Study of the Bonding Characteristics at β-Ga₂O₃(201)/4H-SiC(0001) Interfaces from First Principles and Experiment

Abstract

For the first time, β-Ga₂O₃ were prepared on 4H-SiC (0001) substrates using a low-pressure chemical vapor deposition (LPCVD) technique. The obtained β-Ga₂O₃/4H-SiC heterostructures display strongly preferential growth orientation along the <201> of β-Ga₂O₃. Combining the experimental results, interfacial properties, such as the work of adhesion (Wad), electronic properties and bonding characteristics of β-Ga₂O₃(201)/4H-SiC(0001) heterointerface were systemically studied using first principles. Four different β-Ga₂O₃(201)/4H-SiC(0001) interface models composed of different atom stacking sequences were established. It was found that the interface consisting of silicon terminated of 4H-SiC (0001), and oxygen terminated of β-Ga₂O₃(201) (Si-O) has the lowest relaxation energy and the highest stability. Results indicated that the binding of interface Si and C to the O atoms is stronger than that to the Ga atoms. The results of the difference charge density and electron localization function reveals that the Si and C atoms at interface are bonded with O atoms of β-Ga₂O₃ by covalent bonds, in which Si-O and C-O covalent bonds play a favorable role in the final stable configurations formation. This work will provide a further understanding of the various electronic behaviors of the β-Ga₂O₃(201)/4H-SiC(0001) heterointerface.

1. Introduction

The monoclinic gallium oxide (β-Ga₂O₃) has attracted widespread attention in automobile, high-speed rail power system, new energy, semiconductor power devices, communication electronics and other fields because of its band gap of 4.9 eV [1,2,3]. Unfortunately, its interface thermal management is a downside; this can cause significant damage to electronic devices [4,5]. Thus, there have been intense efforts to study the β-Ga₂O₃/4H-SiC heterointerface, in which the 4H-SiC is used as a high thermal conductivity semiconductor material (4.9 W/cm K) as a thermal conductivity substrate. Worldwide, an enormous amount of research has shown that the interface has a notable influence on the properties of heterogeneous interfaces [6,7]. Thus, the interface formation and the bonding characteristics of the β-Ga₂O₃(201)/4H-SiC(0001) heterointerface are worth investigating.

To date, methods such as laser molecular beam epitaxy (L-MBE) technique [8], radio frequency sputtering [9], ion-cutting technique [10], gallium evaporation in oxygen plasma [11] and pulsed laser deposition (PLD) [12] are used to prepare β-Ga₂O₃/SiC heterostructures. The experiment by Neeraj Nepal et al. [13] found that Ga₂O₃(201) and SiC(0001) formed a clear interface between the two components, and it was believed that this orientation contributed to a better combination between Ga₂O₃ and SiC. Moreover, many researchers have applied the first principle to study the interface between other semiconductor substrates and Ga₂O₃. The first-principle calculation method was used by Yu et al. [14] to study the electronic structure and properties of Ga₂O_3/CuAlO₂ heterojunctions. They found that interface with oxygen–copper terminals is more stable than that oxygen–aluminum terminals. He et al. [15] studied six staking sequences at AlN(0001)/Ga₂O₃(100) interfaces using the first-principCle calculation method which revealed high thermodynamic stability and relatively strong interface bonding for the B-Al, B-N, and C-N models. Unfortunately, it is difficult to use effective experiment characterization to study the interfacial electronic interaction between β-Ga₂O₃ and 4H-SiC. Hence, there is still an urgent need to investigate the interfacial bonding characteristics of β-Ga₂O₃(201)/4H-SiC(0001) heterostructure based on first-principles calculation.

In this paper, the atomic structure and bonding characteristics of β-Ga₂O₃/4H-SiC(0001) have been investigated using first principles and experimental methods. The geometry structures, work of adhesions, stability, bonding characteristics and electronic properties of β-Ga₂O₃(201)/4H-SiC(0001) were deeply explored. It is important that interfacial adhesion and bonding characteristics are taken into account when studying nucleation on substrates. The results show that the work of adhesion of β-Ga₂O₃/4H-SiC(0001) at the oxygen–silicon terminal is stronger than that at the oxygen–carbon terminal. Based on the difference charge density and electron localization function, Si-O polar covalent bonds were formed near the Si-O interface, which made the interface more stable. According to the results, LPCVD can synthesize β-Ga₂O₃ films on 4H-SiC substrates for using in high power electronic devices.

