Background

Beta-gallium oxide (β-Ga2O3) has attracted considerable interests due to its superior material properties [1, 2]. With ultra-wide bandgap (4.6–4.9 eV), the theoretical breakdown electric field (EC) is estimated to be around 8 MV/cm [3, 4]. Combined with its high relative dielectric constant (ε) and electron mobility (μ), the Baliga’s figure of merit (\( \upvarepsilon \upmu {E}_C^3 \)) is triple that of GaN or SiC, reducing the conduction loss significantly [1]. In addition, the availability of large bulk single crystals synthesized via melt-growth and epitaxial techniques delivers significant advantages for industrial applications [5, 6]. By far, β-Ga2O3 has been well demonstrated in a wide range of electronic applications, including light-emitting diodes, gas sensors, photodetectors, as well as field-effect transistors [7,8,9,10]. Very recently, hybrid heterojunctions, i.e., the integration of two-dimensional (2D) materials with three-dimensional (3D) materials, are of particular interest due to the complementary properties of their material systems [11].

To date, diverse 2D layered materials have been stacked on wide bandgap semiconductors to construct hybrid heterojunctions for novel applications with varying functionalities, such as MoS2/GaN, WSe2/GaN, MoS2/SiC, and so on [12,13,14,15]. Structurally, the MoS2 crystal is composed of a Mo atomic layer sandwiched between two sulfur layers, forming a two-dimensional hexagonal trilayer which is bonded to its neighboring layers by weak van der Waals forces [16, 17]. Unlike graphene with a zero bandgap, the thickness-dependent modulation of bandgaps motivated the exploration of MoS2 in optical and electrical devices [18, 19]. Based on the physics of MoS2, the density of states of few-layer MoS2 is three orders of magnitude higher than that of single-layer (SL) MoS2, leading to high drive currents in the ballistic limit. In this context, few-layer MoS2 may deliver significant advantages for transistor applications than SL MoS2 [18]. Thus, the integration of MoS2 with β-Ga2O3 is of great interest for combining respective merits of both the established 2D and 3D materials. And the optical and electrical properties for hybrid heterojunctions are inherently dominated by the interfacial energy band alignment. Consequently, it is quite desirable to have tunable band alignments for improving the performance of heterojunction based devices. In this work, we investigated the band alignment of 2D-MoS2/3D-β-Ga2O3 heterojunctions with and without nitridation treatment via X-ray photoelectron spectroscopy (XPS) characterizations and first principles calculations.

Methods

The SiO2/Si substrate was ultrasonicated with acetone and visopropanol for each 10 min, respectively, followed by rinsing in deionized water and drying with N2. Few-layer MoS2 films were grown on the 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]. The MoO3 and S powder were placed into two separate crucibles with a SiO2/Si substrate in the quartz tube, as shown in Fig. 1a. During the growth process, the quartz tube was held at 800 °C for MoS2 film growth within 5 min. Figure 1b displays the optical microscopic image of uniform MoS2 film on SiO2/Si substrate. After the growth of MoS2 film, it would be transferred to β-Ga2O3 (Tamura Corporation, Japan) substrate via PMMA-assisted method, [22] as sketched in Fig. 1c. During the transfer process, PMMA was first spin-coated on as-grown MoS2 film as a supporting layer, and then the samples were immersed in KOH solution for etching away the SiO2 layer. Subsequently, the PMMA layer with MoS2 film would float on the solution, after which the sample would be rinsed in deionized water for 1 min to remove the residual K+ and further transferred onto β-Ga2O3 substrate. Lastly, the top PMMA layer would be removed away with acetone. For the nitrided MoS2/β-Ga2O3 heterojunction, the nitridation has been implemented on the β-Ga2O3 surface with 50s N2 plasma treatment at a pressure of 3 Pa prior to the MoS2 transfer. The RF power and N2 flow rate were 100 W and 80 sccm, respectively. As a result, four samples were prepared for XPS measurements: (1) uncoated β-Ga2O3 substrate (bulk β-Ga2O3), (2) few-layer MoS2 film on SiO2/Si substrate (few-layer MoS2), (3) transferred MoS2 film on β-Ga2O3 substrate, (4) transferred MoS2 film on nitrided β-Ga2O3 substrate.

