Background

Recently, an oxide semiconductor Ga2O3 has attracted widespread interests because of its unique characteristics such as the large bandgap, high saturation electron velocity, and high temperature resistance [1]. There are five kinds of isomers for Ga2O3: α, β, γ, δ, and ε, where β-Ga2O3 can be grown easier and has been studied widely [2]. In particular, β-Ga2O3 has a larger breakdown electric field than that of traditional third-generation semiconductor materials, such as SiC and GaN [3]. The n-type conductive properties can be modulated by do** Sn [4] or Si [5]. So β-Ga2O3-based devices [6, 7] have broad application prospects in the fields of information technology, energy conservation, and emission reduction. However, β-Ga2O3-based devices have a common limitation: the contact between β-Ga2O3 and most metals tends to be Schottky because of the large barrier induced by the wide bandgap and finite carrier concentration. In recent years, inserting an interlayer, such as ITO [8] and AZO [9], between Ga2O3 and metals is shown to be a valid method to reduce the energy barrier between β-Ga2O3 and metal.

Al-doped zinc oxide (ZnO) has gained much attention because of low resistivity and lower fabrication cost than ITO [10]. In particular, the high thermal stability, high mobility, and carrier concentration make it a promising candidate of the intermediate semiconductor layer (ISL) [11]. So far, Al-doped ZnO films can be grown through the following techniques: molecular beam epitaxy (MBE) [12], magnetron sputtering [13], chemical vapor deposition (CVD) [14], and atomic layer deposition (ALD) [15]. Specially, ALD is a renowned method to prepare nano-thickness film which exhibits large area excellent uniformity and unites growth rate per cycle because of the self-limiting surface reaction including the self-limiting chemical adsorption and self-limiting sequential reaction [16]. Moreover, ALD can reduce interface disorder and more precise modulate the Al do** concentration by changing the ratios of growth cycles.

Note that the conduction band offset (CBO) determines the energy barrier for the electron transport, so a smaller CBO is beneficial to form an Ohmic contact. Based on our previous work [17], by increasing Al do** concentration, the Al-doped ZnO film changes from polycrystalline to amorphous nature, and its bandgap widens as well. However, the band offsets of different Al-doped ZnO/β-Ga2O3 heterojunctions have not been studied widely. In this work, the ZnO films with different Al do** ratios were respectively deposited on β-Ga2O3 substrates by ALD. The results show the VBO and CBO are almost linearly dependent on the Al do** ratio.

Methods

The substrates are bulk β-Ga2O3 (\( \overline{2}01 \)) and the do** concentration is about 3 × 1018/cm3. The cleaning process for Ga2O3 substrates was undergone ultrasonic wash in acetone and isopropanol for each 10 min with repeated three times. Subsequently, the Ga2O3 substrates were rinsed with deionized water. Afterwards, the Al-doped ZnO films were grown onto the Ga2O3 substrate by ALD (Wuxi MNT Micro Nanotech Co., LTD, China). Three kinds of samples were prepared. Firstly, the undoped ZnO films were grown by ALD with the precursors of Zn (C2H5)2 (DEZ) and H2O at 200 oC. Secondly, the Al-doped ZnO films were carried out by adding one pulse of trimethylaluminum (TMA) and H2O every 19th cycle of DEZ and H2O pulsing (denoted as 5% Al do**) at a substrate temperature of 200 oC during ALD. Thirdly, the Al-doped ZnO films of ratio 9:1 (denoted as 10% Al do**) were also prepared. The growth rate of ZnO and Al2O3 was 0.16 and 0.1 nm/cycle, respectively. Every kind film included two different thicknesses, i.e., 40 nm and 10 nm for the thick and thin film, respectively. In addition, the β-Ga2O3 substrate was used to study the bulk material. Ga 2p, Zn 2p CLs, and the valence band maximum (VBM) were measured by X-ray spectroscopy (XPS) (AXIS Ultra DLD, Shimadzu) and the step of resolution XPS spectra is 0.05 eV. To avoid the surface contamination of the sample during the transfer process from ALD to XPS chamber, Ar ion etching was performed before the XPS measurement. Note that the charging effect can shift the XPS spectrum, and the BE of C 1s peak is calibrated at 284.8 eV to solve the problem.

