# Ultra-thin atomic layer deposited HfO2: Tailoring dielectric thickness for low-operating voltage thin-film transistors

Minyu Chen, Huimin Chen, Eduardo Solano, Danile Hermida Merino,

Wenqiang Hua, Xingwei Ding, Bin Wei, Yingjie Liao, Bing Wu

► To cite this version:

Minyu Chen, Huimin Chen, Eduardo Solano, Danile Hermida Merino, Wenqiang Hua, et al. Ultra-thin atomic layer deposited HfO2: Tailoring dielectric thickness for low-operating voltage thin-film transistors. Vacuum, 2025, 233, pp.113923. 10.1016/j.vacuum.2024.113923. hal-05122120

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# Ultra-thin atomic layer deposited HfO₂: Tailoring dielectric thickness for low-operating voltage thin-film transistors

Minyu Chen<sup>a,b,c</sup>, Huimin Chen<sup>b</sup>, Eduardo Solano<sup>d</sup>, Danile Hermida Merino<sup>f</sup>, Wenqiang Hua<sup>e</sup>, Xingwei Ding<sup>b</sup>, Bin Wei<sup>b</sup>, Yingjie Liao<sup>b,**</sup>, Bing Wu<sup>f,g,*</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

**Keywords:**

HfO₂ thin films

X-ray scattering

Atomic layer deposition

Thin film transistors

## ABSTRACT

To achieve low-power consumption and high performance in TFTs, it is crucial to reduce the thickness of the dielectric gate layer while maintaining a defect-free structure and high dielectric properties. This study explores the effects of varying HfO₂ layer thickness on TFT performance. X-ray photoelectron spectroscopy analysis indicates a decrease in oxygen defects as the film thickness increases. The morphological and topological properties of the films were characterized using grazing incidence small and wide-angle X-ray scattering and atomic force microscopy. Impedance spectroscopy was utilized to analyze the dielectric properties, showing that the 10 nm HfO₂ film exhibited a low leakage current density of 13 nA cm⁻² at 1 V and a permittivity of 5.98. The fabricated ZnO TFT with a 10 nm HfO₂ gate dielectric layer operated at an ultra-low voltage of -1.59 V, achieving a high current on/off ratio exceeding 10⁶. These findings highlight the importance of controlling dielectric layer thickness for the development of high-performance TFTs and their potential for low-power microelectronic applications.

## 1. Introduction

As modern microelectronics continue to shrink in size, there is a growing need for low-power, high-performance thin-film transistors (TFTs) to power the next generation of portable devices, which has led to increased research efforts in this area [1–3]. One of the major challenges is electron tunneling effect that would occur when the thickness of the dielectric layer was reduced to a certain threshold (i.e. less than 1.5 nm in SiO₂) [4], resulting in a large magnitude of leakage current that is detrimental for high performance devices [5,6]. Therefore, to achieve both high performance and low power consumption, it is necessary to find the right thickness for the dielectric layer. This thickness should be sufficient to prevent leakage currents but not so thick that it hinders performance [7–10].

The low dielectric permittivity in traditional gate dielectrics, such as SiO₂ ($\epsilon_k$ = 3.9) [11] is insufficient at nanometer thickness to prevent substantial gate leakage currents [12]. In the pursuit of alternative dielectric materials, hafnium oxide (HfO₂)-based substances have emerged as ideal candidates for TFT applications, as they can achieve excellent dielectric performance even at thicknesses below 45 nm [13–16]. Meanwhile, in order to produce high-quality HfO₂ thin films for use as gate dielectrics, researchers have explored various deposition techniques. These include physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD) [17]. Among them, ALD is considered the most promising technique for fabricating HfO₂ films because it offers several advantages over other techniques. These advantages include the ability to produce films with fewer defects, precise control over film thickness, and excellent

<footnote><sup>a</sup> Microelectronic R&D Center, School of Mechanical & Electronic Engineering and Automation, Shanghai University, Shanghai, 200072, China</footnote>

<footnote><sup>b</sup> Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai, 200072, China</footnote>

<footnote><sup>c</sup> Light, Nanomaterials, Nanotechnologies (L2n) Laboratory, CNRS ERL 7004, University of Technology of Troyes, 12 rue Marie Curie, 10004, Troyes, Cedex, France</footnote>