Fig. 1
figure 1

a Schematic illustration of the experimental set-up for CVD-growth of MoS2. b Optical image for the as-grown few-layer MoS2 film on SiO2/Si substrate. c Process flow of PMMA-assisted wet-transfer method for the MoS2/β-Ga2O3 heterojunction formation

Full size image

Results and Discussions

Raman spectroscopy was employed to investigate the quality of few-layer MoS2 film as well as to check relevant layer numbers. The Raman spectra of MoS2 film before and after transfer are presented in Fig. 2, which was characterized by RENISHAW inVia Raman spectroscopy. Two characteristic Raman modes could be observed around 381.91 cm−1 and 405.84 cm−1, corresponding to the in-plane (\( {E}_{2g}^1 \)) mode and out-of-plane (A1g) mode, respectively [23, 24]. Compared with as-grown MoS2 film, there is almost no Raman shift in \( {E}_{2g}^1 \) and A1g modes after transfer process, indicative of undamaged MoS2 after transfer process. The peak at 412.99 cm−1 after transfer process stems from the β-Ga2O3 substrate, in consistent with previous reports [25]. The frequency difference between \( {E}_{2g}^1 \) and A1g mode was deduced to be about 23.93 cm−1, designating four layers of few-layer MoS2 film [26]. Further, as shown in the inset of Fig. 2, the thickness of MoS2 film was verified to be 3 nm approximately (around four layers) by high-resolution transmission electron microscope (HRTEM), which is in good agreement with our Raman spectra. It can be seen from Fig. 3a that a high intensity peak of N 1 s was detected from the nitride β-Ga2O3 substrate, suggesting the presence of nitrogen. Figure 3b shows the SIMS profiles of MoS2/β-Ga2O3 heterojunction with nitridation, where the signals of main components represented by Mo, N, and Ga are plotted against depth. It is observed that the N peak is located at the MoS2/β-Ga2O3 interface, and the N spreading into β-Ga2O3 substrate could be contributed by the N injection into the underlying layer during plasma treatment or primary beam bombardments. The higher Ga profile in the MoS2 layer than β-Ga2O3 substrate probably stems from the different ion yield in the different material matrix [27]. Moreover, the tail of Mo in β-Ga2O3 could be ascribed to the diffusion or depth resolution problem, which is caused by primary beam bombardment [28].

Fig. 2
figure 2

Raman spectra of as-grown MoS2 on SiO2/Si substrate and transferred MoS2 on β-Ga2O3 substrate, respectively. The inset shows cross-section transmission electron microscopy (TEM) image of fabricated MoS2/β-Ga2O3 heterojunction

Full size image
Fig. 3
figure 3

a N 1 s XPS spectrum of β-Ga2O3 substrate with surface nitridation. b SIMS depth profile of fabricated MoS2/β-Ga2O3 heterojunction

Full size image

To obtain the band alignments of MoS2/β-Ga2O3 heterojunctions, XPS measurements with a step of 0.05 eV were carried out on VG ESCALAB 220i-XL system with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The constant pass energy was set at 20 eV. Additionally, the standard C 1 s (284.8 eV) was used for binding energy (BE) calibration [29]. To evaluate the valence band offset (VBO) at the MoS2/β-Ga2O3 interface, Mo 3d and Ga 3d core levels (CLs) were used for few-layer MoS2 and β-Ga2O3 samples, respectively. Figure 4a shows the XPS narrow scan of Mo 3d and valence band spectra from few-layer MoS2 [30]. The binding energy difference (BED) between CLs of Mo 3d5/2 and valence band maximum (VBM) for MoS2 was calculated to be 228.59 ± 0.1 eV. As shown in Fig. 4b, the BE of Ga 3d CL and VBM from few-layer β-Ga2O3 were deduced to be 20.25 ± 0.05 and 3.23 ± 0.05 eV, respectively. The corresponding BED was determined to 17.02 ± 0.1 eV, which is well consistent with that reported by Sun et al. [31]. Figure 4c depicts the measured XPS spectra of Mo 3d and Ga 3d CLs for MoS2/β-Ga2O3 heterojunctions with/without nitridation. It is noted that the Mo 3d5/2 CL shifted from 228.95 ± 0.05 eV for the unnitrided heterojunction toward 229.60 ± 0.05 eV for the nitrided heterojunction while Ga 3d CL shifted from 20.25 ± 0.05 to 20.65 ± 0.05 eV. Based on Kraut’ method,[32] the valence band offset (VBO, ∆EV) of few-layer MoS2/β-Ga2O3 heterojunctions was calculated according to the following equation,