Results and Discussions

The valence band offset (VBO) of Al-doped ZnO/β-Ga2O3 heterojunction can be obtained through the formula as follows [18]:

$$ \Delta {E}_V=\left({E}_{\mathrm{Ga}\ 2p}^{{\mathrm{Ga}}_2{\mathrm{O}}_3}-{E}_{\mathrm{VBM}}^{{\mathrm{Ga}}_2{\mathrm{O}}_3}\right)-\left({E}_{\mathrm{Zn}\ 2p}^{\mathrm{AZO}}-{E}_{\mathrm{VBM}}^{\mathrm{AZO}}\right)-\left({E}_{\mathrm{Ga}\ 2p}^{{\mathrm{Ga}}_2{\mathrm{O}}_3}-{E}_{\mathrm{Zn}\ 2p}^{\mathrm{AZO}}\right) $$
(1)

where\( {E}_{\mathrm{Ga}\ 2p}^{{\mathrm{Ga}}_2{\mathrm{O}}_3} \) refers to the binding energy (BE) of Ga 2p core level (CL) in bulk β-Ga2O3, \( {E}_{\mathrm{VBM}}^{{\mathrm{Ga}}_2{\mathrm{O}}_3} \) refers to the BE of VBM in bulk β-Ga2O3, \( {E}_{\mathrm{Zn}\ 2p}^{\mathrm{AZO}} \) refers to the BE of Zn 2p CL in thick Al-doped ZnO films, \( {E}_{\mathrm{VBM}}^{\mathrm{AZO}} \) refers to the BE of VBM in thick Al-doped ZnO films. The latter \( {E}_{\mathrm{Ga}\ 2p}^{{\mathrm{Ga}}_2{\mathrm{O}}_3} \) and \( {E}_{\mathrm{Zn}\ 2p}^{\mathrm{AZO}} \) refer to the BE of Ga 2p and Zn 2p CLs in thin Al-doped ZnO films, respectively.

Subsequently, based on the Eg and ∆EV, the CBO at the Al-doped ZnO/β-Ga2O3 interface can be calculated by the following equation:

$$ \Delta {E}_C={E}_g^{{\mathrm{Ga}}_2{\mathrm{O}}_3}-{E}_g^{\mathrm{AZO}}-\Delta {E}_V $$
(2)

where\( {E}_g^{{\mathrm{Ga}}_2{\mathrm{O}}_3} \) is the bandgap of Ga2O3 and \( {E}_g^{\mathrm{AZO}} \) is the bandgap of Al-doped ZnO. The bandgaps for undoped, 5% Al-doped ZnO, 10% Al-doped ZnO, and β-Ga2O3 are 3.20 eV, 3.25 eV, 3.40 eV, and 4.65 eV, respectively [17, 1 shows the Ga and Zn element CLs and VBM of bulk β-Ga2O3, thick undoped, and 5% and 10% Al-doped ZnO films. Fitting the linear area and the flat band zone from the VBM spectrum can deduce the VBM [20]. Figure 2 shows Ga 2p and Zn 2p CL from various thin Al-doped ZnO/β-Ga2O3 heterojunctions. The BE differences of Ga 2p and Zn 2p CLs for the undoped, 5% Al-doped ZnO/β-Ga2O3, and 10% Al-doped ZnO/β-Ga2O3 are obtained to be 96.12 eV, 96.16 eV, and 95.94 eV, respectively. Then, the VBOs at the interfaces are determined to be 1.39 eV, 1.52 eV, and 1.67 eV for the undoped, 5% Al-doped ZnO/β-Ga2O3, and 10% Al-doped ZnO/β-Ga2O3 samples, respectively.

Fig. 1
figure 1

High resolution XPS spectra for core level and valence band maximum (VBM) of a Ga 2p core level spectrum and VBM from bare β-Ga2O3, b Zn 2p core level spectrum and VBM from thick pure ZnO/β-Ga2O3, c Zn 2p core level spectrum and VBM from thick 5% Al-doped ZnO/β-Ga2O3, and d Zn 2p core level spectrum and VBM from thick 10% Al-doped ZnO/β-Ga2O3

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Fig. 2
figure 2

The core level spectra of Ga 2p and Zn 2p obtained from high resolution XPS spectra of a thin ZnO/β-Ga2O3, b thin 5% Al-doped ZnO/β-Ga2O3, and c thin 10% Al-doped ZnO/β-Ga2O3

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The systematic band alignment for the 0%, 5%, and 10% Al-doped ZnO/β-Ga2O3 heterojunctions are calculated by the above equations, as shown in Fig. 3. The band offset of undoped ZnO/β-Ga2O3 heterojunction belongs to type I. While both 5% and 10% Al-doped ZnO/β-Ga2O3 heterojunctions have type-II band offsets. Figure 4 depicts the band alignments of Al-doped ZnO/β-Ga2O3 interfaces have a similar linear relationship with Al do** concentration. The CBO varies from 1.39 to 1.67 eV with the Al-doped concentration increasing from 0 to 10%. While the VBO reduces from 0.06 to − 0.42 eV with the Al-doped concentration rising from 0 to 10%. It is noted that the CBO and VBO for sputtered AZO/β-Ga2O3 are 0.79 eV and 0.61 eV, respectively [9]. Both the conduction and valence band shift downward in this work, which could be due to the different composition ratio and crystalline structure introduced by deposited methods.

