High Thermal Stability of κ -Ga 2 O 3 Grown by MOCVD

: We report a high thermal stability of kappa gallium oxide grown on c-plane sapphire substrate by metal organic chemical vapor deposition. Kappa gallium oxide is widely known as a metastable polymorph transitioning its phase when subjected to a high temperature. Here, we show the kappa gallium oxide whose phase is stable in a high temperature annealing process at 1000 ◦ C. These oxide ﬁlms were grown at 690 ◦ C under nitrogen carrier gas. The materials showed high electrical resistivity when doped with silicon, whereas the ﬁlm conductivity was signiﬁcantly improved when doped with both indium and silicon. This work provides a pathway to overcoming limitations for the advance in utilizing kappa gallium oxide possessing superior electrical characteristics.


Introduction
Gallium oxide (Ga 2 O 3 ) has been regarded as a promising next-generation material for realizing future ultra-wide bandgap optoelectronics and/or electronics such as ultraviolet solar blind photodetectors (SBPD) or high-power transistors. Despite of its potential, a complicated nature of Ga 2 O 3 requires further understanding in the structural and electronic properties to advance the application fields to the greater extend. Among five different polymorphic forms (α-, β-, γ-, δ-, and κ-) of Ga 2 O 3 , β-phase Ga 2 O 3 has widely been studied, since it is the most thermally and chemically stable as compared to other structures [1,2]. Moreover, a recent development in fabricating single crystal β-Ga 2 O 3 substrates has been boosting up the expectation for the realization of devices based on such material in near future [3]. Apart from this promising phase of Ga 2 O 3 , growing attention has also been given to the metastable phase of Ga 2 O 3 , which is widely known as εor κ-phase [4,5], since it exhibits similar properties with β-Ga 2 O 3 such as a wide direct bandgap energy of approximately 4.8 eV. Furthermore, various alloys containing other elements such as magnesium, indium, and aluminum can be utilized to tune its bandgap within the ultraviolet C light (UVC) solar blind band (200-280 nm) [6][7][8]. More importantly, the κ-Ga 2 O 3 has been reported to exhibit unique properties, ferroelectric behavior and large polarization, that other polymorphs do not possess [9,10]. For example, the κ-Ga 2 O 3 is predicted to have a large spontaneous polarization value which is about three times larger than the one of aluminum nitride [9]. In spite of numerous advantages of κ-phase, little has been studied making use of κ-Ga 2 O 3 in the application. This is mainly due to the fact that the κ-Ga 2 O 3 transits its phase to the β-Ga 2 O 3 if high temperature energy is applied in order to activate dopants and to decrease the defect density inside layers [2,3]. Thus, further advancement of employing κ-Ga 2 O 3 cannot be made unless the phase transition at high temperature issue is solved. Herein, we report a high thermal stability of κ-Ga 2 O 3 grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The κ-Ga 2 O 3 thin films can withstand a high temperature annealing process at 1000 • C without having a phase transition. Moreover, when the materials are doped with both silicon and indium, the improvement in electrical properties has been observed. This work provides a pathway to overcome limitations for the advance in utilizing kappa gallium oxide possessing superior electrical characteristics.

Materials and Methods
We grow various structures listed in Table 1 under N 2 carrier gas using Trimethylgallium (TMGa), Trimethyl-indium (TMIn), pure H 2 O vapor and Silane (SiH 4 ) as Ga, In, O and Si precursors, respectively. Ga 2 O 3 growth was performed on c-plane sapphire substrates by AIXTRON AIX200/4 horizontal MOCVD reactor at the growth temperature of 690 • C and at the pressure of 50 mbar. The post thermal annealing process was carried out for 1 minute by either ex situ rapid thermal annealing (RTA) or in situ annealing within the MOCVD reactor [11]. The structural property and the corresponding phase of the as-grown and annealed Ga 2 O 3 epilayers were investigated by high-resolution x-ray diffraction (HR-XRD). Field emission scanning electron microscopy (SEM), Hitachi S-4500, was utilized to analyze the surface morphology of samples and also to measure the thickness of respective Ga 2 O 3 epilayers. Electrical characteristics, including resistivity, mobility, and carrier concentration, of the films were obtained by using Van der Pauw Hall technique at room temperature. Optical characterizations were performed by photoluminescence (PL) measurement using an Ar ion laser with excitation wavelength of 244 nm.  Figure 1 shows the XRD spectra of all the as-grown and annealed materials listed in Table 1. The corresponding phase of these as-grown epilayers is identified to be κphase (Figure 1a), aligning with the calculated (002), (004), and (006) planes, similar to the materials grown under H 2 carrier gas as reported in our prior studies [11,12]. However, after the annealing process at 1000 • C, we did not observe any phase transition from κto β-phase. The phase of annealed epilayers remained the same, as shown in Figure 1b, which is opposed to the annealed Ga 2 O 3 films grown under H 2 [11]. This observation indicates a high thermal stability of κ-Ga 2 O 3 epilayers grown under N 2 carrier gas.