$$ \Delta {E}_V=\left({E}_{Mo\ 3{d}_{5/2}}^{Mo{S}_2}-{E}_{VBM}^{Mo{S}_2}\right)-\left({E}_{Ga\ 3d}^{Ga_2{O}_3}-{E}_{VBM}^{Ga_2{O}_3}\right)-{\Delta E}_{CL} $$
(1)
Fig. 4
figure 4

a XPS spectra of Mo 3d CL and valence band from few-layer MoS2. b XPS spectra of Ga 3d CL and valence band from β-Ga2O3 substrate. c XPS spectra of Mo 3d and Ga 3d CLs for fabricated MoS2/β-Ga2O3 heterojunction with/without surface nitridation. d XPS spectra of O 1 s CL energy loss of β-Ga2O3 substrate with/without surface nitridation

Full size image

where \( {E}_{Mo\ 3{d}_{5/2}}^{Mo{S}_2} \) and \( {E}_{VBM}^{Mo{S}_2} \) are binding energies of Mo 3d5/2 CL and VBM from MoS2, \( {E}_{Ga\ 3d}^{Ga_2{O}_3} \), and \( {E}_{VBM}^{Ga_2{O}_3} \) are binding energies of Ga 3d CL and VBM from β-Ga2O3, \( {\Delta E}_{CL}=\Big({E}_{Mo\ 3{d}_{5/2}}^{Mo{S}_2}-{E}_{Ga\ 3d}^{Ga_2{O}_3} \)) is the binding energy difference between Mo 3d5/2 and Ga 3d CLs for MoS2/β-Ga2O3 heterojunctions. Hence, the ∆EV of MoS2 on β-Ga2O3 substrate with and without N2 plasma treatment was calculated to be 2.62±0.1 and 2.87 ± 0.1 eV, respectively.

Figure 4d shows the O 1 s CL energy loss spectra of β-Ga2O3 substrates with and without nitridation. It is noted that the bandgap keeps unchanged after nitridation treatment with a value of 4.70 ± 0.1 eV. Thus, the conduction band offset can be extracted as follows,

$$ {\Delta E}_C={E}_g^{Ga_2{O}_3}-{E}_g^{Mo{S}_2}-{\Delta E}_V $$
(2)

where \( {E}_g^{Ga_2{O}_3} \) and \( {E}_g^{Mo{S}_2} \) are the bandgaps of β-Ga2O3 and few-layer MoS2, respectively. The bandgap of 1.4 ± 0.1 eV for few-layer MoS2 was used in this work.34 According to Eq. (2), the ∆EC between MoS2 and β-Ga2O3 with and without nitridation were deduced to be 0.68 ± 0.1 and 0.43 ± 0.1 eV, respectively. The calculated band diagrams for heterojunctions without/with nitridation are shown in Fig. 5(a) and 5(b), respectively.

Next, the electronic structures of nitrided and unnitrided heterojunctions were further examined through the Vienna ab initio simulation package (VASP) based on density functional theory (DFT) [33,34,35]. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) parameterization was adopted for exchange-correlation function [36, 37]. We used the DFT-D3 dispersion corrections approach to describe the long-distance van der Waals (vdW) interactions [38,39,40]. The projector augmented wave (PAW) pseudopotential method was used to describe the core-valence interaction with a kinetic energy cutoff of 650 eV for plane wave expansion. We employ a 4 × 4 × 1 G-centered k-mesh for structural relaxation of the unit cell, with the smallest spacing between k-points of 0.04 Å−1, which is precise enough by the convergence test with respect to the number of k points. The convergence thresholds are set to 10−4 eV for energy differences of the system and 10−2 eV Å−1 for Hellman-Feynman force. In order to eliminate artificial interactions between two adjacent atomic layers, the thickness of the vacuum layer is set to ~ 15 Å. The eigenvalues of the heterojunctions are further verified by the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional calculations, which improve the precision of eigenvalues via reducing the localization and delocalization errors of PBE and Hartree-Fock (HF) functionals [41]. The mixing ratio is 25% for the short-range HF exchange. The screening parameter is 0.2 Å−1.

