.8 nm兲 grown at 500 ° C. 共b兲 Electron diffraction patterns of a 2.0 at. % Y film grown at 500 ° C and of thickness 4.7 nm 共left兲 and 6.5 at. % Y film grown at 470 ° C and of thickness 15.2 nm 共right兲.

infrared spectra for orthorhombic, tetragonal, or cubic HfO2. From electron diffraction performed on thin films of similar composition, the structure is clearly purely cubic, as shown in Fig. 3共b兲. The film with 2.0 at. % Y contains a mixture of monoclinic and higher-symmetry phases 关left of Fig. 3共b兲兴. The stabilization of a purely cubic phase is thus achieved for 6.5 at. % Y and for a temperature as low as 470 ° C. In ceramics 共of micrometer grain size兲, the same stabilization is achieved for ⬃8 – 10 mol % Y2O3 共or 14.8– 18.2 at. % Y兲 and at a temperature of ⬃1500 ° C or higher. Similar results to ours have been obtained on Zr–Y–O films grown by MOCVD.53–56 Kim et al. achieved a cubic stabilization of ZrO2 for films containing 6 at. % Y and grown at 620 ° C on Si.53 As a matter of fact, the stabilization of higher-symmetry phases in thin films depends not only on the additive content and nature but also on the substrate temperature, film thickness, nature of the substrate, and processing conditions, all of which determine the grain size, distribution of the additive, strain and residual stresses in the film, intrinsic defects 共such as vacancies in the anionic sublattice兲, and nature and content of the impurities. The role of the grain size in the stabilization of a metastable phase is crucial. It is, for example, well known that tetragonal ZrO2 can be stabilized at room temperature in zirconia nanoparticles.57–59 The grain size of the films is in the nanometer range, and strain related to surface tension certainly induces a shift of the boundary between the stability domains. Residual stress and impurities in J. Vac. Sci. Technol. A, Vol. 27, No. 3, May/Jun 2009

the films may also contribute to the lowering of the stabilization temperature and of the threshold amount of additive. We have, for example, observed the stabilization of the cubic phase of HfO2 using 1.1 at. % Sc, which was attributed to the strong carbon incorporation from the precursor in the film. Although the formation of a solid solution between HfO2 and Y2O3 共and, more generally, with the corresponding additive oxide兲 is well established in ceramics, it has not been clearly evidenced in thin films. In XPS analysis, the formation of a solid solution should lead to a slight shift of the Hf 4f doublet. However, the expected change is small and cannot be discriminated from other effects in thin films. To probe the distribution of yttrium in our films, secondary-ion mass spectrometry 共SIMS兲 analyses were performed. Figure 4 shows a typical SIMS profile for Hf, Y, Si, and oxidized Si as a function of erosion time 共for a film with 6.5 at. % Y and a thickness of 15.2 nm兲. A uniform yttrium concentration profile is observed throughout the depth of the film. Hafnium is also uniformly distributed. The tail of Hf in the Si substrate is an artifact of the SIMS measurements. While compared to pure HfO2, no silicon is observed in the film, which indicates that no yttrium silicate is formed in the bulk of the film 共the interface studied by XPS will be discussed later兲. The uniform distribution of yttrium in the film, together with the absence of bulk yttrium silicate or Y2O3 phases, are in favor of a complete incorporation of yttrium in the HfO2 structure. This is confirmed by the evolution of the lattice parameter of the cubic phase with the yttrium content, as shown in Fig. 5. The lattice parameter of the cubic phase was calculated from the 200 peak for the set of thick films 共50– 97 nm兲 grown at 600 ° C up to ⬃40 at. % Y. The substrate peak at 2␪ = 32.958° was taken as an internal reference. Figure 5 shows that the lattice parameter increases continuously with the Y content in the domain of pure cubic-phase stabilization, which is coherent with a solid-solution formation. The lattice parameter of the cubic fluorite-type HfO2 is 5.096 Å,51 while the lattice parameter of Y2O3 cubic structure is 10.604 Å 共Ref. 50兲共one-half of the unit cell, matching the fluorite cell, is 5.302 Å兲. Thus, an increase in the

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FIG. 5. Lattice parameter of Hf–Y–O cubic films as a function of Y content for thick films 共50– 97 nm兲. For the two films with mixed monoclinic and higher-symmetry phases 共2.5 and 5.7 at. % Y兲, the calculated value corresponds to the 2d interplane spacing value calculated from the position of the peak located at ⬃35.45°. The peak at 2␪ = 32.958° of Si was taken as a reference. The horizontal dashed line corresponds to the lattice parameter of referenced cubic HfO2 共Ref. 51兲.