<footnote><sup>d</sup> NCD-SWEET beamline, ALBA Synchrotron, 08290, Cerdanyola del Valles, Barcelona, Spain</footnote>

<footnote><sup>e</sup> Shanghai Advanced Research Institute, Chinese Academy of Sciences, 201210, China</footnote>

<footnote><sup>f</sup> Dutch-Belgian Beamline (DUBBLE), European Synchrotron Radiation Facility (ESRF), 71 Avenue Des Martyrs, CS 40220, Grenoble, 38043, France</footnote>

<footnote><sup>g</sup> Magnetic Resonance Research Center, Institute for Molecules and Materials, Radboud University Nijmegen, the Netherlands</footnote>

<footnote>* Corresponding author. Dutch-Belgian Beamline (DUBBLE), European Synchrotron Radiation Facility (ESRF), 71 Avenue Des Martyrs, CS 40220, Grenoble, 38043, France</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail address: bing.wu@ru.nl (B. Wu).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** AFM characterization of HfO₂ film deposited on SiO₂ substrates. The thicknesses are (a) 5 nm, (b) 10 nm, (c) 20 nm, and (d) 30 nm. (e) SiO₂ substrate. RMS is the root mean square roughness value. The white dots observed in Fig. 1(b–d) are probably HfO₂ particles. As the film thickness surpasses the critical point of around 10 nm, grains start to form within the film. This grain formation leads to a rougher surface as the grains merge together. The white dots visible in Fig. 1(e) are believed to be SiO₂ particles that have been carried over from the substrate.</figcaption></figure>

conformity to the substrate surface [18–21]. Given the pivotal role of the dielectric layer in determining the ultimate performance of TFTs, a comprehensive understanding of the nanostructure and morphology of ALD-deposited HfO₂ films is essential for the design and optimization of next-generation devices [22,23].

In the field of material sciences, several advanced spectroscopic and scattering techniques, such as small angle X-ray scattering (SAXS) [24–26], X-ray photoelectron spectroscopy (XPS) [27–29], atomic force microscopy (AFM) [30–32], nuclear magnetic resonance (NMR) [33–35] have been frequently implemented to understand the morphology and compositions of novel materials. However, to understand the properties of thin HfO₂ films, researchers often use two techniques: grazing incidence small-angle X-ray scattering (GISAXS) and XPS. These techniques provide valuable information about the film’s structure and composition [36]. Stemmer et al. used GISAXS in combination with high resolution transmission electron microscopy (TEM) to investigate the phase separation in hafnium silicate films [23]. The initial growth of HfO₂ was also studied by means of synchrotron-based in situ X-ray fluorescence and GISAXS [37]. Furthermore, Rudenja et al. utilized XPS to study the HfO₂/Si interface made from low temperature deposition [38]. While many studies have investigated the structural changes and interactions between HfO₂ films and other materials, fewer studies have examined how these changes affect the electrical properties of the films and the performance of devices that use them.

In our previous study [39], GISAXS was employed to investigate the correlation between the thickness of the Al₂O₃ layer and the dielectric properties of the fabricated TFTs. We discovered that there are two different thickness thresholds for dielectric films: one for the initial formation of the film, and the other for the film to reach a point where it has the desired electrical properties. In this work, we further extend our study into HfO₂ type thin films. By correlating GISAXS data with AFM and XPS analyses, we carried out a comprehensive investigation of

electronic, morphological, and physical properties of HfO₂ with different thickness. The results can offer great insight into designing HfO₂-based new generation TFTs.

## 2. Experimental

### 2.1. The fabrication of thin films

The HfO₂ dielectric was firstly deposited on cleaned p-type silicon wafer by ALD (TFS-200, Beneq) at 250 °C with Tetrakis (dimethylamino) hafnium (TMAH) as precursor and H₂O as oxygen source. Before being placed in the ALD chamber, the silicon wafer was cleaned with consecutive rinses of acetone, isopropyl alcohol, and de-ionized water in an ultrasonicator for 15 min, and then treated by Ozone for 10 min. The chamber pressure was controlled at approximately less than 10 mbar. The ZnO as the channel of TFTs was also deposited by ALD at 150 °C with a thickness of 20 nm, diethylzinc (DEZn) and H₂O were selected as the source of zinc and oxygen. After deposition of ZnO active layer, about 200 nm Al was deposited by thermal evaporation to form the source and drain electrodes through a shadow-mask with the channel width (W) of 1000 µm and channel length (L) of 100 µm.