2. Experimental and Calculation Methodology

2.1. Calculation Methodology

Based on DFT [16,17], the β-Ga₂O₃/4H-SiC with interfaces connecting β-Ga₂O₃($\overline{2}$01) to 4H-SiC(0001) has been investigated. The exchange-correlation function is selected GGA-PBE [18]. In order to gain a stable configuration and ensure calculation accuracy, the total energy and k-points grid are tested. The optimized plane-wave cutoff energy and k-point grid are set to 500 eV and 2 × 11 × 1, respectively. The β-Ga₂O₃/4H-SiC interface model includes six β-Ga₂O₃ layers and five 4H-SiC carbide diatomic layers, in which the suspended bond is passivated by hydrogen atoms. A vacuum of 15 Å was employed on the two side materials to separate the interaction between periodic slabs. The valence electron is configured: 3d¹⁰4s²4p¹ for Ga, 2s²2p⁴ for O, 2s²2p² for C, 3s²3p² for Si and 1s for H. The maximum total energy is set to 10⁻⁵ eV and forces on atoms in the range of −0.03 eV/Å.

2.2. Experimental

β-Ga₂O₃ films were grown on commercially available 4° ± 0.5° off-axis n-type 4H-SiC substrates (10 mm × 10 mm × 0.35 mm) via a double temperature zone LPCVD system (see Figure 1a). High purity gallium metal (≥99.9999%), oxygen (O₂, 5N) and Argon (Ar, 5N) were used as the gallium source, oxygen precursors and carrier gas, respectively. Growth pressure was always kept at 100 Pa. Figure 1b shows the diagram of the growth process with time dependent on temperature The evaporation temperature of gallium metal and deposition temperature have been maintained at 950°C and 650°C, respectively. The deposition process of β-Ga₂O₃ films were shown in Figure 1c. In Figure 1c, high purity oxygen (5N) reacts with gallium vapor to form gallium oxide, which is transported and deposited onto the surface of a 4H-SiC substrate by high purity Ar (5N).

3. Results and Discussion

3.1. Computational Medels

The optimized bulk material parameters of monoclinic gallium oxide are a = 12.49 Å, b = 3.10 Å, c = 5.91 Å and 4H-SiC are a = b = 3.09 Å, c/a = 3.28. Our optimized lattice constants show a good agreement with experimental and theoretical results [14,19,20,21,22]. According to the XRD θ~2θ and off-specular phi-scan results, the (1 × 1) β-Ga₂O₃(201)–(3×√3) 4H-SiC(0001) interface models were established to simulate the interface between β-Ga₂O₃ and 4H-SiC. The lattice misfit along the b direction is 0.3% with 1b_{β-Ga2O3(010)} matching to 1b_4H-SiC(1120), while along a direction, the mismatch is about 6.9%, with 3a_4H-SiC(1100) matching to 1a_{β-Ga2O3(102)}. The different terminated atoms of (0001)4H-SiC and (201)β-Ga₂O₃ means that there are four kinds of interface models: Si-O terminated, Si-Ga terminated, C-O terminated and C-Ga terminated. The side view and top view of β-Ga₂O₃(201)/4H-SiC(0001) models are illustrated in Figure 2a–h. The lattice constant of β-Ga₂O₃(201)/4H-SiC(0001) interface are: a = 15.56 Å, b = 3.09 Å, c = 36.55 Å, α = β = γ = 90°. The interface models adopted a 5-layer slab of 4H-SiC and 6-layer slab of β-Ga₂O₃(201).