HfO2 – Y2O3 cubic lattice parameter with increasing Y content is expected. Our values are, in general, in good agreement with those reported for cubic hafnia-yttria ceramics, as shown in Table I. The upper limit for the solid solution was not determined in our study due to the lack of compositions in between 40 and 73 at. % Y. For the two films in which the monoclinic phase is present together with a higher-symmetry phase, a clear discontinuity is observed in the calculated lattice parameter values. This observation indicates that the higher-symmetry phase that first formed for low Y contents—and which coexists with the monoclinic one—is rather orthorhombic 共or tetragonal兲 but not cubic. The peak located at 2␪ ⬃ 35.45° for these two films may correspond to

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the 200 monoclinic peak and/or to the 002 orthorhombic one 共Pbca space group兲. The corresponding interplane 2d spacing values, which are plotted in Fig. 5, correspond either to the d001 spacing of the monoclinic phase or to the c lattice parameter of the orthorhombic phase. It is interesting to note that the growth rate concomitantly strongly decreases when the monoclinic phase disappears 共from ⬃21 nm/ ml for 0 – 5.7 at. % Y down to ⬃15 nm/ ml for 9.9– 30 at. % Y兲. From a process perspective, this could be used for following the formation of the cubic phase in real time. The thermal stability of the stabilized cubic HfO2 phase was tested after a thermal annealing of 60 s at 900 ° C under NH3 performed in a rapid thermal-processing furnace. The ATR spectra show that the cubic phase is still present, with no indication of monoclinic phase formation, as shown in Fig. 6 for a film with 9.0 at. % Y. This is particularly important because the CMOS fabrication process includes a hightemperature step. Experiments with tetragonal ZrO2-stabilized HfO2 have shown that the tetragonal phase transforms to a monoclinic phase after the high-temperature step.60 Figure 6 shows that the interface is strongly modified, with the formation of silicon oxinitride共s兲 and yttrium 共and possibly hafnium兲 silicate共s兲. Because the annealing conditions were not controlled in terms of oxygen partial pressure, we did not investigate further the nature of the film after annealing. The nature of the interface in as-deposited films is discussed in the following section. B. Study of chemical environment in films: Nature of interface

XPS measurements were performed on films with different Y contents. Binding energies and/or full width at half

TABLE I. Values of the lattice parameter of the HfO2 – Y2O3 cubic phase for different amounts of yttrium 共given as mol % Y2O3 or as at. % Y兲 in the solid solution 共either films or bulk兲. Our data on films 共50– 97 nm兲 are presented together with data referenced in literature on ceramics or single crystals. The symbol ⬃ denotes values extracted from graphs. Dopant 共mol % Y 2O 3兲 5.2 6.0 6.7 8.0 10.0 12.0 13.0 14.0 15.0 16.0 18.0–18.1 20.0 24.0 25.0 30.0

Lattice parameter 共Å兲 Dopant at. % Y

Our films

9.9

5.106

12.6

5.109

Ref. 20

Ref. 24

Ref. 23

Ref. 25

Ref. 32

Ref. 32

Ref. 26

5.124

5.129

5.137

5.137

5.139

5.150

5.149

5.156

5.180 5.191

5.175

5.218

5.205

⬃5.123 5.118 ⬃5.121 23

5.113

⬃5.1240 ⬃5.1285 ⬃5.1315

⬃5.130 ⬃5.132

5.138 ⬃5.134

30.6

5.149

38.7

5.170

33.0–33.3 40.0

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⬃5.146

5.139

⬃5.160 ⬃5.170

5.160 5.166 5.180

⬃5.140 ⬃5.145 ⬃5.150

⬃5.141 ⬃5.146 ⬃5.152 ⬃5.172 ⬃5.188

5.172

5.196

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FIG. 6. ATR spectra for 共a兲 as-deposited Hf–Y–O film grown at 500 ° C with 8.8 at. % Y and thickness of 6.4 nm; 共b兲 same film after annealing at 900 ° C for 60 s under NH3.

