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A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics

Jiaqi Lu1, §, Shan Li1, §, , Jingpeng Zhang1, Jiaqing Yuan1, Xueqiang Ji1, Zeng Liu2 and Weihua Tang1,

+ Author Affiliations

 Corresponding author: Shan Li, shanli@njupt.edu.cn; Weihua Tang, whtang@njupt.edu.cn

DOI: 10.1088/1674-4926/26050042CSTR: 32376.14.1674-4926.26050042

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Abstract: Ga2O3 is a leading candidate for next-generation high-power electronics and deep-ultraviolet optoelectronics due to its ultrawide bandgap and high theoretical breakdown field. While the stable β-phase has been extensively studied, interest is shifting toward metastable polymorphs to broaden functionality. Among these, the ε/κ phase is distinguished by its non-centrosymmetric orthorhombic structure, offering unique ferroelectric and piezoelectric properties. Despite progress in epitaxial stabilization, a comprehensive understanding of its crystal structure, physical behavior, and device potential remains limited. This review critically examines the ε-κ phase relationship and heteroepitaxial growth mechanisms on Al2O3, GaN, and other substrates, highlighting the roles of supersaturation, doping, and buffer layer engineering in stabilizing metastable phases while suppressing secondary phases. Furthermore, the work explores the unique physical properties of ε/κ-Ga2O3, including the substantial spontaneous polarization, a large piezoelectric coefficient, and the direct-bandgap nature favorable for DUV detection. The practical efficacy of these attributes is showcased across a diverse technological portfolio, including ferroelectric memristors for neuromorphic computing, high-frequency acoustic wave resonators, and high-sensitivity solar-blind photodetectors. Finally, the review identifies remaining challenges-such as rotational domain mitigation and polarization saturation-providing a strategic roadmap for realizing multifunctional ε/κ-Ga2O3-based devices.

Keywords: ε/κ-Ga2O3, crystal Structure, epitaxial growth, ferroelectricity, piezoelectricity, photoelectricity



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Fig. 1.  (Color online) The key physical properties of ε/κ-Ga2O3.

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Fig. 2.  (Color online) (a)early transformation relationship; (b) The transformation relationship after adding the κ phase[59, 60].

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Fig. 3.  (Color online) phase transformation among five crystalline Ga2O3 polymorphs.

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Fig. 4.  (Color online) Schematic representations of the oxygen atomic arrangement of ε-Ga2O3 and κ-Ga2O3, showing the real relationship between them, which is based on actual resolution of the probing techniques.

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Fig. 5.  (Color online) (a) schematic diagram of XRD φ-scan; (b) XRD angle relationships between the orthorhombic κ-Ga2O3 and the hexagonal ε-Ga2O3. The X-ray source is Cu Kα1 (λ = 1.5405 Å); radiation XRD φ scan of ε/κ-Ga2O3 (c) (122) reflection on Al2O3; (d) (122) reflection on GaN; (e) (139) reflection on STO; (f) (0−68) reflection on LiNbO3[66, 68−71].

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Fig. 6.  (Color online) (a) XRD patterns of ε-Ga2O3 on α-Al2O3 substrate; (b) Cross-section TEM image of ε-Ga2O3/α-Al2O3; (c) The epitaxial relationship between the ε-Ga2O3 and α-Al2O3 substrate; (d) The Band alignment of ε-Ga2O3/α-Al2O3[81, 88, 89, 91].

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Fig. 7.  (Color online) Schematic mechanism of the formation of (a)metastable or (b) stable phases depending on Δμ; Sketches of the evolution of MOVPE growth of the Ga2O3 polymorphs: (c) the different nucleation mechanisms related to the growth conditions and the strain state of the interlayer; (d) a scheme of the evolution of the competing β and κ grains[93, 94].

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Fig. 8.  (Color online) the methods of phase control: (a) growth parameters modulation; (b) doping-induced modulation; (c) two-step growth with a buffer layer[101, 112, 121].