Results and Discussion
Crystals 2021, 11, x FOR PEER REVIEW 2 of 5 both silicon and indium, the improvement in electrical properties has been observed. This work provides a pathway to overcome limitations for the advance in utilizing kappa gallium oxide possessing superior electrical characteristics.

Materials and Methods
We grow various structures listed in Table 1 under N2 carrier gas using Trimethylgallium (TMGa), Trimethyl-indium (TMIn), pure H2O vapor and Silane (SiH4) as Ga, In, O and Si precursors, respectively. Ga2O3 growth was performed on c-plane sapphire substrates by AIXTRON AIX200/4 horizontal MOCVD reactor at the growth temperature of 690 °C and at the pressure of 50 mbar. The post thermal annealing process was carried out for 1 minute by either ex situ rapid thermal annealing (RTA) or in situ annealing within the MOCVD reactor [11]. The structural property and the corresponding phase of the asgrown and annealed Ga2O3 epilayers were investigated by high-resolution x-ray diffraction (HR-XRD). Field emission scanning electron microscopy (SEM), Hitachi S-4500, was utilized to analyze the surface morphology of samples and also to measure the thickness of respective Ga2O3 epilayers. Electrical characteristics, including resistivity, mobility, and carrier concentration, of the films were obtained by using Van der Pauw Hall technique at room temperature. Optical characterizations were performed by photoluminescence (PL) measurement using an Ar ion laser with excitation wavelength of 244 nm.  Figure 1 shows the XRD spectra of all the as-grown and annealed materials listed in Table 1. The corresponding phase of these as-grown epilayers is identified to be κ-phase (Figure 1a), aligning with the calculated (002), (004), and (006) planes, similar to the materials grown under H2 carrier gas as reported in our prior studies [11,12]. However, after the annealing process at 1000 °C, we did not observe any phase transition from κ-to βphase. The phase of annealed epilayers remained the same, as shown in Figure 1b, which is opposed to the annealed Ga2O3 films grown under H2 [11]. This observation indicates a high thermal stability of κ-Ga2O3 epilayers grown under N2 carrier gas.   Table 1.

Results and Discussion
Normalized PL spectra were measured from all of the various as-grown and annealed Ga 2 O 3 films in Table 1. As shown in Figure 2a, we could not observe any change in PL peaks as comparing between before and after annealing. However, in our previous papers [11,12], we demonstrated that the difference in PL peaks depending on its phase has been found, exhibiting that the peak of κ-Ga 2 O 3 is around 420 nm, while the β-phase has a 360 nm PL peak. In addition to the PL spectrum, even in the optical bandgap measurement, the materials grown under N 2 did not vary their optical bandgap upon annealing, which is in contrast to the materials grown under H 2 ambience [12]. Both optical characterization techniques confirm that the phase of as-grown materials is κ-phase and this phase can be maintained even after annealing at 1000 • C, suggesting the thermal stable κ-Ga 2 O 3 epilayers can be grown under N 2 carrier gas. Normalized PL spectra were measured from all of the various as-grown and annealed Ga2O3 films in Table 1. As shown in Figure 2a, we could not observe any change in PL peaks as comparing between before and after annealing. However, in our previous papers [11,12], we demonstrated that the difference in PL peaks depending on its phase has been found, exhibiting that the peak of κ-Ga2O3 is around 420 nm, while the β-phase has a 360 nm PL peak. In addition to the PL spectrum, even in the optical bandgap measurement, the materials grown under N2 did not vary their optical bandgap upon annealing, which is in contrast to the materials grown under H2 ambience [12]. Both optical characterization techniques confirm that the phase of as-grown materials is κ-phase and this phase can be maintained even after annealing at 1000 °C, suggesting the thermal stable κ-Ga2O3 epilayers can be grown under N2 carrier gas. Electrical characteristics of all epilayers listed in Table 1 are summarized in Table 2. The as-grown Ga2O3 layers without either silicon or indium dopant (Structure 1 and Structure 2) displayed extremely high resistivity, similar to the as-grown Ga2O3 grown under H2 carrier gas [12]. However, when materials were doped with silicon and the presence of TMIn during the growth (Structure 3), the hall mobility of the sample was remarkably improved. It exhibited an electron concentration (n ≈ 2 × 10 17 cm −3 ) similar to that grown under H2, whereas the obtained hall electron mobility value was as low as around µ = 2 cm 2 V −1 s −1 . Regarding this phenomenon, we recently reported that the indium can play a role of a surfactant, causing the formation of less defective thin films, and thus resulting in the improvement in hall mobility [12]. On the other hand, contrary to our previous results, the annealing process did not induce an improvement of material conductivity. The Ga2O3 epilayers (Structure 1 and Structure 2) still presented a high resistivity while the co-doped Ga2O3 layer (Structure 3) had a slight increase in both carrier concentration (n ≈ 4 × 10 18 cm −3 ) and hall mobility (µ=4 cm 2 V −1 s −1 ). The thicknesses of the epilayers grown under different carrier gases were measured through the scanning electron microscope (SEM). The growth rates of respective epilayers under N2 carrier gas were around 3 nm/min regardless of structures (Figure 3), which is Electrical characteristics of all epilayers listed in Table 1 are summarized in Table 2. The as-grown Ga 2 O 3 layers without either silicon or indium dopant (Structure 1 and Structure 2) displayed extremely high resistivity, similar to the as-grown Ga 2 O 3 grown under H 2 carrier gas [12]. However, when materials were doped with silicon and the presence of TMIn during the growth (Structure 3), the hall mobility of the sample was remarkably improved. It exhibited an electron concentration (n ≈ 2 × 10 17 cm −3 ) similar to that grown under H 2 , whereas the obtained hall electron mobility value was as low as around µ = 2 cm 2 V −1 s −1 . Regarding this phenomenon, we recently reported that the indium can play a role of a surfactant, causing the formation of less defective thin films, and thus resulting in the improvement in hall mobility [12]. On the other hand, contrary to our previous results, the annealing process did not induce an improvement of material conductivity. The Ga 2 O 3 epilayers (Structure 1 and Structure 2) still presented a high resistivity while the co-doped Ga 2 O 3 layer (Structure 3) had a slight increase in both carrier concentration (n ≈ 4 × 10 18 cm −3 ) and hall mobility (µ = 4 cm 2 V −1 s −1 ). The thicknesses of the epilayers grown under different carrier gases were measured through the scanning electron microscope (SEM). The growth rates of respective epilayers under N 2 carrier gas were around 3 nm/min regardless of structures (Figure 3), which is significantly lower than the layers grown under H 2 carrier gas (17 nm/min) [11,12]. In case of structure 3, the film thickness was approximately 600 nm when it was deposited under H 2 . On the other hand, this was considerably reduced to 110 nm under N 2 ambient. The decrease of the growth rate under N 2 carrier gas can be interpreted as follows: according to the chemical reactions under either H 2 or N 2 carrier gas, the decomposition rate under N 2 is much less efficient than under H 2 carrier gas, leading to the low growth rate. This is also supported by the work [13], showing the TMGa is decomposed in different rates depending on the carrier gas. Due to the low decomposition speed, the undissolved organometallic complex would incorporate within epilayers giving the materials a high stable κ-phase. Furthermore, the incorporation of such organometallic complex makes the Si-doped materials compensated on the basis of the electrical properties displaying low carrier mobility and carrier concentration.
Crystals 2021, 11, x FOR PEER REVIEW 4 of 5 significantly lower than the layers grown under H2 carrier gas (17 nm/min) [11,12]. In case of structure 3, the film thickness was approximately 600 nm when it was deposited under H2. On the other hand, this was considerably reduced to 110 nm under N2 ambient. The decrease of the growth rate under N2 carrier gas can be interpreted as follows: according to the chemical reactions under either H2 or N2 carrier gas, the decomposition rate under N2 is much less efficient than under H2 carrier gas, leading to the low growth rate. This is also supported by the work [13], showing the TMGa is decomposed in different rates depending on the carrier gas. Due to the low decomposition speed, the undissolved organometallic complex would incorporate within epilayers giving the materials a high stable κ-phase. Furthermore, the incorporation of such organometallic complex makes the Sidoped materials compensated on the basis of the electrical properties displaying low carrier mobility and carrier concentration.

