Investigation of the Energy Band at the Molybdenum Disulfide and ZrO2 Heterojunctions
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The energy band alignment at the multilayer-MoS2/ZrO2 interface and the effects of CHF3 plasma treatment on the band offset were explored using x-ray photoelectron spectroscopy. The valence band offset (VBO) and conduction band offset (CBO) for the MoS2 /ZrO2 sample is about 1.87 eV and 2.49 eV, respectively. While the VBO was enlarged by about 0.75 eV for the sample with CHF3 plasma treatment, which is attributed to the up-shift of Zr 3d core level. The calculation results demonstrated that F atoms have strong interactions with Zr atoms, and the valence band energy shift for the d-orbital of Zr atoms is about 0.76 eV, in consistent with the experimental result. This interesting finding encourages the application of ZrO2 as gate materials in MoS2-based electronic devices and provides a promising way to adjust the band alignment.
KeywordsEnergy band alignment X-ray photoelectron spectroscopy MoS2/ZrO2 CHF3 treatment
Cambridge Sequential Total Energy Package
Conduction band offset
Chemical vapor deposition
Density functional theory
Density of states
Full width at half maximum
Projector augmented wave
Poly methyl methacrylate
Secondary ion mass spectrometry
Tetrakis Dimethyl Amido Zirconium
Transmission electron microscopy
Transition metal dichalcogenides
Valence band offset
X-ray photoelectron spectroscopy
In the past decades, SiO2/Si-based materials played the dominant role in the manufacture of electronic devices, such as integrated logic circuits, nonvolatile memory, and so on. However, as the size of the devices scaled down ceaselessly from micrometers to below 10 nm, the traditional semiconductors have been hard to satisfy the requirement of enhanced specific capacitance, low gate leakage current, and high carrier mobility. Therefore, the exploration of new semiconductors as the device channels and the high-κ oxides as insulators becomes agog. Since the discovery of graphene, the successful fabrication of two-dimensional (2D) materials, especially the semiconductors with suitable band gap, has provided a promising way to overcome this drawback.
Among the 2D materials, molybdenum disulfide (MoS2) with tunable properties based upon both layer count and the choice of substrate materials has drawn an increasing attention due to not only its good chemical stability and mechanical flexibility but also excellent optical and electrical properties [1, 2]. The energy band gap of the monolayer MoS2 is about 1.80 eV while 1.20 eV for bulk. The promising performance of the electronic and optoelectronic devices made from MoS2 layers, such as field-effect transistors [3, 4, 5], sensors , and photodetectors , proves it to be potential substitute of Si in conventional electronics and of organic semiconductors in wearable and flexible systems [8, 9, 10, 11]. Even though single-layer MoS2-based Field-effect transistors (FETs) have exhibited excellent performances with a high current on/off ratio about 108 and a low subthreshold swing ~ 77 mV/decade , its extensive applications were hindered by the synthesis of large area high-quality single-layer MoS2 and the stability of the devices [12, 13, 14]. Multiple-layer MoS2 could be more attractive due to the high density of states, which contributes to high drive current in the ballistic limit . In addition, the carrier mobility of multilayer MoS2 can be further improved significantly by high-κ oxides owing to the dielectric screening effects [16, 17]. Therefore, it is essential and important to investigate the multilayer MoS2/high-κ oxides heterojunctions.
In heterojunction electronic devices, the electron transport properties are precisely controlled by the energy band profiles at the interface between the semiconductor and insulator oxide in the terms of valence band offset (VBO) and conduction band offset (CBO). The VBO and CBO should be as large as possible to operate as a barrier in order to reduce the leakage current formed by the injection of holes and electrons, especially the CBO plays a pivotal role in the selection of suitable high-k oxides for a gate terminal and should be at least larger than 1 eV to avoid current leakage [18, 19, 20]. Meanwhile, the interface charges located at semiconductor/oxides impose an important effect on the band engineering and needs to be optimized through passivation technology, such as SiH4 passivation, and CHF3 treatment. In this paper, we investigated the band alignment of multilayer MoS2/ ZrO2 systems since the nature of the interface has a direct bearing on the characteristics of the devices, and the effect of CHF3 plasma treatment on the band offset at MoS2/ZrO2 interface was explored.
