# Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO<sub>2</sub> on Anatase TiO<sub>2</sub> (101)

Giulio D'acunto, Rosemary Jones, Lucía Pérez Ramírez, Payam Shayesteh, Esko Kokkonen, Foqia Rehman, Florence Lim, Fabrice Bournel, Jean-Jacques Gallet, Rainer Timm, et al.

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Giulio D'acunto, Rosemary Jones, Lucía Pérez Ramírez, Payam Shayesteh, Esko Kokkonen, et al. Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO<sub>2</sub> on Anatase TiO<sub>2</sub> (101). Journal of Physical Chemistry C, 2022, 126 (29), pp.12210-12221. 10.1021/acs.jpcc.2c02683 . hal-03884304

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# Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO₂ on Anatase TiO₂(101)

Giulio D'Acunto, Rosemary Jones, Lucía Pérez Ramírez, Payam Shayesteh, Esko Kokkonen, Foqia Rehman, Florence Lim, Fabrice Bournel, Jean-Jacques Gallet, Rainer Timm, and Joachim Schnadt*

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Cite This: J. Phys. Chem. C 2022, 126, 12210-12221

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## ACCESS

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Supporting Information

**ABSTRACT:** The atomic layer deposition of HfO₂ on a TiO₂(101) surface from tetrakis(dimethylamido)hafnium and water is investigated using a combination of *in situ* vacuum X-ray photoelectron spectroscopy (XPS) and time-resolved ambient pressure XPS. Precursor pressures and surface temperature are tuned as to map the space state of the deposition. In the initial stages of ALD, a reaction mechanism based on dissociative adsorption dominates over a classic ligand exchange mechanism, typically evoked when metal-amido complexes and water are used as the precursors for metal oxide ALD. Surface species, including a dimethyl ammonium ion and an imine, are identified. It is found that they can be formed only if the active role of the TiO₂(101) surface is taken into consideration. The temperature of the surface

enhances the formation of these species based on an insertion reaction of a hydrogen atom, which then assists the formation of more than the expected monolayer of HfO₂. A HfO₂ overlayer is produced already during the first half-cycle, enabled by a reduction of the TiO₂ support. Dosing water at high pressure allows hydroxyl formation, which marks the transition toward a well-described ligand exchange reaction type. From the experiments performed, we find that the ALD of HfO₂ at room temperature, performed at high pressure, is mainly based on dissociation and that no side reaction occurs. These insights into the ALD reaction mechanism highlight how *in situ* studies can help understand how deposition parameters affect the growth of HfO₂ and how the ALD model for transition metal oxide formation from amido complexes and water can be extended.

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## INTRODUCTION

The high demand for ever-smaller electronic devices,¹ with reduced consumption and superior performance, has shifted attention from silicon-based gate materials in metal-oxide semiconductor field-effect transistors (MOSFETs) to new materials and material combinations.²⁻⁴ A particularly important role is played by high-κ dielectric materials,¹,⁵ which, to varying degrees, allow combining small dimensions with the required high gate-dielectric capacitance, low dielectric loss, and low leakage current. A traditional high-κ material is TiO₂, which exhibits a relative dielectric permittivity of around 100. The relatively high permittivity comes, however, along with a fairly low bandgap and a tendency toward bulk defect formation. Both effects contribute to undesirably high leakage currents. In comparison to TiO₂, HfO₂ has a larger bandgap and greater thermodynamic stability,⁶ and therefore, it exhibits considerably lower leakage currents. Its disadvantage is its limited relative dielectric permittivity of around 25. Thus, while HfO₂ has properties that make it the preferred choice over TiO₂ as a high-κ dielectric, the development of further improved HfO₂ materials that

combine the high relative permittivity of TiO₂ with the low leakage current of HfO₂ would be highly desirable.

Appropriate combinations of high-κ with high-bandgap materials can provide an avenue toward improved gate oxides: for example, a 20.5 nm-thick TiO₂/Al₂O₃/TiO₂ trilayer has been reported to exhibit a relative dielectric permittivity of 44 at a leakage current of 6.5 × 10⁻⁷ A/cm² under an electric field of 1 MV/cm.⁷ Likewise, combining layers of HfO₂ and TiO₂ has the potential of significantly increasing the relative permittivity of the layered compound in comparison to that of pure HfO₂ while efficiently suppressing leakage currents.⁸,⁹ Remarkably, “giant” relative dielectric permittivities of several hundred at low leakage currents can be achieved for multi-layered nanolaminates of both the ¹⁰ Al₂O₃/TiO₂ and ⁵ HfO₂/TiO₂ material combinations.

**Received:** April 19, 2022

**Revised:** June 6, 2022

**Published:** July 14, 2022

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One of the most prominent methods for producing such improved layered materials is atomic layer deposition (ALD).¹¹,¹² Conceptually, the inherent saturative surface chemistry of ALD entails atomic-scale control of the deposition process,¹³ which renders ALD highly suitable for growing nanolayered materials.¹¹⁻¹³ However, in spite of extensive previous work (see, e.g., refs 14–16), the kinetics and surface reaction mechanisms of ALD are still many times not clarified, and generalized ALD schemes are invoked many times rather than specific, verified reaction pathways.¹¹,¹² This means that it is difficult to properly design ALD processes that guarantee interface sharpness and layer uniformity.

In the ALD of HfO₂ and of TiO₂, water not only is often employed as the oxygen source,¹³ but also, oxygen, ozone, and oxygen plasma are frequently used. When metal amide complexes and water are used, the ALD surface chemical reaction is typically assumed to be based on a ligand exchange reaction mechanism¹¹,¹⁷⁻¹⁹ with the general formula AL(a) + BL(b) → AL(b) + BL(a), where, in the present case, A is the metal ion of the metal precursor, B is a surface site, and L(a) and L(b) are the ligands. In more detail, for a HfO₂ ALD process, the two half-cycles are assumed to proceed according to the following (or a similar) scheme²⁰

$$ \text{metal half-cycle: HfL}_4 + n \parallel -(\text{OH}) \\ \rightarrow \parallel -O_n - \text{HfL}_{4-n} + n\text{LH} $$

$$ \text{water half-cycle: } \parallel -O_n - \text{HfL}_{4-n} + (4-n) H_2O \\ \rightarrow \parallel -O_n - \text{Hf} - (\text{OH})_{(4-n)} + (4-n) \text{LH} $$

$$ \text{overall reaction per cycle: HfL}_4 + n \parallel -(\text{OH}) + (4-n) $$

$$ H_2O \rightarrow -O_n - \text{Hf} - (\text{OH})_{(4-n)} + 4\text{LH} \quad (1) $$

Here, || represents the surface, but the surface reaction site is made explicit by writing out the surface oxygen atoms and surface hydroxyls. In terms of the general formula for a ligand exchange reaction stated above, A = Hf, L(a) = L, B = ||-O, and L(b) = H. This reaction, however, does not consider the role of the substrate in the initial deposition steps.

We have recently demonstrated that the support in fact may play a highly active role in the ALD chemistry, which goes far beyond the more static, catalyst-like role in eq 1 above. Specifically, for the case of HfO₂ ALD on oxidized InAs from the same precursors as discussed, here, we have found that the support provides oxygen to the surface already in the first ALD half-cycle, before the introduction of the second precursor, and that HfO₂ is formed already then. This active role of the support is behind the so-called self-cleaning process observed for the native oxides on III-V semiconductor surfaces.²¹⁻²⁴ While the effect is well known and documented, only few experimental and theoretical results have recognized the importance of this process on the formation of the first ALD layers.²⁴ We have also shown that the surface chemical reaction of the initial oxide ALD may follow a completely different pathway from that in eq 1. For the initial ALD of HfO₂ on SiO₂ from tetrakis(dimethylamido)hafnium (TDMA-Hf) and water, we have observed a H transfer reaction mechanism between the ligands of two neighboring TDMA-Hf molecules during the first half-cycle, instead of a ligand exchange reaction with the surface.²⁵

Here, we provide new insight into the surface chemistry of the initial ALD of HfO₂ on the anatase TiO₂(101) surface from TDMA-Hf and water. In situ X-ray photoelectron spectroscopy (XPS), which in this context implies that the X-ray photoelectron (XP) spectra are recorded in vacuum before and after the ALD in the same setup in which the ALD is carried out and thus without breaking the vacuum, and time-resolved ambient pressure XPS (APXPS) recorded during ALD are found to be valuable techniques to study ALD processes on several surfaces and using different precursors.²¹,²²,²⁶,²⁷ Here, we show how the experimental parameters (temperature and pressure) affect the surface chemical reaction. A dissociative adsorption mechanism, based on the splitting-off of one or more DMA⁻ ligands from the TDMA-Hf complex, appears to be the dominating surface chemical process in the early stage of the first ALD half-cycle, independently of the experimental parameters used during the deposition. An exception is the ALD performed at room temperature at 10⁻⁶ mbar precursor pressure, for which no metal deposition is detected on the surface. In all other conditions, the surface chemical reaction implies an interaction between ligands of the same or several molecules on the surface and the surface itself, which leads to the formation of several other surface species. A migration of oxygen atoms from the TiO₂ bulk toward the surface is detected, which allows the formation of HfO₂ already in the first (metal) half-cycle; hence, the surface plays a highly active role in the ALD reaction, which is a stark contrast to the hypothesized static role in general ALD schemes. The reaction with water follows a more direct mechanism based on the removal of surface species. At sufficiently high water pressure, a hydroxyl monolayer results from the reaction with water, enabling further ALD cycles to follow, presumably according to the standard ligand exchange reaction mechanism.

