Energy Band Alignment in Atomic‑Layer‑Deposited ZnO/β‑Ga₂O₃ (over 2 01) Heterojunctions Revealed by XPS

Abstract

We investigated the band alignment of ZnO/β‑Ga₂O₃ (over 2 01) heterojunctions using X‑ray photoelectron spectroscopy (XPS). Atomic‑layer‑deposited (ALD) ZnO films were grown at 150 °C, 200 °C, and 250 °C. All interfaces exhibited a type‑I alignment. The conduction‑band offset (CBO) increased from 1.26 eV to 1.47 eV as the growth temperature rose, while the valence‑band offset (VBO) decreased from 0.20 eV to 0.01 eV. The rise in CBO is attributed to Zn interstitials, whereas the reduction in VBO arises from the acceptor‑type V_Zn + OH complex. These insights provide a quantitative foundation for engineering ZnO/β‑Ga₂O₃ heterostructures in high‑power electronics.

Introduction

Gallium oxide (Ga₂O₃) is emerging as a leading ultrawide‑bandgap (UWBG) semiconductor for next‑generation power devices, thanks to its 4.5–4.9 eV bandgap, exceptional thermal stability, and an 8 MV cm⁻¹ breakdown field that surpasses SiC and GaN [1–4]. Among its polymorphs, monoclinic β‑Ga₂O₃ offers the highest thermal resilience and a bulk electron mobility of ≈100 cm² V⁻¹ s⁻¹, making it attractive for solar‑blind photodetectors and MOSFETs [5–8]. Nevertheless, achieving low‑resistance ohmic contacts remains challenging [9], and inserting an intermediate semiconductor layer (ISL) with a high electron concentration has proven effective in mitigating interfacial barriers [10–12]. Zinc oxide (ZnO) is a compelling ISL candidate: it has a 3.3 eV bandgap, a high exciton binding energy (60 meV), and an electron concentration exceeding 10¹⁹ cm⁻³ [13–15]. The lattice mismatch with β‑Ga₂O₃ is below 5 %, reducing strain. Traditional deposition methods—hydrothermal and CVD—either suffer from slow growth or require temperatures above 900 °C, limiting device integration. Atomic‑layer deposition (ALD), however, offers sub‑nanometer thickness control, excellent step coverage, and low deposition temperatures, making it ideal for fabricating high‑quality ZnO films on β‑Ga₂O₃. Despite theoretical predictions of ZnO/β‑Ga₂O₃ band offsets [20], experimental determination has been lacking. This study fills that gap by measuring the band alignment of ALD‑grown ZnO on β‑Ga₂O₃ and examining the temperature dependence of the offsets.

Methods

Sn‑doped β‑Ga₂O₃ (over 2 01) substrates (≈3 × 10¹⁸ cm⁻³) were diced into 6 × 6 mm² pieces. They were cleaned sequentially in acetone and isopropanol (10 min each, ultrasonic), followed by deionized‑water rinsing. The substrates were then loaded into a Wuxi MNT Micro‑Nanotech ALD reactor. ZnO films (5 nm and 40 nm) were deposited at 150 °C, 200 °C, and 250 °C using Zn(C₂H₅)₂ (DEZ) and H₂O precursors. The growth rate was ~1.6 Å cycle⁻¹. Film thicknesses were verified with a Sopra GES‑5E ellipsometer. A 40‑nm ZnO/β‑Ga₂O₃ sample served as a bulk standard, while 5‑nm ZnO/β‑Ga₂O₃ was used for band‑offset determination; bare β‑Ga₂O₃ was the control. XPS measurements (AXIS Ultra DLD, Shimadzu) were performed with 0.05 eV steps, after a 3‑min Ar ion etch at 2 kV to remove surface contamination. All spectra were calibrated to the C 1s peak at 284.8 eV. Valence‑band maxima (VBM) were extracted by linear extrapolation of the leading edge. UV–VIS transmittance spectra (Lambda 750, PerkinElmer) were recorded to determine optical band gaps via Tauc analysis.

Results and Discussion

Figure 1 shows the Tauc plots (αhv)¹⁄² versus photon energy for bulk β‑Ga₂O₃ and ZnO grown at 200 °C. The extracted optical band gaps are 4.65 eV for β‑Ga₂O₃ and 3.20 eV for ZnO, consistent with literature [22, 23].

Energy Band Alignment in Atomic‑Layer‑Deposited ZnO/β‑Ga₂O₃ (over 2 01) Heterojunctions Revealed by XPS

The VBO was calculated using Kraut’s method:

$$\Delta E_V = (E_{Ga2p}^{Ga_2O_3} - E_{VBM}^{Ga_2O_3}) - (E_{Zn2p}^{ZnO} - E_{VBM}^{ZnO}) - (E_{Ga2p}^{Ga_2O_3} - E_{Zn2p}^{ZnO})$$

Core‑level spectra (Figure 2) reveal symmetric Zn 2p and Ga 2p peaks, confirming uniform bonding. The Zn 2p peak at 1021.09 eV and the Ga 2p peak at 1117.78 eV correspond to Zn–O and Ga–O bonds, respectively. VBM values of 2.11 eV (ZnO) and 2.74 eV (β‑Ga₂O₃) were obtained via linear extrapolation. Applying Eq. (1) gives a VBO of 0.06 eV.

The CBO follows from:

$$\Delta E_C = E_g^{Ga_2O_3} - E_g^{ZnO} - \Delta E_V$$

The resulting band diagram (Figure 3) confirms a type‑I alignment, with both ZnO conduction and valence bands fully nested within the β‑Ga₂O₃ gap.

To assess temperature effects, ZnO films were also grown at 150 °C and 250 °C. High‑resolution O 1s spectra (Figure 4) deconvoluted into Zn–O, oxygen‑vacancy, and –OH components. The oxygen‑vacancy fraction increased from 10.7 % to 15.0 % as temperature rose, while –OH decreased from 5.1 % to 1.9 %. These trends reflect enhanced precursor decomposition and more complete surface reactions at higher temperatures.

Figure 5 summarizes the temperature dependence of the offsets. The CBO rises from 1.26 eV to 1.47 eV, primarily due to increased Zn interstitials that shift the conduction‑band minimum (CBM). Conversely, the VBO drops from 0.20 eV to 0.01 eV as temperature increases, attributed to the diminishing influence of the acceptor complex V_Zn + OH, which is more prevalent at lower temperatures. Thus, lower‑temperature ZnO deposition yields a reduced electron barrier at the interface, beneficial for ISL design.

Conclusions

Atomic‑layer‑deposited ZnO on β‑Ga₂O₃ (over 2 01) exhibits a clear type‑I band alignment. As the ZnO growth temperature increases from 150 °C to 250 °C, the CBO grows from 1.26 eV to 1.47 eV, while the VBO decreases from 0.20 eV to 0.01 eV. These trends are governed by temperature‑dependent defect chemistry—Zn interstitials raise the CBO, whereas the V_Zn + OH complex lowers the VBO. Lower‑temperature ZnO deposition is therefore advantageous as an intermediate semiconductor layer to lower electron barrier heights in β‑Ga₂O₃‑based power devices.