Abstract
The effect of nitridation treatment on the band alignment between few-layer MoS2 and HfO2 has been investigated by X-ray photoelectron spectroscopy. The valence (conduction) band offsets of MoS2/HfO2 with and without nitridation treatment were determined to be 2.09 ± 0.1 (2.41 ± 0.1) and 2.34 ± 0.1 (2.16 ± 0.1) eV, respectively. The tunable band alignment could be attributed to the Mo-N bonding formation and surface band bending for HfO2 triggered by nitridation. This study on the energy band engineering of MoS2/HfO2 heterojunctions may also be extended to other high-k dielectrics for integrating with two-dimensional materials to design and optimize their electronic devices.
Background
Currently, layered transition metal dichalcogenides (TMDCs) have aroused great interest due to their fascinating properties for potential applications in modern electronics and optoelectronics [1, 2]. In particular, molybdenum disulfide (MoS2) has been attracting considerable attention as a promising channel material for continuing the scaling beyond the 7-nm technology node [3, 4]. Structurally, the MoS2 crystal is built up of one hexagonally arranged Mo plane, sandwiched by two hexagonally arranged S planes. A triangular prismatic arrangement was formed via the covalently bonded S-Mo-S units [5, 6]. MoS2 possesses a layer-dependent bandgap, varying from a direct bandgap (1.8 eV) for single-layer (SL) MoS2 to an indirect bandgap (1.2 eV) for bulk MoS2 [7]. Dissimilar to graphene with a zero bandgap, the thickness-dependent modulation of bandgaps motivated the exploration of MoS2 in optical and electrical devices [3, 8]. Based on the physics of MoS2, the density of states of few-layer MoS2 is triple that of single-layer MoS2, resulting in high drive currents in the ballistic limit [8]. In this context, few-layer MoS2 may deliver significant advantages for transistor applications than SL MoS2 [3].
On the other hand, the electronic devices based on traditional silicon dioxide dielectrics are approaching the physical limit because of its low dielectric constant [9]. To obtain a thin equivalent oxide thickness (EOT), it is crucially important to integrate high-k dielectrics with MoS2. To date, many high-k dielectrics have been investigated with MoS2, including Al2O3, ZrO2, HfO2, and h-BN [10,11,12,13,14]. DiStefano et al. obtained the respective conduction and valence band offsets of 3.3 ± 0.2 and 1.4 ± 0.2 eV for few-layer MoS2 grown by oxide vapor deposition on amorphous BN [13]. Tao et al. reported that the conduction band offset (CBO) for the monolayer MoS2/Al2O3 (ZrO2) heterojunction was deduced to be 3.56 eV (1.22 eV), while the valence band offset (VBO) was 3.31 eV (2.76 eV) [15]. And a CBO of 2.09 ± 0.35 eV and VBO of 2.67 ± 0.11 eV at the MoS2/HfO2 interface were reported by McDonnell et al. [12]. Among these gate dielectrics, HfO2 was considered to be one of the most promising candidates owing to its high dielectric constant (k ∼ 20), compatibility with poly-SiGe, TaN gates, and polycrystalline silicon gate [16]. However, HfO2 has a poor thermal stability, large leakage current, high oxide trap density, interface trap density, etc. [17]. These limitations have motivated extensive investigations of searching passivation techniques, such as interface nitridation or fluorination treatment technologies [18, 19]. In this work, we studied the energy band alignments of few-layer MoS2 on HfO2 dielectrics with and without plasma nitridation, in which the effect of surface nitridation was characterized by X-ray photoelectron spectroscopy (XPS).
