Study of Phase Transition in MOCVD Grown Ga₂O₃ from κ to β Phase by Ex Situ and In Situ Annealing

Abstract

We report the post-growth thermal annealing and the subsequent phase transition of Ga₂O₃ grown on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). We demonstrated the post-growth thermal annealing at temperatures higher than 900 °C under N₂ ambience, by either in situ or ex situ thermal annealing, can induce phase transition from nominally metastable κ- to thermodynamically stable β-phase. This was analyzed by structural characterizations such as high-resolution scanning transmission electron microscopy and x-ray diffraction. The highly resistive as-grown Ga₂O₃ epitaxial layer becomes conductive after annealing at 1000 °C. Furthermore, we demonstrate that in situ annealing can lead to a crack-free β-Ga₂O₃.

1. Introduction

Growing attention has been given to gallium oxide (Ga₂O₃) due to its potential for realizing next generation ultra-wide band gap (UWBG) electronic/optoelectronic device applications such as high-power transistors or UV solar blind photodetectors (SBPD). Single crystal Ga₂O₃ can possess different polymorphic forms of α-, β-, γ-, ε -, and κ [1]. Among its five different polymorphs, β-Ga₂O₃ is the thermodynamically most stable with a wide direct bandgap energy of 4.85 eV [2]. Single crystal β-Ga₂O₃ also exhibits a relatively high breakdown voltage compared with those of other wide bandgap materials, such as GaN or SiC. In addition, bandgap engineering within UVC solar blind band (200–280 nm) has also been reported by alloying with other elements such as indium, aluminum, or magnesium [3,4,5]. These unique properties, together with the recent advent of commercially available single crystal substrates by melt growth method, have drawn considerable interest in utilizing β-Ga₂O₃ in a number of important technological applications from transparent electrodes, thin film transistors, and gas sensors to solar blind photodetectors and LEDs emitting in UVC band [6]. For the practical device applications, the growth of high quality Ga₂O₃ on either native Ga₂O₃ substrate or foreign substrates (such as c- or m-plane sapphire substrate) has been investigated by various epitaxial growth techniques such as mist-chemical vapor deposition (mist-CVD) [7,8], molecular beam epitaxy (MBE) [9,10], pulsed laser deposition (PLD) [6,11], hydride vapor phase epitaxy (HVPE) [12,13], and metal organic chemical vapor deposition (MOCVD) [14,15,16,17]. Different types of polymorphs of the epitaxially grown Ga₂O₃ have been reported for different types of crystal growth techniques, growth condition, and substrates. This possibly suggests that the structural properties of the epitaxial Ga₂O₃ can be heavily dependent upon the thermodynamics in the growth process and post-growth processing condition. In a recent report [18], we have shown that a stabilized κ-Ga₂O₃ can be formed on c-plane sapphire substrates by MOCVD process. However, there still has been a lack of the investigation on the thermal stability of this epitaxially grown κ-Ga₂O₃. Thus, in this work, we demonstrated that post-growth thermal annealing at a temperature above 900 °C can induce the phase transition from the epitaxially stabilized κ-Ga₂O₃ to thermodynamically stable β-Ga₂O₃. In addition, we discuss the electrical and optical properties of these, as grown κ-Ga₂O₃ and annealed β-Ga₂O₃.

2. Materials and Methods

The growth of Ga₂O₃ was performed on c-plane sapphire substrates by AIXTRON AIX200/4 horizontal MOCVD reactor at the growth temperature between 610 to 690 °C and at the pressure of 50 mbar, using H₂ as a carrier gas. Trimethyl-Ga (TMGa) and pure H₂O were used as Ga and O precursors, respectively, while SiH₄ was used as a doping precursor. After material growth, post-growth thermal annealing was carried out under N₂ ambience by either an ex situ rapid thermal annealing (RTA) or in situ annealing within the MOCVD reactor. For in situ annealing, the as-grown Ga₂O₃ samples were annealed after the growth without exposure to the ambient air. In situ annealing allows for precise control of the heating and cooling rate in the annealing process, which can minimize undesirable effects, such as the generation of detrimental cracks associated with rapid temperature changes. On the other hand, ex situ RTA generally employs a rapid temperature ramping up and down, which can often result in aforementioned detrimental effects. Structural, optical, and electrical properties of the grown sample were fully analyzed before and after annealing. Field emission scanning electron microscopy (SEM) was used to investigate the surface morphology as well as to measure the thickness of the either as grown and annealed Ga₂O₃ epitaxial layers on sapphire substrates. The surface morphology was further characterized by atomic force microscopy (AFM). In addition, the structural integrity and the corresponding phase of the as-grown and annealed Ga₂O₃ epitaxial layers grown on c-plane sapphire substrates were evaluated by high-resolution x-ray diffraction (HR-XRD). The scanning transmission electron microscopy ((S)TEM) characterization was performed using a probe-corrected JEOL ARM 200CF microscope, which is equipped with bright field (BF) detectors and operated at 200 kV. The beam convergence angle is around 20 mrad, and the collection angle for annular bright field (ABF) imaging ranges from 11 to 22 mrad. The electron transparent cross-sectional samples were prepared by an FEI Helios NanoLab focused ion beam system. Electrical characteristics, including resistivity, mobility, and carrier concentration, of the film 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