## METHODS

The experiments were performed at the NAPXPS end station of Sorbonne Université, Paris, on the TEMPO beamline at the SOLEIL synchrotron facility in Paris, France, and at the HIPPIE beamline of the MAX IV Laboratory synchrotron radiation facility in Lund, Sweden.²⁸ We performed four distinct ALD experiments at two different substrate temperatures (300 and 500 K) and in two different precursor pressure regimes (10⁻⁶ and 10⁻² to 10⁻¹ mbar). We used TDMA-Hf as the metal and water as the oxygen precursor for hafnium oxide ALD. In all experiments, the TDMA-Hf-containing cylinder was held at a temperature of about 350 K to increase the metal precursor vapor pressure to around 1 mbar.²⁹ The experiment performed at 300 K and 10⁻² to 10⁻¹ mbar was conducted at the HIPPIE beamline; all other experiments were conducted at the TEMPO beamline.

It should be noted that TDMA-Hf containers typically contain some dimethyl amine (DMA) in addition to the metal complex. DMA can also be produced in a hydrolysis reaction of TDMA-Hf with residual water in the pipes of the experiment.³⁰ The DMA enters the experimental region together with the TDMA-Hf and is observed in the gas phase components of the ambient pressure X-ray photoelectron (APXP) spectra.³¹

An anatase TiO₂(101) single crystal was cleaned by several cycles of Ar⁺ sputtering (1.5 kV, 1.0 × 10⁻⁵ mbar Ar pressure, 10 mA emission current, and 15 min duration) and annealing at 650–850 K for 20 min. The cleanness of the TiO₂ crystal
<figure><img src="image_9.png" /><figcaption>**Figure 1.** *In situ* O 1s, Ti 2p, N 1s, and Hf 4f XP spectra collected in the (a1—a4) HT-HP, (b1—b4) HT-LP, (c1—c4) LT-HP, and (d1—d4) LT-LP experiments. “LT” designates a “low temperature” of 300 K and “HT” a “high temperature” of 500 K; “LP” refers to a “low pressure” of 10⁻⁶ mbar and “HP” to a “high pressure” of 10⁻¹ to 10⁻² mbar.</figcaption></figure>

was ensured by performing sputtering/annealing cycles until no or only a weak C 1s signal was visible in the XP spectra.

The core-level spectra of interest were collected in ultrahigh vacuum in an *in situ* fashion after the metal and water ALD half-cycles. Hf 4f, C 1s, N 1s, Ti 2p, and O 1s spectra were recorded at 650 eV photon energy, with the exception of the experiment at 500 K/10⁻² to 10⁻¹ mbar, in which we used photon energies of 400 eV for measurement of the Hf 4f and C 1s, 500 eV for the N 1s, 580 eV for the Ti 2p, and 650 eV for the O 1s core level. The overall experimental resolution was estimated to be 100 meV for all the experiments except for the experiment at 500 K/10⁻² to 10⁻¹ mbar, for which the resolution is estimated to be around 200 meV. To be able to directly compare the results from these latter experiments to the others, the intensities of the individual spectra obtained at

500 K/10⁻² to 10⁻¹ mbar were normalized to a survey spectrum acquired at 650 eV. All spectra were calibrated to the Fermi edge of a gold foil mounted in good electrical contact with the sample. In all experiments, the sample was raster-scanned while acquiring the X-ray photoelectron (XP) spectra to prevent the occurrence of spectral features related to beam damage or X-ray induced deposition.

## RESULTS

We will present the results from each of the experiments independently to later discuss how the chosen parameters—temperature and pressure—influence the ALD surface chemistry. “LT” will be used to designate a “low temperature” of 300 K and “HT” a “high temperature” of 500 K; “LP” will be
used to refer to a “low pressure” regime of 10⁻⁶ mbar and “HP”
the “high pressure” one at 10⁻¹ to 10⁻² mbar.

**HT-HP.** The XP spectra acquired in vacuum after the first TDMA-Hf and after the first water half-cycles are shown in Figure 1a1–a4. In black, we report the surface spectra recorded before the deposition, in red, those taken after the TDMA-Hf, and in blue, those measured after the water pulse. The surface prior to the deposition is characterized by a Ti 2p₃/₂ line with the position of maximum intensity at 459.2 ± 0.1 eV binding energy and an O 1s line at 530.5 ± 0.1 eV (Figure 1a1,a2), in excellent agreement with previous studies.³² Moreover, from the intensities normalized to the subshell photoionization cross sections, we find a Ti:O ratio of 1:2, which indicates a pristine surface with the expected TiO₂ stoichiometry. A small shoulder toward low binding energy in the Ti 2p₃/₂ spectra, with an intensity of around 4% of the overall Ti 2p XP signal, indicates the presence of some minor defects, associated with Ti ions in the Ti²⁺ and Ti³⁺ oxidation states.³²⁻³⁵ Weak N 1s and C 1s signals are also visible in the spectra, which we assign to a slight surface contamination due residual gas adsorption after preceding ALD experiments in the experimental setup. No Hf signal is visible in the spectra.

**Metal Half-Cycle.** After the TDMA-Hf pulse, a shift of about +0.25 eV of the Ti 2p₃/₂ main line is observed as well as the appearance of a low-binding energy shoulder in the spectrum in Figure 1a2. The defects shoulder increases to around 6% of the overall Ti 2p core-level intensity. Further, in the O 1s spectra, normalized to the Ti 2p signal, we see an increase in intensity by +175% (Figure 1a1). Together, the apparent intensity increase in the O 1s core level and the growth of the lower-oxidation state shoulder in the Ti 2p core-level spectrum—indicative of a reduction of the TiO₂ support—strongly suggest oxygen transport to the surface, where the oxygen reacts with the Hf complex. Moreover, the O 1s peak shape becomes slightly more asymmetric toward high binding energy. This increase in asymmetry could either be due to the presence of surface hydroxyls already after the metal half-cycle or an increase in the intrinsic asymmetry of the O 1s line, the origin of which has not yet been clarified.³²

The N 1s and C 1s core-level spectra show the presence of both reacted and unreacted TDMA-Hf ligands on the TiO₂ surface (cf. Figure 2a for the chemical structure of TDMA-Hf). In the N 1s core-level spectrum (Figure 1a3), a dominant low-binding energy component with a binding energy of 398.6 ± 0.1 eV is visible together with a smaller one at around 396.7 ± 0.1 eV. We attribute these two components to the original amido ligand (DMA⁻, (N(CH₃)₂; Figure 2b) and to methyl-methylene imine (MMI, NCH₃CH₂; Figure 2c) of TDMA-Hf, respectively. The assignment agrees with our previous work²⁷ on the ALD of TiO₂ on RuO₂(110) from tetrakis-(dimethylamido) titanium and H₂O and that²⁵ of the ALD of HfO₂ on SiO₂ from TDMA-Hf and H₂O. The latter of these two works²⁵ as well as other previous work³⁶ shows that MMI can be formed either as a three-membered ring²⁵,³⁶ bonding to a single metal ion or, alternatively, as a bridge between two metal ions (X-CH₂-N(CH₃)-X'), where X and X' can be either Hf and/or Ti. This is depicted in Figure 2c. The overall C 1s:N 1s intensity ratio is around 2, in agreement with the stoichiometry of the ligands. The intensity ratio of the two N 1s components is around I<sub>DMA⁻</sub><sup>N 1s</sup> : I<sub>MMI</sub><sup>N 1s</sup> = 3: 7.

Given that N 1s spectrum suggests the presence of MMI
with both a methylene and a methyl group, the C 1s line is

<figure><img src="image_10.png" /><figcaption>**Figure 2.** Scheme of (a) TDMA-Hf, (b) DMA and DMA⁻, (c) MMI, which can be in 3MR configuration and in the bridge configuration between one or two metal complexes, and (d) DMA⁺.</figcaption></figure>

expected to exhibit a methyl component at around 286 eV binding energy and a smaller methylene component at around 285 eV. Hence, the C 1s line should be asymmetric toward low binding energy. The *in situ* C 1s spectrum, as shown in Figure S4 of the Supporting Information, does not agree with these expectations, neither in terms of its position nor in terms of the observed asymmetry, which is toward the high-binding energy side. A comparison of one of the C 1s APXP spectra acquired during the deposition and the *in situ* XP spectrum recorded after the deposition shows clearly that the lineshape has changed in between the two measurements. The C 1s APXP spectrum acquired during the deposition is, indeed, asymmetric toward low binding energy, and the components are found at the expected positions. We conclude that the adsorption of unwanted carbon species on the surface has taken place during the time in between the deposition and the acquisition of the *in situ* C 1s XP spectrum. This alters strongly the appearance of the C 1s spectrum, but it affects the other core levels to a much lesser degree. These additional surface carbon species result from the presence of residual gases in the vacuum chamber. The position of maximum intensity of the Hf 4f line at 17.8 ± 0.1 eV binding energy suggests that the Hf ions are already partially in the HfO₂ configuration.

