High Thermal Stability of κ-Ga₂O₃ Grown by MOCVD

Abstract

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.

1. Introduction

Gallium oxide (Ga₂O₃) 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 Ga₂O₃ 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 Ga₂O₃, β-phase Ga₂O₃ 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-Ga₂O₃ 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 Ga₂O₃, growing attention has also been given to the metastable phase of Ga₂O₃, which is widely known as ε- or κ-phase [4,5], since it exhibits similar properties with β-Ga₂O₃ 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-Ga₂O₃ has been reported to exhibit unique properties, ferroelectric behavior and large polarization, that other polymorphs do not possess [9,10]. For example, the κ-Ga₂O₃ 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-Ga₂O₃ in the application. This is mainly due to the fact that the κ-Ga₂O₃ transits its phase to the β-Ga₂O₃ 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-Ga₂O₃ cannot be made unless the phase transition at high temperature issue is solved. Herein, we report a high thermal stability of κ-Ga₂O₃ grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The κ-Ga₂O₃ 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.

2. Materials and Methods

We grow various structures listed in

Table 1. Detail information about various growth structures. Nitrogen was used as a carrier gas during the growth.

Structure	Components	TMGa	TMIn	H2O	SiH4
Structure 1	Ga2O3 [Reference]	5 sccm (N2)	0 sccm (N2)	1600 sccm (N2)	20 sccm (N2)
Structure 2	Superlattice [Ga2O3 (30 s)/In2O3 (1 min)]	5 sccm (N2)	70 sccm (N2)	1600 sccm (N2)	0
Structure 3	Superlattice [Ga2O3 (30 s)/In2O3 (1 min)]	5 sccm (N2)	70 sccm (N2)	1600 sccm (N2)	20 sccm (N2)

under N₂ carrier gas using Trimethyl-gallium (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 as-grown 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.

3. Results and Discussion

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 β-phase. 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.

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. Summary of the hall measurement data for the epitaxial layers listed in Table 1.

	Before Annealing		After Annealing
	Hall Mobility	Carrier Concentration	Hall Mobility	Carrier Concentration
Structure 1	Highly resistive	Highly resistive	Highly resistive	Highly resistive
Structure 2	Highly resistive	Highly resistive	Highly resistive	Highly resistive
Structure 3	2 cm2/V·s	2 × 1017 cm−3	4 cm2/V·s	4 × 1018 cm−3

. 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 × 10¹⁷ cm⁻³) similar to that grown under H₂, whereas the obtained hall electron mobility value was as low as around µ = 2 cm²V⁻¹s⁻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 × 10¹⁸ cm⁻³) and hall mobility (µ = 4 cm²V⁻¹s⁻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 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 Si-doped materials compensated on the basis of the electrical properties displaying low carrier mobility and carrier concentration.

4. 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 κ-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.