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. 2022 Nov 24;7(48):43603–43608. doi: 10.1021/acsomega.2c04387

Epitaxial Growth of β-Ga2O3 Thin Films on Si with YSZ Buffer Layer

Hyung-Jin Choi †, Jun Young Lee †, Soo Young Jung †,‡, Ruiguang Ning †,§, Min-Seok Kim †,∥, Sung-Jin Jung †, Sung Ok Won ⊥, Seung-Hyub Baek †,§,#,∇,*, Ji-Soo Jang †,*
PMCID: PMC9730745  PMID: 36506186

Abstract

graphic file with name ao2c04387_0006.jpg

We report the epitaxial growth of (2̅01)-oriented β-Ga2O3 thin films on a (001) Si substrate using the pulsed laser deposition technique employing epitaxial yttria-stabilized zirconia (YSZ) buffer layers. Epitaxial β-Ga2O3 thin films possess a biaxial compressive strain on YSZ single-crystal substrates while they exhibit a biaxial tensile strain on YSZ-buffered Si substrates. Post-annealing improves the crystalline quality of β-Ga2O3 thin films. High-resolution X-ray diffraction analyses reveal that the epitaxial (2̅01) β-Ga2O3 thin films on Si have eight in-plane domain variants to accommodate the large difference in the crystal structure between monoclinic β-Ga2O3 and cubic YSZ. The results provide a pathway to integrate epitaxial β-Ga2O3 thin films on a Si gold standard substrate, which will expand the application scope beyond high-power electronics.

Introduction

Recently, Ga2O3 has attracted significant attention as a promising wide-bandgap semiconductor for applications in high-power devices, ultraviolet photodetectors, and gas sensors. Among the five Ga2O3 polymorphs (α, β, γ, δ, and ε), monoclinic β-Ga2O3 is the most thermodynamically stable and exhibits excellent physical properties, such as a wide band gap (∼4.9 eV), optical transparency, n-type semiconducting properties, high electrical breakdown voltage (∼8 MV/cm), and radiation resistance.1−3

The epitaxial growth of Ga2O3 thin films is important for the realization of high-performance devices and understanding their intrinsic properties. Thus far, many growth techniques have been reported for the epitaxial growth of Ga2O3 thin films, including pulsed laser deposition (PLD),4,5 molecular beam epitaxy,6 metal–organic chemical vapor deposition (CVD),7 mist-CVD,8 sol–gel,9 and sputtering.10 For high-power devices, homoepitaxial Ga2O3 structures, wherein both the film and substrate are β-Ga2O3, are desirable because a key feature of high-power electronics is a high resistance to electrical breakdown under high voltage conditions. Several fabrication techniques for β-Ga2O3 bulk single crystals have been developed, such as Verneuil,11 floating,12 Czochralski,13 edge-defined film growth,14 and vertical Bridgman methods.15 However, β-Ga2O3 single-crystal substrates are very expensive compared to other single-crystal oxide and semiconductor substrates, which limits the application of homoepitaxial Ga2O3 structures. In addition, heteroepitaxial structures are formed when β-Ga2O3 thin films are grown on different substrates, such as MgO,16 CeO2,17 and Al2O3.18 These have also been widely studied.

In this study, we investigated the epitaxial growth of β-Ga2O3 thin films on Si substrates, the gold standard single crystal of modern electronics19−21 using the PLD technique. (2̅01)-Oriented epitaxial β-Ga2O3 thin films were grown on yttria-stabilized zirconia (YSZ) and YSZ-buffered Si substrates. Using high-resolution X-ray diffraction (HRXRD), we analyzed domain structures and strain states of monoclinic β-Ga2O3 thin films on cubic YSZ-buffered Si substrates and demonstrated the improved crystalline quality through a post-annealing process. The results provide a pathway for integrating the functionalities of β-Ga2O3 thin films onto Si, which can broaden the scope of β-Ga2O3 applications beyond high-power electronics.

Results and Discussion

Figure 1a shows schematics of the β-Ga2O3, YSZ, and Si unit cells. YSZ has a fluorite structure (Fm3®m, cubic), with a lattice parameter of 5.143 Å. Owing to the small lattice mismatch with Si (Fm3̅m, cubic, 5.431 Å), YSZ can be grown epitaxially on Si. The epitaxial YSZ layers on Si can function as a buffer layer to integrate additional functional oxide overlayers onto Si.22−25 Owing to their unique growth process, involving the scavenging effect, epitaxial YSZ buffer layers can be deposited on Si using low-cost deposition processes, such as PLD and sputtering. β-Ga2O3 belongs to a monoclinic crystal system (C2/m, a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7°) and has a large lattice mismatch with cubic YSZ and Si substrates. Typically, complex domain structures evolve when a material with low symmetry is epitaxially grown on a substrate with high symmetry; for example, epitaxial BiFeO3 (rhombohedral) thin films have four structural variants on SrTiO3 (cubic) substrates.26 Therefore, it is expected that epitaxial β-Ga2O3 thin films on YSZ and YSZ-buffered Si substrates will exhibit a complex domain structure. In addition to the lattice mismatch, thermal mismatch affects epitaxial β-Ga2O3 thin films, particularly their strain state. A large difference exists in the thermal expansion coefficients of β-Ga2O3 (∼5 × 10–6/K), YSZ (∼9 × 10–6/K), and Si (∼3 × 10–6/K). Typically, epitaxial oxide thin films grown on Si possess tensile strain at room temperature because of the thermal mismatch due to cooling to room temperature. To study this effect, we grew epitaxial β-Ga2O3 thin films on both YSZ and YSZ-buffered Si substrates via PLD, as shown in Figure 1b.