maximum 共FWHM兲 of the XPS core lines are influenced by the local order and by the character of the chemical bonds of the atom that undergoes the photoemission process. Thus, valuable information can be obtained on each element’s environment. Figure 7 shows the Hf 4f, Y 3d, and Si 2p corelevel spectra of a 3.0 nm thick film with 8.5 at. % Y. These spectra are representative of the measured films with a single HfO2 cubic phase. The film with 2.0 at. % Y—mixed monoclinic and high-symmetry phase—is quite different and will be commented on separately 关its Y 3d spectrum is shown in Fig. 7共c兲兴. All profiles were fitted using a Voigt function convolving Gaussian and Lorentzian functions. The spin-orbit splitting for Hf 4f 7/2-5/2 and Y 3d5/2-3/2 were 1.66 and 2.00 eV, respectively. The position and FWHM of the peaks are reported in Table II. All measurements were performed at a take-off angle of 45° except for the film with 21 at. % Y, which was measured using a take-off angle of 90°. The Hf 4f 7/2 profiles could be fitted using a single peak, situated between 17.79 and 18.07 eV, for the films containing the HfO2 cubic phase. No significant change of the peak

FIG. 7. XPS spectra of different Hf–Y–O films grown on Si/ SiO2: 共a兲 Hf 4f core level for 8.5 at. % Y; 共b兲 Si 2p core level for 8.5 and 26.0 at. % Y; 共c兲 Y 3d core level for 2.0 and 8.5 at. % Y. The growth temperature and films thickness are given in Table II.

position is observed with increasing amount of yttrium in the films. Values of 16.4– 16.7 eV are reported for Hf 4f 7/2 in bulk monoclinic HfO2.61,62 However, the reported values for Hf 4f 7/2 in HfO2 thin films are larger and very much scattered. Wilk et al. had a value of 17.3 eV for a thick HfO2 film 共however, the doublet is not well defined, which may indicate a multiple binding environment for Hf兲.2 Cosnier et al. obtained the same value of 17.3 eV for a 5 nm thick film.63 Lee et al. reported a value of 17.55 eV for a similar thickness.64 Using high-resolution synchrotron radiation with various photon energies, they did not evidence any shift or change with the probing depth. Higher values of 17.9,65,66 18.10,67 18.16,68 or 18.3 eV 共Ref. 69兲 have also been re-

TABLE II. Position of the peaks of different core-level XPS spectra and full width at half maximum 共⌬␻兲 for films containing different amounts of Y. The positions are determined from the fitting of the experimental spectra.

at. Deposition Thickness % Y conditions 共nm兲 Rey 17 monoclinic ⫹ orthorhombic HfO2

2.0

500 ° C 1 Hz

Rey 32 cubic HfO2 Rey 74 cubic HfO2 Rey 93 共90°兲 cubic HfO2 Rey 94 cubic HfO2

8.8

500 ° C 1 Hz 500 ° C 0.33 Hz 450 ° C 0.33 Hz 450 ° C 0.33 Hz

8.5 21.0 26.0

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4.7

6.4 3.0 7.3 6.1

Hf 4f 7/2 共eV兲 18.0 18.5 ⌬␻ = 1.0° ⌬␻ = 1.0° 18.1 ⌬␻ = 1.1° 17.85 ⌬␻ = 1.2° 17.95 ⌬␻ = 1.2° 17.8 ⌬␻ = 1.25°

Y 3d5/2 Y 3d5/2 contribution 1 contribution 2 共eV兲共eV兲 157.70 ⌬␻ = 1.15°

158.1 ⌬␻ = 1.0° 158.4 ⌬␻ = 1.2° 158.4 ⌬␻ = 1.1° 158.3 ⌬␻ = 1.25°

158.75 159.6 ⌬␻ = 1.9° ⌬␻ = 1.9° 158.6 ⌬␻ = 1.6° 158.9 ⌬␻ = 1.5° 158.9 ⌬␻ = 1.7° 159.0 ⌬␻ = 1.5°