### 2.2. The characterization of thin films

The surface morphology and roughness of thin HfO₂ film was measured by Atomic Force Microscope (AFM) (Dimension XR, Bruker Corporation) in the tapping mode. The thickness (d) and capacitance (C) of HfO₂ films were measured by an ellipsometer (FilmTek SE, Bruker Corporation) and an impedance analyzer (E4990A, Keysight). The equation, $ε = 4πkdC/S$, is used to calculate the dielectric constant ($ε$), where k is static constant, and S is film area. Additionally, the electrical properties of the then-prepared TFTs were measured by semiconductor parametric analyzer (Keithley 4200A-SCS Parameter Analyzer,
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Normalized XPS high resolution spectra of (a) Hf 4f region and (b) O 1s region of all deposited thin films. The dashed lines indicate the position of deconvoluted peaks.</figcaption></figure>

Tektronix) at room temperature. The composition of the thin-film was characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha+, Waltham, MA, USA).

GISAXS investigation was performed at the BL11 NCD-SWEET X-ray beamline at ALBA Synchrotron (Cerdanyola del Vallès, Spain). A monochromatic X-ray beam of 12.4 keV was set using a Si (1 1 1) channel cut monochromator, and further collimated using an array of Be lenses. The sample to detector distance was $D_{SD} = 2599$ mm, calibrated using Silver Behenate as reference. The detector employed was a Pila tus3 S 1M single photon counting detector from Dectris®. For the measurements an incident angle of the X-ray beam with respect to the sample surface was $0.18^\circ$ to resolve the reflect angle of HfO$_2$ and the critical angle of the substrate and to minimize bulk signal. The GISAXS patterns were measured at room temperature.

GIWAXS experiments were carried out at BL16B1 X-ray beam line of

the Shanghai Synchrotron Radiation Facility at Shanghai, China. The measurements were performed at an energy of 10 keV. An incident angle of $\alpha_i = 0.40^\circ$, which was above the critical angle of HfO$_2$ (0.315°) at this energy. The WAXS detector was PILATUS3 900K from Dectris. Measurements were performed at room temperature.

The XPS measurements were conducted using Al Kα radiation as the excitation source, with an electron emission angle of 45°. The analyzed area was 700 µm by 300 µm. Prior to analysis, the samples were subjected to sputter-etching with an Ar + energy of 2 keV at an incidence angle of 30°. The base pressure during the analysis was maintained at $10^{-9}$ mbar to ensure a high vacuum environment. Additionally, a low-energy electron flood gun was employed as a charge neutralizer to prevent charging effects on the sample surface. The XPS banding energy scale is referenced according to literature [40], the $\phi_{SA}$ varies from 2.5 to 3.5 eV, and $E_B$ varies from 286.48 to 287.48 eV.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Normalized XPS high resolution spectra of Hf 4f region (a, b) and O 1s region (c, d) of HfO$_2$ deposited (5 nm & 30 nm) SiO$_2$ films, and their relevant fitting (Hf spectra: Hf suboxide: Hf 4f$_{7/2}$: 16.65 eV; Hf 4f$_{5/2}$: 18.25 eV; HfO$_2$: Hf 4f$_{7/2}$: 17.24 eV; Hf 4f$_{5/2}$: 18.82 eV; O spectra: Oxygen in HfO$_2$: O 1s: 530.11 eV; Non-HfO$_2$ Oxygen: O 1s:531.81 eV).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (a) The GISAXS patterns of HfO₂ films deposited on the Si substrates, measured at room temperature. The color scale is logarithmic, and the deposited thickness is given on each figure. The colors represent the intensity of the scattered X-rays. (b) In-plane and (c) out-of-plane scattering profiles extracted from the GISAXS patterns in Fig. 3(a). The equivalent thicknesses are (from left to right) 5, 10, 20 and 30 nm. The axes $q_y$ and $q_z$ represent the components of the scattering vector $q$ in the plane of the sample and perpendicular to it, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

