As an ultrawide bandgap semiconductor material for next-generation high power electronics,β-Ga₂O₃ has attracted enormous attention due to its high breakdown field strength of 8 MV/cm, a large Baliga’s figure of merit of 3444, and the availability of high-quality melt-grown native substrates^[1-3]. Manyβ-Ga₂O₃ electronic devices such as Schottky barrier diodes^[4-6], MOSFETs^[7,8], FinFETs^[9-11], and CAVETs^[12,13] have already been successfully demonstrated, which reveal their great potential in high-power electronics. To further improve their performance, it is essential to obtain epitaxialβ-Ga₂O₃ films with high crystal quality. To date, the homoepitaxy ofβ-Ga₂O₃ films is still confronted with various challenges such as twins and stacking faults, despite the perfect crystalline match between the film and the substrate due to immature research status^[14-17]. Various epitaxial growth techniques forβ-Ga₂O₃ films have been demonstrated over the past few years, such as halide vapor phase epitaxy (HVPE)^[18,19], molecular beam epitaxy (MBE)^[20,21], low-pressure chemical vapor deposition (LPCVD)^[22,23], metalorganic chemical vapor deposition (MOCVD)^[24-26], and carbothermal reduction rapid growth method^[27]. With the available (100), (010), (001), and ( 2¯01 ) orientated substrates^[28], the homoepitaxy ofβ-Ga₂O₃ films exhibits a great development prospect. Owing to the anisotropic characteristics, the crystal orientation ofβ-Ga₂O₃ substrates has an enormous impact on the quality of the epilayers. Among them, (100) oriented substrates can be easily prepared, but its epitaxy is still challenging due to the preferred cleavage plane with the lowest surface energy^[29]. 2D island growth mode usually plays a dominant role due to the limited diffusion length of the (100) surface adatoms^[30], which causes deterioration of the crystal quality. Many efforts have been made on the homoepitaxial growth of (100)β-Ga₂O₃ thin films^[20,31,32]. In addition, the high concentration n-type doping inβ-Ga₂O₃ still remains challenging, and Si-ion implantation is usually used in the Ohmic contact region nowadays^[33-35]. Therefore, the homoepitaxial growth of (100) Si-dopedβ-Ga₂O₃ thin films with high concentration is important forβ-Ga₂O₃ devices.

In this work, (100)β-Ga₂O₃ films with Si-doping were grown on semi-insulating (100) substrates by MOCVD, which is a preferred growth method for semiconductor device technologies at the industrial level. The suppression of the 2D island growth mode was demonstrated through the optimization of the growth conditions, such as VI/III ratio, temperature and chamber pressure. A promotion of the surface morphology of the epitaxial films from Si doping were confirmed, which proved the facilitation of the growth kinetics. The obtained (100)β-Ga₂O₃ films exhibited a smooth surface with high crystal quality, even for the heavy doped films. Excellent Ohmic contacts and high current density were achieved.

The (100)β-Ga₂O₃ epitaxial films were grown via MOCVD on semi-insulatingβ-Ga₂O₃ substrates with no intentional offcut produced by Hangzhou Fujia Gallium Technology Co., Ltd. Before the epi-growth, the substrates were immersed in piranha solution and then hydrofluoric acid to remove the possible Si contaminants on the substrate surface and followed with in-situ annealing for 15 min at 800 °C under an O₂ atmosphere. Triethylgallium (TEGa), high-purity oxygen gas (O₂), and silane (SiH₄) were used as Ga and O precursors and Si dopants, respectively. High-purity nitrogen (N₂) was used as the carrier gas. The growth temperature was set at 800–900 °C with the pressure kept at 10 kPa. The growth rates of the epitaxial films were in the range of 4–7 nm/min. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and Hall measurement were applied to characterize the material properties of the (100)β-Ga₂O₃ films. A Keysight B1505A Power Device Analyzer was used to measure the electrical characteristics of the devices based on the epi-films.