Additional information is received from the time-resolved APXPS data acquired during the deposition. In Figure 3a, the O 1s, Ti 2p, N 1s, C 1s, and Hf 4f intensities from the HT-HP experiment, normalized by the corresponding subshell photoionization cross sections, are plotted as a function of exposure time. As soon as the metal complex reaches the surface, the signal from the surface (Ti 2p and O 1s lines) decreases, and the signal from the precursor (Hf 4f, N 1s, and C 1s lines) rises. However, at *t* = 6 min, just when the O 1s signal has reached its minimum, its intensity starts to grow with the same rate as the intensities of the N 1s, C 1s, and Hf 4f lines. At around *t* = 43 min, the reaction stops due to the evacuation of the analysis chamber; after that moment, all intensities remain constant, with the exception of that of the C 1s line. Likely, the decrease in the C 1s intensity is due to the partial desorption of C species; moreover, the C 1s spectrum measured in vacuum after the evacuation appears different in composition compared
<figure><img src="image_11.png" /><figcaption>**Figure 3.** (a) Intensities of various core levels during the TDMA-Hf half-cycle in the HT-HP experiment. (b) Fraction of DMA⁻ and MMI in the N 1s core level during the first 10 min of acquisition; in red, the intensity of the O 1s core level normalized to its initial intensity is shown. (c) *In situ* N 1s XP spectra measured at different times during the TDMA-Hf half-cycle.</figcaption></figure>

to the one acquired during the deposition, suggesting that other carbon contaminants reach the surface after a partial desorption of carbon species (see above; more details in Section S4 of the Supporting Information). In Figure 3b, the intensities of the MMI and DMA⁻ components in the N 1s core-level spectrum are shown during the first 12 min of data acquisition. The MMI starts to be the dominant component when the O 1s intensity, shown in red, is growing. Already when the intensity of the O 1s line is in its minimum, at around *t* = 6 min, more than 50% of the N 1s intensity is due to MMI. The development is also seen from Figure 3c, which shows the N 1s spectrum for different exposure times.

**Water Half-Cycle.** During the water pulse (blue spectra in Figure 1a1–a4), the Ti 2p core level shifts back to its original position, and a reoxidation of the surface is evident from the attenuation of the low-binding energy shoulder of the Ti 2p line. We find that its overall contribution is less than 2%. In the O 1s spectrum in Figure 1a1, a signal attributable to surface hydroxyls clearly appears as a shoulder to the main line at around 532.5 eV binding energy.³² The N 1s signal is strongly attenuated, as is seen from an intensity loss of 70% in comparison to the N 1s XP spectrum acquired after the metal half-cycle. Hence, as expected from basic models of the ALD surface chemistry, the water pulse removes the surface ligands and leaves behind an –OH-saturated surface. The presence of surface hydroxyls after the water cycle marks a clear transition toward a ligand exchange behavior of the surface chemical reaction.¹¹ However, two prominent peaks remain at around 396.5 and 399.3 eV binding energies. Although the positions of these two peaks are similar to those of the N 1s DMA⁻ and MMI components observed after the metal half-cycle, the differences in binding energies point to the fact that they cannot be assigned as before. Indeed, the interaction between

the surface and water is expected to generate a reorganization of the surface species that can cause the incorporation of nitrogen species into the film.

Finally, after the water half-cycle, the Hf 4f signal is shifted by about 0.25 eV toward a lower binding energy. The shift is the same as that of the Ti 2p core level. Moreover, the Hf 4f line has become narrower and is now centered at 17.60 eV, in the range of literature values previously reported for HfO₂.²²,³⁷⁻³⁹ We suggest that the narrowing can be attributed to the Hf ions' chemical environments, overall, becoming more homogenous, in line with the reduction in the variety of ligands seen from the N 1s level. A calculation of the thickness of HfO₂ shows that the surface carries approximately six monolayers⁴⁰ of HfO₂ (around 16 Å) after the water half-cycle (more details in Section S5 of the Supporting Information).

**HT-LP.** The *in situ* XP spectra acquired at 500 K and 10⁻⁶ mbar, together with reference spectra taken before the deposition, are shown in Figure 1b1–b4. As before, the reference spectra are displayed in black. The positions of maximum intensity of the Ti 2p and O 1s lines are found at 459.2 ± 0.1 and 530.6 ± 0.1 eV, respectively, values that are comparable to those found in the HT-HP experiment and in agreement with literature values.³² The low-binding energy shoulder, at around 15% of the overall intensity, in the Ti 2p XP spectrum before the deposition shows that the surface is already partially reduced before the experiment, which reflects that commercial TiO₂(101) single crystals of different batches vary rather strongly in terms of stoichiometry, degree of order, and, often, content of different impurities.

**Metal Half-Cycle.** The red XP spectra in Figure 1b1–b4 were recorded after the first ALD half-cycle. In comparison to the reference spectrum measured prior to the exposure to TDMA-Hf, the surface is even more reduced: the relative intensity of the low-binding energy shoulder has increased from 15 to 30% of the overall intensity of the Ti 2p line. The intensity of the O 1s line, normalized to the Ti 2p intensity, increases by about 100%, in the same way as in the HT-HP experiments. In the N 1s component, we can identify at least three components: one at 396.7 ± 0.1 eV binding energy, corresponding to MMI surface species, one at 398.6 ± 0.1 eV, due to DMA⁻ surface species, and, finally, a shoulder at around 402.0 ± 0.5 eV. Closer inspection of the latter shoulder suggests (cf. Figure S3 of the Supporting Information) that it likely is composed of two distinct components, one at around 400.6 eV binding energy due to dimethylamine (HN(CH₃)₂, DMA; Figure 2b) and one at 402.8 eV attributable to dimethylammonium ions ((H₂N(CH₃)₂)⁺, DMA⁺, cf. Figure 2d). We have observed the ammonium species also in TiO₂ ALD from TDMA-Ti,²⁷ but we did not in the ALD of HfO₂ from TDMA-Hf on InAs or on SiO₂.²²,²³,²⁵ The C 1s:N 1s ratio is around 2, in agreement with the stoichiometry of the ligands. The Hf 4f line is found at 17.6 ± 0.1 eV binding energy.

**Water Half-Cycle.** The APXP spectra in blue were recorded after the water pulse, which induces a reoxidation of the surface. This is clearly visible from the attenuation of the Ti³⁺ and Ti²⁺ signals in the Ti 2p spectrum. Interestingly, the O 1s signal preserves its intensity, and no presence of –OH groups is detected. The N 1s line (Figure 1b3) loses almost all its intensity: its intensity is decreased by 82% in comparison to the situation after the TDMA-Hf pulse. DMA and DMA⁺ are completely desorbed from the surface, and also, the DMA⁻ ligands are nearly completely gone. In contrast, a component at
<table><caption>**Table 1. Summary of Results**</caption><thead><tr><th rowspan="2"></th><th colspan="3">Ti<sup>3+</sup> and Ti<sup>2+</sup> intensity in the Ti 2p line, related to vacancies (% of total intensity)</th><th colspan="2">C 1s:N 1s intensity ratio</th><th colspan="2">N 1s:Hf 4f intensity ratio</th><th colspan="2">DMA<sup>-</sup> (% of N 1s intensity)</th><th rowspan="2">layer thickness (Å)<br/>after the second half-cycle</th></tr><tr><th>pristine</th><th>after the first half-cycle</th><th>after the second half-cycle</th><th>after the first half-cycle</th><th>after the first half-cycle</th><th>after the first half-cycle</th><th>after the first half-cycle</th><th>after the first half-cycle</th><th>after the first half-cycle</th></tr></thead><tbody><tr><td>HT-HP</td><td>4</td><td>6</td><td>2</td><td>1.9</td><td>0.4</td><td>0.4</td><td>28</td><td>28</td><td>16</td><td></td></tr><tr><td>HT-LP</td><td>15</td><td>30</td><td>22</td><td>1.8</td><td>0.3</td><td>0.3</td><td>27</td><td>27</td><td>13</td><td></td></tr><tr><td>LT-HP</td><td>18</td><td>36</td><td>15</td><td>2.2</td><td>1.3</td><td>1.3</td><td>71</td><td>71</td><td>5</td><td></td></tr></tbody></table>

396.7 eV persists, i.e., at the position of the component attributed to MMI that is observed after the metal half-cycle. The consistency of the binding energies makes it likely that the component observed now stems from MMI residuals at the surface, which is in contrast to the HT-HP case, in which a shift is observed, indicative of a modification of the MMI surface species. The intensity of the Hf 4f core level (Figure 1b4) does not vary with the water pulse, but the width of its components decreases very clearly, which again suggests that the Hf ions become chemically more alike. Concerning the position, the Hf 4f$_{7/2}$ is centered at 17.60 eV, indicative of HfO$_2$ as after the HT-HP exposure. A calculation of the thickness of HfO$_2$ shows that approximately five monolayers of HfO$_2$ (around 13 Å) have been grown on the surface.

**LT-HP.** In the LT-HP experiment, the XP spectra were acquired at room temperature and a TDMA-Hf pressure of $10^{-2}$ mbar in the metal half-cycle, while a water pressure of $10^{-1}$ mbar was used in the water half-cycle. The *in situ* XP spectra recorded during this experiment are shown in Figure 1c1-d4. Again, the reference spectra taken before the TDMA-Hf half-cycle are shown in black, while the spectra recorded after the TDMA-Hf half-cycle are displayed in red and those after the water half-cycle in blue. We would like to underline that the LT-HP experiment was performed at another beamline compared to all the other experiments (cf. Methods Section), at a higher resolving power. This implies that the widths of lines presented in this subsection cannot be directly compared to those of the other experiments.