Si–O 共eV兲 103.3 ⌬␻ = 1.5°

103.1 ⌬␻ = 1.7° 103.0 ⌬␻ = 1.7° 102.9 ⌬␻ = 1.6° 102.8 ⌬␻ = 1.6°

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ported. The value of the Hf 4f 7/2 peak also depends on film thickness.70–74 Renault et al., and Barrett et al. reported values of 17.06, 17.15, and 17.65 eV, respectively, for 0.6, 1.0, and 2.5 nm thick films grown by atomic layer deposition 共ALD兲.70,71 Ulrich et al. measured 18.17 and 17.71 eV on ultrathin and thick plasma-enhanced MOCVD films, respectively.72 Cho et al. measured 17.7 and 16.8 eV on 1.7 and 26.5 nm thick ALD films.73 The formation of hafnium silicate has been shown to induce a shift toward higher binding energies.63,71,75 However, in the XPS work by Suzer et al. using synchrotron radiation 共Hf 4f 7/2 at 18.16 eV兲, HfO2 results from the oxidation of a Hf foil, which excludes hafnium silicate formation.68 In the work of Toyoda et al., a value of 18.3 eV is reported for a HfO2 film with little Hf-silicate component, whereas 17.4 eV is measured on a film containing mainly hafnium silicate.69 The scattered values for pure HfO2 films reflect the difficulty to measure accurately binding energies in oxides due to charging effects 共different calibrations are used兲 and also the fact that the binding energy of an element is highly sensitive to its chemical environment, which may differ depending on the growth technique and film thickness, both determining the possible impurities and the oxygenation of the films. Chen et al. showed, for example, that plasma N2O / He or N2 / He treatments induce a shift toward higher binding energies in MOCVD-grown films,74 which they attributed to a better oxidation of the films. Now, considering the addition of an element in the HfO2 structure, one should expect a shift of the binding energy of Hf 4f. Overall, there are quite few XPS data for HfO2 films with additives. Addition of alumina is found to increase the Hf 4f 7/2 binding energy.76–78 Cho et al. mentioned that the shift to 17.8 eV is similar to the one obtained after addition of SiO2 共hafnium silicate兲.77 For yttrium addition, Yang et al. found a value of ⬃18.1 eV for a 共HfO2兲0.895共Y2O3兲0.105 film deposited on GaAs 共001兲.44 The difference in electronegativity between Hf 共1.30兲 and the additive 共1.22 for Y兲 is usually rather small, and the expected shift is not large enough to be discriminated from other effects. A direct comparison is needed with a pure HfO2 film of similar thickness and grown in the same conditions. In our case, we have a position of 17.70– 17.80 eV for a 3.1 nm thick HfO2 film grown in the same conditions. From all these considerations, it is difficult to conclude on the influence of yttrium incorporation on the Hf binding energy or on the presence of hafnium silicate. For the film with 2.0 at. % Y, the fitting requires the addition of a peak located at 18.47 eV. On the other hand, the Si 2p spectrum shows no indication for silicate in this film. The two Hf 4f contributions could originate in the presence of two HfO2 phases in this film 共monoclinic and highersymmetry phases兲. Let us now consider the Si 2p region. For the film containing only 2.0 at. % Y, the Si–O peak is located at 103.3 eV, which corresponds to the position expected for SiO2 共Si4+兲. Thus, no silicate is detected in this film within our resolution. As the yttrium content increases, the Si–O peak maximum shifts toward lower binding energies, which JVST A - Vacuum, Surfaces, and Films