Methods
The SiO2 (280 nm)/Si wafer was alternately cleaned with acetone and isopropanol by ultrasonic cleaning for each 10 min, followed by deionized water rinse and N2 dry. The few-layer MoS2 films were deposited on SiO2/Si substrate by chemical vapor deposition (CVD) using precursors of MoO3 (0.08 mg, 99%, Alfa Aesar) and S powder (1 g, 99%) [20, 21]. After the growth procedure, the MoS2 film would be transferred to HfO2/Si substrate by the poly (methyl methacrylate) (PMMA) method [22], as depicted in Fig. 1a. In this process, PMMA was first spin-coated on MoS2/SiO2/Si samples as a supporting layer. Then, the samples were immersed in KOH solution for etching away the SiO2, after which the MoS2 layer with PMMA would float to the top of the solution. In the end, the PMMA layer would be dissolved in acetone after the sample was transferred onto HfO2/Si substrate. The HfO2 films were grown on the silicon wafer by atomic layer deposition (ALD) at a temperature of 200 °C using Hf [N (CH3)(C2H5)]4 [tetrakis (ethylmethylamido) hafnium, TEMAH] and H2O vapor as precursors [23, 24]. During the optimization process of the plasma treatment time, it was found that the nitrogen would diffuse into the oxide greatly after 70 s nitridation treatment by SIMS measurements, which would severely deteriorate the oxide quality. While the plasma treatment time is 30 s, no obvious N peak at the oxide surface was observed from the SIMS results. For the control sample, 50 s N2 plasma treatment was implemented on HfO2/Si substrate at a pressure of 3 Pa before the MoS2 transfer. Under the plasma condition, the resultant N dose is about 8.4 × 1014 atoms/cm2 estimated from the secondary ion mass spectrometry (SIMS) results. And the concentration of nitrogen was calculated to be about 1.5% after nitridation based on the XPS data. Four samples 1–4# were prepared for XPS measurements: 1# few-layer MoS2 film on SiO2/Si substrate (few-layer MoS2), 2# thick HfO2 film on Si substrate (bulk HfO2), 3# transferred MoS2 film on as-grown HfO2/Si substrate (as-grown MoS2/HfO2 heterojunction), and 4# transferred MoS2 film on N2 plasma-treated HfO2/Si substrate (nitrided MoS2/HfO2 heterojunction).
Results and Discussions
RENISHAW inVia Raman spectroscopy was employed to characterize the Raman spectra of few-layer MoS2 film before and after transfer procedure, as illustrated in Fig. 1b. Two Raman peaks can be seen at around 382.86 cm−1 and 406.43 cm−1, corresponding to the in-plane (\( {E}_{2g}^1 \)) and out-of-plane (A1g) modes, respectively [25, 26]. It was found that there is nearly no Raman shift in \( {E}_{2g}^1 \) and A1g mode frequencies after transfer process, indicating minimal structure modification. The frequency difference (∆k) between \( {E}_{2g}^1 \) and A1g mode was deduced to be about 23.57 cm−1, designating around four to five layers of MoS2 film [27]. As shown in the inset of Fig. 1b, the thickness of MoS2 film was verified to be approximately 2.8 nm by high-resolution transmission electron microscope (HRTEM), which is in consistent with the abovementioned Raman spectra. Moreover, we presented SIMS depth profiles of transferred MoS2 film on nitrided HfO2/Si substrate. SIMS measurement was performed on a Physical Electronics ADEPT 1010 SIMS instrument with Cs primary ion beam at the energy of 1 keV, in which positive ions were collected and charge compensation was carried out. In this SIMS measurement, the nitrogen element was quantified while the other elements (Mo, Hf, and Si) are only meant as layer markers and not quantified. As illustrated in Fig. 2a, the depth profiles for transferred MoS2 film on nitrided HfO2/Si substrate were determined by SIMS, in which signals of main components represented by Mo, N, Hf, and Si are plotted against the depth. The spreading of N into the HfO2 layer was observed, which could be intrigued by the N injection into the underlying layer during primary beam bombards or plasma treatments. It is also worth noting that depth profiles near the surface layer are normally complicated and meaningless because of the surface contamination and surface effects, e.g., the abnormal intensity of N element near the surface [28]. The higher signal of N profile near the HfO2/Si interface could be ascribed to that the nitrogen tends to diffuse to the HfO2/Si interface, leading to the accumulation of N near the interface [29]. The tail of Mo in HfO2 film could be mainly caused by primary beam bombardments in SIMS measurements [30]. Figure 2b illustrates the respective N 1s XPS spectra for sample 3# and 4#; the high-intensity peaks for both heterojunctions were Mo 3p3/2 while a low-intensity peak at ~ 395.80 eV was detected for the nitrided heterojunction, indicating the formation of Mo-N bonding [31].
