Dopants in Chemical Solution-Deposited HfO2 Films

CHAPTER 3.4

Dopants in Chemical Solution-Deposited HfO2 Films € ttger, Sergej Starschich, David Griesche, Theodor Schneller Ulrich Bo Institute of Electronic Materials (IWE 2), RWTH Aachen University, Aachen, Germany

3.4.1 Introduction Chemical solution deposition (CSD) represents an interesting alternative fabrication technique for functional inorganic oxide thin films [1], such as the ferroelectric binary oxide derivatives discussed in the present book or more general inorganic chalcogenide oxide thin films. Primarily CSD typifies an umbrella term comprising a number of chemically different methods such as sol-gel [2, 3], metallo-organic decomposition (MOD) [4], chemical bath deposition [5, 6], etc. The term “metallo-organic” has been introduced to indicate that in the precursors typically used for this method, the organic residue is bound via oxygen atoms (e.g., barium propionate (Ba (OOCCH2CH3)2)) to the center metal instead of being directly bound via the carbon atom as in classical metal-organic chemistry (e.g., trimethyl aluminum (Al(CH3)3)). Common to all CSD techniques is the deposition of a precursor material from the liquid phase onto a suitable substrate in contrast to the typically applied gas phase-based methods of atomic layer deposition (ALD), (metal-organic) chemical vapor deposition (MO)CVD, or sputtering. By various kinds of thermal treatments, the as-deposited films are subsequently transferred to the desired typically crystalline phase. The required coating solutions are generally based on precursor molecules that are dissolved in a common solvent by established chemical synthesis methods. Differences in the routes are related to the involved chemistry, solvent, temperatures, and heating processes. The main educts are salts, carboxylates, or other metallo-organic compounds, such as metal alkoxides and metal βdiketonates, which can often be purchased commercially or synthesized in house by typical chemical synthesis strategies. Often additives, such as chemical stabilizers, are included during synthesis to adjust the properties of the final coating solution. Depending on the procedures utilized during coating solution preparation, the gelation behavior of the deposited film, the

Ferroelectricity in Doped Hafnium Oxide https://doi.org/10.1016/B978-0-08-102430-0.00010-3

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Ferroelectricity in Doped Hafnium Oxide

reactions that take place during thermal annealing, and the various chemical routes utilized for the preparation of functional metal oxide films can be categorized into three main groups [7]: • Classical sol-gel processes that use alkoxide precursors that undergo primarily hydrolysis and polycondensation. • MOD routes that utilize carboxylate or acetylacetonate educts that do not undergo significant condensation reactions during either solution preparation or film deposition. • Hybrid routes that consist of mixtures of metal alkoxides and carboxylates/β-diketonates and do exhibit condensation reactions at several process stages. Frequently, this route is used when multicomponent oxide films (e.g., perovskite materials) are prepared from multiple educt types or when chelating ligands are added to solutions with such multiple precursor types. These categories are certainly too imprecise to classify all conceivable CSD routes to metal oxides exactly, and in many cases, the route under study comprises aspects of more than one of these categories. However, to understand the underlying chemistry of a particular CSD route, it is beneficial to discuss the various approaches that have been utilized from this viewpoint. From this brief general introduction—for a deeper insight, the reader is referred to the literature, for example, [1, 8–12]—the following advantages of CSD can be easily identified. If no conformal step coverage of narrow 3D structures and no ultrathin films (thickness ≪ 15 nm) are required, “device quality” films at moderate precursor and investment cost can be prepared by CSD. The latter are lower by about one to two orders of magnitude because no sophisticated vacuum equipment is required. Due to the high flexibility with regard to the coating method and stoichiometry as well as the economic advantage, CSD offers excellent possibilities in research labs for exploring new thin film materials. It is furthermore considered to be extremely useful for a quick check of the effect of any crystal chemistry modification of the host material by the addition of iso- or aliovalent dopant(s). Hence the method is also ideally suited for small series production and for small- and medium-sized enterprises. Application examples may be found in the fields of microelectromechanical systems [13], high-Tc superconductors [14, 15], transparent conducting oxides [16], and solid oxide fuel cells [17]. It should be mentioned that with regard to the nonclassical ferroelectric binary oxides, initially in all studies gas-phase methods ((MO)CVD, ALD, sputtering) have been applied because it was believed that ultrathin layers (approx. 10 nm) are one prerequisite to obtain

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ferroelectricity. Although this thickness range would normally rule out using CSD, it was possible by just using CSD for the deposition of thicker films to demonstrate that ferroelectricity in this material is not directly related to ultrathin film thicknesses [18]. Moreover, the stoichiometric flexibility of CSD enabled a detailed study of the dopant influence on the ferroelectric properties. While initially a hybrid-type solution was applied, an optimized, even more stable MOD route was established to introduce a bunch of dopants into hafnium oxide [19]. In the following sections, various aspects of the developed CSD process and their impact on the ferroelectric and piezoelectric properties are discussed in detail.

