Exploring heteroepitaxial growth and electrical properties of α-Ga2O3 films on differently oriented sapphire substrates

Wei Wang; Shudong Hu; Zilong Wang; Kaisen Liu; Jinfu Zhang; Simiao Wu; Yuxia Yang; Ning Xia; Wenrui Zhang; Jichun Ye

doi:10.1088/1674-4926/44/6/062802

1.　Introduction

As an emerging third-generation semiconductor material, gallium oxide (Ga₂O₃) has superior properties such as wide band gap, high breakdown electric field, and strong radiation resistance, which can more effectively serve the emerging needs of power electronic device development, such as Schottky barrier diodes (SBDs)^[1,2], heterojunction diodes^[3,4], metal-oxide semiconductor field-effect transistors (MOSFETs)^[5,6], etc. compared with Si, SiC and GaN^[5,7-9]. It has a wide range of application prospects in the field of advanced information technology and new energy.

Ga₂O₃ has five crystalline phases:α,β,δ,γ andε. Among them, the monoclinic structureβ-Ga₂O₃ is a thermally stable phase at high temperature, and the other four phases are all metastable crystalline phases^[10-16]. But, due to the lack of symmetry of monoclinicβ-Ga₂O₃ compared with the hexagonal system, there is a problem of "twisting" in the heteroepitaxial, which is a major difficulty in the crystal quality of heteroepitaxial thin films^[17]. Compared withβ-Ga₂O₃, the corundom structureα-Ga₂O₃ is one of the metastable phases and has a wider band gap and higher breakdown field strength, making it more suitable for the fabrication of power electronic devices^[18]. Bothα-Ga₂O₃ (a = 4.98 Å,c = 13.43 Å)^[19] and sapphire (a = 4.76 Å,c = 12.99 Å) (PDF: 46-1212) belong to a corundum structure, and the lattice mismatch between them is small. Therefore, sapphire is well suited as a substrate for epitaxialα-Ga₂O₃. We have mainly selected sapphire substrates with three different crystal orientations: a-plane ( $11 \bar{2} 0$ ), m-plane ( $10 \bar{1} 0$ ) and r-plane ( $01 \bar{1} 2$ ). The study of the electrical properties ofα-Ga₂O₃ thin films is an important basis for the fabrication of high-performance devices. However, the effect of substrate crystal orientation on the thin film properties remains contentious, including the crystalline quality and the electrical properties of the thin films^[7,20-23]. The anisotropy of the electrical properties of Ga₂O₃ may be follows from the anisotropy of the long-range electron–phonon interaction, as well as from the conduction band anisotropy^[24]. On the other hand, the defects also play a crucial role in the electrical properties^[25]. Therefore, there is a significant need to investigate the effect of substrate crystal orientation on the heteroepitaxial and electrical properties of the films.

The main methods currently used to grow Ga₂O₃ films are radio frequency magnetron sputtering (RFMS)^[25,26], metal organic chemical vapor deposition (MOCVD)^[14,27-29], molecular beam epitaxy (MBE)^[7,8]. Compared to the above methods, pulsed laser deposition (PLD) has the following outstanding advantages: fast film growth, easy doping, easy preparation of films with the same stoichiometric ratio as the target, easy adjustment of the growth process^{[10,11,13,30]}.

In this work, we have systematically investigated the epitaxial relationships and the differences in electrical properties of epitaxialα-Ga₂O₃ films on a-plane, m-plane, and r-plane sapphire substrates using the PLD technique. We found that the epitaxial films have the same corundum structure and the growth orientation as the substrate, respectively. The conductivity shows an increasing trend from a-plane to m-plane to r-plane, which may be related to oxygen vacancy.