Methods and Experiments
In the experiments, the multilayer MoS2 films were grown on SiO2/Si substrates by chemical vapor deposition (CVD) systems with MoO3 and sulfur powder as the Mo sources and S precursors, respectively. During the growth process, Ar gas was used as the carrier gas and the growth temperature was 800 °C for 5 min. Then the MoS2/ZrO2 samples were prepared by transferring the large area multilayer MoS2 film onto the ZrO2/Si substrates using the poly methyl methacrylate (PMMA) method. The ZrO2 oxide (15 nm) was deposited on Si at 200 °C using atomic layer deposition (BENEQ TFS-200) system with Tetrakis Dimethyl Amido Zirconium (TDMAZr) precursor as the zirconium source and water (H2O) as the oxygen source. In order to investigate the effects of CHF3 treatment on the band alignment at MoS2/ZrO2 interfaces, for one sample, the ZrO2/Si substrate was treated by CHF3 plasma with RF power about 20 W and flow rate about 26 sccm. Meanwhile, the plasma treatment time is about 60 s and the pressure was kept at 1 Pa during the process. Consequently, the resulted F dose is about 2.0 × 1014 atoms/cm2 estimated by secondary ion mass spectrometry (SIMS) measurements. During the optimization process of the plasma treatment time, the CHF3 plasma seriously deteriorated the material quality by introducing fluorine diffused into ZrO2 largely when the time was set at 70 s. While when the plasma treatment time was 50 s, smaller than 60 s, SIMS results demonstrated no obvious F peak at the oxide surface. For the other sample, no CHF3 plasma treatment was implemented. The Raman characteristics of the samples were taken in a RENISHAW system at room temperature. The X-ray photoelectron spectroscopy (XPS) was measured using a VG ESCALAB 220i-XL system. The photon energy of the monochromatized Al Kα x-ray source is about 1486.6 eV. During the measurements, the pass energy was set at 20 eV in order to obtain the XPS spectra. In addition, C 1 s peak (284.8 eV) was used to correct the core-level binding energy in order to eliminate the sample surface differential charging effect.
Results and Discussions
In this paper, we explored the energy band engineering at the multilayer MoS2/ZrO2 interface and investigated the effects of CHF3 treatment using x-ray photoelectron spectroscopy. The results demonstrated that a type I alignment was formed at the MoS2/ZrO2 heterojunction interface with CBO and VBO about 2.49 eV and 1.87 eV, respectively. While the CHF3 plasma treatment increases the VBO by about 0.75 ± 0.04 eV mainly due to the up-shift of Zr 3d core-level energy, which is consistent with the calculation results. This work proves the great potential applications of high-κ ZrO2 oxide in multilayer MoS2-based devices and provides a possible way to modify the interface energy band alignment.
We thank the reviewers for their valuable comments, and the Photonics Center of Shenzhen University for technical support.
The National Key Research and Development Program of China (2017YFB0404100), the National Natural Science Foundation of China (61504083,61804086), the PhD Start-up Fund of Natural Science Foundation of Guangdong Province (2017A030310424), the Science and Technology Foundation of Shenzhen (JCYJ20160226192033020), the National Taipei University of Technology-Shenzhen University Joint Research Program (2018001), and the Natural Science Foundation of Shandong Province (Grant No. ZR2017LF022, ZR2016AM25) act as guide to the design of the study and the collection, analysis, and interpretation of the data and the publication of the study.
Availability of data and materials
All data generated or analyzed during this study are included in this published article.
CH, KLL, WJW, ZWL, and QL carried out the related experiments and data analysis. KLL drafted the manuscript. XKL supervised the experiments and the writing of the manuscript. JPA, WH, JW, WJY, and RJC provided suggestions and guidance for the experiments and data analysis. ZWL and WM carried out the calculation. All authors read and approved the final manuscript.
XKL is an associate professor in materials physics. CH and ZWL are students in material growth and calculation. KLL and WJW are associate professors in material characterization and analysis. JPA, JW, WH, WJY, and RJC are professors in materials science. WM is an associate professor in calculation and analysis. QL is assistant research in material growth and characterization.
The authors declare that they have no competing interests.
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