Prior to deposition, some minor surface contamination is visible in the C 1s and N 1s regions, without further significance for the experiments. As in the HT-LP case, the position of the maximum intensity of the Ti 2p line, found at 459.2 ± 0.1 eV, is characterized by the presence of Ti$^{3+}$ and Ti$^{2+}$ components (18% of the overall intensity), clearly showing that the surface is not fully stoichiometric. As in the other experiments, the O 1s line is found at 530.5 ± 0.1 eV binding energy.

**Metal Half-Cycle.** From the N 1s core-level spectrum, it is seen that the TDMA-Hf pulse (red spectra) leads to the formation of DMA$^{-}$ and MMI surface species, with the former clearly being in majority (71% of the overall intensity), with binding energies in agreement with the other experiments (398.6 eV for DMA$^{-}$ and 396.7 eV for MMI). In the Ti 2p spectrum, a drastic increase in the Ti$^{2+}$ and Ti$^{3+}$ components (+100%) is observed, together with a small shift of about 0.1 eV of the main component. As for the other experiments, the O 1s line increases in intensity, and it is found at around +0.1 to +0.2 eV binding energy in comparison to that of the pristine surface. The C 1s:N 1s intensity ratio is around 2, in agreement with the stoichiometry of the ligands. The Hf 4f line is found at 17.6 ± 0.1 eV binding energy.

**Water Half-Cycle.** The water pulse (blue spectra in Figure 1c1,c2,c3,c4) reoxidizes the surface as it is clearly visible from

the intensity decrease in the Ti 2p low-binding energy shoulder (15% of the overall intensity). Both the O 1s line and the main component of the Ti 2p line shift back to their original positions. In the O 1s spectrum, a very distinct shoulder is now found at around 532.5 eV binding energy. It shows the presence of hydroxyl groups on the surface.$^{41,42}$ The N 1s line gets strongly reduced in intensity; only a small component at 396.7 eV binding energy is left, as in the HT-LP case at the same binding energy at which the MMI component is found after the metal half-cycle. Again, we take this as an indication that the component stems from MMI residuals at the surface. In addition, a very minor component is seen at around 400.5 eV binding energy. This component is assigned to a N surface contamination. A similar contamination was visible also before the first half-cycle was carried out. The main component of the C 1s line shifts to 285.5 eV (Figure S4 in the Supporting Information), and a small peak appears at 289.0 eV binding energy. This is likely associated to the appearance of carboxylic groups;$^{43}$ however, as observed before, the *in situ* C 1s XP spectrum is heavily affected by adsorption of carbon residuals in the analysis chamber. We therefore refrain from a detailed analysis of the spectrum. The Hf 4f line, found at 17.6 eV, loses intensity slightly and shifts toward lower binding energy by around 0.1 eV; the overall shape becomes narrower, which suggests that the Hf ions become more similar to each other, in line with the observations from the HT-HP and HT-LP experiments. The thickness of the HfO$_2$ layer after the water half-cycle is approximately two monolayers (around 5 Å and one unit cell of HfO$_2$).

**LT-LP.** The LT-LP experiment was performed with the sample at room temperature and a TDMA-Hf pressure of $10^{-6}$ mbar. Figure 1d1-d4 displays the reference spectra prior to the TDMA-Hf pulse in black together with those after the pulse in red. No Hf signal is detected (Figure 1d4) after the TDMA-Hf pulse, and therefore, no water pulse was performed. In the N 1s APXP spectra, new intensity is, however, observed after the TDMA-Hf pulse. The peak has the binding energy typical for DMA, and, hence, the metal half-cycle pulse leads to the adsorption of DMA on the surface. Indeed, DMA is expected to be present in the gas phase as a contamination of the TDMA-Hf vapor, as stated in the Methods section. The O 1s line remains almost unchanged, but a small shoulder appears in the Ti 2p spectrum. This shoulder could, however, also be related to an inhomogeneity of the TiO$_2$(110) surface. Irrespective of the latter spectral changes, we conclude that ALD cannot be performed using LT-HP conditions, in line with expectations.

To investigate the role of surface defects, we performed a similar experiment, under the same conditions, on a defect-rich TiO$_2$ surface. The defects were induced by sputtering with Ar$^{+}$ ions (Figure S1 in the Supporting Information). No presence of hafnium was found on the surface, and hence, the surface
defects do not provide a sufficient reactivity to allow ALD under LT-HP conditions.

**Summary of the Results.** Table 1 summarizes the main results of the analysis provided in the Results section. The table specifies the fraction of Ti 2p intensity related to Ti oxidation states lower than +4, before deposition, after the TDMA-Hf half-cycle, and after the water half-cycle. The entire low-oxidation state intensity is assumed to be related to vacancies. The assumption is justified in view of the fact that Ti interstitials are not stable⁴⁴ in the anatase modification of TiO₂. Further, the table provides the as-observed C 1s:N 1s and N 1s:Hf 4f intensity ratios after the TDMA-Hf half-cycle as well as the fraction of DMA⁻N 1s intensity after the same half-cycle. Finally, the estimated layer thickness after the water half-cycle is given.

## DISCUSSION

**Influence of Oxygen Vacancies.** The above results make it clear that both the support temperature and the pressure have significant influence on the ALD surface chemistry, quite as expected and in line with previous knowledge.²¹ In view of the fact that the pristine surfaces used in the various experiments exhibited clearly differing starting conditions in terms of the (sub-) surface defect densities (cf. Table 1), it is conceivable that, also, this density could be a parameter with significant influence on the ALD surface chemistry. Closer inspection of the results in Table 1, however, does not suggest any clearly visible correlation between the defect density and the way the deposition proceeds: The results for the HT-HP and HT-LP experiments are very similar in terms of the C 1s:N 1s and N 1s:Hf 4f intensity ratios, fraction of DMA⁻ species, and resulting layer thickness, in spite of a large difference in initial defect density. On the contrary, the starting conditions in terms of the defect densities are rather similar for the HT-LP and LT-HP experiments, but the resulting surfaces are quite different in terms of these parameters. It should be noted, though, that Table 1 does not catch the differences between the HT-HP and HT-LP results in terms of the additionally found DMA and DMA⁺ species after the TDMA-Hf half-cycle in the HT-LP, but not the HT-HP experiment, and in terms of the higher-binding energy shoulder in the O 1s XP spectrum (cf. Figure 1a1 and Figure S2 in the Supporting Information), which likely can be attributed to surface hydroxyls. The two latter observations notwithstanding, we tentatively suggest that the defect density—primarily related to oxygen vacancies⁴⁵— does not exert any significant direct influence on the ALD surface chemistry. This is in line with the fact that, on the anatase TiO₂(101) surface, vacancies are primarily found subsurface,⁴⁶ which stands in clear contrast to, e.g., the rutile TiO₂(110) surface where bridging oxygen vacancies at the surface have been shown to modify the surface chemistry in comparison to the stoichiometric surface.⁴⁷

**Potential Role of Surface Hydroxyls.** Surface hydroxyls may exert significant influence on the surface chemistry of the TiO₂(101) surface. Due to the absence of any hydroxyl peak from the O 1s XP spectra of the pristine surfaces in Figure 1a1,b1,c1, it is clear that the degree of hydroxylation is negligible initially. This does not imply that surface hydroxyls cannot be formed during the half-cycles. This is very evident not only from the O 1s XP spectra taken after the water half-cycle in the HT-HP and LT-HP experiments but also from those measured after the metal half-cycle in the HT-HP case. In contrast, the O 1s XP spectra measured after the metal half-cycle in the HT-LP and LT-HP experiments do not exhibit any spectral fingerprints of surface hydroxyls. At first sight, the observation of hydroxyls in the HT-HP case appears surprising since surface hydroxyls are not expected to be stable to any significant degree on the stoichiometric TiO₂(101) surface at 500 K (i.e., at HT).⁴⁸ Subsurface defects can, however, stabilize hydroxyls at temperatures beyond 700 K.⁴⁹ It needs to be taken into account, though, that proton migration toward the bulk is favorable.⁵⁰

**TDMA-Hf Half-Cycle.** In the following, we analyze the influence of the precursor pressure and support temperature parameters on the initial ALD chemistry, neglecting any possible influence of the presence of surface defects. These two parameters lead to strong differences in surface species distribution in the different experiments, where the possible surface species are illustrated in Figure 2. For the initial TDMA-Hf half-cycle, we can identify three principal pathways for the chemistry that underlies the formation of some, or all, of these surface species:

* A ligand exchange reaction mechanism as described in the Introduction section.¹²

* A reaction mechanism that is based on dissociative adsorption.¹²

* A reaction mechanism that is based on initial physisorption and which may evolve further due to inter- or intramolecular chemical reactions.²²,²⁵

In the ligand exchange reaction mechanism (cf. Introduction section), the surface is required to provide suitable ligands that can participate in the ligand exchange reaction.¹¹,¹⁷⁻¹⁹ Initially, none of the O 1s XP spectra of the TiO₂ surfaces in the four different experiments (Figure 1 and Figure S2 in the Supporting Information) contain any hydroxyl signal, and therefore, a ligand exchange reaction mechanism cannot be evoked for the TDMA-Hf half-cycle. The presence of a line at the hydroxyl binding energy in the O 1s spectrum after the metal half-cycle in the HT-HP experiments suggests, though, that hydroxyls were formed during the half-cycle. Naturally, these surface hydroxyls could lead to a ligand exchange in the course of the first half-cycle.

