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).

Fig. 1
figure 1

a Process flow of PMMA-assisted wet transfer method for the MoS2/ALD-HfO2 heterojunction formation. b Respective Raman spectra of as-grown and transferred MoS2 film. The inset is the cross-section transmission electron microscopy images of as-grown MoS2 on SiO2/Si substrate

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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].

Fig. 2
figure 2

a SIMS depth profiles of transferred MoS2 film on nitrided HfO2/Si substrate. b N 1s XPS spectra for MoS2/HfO2 heterojunctions with and without nitridation treatment, respectively

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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:

$$ \Delta {E}_V=\left({E}_{\mathrm{Mo}\ 3{\mathrm{d}}_{5/2}}^{\mathrm{Mo}{\mathrm{S}}_2}-{E}_{\mathrm{VBM}}^{\mathrm{Mo}{\mathrm{S}}_2}\right)-\left({E}_{\mathrm{Hf}\ 4{\mathrm{f}}_{7/2}}^{{\mathrm{Hf}\mathrm{O}}_2}-{E}_{\mathrm{VBM}}^{{\mathrm{Hf}\mathrm{O}}_2}\right)-{\Delta E}_{\mathrm{CL}} $$
(1)

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.

Fig. 3
figure 3

a XPS spectra of Mo 3d CL and valence band for the few-layer MoS2. b XPS spectra of Hf 4f CL and valence band for bulk HfO2. XPS spectra of c Mo 3d, Hf 4f, and d O 1s CLs for transferred MoS2 film on bulk HfO2 with/without nitridation treatment

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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:

$$ {\Delta E}_C={E}_g^{{\mathrm{HfO}}_2}-{E}_g^{\mathrm{Mo}{\mathrm{S}}_2}-{\Delta E}_V $$
(2)

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.

Fig. 4
figure 4

Band diagrams of MoS2/HfO2 heterojunction a without nitridation treatment and b with nitridation treatment

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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.