3.4.2 Thin Film Deposition 3.4.2.1 Workflow Spin coating of the precursor solutions is used for the deposition of hafnium oxide or related compositions on electrodized silicon wafers. Suitable bottom electrodes are, for instance: (i) platinum, which is DC sputtered with a thickness of about 100 nm—in this case, the deposition of an additional layer of 10 nm TiOx or AlOx at 150°C is needed to guarantee adhesion of Pt on the oxide surface of the silicon, or (ii) titanium nitride, which is reactively RF sputtered with a thickness of 30 nm from a metallic titanium target at room temperature with 10% nitrogen in the process gas. Each hafnia coating is followed by a heating step at about 215°C in ambient atmosphere for 5 minutes. Typically, a final functional thickness of 45 nm needs three coating steps. The crystallization was realized in a rapid thermal processing (RTP) step at 800°C for 90 seconds in an argon/oxygen (1:1) atmosphere showing the best electrical results. Afterward the patterned 50 nm platinum top electrodes were deposited at room temperature by a negative lift-off process. The different steps during the preparation procedure are shown in Fig. 3.4.1.

3.4.2.2 Precursor Solutions A suitable approach for the preparation of doped hafnium or zirconium oxide precursor solutions is based on a pure MOD routine [7]. The method starts with weighing of the desired amount of hafnium 2,4-pentanedionate and the 2,4-pentanedionate of the corresponding doping metal into a Schlenk flask in a glove box with an inert gas atmosphere. A typical purity of commercial available pentanedionates is about 98%, but for lower

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Ferroelectricity in Doped Hafnium Oxide

Repeated steps Metallization of substrate

Heating

Spin coating

Deposition of top electrodes

Crystallization

Fig. 3.4.1 Process steps of sample preparation: deposition of the bottom electrode on the substrate, spin coating of the precursor solution, heating on a hot plate, crystallization in an RTP oven, and top electrode deposition with subsequent negative lift-off process. The desired thickness is achieved by repetition of the second and third steps [19].

impurity concentrations in these educts additional classical chemical cleaning procedures such as recrystallization may be applied prior to solution synthesis. Subsequently, the two educts were dissolved in a 5:1 mixture of propionic acid and propionic acid anhydride at 130°C. A final concentration of 0.25 mol/L was adjusted by the addition of propionic acid, resulting in a yellow precursor solution. More detailed information is given in Ref. [19]. A similar procedure can be used for the fabrication of mixed hafnium zirconium oxide solutions. The required amount of hafnium 2,4pentanedionate and zirconium 2,4-pentanedionate is dissolved with a ratio of propionic acid and propionic acid anhydride (5:3) at 140°C [20]. Thereby, the duration of the solution heating determines the ferroelectric behavior of the ZrO2 films: the remanent polarization is increased with increasing the heating duration, reaching a maximum at 8 hours as shown in Fig. 3.4.2A. This is accompanied by a change of the solution color,

ZrO2

10 0

Solution heating

–10

–4

0.5 h 8h

–20

(A)

Nd0.05 :HfO2

20 Polarization (µC/cm²)

Polarization (µC/cm²)

20

–2

0

2

0 –10

Method

–20

20 h

Electrical field (MV/cm)

10

–4

4

(B)

–2

0

2

4

Electrical field (MV/cm)

Fig. 3.4.2 (A) Influence of the duration of the solution heating on the ferroelectric properties for pure ZrO2 at a temperature of 140°C and accompanied color change, and (B) comparison between the hybrid and the MOD preparation techniques exemplarily shown for 5% neodymium-doped hafnium oxide [21]. All hystereses are taken at 1 kHz after 1000 wake-up cycles.

Dopants in Chemical Solution-Deposited HfO2 Films

131

obviously indicating the presence of chemical reactions that influence the final ferroelectric properties; see inset of Fig. 3.4.2A. The color change of the solution is also observed for the yttrium-doped hafnium oxide; however, no influence on the ferroelectric properties is found. Although this fact is very interesting from the chemical point of view, detailed investigations are still missing. An alternative procedure for doped hafnium oxide precursors is a hybrid routine consisting of a combination of the MOD technique with sol-gel processing [7]. The sol-gel educt is provided by a hafnium ethoxide with a high purity not less than 99.9%. The processing of hafnium ethoxide is carried out under an inert gas atmosphere by means of standard Schlenk techniques and a glove box. The required amount of hafnium ethoxide is weighted into a Schlenk flask with the addition of dry ethanol. After heating at 60°C for 30 minutes in an oil bath, a transparent solution is formed that is stabilized by adding one equivalent of 2,4-pentanedionate per hafnium ion. The MOD educt corresponds to the desired dopant 2,4-pentanedionate. The dopant 2,4-pentanedionate is dissolved in a mixture of propionic acid and propionic anhydride (5:1) at 100°C. Finally, the two solutions are combined and the concentration is adjusted with propionic acid. Both methods result in a comparable ferroelectric behavior as exemplarily shown by the hysteresis loops P(V ) for 5% neodymium doped hafnium oxide in Fig. 3.4.2B. Except for the precursor preparation, all subsequent processing steps of the deposition are identical. The polarization measurements were carried out using an aixACCT Systems TF Analyzer 2000. No noteworthy difference for the remanent polarization can be found; the only considerable variation is the slight coercive field shift for the hybrid method. Nevertheless, the “simpler” processing favors the MOD technique over the hybrid method due to the absence of the demand for an inert gas atmosphere. All further discussed thin films in this chapter are prepared by means of MOD type precursors.