3.1. Growth of κ-Phase Ga₂O₃ on Sapphire Substrate

The temperature dependent growth study was carried out at a fixed VI/III molar flow ratio, while varying the growth temperature from 610 to 690 °C. A significant improvement in the surface morphology was observed as the growth temperature increased from 610 to 690 °C, evidenced by both top-view SEM and AFM images shown in Figure 1(a-1),(b-1),(c-1). A reduced root-mean-square (RMS) roughness (2 nm) was obtained from the sample grown at 690 °C. In addition, narrower full-width-at-half-maximum (FWHM) values in the XRD peaks were obtained as a higher growth temperature was employed (Figure 1(a-2),(b-2),(c-3)). These XRD peaks were well aligned with the calculated (002), (004) and (006) planes of κ-Ga₂O₃ peak positions shown in

Table 1. Summary of measured and calculated reflection angle from the Ga₂O₃ grown at 690 °C on c-plane sapphire substrate (lattice parameter of orthorhombic κ-Ga₂O₃: a = 5.12 Å, b = 8.78 Å, c = 9.4 Å).

Phase	(h k l)	d-Spacing (Å)	Calculated Bragg’s Angle	Measured Peak Position
κ-Ga2O3 (Orthorhombic)	(002)	4.705	9.4°	9.6°
κ-Ga2O3 (Orthorhombic)	(004)	2.353	19.1°	19.4°
κ-Ga2O3 (Orthorhombic)	(006)	1.568	29.4°	29.96°

.

3.2. Effect of Annealing Condition on the Structural Properties

Systematic annealing studies were carried out to investigate the thermal stability of the epitaxial κ-Ga₂O₃, which was grown at 690 °C. This sample was subjected to annealing at varying temperatures ranging from 800 to 1000 °C under N₂ ambience. These annealing conditions are also summarized in

Table 2. The annealing condition to investigate the effect of annealing temperature for the sample grown at 690 °C.

Annealing Temp. [°C]	Annealing Type	Ambience	Duration (s)
800	in situ annealing	N2	30
800	ex situ RTA	N2	30
900	in situ annealing	N2	30
900	ex situ RTA	N2	30
1000	in situ annealing	N2	30
1000	ex situ RTA	N2	30

. For in situ annealing, the heating and cooling were performed for 20 min and 10 min, respectively.

Shown in Figure 2a are the XRD patterns for the samples annealed in situ at various temperature for 30 s ranging from 800 to 1000 °C, while those annealed by ex situ RTA are shown in Figure 2b. Regardless of the annealing method, the evidence of phase transition from κ to β phase was not observed from the samples annealed at 800 °C, based on the same observed peak positions, which corresponds to (002), (004), and (006) of orthorhombic κ-Ga₂O₃.

On the other hand, when the annealing temperature exceeds 900 °C, these peaks disappeared, and new peaks near 19 and 40° started to evolve, which are in close agreement with calculated (310) and (620) planes of monoclinic β-Ga₂O₃ peak positions, as summarized in

Table 3. Summary of measured and calculated reflection angle from the annealed Ga₂O₃ grown on c-plane sapphire substrate (lattice parameter of monoclinic β-Ga₂O₃: a = 12.23 Å, b = 3.04 Å, c = 5.8 Å, and β = 103.7°).

Phase	(h k l)	d-Spacing (Å)	Calculated Bragg’s Angle	Measured Peak Position
β-Ga2O3 (Monoclinic)	(310)	2.412	18.6°	18.5°
β-Ga2O3 (Monoclinic)	(620)	1.206	39.7°	39.5°

. When the annealing temperature of 1000 °C was employed, the most distinguishable contrast in the intensities of β-Ga₂O₃ (310) and (620) peaks were observed.

When the annealing temperature of 1000 °C was used, a number of cracks on the surface was observed from annealed sample by RTA, as shown in Figure 3b, while the in situ annealed sample exhibited nearly crack-free surface (Figure 3a). This observation is attributed to a slower heating/cooling rates employed in in situ annealing process, in comparison to those of RTA. This result also suggests that in situ annealing with well-controlled heating and cooling rates will help avoid the generation of severely extended cracks and defects, which can be a problematic issue for the practical device application.