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may be assigned to the presence of yttrium silicate, with a continuous yttrium enrichment 共the Si–O peak is shifted down to 102.8 eV for 26 at. % Y兲. The expected range of Si 2p in Y–O–Si binding is indeed in the range of 102– 103 eV depending on the silicate composition.61 Our values are in good agreement with the value of 102.9 eV reported by Chambers and Parsons for Y–O–Si films formed by the oxidation of yttrium deposited onto Si.79 The Si 2p peak for Y–O–Si bond is measured at a lower binding energy than SiO2 due to a lower electronegativity of yttrium 共1.22兲 compared to that of silicon 共1.90兲.80,81 The FWHM of the Si–O peak, which is 1.5 eV for SiO2, increases slightly to 1.6 or 1.7, which shows that the Si–O bonding environment is well defined. The deconvolution of the Si–O peak shows that the Si4+ contribution is still present, which indicates the presence of SiO2 either underneath the yttrium silicate or mixed with it. A similar result has been obtained by Durand et al.80,81 for yttrium oxide films deposited by MOCVD on Si/ SiO2. The analysis for a 6.1 nm thick film with 26.0 at. % Y is also shown in Fig. 7共b兲. Spectra from the Si 2p region are normalized to have the same Si 2p substrate peak area. This representation is used to estimate the thickness evolution of the oxidized Si-based compounds. We clearly observe that the yttrium silicate thickness increases with increasing amount of yttrium in the films. Figure 7共c兲 shows the Y 3d doublet 共Y 3d5/2-3/2兲. The Y 3d5/2 peak in Y2O3 is expected at 156.6 eV.82 The average position of the Y 3d5/2 peak maximum in our films is located at ⬃158.6 eV and is in good agreement with that found in a 共HfO2兲0.895共Y2O3兲0.105 film deposited on GaAs 共001兲.44 However, if a constant splitting of 2.0 eV is assumed for the doublet, the fitting of the Y 3d spectra requires at least two components for each peak of the doublet, which indicates a multiple yttrium bonding environment. The film with 2 at. % Y requires at least three components. Considering the Y 3d5/2 peak of the HfO2 cubic films, a first component is situated at ⬃158.1– 158.4 eV and a second component at ⬃158.6– 159.0 eV. The first component can be attributed to the Y–O–Hf bonds. For ZrO2共Y2O3兲, which is a similar system to HfO2共Y2O3兲 from the perspective of the crystalline structure 共Hf and Zr also have similar electronegativity兲, XPS studies on single crystals have shown that the Y 3d5/2 peak is shifted toward higher binding energies at a value of 157.15 eV 共Ref. 83兲 and does not significantly change with the content of Y2O3. A higher shift, up to 157.7 eV, is reported by Hughes and Sexton for polycrystalline samples 共10 mol % Y2O3兲.84 Studies on ZrO2共Y2O3兲 films give scattered values of ⬃157.0 eV,85 157.4 eV,86 157.8 eV,87 and up to 158.6 eV.88 The second component can be attributed to three possible types of bonds: Y–O–Si, Y–O–C, and Y–O–H bonds. The silicate presence has been evidenced from the Si 2p spectra. Our Y 3d5/2 peak positions are in good agreement with reported values. Chambers and Parsons found a value of 159.0 eV in Y–O–Si films of ⬃0.8 up to 4 nm.79 Durand et al. found values of 158.7– 158.8 eV for yttrium silicate films grown by MOCVD.80 Additional contributions come from Y–O–C and Y–O–H bonds due to surface contamina-