3.2. Work of Adhesion

Interface bonding can be evaluated by work of adhesion (Wad) and be expressed as the work to hive off a complete interface into two free slabs. The Wad [23,24,25] is determined by Equation (1):

$$W_{ad}=(E_{\beta -G{a}_2{O}_3}+E_{4H-SiC }-E_{\beta -G{a}_2{O}_3/ 4H-SiC })/A_{interface}$$

(1)

where $E_{\beta -G{a}_2{O}_3}$ and $E_{4H-SiC }$ are the total energy of the isolated β-Ga₂O₃ layer and 4H-SiC layer, respectively. $E_{\beta -G{a}_2{O}_3/ 4H-SiC}$ devotes total energy of the heterointerface. A_interface is the interfacial area of β-Ga₂O₃($\overline{2}$01)/4H-SiC(0001). A positive value of W_ad represents that the reversible work necessary to separate theβ-Ga₂O₃(201)/4H-SiC(0001) interface into β-Ga₂O₃ slab and 4H-SiC slab. According to formula (1), W_ad of four different interface models are calculated. The calculated Wad, total energy, bond length and interface spacing are shown in

Table 1. The total energy (eV), Interface spacing (Å), Bond length (Å) and Wad (J/m²) for interfaces.

Inteface	Energy/eV	Interface Spacing (Å)	Unrelaxed Bond Length (Å)	Relaxed Bond Length (Å)	Wad/(J/m2)
Si-O	−707.72901323	1.7	1.80	1.66	8.5
Si-Ga	−686.20897765	2.3	2.31	2.44	3.1
C-O	−691.84100188	1.5	1.60	1.47	4.3
C-Ga	−684.10880119	1.9	1.91	1.95	3.4

with different configurations. It shows that the calculated W_ad of Si-O, Si-Ga, C-O and C-Ga interface is 8.5, 3.1, 4.3 and 3.4 J/m², respectively. As shown in Table 1, the optimized interface spacing of Si-O, Si-Ga, C-O and C-Ga interface is 1.7 Å, 2.3 Å, 1.5 Å and 1.9 Å, respectively. Obviously, the Si-O interface presents the lowest relaxation energy and largest Wad among the four different terminals, indicating that the bonding strength at this interface is the strongest. The Si-Ga model and C-Ga model have smaller work of adhesion. Thus, among the four structures, it can be seen that the stability of Si-O interface system is strongest. There may be a difference in the oxidation process between the 4H-SiC surface at the Si-terminated and the 4H-SiC surface at the C-terminated, resulting in a stronger Si-O bond than a C-O bond on the 4H-SiC surface [26]. The larger the Wad, the more difficult the interface is to separate, indicating a more stable interface structure, which means that bonding strength is as follows: Si-O > C-O > C-Ga > Si-Ga.

3.3. Bonding Characteristics

It is well known that high stability and strength can be bound up to the chemical bond type at the interfaces. To understand the electron transfer at the four interfaces, the difference charge density of the β-Ga₂O₃/4H-SiC heterointerfaces were calculated (see Figure 3). The difference charge density along the z direction Δρ (z) [27,28] is as follows:

$$\Delta \rho (\mathrm{z})=\Delta \rho {(\mathrm{z})}_{\beta -G{a}_2{O}_3/ 4H-SiC }-\Delta \rho {(\mathrm{z})}_{\beta -G{a}_2{O}_3 }-\Delta \rho {(\mathrm{z})}_{4H-SiC }$$

(2)

where $\Delta \rho {(\mathrm{z})}_{\beta -G{a}_2{O}_3/ 4H-SiC}$ is the total charge density of the β-Ga₂O₃/4H-SiC interface, $\Delta \rho {(\mathrm{z})}_{\beta -G{a}_2{O}_3}$ and $\Delta \rho {(\mathrm{z})}_{4H-SiC }$ are the charge density of individual β-Ga₂O₃(201) slab and 4H-SiC(0001) slab, respectively. The four figures show that there is a significant charge transfer at the interface atoms. The yellow and blue sections present the aggregation and depletion of charges, respectively. The distinct charge accumulation between Si and O is closer to O, suggesting a polar covalent bond in Figure 3a. There is the charge accumulated between Si and Ga atoms, implying a covalent bond in Figure 3b. Figure 3c shows the electrons of O atoms on β-Ga₂O₃ and the electrons of C atoms on 4H-SiC side are transferred to the middle of C and O atoms at the interface. The electrons are accumulated at C atoms and Ga atoms at interface in Figure 3d. It is shown that Si and C prefer to combine with O atom rather than Ga atom of β-Ga₂O₃(201). Above all charge transfer mainly occurs between several atomic layers at the heterointerface.

