Introduction

Gallium oxide (Ga2O3) has been widely investigated as a promising ultrawide bandgap semiconductor material for next generation power electronic devices due to its unique properties [1]. Among various polymorphs (α, β, γ, δ, and ε), monoclinic β-Ga2O3 has the most thermal stability [2]. In addition, β-Ga2O3 has a room temperature bandgap of 4.5~4.9 eV, and excellent chemical stability [3]. Especially, β-Ga2O3 has a high bulk electron mobility of ∼100 cm2/V·s, much higher breakdown field of 8 MV/cm than that of SiC (3.18 MV/cm) or GaN (3 MV/cm) [4], and the carrier concentration can be easily modulated by doping Sn and Si [5, 6]. Therefore, β-Ga2O3-based devices including solar-blind photodetectors [7] and metal-oxide-semiconductor field-effect transistors (MOSFETs) [8] have been reported. However, limitations still exist in β-Ga2O3-based devices, such as the poor ohmic contact between the metal and β-Ga2O3 [9]. In recent year, inserting a high electron concentration metal-oxide-semiconductor interlayer, i.e., intermediate semiconductor layer (ISL) between the metal and Ga2O3, has been shown to be an effective resolution because the modulation of energy barrier at the interface [10,11,12].

Zinc oxide (ZnO) has attracted much attention because it has a large exciton binding energy of 60 meV, a high electron concentration of > 1019 cm−3, and a strong cohesive energy of 1.89 eV. [13, 14] Additionally, the lattice mismatch between ZnO and Ga2O3 is within 5% [15]. Various deposition techniques have been developed to prepare ZnO film, including hydrothermal method [16, 17] and chemical vapor deposition (CVD). [18] However, hydrothermal method need a complicated process and the grow rate is quiet slow, and CVD generally requires quiet high growth temperature above 900 °C. These drawbacks make it challenging to be applied in devices. Recently, atomic layer deposition (ALD) has emerged as a promising technique, which exhibits excellent step coverage, atomic scale thickness controllability, good uniformity, and a relatively low deposition temperature. Consequently, atomic-layer-deposited ZnO on wide-bandgap semiconductors can reduce interface disorder and yield more controllable sample to examine the energy band alignment, which plays an important role in the carrier transport process [19]. Up to now, band alignment between Ga2O3 and atomic-layer-deposited ZnO has not been studied by experiment, although there are some reports about the theoretical band alignment of ZnO and Ga2O3. [20] Therefore, understanding the energy band alignment of atomic-layer-deposited ZnO/β-Ga2O3 heterojunction is highly desirable for the design and physical analysis of relevant devices in the future. In this work, the energy band alignment of atomic-layer-deposited ZnO on β-Ga2O3 was characterized by X-ray photoelectron spectroscopy (XPS). Moreover, the influence of growth temperature of ZnO on the band alignment was also addressed.

Methods

β-Ga2O3 (\( \overline{2}01 \)) substrates with a Sn doping concentration of ~ 3 × 1018/cm3 were diced into small pieces with the size of 6 × 6 mm2. The diced samples were alternately cleaned in acetone, isopropanol by ultrasonic cleaning for each 10 min, subsequently rinsed with deionized water to remove residual organic solvents. After that, Ga2O3 substrates were transferred into an ALD reactor (Wuxi MNT Micro Nanotech co., LTD, China). The growth rate of ZnO films was ~ 1.6 Å/cycle. Both 40 and 5 nm ZnO films were grown on cleaned β-Ga2O3 using Zn (C2H5)2 (DEZ) and H2O at each temperature of 150, 200, and 250 °C, respectively. The thickness of prepared ZnO films was measured by Ellipsometer (Sopra GES-5E). The ZnO(40 nm)/β-Ga2O3 was used as bulk standard, and the ZnO(5 nm)/β-Ga2O3 was used to determine the band alignment, in the meanwhile the bare bulk β-Ga2O3 was used as the control sample. XPS (AXIS Ultra DLD, Shimadzu) measurements with a step of 0.05 eV were performed to measure the valence band maximum (VBM), Ga 2p and Zn 2p spectra. To avoid interference of surface oxidation and contamination, all samples were etched by Ar ion for 3 min with a voltage of 2 kV before XPS measurement. Note that all the XPS spectra were calibrated by C 1s peak at 284.8 eV for compensating the charging effect. To identify the bandgap, the optical transmittance spectra of Ga2O3 and ZnO were measured by ultraviolet-visible (UV-VIS) spectroscopy (Lambda 750, PerkinElmer, USA).