3.4.2.3 Thermal Analysis Thermogravimetric analysis (TG) is a technique to determine the change of the mass of a sample as the ambient temperature is changed. Such measurements provide information on absorption and desorption, vaporization, sublimation, and mass loss due to precursor decomposition, and is in particular a suitable tool to evaluate pyrolysis and/or crystallization temperatures of the prepared solution. TG is often combined with differential thermal analysis

132

Ferroelectricity in Doped Hafnium Oxide

(

)

( )

(DTA), which compares the temperature signal of the sample and a reference, for example, Al2O3 at an applied temperature ramp. Exothermic as well as endothermic changes of the sample provide information about crystallization, melting, sublimation, or glass transitions. In a DTA plot, the area under a signal peak is given by the enthalpy change. The partially dried solution is placed in a furnace with a thermocouple for accurate temperature measurement. Furthermore, a precise balance is used to measure the mass loss. For better accuracy an inert reference sample (in this case aluminum oxide) is used. For further information, the reader is referred to Ref. [22]. Fig. 3.4.3 illustrates the results of a solution prepared by the MOD method with hafnium and yttrium 2,4-pentanedionate with 5.2% yttrium doping. As indicated by the endothermic peaks in the DTA signal, vaporization of small amounts of residual solvent occurs in a temperature range below 160°C. With increasing temperature, further weight loss continues, accompanied by the onset of the exothermic decomposition of the metalloorganic precursor molecules indicated by the increase of the DTA signal. This pyrolysis starts at approximately 250°C, passes through a maximum at 415°C, and is completed at approximately 490°C. Above this temperature no further weight loss is detected. The largest weight loss of about 40% is observed, which is significantly lower than the expected weight loss during pyrolysis of 63.6% resulting from the assumption that the yttrium and hafnium 2,4-pentanedionate compounds are solely dissolved during the solution preparation.

(

)

Fig. 3.4.3 Thermogravimetric analysis of HfO2 with 5.2% yttrium doping: The dark curve shows the differential thermal analysis (DTA) where values above 0 μV indicate an exothermic process and below 0 μV an endothermic process. The light curve illustrates the percentage mass loss of the investigated material [19].

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The discrepancy can be explained by the chemical reaction of the initial metal acetyl acetonato complexes and the propionic acid used as a solvent during solution synthesis. In addition, pure yttrium 2,4-pentanedionate does not dissolve permanently in propionic acid/propionic anhydride, but in combination with the hafnium 2,4-pentanedionate it dissolves well. Similar observations have been reported in the literature for zirconium acetylacetonate-based precursor solutions [23–25]. Because hafnium and zirconium are nearly identical from the chemical point of view, a reaction with propionic acid is probable here as well. However, a simple complete ligand substitution by propionate groups can be ruled out because the expected mass loss of 55.4% is significantly higher than the observed one of 40%. It is assumed that a more complex Hf cluster structure containing bridging oxo groups and carboxylate groups is formed, similar to what has been found in case of the reaction of Zr(acac)4 with propionic acid [23]. Moreover, the yttrium compound should be linked to this hafnium cluster, for example, via a carboxylate group. The fact that yttrium acetyl acetonate only dissolves well in propionic acid in the presence of the Hf-cluster points to this opportunity, but the exact structure of the precursor molecule is not known yet. The small exothermic hump above 550°C with its maximum at 586°C probably indicates the crystallization of the powder.

3.4.2.4 CSD Processing The evolution of a single coating film can be visualized by the model in Fig. 3.4.4 based on X-ray reflectance measurements (XRR). The wet as-deposited layer after spin coating has a thickness of 114 nm. It contains the metallo-organic hafnium precursor clusters distributed in a considerable amount of propionic acid. Low-temperature heating at Th1 ¼ 215°C initiates a film densification, leading to a film thickness reduction of 38 nm; see Fig. 3.4.4B. The higher density is indicated by the enhancement of the critical angle Θc1 in the XRR measurements. By the results of thermal analysis, it is most likely that the vaporization of the solvent occurs predominantly and almost no pyrolytic decomposition takes place. Higher temperatures than 215°C increase the pyrolytic decomposition and the thickness reduction. In the second heating step at Th2 ¼ 800°C, the main part of the pyrolysis and finally the crystallization take place, leading to a hafnium oxide layer with a layer thickness of 15 nm that is assumed to be carbon-free due to thermodynamic reasons (Fig. 3.4.4C). Further investigation shows that Th1 ¼ 215°C is the best choice. Lower temperatures at multicoating processing

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Ferroelectricity in Doped Hafnium Oxide

