Growth and characterization of α-, β-, and ϵ-phases of Ga₂O₃ using MOCVD and HVPE techniques

Figures & data

Table 1. Crystal parameters for α-GaO, β-GaO, and ϵ-GaO [5,23,24].

Phase	Crystal System	Point Group	a (Å)	b (Å)	c (Å)	β ()
α	Trigonal	Rc	4.983	–	13.433	–
β	Monoclinic	C2/m	12.214(3)	3.0371(9)	5.7981(9)	103.83(2)
ϵ	Hexagonal	P6mc	2.907	–	9.2559	–

Table 2. Summary of epitaxial growths of α- and ϵ-GaO reported in the literature.

Method	Substrate	Precursors	Temperature	Pressure	Film Growth
MBE, PLD, MOCVD [25]	c-plane sapphire		575C (MBE) 650C (PLD) 800C (MOCVD)	Low to moderate	3 monolayer pseudomorphic α-GaO followed by β-GaO
RF magnetron sputtering [26]	a-plane sapphire	O	>600C	N.R.	(110) α-GaO
Ultrasonic mist CVD [27]	c-plane sapphire	Ga(CHO), DI water, HCl, N carrier gas	430–470C (amorphous at lower T, includes β-GaO at higher T)	Atmospheric pressure	α-GaO
HVPE [28]	c-plane sapphire	Gallium chloride (Ga metal + HCl), O, N carrier gas	525–650C	Atmospheric pressure	α-GaO
Plasma-assisted MBE [16]	c-plane sapphire	Ga and O plasma	500–1000C	Gallium beam equivalent pressure (BEP) = –	β-GaO with α-GaO inclusions
MOCVD [29,30]	c-plane sapphire	TMGa and HO	650C	100 mbar	ϵ-GaO
ALD [30]	c-plane sapphire	TMGa and HO	550C	N.R.	ϵ-GaO
HVPE [31]	GaN (0001), AlN (0001), and β-GaO (01)	GaCl and O	550C	N.R.	ϵ-GaO
Mist CVD [32]	MgO (111), YSZ (111)	Ga(CHO), DI water, HCl, N carrier gas	400–700C	N.R.	ϵ-GaO
HVPE (This work)	c-plane sapphire	GaCl (Ga metal + HCl), O, Ar carrier gas	650–850C	500 Torr	α-GaO and ϵ-GaO
HVPE (This work)	c-plane sapphire	GaCl, O	650–850C	700 Torr	α-GaO and ϵ-GaO

Note: MBE, molecular beam epitaxy; PLD, pulsed-laser deposition; RF, radio frequency; MOCVD, metal organic chemical vapor deposition; HVPE, hydride vapor phase epitaxy; ALD, atomic layer deposition; N.R., not reported.

Figure 1. XRD of (a) β, (b) α, and (c) ϵ-phases. Growth conditions are shown in Table 3.

Table 3. Growth conditions for β, α, and ϵ-GaO films corresponding to the XRD patterns shown in Figure 1(a)–(c), respectively.

			Flow rate (sccm)
Run #	Growth method	Phase	15% HCl in Ar	TMGa	O	Pressure (Torr)	Growth rate (nm/min)
105	HVPE	α	138	–	80	550	9.5
49	MOCVD	β	–	85	2800	45	1
114	HVPE	ϵ	300	–	800	550	18

Note: The β-GaO films were grown via MOCVD; the α- and ϵ-GaO were deposited using HVPE. All films were grown at 650C for an hour.

Figure 2. SEM images showing a series of heteroepitaxial GaO films having respective sample #s 86–89 and grown by HVPE on c-plane sapphire. The Ar and O flow rates are indicated. The inset of (b) shows a magnified image of the hexagonal islands achieved under these particular conditions; they were also present in the film shown in (a). (e) XRD of samples shown in (a)–(d), respectively.

Figure 3. (a) Cross-section TEM of β-GaO epitaxial layer grown by MOCVD on sapphire (0001) at 650C. The epitaxial relationship is [01] β-GaO [0001] α-AlO. (b) Cross-section HRTEM of GaO on c-plane sapphire along the AlO [] zone axis (sample #89). An ∼10nm thick interfacial layer of α-GaO grew directly on the substrate, followed by a thicker layer of ϵ-GaO. Insets shows selected area diffraction corresponding to (i) ϵ-GaO [] and (ii) α-GaO []. The epitaxial relationship was found to be [] ϵ-GaO [] α-GaO [] α-AlO. (c) Cross-section HRTEM of α-GaO on c-plane sapphire along the α-AlO [] zone axis. Inset is magnified image of misfit dislocation. (d) Cross-section HRTEM of ϵ-GaO on α-GaO along the α-GaO [] zone axis.

Figure 4. (a) XRD of α-GaO grown on c-plane sapphire before and after annealing at 700C for 1 h. Insert shows an optical image of the surface microstructure of the β-GaO that transformed from the α-GaO after annealing at 700C for 1 h. (b) XRD of ϵ-GaO grown on c-plane sapphire before and after annealing at 1000C for 1 h.

Figure 5. Cl⁻ concentration in atoms/cc and Ga, Al and O concentration in arbitrary units, measured by SIMS for samples (a) #105, (b) #112, and (c) #165, respectively. (d) Cl⁻ concentration in three samples measured by SIMS. Sample #105 shows α-phase and was grown at 650C, sample #112 shows ϵ-phase and was grown at 650C. Sample #165 shows ϵ-phase and was grown at 850C.

Table 4. Growth conditions for three growth runs of heteroepitaxial GaO on c-plane sapphire using HVPE.

Run #	Phase	Temp. (C)	HCl % of total flow	HCl to O ratio	Thickness (nm)	Growth rate (nm/min)	SIMS Aver. Cl Conc. (atoms/cc)
105	α	650	0.37%	0.25875	350	11.7
112	ϵ	650	0.69%	0.05625	900	30
165	ϵ	850	0.71%	0.075	1250	41.7

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