Compared to (010) and (001)β-Ga₂O₃, the homoepitaxy of (100)β-Ga₂O₃ films is up against more issues owing to the low surface energy and the facile cleavage plane^[36]. Slow growth rate^[28] and 2D island growth mode^[37] are commonly seen in the (100)β-Ga₂O₃ films growth process. As the absorption and diffusion length of Ga adatoms on the (100) surface define low incorporation efficiency, random nucleation and island formation will easily occur. And the lack of energetically favorable lattice sites induces more formation of 3D islands by the encounter of Ga adatoms^[30], resulting in the roughening of the surface. To improve the surface morphology and get a smooth film, suppressing the formation of 3D islands is essential. Usually, an O-rich regime is adopted in the growth environment, and the decrease of the O/Ga ratio can bring an increasing diffusion length of Ga adatoms^[31]. In the experiments, the TEGa molar flow rate was 68.2μmol/min. Theβ-Ga₂O₃ films grown in different conditions with unintentional doping were labeled as sample A₁, A₂, A₃ and A₄, respectively.Fig. 1(a) shows the AFM image of sample A₁ grown at 10 kPa and 850 °C with the O/Ga ratio of 1308, showing a root-mean-square (RMS) roughness of 4.5 nm in a 5 × 5μm² scanning area. Scattered islands on the surface contribute to the large roughness and deteriorate the crystal quality. Similar morphology was reported in Ref. [36]. In addition, the islands, 0.3–1.5μm long and 0.2–0.7μm wide, with a height of no more than 27 nm, show a fixed orientation along the [010] direction, which is consistent with the stripes in the region without islands. The aligned islands also indicate strong anisotropic characteristics in growth. To suppress the formation of islands, a slightly smaller O/Ga ratio of 1250 was implemented. As can be seen fromFig. 1(b), islands get smaller with an increasing diffusion length of Ga adatoms, accompanied by a smaller RMS roughness of 3.2 nm. To further eliminate the islands, the growth temperature was decreased from 850 to 800 °C. It is found that the big islands disappear and small grains appear instead. The grains have the same arrangement oriented along the [010] direction and some of them even stack as hills. As exhibited inFig. 1(c), the film surface was filled with aligned grains and had a smaller RMS roughness of 2.9 nm. As the pressure drops to 8 kPa, the length of the grains extends along the [010] direction with granular features, which implies that lateral growth along the [010] direction gets stronger. The granular surface morphology is shown inFig. 1(d), with a length of ~500 nm, a width of ~120 nm, and a height of ~6 nm. The transition from island morphology to granular morphology attributes to the relatively low RMS roughness of 2.3 nm.Figs. 1(e) and 1(f) are the 3D AFM images corresponding toFigs. 1(a) and 1(d), respectively. It can be intuitively seen that oriented 3D islands disperse on the film surface inFig. 1(e) and oriented granules align on the film surface inFig. 1(f). The feature of the [010] orientation confirms the anisotropic growth ofβ-Ga₂O₃. Similar morphology was also reported in the homoepitaxy of ( 2¯01 )β-Ga₂O₃ films^[38,39].

Subsequently, the homoepitaxial growth of (100)β-Ga₂O₃ films with different Si-doping concentration was investigated. The samples are labeled as B₁, B₂, B₃, B₄, B₅, B₆ and B₇, with the flow rate of SiH₄ in the growth of 0, 0.5, 1.2, 3, 6, 10, and 20 sccm, respectively. The growth rate of the films was almost unchanged with the introduction of SiH₄.Fig. 2 shows the surface morphologies of the films. Sample B₁ is undoped and has a similar surface morphology to sample A₄, therefore its AFM image is not shown here. It can be seen that the introduction of Si atoms brings a decrease of RMS roughness to the epitaxial films. With 0.5 sccm SiH₄ flow rate, the morphology transforms from granules into narrow and continuous stripes with a width of ~80 nm, as shown inFig. 2(a). The RMS roughness of sample B₂ is 2.4 nm and stays almost unchanged. Thinner continuous stripes with a width of ~60 nm were found for sample B₃, the films grown with 1.2 sccm SiH₄, resulting in a decreased RMS roughness of 1.7 nm, shown inFig. 2(b). By gradually further increasing the flow rate of SiH₄, similar morphologies with oriented stripes appeared on the film surface.Figs. 2(c)–2(e) display the surface morphologies of sample B₄, B₅, and B₆, whose RMS roughness is 1.9, 1.3, and 1.4 nm, respectively. The incorporation of Si atoms can suppress random nucleation and island formation during (100)β-Ga₂O₃ growth to a certain extent through the preferred bonding species of Si adatoms to the O atoms, where a similar mechanism was reported in the (Al_xGa_1–x)₂O₃ growth^[30]. As a result, a smooth and flat surface was obtained through light Si doping. The coalescence of the original 2D islands was promoted by the distributed Si nucleation sites on the growth surface, facilitating the step-flow growth mode^[16]. However, when the doping amount exceeds a certain degree, such as 20 sccm SiH₄, the stripes start to lose their orientation and reveal an undulating surface with increasing RMS roughness.Fig. 2(f) depicts the surface morphology of sample B₇, which has curly stripes and an RMS roughness of 4.3 nm. The weak anisotropic growth may contribute to the above morphology change.