To obtain the band alignments between few-layer MoS2 and HfO2 with and without nitridation treatment, XPS measurements with a step of 0.05 eV were carried out on VG ESCALAB 220i-XL system using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The constant pass energy was set at 20 eV. Additionally, the standard C 1s (284.8 eV) was used for binding energy (BE) calibration [32]. To evaluate VBO values for MoS2/HfO2 heterojunctions, Mo 3d and Hf 4f core levels (CLs) were selected for sample 1–4#, respectively. Figure 3a presents the XPS narrow scan of Mo 3d and valence band spectra from sample 1# [33]. Thus, the binding energy difference (BED) between Mo 3d5/2 core level and valence band maximum (VBM) for sample 1# was calculated to be 228.49 ± 0.1 eV. Figure 3b illustrates the CLs of Hf 4f7/2 and VBM for sample 2#; the corresponding BED was determined to be 14.10 ± 0.1 eV. Figure 3c depicts the measured XPS spectra of Mo 3d and Hf 4f CLs for MoS2/HfO2 heterojunctions with/without nitridation treatment. It is noted that the Mo 3d5/2 CL shifted from 229.45 ± 0.05 eV for sample 3# to 229.90 ± 0.05 eV for sample 4#. This could be ascribed to that a nitridation interfacial layer was introduced at the MoS2/HfO2 interface after plasma treatment, resulting in the abovementioned Mo-N bonding. With the presence of Mo-N bonding, the consequent charge transfer between Mo and N elements contributed to the measured Mo 3d5/2 CL shift. Additionally, the Hf 4f7/2 CL of 17.40 ± 0.05 eV for sample 3# was shifted to a higher binding energy of 17.60 ± 0.05 eV for sample 4# while O 1s also showed a shift of 0.20 eV to a higher BED, as shown in Fig. 3d. These peak shifts implied the downward band bending at the HfO2 surface, which could be interpreted as that the nitrogen plasma induced donor-like defects for HfO2 [34]. Based on the Kraut method [35], the VBO (∆EV) values can be calculated from the following equation:
where \( {E}_{\mathrm{Mo}\ 3{\mathrm{d}}_{5/2}}^{\mathrm{Mo}{\mathrm{S}}_2} \) and \( {E}_{\mathrm{VBM}}^{\mathrm{Mo}{\mathrm{S}}_2} \) are binding energies of Mo 3d5/2 CL and VBM for MoS2, \( {E}_{\mathrm{Hf}\ 4{\mathrm{f}}_{7/2}}^{{\mathrm{Hf}\mathrm{O}}_2} \) and \( {E}_{\mathrm{VBM}}^{{\mathrm{HfO}}_2} \) are binding energies of Hf 4f7/2 CL and VBM for ALD-HfO2, ∆ECL =\( {E}_{\mathrm{Mo}\ 3{\mathrm{d}}_{5/2}}^{\mathrm{Mo}{\mathrm{S}}_2}-{E}_{\mathrm{Hf}\ 4{\mathrm{f}}_{7/2}}^{{\mathrm{Hf}\mathrm{O}}_2} \) refers to the BED between Mo 3d5/2 and Hf 4f7/2 CLs for ALD-HfO2/MoS2 heterojunctions. Hence, the ∆EV of MoS2 on ALD-HfO2 with and without nitridation treatment were calculated to be 2.09 ± 0.1 and 2.34 ± 0.1 eV, respectively.
