Ting Lu received her BS degree from Chongqing Normal University in 2022 and is now a master's candidate at Hunan University under Professor Song Liu. Her research direction is two-dimensional materials and devices.
Zhuojun Duan received her M.Sc degree of Physics from Xiangtan University in 2021. She is a Ph.D. student of chemistry in Hunan University supervised by Professor Song Liu. Her research mainly focuses on controllable synthesis of two-dimensional materials by CVD method.
Song Liu received his BS degree from Nankai University in 2006 and PhD degree at Peking University in 2011. Then he joined the team of Professor Liming Dai at Case Western Reserve University in the United States as a postdoctoral fellow. He joined the National University of Singapore as a research fellow. He joined Hunan University in 2016 as a full professor. His research interests include controlled synthesis of low-dimensional layered materials, application research of functional devices, and flexible wearable sensors.1. Introduction
In recent years, complementary metal-oxide semiconductors (CMOS) have been driving the advancement of modern computers. According to Moore’s Law, transistor size needs to continue shrinking in order to accommodate more transistors on silicon chips[1−3]. However, the reduction in transistor size leads to shorter channel lengths in field effect transistors (FETs), resulting in increased gate leakage current and significant performance degradation due to short channel effects and quantum tunneling effects[4, 5]. There are currently three ideas to solve the above problems: replace traditional metal-oxide-semiconductor field-effect transistors (MOSFETs) with Fin field-effect transistors (FinFETs), replace three-dimensional (3D) silicon crystals with two-dimensional (2D) semiconductor materials with high mobility[6, 7], and use gate dielectric layers with high-κ dielectric constants[8]. Extensive exploration has led to the identification of high-κ materials as ideal gate dielectrics to replace silicon dioxide. High-κ materials have a smaller physical thickness limit at the same equivalent oxide thickness (EOT), enabling further transistor scaling[9−11].
In recent years, the research on high-κ materials has expanded, with mica, TiO2[12], GaS[13], CaF2[14], MoO3[15], Ta2O5[16], Bi2SeO5[17], HfOx[8], and ZrOx[18] being among the most extensively studied materials. Single-crystal zirconia (ZrO2) has emerged as a promising gate dielectric material with desirable properties, including a suitable dielectric constant (theoretical κ = 25), wide band gap (Eg = 5.8−7.8 eV), ideal conduction band offset, and good thermodynamic stability when in direct contact with silicon substrates[19].
Dielectric materials are mainly amorphous and single-crystal structures, and the preparation method is one of the important factors affecting the structure and properties of ZrO2 gate dielectric during the preparation of MOS devices. Atomic layer deposition (ALD) is the most common method for preparing amorphous dielectric materials and has universal applicability[20]. However, ALD is not compatible with 2D materials, and ALD on 2D materials is usually achieved by random nucleation at defects, edges, and impurities, so it is easy to form highly porous films[21]. The solution is to vaporize the metal oxide buffer[22], treat the surface with ozone or O2 plasma to create an interface activation layer[23], which provides a nucleation site for further ALD growth of the oxide. However, these methods can destroy or change the electronic properties of the underlying 2D material, especially when the 2D material is thinned down to the thickness of a single cell. Single-crystal ZrO2 nanosheets can be prepared by thermal oxidation and chemical vapor deposition (CVD) as well. CVD offers advantages such as low processing temperature, reasonable deposition rate, uniform nanosheet deposition, reproducibility, good adhesion, and compatibility with large-area growth, precise composition control, and conformal coverage on complex substrates[24, 25]. Yamada et al. achieves the growth of single-crystal m-ZrO2 nanosheets by thermal conversion of NH4Zr2F9[26]. We successfully prepares high-κ single-crystal m-ZrO2 by controlled oxidation of ZrS2 grown by CVD[27]. However, neither of the above two methods can prepare single-crystal ZrO2 nanosheets in one step, and the operation is more complicated.
In this study, we successfully prepare high-quality 2D single-crystal m-ZrO2 nanosheets using a one-step CVD method. 5A molecular sieves participate in the CVD process at a temperature of 830 °C. Comparative experiments confirm the oxygen supply effect of 5A molecular sieves during the deposition process. Using mechanically exfoliated thin-layer MoS2 as the channel material, we fabricate a MoS2−ZrO2 heterojunction through dry transfer. FETs are successfully constructed, and their electrical properties are investigated. The FETs exhibit a high carrier mobility of up to 5.50 cm2·V−1·s−1 and Ion/off is about 104, meeting the standards for current logic circuits. These results demonstrate the potential application value of ZrO2 nanosheets as gate dielectrics. Our work presents a viable approach to prepare single-crystal m-ZrO2, laying the foundation for high-κ gate dielectric applications and creating conditions for the development of high-performance semiconductor devices.