In contrast, dissociative adsorption, such as observed in the chemisorptive reaction of⁵¹ O₂ or of metalorganic precursors such⁵² as Ru(Cp)₂ with a metal surface, is conceivable in all four experiments. Dissociative adsorption is known to occur in certain ALD processes. For example, trimethylaluminium (TMA) has been observed to adsorb dissociatively on the oxygen bridges of silica.⁵³ In the present case, dissociative adsorption would proceed via splitting-off of a DMA⁻ ligand and bond formation between the Hf ion of the precursor and a surface oxygen ion

$$ \mathrm{Hf}(\mathrm{N}(\mathrm{CH}_3)_2)_4 + \|\mathrm{TiO}_x \\ \rightarrow |(\mathrm{Ti} - (\mathrm{N}(\mathrm{CH}_3)_2)_x + \|\mathrm{O}_x - \mathrm{Hf}(\mathrm{N}(\mathrm{CH}_3)_2)_4)_x| $$

Among the surface species observable from the N 1s XP spectra (cf. Figure 2), DMA⁻ is the only one that features in a pure dissociative adsorption reaction mechanism. Hence, such a reaction mechanism alone cannot explain the occurrence of MMI, DMA, and DMA⁺ surface species, but surface-adsorbed DMA⁻ can provide a starting point for further reaction.

DMA⁻ is also the only surface species that would be observed after the TDMA-Hf half-cycle if the only reaction that occurred was the physisorption of the metal precursor.
<figure><img src="image_12.png" /><figcaption>**Figure 4.** Reaction paths toward the formation of MMI. (a) Formation of MMI and DMA in an intra- or intermolecular reaction mechanism. (b) Formation of MMI and a surface hydroxyl using a surface oxygen. (c) Consumption of the surface hydroxyl formed in pathway (b) by reaction with a second DMA⁻ ligand. (d) Reaction path for the formation of DMA⁺.</figcaption></figure>

While it is not clear that dissociative adsorption and physisorption can be distinguished from their N 1s signatures, the appearance of the Hf 4f XP spectra provides further information on the reaction path: The initial physisorbed state is characterized by a high Hf 4f binding energy of around 17.4–17.6 eV.²²,⁵⁴ The physisorption is followed by an attack of the lone pair of a ligand nitrogen onto the surface, which causes a downshift of the Hf 4f binding energy by around 0.5 eV. The attack leaves the surface-adsorbed TDMA-Hf complex in an intact state, which, however, is more strongly bonded to the surface than the initially physisorbed molecule. The formation of a surface oxygen-hafnium ion bond—and thus the establishment of the chemisorbed state—is then seen from an upshift of the Hf 4f binding energy by around 0.4 eV. In the present case, the *in situ* Hf 4f binding energy after the first half-cycle suggests that the Hf complex is in this chemisorbed state, for all the examined samples.

Physisorbed TDMA-Hf, but likely also the chemisorbed complexes, can also evolve further in an intra- or intermolecular chemical reaction: the proton of a DMA⁻ ligand may be inserted into the bond of another DMA⁻ ligand, either on the same TDMA-Hf complex or a neighboring complex. This reaction leads to the formation of MMI,²⁵ in a three-membered ring configuration if the reaction is unimolecular or a bridge configuration if the reaction is bimolecular (cf. Figure 2), and DMA. The latter compound is expected to have a desorption temperature on a rutile TiO₂ surface of the order of 180 °C,⁵⁵ and a similar desorption temperature is expected for the anatase TiO₂(101) surface. Alternatively, it is conceivable that O ions from the TiO₂ surface could provide a suitable hydrogen acceptor and that MMI was formed by reaction of a DMA⁻ ligand and an oxygen ion. Such a reaction could potentially induce migration of oxygen ions to the surface, as, indeed, visible from the increase in defect density in all three experiments with TDMA-Hf reaction (cf. Figure 1a2,b2,c2 and Table 1). We note that the MMI intensity ratio is around 50% of the overall N 1s core-level intensity already when the O 1s intensity is at its minimum (Figure 3) and therefore prior to oxygen migration.

The different reaction pathways toward formation of MMI are summarized in Figure 4. The reactions are initiated by TDMA-Hf physisorption, but they could also take place between DMA⁻ ligands of either Hf or Ti ions after dissociative adsorption. Pathway (a) is the intra- or intermolecular reaction path, and MMI and DMA are formed. Pathway (b), involving oxygen ions that have migrated from the bulk to the surfaces, leads to the formation of MMI and a surface hydroxyl. The surface hydroxyl can be consumed in a reaction with a further DMA⁻ ligand (pathway (c)). Both pathways are open for continued chain reactions that involved

the remaining DMA⁻ ligands as well as those of neighboring physisorbed TDMA-Hf complexes. As shown by reaction (d), DMA⁺ can be formed if the proton of a surface hydroxyl is transferred to a DMA adsorbate.²⁶,⁵⁶

Based on the above considerations, the TDMA-Hf half-cycle of the LT-HP experiment is rationalized relatively easily. The N 1s XP spectrum is dominated strongly by the DMA⁻ component, with only a small MMI component visible (cf. Table 1). The dominance of the DMA⁻ component points to the prevalence of either dissociative adsorption or of physisorption. The N 1s:Hf 4f intensity ratio of a little more than 1 seems to suggest dissociative adsorption, given that this ratio would be expected to be 4 for the intact TDMA-Hf complex. However, the ratio is calculated at the end of the reaction and it therefore does not provide any information at the start of the surface reaction; the ratio could very well be higher than, which would be an indication of initial physisorption.²⁵ Moreover, the intensity ratio needs to be considered with care since the measured intensities are not corrected for the kinetic energy dependence of the electron analyzer transmission (but they are corrected for the differing subshell photoionization cross sections). Therefore, while it is clear that the N 1s:Hf 4f intensity ratio is larger in the N 1s XP spectrum after the TDMA-Hf half-cycle than in the corresponding N 1s XP spectra of the HT-HP and HT-LP experiments, it is not equally clear that it provides a true representation of the sample stoichiometry. The observation of oxygen migration toward the surface, observed from an increased O 1s intensity after the TDMA-Hf half-cycle and a very much increased low-oxidation state shoulder in the corresponding Ti 2p XP spectrum, indicative of a reduction of the sample, would, however, be in line with a dissociative adsorption mechanism and the formation of Ti–O–Hf bonds. As outlined in Figure 4b, the oxygen migration could even contribute to the formation of MMI. Alternatively, the formation of MMI could also be entirely due to a side reaction that proceeds according to the scheme in Figure 4a.

In contrast, in the HT-HP experiment, the MMI component dominates rather strongly over the DMA⁻ component in the N 1s XP spectrum measured after the TDMA-Hf half-cycle. In addition, a high-binding energy component is observed in the corresponding O 1s XP spectrum (cf. Figure S2 in the Supporting Information), indicative of surface hydroxyls. These two findings together suggest that pathway (b) in Figure 4 might be active, possibly aided by an intra- or intermolecular side reaction, as shown in Figure 4a. Also, in the HT-HP case, a reduction of the TiO₂ support and concomitant oxygen transport is seen in the TDMA-Hf half-cycle, visible from the increase in the low-binding energy shoulder of the Ti 2p line and the increase in the O 1s intensity.
Finally, the N 1s XP spectrum measured after the TDMA-Hf half-cycle in the HT-LP experiment differs quite strongly from the other two cases. The MMI:DMA⁻ ratio is similar to that observed in the HT-HP case, but now, also, DMA and DMA⁺ components are observed. Their presence on the surface is somewhat unexpected, given that the desorption temperature for DMA on the rutile TiO₂(110) surface is below 180 °C. While it is conceivable that the desorption temperature is higher on the TiO₂(101) surface—not least given that oxygen adatoms seem to be readily available as a result of oxygen migration from the bulk to the surface—it is striking that the DMA signal is observed in the HT-LP experiment, but not in the HT-HP experiment, although it is carried out at the same temperature. If we rule out any influence of surface defects (see above), then the only difference is the precursor pressure used during the TDMA-Hf half-cycles. If the adsorption energy of TDMA-Hf exceeds that of DMA—and this seems likely even for physisorbed TDMA-Hf—then the high flux of impinging TDMA-Hf molecules on the surface at high pressure could lead to the displacement of more weakly adsorbed DMA (and DMA⁺) from the surface. Conversely, the much lower flux of TDMA-Hf molecules in the HT-LP experiment could explain the survival of DMA on the surface. DMA⁺ can then be formed in the interaction between a DMA molecule and a surface hydroxyl.

From the above, it is clear that oxygen migration plays an important role for the chemistry initial TDMA-Hf half-cycle in the HT-HP, HT-LP, and LT-HP experiments, visible foremost from a strong increase in the overall O 1s intensity, relative to that of the Ti 2p line. The here observed migration of oxygen ions and corresponding reduction of the TiO₂ support contrasts with what has been observed previously for the deposition of HfO₂ from TDMA-Hf and water on SiO₂, for which no such effect was observed.²⁵ The difference can be related to the different enthalpies of formation of the two surfaces. Following the work made by Nagamori et al.⁵⁷ and experimental data from NIST,⁵⁸ the heat of formation of Si-O and Ti-O systems is reported in Figure 5. For the TiₓOᵧ systems, the enthalpy difference between TiO₂ and Ti₃O₅ is 7.7 kJ/mol and between TiO₂ and Ti₂O₃ is 10.8 kJ/mol. The enthalpy difference between SiO₂ and Si₂O₃ is around 32 kJ/mol, and hence, the energy difference is much larger between the different SiₓOᵧ configuration than the corresponding TiₓOᵧ ones. The oxidation state is more easily changed in the TiₓOᵧ

<figure><img src="image_13.png" /><figcaption>**Figure 5.** Standard heat of formation of the Si-O and Ti-O systems as a function of oxygen concentration.</figcaption></figure>

system, and the support takes on an active role in the ALD chemistry—the surface can no longer be considered as static.