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tion, as well as bulk impurities. Indeed, the presence of small amounts of carbon and H or OH groups inside the films cannot be precluded during the CVD with organic precursors/solvent. Moreover, the film with 2 at. % Y, in which no silicate was evidenced on the Si 2p spectrum, clearly shows a strong contribution around 159.6 eV, as shown in Fig. 7共c兲. The Y–O–C Y 3d peak has been reported at a value of ⬃159.0 eV for films grown at 350 ° C by plasma-assisted MOCVD using Y共tmhd兲3 precursor diluted in cyclohexane solvent.80,81 Y–O–H bonds have been reported to produce much larger binding-energy shifts than does the Si second-neighbor shift.89 Ulrich et al. reported positions of Y 3d5/2 peak ranging from 158.81 up to 159.65 eV for yttrium oxide films grown at 400 ° C by plasma-assisted MOCVD using sublimated Y共tmhd兲3 precursor.89 For the HfO2 cubic films, it is not possible to determine the relative contribution of each type of bond in the second component. However, the Y 3d doublet is well defined, which indicates a well-defined Y binding environment both in HfO2共Y2O3兲 and in the yttrium silicate layer. Comparing the two components, the FWHM for the Y–O–Hf peak is narrower than the one for Y–O–Si 共+Y – O – C + Y – O – H兲 and similar to the one measured for the Hf 4f 7/2 peak, which is consistent with a crystalline structure for HfO2共Y2O3兲 versus a broad distribution of bonds in the second component. Also, the relative positions of both contributions are consistent with the electronegativities of Hf 共1.30兲 and Si 共1.90兲 or C 共2.55兲. Finally, the relative intensities of both components are also consistent with the proposed picture of a HfO2共Y2O3兲 film with a yttrium silicate interfacial layer. In the film with 21 at. % Y measured at a take-off angle of 90°, the first component at 158.4 eV is larger than the component at 158.9 eV, whereas it is contrary for the film with 26 at. % Y measured at a take-off angle of 45° 共when the take-off angle decreases, the effective probed depth decreases as well, so that the interface contribution is decreased兲. The presence of silicate was confirmed from ATR analysis.40 A broad band is observed for thick films in the range of 800– 1000 cm−1, whose intensity increases with increasing amount of Y. The film grown without injecting hafnium precursor 共99.5 at. % Y兲 exhibits a large peak centered around 944 cm−1, which is attributed to an amorphous silicate. The amorphous nature of the silicate is deduced from the absence of diffraction peaks on the ␪ / 2␪ spectra. Spectra measured on thinner films also show the increase in a contribution in the same spectral range. Figure 8 shows TEM images for four films presented from top to bottom by increasing the amount of yttrium content. Three of these samples 共Rey 17, Rey 32, and Rey 94兲 were analyzed by XPS 共see Table II兲. The fourth film 共Rey 91兲 was grown at 470 ° C and contains 6.5 at. % Y 共analyzed by SIMS in Fig. 4兲. The electron diffraction 共ED兲 pattern of sample Rey 17 is shown on the left of Fig. 3共b兲. The ED patterns of the three other samples are identical and represented on the right of Fig. 3共b兲, showing a pure cubic phase. The increase in the interfacial layer thickness 共yttrium silicate兲 with increasing Y J. Vac. Sci. Technol. A, Vol. 27, No. 3, May/Jun 2009

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FIG. 8. Transmission electron microscopy images of different films: Rey17 has 2.0 at. % Y, Rey91 has 6.5 at. % Y, Rey32 has 8.8 at. % Y, and Rey94 has 26.0 at. % Y. The growth temperature and film thickness are given in Table II, except for Rey91 共470 ° C, 15.2 nm兲.

content is confirmed by TEM images of samples Rey 17, Rey 32, and Rey 94 共0.7, 1.0, and 1.9 nm, respectively, for 2.0, 8.8, and 26 at. % Y and for a total thickness of ⬃5 nm兲. The lowering of the growth temperature to 450 ° C for sample Rey 94 does not prevent the formation of a large interfacial layer. Finally, the image of sample Rey 91 共this sample contains less yttrium than Rey 32 and was grown at a slightly lower temperature兲 clearly shows that the interfacial layer thickness also increases with film thickness 共the thickness of the interface is 1.7 nm, compared to 1.0 nm for Rey 32兲. To conclude, the combined studies by different techniques show that HfO2共Y2O3兲 cubic films can be stabilized for yttrium content of 6.5 at. % and higher and for a temperature as low as 470 ° C. A solid solution is formed, with an increasing lattice parameter for increasing yttrium content. The cubic phase is stable upon high-temperature anneal 共tested up to 900 ° C兲. The interfacial layer consists of yttrium silicate and SiO2, and its composition becomes yttrium richer as the yttrium content in the film increases. The interfacial layer thickness increases with increasing film thickness and with increasing yttrium content. For the minimization of the

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FIG. 9. C-V curves for an as-deposited Hf–Y–O film grown on Si/ SiO2, with 8.8 at. % Y 共Rey32兲. The film thickness is 6.4 nm. The C-V curves after annealing at 900 ° C for 60 s under NH3 are also shown.