2. Experimental Section
5A molecular sieves pretreatment: 5A molecular sieves (General Reagent) are packed into an alumina porcelain boat and placed in the center of a tubular furnace. Sufficient high-purity argon is injected into the quartz tube for gas washing. Then, 100 sccm of high-purity argon is injected, and the program is initiated. The temperature is ramped up to 350 °C at a rate of 20 °C/min and maintained at 350 °C for 6 h. Afterward, the tube furnace is allowed to cool naturally to 150 °C. The boat is then removed and left in a clean and dry air environment for 2 days to ensure full adsorption of oxygen from the air.
Synthesis of ZrO2 flakes: Initially, the silicon wafer is subjected to O2 plasma treatment (power: 100 W, t: 1 min) to make the surface hydrophilic. Next, a prepared 1 g/L K3[Fe(CN)6] solution is evenly spun onto the silicon wafer substrate (3000 rpm, 30 s). ZrCl4 powder (20 mg, ≥99.9%, Aladdin), S powder (220 mg, Sigma-Aldrich), and the pre-treated 5A molecular sieves are loaded into the alumina porcelain boat, positioned near the inlet of the quartz tube. After flushing the tube with sufficient high-purity argon gas to remove residual air, Ar (100 sccm) and H2 (9 sccm) are introduced. The tube is heated to 830 °C at a rate of 20 °C/min. Once the tube reaches 830 °C, the porcelain boat is quickly pushed into the reaction area to initiate the reaction, and the temperature is maintained for 6 min. Following the completion of the reaction, the tube furnace is allowed to cool naturally to room temperature.
Characterization: The as-grown 2D ZrO2 flakes are examined using an optical microscope (H600L, Nikon microscope), 532 nm laser Raman spectroscopy (Bruker SENTERRA Ⅱ), atomic force microscopy (AFM) (Bruker Dimension Icon), scanning electron microscopy (SEM) (Regulus 8100), X-ray diffraction (XRD) (Bruker Dimension Icon D8 Advance system, 40 kV, 40 mA Cu Kα radiation), X-ray photoelectron spectroscopy (XPS) (AXIS SUPRA) (λ = 1486.6 eV), and transmission electron microscopy (TEM) (JEM-2100Plus).
Fabrication of MoS2−ZrO2 heterojunction and FET devices: Initially, a thin layer of MoS2 is obtained by mechanical exfoliation onto a SiO2/Si surface. Subsequently, a thin layer with a larger size of ZrO2 nanosheets is directly separated from the silicon wafer using polydimethylsiloxane (PDMS). The ZrO2 nanosheet is transferred to the MoS2 layer using a dry transfer platform (E1-T), completing the preparation of the heterojunction. Next, the electrode is prepared using the In/Au (5/20 nm) hot evaporation method. Any residual photoresist is removed using acetone, followed by washing with isopropyl alcohol to eliminate any remaining acetone residue.
3. Results
The preparation method of ZrO2 nanosheets is illustrated in Fig. 1(a). A SiO2/Si substrate is initially spin-coated with a K3[Fe(CN)6] solution (1 g/L), serving as crystal seeding agents for ZrO2 nanosheet growth[27]. Upstream of the substrate, the precursor ZrCl4 powder, S powder, and 5A molecular sieves are placed, while Ar/H2, a gas mixture, is used as the carrier gas. The entire experimental setup is heated to 830 °C, at which point the porcelain boat containing the precursor is rapidly inserted into the reaction zone for the reaction to take place. Initially, ZrCl4 and S powder react to form ZrS2. As the molecular sieves are heated, they release oxygen, which gradually replaces the sulfur atoms. Consequently, an oxygen atmosphere is formed in the furnace and ZrS2 begins to oxidize. After 6 min of growth and gradual oxidation, ZrO2 nanosheets are obtained. The corresponding optical image is displayed in Fig. 1(b). Both vertical and horizontal growth samples are distributed on the substrate. The thin-layer samples are mostly distributed in regular hexagonal and semi-hexagonal shapes, with sizes ranging from 6 to 8 μm. Vertically grown samples can also be transferred to a new substrate by physical pressing to form a flat sample, and the size is larger than the flat grown sample. Raman spectra of the ZrO2 nanosheets are presented in Fig. 1(c). The dominant peak observed is at 179 cm−1. Additionally, nine characteristic peaks (188, 222, 334, 349, 381, 480, 617, and 632 cm−1) closely align with the vibrational modes of m-ZrO2. Specifically, 179, 188, 381, 480, and 632 cm−1 correspond to the Ag vibration mode, while 334 and 617 cm−1 correspond to the Bg vibration mode[28]. Fig. 1(d) exhibits a typical scanning electron microscope (SEM) image of a ZrO2 wafer on a SiO2/Si substrate. The contrast between light and dark regions reveals differences in sample thickness, suggesting a more uniform growth with thicker edges compared to the inner regions. The atomic force microscopy (AFM) image of the ZrO2 nanosheets, shown in Fig. 1(e), indicates a minimal thickness of 16.1 nm, making it suitable as a gate dielectric for FETs.