Figure 1.

Figure 1

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(a) Schematics of unit cells for β-Ga2O3, YSZ, and Si. (b) Schematic of the fabrication process of epitaxial β-Ga2O3 films on two types of substrates: (i) YSZ single-crystal and (ii) YSZ-buffered (001) Si substrates.

β-Ga2O3 thin films were grown via PLD using a KrF excimer laser (λ = 248 nm) at 100 mTorr O2 partial pressure with a laser energy density of 1.5 J/cm2 and frequency of 5 Hz at 750 °C. A Ga2O3 ceramic target was used with a sample-to-target distance of 5 cm. Epitaxial β-Ga2O3 thin films were grown on two different substrates: (001) YSZ and YSZ-buffered (001) Si single-crystal (YSZ-buffered Si) substrates. A 45 nm-thick epitaxial YSZ buffer layer was grown via PLD at 0.1 mTorr O2 partial pressure with a laser energy density of 1.5 J/cm2 and frequency of 5 Hz at 750 °C. Note that the 45 nm-thick epitaxial YSZ buffer layer was selected considering the crystallinity of each thickness sample (Figure S2). A YSZ ceramic target with a composition of 20%Y-ZrO2 was used with a sample-to-target distance of 5 cm. The growth rate was maintained at 6 nm/min. Before Ga2O3 deposition, both the YSZ and YSZ-buffered Si substrates were cleaned with acetone, isopropyl alcohol, and DI water, followed by N2 drying.

Commercial atomic force microscopy (AFM, Digital Instrument Dimension 3100, equipped with a Nanoscope IV controller) was used to investigate the surface morphology of β-Ga2O3 thin films in the tapping mode. Figure 2a,b show AFM images of β-Ga2O3 thin films grown on YSZ and YSZ-buffered Si substrates, respectively. The β-Ga2O3 film on the YSZ substrate exhibited a smooth surface with a height variation of ±2 nm. In contrast, the β-Ga2O3 film on YSZ-buffered Si substrates exhibited a slightly rougher surface with a height variation of ±4 nm. These results are also supported by SEM results (see Figure S1).

Figure 2.

Figure 2

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θ–2θ XRD pattern and surface morphologies measured via AFM of epitaxial β-Ga2O3 thin films on (a, c) YSZ and (b, d) YSZ-buffered Si substrates.

A high-resolution X-ray diffractometer (Bruker Discovery D8) equipped with a two-channel cut Ge(220) and four crystal monochromators (λ = 1.5406 Å, 30 kV) was used for the structural analysis of the epitaxial β-Ga2O3 thin films. Figure 2c,d show the XRD θ–2θ scan spectra of the β-Ga2O3 thin films deposited on YSZ and YSZ-buffered Si substrates, respectively. In both cases, the XRD θ–2θ patterns show only {2̅01} diffraction peaks. These results clearly indicate that both β-Ga2O3 thin films grown on the YSZ and YSZ-buffered Si substrates are epitaxially grown with a (2̅01) orientation along the out-of-plane direction.

To study the effect of thermal treatment on the crystalline quality and strain states of β-Ga2O3 thin films, we performed post-annealing processes. The samples were annealed in a tube furnace at 1100 °C under O2 flow (20 sccm) for 5 h at a heating and cooling rate of ∼1 °C/min. This annealing condition did not cause cracks in β-Ga2O3 films. To evaluate the crystalline quality of the β-Ga2O3 films, we measured the full width at half maximum (FWHM) of the XRD rocking curve of the (2̅01) peak, which is the most intense peak. The strain states of β-Ga2O3 thin films were characterized by a shift in the (6̅03) diffraction peak. Figure 3a,b show the XRD θ–2θ scan (57–61°) of the β-Ga2O3 thin films deposited on the YSZ and YSZ-buffered Si substrates, respectively, before and after annealing. For both cases, the (6̅03) diffraction peak intensity increased by approximately one order of magnitude owing to thermal annealing, which indicates an improvement in the crystalline quality. The FWHMs of the (2̅01) rocking curves of β-Ga2O3 thin films deposited on the YSZ and YSZ-buffered Si substrates also increased from 1.190 to 0.257° (Figure 3c) and from 1.300 to 0.543° (Figure 3d), respectively, owing to thermal annealing. This indicates that the crystalline quality of β-Ga2O3 thin films on YSZ substrates is significantly improved through thermal annealing, as shown in Figure 3e.

