† Corresponding author. E-mail:
Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB2200103).
Hafnium disulfide (HfS2) is a promising two-dimensional material for scaling electronic devices due to its higher carrier mobility, in which the combination of two-dimensional materials with traditional semiconductors in the framework of CMOS-compatible technology is necessary. We reported on the deposition of HfS2 nanocrystals by remote plasma enhanced atomic layer deposition at low temperature using Hf(N(CH3)(C2H5))4 and H2S as the reaction precursors. Self-limiting reaction behavior was observed at the deposition temperatures ranging from 150 °C to 350 °C, and the film thickness increased linearly with the growth cycles. The uniform HfS2 nanocrystal thin films were obtained with the size of nanocrystal grain up to 27 nm. It was demonstrated that higher deposition temperature could enlarge the grain size and improve the HfS2 crystallinity, while causing crystallization of the mixed HfO2 above 450 °C. These results suggested that atomic layer deposition is a low-temperature route to synthesize high quality HfS2 nanocrystals for electronic device or electrochemical applications.
Two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered as promising candidate materials for optoelectronic devices because of their fascinating electronic, optical, and mechanical properties.[1–5] As a member of TMDs family, hafnium disulfide (HfS2) is a layered material in which the layers are connected by weak van der Waals forces and Hf and S atoms in the layers are connected by strong chemical bonds. HfS2 possesses ultrahigh room-temperature carrier mobility, which is necessary for the realization of high-performance electronic devices.[6] However, instability of HfS2 makes it more difficult to synthesis and be applied in atmosphere ambient. Environmental sensitivity study performed on HfS2 proves that oxidation due to moisture and oxygen adsorption occurs quickly.[7,8]
Controllable synthesis of TMDs is necessary for obtaining high quality, uniform, atomic thin films to realize business application. Currently, the synthesis methods of two-dimensional materials are mainly divided into top-down methods (such as mechanical exfoliation and chemical exfoliation) and bottom-up methods (such as chemical vapor deposition and atomic layer deposition).[4,9] In current scientific research, most of the 2D TMDs have been obtained by means of mechanical exfoliation, in which single-layer or few layers of HfS2 are divided by a tape. HfS2 layers obtained by mechanical exfoliation have high quality, but low yield and poor repeatability, making it difficult to obtain large size films. HfS2 films with large size and high uniformity can be achieved by chemical vapor deposition using HfCl4 powders and sulfur pieces or sulfur powders.[10–17] However, the growth temperature of chemical vapor deposition is high, and the quality of the films is greatly affected by the environment inside the reaction chamber.
As a special chemical vapor deposition method, the atomic layer deposition (ALD) has a growth temperature below 500 °C, so it is well compatible with the complementary metal–oxide–semiconductor (CMOS) process. ALD is an indispensable method to combine two-dimensional materials with traditional semiconductors. In addition, ALD has unique self-limiting reaction mechanism, which can grow films with high uniformity, high shape preserving, and precise thickness control. Therefore, ALD has been widely used in the synthesis of 2D TMDs, such as MoS2,[18–26] WS2,[27–29] SnS2,[30,31] and ReS2.[32,33] So far, HfS2 and ZrS2 thin films have been deposited by ALD using HfCl4 and ZrCl4 with H2S as the precursors.[34]
In this work, we synthesized HfS2 films by remote plasma enhanced atomic layer deposition (RPALD) at lower temperature, with Hf[(C2H5)(CH3)N]4 (TEMAH) and H2S as the reaction precursors. The influence of the deposition temperature on the surface morphology and crystallinity of HfS2 thin films were analyzed by various characterization techniques. The temperature range of self-limiting reaction behavior and the growth rate were determined. The uniform HfS2 nanocrystals were deposited on sapphire, thermally oxidized 300 nm thick silicon dioxide layer (SiO2/Si), and silicon (Si) substrates, respectively, and characterized by Raman and x-ray diffraction. The HfS2 film mixed with hafnium dioxide (HfO2) was distinguished, which was easily oxidized under ambient conditions.