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To further confirm the crystal structure of the sample, X-ray diffraction (XRD) characterization is performed to obtain the pattern of the grown sample, with 2θ ranging from 15° to 60° (Fig. 2(a)). The peaks observed at 28.1° and 31.5° correspond perfectly to the (−111) and (111) crystal plane structures of monoclinic ZrO2, excluding the substrate peaks. This indicates that the obtained ZrO2 nanosheets are of the monoclinic phase. The presence of substrate peaks in the XRD pattern can be attributed to the incomplete coverage of the ZrO2 nanosheets on the substrate during deposition, leaving a portion of the substrate exposed and detected. Notably, the dominant intensity of the (−111) peak compared to the (111) peak suggests preferential growth along the (−111) crystal plane direction.
X-ray photoelectron spectroscopy (XPS) is employed to characterize the chemical composition of the prepared ZrO2 nanosheets (Fig. 2(b)). The presence of Zr and O elements is identified as the main components of the sample. Narrow XPS spectra of the Zr 3d and O 1s regions of the ZrO2 nanosheets are depicted in Figs. 2(c) and 2(d), respectively. The 3d spectrogram of Zr displays two peaks, namely Zr 3d3/2 and Zr 3d5/2. In the case of m-ZrO2, the binding energies at Zr 3d3/2 and 3d5/2 are typically 184.44 and 182.00 eV, respectively. The narrow XPS spectrum of the O 1s region shows two distinct peaks: one near 530.00 eV corresponding to lattice oxygen (O−Ⅰ) with low binding energy, and the other near 532.00 eV associated with low coordination or hydroxy-oxygen anions (O−Ⅱ) with high binding energy[29]. The peak areas of these two elements are extracted and analyzed, revealing an element ratio of approximately 2 : 1. This analysis confirms that the material is pure m-ZrO2.
The morphology and microstructure of ZrO2 nanosheets are characterized using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), as depicted in Figs. 3(a) and 3(b), respectively. From the images, it is evident that the transferred sample predominantly exhibits a semi-hexagonal shape. In the HRTEM image (Fig. 3(b)), the lattice arrangement is clearly discernible, indicating the high crystal quality of the grown ZrO2 nanosheets. The white lines in Fig. 3(b) correspond to the (−111) and (111) crystal faces of ZrO2, respectively. It is notable that the ZrO2 nanosheets primarily grow along the (−111) plane direction, consistent with the XRD diffraction results. By measuring and calculating the diagonal lattice spacing in selected area electron diffraction (SEAD) (Fig. 3(c)), it is determined that the two sets of diffraction points correspond to the (−111) and (022) crystal faces, respectively. However, due to the limited holding time in this particular test, the sample do not exhibit sufficient crystallinity, resulting in tailing in the SEAD pattern. Qualitative and quantitative analysis of ZrO2 nanosheets is conducted using energy-dispersive X-ray spectroscopy (EDS) (Fig. 3(d)). The results indicate that the detected Zr to O content ratio is approximately 1 : 1.89, further confirming the successful growth of high-quality ZrO2 nanosheets through one-step CVD.
To elucidate the crucial role of 5A molecular sieves in the preparation of ZrO2 nanosheets, comparative experiments are conducted. Fig. 4(a) illustrates the optical image of the sample obtained without the addition of 5A molecular sieves, and its corresponding Raman spectrum (Fig. 4(b)) exhibits characteristic peaks at 313 and 333 cm−1, corresponding to ZrS2[30]. In contrast, Fig. 4(c) shows the optical image of the sample obtained with the addition of 5A molecular sieves, and its corresponding Raman spectrum (Fig. 4(d)) displays visible peaks that align with the characteristic Raman peaks of ZrO2. These comparative experiments effectively confirm the role of 5A molecular sieves in facilitating oxygen supply during the growth of ZrO2 nanosheets via one-step CVD.