Results and Discussion

Figure 1 shows the variation of (αhv)1/n as a function of photon energy for bulk β-Ga2O3 and the as-grown ZnO film deposited at 200 °C. The optical band gap (Eg) of the ZnO film and β-Ga2O3 can be determined by the Tauc’s relation [21]: (αhv)1/n = A(hv − Eg), where α is the absorption coefficient, A is a constant, hv is the incident photon energy, Eg is the optical energy bandgap, n is 1/2 for the direct bandgap, and 2 for the indirect bandgap. Here, both ZnO and β-Ga2O3 have typical direct band gap that make the value of n is 1/2. Subsequently, Eg can be extracted by extrapolating the straight line portion to the energy bias at α = 0. Therefore, the extracted Eg of ZnO and β-Ga2O3 are 3.20 eV and 4.65 eV, respectively, in good agreement with the reported. [22, 23]

Fig. 1
figure 1

The plot of (αhv)2 versus hv for a ZnO film grown on quartz glass b β-Ga2O3 substrate. The inset shows the optical transmission spectra of ZnO and β-Ga2O3, respectively

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The valence band offset (VBO) can be determined by Kraut’s method using the following formula [24]

$$ \Delta {E}_V=\left({E}_{Ga\ 2p}^{Ga_2{O}_3}-{E}_{VBM}^{Ga_2{O}_3}\right)-\left({E}_{Zn\ 2p}^{Zn O}-{E}_{VBM}^{Zn O}\right)-\left({E}_{Ga\ 2p}^{Ga_2{O}_3}-{E}_{Zn\ 2p}^{Zn O}\right), $$
(1)

where \( {E}_{Ga\ 2p}^{Ga_2{O}_3}-{E}_{VBM}^{Ga_2{O}_3} \) \( \Big({E}_{Zn\ 2p}^{Zn O}-{E}_{VBM}^{Zn O} \)) represents to the energy difference between Ga 2p (Zn 2p) core level (CL) and VBM of bulk β-Ga2O3 (ZnO), and \( {E}_{Ga\ 2p}^{Ga_2{O}_3}-{E}_{Zn\ 2p}^{Zn O} \) denotes as the energy difference between Ga 2p and Zn 2p core levels. Figure 2 shows all CL spectra including Zn 2p of ZnO (40 nm)/β-Ga2O3 and ZnO (5 nm)/β-Ga2O3, Ga 2p of bulk Ga2O3 and ZnO (5 nm)/β-Ga2O3, as well as valence band spectra from bulk Ga2O3 and ZnO (40 nm)/β-Ga2O3. Figure 2a presents the CL spectra of Zn 2p on the ZnO (40 nm)/β-Ga2O3, which is quiet symmetrical indicating the uniform bonding state, and the peak locates at 1021.09 eV corresponds the Zn-O bond [25]. The VBM can be determined using a linear extrapolation method [26]. The VBM of ZnO is located at 2.11 eV. In Fig. 2b, the peak located at 1117.78 eV corresponds to the Ga-O bond [27] and the VBM of Ga2O3 is deduced to be 2.74 eV according to the method mentioned above. The CLs of Zn 2p and Ga 2p in the ZnO (5 nm)/β-Ga2O3 are shown in Fig. 2c. According to Eq. (1), the VBO at the interface of ZnO/Ga2O3 is determined to be 0.06 eV.

Fig. 2
figure 2

High-resolution XPS spectra for core level and valence band maximum(VBM) of a Zn 2p core level spectrum and VBM from 40 nm ZnO/β-Ga2O3, b Ga 2p core level spectrum and VBM from bare β-Ga2O3, and c the core level spectra of Ga 2p and Zn 2p obtained from high-resolution XPS spectra of 5 nm ZnO/β-Ga2O3

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Based on the calculated Eg and ∆EV, the conduction band offset (CBO) at the ZnO/Ga2O3 interface can be easily deduced from the following equation:

$$ \Delta {E}_C={E}_g^{Ga_2{O}_3}-{E}_g^{ZnO}-\Delta {E}_V, $$
(2)

where\( {E}_g^{Ga_2{O}_3} \) and \( {E}_g^{ZnO} \) are the energy bandgap for β-Ga2O3 and ZnO, respectively. The detailed energy band diagram for ZnO/β-Ga2O3 is depicted in Fig. 3. The interface has a type-I band alignment, where both conduction and valence band edges of ZnO are located within the bandgap of β-Ga2O3.