To further calculate the net charge transfer of interfacial atoms on β-Ga₂O₃/4H-SiC interfaces, we analyzed the Bader charge. The Bader charge of Si-O interface, Si-Ga interface, C-O interface and C-Ga interface are 4.63 e, 0.157 e, 2.86 e and −1.61 e, respectively. For Si-O interface, 4.63 e is transferred from 4H-SiC to β-Ga₂O₃, each O atom at the interface receive about 0.77 e from Si atoms. While only 0.157 e is also transferred from 4H-SiC to β-Ga₂O₃ at the Si-Ga interface, suggesting gallium atoms and silicon atoms almost share electrons. For the C-O model, there are 2.86 e transferred from 4H-SiC to β-Ga₂O₃. The 3.11 e is transferred from Si atoms to interfacial C atoms and then 2.86 e is further transferred from interfacial C atoms to O atoms of β-Ga₂O₃. For C-Ga interface, the 1.61 e transferred from Ga atoms to interfacial C atoms. In conclusion, the charge tends to transfer to atoms with greater electronegativity. These results explain that, compared with the other three interface models, the Si-O interface models transferred the most electrons, and is the most stable.

Figure 4 shows the electron localization function of β-Ga₂O₃(201)/4H-SiC(0001). High-localized electrons are represented by red, and high-delocalized electrons are represented by blue. It is assumed that charge transfer takes place between only the nearest atomic layers [29]. Obviously, in all heterojunction systems, the bonds between atoms are covalent bonds. For the Si-O interface, O atoms have a strong charge accumulation and there is a low localization degree around Si atoms, resulting in a strong Si-O polar covalent bond with bond length 1.66 Å. In Figure 4b, there is a high localization degree in the middle of Si atoms and Ga atoms, indicating Si-Ga covalent bond and an average bond length of 2.44 Å, which is highly consistent with the result of the Bader charge. From Figure 4c, the high local state between C and O indicates that there is a C-O polar covalent bond at β-Ga₂O₃(201)/4H-SiC(0001) interface, with the bond length 1.47 Å. Obviously, according to the localization degree between the C and Ga atom, the interaction for C-Ga seems relatively weak in Figure 4d, which is weak a C-Ga covalent bond with an average bond length of 1.95 Å. In Table 1, after optimizing the interfacial models, it is clearly observed that the bond length of Si-O terminated and C-O terminated is reduced, resulting in a more stable structure. The results show that the bonding mechanism of β-Ga₂O₃(201)/4H-SiC(0001) is a covalent bond, and the bonding strength between silicon atoms and oxygen atoms is the stronger than other interfacial models. Si-O and C-O polar covalent bonds play a favorable role in the formation of stable configurations.

Based on the above growth conditions, we prepared single crystal β-Ga₂O₃ thin films on (0004) SiC substrate using the LPCVD technique. Figure 5a shows the XRD pattern of β-Ga₂O₃/4H-SiC. The diffraction peak near 35.58° corresponded to (0004) SiC substrate. The other three different peaks observed at 18.94°, 38.40° and 59.10° are from the (201), ($\overline{4}$02) and ($\overline{6}$03), respectively. The sample was indexed according to JCPDS No. 43-1012 [30]. The XRD result indicates that the as-prepared β-Ga₂O₃ films via LPCVD on 4H-SiC substrate exhibited preferentially selective orientation along <201>. The FWHM is the full width at half of the peak maximum. The full width at half maximum (FWHM) was calculated to be 0.155° by fitting with Gaussian functions of the lower angle diffraction peak of (201) in Figure 5b. The average grain size (D) of β-Ga₂O₃ films is calculated by the following expressions [31]:

$$D=\frac{k\lambda }{FWHM\cos \theta }$$

(3)