To assess the influence of N2 plasma treatment on the conduction band offset (CBO, ∆EC) between ALD-HfO2 and few-layer MoS2, the bandgaps of 5.9 ± 0.1 eV for HfO2 and 1.4 ± 0.1 eV for MoS2 were used here, respectively [7, 36]. Thus, the CBO can be attained by the following equation:
where \( {E}_g^{{\mathrm{HfO}}_2} \) and \( {E}_g^{\mathrm{Mo}{\mathrm{S}}_2} \) are the bandgaps of HfO2 and MoS2, respectively. According to Eq. (2), the ∆EC between MoS2 and ALD-HfO2 with and without nitridation treatment were calculated to be 2.41 ± 0.1 and 2.16 ± 0.1 eV, respectively. The corresponding band diagrams are illustrated in Fig. 4. Remarkably, both VBO and CBO values of these two heterojunctions provide excellent electron and hole confinements, ensuring their suitability for MoS2-based FETs [37]. Moreover, the nitrided heterojunction has a higher CBO compared with unnitrided heterojunction, which is better for n-channel FETs applications.
Conclusions
In conclusion, the XPS measurements revealed that the band alignment at the MoS2/HfO2 interface could be modified by introducing nitridation to HfO2 surface prior to stacking MoS2 film. The CBO and VBO were determined to be 2.16 ± 0.1 and 2.34 ± 0.1 eV for the unnitrided MoS2/HfO2 heterojunction, whereas the CBO was altered up to 2.41 ± 0.1 eV and the VBO was altered down to 2.09 ± 0.1 eV for the nitrided MoS2/HfO2 heterojunction, respectively. A nitridation interfacial layer was introduced at the interface, which was found to result in the Mo-N bonding formation. Additionally, the nitrogen plasma could induce donor-like defects, leading to the surface band bending for HfO2. In this way, the interfacial band alignment engineering would supply promising routes toward the flexible deign and optimization of modern electronics.
Availability of Data and Materials
The datasets supporting the conclusions of this manuscript are included within the manuscript.
Abbreviations
- ALD:
-
Atomic layer deposition
- BE:
-
Binding energy
- BED:
-
Binding energy difference
- CBO:
-
Conduction band offset
- CL:
-
Core level
- CVD:
-
Chemical vapor deposition
- FET:
-
Field-effect transistor
- HfO2:
-
Hafnium oxide
- HRTEM:
-
High-resolution transmission electron microscope
- MoS2 :
-
Molybdenum disulfide
- PMMA:
-
Poly (methyl methacrylate)
- SIMS:
-
Secondary ion mass spectrometry
- SL:
-
Single-layer
- TEMAH:
-
Tetrakis (ethylmethylamido) hafnium
- TMDC:
-
Transition metal dichalcogenide
- VBM:
-
Valence band maximum
- VBO:
-
Valence band offset
- XPS:
-
X-ray photoelectron spectroscopy
References
Wang QH, Kalantar-Zadeh K, Kis A, Coleman JN, Strano MS (2012) Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nanotechnol 7(11):699
Jariwala D, Sangwan VK, Lauhon LJ, Marks TJ, Hersam MC (2014) Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides. ACS Nano 8(2):1102–1120
Ganatra R, Zhang Q (2014) Few-layer MoS2: a promising layered semiconductor. ACS Nano 8(5):4074–4099
Liu Y, Duan X, Huang Y, Duan X (2018) Two-dimensional transistors beyond graphene and TMDCs. Chem Soc Rev 47(16):6388–6409
Eda G, Fujita T, Yamaguchi H, Voiry D, Chen M, Chhowalla M (2012) Coherent atomic and electronic heterostructures of single-layer MoS2. ACS Nano 6(8):7311–7317
Zhou S, Wang S, Li H, Xu W, Gong C, Grossman JC, Warner JH (2017) Atomic structure and dynamics of defects in 2D MoS2 bilayers. ACS Omega 2(7):3315–3324