equivalent oxide thickness, the formation of a thick interface is, of course, not desirable. Yttrium silicate formation is, however, favored when yttria is in contact with SiO2.90 C. Electrical properties

Electrical measurements were performed on MOS structures with top gold electrodes. C-V curves were measured at frequencies ranging from 0.1 to 100 kHz. Typical curves are shown in Fig. 9. For as-deposited films, the curves are frequency dependent, and a shoulder characteristic for interfacial traps is present. A significant shift in the flat-band voltage VFB is observed toward positive voltage in as-deposited films, indicating the presence of negative charges in the films 共the expected VFB value is 0.22 V considering a work function of 5.1 eV for the gold electrode兲. From the capacitance in accumulation mode Cox and the shift in the flat-band voltage ⌬VFB, the number of total fixed and trapped charges Qtot in the oxide was determined using the following relationship: ⌬VFB = Qtot/Cox . There is no significant difference in Qtot values determined for pure monoclinic HfO2 and cubic Hf–Y–O films for films of thickness below ⬃10 nm, as illustrated in Fig. 10. For higher thicknesses, a slight increase is found for Hf–Y–O films compared to pure HfO2. Below ⬃10 nm, values are of the order of ⬃5.5⫻ 1012 cm−2. These rather high values for as-deposited films are decreased after high-temperature anneal down to ⬃7 ⫻ 1010 cm−2 共after annealing, the flat-band voltage is close to the expected value, as seen in Fig. 10兲. We did not find either a significant difference in the interfacial trap densities Dit 共peak value兲 after Y addition. The Dit values for as-deposited films are on the order of ⬃2 ⫻ 1012 eV−1 cm−2. After annealing, the Dit values decrease to ⬃5 ⫻ 1010 eV−1 cm−2. The equivalent oxide thickness 共EOT兲 of the as-deposited films was determined from the capacitance in accumulation mode Cox. The EOT values were plotted as a function of film thicknesses for Y contents of 2.0 at. % 共⫾0.2兲 and 9.0 at. % 共⫾0.2兲.40 A linear relationship was observed between the JVST A - Vacuum, Surfaces, and Films

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FIG. 10. Total oxide charge density Qtot 共absolute value兲 as a function of film thickness for as-deposited Hf–Y–O films grown on Si/ SiO2, with ⬃9.0 at. % Y 共⫾0.2兲 and for pure HfO2 monoclinic films. The value for an annealed film 共900 ° C, 60 s, NH3兲 is also shown.

EOT and film thickness. From the intercept with the origin, equivalent electrical interfacial layer thicknesses of ⬃1.1 and 2.0 nm were determined for 2.0 and 9.0 at. % Y, respectively. The EOT is given by the following expression: EOT = ETIL + 共3.9/␬兲 ⫻ t, where t is the film thickness, ␬ is the relative dielectric permittivity of the film, and ETIL is the equivalent electrical interfacial layer thickness. The permittivity of the different films was calculated considering a value of 1.1, 2.0, or 2.2 nm for ETIL depending on the Y content and film thickness. The dependence of ␬ on the Y content is shown in Fig. 11. A maximum of ⬃30 共⫾3兲 is observed for ⬃8.8 at. %. This value is in good agreement with the value of 26–29 expected for the cubic HfO2 phase.5,6 No ab initio calculation exists for solid-solution cubic phases of HfO2. Similar permittivities have been reported for cubic films stabilized with Y, although obtained for completely different Y contents: values of ⬃27– 32 are reported for ⬃20 at. % Y,41,43 while a value of 27 is reported for 4 at. % Y.39 A value of 32 has also been reported for trivalent additives such as Dy or Sc.91

FIG. 11. Dielectric relative permittivity of the Hf–Y–O films grown on Si/ SiO2 plotted as a function of Y content.

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result points out that the introduction of Y3+ in place of Hf4+, which is expected to lead to the formation of anionic vacancies, is not detrimental for leakage currents in the investigated thickness range 共艋15 nm兲. IV. CONCLUSION

FIG. 12. Leakage current density J as a function of applied voltage for Hf–Y–O films grown on Si/ SiO2 with different Y contents and compared to a monoclinic HfO2 film.