To summarize, due to the absence of surface hydroxyls on the TiO₂(101) surface, the first metal half-cycle proceeds according to the reaction mechanism other than ligand exchange. The exact reaction mechanisms depend on the experimental parameters, i.e., temperature and pressure. This is very obvious from the fact that the main nitrogen surface species after the LT-HP half-cycle is DMA⁻, while several other species are observed in the HT-HP and HT-LP cases.

**Water Half-Cycle.** The water half-cycle brings about very similar changes in all examined systems, irrespective of the pressure and temperature, quite in contrast to what is observed for the metal half-cycle. The Hf 4f core level exhibits a decreased broadening, with the line found at 17.60 eV binding energy in the HT-LP and LT-HP cases and at a somewhat higher binding energy of 17.80 eV in the HT-HP case. This shift is also seen in the Ti 2p, O 1s, and N 1s core-level spectra and therefore likely to be due to a band-banding effect. The Hf 4f spin-orbit components seem to be narrower in the LT-HP case than the HT-LP and HT-HP ones. This is, however, largely a result of the increased energy resolution of the LT-HP experiment. The positions of the Hf 4f lines are compatible with HfO₂, and the lack of shift observed before and after the water half-cycle is further evidence for the formation of hafnium oxide already in the TDMA-Hf half-cycle.²²,³⁷⁻³⁹

It is worth noting that the layer thicknesses achieved in the different experiments differ quite strongly: In the experiment carried out under LT-HP conditions, an oxide layer is produced with a thickness of around 5 Å, comparable to the typical lattice constant of HfO₂ and thus to two monolayers of HfO₂. The two experiments at high temperature exceed this value by almost a factor of 3, and layers with a thickness of around 15 Å are formed (cf. Table 1). The reason for this difference is not clear but could be related to the fact that more reaction paths exist under HT than LT conditions. This could favor the adsorption of more TDMA-Hf complexes.

The standardly used ligand exchange model of ALD suggests that the water half-cycle should create a surface with an -OH termination. The corresponding O 1s component is found at a binding energy of 532.5 eV.¹¹ Such a component is observed only in the HT-HP and LT-HP cases, and, hence, the formation of surface hydroxyl is pressure-dependent. A low water pressure is not sufficient for preparing the surface for subsequent ALD half-cycles that make use of the ligand exchange reaction mechanisms. However, an intra- or intermolecular reaction, based on hydrogen insertion as described in Figure 4a, would allow subsequent ALD half-cycles. It is also conceivable that the formed HfO₂ could act as an oxygen reservoir, allowing reaction pathway (b) in Figure 4 in a hydroxyl-free process. The reservoir would be replenished by further water half-cycles. A similar reaction has been observed in the ALD⁵⁹ of Fe₂O₃. Indeed, as our results on the initial TDMA-Hf half-cycle show, alternative reaction pathways exist so that ALD is possible also on hydroxyl-free surfaces. Alternatively, once a surface hydroxyl is formed on the surface, it can protonate a DMA⁻ ligand, as shown in Figure 4b. If the water pressure is insufficient, then this process is favorable and leads to a net reaction without hydroxyl when all the ligands are removed. The mechanism is also observed in the ALD of HfO₂ on InAs.⁵⁴

The N 1s core-level line loses most of its initial intensity due to the interaction of the surface with water. This interaction,
effective at removing the nitrogen species, is not at all as efficient for the removal of carbon species (cf. Section S4 in the Supporting Information), a well-known problem of ALD with carbon containing molecules.<sup>60,29,61</sup>

## ■ CONCLUSIONS

One full cycle of ALD of HfO₂ on TiO₂(101) was studied and analyzed in four different experimental conditions using in situ XPS and time-resolved ambient pressure XPS. We map the surface state and surface species in real time, while the ALD process is ongoing. The experiments are used to map the influence of surface temperature and precursor pressure on the final HfO₂ layer. We demonstrate that the TiO₂ surface has an active role during the ALD, which goes clearly beyond the catalyst-like role stipulated in the ligand exchange reaction mechanism: the surface provides oxygen as an oxidant source to the TDMA-Hf to form HfO₂ already in the first half-cycle. The effect is related to the so-called interfacial self-cleaning of the native oxides on III-V semiconductor surfaces observed in a wide range of oxide ALD processes. The enthalpy of formation can help in deciding whether the support is static, as in the case of SiO₂, or active, as in the presented case. Hence, it provides a tool to predict the migration of oxygen atoms and so foresee the formation of HfO₂ already from the first half-cycle.

Largely due to the absence of surface hydroxyls, the first ALD half-cycle of HfO₂ on TiO₂ cannot be described by the ligand exchange reaction mechanism but instead by a mechanism involving dissociative adsorption followed by hydrogen insertion reactions. Thus, the surface chemical reaction of the ligands is found to be mono- and bi-molecular in nature, leading to surface species not predicated by standard ALD models that describe the reaction between metal-amido complexes and a surface. Concerning the second half-cycle, at sufficiently high water pressure, the surface is hydroxylated, which likely leads to a dominance of ligand exchange in subsequent cycles. However, even without surface hydroxyls, i.e., if the water pressure of the water half-cycle is too low for their formation, the ALD reaction can proceed along the lines discussed here for the initial ALD cycle.

While our study provides details on a specific ALD experiment, our findings also result in more general insight into how the commonly used model of ALD chemistry should be seen. Standard ALD models of transition metal oxide ALD from metal amido complexes and water are typically based on (i) the static, catalyst-like role of the support, (ii) the limited thickness of a monolayer or less after a full cycle, and (iii) the ligand exchange reaction mechanism. Our results, in contrast, imply that the role of the support might be either active or static, depending on the surface. An active support delivers oxygen to the chemical reaction already in the first half-cycle. Whether the support is static or active can be judged from its enthalpy of formation. It is entirely clear that the ligand exchange reaction mechanism is not applicable in the absence of suitable surface ligands such as hydroxyls. Indeed, it is necessary to consider that many oxide surfaces are not hydroxylated under ALD reaction conditions, i.e., at room temperature and above.

## ■ ASSOCIATED CONTENT

### ■ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c02683.

LT-LP experiment on a sputtered TiO₂(101) surface, comparison of O 1s XP spectra, N 1s core-level deconvolution, C 1s XP spectra, and overlayer model to calculate the thickness of the HfO₂ layer (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Author

Joachim Schnadt – Division of Synchrotron Radiation
Research and NanoLund, Department of Physics, Lund
University, Lund 221 00, Sweden; MAX IV Laboratory,
Lund University, Lund 221 00, Sweden; ● orcid.org/0000-
0001-9375-831X; Email: joachim.schnadt@sljus.lu.se

### Authors

Giulio D'Acunto – Division of Synchrotron Radiation
Research and NanoLund, Department of Physics, Lund
University, Lund 221 00, Sweden; MAX IV Laboratory,
Lund University, Lund 221 00, Sweden; ● orcid.org/0000-
0002-0564-6906

Rosemary Jones – Division of Synchrotron Radiation
Research and NanoLund, Department of Physics, Lund
University, Lund 221 00, Sweden; MAX IV Laboratory,
Lund University, Lund 221 00, Sweden; ● orcid.org/0000-
0001-6273-4243

Lucía Pérez Ramírez – CNRS Laboratoire de chimie
physique-Matière et rayonnement, Sorbonne Université, Paris
75005, France; ● orcid.org/0000-0002-1357-5650

Payam Shayesteh – Division of Synchrotron Radiation
Research and NanoLund, Department of Physics, Lund
University, Lund 221 00, Sweden; MAX IV Laboratory,
Lund University, Lund 221 00, Sweden

Esko Kokkonen – MAX IV Laboratory, Lund University,
Lund 221 00, Sweden; ● orcid.org/0000-0002-3674-7486

Foqia Rehman – Division of Synchrotron Radiation Research
and NanoLund, Department of Physics, Lund University,
Lund 221 00, Sweden; MAX IV Laboratory, Lund
University, Lund 221 00, Sweden

Florence Lim – CNRS Laboratoire de chimie physique-
Matière et rayonnement, Sorbonne Université, Paris 75005,
France

Fabrice Bournel – CNRS Laboratoire de chimie physique-
Matière et rayonnement, Sorbonne Université, Paris 75005,
France; Synchrotron SOLEIL, Saint-Aubin 91192, France;
● orcid.org/0000-0001-6452-3415

Jean-Jacques Gallet – CNRS Laboratoire de chimie physique-
Matière et rayonnement, Sorbonne Université, Paris 75005,
France; Synchrotron SOLEIL, Saint-Aubin 91192, France

Rainer Timm – Division of Synchrotron Radiation Research
and NanoLund, Department of Physics, Lund University,
Lund 221 00, Sweden; MAX IV Laboratory, Lund
University, Lund 221 00, Sweden; ● orcid.org/0000-0001-
8914-5924

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcc.2c02683

### Notes

The authors declare no competing financial interest.