The effect of Si incorporation on the crystal quality of the epitaxial films was studied through HRXRD. Rocking curves of the (400) diffraction peak ofβ-Ga₂O₃ were shown inFig. 3. All the films show good crystal quality with the full-width at high maximum (FWHM) of the (400) peak below 60 arcsec, except for the highly doped sample B₇, whose SiH₄ flow rate is 20 sccm. It should be noted that all the prepared films have a thickness of ~400 nm, most of the diffraction signal may come from the substrate as a result. However, if the Si doping leads to a deterioration in crystal quality, a large FWHM should be present, such as for sample B₇, which has a FWHM of 119 arcsec. This suggests new defects are generated in the epitaxial films, and the excess incorporation of Si atoms deteriorates the crystal quality of theβ-Ga₂O₃ films. Therefore, to ensure the high crystal quality of the film, the flow rate of SiH₄ in the growth should not exceed 20 sccm.

ToF-SIMS was applied to identify the concentration of Si atoms in the grownβ-Ga₂O₃ films. As depicted inFig. 4, the concentrations of Si atoms of all the films are in the range of 1.57 × 10¹⁷ – 2.83 × 10²⁰ cm⁻³. Note that the detection limit of Si is 1.57 × 10¹⁷ cm⁻³, and a Si-ion implanted film with a concentration of 5.0 × 10¹⁹ cm⁻³ is used as the standard sample. For sample B₄, B₅, B₆ and B₇, their Si concentrations are 2.63 × 10¹⁹, 3.46 × 10¹⁹, 7.73 × 10¹⁹, and 2.83 × 10²⁰ cm⁻³, respectively. Such high doping levels with high crystal quality could be achieved only by Si-ion implantation previously^{[8,13,35,40-42]}. While ion implantation usually results in large lattice damage and requires high-temperature activation^[40], which may cause adverse effects on the subsequent process^[43,44]. As a result, the technical route of Si doping in the process of epitaxy can bring more convenience and solutions to semiconductor processing technology.

A Ti (50 nm)/Au (100 nm) contact was deposited on the film surface as the Ohmic electrode, followed by 475 °C annealing in N₂ atmosphere for 1 min. And a Ni (50 nm)/Au (100 nm) contact was deposited as the Schottky electrode subsequently. Capacitance–voltage (C–V) characteristics of the epitaxialβ-Ga₂O₃ films were measured, and the effective net doping concentration (N_D–N_A) can be extracted from the slope of the1/C²–V plots inFig. 5. The equation is given as