Mak KF, Lee C, Hone J, Shan J, Heinz TF (2010) Atomically thin MoS2: a new direct-gap semiconductor. Phys Rev Lett 105(13):136805
Kim S, Konar A, Hwang W, Lee JH, Lee J, Yang J, Jung C, Kim H, Yoo J, Choi J, Jin YW, Lee SY, Jena D, Choi W, Kim K (2012) High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals. Nat Commun 3:1011
Manchanda L, Morris MD, Green ML, Van Dover RB, Klemens F, Sorsch TW, Silverman PJ, Wilk G, Busch B, Aravamudhan S (2001) Multi-component high-K gate dielectrics for the silicon industry. Microelectron Eng 59(1–4):351–359
Liu X, He J, Tang D, Liu Q, Wen J, Yu W, Lu Y, Zhu D, Liu W, Cao P, Han S, Pan J, Liu W, Ang KW, He Z (2015) Band alignment of atomic layer deposited high-k Al2O3/multilayer MoS2 interface determined by X-ray photoelectron spectroscopy. J Alloy Compd 650:502–507
Pradhan SK, Xiao B, Pradhan AK (2016) Energy band alignment of high-k oxide heterostructures at MoS2/Al2O3 and MoS2/ZrO2 interfaces. J Appl Phys 120(12):125305
Mcdonnell S, Brennan B, Azcatl A, Lu N, Dong H, Buie C, Kim J, Hinkle CL, Kim MJ, Wallace RM (2013) HfO2 on MoS2 by atomic layer deposition: adsorption mechanisms and thickness scalability. ACS Nano 7(11):10354–10361
Distefano J, Lin Y, Robinson J, Glavin NR, Voevodin AA, Brockman J, Kuhn M, French B, King SW (2016) Band alignment at molybdenum disulphide/boron nitride/aluminum oxide interfaces. J Electron Mater 45(2):983–988
Illarionov YY, Rzepa G, Waltl M, Knobloch T, Grill A, Furchi MM, Mueller T, Grasser T (2016) The role of charge trapping in MoS2/SiO2 and MoS2/hBN field-effect transistors. 2D Mater 3(3):35004
Tao J, Chai JW, Zhang Z, Pan JS, Wang SJ (2014) The energy-band alignment at molybdenum disulphide and high-k dielectrics interfaces. Appl Phys Lett 104(23):232110
Kang CS, Cho H, Onishi K, Nieh R, Choi R, Gopalan S, Krishnan S, Han JH, Lee JC (2002) Bonding states and electrical properties of ultrathin HfOxNy gate dielectrics. Appl Phys Lett 81(14):2593–2595
Wong H, Zhan N, Ng KL, Poon MC, Kok CW (2004) Interface and oxide traps in high-k hafnium oxide films. Thin Solid Films 462:96–100
Wu WC, Lai CS, Wang JC, Chen JH, Ma MW, Chao TS (2007) High-performance HfO2 gate dielectrics fluorinated by postdeposition CF4 plasma treatment. J Electrochem Soc 154(7):H561–H565
Yu T, Jin CG, Yang Y, Zhuge LJ, Wu XM, Wu ZF (2013) Effect of NH3 plasma treatment on the interfacial property between ultrathin HfO2 and strained Si0.65Ge0.35 substrate. J Appl Phys 113(4):44105
Zobel A, Boson A, Wilson PM, Muratov DS, Kuznetsov DV, Sinitskii A (2016) Chemical vapour deposition and characterization of uniform bilayer and trilayer MoS2 crystals. J Mater Chem C 4(47):11081–11087
Wong SL, Liu H, Chi D (2016) Recent progress in chemical vapor deposition growth of two-dimensional transition metal dichalcogenides. Prog Cryst Growth Ch 62(3):9–28
Ma D, Shi J, Ji Q, Chen K, Yin J, Lin Y, Zhang Y, Liu M, Feng Q, Song X (2015) A universal etching-free transfer of MoS2 films for applications in photodetectors. Nano Res 8(11):3662–3672
Kukli K, Ritala M, Lu J, Haårsta A, Leskelä M (2004) Properties of HfO2 thin films grown by ALD from hafnium tetrakis (ethylmethylamide) and water. J Electrochem Soc 151(8):F189–F193
Chen W, Sun Q, Xu M, Ding S, Zhang DW, Wang L (2007) Atomic layer deposition of hafnium oxide from tetrakis (ethylmethylamino) hafnium and water precursors. J Phys Chem C 111(17):6495–6499