2.　Experimental

The 1% Sn-dopedα-Ga₂O₃ thin films were deposited with the same growth condition on a-plane, m-plane, and r-plane sapphire substrates by PLD (SKY Technology Development Co., Ltd. Chinese Academy of Sciences), at about 800 °C, the oxygen pressure was fixed at 0.1 mTorr. The epitaxial growth relationship and crystalline structure ofα-Ga₂O₃ film were analyzed by X-ray diffraction (XRD, D8 Discover). The structure of theα-Ga₂O₃ thin films was also confirmed by Raman spectroscopy (Renishaw inVia Reflex) at room temperature using a laser beam of 532 nm. Film transmission in the range of 200–800 nm was measured by a UV-visible spectrophotometer (UV-vis, Lambda 1050). The elemental composition and chemical state were characterized by X-ray photoelectron spectroscopy (XPS, Kratos AXIS SUPRA) using Mg Kα (hν = 1253.6 eV) as the excitation source. The electrode spacing and effective electrode area were controlled using a direct-write optical lithography machine (MicroWriter ML3) and a lift-off process. The electrical properties at room temperature and varies temperatures were measured in the Hall measurement system (HL-5500PC) with a 0.5 T magnet, and in a semiconductor parameter analyzer (Keithley 4200-SCS), respectively.

3.　Results and discussion

The schematic diagram of different crystal planes in the corundom structure are shown inFig. 1(a). The lattice mismatch (δ) is calculated according to the following equation:δ = ( $a$ _e – $a$ _s)/ $a$ _s. Here, $a$ _e and $a$ _s indicate the lattice constants of epitaxial thin films and substrates, respectively. The lattice mismatches between different crystal planesα-Ga₂O₃ films and corresponding sapphire substrates are shown inFig. 1(b). It is easy to find that the lattice mismatch between r-plane sapphire and r-planeα-Ga₂O₃ of the long side is larger than that of the a-plane and m-plane, which may be the reason why it is more difficult to epitaxy Ga₂O₃ on r-plane sapphire.

Figure 1.(Color online) (a) Schematic diagram of various crystal planes in the corundom structure. (b) The lattice mismatch between different crystal planes α-Ga₂O₃ films and corresponding sapphire substrates.

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We determine the crystalline phase of Ga₂O₃ using XRD and Raman scattering. As shown inFig. 2(a), the ( $11 \bar{2} 0$ ) and ( $22 \bar{4} 0$ ) diffraction peaks ofα-Ga₂O₃ are accompanied by ( $11 \bar{2} 0$ ) and ( $22 \bar{4} 0$ ) diffraction peaks of a-plane sapphire, the ( $30 \bar{3} 0$ ) diffraction peak ofα-Ga₂O₃ are accompanied by (30 $\bar{3}$ 0) diffraction peak of m-plane sapphire, and the ( $01 \bar{1} 2$ ), ( $02 \bar{2} 4$ ) and ( $03 \bar{3} 6$ ) diffraction peaks ofα-Ga₂O₃ are accompanied by ( $01 \bar{1} 2$ ), ( $02 \bar{2} 4$ ) and ( $03 \bar{3} 6$ ) diffraction peaks of r-plane sapphire. The structure ofα-Ga₂O₃ thin films is also confirmed by Raman scattering. The XRDθ–2θ scan patterns of the above three samples all show that the crystal orientation of theα-Ga₂O₃ films is almost consistent with the sapphire substrate.

Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga₂O₃ thin films grown on a-, c- and r-plane sapphire substrates.

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Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga₂O₃ thin films grown on a-, c- and r-plane sapphire substrates.

Substrate Thickness (nm) σ (S/cm) μ (cm²/(V·s)) n (10¹⁷ cm⁻³)
a-sapphire 302 Exceed test limit (<0.03) – –
m-sapphire 280 0.29 12.2 1.45
r-sapphire 273 2.71 1.35 125

Figure 2.(Color online) (a) XRDθ–2θ full scans ofα-Ga₂O₃ thin films grown on a-plane, m-plane, and r-plane sapphire substrates. (b) XRD $ϕ$ -scans of ( $30 \bar{3} 0$ ) plane, ( $02 \bar{2} 4$ ) plane, and ( $11 \bar{2} 0$ ) plane of correspondingα-Ga₂O₃ films and the underlying sapphire, respectively. (c) Raman scattering spectra ofα-Ga₂O₃ thin films grown on a-plane, m-plane, and r-plane sapphire substrates. (d) XRD rocking curves around theα-Ga₂O₃ ( $11 \bar{2} 0$ ) peak grown on a-plane sapphire,α-Ga₂O₃ ( $30 \bar{3} 0$ ) peak grown on m-plane sapphire, andα-Ga₂O₃ ( $02 \bar{2} 4$ ) peak grown on r-plane sapphire.