1(1C)2=2(Vbi+VR)eεs(ND−NA),

whereC is the capacitance per unit area, e is the electron charge, Vbi is the built-in potential difference, VR is the applied voltage, and εs is the dielectric constant ofβ-Ga₂O₃. The values ofN_D–N_A of the prepared films are 5.41 × 10¹⁵, 2.30 × 10¹⁶, 2.01 × 10¹⁷, 6.65 × 10¹⁸, 1.17 × 10¹⁹, 1.64 × 10²⁰, and 1.74 × 10²⁰ cm⁻³, corresponding to the samples B₁, B₂, B₃, B₄, B₅, B₆ and B₇, respectively. The1/C²–V plots of the lightly doped films are not shown here. The small changes of theN_D–N_A between the films grown with 10 and 20 sccm SiH₄ further confirm the excessive Si-doping in B₇. Hall measurement was also used to acquire the effective carrier concentration of the epitaxial films. A high electron mobility value of 51 cm²/(V·s) at room temperature was achieved for sample B₅ with a carrier concentration of 7.19 × 10¹⁸ cm⁻³. And a high activation efficiency of about 61.5% is obtained for sample B₅ meanwhile.Fig. 6 compares the concentrations extracted fromC–V, SIMS, and Hall measurements. The extracted curves have a similar trend, which means the efficiency of Si incorporation gets smaller when the SiH₄ flow rate increases.

Current density–voltage (J–V) characteristics of the epitaxialβ-Ga₂O₃ films were measured.Fig. 7(a) shows theJ–V characteristics acquired on transmission line model (TLM)^[45] patterns of the epitaxialβ-Ga₂O₃ films with different Si-doping concentrations in a linear plot. It is clear that the grown films with high doping exhibit good Ohmic contacts. With the proper increase of the flow rate, the resistivity of the film decreases, and B₆ gets the minimum. The significant increase in resistivity of B₇ compared to B₅ is due to the deteriorating film quality caused by excessive doping.Fig. 7(b) shows theJ–V characteristics of the epitaxialβ-Ga₂O₃ films in a semi-log plot. The current density of the high Si-doped film is greater than 10 A/cm². The relationship between the total resistance (R_T) and the pad spacing determined from theJ–V curves of samples with different Si doping amounts is shown inFig. 7(c), and the data were fitted linearly. Based on the TLM measurements, the contact resistivities, the sheet resistances, the transfer lengths, and the specific contact resistances are extracted, as presented inTable 1. From the fits, theR_c = 3.73 Ω,L_T = 3.44µm andR_SH = 1084.55 Ω/□ of sample B₆ were extracted. The specific contact resistanceρ_c = 1.29 × 10⁻⁴ Ω·cm² is obtained by this analysis. These excellent electrical conductivities are comparable to the sample Ref in the table, which is the Si-implanted film with a concentration of 5.0 × 10¹⁹ cm⁻³, confirming the effective Si doping in the MOCVD epitaxy. The low specific contact resistance and high current density indicate that apart from ion implantation, Si-doped epitaxy can also achieve excellent Ohmic contacts.

Homoepitaxial growth of (100)β-Ga₂O₃ films and Si doping with different concentration through MOCVD were studied in this work. By adjusting the Ⅵ/Ⅲ ratio, chamber temperature and pressure, the diffusion length of Ga adsorption atoms was increased, inhibiting the 2D island growth mode and resulting in smooth surface morphology. The gradient Si-doping films indicate that the introduction of Si atoms facilitates the growth kinetics and promotes the coalescence of original 2D islands, which is beneficial to the formation of a smooth surface. Apart from the over-doped sample, most of the epitaxialβ-Ga₂O₃ films with Si-doping exhibit good crystal quality, with (400) FWHM less than 60 arc sec. Based on theC–V characteristics, an effective net doping concentration from 5.41 × 10¹⁵ to 1.74 × 10²⁰ cm⁻³ was realized. And a high Hall mobility of 51 cm²/(V·s) was also achieved for the sample with a carrier concentration of 7.19 × 10¹⁸ cm⁻³ and a high activation efficiency of about 61.5%. The epitaxial film demonstrates a low specific contact resistance of 1.29 × 10^–4 Ω·cm², which is comparable to the Si-implanted film, indicating the effective Si doping in the MOCVD epitaxy. It proves that Si doping in the MOCVD epitaxy ofβ-Ga₂O₃ films can meet the demand of both a high concentration doping and high crystal quality, which provides a solution for the future development ofβ-Ga₂O₃ power devices apart from Si-ion implantation.