Li H, Zhang Q, Yap CCR, Tay BK, Edwin THT, Olivier A, Baillargeat D (2012) From bulk to monolayer MoS2: evolution of Raman scattering. Adv Funct Mater 22(7):1385–1390
Najmaei S, Liu Z, Ajayan PM, Lou J (2012) Thermal effects on the characteristic Raman spectrum of molybdenum disulfide (MoS2) of varying thicknesses. Appl Phys Lett 100(1):1271
Muratore C, Hu JJ, Wang B, Haque MA (2014) Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition. Appl Phys Lett 104(26):261604
Stingeder G, Grundner M, Grasserbauer M (1988) Investigation of surface contamination on silicon wafers with SIMS. Surf Interface Anal 11(8):407–413
Wang JC, Lei TF, Lee CL (2003) Characterization of temperature dependence for HfO2 gate dielectrics treated in NH3 plasma. Electrochem Solid-State Lett 6(10):F34–F36
Leng Y (2008) Materials characterization: introduction to microscopic and spectroscopic methods. Wiley, New York
Su T, Lin Y (2016) Effects of nitrogen plasma treatment on the electrical property and band structure of few-layer MoS2. Appl Phys Lett 108(3):33103
Ebel MF, Ebel H (1974) About the charging effect in X-ray photoelectron spectrometry. J Electron Spectrosc Relat Phenom 3(3):169–180
Chambers SA, Droubay T, Kaspar TC, Gutowski M (2004) Experimental determination of valence band maxima for SrTiO3, TiO2, and SrO and the associated valence band offsets with Si (001). J Vac Sci Technol B: Microelectron Nanometer Struct Process, Meas, Phenom 22(4):2205–2215
Yang J, Eller BS, Zhu C, England C, Nemanich RJ (2012) Comparative band alignment of plasma-enhanced atomic layer deposited high-k dielectrics on gallium nitride. J Appl Phys 112(5):53710
Kraut EA (1980) Precise determination of the valence-band edge in X-ray photoemission spectra: application to measurement of semiconductor Interface potentials. Phys Rev Lett 44(24):1620–1623
Cheynet MC, Pokrant S, Tichelaar FD, Rouvière J (2007) Crystal structure and band gap determination of HfO2 thin films. J Appl Phys 101(5):54101
Robertson J (2000) Band offsets of wide-band-gap oxides and implications for future electronic devices. J Vac Sci Technol B: Microelectron Nanometer Struct Process, Meas, Phenom 18(3):1785–1791
Acknowledgements
The authors would like to acknowledge the financial support in part by the National Natural Science Foundation of China (Nos. 61774041, 61704029, 61474027, 61704029, 61874028, and 61834009) and in part by the National Key Technologies Research and Development Program of China (No. 2017YFB0405600).
Author information
Authors and Affiliations
Contributions
WHY performed the experiment, data processing, and manuscript drafting. WJL, XYX, and SJD modified the manuscript. Other authors help review and discuss the manuscript. All authors read and approved the final manuscript.
Corresponding authors
Ethics declarations
Competing Interests
The authors declare that they have no competing interests.
Additional information
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
About this article
Cite this article
Huan, YW., Liu, WJ., Tang, XB. et al. Investigation of Nitridation on the Band Alignment at MoS2/HfO2 Interfaces. Nanoscale Res Lett 14, 181 (2019). https://doi.org/10.1186/s11671-019-3020-0
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/s11671-019-3020-0