Investigation of Energy Band at Atomic-Layer-Deposited ZnO/β-Ga2O3 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{2}01 $$\end{document}2¯01) Heterojunctions

The energy band alignment of ZnO/β-Ga2O3 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{2}01 $$\end{document}2¯01) heterojunction was characterized by X-ray photoelectron spectroscopy (XPS). The ZnO films were grown by using atomic layer deposition at various temperatures. A type-I band alignment was identified for all the ZnO/β-Ga2O3 heterojunctions. The conduction (valence) band offset varied from 1.26 (0.20) eV to 1.47 (0.01) eV with the growth temperature increasing from 150 to 250 °C. The increased conduction band offset with temperature is mainly contributed by Zn interstitials in ZnO film. In the meanwhile, the acceptor-type complex defect Vzn + OH could account for the reduced valence band offset. These findings will facilitate the design and physical analysis of ZnO/β-Ga2O3 relevant electronic devices.


Introduction
Gallium oxide (Ga 2 O 3 ) 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-Ga 2 O 3 has the most thermal stability [2]. In addition, β-Ga 2 O 3 has a room temperature bandgap of 4.5~4.9 eV, and excellent chemical stability [3]. Especially, β-Ga 2 O 3 has a high bulk electron mobility of ∼100 cm 2 /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-Ga 2 O 3 -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-Ga 2 O 3 -based devices, such as the poor ohmic contact between the metal and β-Ga 2 O 3 [9]. In recent year, inserting a high electron concentration metal-oxide-semiconductor interlayer, i.e., intermediate semiconductor layer (ISL) between the metal and Ga 2 O 3 , 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 > 10 19 cm −3 , and a strong cohesive energy of 1.89 eV. [13,14] Additionally, the lattice mismatch between ZnO and Ga 2 O 3 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 Ga 2 O 3 and atomic-layer-deposited ZnO has not been studied by experiment, although there are some reports about the theoretical band alignment of ZnO and Ga 2 O 3 . [20] Therefore, understanding the energy band alignment of atomic-layer-deposited ZnO/β-Ga 2 O 3 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-Ga 2 O 3 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-Ga 2 O 3 ( 201 ) substrates with a Sn doping concentration of~3 × 10 18 /cm 3 were diced into small pieces with the size of 6 × 6 mm 2 . 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, Ga 2 O 3 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-Ga 2 O 3 using Zn (C 2 H 5 ) 2 (DEZ) and H 2 O 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-Ga 2 O 3 was used as bulk standard, and the ZnO(5 nm)/β-Ga 2 O 3 was used to determine the band alignment, in the meanwhile the bare bulk β-Ga 2 O 3 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 Ga 2 O 3 and ZnO were measured by ultraviolet-visible (UV-VIS) spectroscopy (Lambda 750, PerkinElmer, USA). Figure 1 shows the variation of (αhv) 1/n as a function of photon energy for bulk β-Ga 2 O 3 and the as-grown ZnO film deposited at 200°C. The optical band gap (E g ) of the ZnO film and β-Ga 2 O 3 can be determined by the Tauc's relation [21]: (αhv) 1/n = A(hv − E g ), where α is the absorption coefficient, A is a constant, hv is the incident photon energy, E g is the optical energy bandgap, n is 1/2 for the direct bandgap, and 2 for the indirect bandgap. Here, both ZnO and β-Ga 2 O 3 have typical direct band gap that make the value of n is 1/2. Subsequently, E g can be extracted by extrapolating the straight line portion to the energy bias at α = 0. Therefore, the extracted E g of ZnO and β-Ga 2 O 3 are 3.20 eV and 4.65 eV, respectively, in good agreement with the reported. [22,23] The valence band offset (VBO) can be determined by Kraut's method using the following formula [24]

Results and Discussion
represents to the energy difference between Ga 2p (Zn 2p) core level (CL) and VBM of bulk β-Ga 2 O 3 (ZnO), and E Ga 2 O 3 Ga 2p −E ZnO Zn 2p denotes as the energy difference between Ga 2p and Zn 2p core levels.  Zn 2p on the ZnO (40 nm)/β-Ga 2 O 3 , 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 Ga 2 O 3 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-Ga 2 O 3 are shown in Fig. 2c. According to Eq. (1), the VBO at the interface of ZnO/Ga 2 O 3 is determined to be 0.06 eV.
Based on the calculated E g and ΔE V , the conduction band offset (CBO) at the ZnO/Ga 2 O 3 interface can be easily deduced from the following equation: where E Ga 2 O 3 g and E ZnO g are the energy bandgap for β-Ga 2 O 3 and ZnO, respectively. The detailed energy band diagram for ZnO/β-Ga 2 O 3 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-Ga 2 O 3 .
To further examine the effect of the growth temperature on the band alignment between ZnO and β-Ga 2 O 3 , 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]. Figure 5 shows the band offsets of ZnO/β-Ga 2 O 3 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, V zn + OH is favorable to be presented duo to the low formation Fig. 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-Ga 2 O 3 , b Ga 2p core level spectrum and VBM from bare β-Ga 2 O 3 , and c the core level spectra of Ga 2p and Zn 2p obtained from high-resolution XPS spectra of 5 nm ZnO/β-Ga 2 O 3 energy, [34] V zn + 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 V Zn 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 E V of ZnO, thus the ΔE V 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 Ga 2 O 3 .