Figure 4a shows an annular bright filed scanning transmission electron microscopy (ABF-STEM) image of the thin film before annealing. The nominal thickness of the Ga₂O₃ film is around 450 nm. Figure 4b,c show electron diffraction patterns (EDPs) taken from only the Al₂O₃ substrate and Ga₂O₃ film, respectively. EDP analysis confirmed the presence of κ phase of Ga₂O₃, which agrees well to above XRD analyses. Additionally, κ-Ga₂O₃ phase can keep a good orientation relationship (OR) with the α-Al₂O₃ matrix, which can be confirmed based on the composite EDPs shown in Figure 4d. This specific orientation relationship (OR) can be described as [100]_Al2O3//[100]_κ-Ga2O3 and (001)_Al2O3//(001)_κ-Ga2O3, which is consistent with our previous work [18].

Figure 5a shows an ABF-STEM image of the thin film after annealing. Figure 5b,c EDPs were taken from the α-Al₂O₃ matrix and composite/interface, respectively. Based on Figure 5c EDPs, it can be shown that the thin film can be indexed as β-Ga₂O₃ along [132]_β-Ga2O3 direction. Additionally, the newly transformed β-Ga₂O₃ keeps a good OR with matrix as well, which can be described as [210]_Al2O3//[132]_β-Ga2O3 and (001)_Al2O3//(3$\overline{1}$0)_β-Ga2O3. Figure 5d EDPs were taken from the β-Ga₂O₃ thin film only, which can be indexed consistently invoking just the innate twin structure. The twin plan is (3$\overline{1}$0)_β-Ga2O3, which is indicated in Figure 5a. Additionally, in Figure 4a and Figure 5a ABF images, there are some holes/gaps along the interface as indicated by the circles. The formation of the hole likely resulted from the large lattice misfit between Ga₂O₃ and Al₂O₃, which was reported and discussed in our previous work [18].

3.3. Effect of Annealing Condition on the Optical and Electrical Properties

Figure 6a shows the photoluminescence (PL) spectra measured from either as-grown or in situ annealed samples, while Figure 6b shows those annealed by RTA. Both as-grown sample and the sample annealed at 800 °C exhibited peak position near 420 nm. On the other hand, as the annealing temperature exceeds 900 °C, another peak near 370 nm started to evolve. A prior study has claimed that the PL peak near 380 nm is related to the transition levels between the oxygen vacancy and unintended N impurities introduced during N₂ annealing [19]. On the other hand, the other studies contended the PL peaks near 416, 442, or 464 nm originated from the electron-hole recombination formed by oxygen vacancies or to the recombination of Ga-O vacancy pair [20,21]. While finding the origin of these emissions is a subject of our ongoing study, the comparison between the PL spectrum of (010) Ga₂O₃ substrate and that of the annealed samples at 1000 °C reveals analogy in their PL spectrum.

Electrical properties of Ga₂O₃ epitaxial layer before and after annealing were characterized by Van der Pauw Hall measurements. While the as-grown Ga₂O₃ films were highly resistive, the Ga₂O₃ layers become conductive when annealed at 1000 °C under N₂. N-type conductivity was obtained after the post growth annealing by either in situ or ex situ RTA. The obtained electron concentration was in the range of a few times ${10}^{18} {\mathrm{cm}}^{-3}$ with the mobility values ranging from 22 to 43 cm²/V-s, depending on the electron concentrations.

4. Conclusions

High-quality Ga₂O₃ thin films were grown at 690 °C on sapphire substrate by low-pressure MOCVD using H₂ as a carrier gas, TMGa, and H₂O as Ga and oxygen precursors. SiH₄ was used a doping precursor as well. After material growth, the material was annealed either in situ or ex situ under N₂ ambience. Structural, optical, and electrical properties of the grown sample were fully analyzed before and after annealing. A systematic annealing study was performed, which showed that when the annealing temperature was higher than 900 °C, the evidence of phase transition from κ to β phase was observed by XRD and STEM. When in situ annealing was employed, a crack-free surface was obtained. The as-grown sample was highly resistive. After annealing at 1000 °C, this as-grown material became highly conductive with the electron concentration in the range of a few times ${10}^{18} {\mathrm{cm}}^{-3}$ and mobilities ranging from 22~43 cm²/V-s. After annealing, the PL spectra of the epitaxially grown Ga₂O₃ was compared with that of β-Ga₂O₃ substrate, and a close similarity was observed.