Fig. 3.4.4 Scheme of the evolution of a hafnia film 5.2% Y doping and corresponding XRR for (A) a wet as-deposited layer, (B) a partially pyrolized layer, and (C) fully crystallized film. The row part of the figure illustrates the accompanying thickness reduction from 114 to 15 nm. The lower row represents the XRR data with the critical angles Θc1 for the film indicating the surface density and Θc2 for the platinum substrate, which is constant for all conditions [19].

lead to a dissolution of the first layer in the subsequent spin-on process and make a film thickness enlargement impossible. Higher temperatures cause a considerable reduction of the remanent polarization. It is assumed that the presence of the organic part prevents the growth of larger inhomogeneously composed clusters. This in turn might help to maintain a solution like homogeneity of the metallo-organic precursors in the dried or only partially pyrolyzed films up to higher temperatures, where sufficient energy is available for the crystallization process. Thus, more homogeneously composed ferroelectric films with superior properties result. The dependence of the crystallization temperature Th2 on the phase formation is shown by the grazing incidence x-ray diffraction (GI-XRD) pattern in Fig. 3.4.5, which makes obvious the fundamental problem of the identification of the ferroelectric orthorhombic phase. Generally, as the monoclinic and a higher symmetric cubic/tetragonal phase commonly coexist, the discrimination of an orthorhombic phase from the cubic/tetragonal phase is difficult [26, 27]. For CSD-prepared films, it is characteristic

Dopants in Chemical Solution-Deposited HfO2 Films

135

(B)

(A)

(C)

Fig. 3.4.5 Influence of the annealing temperature of 5.2% doped HfO2 on (A) the crystal structure measured with GIXRD, (B) corresponding P(V ) hysteresis loops, and (C) related I(V ) curves [21].

that a strong cubic phase is present as long as the crystallization temperatures is between 600 and 800°C. In case of crystallization at 900°C, some reflections may be attributed to the orthorhombic phase coexisting with the still dominant cubic phase. The trend to a more pronounced orthorhombic phase formation with increasing temperature is backed up by hysteresis measurements. The hysteresis for the 600°C crystallization probably suffers from high leakage currents due to carbon residuals in the layer. For higher temperatures the leakage current is strongly reduced and an enhancement of the remanent polarization is found. For a crystallization at 800°C the best electrical results are observed. At 900°C, where the highest orthorhombic fraction is given, electrical results are not available due to an accelerated breakdown behavior during the wake-up procedure.

3.4.3 Properties of CSD-Prepared Films 3.4.3.1 Effect of Dopants Hafnia or zirconia films as well as mixtures of both have a characteristic doping or composition window in which ferroelectricity is present. Fig. 3.4.6A and B illustrates this feature by the measured hysteresis loops of a series of HfO2 films doped with varying yttrium concentrations. Pure HfO2 behaves

136

Polarization (µC/cm²)

30 15 0 –15 –30 –15 –10 –5

(A)

6% 7.5% 11%

0-% 3.75-% 5-%

0

5

Voltage (V)

10

15 –15 –10 –5

(B)

0

5

Voltage (V)

10

Remanent polarization (µC/cm²)

Ferroelectricity in Doped Hafnium Oxide

20 ALD 15

PVD CSD

10 5 0 0

15

(C)

5 10 15 Yttrium content (mol %)

Fig. 3.4.6 Dependence of the ferroelectric hysteresis on the yttrium content for (A) 0%–5%, (B) 6%–11%, and (C) remanent polarization as a function of the yttrium concentration for different deposition techniques. (Data taken from J. Mueller, U. Schroeder, T.S. Boescke, I. Mueller, U. Boettger, L. Wilde, J. Sundqvist, M. Lemberger, P. Kuecher, T. Mikolajick, L. Frey, Ferroelectricity in yttrium-doped hafnium oxide, Appl. Phys. Lett. 110 (2011) 114113/1–5 (ALD) and T. Olsen, U. Schroeder, S. Mueller, A. Krause, D. Martin, A. Singh, J. Mueller, M. Geidel, T. Mikolajick, Co-sputtering yttrium into hafnium oxide thin films to produce ferroelectric properties, Appl. Phys. Lett. 101 (2012) 082905/1–4 (PVD).)

as a dielectric; a ferroelectric hysteresis is missing. With increasing yttrium content, the remanent polarization increases because a maximum is reached for a yttrium concentration of 5.2%. Further rise of the yttrium content leads to a reduction of the remanent polarization; at 11% the ferroelectric properties vanish. The reason for this dependence of the doping concentration on the hysteresis is not fully understood yet. A similar behavior is also observed for other dopants such as rare earth metals, boron group elements, or alkaline earth metals that may differ among others in size, valence state, chemical properties, and/or electronegativity, and in the ferroelectric behavior. The variety is exemplarily depicted by the hysteresis curves in Fig. 3.4.7A and B. Each P(V )-curve represents the ferroelectric response of a doped HfO2 film with that concentration evoking the highest remanent polarization for a particular doping element. In the case of the alkaline earth metals, it was found at 7.5 mol%, whereas for all other dopants the maximum of Pr is given at 5.2 mol%. In Fig. 3.4.7C, the plot of the remanent polarization versus the dopants’ ionic radii [28] in a broad range from 54 pm (Al) to 135 pm (Ba) shows a clear dependence on the ferroelectric behavior for CSD-prepared films. Dopants larger than approximately 85 pm induce almost four times more pronounced ferroelectricity than smaller ones. Further size increase seems to result in a slight reduction of the remanent polarization.