Fig. 5
figure 5

Band diagrams of MoS2/β-Ga2O3 heterojunction a without and b with surface nitridation

Full size image

The MoS2/β-Ga2O3 heterojunctions were constructed as shown in Fig. 6a. The universal binding energy relation (UBER) method, which provides a simple universal form for the relationship between binding energy and atomic separation, [42] was applied to determine the energetically stable structure before electronic structure calculation. Various interlayer distances were considered and the surface adhesion energy Wad for the heterojunctions are shown below,

$$ {W}_{ad}=\frac{E_{Ga_2{O}_3}+{E}_{Mo{S}_2}-{\mathrm{E}}_{Ga_2{O}_3/ Mo{S}_2}}{A} $$
Fig. 6
figure 6

Atomic structure and charge-density distributions of β-Ga2O3-MoS2 stacked heterostructures a without and b with nitrogen dopants in a 4 × 4 × 1 supercell from a side view. Ga (O) atoms are in red (gray) and Mo (S) atoms in blue (orange). Band structures of MoS2/β-Ga2O3 heterostructures c without and d with nitrogen dopants

Full size image

where A is the interface area, \( {E}_{Ga_2{O}_3} \), \( {E}_{Mo{S}_2} \), and \( {E}_{Ga_2{O}_3/ Mo{S}_2} \) are the total energies of β-Ga2O3, monolayer MoS2 and the MoS2/β-Ga2O3 heterojunction, respectively. Once the Wad reaches a maximum, the optimal interlayer distance will be obtained. After structure optimizations, a nitrogen atom is substitutionally doped in the original MoS2/β-Ga2O3 heterojunction, as shown in Fig. 6b. The concentration of nitrogen in DFT calculation is around 4.17%, which is close to that (3.61%) in experiments. The electronic structures for both nitrided and unnitrided MoS2/β-Ga2O3 heterojunctions were calculated as illustrated in Fig. 6c and d. It was seen that mid-gap states were introduced, which may enhance the charge transfer across the MoS2/β-Ga2O3 interface, and the resulting interface dipole contributed to the measured binding energy shift. Furthermore, the calculated conduction band offsets ∆EC (\( \Delta {E}_C={E}_{CB}^{Mo{S}_2}-{E}_{CB}^{Ga_2{O}_3} \)) for undoped- and doped-β-Ga2O3/MoS2 heterojunctions are 0.82 and 1.0 eV respectively, showing the same trend with the experimental results. We have also calculated the eigenvalues of \( {E}_{CB}^{Mo{S}_2} \) and \( {E}_{CB}^{Ga_2{O}_3} \) using the HSE06 method to further confirm the above conclusion, and find that the corrected ∆EC are 0.87 and 1.08 eV for undoped- and doped-β-Ga2O3/MoS2 heterojunctions respectively.

Conclusions

In conclusion, respective MoS2 film has been transferred onto unnitrided and nitride β-Ga2O3 for constructing MoS2/β-Ga2O3 heterojunctions. Raman spectroscopy was used to investigate the quality of transferred MoS2 film, and SIMS study was performed to probe the elemental depth profiles of the MoS2/β-Ga2O3 heterojunction with nitridation. The VBOs were determined to be 2.62 ± 0.1 eV for nitrided MoS2/β-Ga2O3 heterojunction and 2.87 ± 0.1 eV for unnitrided heterojunction by XPS, respectively. The resultant CBOs were deduced to be 0.68 ± 0.1 and 0.43 ± 0.1 eV, which was in the same trends with the DFT calculations. These findings demonstrated that the band offsets can be modified via surface nitridation process. This study offers glorious perspectives on the implementation of designed electronic devices based on 2D/3D vertical heterojunctions.