# 3. Results

## 3.1. AFM analyses

AFM is a technique that can be used to measure the tiny bumps and valleys on the surface of a film. It's very sensitive, so it can detect even very small variations in height. Fig. 1 presents AFM images depicting SiO₂ substrates coated with varying thicknesses of HfO₂ films. The surface of all four HfO₂ films was quite smooth. As the film thickness increased from 5 to 30 nm, the roughness increased slightly, from 0.22 to 0.32 nm. This indicates that the films grew smoothly and evenly on the surface. The surfaces of the deposited HfO₂ films exhibited only minute grains, in stark contrast to the significantly rougher surface of the SiO₂ substrate. This counterintuitive finding can be attributed to the atomic-level control inherent in the atomic layer deposition (ALD) process. During the initial stages of deposition, HfO₂ atoms exhibit a strong affinity for the substrate, resulting in the replication of its surface morphology. As the film thickness exceeds the critical threshold of approximately 10 nm, grain nucleation and coalescence occur, leading to an increase in surface roughness. The smoother surface of the 5 nm film suggests that at this thickness, the surface energy plays a significant role in the total energy of the system, suppressing the structural transformation from amorphous to polycrystalline and resulting in a more uniform and smoother surface. This surface roughness significantly influences the nanostructure of the oxide, creating defect-rich sites conducive to enhanced vacancy formation and oxygen ion mobility. These factors contribute to favorable switching properties in the

material.

## 3.2. High resolution XPS analyses

The chemical state and composition of the HfO₂ films were analyzed using XPS. High-resolution core level XPS scans were conducted on the Hf 4f and O 1s peaks for all HfO₂-deposited films with varying deposition layer thicknesses, see Fig. 2. Subsequently, the obtained spectra were deconvoluted based on established methodologies from previous studies [4,41].

As depicted in Fig. 3, two distinct sets of peaks are discernible in the Hf 4f spectra for all deposited films following meticulous deconvolution. One set of these peaks corresponds to HfO₂ (depicted by the red dashed line in Fig. 2a), while the other set originates from HfOₓ (illustrated by the blue dashed line in Fig. 2a). This observation aligns with a prior study utilizing ARXPS [42], the occurrence of HfOₓ is attributed to the higher electronegativity of Si compared to Hf. Consequently, its presence typically intensifies with the augmentation of the SiO₂/HfO₂ interface area. Additionally, thorough analyses of the O 1s spectra reveal the existence of two distinct species. These can be identified as O within HfO₂ (indicated by the blue dashed line in Fig. 2b) and O outside of HfO₂ (depicted by the red dashed line in Fig. 2b), consistent with discussions in a previous study [43]. These non-HfO₂ species can be SiO₂ substrate as well as some C-O and absorbed oxygen species (e.g. O⁻, O⁻², or O₂⁻). The ratio between O within HfO₂ and O outside of HfO₂ experiences an increase with the progressive thickening of the HfO₂ layer.
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Evolution of the scattering intensity in vertical direction for HfO₂, illustrating the Yoneda peaks of the Si substrate in the first stages (dark curves, peaks at around $q_z \approx 0.155 \text{ nm}^{-1}$) and the evolution of the strong Yoneda peak intensity of the HfO₂ layer evolving at $q_z \approx 0.365 \text{ nm}^{-1}$. Afterwards, the HfO₂ Yoneda peak is partly overlapped by the Kiessig fringes developing due to the growing film thickness.</figcaption></figure>

### 3.3. GISAXS and GIWAXS analyses

As a powerful technique, scattering techniques have been frequently used in the material characterizations [24,44,45]. In this investigation, GISAXS analyses were employed to gain specific insights into the growth dynamics of HfO₂ thin films on the substrate. The measurements were conducted at an incident angle of the X-ray beam with respect to the sample surface set at 0.18°. This angle was carefully chosen to discern the reflection angle of HfO₂ and the critical angle of the substrate while minimizing bulk signal interference.

The observed augmentation in the scattering signal, as depicted in Fig. 4a, provides confirmation of thin film growth. This phenomenon is attributed to the deposition of electronically dense HfO₂ through ALD. Moreover, the augmentation of the diffuse scattering corroborated the sample roughness increase when increasing the number of ALD cycles, which agrees with the AFM results. As the HfO₂ coverage is elevated, distinctive vertical interference fringes emerge in the perpendicular direction. These fringes exhibit increased intensity and are positioned at lower $q_z$ values, providing clear evidence of the continuous growth of the thin film in the surface normal direction. This observation points to a horizontal lamellar structure, specifically indicating the ongoing deposition with additional ALD cycles [46].