The leakage current densities measured as a function of applied voltage for different Y contents are compared to that of a pure monoclinic HfO2 film in Fig. 12 共similar film thicknesses are considered, along with the same capacitor area of 2810 ␮m2兲. For films of ⬃6 nm 关see Fig. 12共a兲兴, the addition of Y does not significantly change the leakage current density, especially at low voltages 共−1 V兲, which is in agreement with results reported for 10 at. % Dy or Sc addition.91 However, we observe an increase in the electrical breakdown field Ebd as compared to pure HfO2 共Ebd ⬎ 12.5 MV/ cm for a 6.4 nm film with 8.8 at. % Y兲. The film with 2.4 at. % Y—with mixed monoclinic and tetragonal or orthorhombic phases—exhibits a similar current density–voltage 共J-V兲 behavior as a pure monoclinic film of similar thickness, but again, the electrical breakdown field is increased 共5.0 and 6.3 MV cm−1, respectively兲. Note that the Ebd values depend on film thickness and area of the capacitor. For cubic films of thickness larger than 10 nm, a decrease in the leakage current density is observed with increasing amount of Y, especially for high voltages 关Fig. 12共b兲兴. An opposite behavior was reported by Adelmann et al. for 8.6 nm thick films with 10 at. % Dy or Sc.91 The interfacial silicate layer could play a role in the lowering observed here. However, for films with 6.5 up to 10 at. % Y, the silicate interfacial layer is not significantly different. Thus, we can conclude that Y addition does not lead to an increase in the leakage currents. This J. Vac. Sci. Technol. A, Vol. 27, No. 3, May/Jun 2009

In summary, the addition of yttrium in HfO2 thin films deposited on 共100兲 Si/ SiO2 substrates by injection MOCVD was investigated. A pure cubic phase is stabilized for a minimum Y content of ⬃6.5 at. % and a growing temperature as low as 470 ° C. The cubic fluorite-type phase consists of a solid solution of HfO2 – Y2O3, which is formed up to 40 at. % Y. This cubic phase is stable upon high-temperature rapid thermal annealing at 900 ° C under NH3. A strong dependence of the permittivity is found as a function of yttrium content, with a maximum of 30 for 8.8 at. %. The optimal additive content lies in the range ⬃6.5– 10 at. %. The higher ␬ values obtained for films within ⬃6.5– 10 at. % Y give a scaling advantage in view of further reducing the EOT. The formation of silicate should be controlled so that its thickness does not exceed a few angstroms. Note that if SiO2 is preferred at the interface with silicon, it is possible to adapt the process by first injecting Hf precursor solely in the first stages of the growth to prevent the contact of yttrium with SiO2. Finally, we showed that the leakage currents are not increased by the presence of Y3+ ions in the films. Further work will include the study of the mobility in HfO2 – Y2O3-based transistors. It was shown that Y2O3 / HfO2 stacks exhibit an improved channel mobility,92 which was attributed to reduced phonon and Coulomb scatterings.93 A possible tuning of the metal gate work function ␾m by yttrium addition will also be investigated. Y-based interlayers have been shown to modulate ␾m.94,95 With our injection process, different locations for yttrium addition can be easily investigated. Finally, the use of the HfO2 – Y2O3 solid solution is of high interest for realizing high-capacitance MIM structures. Indeed, HfO2-based MIM capacitors show reasonable capacitance values and low leakage current; however, they suffer from low electrical breakdown fields Ebd. In contrast, Y2O3-based capacitors exhibit high Ebd values but rather low-capacitance values.96 Thus, the superior Ebd values as compared to pure HfO2 and the enhanced ␬ values should benefit MIM structures. ACKNOWLEDGMENTS This study was performed within the projects MEDEA+ T207 and FOREMOST, in close collaboration with STMicroelectronics 共Crolles兲 and SAFC Hitech. The authors acknowledge the French Ministère de la Recherche and the French Ministère de l’Industrie for financial support. N. Rochat from LETI-CEA in Grenoble is acknowledged for access to the ATR equipment. B. Pelissier from LTM-CNRS is acknowledged for XPS measurements. M. Hopstaken from STMicroelectronics is acknowledged for SIMS measurements.

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