Conclusions
Highly thermal stable κ-Ga2O3 epilayers grown on c-plane sapphire substrate by MOCVD using N2 carrier gas were demonstrated through the various characterization techniques such as structural (XRD and SEM), optical (PL and transmission), and electrical (Hall measurement) measurement. Structural as well as optical characterization data have confirmed that the metastable k-phase Ga2O3 can withstand its phase even when subjected to the annealing at 1000 °C, which is opposed to the case grown under H2 carrier gas where the phase transition occurred from κ-to β-phase. If the materials were grown with either silicon dopant or TMIn, both of the as-grown and annealed Ga2O3 films showed high resistive properties. However, the materials became conductive when doped with both silicon and indium. The growth rate under N2 carrier gas was significantly decreased as compared to those grown under H2, which can be attributed to different TMGa decomposition rates depending on the carrier gas. Despite the reduced growth rate, this observation suggests a pathway to maintain metastable κ-Ga2O3 possessing tremendous potential for further exploitation in the semiconductor industry.

Conclusions
Highly thermal stable κ-Ga 2 O 3 epilayers grown on c-plane sapphire substrate by MOCVD using N 2 carrier gas were demonstrated through the various characterization techniques such as structural (XRD and SEM), optical (PL and transmission), and electrical (Hall measurement) measurement. Structural as well as optical characterization data have confirmed that the metastable κ-phase Ga 2 O 3 can withstand its phase even when subjected to the annealing at 1000 • C, which is opposed to the case grown under H 2 carrier gas where the phase transition occurred from κto β-phase. If the materials were grown with either silicon dopant or TMIn, both of the as-grown and annealed Ga 2 O 3 films showed high resistive properties. However, the materials became conductive when doped with both silicon and indium. The growth rate under N 2 carrier gas was significantly decreased as compared to those grown under H 2 , which can be attributed to different TMGa decomposition rates depending on the carrier gas. Despite the reduced growth rate, this observation suggests a pathway to maintain metastable κ-Ga 2 O 3 possessing tremendous potential for further exploitation in the semiconductor industry.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.