To investigate the in-plane and out-of-plane growth characteristics of ALD-grown HfO₂ layer, vertical and horizontal line cuts were executed, as illustrated in Fig. 4b and c. The lack of distinct features in the scattering profiles along the $q_y$ direction suggests the absence of cluster ordering at the observed length scales in the in-plane direction. In contrast, the out-of-plane scattering profiles ($q_z$) exhibited distinct fringes with evident peak maxima that evolved towards lower values of $q_z$ throughout the deposition process. This evolution is indicative of the anticipated growth of the thin film along the in-plane direction.

The average thickness $\delta_{HfO_2}$ for each deposition thickness can be

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** The vertical cuts at $q_y = 0 \text{ nm}^{-1}$ along the different incident angle with corresponding film thickness. The equivalent thicknesses are (from a to f) 1, 3, 5, 10, 20 and 30 nm.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. (a) GIWAXS patterns of HfO₂ films with different thickness, which were deposited on Si substrates and measured at room temperature. The color scale is logarithmic and the deposited thickness is given on each figure (b) The in-plane and (c) out-of-plane linecuts of the GIWAXS patterns. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

determined from the scattering peaks using the equation $\delta = 2\pi\Delta q_z^{-1}$, where $\Delta q_z$ represents the difference between two neighboring maxima or minima [47]. The obtained GISAXS data allows the determination of the final film thickness, $\delta_{HfO_2}$ (final), to be approximately 4.97, 9.12, 20.93, 29.66 nm after 75, 150, 300, and 450 deposition cycles, respectively. These values align with the expected thin film thicknesses. Additionally, the Yoneda-Vineyard peak position (Fig. 5), derived from the lower $q_z$-range of the vertical line cuts, exhibits a gradual shift from the Si substrate position in the initial stages (peaks at around $q_z \approx 0.155 \text{ nm}^{-1}$) to the HfO₂ layer evolving at $q_z \approx 0.365 \text{ nm}^{-1}$. This shift provides evidence of complete surface coverage during the ALD procedure as the thin film undergoes growth.

To assess the nanostructural uniformity on both the surface and bulk of these films, the incident angle was varied from 0.16° to 0.22° in increments of 0.02°. As illustrated in Fig. 6, the determined Yoneda peak position gradually shifted to higher values with an increase in the

incident angle. While the scattering intensity noticeably increased at larger incident angles, the consistent shape of the profiles indicated a uniform structure extending from the surface to the bottom of the films.

GIWAXS measurements were conducted at an incident angle $\alpha_i = 0.40^\circ$ with an energy of 10 keV. Fig. 7 presents representative GIWAXS patterns at different thicknesses along with corresponding in-plane and out-of-plane profiles. Notably, a faint Bragg peak at approximately $q_y = 2.4 \AA^{-1}$ is discernible when the film thickness is 5 nm, suggesting an amorphous structure for HfO₂. Scattering intensity increase with the film thickness. As the thickness progresses from 5 to 10 nm, a distinct Bragg peak at $q_y = 2.3 \AA^{-1}$ emerges, indicating the presence of ordered HfO₂ domains. The intensity of this peak exhibits a growth with increasing film thickness. However, it remains broad, and no sharp peaks around $q_y = 2.3 \AA^{-1}$ are observed even as the film thickness reaches up to 30 nm. This suggests that HfO₂ demonstrates a tendency

<figure><img src="image_10.png" /><figcaption>Fig. 8. (a) Water contact angle of HfO₂ films on silicone substrate. (b) Dielectric constant of HfO₂ with different thickness.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Dielectric Properties of HfO₂ films: leakage current density characteristics.</figcaption></figure>

towards crystallization but maintains poor crystallinity during ALD deposition.