Fig. 3
figure 3

Schematic band alignment diagram of a pure ZnO/β-Ga2O3, b 5% Al-doped ZnO/β-Ga2O3, and c 10% Al-doped ZnO/β-Ga2O3

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Fig. 4
figure 4

The conduction and valence band offsets of atomic-layer-deposited AZO/β-Ga2O3 heterojunctions fabricated at different Al do** ratios

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Other than that, first-principle simulations were performed by the Vienna Ab-initio Simulation Package (VASP) [21,22,23,24] to investigate the electronic band structure and band alignment of Al-doped ZnO/Ga2O3 heterojunctions. During the calculation, the electron-ion interactions were treated by the ultra-soft pseudo-potentials, and the wave functions and potentials were expanded by the plane-wave basis [25]. Besides, generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE) was implemented to describe the exchange-correlation energies [26]. Prior to initiating the simulation, converging tests were performed. It showed that the cutoff energy of 450 eV for the plane-wave basis and k-space grids of 3 × 3 × 3 with the Monkhorst Pack scheme gave the well-converged results. In the structure optimization, a conjugate gradient method was used and the residual force was released until it was less than 0.01 eV/Å. Moreover, the hybrid density functions based on the semi-local PBE approximation were implemented. To correct the underestimated bandgap, 35% of PBE exchange was replaced with the exact one [27]. To identify the band edge shift with the change of the Al do** level, the average electrostatic potential (AEP) was calculated and aligned to the vacuum level which was scaled to 0 V. The VBM and conduction band minimum (CBM) were consequently aligned to the AEP based on the band diagram [5 a–c shows the calculated band diagrams of the undoped, 3.21% Al-doped ZnO, and 6.25% Al-doped ZnO structures, respectively. It clearly shows that ZnO is a direct bandgap semiconductor with the bandgap of 3.42 eV, and the CBM as well as the VBM was located at the Γ point of Brillouin zone. These theoretical simulation results match the experimental value quite well [29]. With the Al do**, it could be found that the Fermi levels shifted upwards into the conduction band, which converts the pure ZnO into an n-type semiconductor. In the meanwhile, the bandgaps also increased to 4.83 eV and 5.42 eV for 3.21% Al-doped ZnO and 6.25% Al-doped ZnO, respectively. Although the bandgaps here for the doped ZnO are higher than our experimental results; however, this could be ascribed to the neglecting of interfacial defect states as well as other crystal defects.

Fig. 5
figure 5

The calculated band diagram of a undoped ZnO, b 3.21% Al-doped ZnO, and c 6.25% Al-doped ZnO structure. The Fermi levels were set to 0 eV

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Figure 6 a–c presents the band alignments of undoped, 3.21% Al-doped ZnO, and 6.25% Al-doped ZnO to the vacuum level. For the conduction bands of the materials, due to the strong electron mixing between the Al and O element, it could be found that the energy level decreases from − 6.19 eV of the ZnO to − 6.81 eV for the 3.21% Al-doped ZnO (ΔE = 0.62 eV ) and further decreases to − 7.48 eV for the 6.25% Al-doped ZnO (ΔE = 1.29 eV ). In the meanwhile, due to the opening up of the bandgap, it also could be found that the valence band edge moves downwards from − 9.59 eV for the ZnO to − 11.64 eV for 3.21% Al-doped ZnO (ΔE = 2.05 eV ) and − 12.9 eV for the 6.25% Al-doped ZnO (ΔE = 3.31 eV ). In all, ascribed to the strong Al and O electron mixing, it could be understood that incorporating Al in the ZnO would open up the band gaps. Moreover, it would shift both the conduction band and valence band edge towards the lower energy level when aligned to the vacuum level.

Fig. 6
figure 6

The band alignment of AZO/β-Ga2O3 heterojunctions with a undoped, b 3.21%, and c 6.25% Al-doped ZnO. The vacuum levels were scaled to 0 eV

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Conclusions

In conclusion, the band alignments of different Al-doped ZnO/β-Ga2O3(\( \overline{2} \)01) interfaces have been investigated by XPS. A type-I band alignment forms at the interface of ZnO/β-Ga2O3 heterojunction. While the AZO/β-Ga2O3 interface has a type-II band alignment. The CBOs vary from 1.39 to 1.67 eV and the VBOs reduce from 0.06 to − 0.42 eV with the Al-doped concentration rising from 0 to 10%. Moreover, the density function calculations show that band offsets change due to strong Al and O electron mixing when Al is incorporated into ZnO. These results suggest that the pure ZnO is a valid ISL to reduce the barrier height and promote the electron transport.