## ■ ACKNOWLEDGMENTS

The French national synchrotron radiation light source SOLEIL is acknowledged for beamtime on the TEMPO beamline under proposal no. 99200128. We acknowledge the
MAX IV Laboratory for beamtime on the HIPPIE beamline under proposal no. 20180354. Research conducted at MAX IV, a Swedish national user facility, is supported by Vetenskapsrådet (Swedish Research Council, VR) under contract 2018-07152, VINNOVA (Swedish Governmental Agency for Innovation Systems) under contract 2018-04969, and Formas under contract 2019-02496. VR is acknowledged for project funding under grant nos. 2017-03871, 2017-4108, and 2016-6186.

## REFERENCES

(1) International Technology Roadmap for Semiconductors (ITRS) More Moore. *ITRS* 2015, 1–52.

(2) Won Seo, J.; Baik, S. J.; Kang, S. J.; Hong, Y. H.; Yang, J. H.; Lim, K. S. A ZnO Cross-Bar Array Resistive Random Access Memory Stacked with Heterostructure Diodes for Eliminating the Sneak Current Effect. *Appl. Phys. Lett.* **2011**, *98*, 233505.

(3) Kwon, D. H.; Kim, K. M.; Jang, J. H.; Jeon, J. M.; Lee, M. H.; Kim, G. H.; Li, X. S.; Park, G. S.; Lee, B.; Han, S.; Kim, M.; Hwang, C. S. Atomic Structure of Conducting Nanofilaments in TiO₂ Resistive Switching Memory. *Nat. Nanotechnol.* **2010**, *5*, 148–153.

(4) Prakash, A.; Jana, D.; Maikap, S. TaOx-Based Resistive Switching Memories: Prospective and Challenges. *Nanoscale Res. Lett.* **2013**, *8*, 1–17.

(5) Auciello, O.; Lee, G.; Wu, C.; Chen, Y.; Alcantar-Peña, J. J.; Mejia, I.; De Obaldía, E. Super High-Dielectric-Constant Oxide Films for next-Generation Nanoelectronics and Supercapacitors for Energy Storage. *MRS Bull.* **2020**, *45*, 231–238.

(6) Zeman, M. C.; Fulton, C. C.; Lucovsky, G.; Nemanich, R. J.; Yang, W. C. Thermal Stability of TiO₂, ZrO₂, or HfO₂ on Si(100) by Photoelectron Emission Microscopy. *J. Appl. Phys.* **2006**, *99*, 023519.

(7) Jeon, W.; Chung, H. S.; Joo, D.; Kang, S. W. TiO₂ Al₂O₃ TiO₂ Nanolaminated Thin Films for DRAM Capacitor Deposited by Plasma-Enhanced Atomic Layer Deposition. *Electrochem. Solid-State Lett.* **2008**, *11*, 19–22.

(8) Honda, K.; Sakai, A.; Sakashita, M.; Ikeda, H.; Zaima, S.; Yasuda, Y. Pulsed Laser Deposition and Analysis for Structural and Electrical Properties of HfO₂-TiO₂ Composite Films. *Jpn. J. Appl. Phys.* **2004**, *43*, 1571–1576.

(9) Seo, M.; Kim, S. K.; Han, J. H.; Hwang, C. S. Permittivity Enhanced Atomic Layer Deposited HfO₂ Thin Films Manipulated by a Rutile TiO₂ Interlayer. *Chem. Mater.* **2010**, *22*, 4419–4425.

(10) Li, W.; Auciello, O.; Premnath, R. N.; Kabius, B. Giant Dielectric Constant Dominated by Maxwell-Wagner Relaxation in Al₂O₃/ TiO₂ Nanolaminates Synthesized by Atomic Layer Deposition. *Appl. Phys. Lett.* **2010**, *96*, 162907.

(11) Zaera, F. Mechanisms of Surface Reactions in Thin Solid Film Chemical Deposition Processes. *Coord. Chem. Rev.* **2013**, *257*, 3177–3191.

(12) Richey, N. E.; De Paula, C.; Bent, S. F. Understanding Chemical and Physical Mechanisms in Atomic Layer Deposition. *J. Chem. Phys.* **2020**, *152*, 040902.

(13) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of Inorganic Films Grown by Atomic Layer Deposition: Overview and General Trends. *J. Appl. Phys.* **2013**, *113*, 2.

(14) Lin, J. M.; Teplyakov, A. V. Computational Investigation of Surface Reactivity of Functionalized Silicon Surfaces in Deposition Processes. *Theor. Chem. Acc.* **2013**, *132*, 1–14.

(15) Barry, S. T.; Teplyakov, A. V.; Zaera, F. The Chemistry of Inorganic Precursors during the Chemical Deposition of Films on Solid Surfaces. *Acc. Chem. Res.* **2018**, *51*, 800–809.

(16) Duan, Y.; Lin, J. M.; Teplyakov, A. V. Computational Investigation of Electronic and Steric Effects in Surface Reactions of Metalorganic Precursors on Functionalized Silicon Surfaces. *J. Phys. Chem. C* **2015**, *119*, 13670–13681.

(17) Methaapanon, R.; Bent, S. F. Comparative Study of Titanium Dioxide Atomic Layer Deposition on Silicon Dioxide and Hydrogen-Terminated Silicon. *J. Phys. Chem. C* **2010**, *114*, 10498–10504.

(18) Pinna, N.; Hochepied, J. F.; Niederberger, M.; Gregg, M. Chemistry and Physics of Metal Oxide Nanostructures. *Phys. Chem. Chem. Phys.* **2009**, *11*, 3607.

(19) Wang, H.; Jiang, X.; Fu, K.; Willis, B. G. Nucleation, Hydroxylation, and Crystallization Effects in ALD SrO. *J. Phys. Chem. C* **2013**, *117*, 11578–11583.

(20) Niemelä, J. P.; Marin, G.; Karppinen, M. Titanium Dioxide Thin Films by Atomic Layer Deposition: A Review. *Semicond. Sci. Technol.* **2017**, *32*, 093005.

(21) Timm, R.; Head, A. R.; Yngman, S.; Knutsson, J. V.; Hjort, M.; McKibbin, S. R.; Troian, A.; Persson, O.; Urpelainen, S.; Knudsen, J.; Schnadt, J.; Mikkelsen, A. Self-Cleaning and Surface Chemical Reactions during Hafnium Dioxide Atomic Layer Deposition on Indium Arsenide. *Nat. Commun.* **2018**, *9*, 1–9.

(22) D'Acunto, G.; Troian, A.; Kokkonen, E.; Rehman, F.; Liu, Y. P.; Yngman, S.; Yong, Z.; McKibbin, S. R.; Gallo, T.; Lind, E.; Schnadt, J.; Timm, R. Atomic Layer Deposition of Hafnium Oxide on InAs: Insight from Time-Resolved in Situ Studies. *ACS Appl. Electron. Mater.* **2020**, *2*, 3915–3922.

(23) D'Acunto, G.; Kokkonen, E.; Shayesteh, P.; Boix, V.; Rehman, F.; Mosahebfard, Z.; Lind, E.; Schnadt, J.; Timm, R. Oxygen Relocation during HfO₂ ALD on InAs. *Faraday Discuss.* **2022**, DOI: 10.1039/d1fd00116g.

(24) Myers, T. J.; Cano, A. M.; Lancaster, D. K.; Clancey, J. W.; George, S. M. Conversion Reactions in Atomic Layer Processing with Emphasis on ZnO Conversion to Al₂O₃ by Trimethylaluminum. *J. Vac. Sci. Technol.*, **A** **2021**, *39*, 021001.

(25) D'Acunto, G.; Tsyshevsky, R.; Shayesteh, P.; Gallet, J.-J.; Bournel, F.; Rochet, F.; Pinsard, I.; Timm, R.; Head, A. R.; Kukla, M.; Schnadt, J. *Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO2*. Lund University Submitted 2022.

(26) Head, A. R.; Johansson, N.; Niu, Y.; Snezhkova, O.; Chaudhary, S.; Schnadt, J.; Bluhm, H.; Chen, C.; Avila, J.; Asensio, M.-C. In Situ Characterization of the Deposition of Anatase TiO₂ on Rutile TiO₂ (110). *J. Vac. Sci. Technol.*, **A** **2018**, *36*, 02D405.

(27) Head, A. R.; Chaudhary, S.; Olivieri, G.; Bournel, F.; Andersen, J. N.; Rochet, F.; Gallet, J. J.; Schnadt, J. Near Ambient Pressure X-Ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO₂ on RuO₂(110). *J. Phys. Chem. C* **2016**, *120*, 243–251.

(28) Zhu, S.; Scardamaglia, M.; Kundsen, J.; Sankari, R.; Tarawneh, H.; Temperton, R.; Pickworth, L.; Cavalca, F.; Wang, C.; Tissot, H.; Weissenrieder, J.; Hagman, B.; Gustafson, J.; Kaya, S.; Lindgren, F.; Källquist, I.; Maibach, J.; Hahlin, M.; Boix, V.; Gallo, T.; Rehman, F.; D'Acunto, G.; Schnadt, J.; Shavorskiy, A. HIPPIE: A New Platform for Ambient-Pressure X-Ray Photoelectron Spectroscopy at the MAX IV Laboratory. *J. Synchrotron Radiat.* **2021**, *28*, 624–636.