Fig. 3
figure 3

Schematic band alignment diagram of the ZnO (200 °C)/β-Ga2O3 heterojunction

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To further examine the effect of the growth temperature on the band alignment between ZnO and β-Ga2O3, the ZnO films are also grown at 150 and 250 °C. Note that ZnO films prepared by ALD at the temperatures of 150–250 °C have poly-crystalline nature. Figure 4 shows the high-resolution O 1s XPS spectra of the ZnO films grown at different temperatures. Each O 1s spectrum can be well separated into three components using Gaussian-Lorentzian function. The peaks centered at 530.0 (O1), 531.6 (O2), and 532.4 (O3) eV correspond to the Zn-O bands, oxygen vacancies, and –OH group [28, 29], respectively. The relative percentage of different components is also calculated according to the peak area, digested in Fig. 4. It shows that the relative content of oxygen vacancies increases from 10.7 to 15.0% due to the decomposition of precursors and the increase of Zn interstitials. However, the –OH counterpart reduces from 5.1 to 1.9% because of more complete reactions between DEZ precursors and surface –OH groups in this temperature range [30].

Fig. 4
figure 4

High-resolution O 1 s XPS spectra of the ZnO films grown at a 150 °C, b 200 °C, and c 250 °C, respectively

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Figure 5 shows the band offsets of ZnO/β-Ga2O3 heterojunctions as a function of growth temperature. The CBO increases from 1.26 to 1.47 eV with the growth temperature varying from 150 to 250 °C. The native donor defects include the Zn anti-position, oxygen vacancies, and Zn interstitials. However, the formation energy of anti-position atoms is so high that its concentration is extremely low. The Zn interstitials have more influence on the conduction band minimum (CBM) than that of the oxygen vacancy because the CBM is mainly dominated by the 4s orbit of Zn atom. [31] As a result, the increased CBO of 0.21 eV could be mainly contributed by Zn interstitials. On the other hand, the VBO reduces from 0.20 to 0.01 eV with the growth temperature increasing from 150 to 250 °C. The native acceptor defects include the O anti-position, Zn vacancies, and oxygen interstitials [32], whose formation energies are high and their number can be even negligible. Furthermore, the most native acceptor levels are deep within the ZnO bandgap, thus they have little effect on the VBM [33]. However, Vzn + OH is favorable to be presented duo to the low formation energy, [34] Vzn + OH may occur with an electron belonging to OH bonds. The lattice hydrogen H+ ion acts as a compensating center, and it can bind with the VZn around the dislocation and stacking faults core, ensuring the acceptor-type complex defect for p-type conductivity [35]. More residual –OH groups in the ZnO film are obtained at a lower growth temperature, i.e., 150 °C [36]. The acceptor level near the VBM reduces with the temperature, leading to an effectively downward shift in EV of ZnO, thus the ∆EV becomes lower. Therefore, the ZnO deposited at lower temperature could be more efficiently to reduce the barrier height at the interface between the metal and Ga2O3.

Fig. 5
figure 5

The conduction and valence band offsets of atomic-layer-deposited ZnO/β-Ga2O3 heterojunctions fabricated at different temperatures

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Conclusions

In summary, the energy band alignment at atomic-layer-deposited ZnO/β-Ga2O3 (\( \overline{2}01 \)) was characterized by XPS. A type-I band alignment formed at the ZnO/β-Ga2O3 interface. The conduction band offset increased from 1.26 to 1.47 eV while the valence band offset decreased from 0.20 to 0.01 eV with the temperature increasing from 150 to 250 °C. These observations suggest that the ZnO deposited at lower temperature is favorable to be a promising ISL to reduce the electron barrier height at the ZnO/β-Ga2O3 interface.