where θ is the Bragg’s angle, λ corresponds to the X-ray wavelength (λ = 0.15406 nm) and k = 0.89. The calculated average grain size D is 51.4 nm. Crystallinity is increased and crystal quality is improved with a smaller FWHM. Figure 5c shows the off-specular phi-scan of β-Ga₂O₃(111) and 4H-SiC(101) planes. For β-Ga₂O₃(111) plane, the Bragg angle θ and rotation angle χ are 17.59° and 80.53°, respectively. It is well known that the (111) plane of β-Ga₂O₃ single crystal has twofold symmetry along the normal direction of <010>. As shown in Figure 5c, there are six diffraction peaks separated by 60° of β-Ga₂O_3, indicating that a tripartite domain structure exists inside the β-Ga₂O₃ film [32,33]. The Bragg angle θ and rotation angle χ of 4H-SiC are17.39° and 75.14°, respectively. Due to the 4H-SiC is six-fold symmetry along [0001] direction, there is a diffraction peak every 60° in the diffraction pattern. In comparing the off-specular phi-scan of β-Ga₂O₃(111) to 4H-SiC (101) planes, it is found that the six φ angles are identical in Figure 5c. Therefore, the 4H-SiC (0001) plane is parallel to (201) plane of the monoclinic gallium oxide (β-Ga₂O₃(201)||4H-SiC(0001)). Thus, high-quality (201) β-Ga₂O ₃ film was successfully deposited on the (0001) 4H-SiC substrate. The chemical composition states of β-Ga₂O₃ thin films were studied by XPS measurement. Figure 5d displays a survey scan of the as-deposited β-Ga₂O₃ thin films, including photoelectron lines Ga 2p, Ga 3s, Ga 3d, Ga 3p, O 1s and C 1s, respectively [34]. No peak of Si belonging to 4H-SiC substrate was observed as the epitaxial Ga₂O₃ was thick enough. It is worth noting that the element energy shifts of the C, O and Ga peaks were calibrated by C1s peak at 284.8 eV, which was considered to have originated from the absorption of hydrocarbons or CO₂ on the surface. Figure 5e shows the optical transmittance spectrum of the β-Ga₂O₃/4H-SiC heterojunction in the test coverage of 200 to 800 nm. It is possible to calculate the optical band gap of materials by measuring their transmittance. The smaller band gap of 4H-SiC than β-Ga₂O₃ means that light sources less than 300 nm cannot pass through the substrate and are all absorbed. Thus, we grow gallium oxide thin films on quartz to measure their optical band gap. Transmittance of β-Ga₂O₃ film is more than 90% and has weak absorption for wavelengths over 300 nm. In addition, the β-Ga₂O₃ thin films have high absorptivity in the deep ultraviolet region (200–300 nm) with an absorption edge located at around 270 nm. As can be seen in the inset of Figure 5e, the optical band gap (4.88 eV) is estimated by using the Tauc plot [35], which almost consistent with the values of 4.86 eV [14].

4. Conclusions

In summary, the electronic properties and bonding strength of four different models of β-Ga₂O₃(201)/4H-SiC(0001) have been deeply studied by calculations and experimental investigations in this work. The crystalline structures, chemical compositions, optical properties and interface bonding characteristics of the as-prepared heterostructures were all comprehensively investigated using experiment and first principles. β-Ga₂O₃ films are pure phase with highly preferential growth orientation along the <201> and show high transmittance at wavelengths over 280 nm. The XRD phi-scan reveals that epitaxial relationship of β-Ga₂O₃ and 4H-SiC is β-Ga₂O₃(201)||4H-SiC(0001). The results show that the W_ad of β-Ga₂O₃(201) with an oxygen–silicon terminal of 4H-SiC(0001) is stronger than that of the other three models, in which it can be seen that the stability of Si-O interface system is strongest. The electron localization function analysis shows that the dominant interfacial chemical bonds are the Si–O polar covalent bond and C–O covalent bond for the Si-termination and C-termination of β-Ga₂O₃(201)/4H-SiC(0001) interface, respectively. The results show that the Si-O and C-O covalent bonds play a dominant role in the formation of stable configurations. The results obtained in this work also illustrate that LPCVD is a suitable method to synthesize β-Ga₂O₃ films on 4H-SiC substrates for high power electronic devices.