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XRD $ϕ$ -scan mode measurements are used to further determine the in-plane epitaxial growth relationship ofα-Ga₂O₃ film relative to sapphire substrates. As shown inFig. 2(b), the characteristic ( $30 \bar{3} 0$ ) of a-planeα-Ga₂O₃ thin film and ( $30 \bar{3} 0$ ) of a-plane sapphire substrate appear at the same rotational angle of $ϕ$ , suggesting that theα-Ga₂O₃ thin film has the same corundom structure as the sapphire substrate. The ( $02 \bar{2} 4$ ) of the m-planeα-Ga₂O₃ thin film and sapphire substrate, and the ( $01 \bar{1} 2$ ) of r-planeα-Ga₂O₃ thin film and sapphire substrate show similar results to those above.

Fig. 2(c) shows the Raman shift ofα-Ga₂O₃ thin films grown on a-, m-, r-plane sapphire substrates. The Raman peaks belonging toα-Ga₂O₃ thin film are mainly located at 430.4, 575.8 cm⁻¹, which is consistent with reported values^[31,32]. The Raman results are consistent with the XRD results. The XRD rocking curves of a-planeα-Ga₂O₃ ( $11 \bar{2} 0$ ) peak, m-planeα-Ga₂O₃ (30 $\bar{3}$ 0) peak, and r-planeα-Ga₂O₃ ( $02 \bar{2} 4$ ) peak are shown inFig. 2(d). The fullwidth at half maximum (FWHM) of theω-scan rocking curves of ( $11 \bar{2} 0$ ) peak, (30 $\bar{3}$ 0) peak, and ( $02 \bar{2} 4$ ) peak are 0.34°, 0.57°, and 0.85°, respectively. This indicates that the crystalline quality of a-planeα-Ga₂O₃ is greater at the a-plane than m-plane and r-plane, which is affected by multiple factors, including the different crystalline quality of the substrate with different orientations, the lattice mismatch, and the nucleation kinetics for different orientations^[17,33,34].

Based on the above test results, it is not difficult to deduce the growth behaviors ofα-Ga₂O₃ thin films grown on differently-oriented sapphire substrates. As shown inFig. 3, the unit cell arrangement ofα-Ga₂O₃ thin films is almost the same as that of the corresponding sapphire substrates.

Figure 3.(Color online) Schematic of growth relation of α-Ga₂O₃ grown on (a) a-plane, (b) m-plane, and (c) r-plane sapphire substrates by PLD.

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Then, we probed the defect density and distribution of differently-orientedα-Ga₂O₃ films using XPS.Fig. 4 shows the O 1s core level spectra of various planeα-Ga₂O₃ thin films. We calibrate the data with a standard binding energy of 284.8 eV for the C 1s peak^[35]. The XPS O 1s core level spectra of the films are divided into two individual peaks and well fitted with the sum Gaussian functions. The positions of the two peaks correspond to lattice oxygen of Ga₂O₃ (O_I) and oxygen vacancies (O_II), respectively^[26]. We can use O_II/(O_I+O_II) to reflect the concentration of oxygen vacancies. The oxygen vacancy concentration of the a-plane, m-plane, and r-planeα-Ga₂O₃ are 11.42%, 20.31%, and 19.29%, respectively.

Figure 4.(Color online) XPS spectra of O 1s peak of α-Ga₂O₃ films grown on (a) a-plane, (b) m-plane, and (c) r-plane sapphire substrates.

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Furthermore, the UV-vis transmission spectra for as grownα-Ga₂O₃ epitaxial films on differently-oriented sapphires are illustrated inFig. 5(a). Both films have high transparency (>80%) in the visible and near-UV regions. As shown inFig. 5(b), the optical band gap (E_g) of theα-Ga₂O₃ films can be evaluated using the plot of (αhν)² vshν, whereα is the absorption coefficient andhν is the photon energy. a-plane, m-plane, and r-planeα-Ga₂O₃ films have an estimated bandgap of 5.16, 5.12, and 5.02 eV, respectively.

Figure 5.(Color online) (a) Transmittance spectra and (b) Tauc plot of the a-plane, m-plane, and r-plane α-Ga₂O₃ thin films grown on various plane sapphire substrates.