Polarization (µC/cm2)

Dopants in Chemical Solution-Deposited HfO2 Films

20

(A)

(B)

10 0 Er La Ba Sr

–10 –20 2 –2 0 Electrical field (MV/cm)

Remanent polarization (µC/cm2)

137

15

Al Ga Mg In

2 –2 0 Electrical field (MV/cm)

(C)

10

Y and rare earth metals

Hf

5

0

Al

Ga Co Mg In Er Y Nd Sm La 60

80

100 Ionic radius (pm)

Sr 120

Ba 140

Fig. 3.4.7 Ferroelectric hysteresis of hafnium oxide with various dopants having ionic radii (A) larger than 85 pm, and (B) smaller than 85 pm. The curves are taken at 1 kHz after 1000 wake-up cycles. The chosen concentration for the specific doping element is the one that evokes the highest remanent polarization: 5.2% for Y, Co, the rare earth elements (Er, Nd, Sm, La) and the boron group elements (Al, Ga, In); and 7.5% for the alkaline earth metals (Mg, Sr, Ba). (C) Remanent polarization versus ionic radius for different dopants, whereby the coordination number of VI is used for all elements for the ionic radius [19, 31].

The size influence of different dopants was discussed earlier by Schroeder, concluding for small dopants an antiferroelectric-like behavior due to a pinched hysteresis, stable even after wake-up cycling [29]. Later, a fieldinduced phase transition from tetragonal to orthorhombic phase was suggested [30]; see also Chapter 3.1.

3.4.3.2 Processing Influence As already mentioned in the previous chapters, the observation of the ferroelectric phase for doped hafnium oxide is not restricted to film fabrication by the CSD technique. The same behavior was found using ALD [32],

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Ferroelectricity in Doped Hafnium Oxide

see Chapter 3.1; sputter deposition [33], see Chapter 3.2; pulsed layer deposition [34]; or chemical vapor deposition (for Hf1xZrxO2) [35]. It can be concluded that the stabilization of the ferroelectric phase is based on “intrinsic” properties and not caused by a specific preparation technique. For CSD processing and ALD preparation (including a postdeposition anneal [32]), the positions of the doping window and the composition with the maximal remanent polarization for Y-doped HfO2 are identical. Layers resulting from sputtering (PVD) show a shift of the doping window and the Pr maximum to lower yttrium contents [33]. It is still under discussion whether the higher kinetic energy transfer during sputtering is responsible. This may result in a different concentration of oxygen vacancies playing an important role for the ferroelectric behavior [36]. Fig. 3.4.6C summarizes the comparison between three different deposition techniques for yttrium-doped HfO2. The statements made so far about the simple dependence of dopant size and the independence of processing/deposition technique cannot be generalized to any case. For example, ALD preparation of Al-doped HfO2 films leads to a remanent polarization of 6 μC/cm2 [37], which is twice the value found by CSD. An enhancement to Pr  20μC/cm2 for the ALD films is even possible only by process optimization [38]. Doping with silicon having an ionic radius of 39 pm (unfortunately not available for CSD processing due to missing suitable precursor solutions) is able to evoke a remanent polarization above 20 μC/cm2 at a thickness of 9 nm because the crystallization temperature is 1000°C [39]. The stabilization of the ferroelectricity is favored by the encapsulation effect of the top electrode during crystallization [40]. High-temperature crystallization and/or top electrode encapsulation are not applicable by CSD, as explained in Section 3.4.2.4. Otherwise, it could not be excluded that higher polarization values, even by CSD, are to be expected, particularly for small-size dopants.

3.4.3.3 Thickness Dependence The ferroelectric properties of yttrium-doped hafnia films prepared by CSD do not significantly vary under a thickness change from 29 to 66 nm, as illustrated in Fig. 3.4.8A [21, 31]. This is in contrast to layers deposited by ALD despite the very similar behavior to the doping concentration as shown in Fig. 3.4.6C and may be caused by three reasons: (i) the maximum precrystallization temperature of CSD is lower (215°C) than that of ALD (300°C) [39], (ii) before crystallization, CSD suffers from carbon residuals leading to a

Dopants in Chemical Solution-Deposited HfO2 Films

(A) Y-doped HfO2

30 Polarization (µC/cm2)

Polarization (µC/cm2)

30 15 0 –15 –30

(C) 100 nm Hf1-xZrxO2

15 0 –15 –30

–2 2 0 Electrical field (MV/cm)

(B) 44 nm Hf1-xZrxO2

139

100% 95% 90% 80% 50%

–2 2 0 Electrical field (MV/cm)

–2 2 0 Electrical field (MV/cm)