Figure 3.

Figure 3

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(a) β-Ga2O3 (2̅01) θ–2θ XRD pattern of the as-grown and annealed β-Ga2O3 on the YSZ substrate. (b) β-Ga2O3 (2̅01) θ-2θ XRD pattern of the as-grown and annealed β-Ga2O3 on the YSZ-buffered Si substrate. (c) Rocking curve of Ga2O3(2̅01) peaks for as-grown and annealed β-Ga2O3 on the YSZ substrate. (d) Rocking curve of β-Ga2O3(2̅01) peaks for as-grown and annealed β-Ga2O3 on the YSZ-buffered Si substrate. (e) FWHM values of the (2̅01) plane of the as-grown and annealed β-Ga2O3 thin films. (f) Out-of-plane lattice parameter of the (2̅01) plane of the as-grown and annealed β-Ga2O3 thin films.

Notably, the (6̅03) peak position in the θ–2θ scan of β-Ga2O3 thin films on YSZ-buffered Si substrates is higher than that of β-Ga2O3 thin films on YSZ substrates. Moreover, after thermal annealing, the peak of β-Ga2O3 thin films on the YSZ-buffered Si substrate shifted toward a higher angle, which indicates that the out-of-plane lattice parameter became smaller owing to thermal annealing. These results, summarized in Figure 3f, originate from the thermal stress that evolves from the difference in thermal expansion coefficients of β-Ga2O3 and Si.

To investigate the in-plane epitaxial relationship between a β-Ga2O3 thin film, YSZ buffer layer, and Si substrate, we performed XRD azimuthal φ scans of β-Ga2O3(4̅01), YSZ (202), and Si(202) peaks, respectively, as shown in Figure 4a. For YSZ and Si, four peaks appear at the same φ angles at 90° intervals, indicating in-plane epitaxy with a cube-on-cube epitaxial relationship between the YSZ buffer layer and Si substrate. In contrast, eight β-Ga2O3(4̅01) diffraction peaks appeared at two distinct intervals of 31 and 28°. Note that the (4̅01) plane is unique without family planes in the monoclinic crystal structure. Therefore, one (4̅01) peak in the φ scan represents a particular domain of β-Ga2O3. Therefore, epitaxial β-Ga2O3 thin films on YSZ-buffered Si substrates had eight domain variants (Figure 4b). With respect to each of the four {100} directions of the YSZ-buffered Si unitcell, two domains exist, with the [102] direction of β-Ga2O3 rotated by ±31°. Based on the XRD results, all eight domains are summarized in Figure 4b. This complex domain structure is attributed to the large mismatch between crystal structures of β-Ga2O3 and YSZ. As a potential application of the β-Ga2O3 films, we carried out photo current test. As shown in Figure S3, the β-Ga2O3 films on interdigitated electrode clearly showed photo-resistive characteristics which can be potentially applied to photodetector.

Figure 4.

Figure 4

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(a) φ-scan of β-Ga2O3(4̅01) plane (red), YSZ (202) plane (blue), and Si(202) plane (black) of β-Ga2O3 on the YSZ-buffered Si substrate. (b) Schematic of epitaxial relationship between YSZ (001) plane and β-Ga2O3(2̅01) plane.

In summary, we successfully grew (2̅01)-oriented epitaxial β-Ga2O3 thin films on Si substrates by employing epitaxial YSZ buffer layers. Biaxial compressive strain evolved in the β-Ga2O3 thin films on the YSZ substrate, whereas biaxial tensile strain evolved in the β-Ga2O3 thin films on the YSZ-buffered Si substrate. To further improve the crystalline quality, post-annealing was performed. Finally, we reveal that epitaxial β-Ga2O3 thin films have a complex domain structure with eight domain variants. These results will provide a pathway to integrate epitaxial β-Ga2O3 thin films on Si, which can broaden the scope of β-Ga2O3 applications beyond high-power electronics and toward UV photodetectors and gas sensors.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (NRF 2020M3F3A2A01081572 and NRF-2020M3D1A2101933). This work was supported by the Technology Innovation Program (00144157, Development of Heterogeneous Multi-Sensor Micro-System Platform) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c04387.

  • Additional experimental results, including SEM, FWHM, and photo-detector data (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c04387_si_001.pdf (374.3KB, pdf)

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Supplementary Materials

ao2c04387_si_001.pdf (374.3KB, pdf)

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