HfS2 thin films were deposited in a commercial PRALD setup (R200 Advanced) from PICOSUN. The growth was performed on diverse substrates: sapphire, SiO2/Si, and Si. Sapphire and 300 nm SiO2/Si substrates were cleaned by acetone, ethanol, and deionized water in an ultrasonic bath for three times. Si was cleaned by boiled H2SO4/H2O2 (4 : 1) for 10 min, then etched by HF/H2O (1 : 20) for 4 min, and washed in deionized water. The Hf precursor employed was TEMAH, which was heated to 120 °C and delivered to the reactor chamber at 130 °C. H2S (3.5%, balance Ar) with a flow rate of 150 standard cubic centimeter per minute (sccm) was used as the S source. A remote plasma source operating at the power of 1000 W was used for the H2S plasma step. The deposition temperature was varied from 150 °C to 500 °C and the chamber pressure was controlled at 6 hundred Pa (hPa), regardless of the deposition temperature. Two precursors were alternately exposed to the substrates and subsequently purged for each precursor in an ALD cycle. One growth cycle consists of four main steps: (i) 1.6 s exposure to TEMAH, (ii) 10 s N2 purge, (iii) 10 s exposure to H2S, (iv) 10 s N2 purge. By repeating these steps, HfS2 thin films with desired layers can be obtained. The possible reaction is as follows:
The thickness of the HfS2 films was measured by x-ray reflectivity using a PANalytical instrument. Atomic force microscope (AFM) was performed with tapping mode on a SPA400-Nanonavi instrument. Raman spectra were measured with 532 nm excitation under ambient conditions using IDSpec ARCTIC, and the power levels on the sample were 0.1 mW. The x-ray diffraction (XRD) was performed on a Rigaku IV instrument. The x-ray photoelectron spectroscopy (XPS) was performed on a PHI Quantum-2000 instrument, and a conventional Al Kα anode was used as the source of x-ray radiation.
The self-limiting growth behavior of the ALD process for HfS2 thin films was investigated at temperatures ranging from 150 °C to 500 °C. Figure
The optical images of samples grown on 300 nm SiO2/Si, sapphire, and Si substrates by RPALD are shown in Fig.
We also examined the surface morphology of the HfS2 films by AFM and SEM measurements. The AFM images of the as-grown HfS2 films deposited on Si substrates for 100 cycles at various temperatures are shown in Figs.
In order to study the effect of substrate on the surface morphology of the deposited HfS2 films, the AFM images of the samples grown on Si and sapphire substrates are compared. Figures
Auger electron spectroscopy was used to analyze the element fraction in the HfS2 thin films. Figure
In order to further determine the chemical states of Hf, S, and O in the films, XPS was used to evaluate the formation of chemical bonding in the HfS2 films. To remove oxidation of the surface, 5 nm HfS2 films on the surface were etched away by Ar ions. The XPS spectra of the surface and at 5 nm depth of the HfS2 films were tested. Figure
According to the XPS measurement of the surface and at 5 nm depth of the HfS2 films, the atomic percentages of Hf, S, and O in the HfS2 thin films were evaluated by the sensitivity factor method, and the results are listed in Table
Raman spectra of the HfS2 films grown on SiO2/Si for 300 cycles at various temperatures are shown in Fig.
Grazing incident XRD was also used to study the structural properties and crystalline nature of the HfS2 films. Grazing incident XRD patterns of the as-grown HfS2 films with small signal from the oxide phase are shown in Fig.
In summary, we have successfully deposited HfS2 nanocrystal thin films by RPALD at low temperature with TEMAH and H2S as the precursors. Self-limiting growth behavior is observed in the temperature range of 150 °C to 350 °C with a growth rate of about 0.11 nm/cycle. The surface of the deposited HfS2 is smooth with the appearance of uniformly nanocrystal grains. The size of the nanocrystal grain depends on the growth temperature, the kind of substrate, and the HfS2 film thickness. The crystalline HfS2 is achieved at lower temperature down to 250 °C, and the crystal quality is significantly improved with increasing deposition temperature. However, the as-deposited HfS2 films are almost oxygen-rich, in which crystallized HfO2 is detected for the sample deposited at higher temperature. In addition, the as-deposited HfS2 is easily oxidized in the atmosphere. Further investigation is needed to lower the oxygen content and increase the crystallinity of the HfS2 films. Low temperature atomic layer deposition is believed to be a promising route to large area HfS2 nanocrystal thin films growth for electronic or electrochemical applications in the future.
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