On the other hand, another Bragg peak ($q_y = 0.25 \AA^{-1}$) is originated from the arrangement of HfO₂ grains. The calculated domain spacing of HfO₂ grains is approximately 2.5 nm, determined using the Bragg equation. This peak widens as the film thickness increases, indicating the growth of HfO₂ grains with thickness. These findings align with the morphology investigation using AFM (see Fig. 1). The out-of-plane linecuts of these HfO₂ films exhibit oscillations when the $q_z$ value is below $1 \AA^{-1}$, indicating a smooth surface. The scattering intensity at $q_z = 2.3 \AA^{-1}$ significantly enhances as the film thickness reaches 10 nm, suggesting that the growth direction of HfO₂ nucleation is not fixed.

As the film grain size begins to increase, the structure becomes more compact, and defects such as voids decrease, the density of the film improves, but a crystalline structure has not yet appeared. Materials exhibiting a high degree of crystallinity possess low carrier concentrations, resulting in significantly reduced leakage current in TFT devices. Additionally, the enhanced stability of the crystalline structure renders these materials suitable for TFT applications.

An increase in HfO₂ film thickness resulted in the emergence of distinctive vertical interference fringes in the perpendicular direction, indicative of continuous growth along the surface normal. Conversely, the scattering profiles along the in-plane direction exhibited distinct fringes with peak maxima shifting towards lower values, suggesting in-plane growth. These observations collectively indicate a strong

<table><caption>**Table 1**

Threshold voltage, mobility and switching ratio of the TFT devices at different temperatures.</caption><thead><tr><th>Temperature (°C)</th><th>V<sub>th</sub> (V) ±0.5</th><th>μ (cm<sup>2</sup>/Vs) ±1</th><th>I<sub>on</sub>/I<sub>off</sub></th></tr></thead><tbody><tr><td>25</td><td>-1.59</td><td>7.81</td><td>1.43*10<sup>6</sup></td></tr><tr><td>50</td><td>-1.38</td><td>7.26</td><td>1.41*10<sup>6</sup></td></tr><tr><td>75</td><td>-1.23</td><td>8.01</td><td>1.36*10<sup>6</sup></td></tr><tr><td>100</td><td>-1.05</td><td>7.91</td><td>1.06*10<sup>6</sup></td></tr></tbody></table>

dependence of the film structure on thickness.

### 3.4. Water contact angle and dielectric properties

As shown in Fig. 8a, the water contact angle test suggests that the critical thickness for physical film formation of HfO₂ is around 5–10 nm, which is similar to the value obtained for Al₂O₃ type thin film [39]. On the other hand, the electrical properties and transfer characteristics of HfO₂-based TFT were measured by a semiconductor parametric analyzer (Keithley 4200) at room temperature. Fig. 8b reveals the dielectric properties of HfO₂ films with different thickness. It was observed that the leakage current exhibited an exponential increase with bias for thicknesses below 20 nm, whereas a more gradual increase was observed for thicknesses exceeding 20 nm. (See Fig. 9). Therefore, a critical thickness range of 10–20 nm was identified for the transition from electrically conductive to dielectric behavior in HfO₂ films, significantly lower than the 30 nm threshold observed for Al₂O₃ in our previous study. Furthermore, the dielectric constant of HfO₂ thin films ($\epsilon_k = 8.6$) was considerably higher than that of Al₂O₃ ($\epsilon_k = 6.4$) [39]. This value is also higher than most of widely-used dielectric thin films of the same thickness.

Fig. 9 shows the leakage characteristics of the HfO₂ films with different thickness. The leakage currents of 5, 10 and 20 nm HfO₂ dielectric layers are $8.36 \times 10^{-6}$, $9.37 \times 10^{-9}$ and $6.84 \times 10^{-10} \AA/cm^2$ at a field strength of 2 V/cm respectively. And the breakdown electric field is 7.5, 11.5, and 14 V/cm. In the summary, for a 10 nm-thin HfO₂ film, when the voltage applied to the gate is at a minimum of 2 V/cm, the leakage current density of the dielectric layer is lower than $10^{-7} \AA/cm^2$, meeting the requirements for TFT operation, and it also has a higher breakdown voltage.