Based on the aforementioned findings, we propose the following growth mechanism: as the temperature in the quartz tube gradually rises to the deposition temperature, the adsorption capacity of the molecular sieves diminishes, causing the release of oxygen adsorbed within the micropores into the quartz tube. During the subsequent heat preservation period, ZrCl4 and S react on the SiO2/Si substrate, preferentially yielding ZrS2 nanosheets. As a result of the oxygen atmosphere within the tube due to oxygen resolution in the molecular sieves, the ZrS2 nanosheets deposited on the substrate undergo rapid thermal oxidation, ultimately leading to the formation of ZrO2 nanosheets. The deposition temperature plays a vital role in the growth of 2D materials, affecting parameters such as the deposition rate and sample size. To determine the optimal deposition temperature, three different temperatures 780, 830, and 880 °C are selected for comparison regarding their effects on the growth of ZrO2 nanosheets. By examining the optical images (Fig. 4(e)), it is evident that the size of ZrO2 nanosheets increases with higher deposition temperatures. At 780 °C, the ZrO2 sample appears more granular than sheet-like, with a size of only 2−3 μm. This is attributed to the low deposition temperature, slow evaporation rate of the Zr source within the limited holding time, and minimal deposition on the substrate, resulting in small-sized ZrO2 samples. Conversely, at 880 °C, the increased evaporation rate of the Zr source leads to a substantial amount of Zr being transported to the substrate surface. This excess Zr continuously reacts with S powder on the substrate surface, causing the stacking of ZrO2 nanosheets layer by layer and a significant increase in sample thickness. At 830 °C, thin and regularly shaped hexagonal ZrO2 nanosheets with fewer layers can be obtained, making it the most ideal deposition temperature through comparison.
Based on our previous research, it is shown that the dielectric constant of m-ZrO2 is 19.3, and this value is stable and does not decrease with the decrease of thickness[27]. To investigate the electrical properties of single-crystal m-ZrO2 as a gate dielectric, we fabricate a heterojunction FET using a thin layer of mechanically exfoliated MoS2 as the channel material and single-crystal m-ZrO2 as the gate dielectric, as illustrated in Fig. 5(a). The optical microscopy image of the entire device is shown in Fig. 5(b), and the attached figure displays the AFM image of the device, showing that the thickness of ZrO2 is 56.7 nm. Following dry transfer, the ZrO2 nanosheets exhibit a size of 5−6 μm, a flat surface devoid of defects and impurities, and maintain their morphology without any ripples, indicating excellent adhesion with MoS2. The electrode edges are clearly defined, and there is no adhesion between them.
Fig. 5(c) exhibits the output characteristic curve of the MoS2−ZrO2 heterojunction FET. The curve displays a distinct linear behavior and exhibits excellent symmetry on both sides, indicating that the device electrodes establish good Ohmic contact. Fig. 5(d) presents the transfer characteristic curve of the MoS2−ZrO2 heterojunction FET. At a drain voltage (Vd) of 1 V, the FET demonstrates a carrier mobility of 5.5 cm2·V−1·s−1 and Ion/off of approximately 104, which is calculated by formula:
| μ=1CiLW∂Id∂Vg1Vd. | (1) |
These results align with the requirements for practical logic circuits. The fabricated MoS2−ZrO2 heterojunction FET offers electrical properties, with the ZrO2 nanosheets serving as a high-quality gate dielectric material. The device exhibits excellent Ohmic contact, linear output characteristics, and desirable carrier mobility and current switching ratio. These findings highlight the potential of single-crystal m-ZrO2 as an effective gate dielectric in electronic devices, but also indicate that the poor quality of the heterojunction interface of MoS2−ZrO2 prepared by dry transfer leads to poor electrical properties, and there is still much room for improvement.
4. Conclusion
By introducing 5A molecular sieves as oxygen source instead of air thermal oxidation, single-crystal m-ZrO2 nanosheets are successfully grown by one-step CVD with inorganic material ZrCl4 as precursor. This method is easier to operate and the preparation environment is cleaner. At the same time, it fills the vacancy of direct preparation of thin layer single-crystal m-ZrO2 nanosheets by inorganic materials as precursor CVD. The MoS2 FET with m-ZrO2 as the gate dielectric is directly constructed by the dry transfer method. The surface is flat without defects and impurities, and the two have good adhesion. The carrier mobility and Ion/off rate of the device meet the current standard of logic circuits, indicating that the m-ZrO2 nanosheets developed by the device have a good application prospect as the gate dielectric. It creates conditions for the preparation of high-performance semiconductor devices.
Acknowledgments
The work gratefully acknowledges financial support from the National Natural Science Foundation of China (No. 21975067) and Shenzhen Science and Technology Program (No. JCYJ20220530160407016).
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