Fig. 3.4.8 Influence of the film thickness and composition on the ferroelectric properties of (A) a Y-doped HfO2 film, (B) a Hf1xZrxO2 film with a thickness of 44 nm, and (C) a Hf1xZrxO2 film with a thickness of 100 nm. (Redrawn from S. Starschich, T. Schenk, U. Schroeder, U. Boettger, Ferroelectric and piezoelectric properties of Hf1xZrxO2 and pure ZrO2 films, Appl. Phys. Lett. 110 (2017) 182905/1–5; S. Starschich, Ferroelectric, Pyroelectric and Piezoelectric Effects of Hafnia and Zirconia Based Thin Films (Ph.D. thesis), RWTH Aachen, Germany, 2017.)

crystallization shrinking much stronger than in amorphous ALD layers, or (iii) the grain size for different thicknesses is similar (see Chapter 3.5). Thin films with compositions Hf1xZrxO2 show a thickness dependence of the remanent polarization whose trend is not so obvious at first glance. In Fig. 3.4.8B and C, the ferroelectric hysteresis of CSD-prepared layers for different hafnium contents and two different thicknesses of 44 and 100 nm, respectively, are depicted. For the thicker layer a maximum remanent polarization of 12.5 μC/cm2 is observed for pure ZrO2, and almost no remanent polarization is found for 80% zirconium content. In the case of the 44 nm thick film a maximum remanent polarization of 15 μC/cm2 is found for 95% zirconium content and a remanent polarization of 6 μC/cm2 is still present for 20% Zr. A very suitable tool to explain the thickness dependence of columnar ALD films is based on the related variation of the film microstructure resulting in specific surface energies due to the surface to volume ratio A/V of each grain. A detailed discussion is given in Chapter 3.5. Following the free energy model of Materlik et al. [30] such energy terms may favor the stabilization of the ferroelectric phase Pca21 under distinct circumstances in the way that thinner films need Hf-richer compositions and thicker films higher Zr contents; see also Fig. 3.5.10. Due to the granular structure of CSD-prepared layers that is illustrated in Fig. 3.5.8A, the grain size controlling by thickness variation is clearly reduced. The mean grain radius is determined to be 10 nm for 390 nm thick film and about 5 nm for thinner films—in contrast to the strong dependence

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Ferroelectricity in Doped Hafnium Oxide

in case of ALD processing; see Fig. 3.5.5B. A simple transfer to the abovementioned free energy model is not possible. Nevertheless, there are (weak) trends for increasing the film thickness: reduction of grain size, reduction of the m-phase from 20% to 16%, accompanied with a reduction of the remanent polarization from 12 to 9 μC/cm2; see Figs. 3.5.8B and 3.5.9A. Samples prepared by CSD generally show a much more ZrO2-side shifted ferroelectric performance in the HfO2-ZrO2 compared with other techniques. Possible reasons for the origin of this shift are the relatively low precrystallization temperature of 215°C, the smaller grain size, the existence of carbon residuals before crystallization as well as a different influence on the crystal growth by the used Pt bottom electrodes. Even 390 nm thick pure ZrO2 films have pronounced ferroelectric as well as piezoelectric properties with a remanent polarization Pr ¼ 9μC/cm2 and estimated small signal coefficients d33  25 pm/V at the initial state or d33  12 pm/V after 103 cycles, respectively [31]. It should be mentioned that “thick” films are needed for some applications of piezoelectric devices; however, they suffer from high operation voltages in order to guarantee sufficient high electrical fields.

3.4.4 Conclusion In this contribution the CSD is presented, in general, as a technique for flexible, high-quality, and cost-saving thin film production, and, in particular, as a suitable tool for the fabrication of hafnia- and/or zirconia-based ferroelectric layers. Two equivalent methods are discussed, whereby the MOD approach offers some practical benefits over the hybrid concept. The MOD technique is applicable in a wide thickness range between 14 and 390 nm, which is definitely significantly larger than a layer thickness of 50 nm achieved by ALD. A material and concentration screening reveals that numerous dopants induce the stabilization of the ferroelectric phase in hafnium oxide for a specific concentration window. Thereby, the ionic radius of the dopant has a strong influence on the strength of ferroelectricity. Larger ions cause higher polarization than smaller ones. The origin of the concentration window is not yet understood. In contrast to doped hafnia, the film thickness influences the formation of the ferroelectric phase in hafnia-zirconia mixtures. With increasing the layer thickness, the maximum remanent polarization is found for zirconiumricher compounds. The application of a free energy model including the

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surface energy of individual grains enables only statements about trends of the phase formation and the electrical behavior, respectively. No such dependence on the film thickness, and the grain size, respectively, was found for doped HfO2. Therefore, it can be assumed that the ferroelectric phase stabilization originates from significant energy contribution by the dopant. Further parameters such as chemical energy from dopants and oxygen vacancies need to be taken into account as well to understand the stabilization of the ferroelectric phase in doped hafnium oxide.

Acknowledgments This work was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft) within the scope of the project “Inferox” (Project No. BO 1629/10-1).