### 3.5. Dielectric performance of the TFT device

We further tested the dielectric performance of HfO₂ films using ZnO-based TFT, as schematically illustrated in Fig. 10a. The ZnO as the channel of TFTs was also deposited by ALD at 150 °C with a thickness of 20 nm, DEZn and H₂O were selected as the source of zinc and oxygen. After deposition of ZnO active layer, about 200 nm Al was deposited by thermal evaporation to form the source and drain electrodes through a

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** (a) Schematic of TFT; (b) The transfer characteristic curves ($I_{DS} - V_{GS}$ curves) of TFT devices using 10 nm HfO₂ film as the dielectric layer at different temperature.</figcaption></figure>
shadow-mask with the channel width (W) of 1000 µm and channel length (L) of 100 µm. The output curves of the TFT with increasing temperature using 10 nm HfO₂ film as a dielectric layer is shown in Fig. 10b. The TFT exhibit good electrical performance at room temperature, with a high current on/off ratio of 10⁷, a low threshold voltage of –1.59 V and high mobility of 7.81 cm²/Vs.

A threshold voltage shift of 0.54 V is observed under the temperature stress from 25 to 100 °C (see Table 1), indicating the 10 nm-HfO₂-based TFT is also demonstrated to have thermal stability.

The threshold voltage shifts of 0.54 V mentioned in Table 1 indicates the change in threshold voltage from room temperature (25 °C) to 100 °C. Thin-film transistors (TFTs) characterized by lower threshold voltages exhibit reduced power consumption, thereby enhancing energy efficiency in electronic circuits. Furthermore, the low threshold voltage facilitates compatibility with low-voltage systems, enabling efficient and reliable operation even at reduced supply voltages.

Notwithstanding the observed shift, the overall performance of the device, including the on/off current ratio, remained stable and met the prescribed specifications. Furthermore, the TFT parameters were found to remain within acceptable ranges for functional operation, even when subjected to temperature stress up to 100 °C. Specifically, the on/off current ratio was maintained at a value of 10⁶, satisfying the requirements for TFT devices employed in various applications.

In summary, the 10 nm-HfO₂-based TFT exhibited excellent thermal stability, preserving critical electrical parameters essential for device functionality. Although a slight decrease in the on/off current ratio was observed, a concomitant reduction in threshold voltage and an increase in mobility were noted.

## 4. Conclusions

In conclusion, this study investigated the characteristics of HfO₂ films deposited via ALD as a function of film thickness. XPS analysis indicated a decrease in the HfOₓ/HfO₂ ratio and the concentration of oxygen species outside the HfO₂ layer with increasing film thickness. The nucleation of HfO₂ grains exhibits a non-directional growth pattern, with an inter-grain spacing of approximately 2.5 nm. By integrating the observed structural morphology with the electrical performance data, it was determined that a 10 nm-thick ALDed HfO₂ dielectric film offers an optimal balance of dielectric properties and low leakage current. This high-quality dielectric property is further confirmed by using 10 nm HfO₂ film as a dielectric layer of a ZnO-based TFT. The then made TFT demonstrated exceptional electrical performance at room temperature, exhibiting a high current on/off ratio of 10⁶, a low threshold voltage of –1.59 V and high mobility of 7.81 cm²/Vs. Furthermore, this HfO₂-based TFT is also demonstrated to have excellent thermal stability even at 100 °C. This approach offers the feasibility of controlling the thickness of the dielectric layer while maintaining adequate high-quality dielectric properties, which are essential for achieving high-performance TFTs and their potential applications in low-power consumption microelectronics.

### CRediT authorship contribution statement

**Minyu Chen:** Writing – original draft, Methodology, Investigation, Data curation.
**Huimin Chen:** Investigation, Data curation.
**Eduardo Solano:** Investigation.
**Danile Hermida Merino:** Investigation.
**Wen-qiang Hua:** Investigation.
**Xingwei Ding:** Investigation.
**Bin Wei:** Investigation.
**Yingjie Liao:** Writing – review & editing, Methodology, Investigation.
**Bing Wu:** Writing – review & editing, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence

the work reported in this paper.

### Acknowledgements

Authors acknowledge funding from the EU Horizon2020 Marie Curie Cofund project (Grant No. 713279). The authors also acknowledge beamline BL16B1 at the Shanghai Synchrotron Radiation Facility for providing GIWAXS beam time. The authors thank BL11 NCD-SWEET beamline at ALBA Synchrotron for GISAXS experiments.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.vacuum.2024.113923.

### Data availability

Data will be made available on request.

### References

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