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In order to explore the electrical transport properties of differently-orientedα-Ga₂O₃ thin films, Hall measurement is performed in a conventional van der Pauw geometry.Table 1 shows the results of Hall measurements. The conductivity (σ) of theα-Ga₂O₃ thin film epitaxial on a-plane sapphire exceeds the measurement capacity of the Hall tester. Forα-Ga₂O₃ thin film epitaxial on m-plane sapphire, Hall measurements indicate that the film is n-type conductive with a high conductivity of 0.29 S/cm, a mobility (μ) of 12.2 cm²/(V·s) and a carrier concentration (n) of 1.45 × 10¹⁷ cm⁻³. While theα-Ga₂O₃ thin film epitaxial on r-plane sapphire has a higher conductivity of 2.71 S/cm, a mobility of 1.35 cm²/(V·s) and a high carrier concentration of 1.25 × 10¹⁹ cm⁻³.

The temperature dependence of the resistance in the range from 295 to 473 K for the Sn-dopedα-Ga₂O₃ thin films are shown inFig. 6. The activation energy (E_a) is derived from the linear fit of the ln(R)–T plot usingR ∝ exp (E_a/k_BT), whereR is the resistance,k_B is the Boltzmann constant, andT is the temperature^[10,36]. The estimated activation energy forα-Ga₂O₃ epitaxial on a-plane, m-plane and r-plane sapphire substrates are 0.266, 0.079, and 0.075 eV, respectively. The value of activation energy is used to quantify the thermal ionization of defect states^[28].

Figure 6.(Color online) Fitting plots of temperature-dependent resistance deriving the activation energy for a-plane, m-plane, and r-plane α-Ga₂O₃ thin films.

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Theα-Ga₂O₃ thin films on m-plane and r-plane sapphire substrates are far more conducting thanα-Ga₂O₃ thin film on a-plane sapphire substrate under similar thin film growth conditions. It is interesting that the oxygen vacancy concentration ofα-Ga₂O₃ thin films on m-plane and r-plane sapphire substrates are also higher thanα-Ga₂O₃ thin film on a-plane sapphire substrates. It is speculated that the conductivity behavior ofα-Ga₂O₃ thin films may be related to the oxygen vacancy concentration, and the formation energy could strongly depend on the lattice orientation, thus leading to drastical differences in the dopant activation and carrier transport^[29,37,38]. Further studies on the mechanism of the effect of anisotropy and crystallinity on electrical conductivity are expected.

4.　Conclusions

We have investigated the Sn-dopedα-Ga₂O₃ thin films deposited on a-plane, m-plane, and r-plane sapphire substrates by PLD. Due to the same corundum structure, the out-of-plane and in-plane alignment ofα-Ga₂O₃ films is almost identical to that of the corresponding oriented sapphire substrate. The FWHMs ofω-scan rocking curves of the a-plane, m-plane, and r-planeα-Ga₂O₃ thin films are 0.34°, 0.57°, and 0.85°, respectively. The r-planeα-Ga₂O₃ thin film has the highest conductivity, the m-planeα-Ga₂O₃ thin film the second, and the a-planeα-Ga₂O₃ thin film dramatically more insulating. A similar trend is also observed in their activation energy. This could be understood by that the orientation difference leads to the concentration variation of oxygen defects in differently orientedα-Ga₂O₃ thin films, further causing differences in the electrical properties of the thin films.

Category: Articles

Received: Dec. 28, 2022

Accepted: --

Published Online: Jul. 6, 2023

The Author Email: Zhang Wenrui (zhangwenrui@nimte.ac.cn), Ye Jichun (jichun.ye@nimte.ac.cn)

DOI:10.1088/1674-4926/44/6/062802

Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga2O3 thin films grown on a-, c- and r-plane sapphire substrates.

Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga2O3 thin films grown on a-, c- and r-plane sapphire substrates.

Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga₂O₃ thin films grown on a-, c- and r-plane sapphire substrates.

Table 1. Film thicknesses, conductivities, mobilities, and carrier concentrations for α-Ga₂O₃ thin films grown on a-, c- and r-plane sapphire substrates.