References [1] T. Schneller, R. Waser, M. Kosec, D. Payne, Deposition of Functional Oxide Thin Films, Springer, New York, NY, 2013. [2] K.D. Budd, S.K. Dey, D.A. Payne, Sol-gel processing of PbTiO3, PbZrO3, PZT, and PLZT thin films, Br. Ceram. Proc. 36 (1985) 107–121. [3] H. Dislich, Sol-gel: science, processes and products, J. Non-Cryst. Solids 80 (1–3) (1986) 115–121. [4] R.W. Vest, Metallo-organic decomposition (MOD) processing of ferroelectric and electro-optic films: a review, Ferroelectrics 102 (1990) 53–68. [5] T.P. Niesen, M.R.D. Guire, Review: deposition of ceramic thin films at low temperatures from aqueous solutions, J. Electroceram. 6 (2001) 169–207. [6] G. Hodes, Semiconductor and ceramic nanoparticle films deposited by chemical bath deposition, Phys. Chem. Chem. Phys. 9 (2007) 2181–2196. [7] T. Schneller, R. Waser, Chemical solution deposition of ferroelectric thin films—state of the art and recent trends, Ferroelectrics 267 (2002) 293–301. [8] M.D. Losego, J.F. Ihlefeld, J. Maria, Importance of solution chemistry in preparing solgel PZT thin films directly on copper surfaces, Chem. Mater. 20 (1) (2008) 303–307. [9] B. Malic, M. Kosec, I. Arcon, A. Kodre, Homogeneity issues in chemical solution deposition of Pb(Zr,Ti)O3 thin films, J. Eur. Ceram. Soc. 25 (12) (2005) 2241–2246. [10] R.W. Schwartz, T. Schneller, R. Waser, Chemical solution deposition of electronic oxide films, C. R. Chim. 7 (2004) 433–461. [11] R.W. Schwartz, Chemical solution deposition of perovskite thin films, Chem. Mater. 1 (1997) 2325–2340. [12] M. Klee, R. Eusemann, R. Waser, W. Brand, H. van Hal, Processing and electrical properties of Pb (ZrxTi1x)O3 (x ¼ 0.2–0.75) films: comparison of metallo-organic decomposition and sol-gel processes, J. Appl. Phys. 72 (4) (1992) 1566–1576. [13] P. Muralt, Recent progress in materials issues for piezoelectric MEMS, J. Am. Ceram. Soc. 91 (5) (2008) 1385–1396. [14] I.V. Driessche, J. Feys, S.C. Hopkins, P. Lommens, X. Granados, B.A. Glowacki, S. Ricart, B. Holzapfel, M. Vilardell, A. Kirchner, M. Baecker, Chemical solution deposition using ink-jet printing for YBCO coated conductors, Supercond. Sci. Technol. 25 (2012) 065017.