(29) Hausmann, D. M.; Kim, E.; Becker, J.; Gordon, R. G. Atomic Layer Deposition of Hafnium and Zirconium Oxides Using Metal Amide Precursors. *Chem. Mater.* **2002**, *14*, 4350–4358.

(30) Hidayat, R.; Kim, H.-L.; Kim, H.; Byun, Y.; Lee, J.; Lee, W.-J. Surface Reaction of the Hafnium Precursor with a Linked Amidocyclopentadienyl Ligand: A Density Functional Theory Study. *J. Vac. Sci. Technol.*, **A** **2021**, *39*, No. 032410.

(31) Shayesteh, P.; Tsyshevsky, R.; Urpelainen, S.; Rochet, F.; Bournel, F.; Gallet, J. J.; Kuklja, M. M.; Schnadt, J.; Head, A. R. Experimental and Theoretical Gas Phase Electronic Structure Study of Tetrakis(Dimethylamino) Complexes of Ti(IV) and Hf(IV). *J. Electron Spectrosc. Relat. Phenom.* **2019**, *234*, 80–85.

(32) Jackman, M. J.; Thomas, A. G.; Muryn, C. Photoelectron Spectroscopy Study of Stoichiometric and Reduced Anatase TiO₂(101) Surfaces: The Effect of Subsurface Defects on Water Adsorption at near-Ambient Pressures. *J. Phys. Chem. C* **2015**, *119*, 13682–13690.

(33) Armitage, D. A.; Grant, D. M. Characterisation of Surface-Modified Nickel Titanium Alloys. *Mater. Sci. Eng.*, **A** **2003**, *349*, 89–97.
(34) Lu, G.; Bernasek, S. L.; Schwartz, J. Oxidation of a Polycrystalline Titanium Surface by Oxygen and Water. *Surf. Sci.* **2000**, *458*, 80–90.

(35) Bertóti, I.; Mohai, M.; Sullivan, J. L.; Saied, S. O. Surface Characterisation of Plasma-Nitrided Titanium: An XPS Study. *Appl. Surf. Sci.* **1995**, *84*, 357–371.

(36) Li, K.; Li, S.; Li, N.; Dixon, D. A.; Klein, T. M. Tetrakis(Dimethylamido)Hafnium Adsorption and Reaction on Hydrogen Terminated Si(100) Surfaces. *J. Phys. Chem. C* **2010**, *114*, 14061–14075.

(37) Renault, O.; Samour, D.; Damlencourt, J. F.; Blin, D.; Martin, F.; Marthon, S.; Barrett, N. T.; Besson, P. HfO₂/SiO₂ Interface Chemistry Studied by Synchrotron Radiation x-Ray Photoelectron Spectroscopy. *Appl. Phys. Lett.* **2002**, *81*, 3627–3629.

(38) Lee, J. C.; Oh, S. J.; Cho, M.; Hwang, C. S.; Jung, R. Chemical Structure of the Interface in Ultrathin HfO₂/Si Films. *Appl. Phys. Lett.* **2004**, *84*, 1305–1307.

(39) Aygun, G.; Yildiz, I. Interfacial and Structural Properties of Sputtered HfO₂ Layers. *J. Appl. Phys.* **2009**, *106*, No. 014312.

(40) Wang, Y.; Zahid, F.; Wang, J.; Guo, H. Structure and Dielectric Properties of Amorphous High-κ Oxides: HfO₂, ZrO₂, and Their Alloys. *Phys. Rev. B* **2012**, *85*, 1–5.

(41) Fan, C.; Chen, C.; Wang, J.; Fu, X.; Ren, Z.; Qian, G.; Wang, Z. Black Hydroxylated Titanium Dioxide Prepared via Ultrasonication with Enhanced Photocatalytic Activity. *Sci. Rep.* **2015**, *5*, 1–10.

(42) McCafferty, E.; Wightman, J. P. Determination of the Concentration of Surface Hydroxyl Groups on Metal Oxide Films by a Quantitative XPS Method. *Surf. Interface Anal.* **1998**, *26*, 549–564.

(43) Pletincx, S.; Marcoen, K.; Trotochaud, L.; Fockaert, L. L.; Mol, J. M. C.; Head, A. R.; Karslioğlu, O.; Bluhm, H.; Terryn, H.; Hauffman, T. Unravelling the Chemical Influence of Water on the PMMA/Aluminum Oxide Hybrid Interface in Situ. *Sci. Rep.* **2017**, *7*, 1–11.

(44) Pan, X.; Yang, M. Q.; Fu, X.; Zhang, N.; Xu, Y. J. Defective TiO₂ with Oxygen Vacancies: Synthesis, Properties and Photocatalytic Applications. *Nanoscale* **2013**, *5*, 3601–3614.

(45) Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W.; Aschauer, U.; Selloni, A. (Sub)Surface Mobility of Oxygen Vacancies at the TiO₂ Anatase (101) Surface. *Phys. Rev. Lett.* **2012**, *109*, 1–5.

(46) Cheng, H.; Selloni, A. Surface and Subsurface Oxygen Vacancies in Anatase TiO₂ and Differences with Rutile. *Phys. Rev. B* **2009**, *79*, 2–5.

(47) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Direct Visualization of Defect-Mediated Dissociation of Water on TiO₂(110). *Nat. Mater.* **2006**, *5*, 189–192.

(48) Walle, L. E.; Borg, A.; Johansson, E. M. J.; Plogmaker, S.; Rensmo, H.; Uvdal, P.; Sandell, A. Mixed Dissociative and Molecular Water Adsorption on Anatase TiO₂(101). *J. Phys. Chem. C* **2011**, *115*, 9545–9550.

(49) Schaefer, A.; Lanzilotto, V.; Cappel, U. B.; Uvdal, P.; Borg, A.; Sandell, A. Defect-Induced Water Bilayer Growth on Anatase TiO₂(101). *Langmuir* **2018**, *34*, 10856–10864.

(50) Aschauer, U.; Selloni, A. Influence of Subsurface Ti Interstitials on the Reactivity of Anatase (101). *Proc. SPIE* **2010**, *7758*, 77580B.

(51) Knudsen, J.; Gallo, T.; Boix, V.; Strømsheim, M. D.; D'Acunto, G.; Goodwin, C.; Wallander, H.; Zhu, S.; Soldemo, M.; Lömker, P.; Cavalca, F.; Scardamaglia, M.; Degerman, D.; Nilsson, A.; Amann, P.; Shavorskiy, A.; Schnadt, J. Stroboscopic Operando Spectroscopy of the Dynamics in Heterogeneous Catalysis by Event-Averaging. *Nat. Commun.* **2021**, *12*, 1–8.

(52) Phung, Q. M.; Pourtois, G.; Swerts, J.; Pierloot, K.; Delabie, A. Atomic Layer Deposition of Ruthenium on Ruthenium Surfaces: A Theoretical Study. *J. Phys. Chem. C* **2015**, *119*, 6592–6603.

(53) Puurunen, R. L. Surface Chemistry of Atomic Layer Deposition: A Case Study for the Trimethylaluminum/Water Process. *J. Appl. Phys.* **2005**, *97*, 9.

(54) D'Acunto, G.; Shayesteh, P.; Kokkonen, E.; Boix, V.; Rehman, F.; Mosahebfard, N.; Lind, E.; Schnadt, J.; Timm, R. *Time Evolution of Surface Species during the ALD of High-k Oxide on InAs. Submitted* 2022.

(55) Farfan-Arribas, E.; Madix, R. J. Characterization of the Acid-Base Properties of the TiO₂(110) Surface by Adsorption of Amines. *J. Phys. Chem. B* **2003**, *107*, 3225–3233.

(56) Sperling, B. A.; Kimes, W. A.; Maslar, J. E. Reflection Absorption Infrared Spectroscopy during Atomic Layer Deposition of HfO₂ Films from Tetrakis(Ethylmethylamido)Hafnium and Water. *Appl. Surf. Sci.* **2010**, *256*, 5035–5041.

(57) Nagamori, M.; Boivin, J. A.; Claveau, A. Gibbs Free Energies of Formation of Amorphous Si₂O₃, SiO and Si₂O. *J. Non-Cryst. Solids* **1995**, *189*, 270–276.

(58) Chase, Jr., M. W. NIST-JANAF Thermochemical Tables https://janaf.nist.gov/ (accessed Mar 4, 2022).

(59) Schneider, J. R.; Baker, J. G.; Bent, S. F. The Influence of Ozone: Superstoichiometric Oxygen in Atomic Layer Deposition of Fe₂O₃ Using Tert-Butylferrocene and O₃. *Adv. Mater. Interfaces* **2020**, *7*, 1–10.

(60) Schmeisser, D.; Tallarida, M.; Henkel, K.; Müller, K.; Mandal, D.; Chumakov, D.; Zschech, E. Characterization of Oxidic and Organic Materials with Synchrotron Radiation Based XPS and XAS. *Mater. Sci. Pol.* **2009**, *27*, 141–157.

(61) Li, K.; Li, S.; Li, N.; Klein, T. M.; Dixon, D. A. Tetrakis(Ethylmethylamido) Hafnium Adsorption and Reaction on Hydrogen-Terminated Si(100) Surfaces. *J. Phys. Chem. C* **2011**, *115*, 18560–18571.

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