142

Ferroelectricity in Doped Hafnium Oxide

[15] X. Obradors, T. Puig, A. Pomar, F. Sandiumenge, N. Mestres, M. Coll, A. Cavallaro, N. Roma, J. Gazquez, J.C. Gonzalez, O. Castano, J. Gutierrez, A. Palau, K. Zalamova, S. Morlens, A. Hassini, M. Gibert, S. Ricart, J.M. Moreto, S. Pinol, D. Isfort, J. Bock, Progress towards all-chemical superconducting YBa2Cu3O7-coated conductors, Supercond. Sci. Tech. 19 (3) (2006) S13–S26. [16] G.J. Exarhos, X. Zhou, Discovery-based design of transparent conducting oxide films, Thin Solid Films 515 (2007) 7025–7052. [17] M. Biswas, P.C. Su, Chemical solution deposition technique of thin-film ceramic electrolytes for solid oxide fuel cells, in: N.N. Nikitenkov (Ed.), Modern Technologies for Creating the Thin-Film Systems and Coatings, InTech, Rijeka, 2017, pp. 319–343. [18] S. Starschich, D. Griesche, T. Schneller, R. Waser, U. Boettger, Chemical solution deposition of ferroelectric yttrium-doped hafnium oxide films on platinum electrodes, Appl. Phys. Lett. 104 (2014) 202903/1–4. [19] S. Starschich, D. Griesche, T. Schneller, U. B€ ottger, Chemical solution deposition of ferroelectric hafnium oxide for future lead free ferroelectric devices, ECS J. Solid State Sci. Technol. 4 (2015) 419–423. [20] S. Starschich, T. Schenk, U. Schroeder, U. Boettger, Ferroelectric and piezoelectric properties of Hf1xZrxO2 and pure ZrO2 films, Appl. Phys. Lett. 110 (2017) 182905/1–5. [21] S. Starschich, Ferroelectric, Pyroelectric and Piezoelectric Effects of Hafnia and Zirconia Based Thin Films (Ph.D. thesis), RWTH Aachen, Germany, 2017. [22] A.W. Coats, J.P. Redfern, Thermogravimetric analysis. A review, Analyst 88 (1963) 906–924. [23] S. Petit, S. Morlens, Z. Yu, D. Luneau, G. Pilet, J. Soubeyroux, P. Odier, Synthesis and thermal decomposition of a novel zirconium acetato-propionate cluster: [Zr12], Sol. Stat. Sci. 13 (3) (2011) 665–670. [24] K. Knoth, R. H€ uhne, S. Oswald, L. Schultz, B. Holzapfel, Detailed investigations on La2Zr2O7 buffer layers for YBCO-coated conductors prepared by chemical solution deposition, Acta Mater. 55 (2) (2007) 517–529. [25] Z. Yu, P. Odier, L. Ortega, L. Zhou, P. Zhang, A. Girard, La2Zr2O7 films on Cu-Ni alloy by chemical solution deposition process, Mater. Sci. Eng. B 130 (2006) 126–131. [26] T.S. Boescke, J. Mueller, D. Braeuhaus, U. Schroeder, U. Boettger, Ferroelectricity in hafnium oxide thin films, Appl. Phys. Lett. 99 (10) (2011) 102903/1–3. [27] M. Hoffmann, U. Schroeder, T. Schenk, T. Shimizu, H. Funakubo, O. Sakata, D. Pohl, M. Drescher, C. Adelmann, R. Materlik, A. Kersch, T. Mikolajick, Stabilizing the ferroelectric phase in doped hafnium oxide, J. Appl. Phys. 118 (2015) 72006/1–9. [28] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (1976) 751–767. [29] U. Schroeder, E. Yurchuk, J. Muller, D. Martin, T. Schenk, P. Polakowski, C. Adelmann, M.I. Popovici, S.V. Kalinin, T. Mikolajick, Impact of different dopants on the switching properties of ferroelectric hafniumoxide, Jpn. J. Appl. Phys. 53 (8S1) (2014) 08LE02/1–5. [30] R. Materlik, C. K€ unneth, A. Kersch, The origin of ferroelectricity in Hf1-xZrxO2: a computational investigation and a surface energy model, J. Appl. Phys. 117 (2015) 134109/1–15. [31] S. Starschich, U. Boettger, An extensive study of the influence of dopants on the ferroelectric properties of HfO2, J. Mater. Chem. C 5 (2017) 333–338. [32] J. Mueller, U. Schroeder, T.S. Boescke, I. Mueller, U. Boettger, L. Wilde, J. Sundqvist, M. Lemberger, P. Kuecher, T. Mikolajick, L. Frey, Ferroelectricity in yttrium-doped hafnium oxide, Appl. Phys. Lett. 110 (2011) 114113/1–5.

Dopants in Chemical Solution-Deposited HfO2 Films

143

[33] T. Olsen, U. Schroeder, S. Mueller, A. Krause, D. Martin, A. Singh, J. Mueller, M. Geidel, T. Mikolajick, Co-sputtering yttrium into hafnium oxide thin films to produce ferroelectric properties, Appl. Phys. Lett. 101 (2012) 082905/1–4. [34] T. Shimizu, K. Katayama, T. Kiguchi, A. Akama, T.J. Konno, H. Funakubo, Growth of epitaxial orthorhombic YO1.5-substituted HfO2 thin film, Appl. Phys. Lett. 107 (3) (2015) 32910/1–5. [35] T. Shimizu, T. Yokouchi, T. Shiraishi, T. Oikawa, P.S. Sankara Rama Krishnan, H. Funakubo, Study on the effect of heat treatment conditions on metalorganicchemical-vapor-deposited ferroelectric Hf0.5Zr0.5O2 thin film on Ir electrode, Jpn. J. Appl. Phys. 53 (2014) 9PA04/1–6. [36] S. Starschich, S. Menzel, U. B€ ottger, Evidence for oxygen vacancies movement during wake-up in ferroelectric hafnium oxide, Appl. Phys. Lett. 108 (2016) 032903/1–5. [37] S. Mueller, J. Mueller, A. Singh, S. Riedel, J. Sundqvist, U. Schroeder, T. Mikolajick, Incipient ferroelectricity in Al-doped HfO2 thin films, Adv. Funct. Mater. 22 (11) (2012) 2412–2417. [38] P. Polakowski, S. Riedel, W. Weinreich, M. Rudolf, J. Sundqvist, K. Seidel, J. Mueller, Ferroelectric deep trench capacitors based on Al:HfO2 for 3D nonvolatile memory Applications, IEEE 6th International Memory Workshop (IMW), 2014, pp. 1–4. [39] E. Yurchuk, J. Mueller, S. Knebel, J. Sundqvist, A.P. Graham, T. Melde, U. Schroeder, T. Mikolajick, Impact of layer thickness on the ferroelectric behaviour of silicon doped hafnium oxide thin films, Thin Solid Films 533 (2013) 88–92. [40] T.S. Boescke, S. Teichert, D. Braeuhaus, J. Mueller, U. Schroeder, U. Boettger, T. Mikolajick, Phase transitions in ferroelectric silicon doped hafnium oxide, Appl. Phys. Lett. 99 (2011) 112904/1–3.