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Fig. 9.  (Color online) (a) XRD patterns of κ-Ga2O3 on GaN substrate; (b) Cross-section TEM image of κ-Ga2O3/GaN; (c) The epitaxial relationship between the κ-Ga2O3 and GaN substrate; (d) The Band alignment of κ-Ga2O3/GaN[124−126].

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Fig. 10.  (Color online) (a) Capacitance and G/ω in dependence of the applied voltage of 17 min grown ε-Ga2O3 layers on GaN/Al2O3 on-axis template; (b) Simulation of bandgap bending and calculation of electron density in case of ε-Ga2O3 /GaN interfaces[127].

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Fig. 11.  (Color online) The epitaxial relationship between the ε/κ-Ga2O3 and (a) AlN; (b) 4H-SiC; (c) ITO; (d) Cubic substrate including MgO, YSZ and STO[71, 84−86].

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Fig. 12.  (Color online) (a) Evolution of Polarization calculated along the pathway from the centrosymmetric parent phase to non-centrosymmetric ε-Ga2O3; (b) The activation energy and spontaneous polarization along the transition path of ε-Ga2O3[52, 129].

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Fig. 13.  (Color online) (a) General signal characteristics of a DHM measurement (b) DHM measurement of the electrical polarization; (c) P−V loops measurement under different applied voltage; (d) Frequency-dependent P−V loops; (e) polarization switching; (f) P−V loop acquired by PUND measurement[50, 51, 125].

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Fig. 14.  (Color online) (a) Vdc-dependence of the phase and amplitude of PFM measurement; (b) Vdc-dependence of the piezoelectric displacement of the AlN/ε-Ga2O3; (c) Comparison of piezoelectric semiconductors AlN, ε-Ga2O3, and ZnO[134].

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Fig. 15.  (Color online) (a) Band structure of κ-Ga2O3, calculated using the hybrid functional B3LYP potential; (b) Transmittance spectra of κ-Ga2O3 thin film on a sapphire substrate. Inset shows a Tauc plot for bandgap of κ-Ga2O3; (c) Photoluminescence spectra of the κ-Ga2O3; (d) the recombination process in κ-Ga2O3 under illumination[145−148].

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Fig. 16.  (Color online) (a) I−V characteristics of FE memristor based on ε/κ-Ga2O3; (b) pulse-induced current serves as the post-synaptic; (c) Schematic image of information transmission in the human brain; (d) PPF index variation as a function of electrical pulse width. Inset shows the changes in synaptic current triggered by a pair of light pulses[49, 102, 107].

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Fig. 17.  (Color online) (a) Current−voltage characteristics in Pt/κ-Ga2O3/ NSTO device; (b) Reproducible resistive switching under programming voltages of ±6 V; (c) Switching cycling stability; (d) Retention measurement[51].

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Fig. 18.  (Color online) (a) The schematic of the SAW resonator fabricated using the ε-Ga2O3 on-sapphire structure, and the inset shows the cross-sectional SEM image of the epitaxial ε-Ga2O3 film; (b) The measured S11 parameter of the fabricated SAW resonator. The inset shows the simulated displacement mode shapes at the resonant frequencies; (c) The schematic of the ε-Ga2O3 SAW deep-UV PD; (d) The S11 curves of the Rayleigh mode signal under 254 nm UV illumination with different light intensities[164, 166].

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Fig. 19.  (Color online) (a) The schematic diagram of a 3D ε-Ga2O3-based BAW resonator; (b) the schematic of the HBAR with only top electrodes fabricated using the epitaxial ε-Ga2O3 on SiC substrate[167, 168].

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Fig. 20.  (Color online) Different device geometries for ε/κ-Ga2O3-based PDs along with their I−V curves under DUV illumination: (a) and (b) MSM structure; (c) and (d) Schottky structure; (e) and (f) Heterojunction structure[171−173].

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Fig. 21.  (Color online) ε-Ga2O3 PD application: (a) UV imaging; (b) UV communication[174, 175].

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Fig. 22.  (Color online) The device design concept, its structure, and PPC characterization. Schematic diagram of the functional classification of the device, including UV sensing, synaptic memory, device authentication and neuromorphic computing[150].

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Table 1.   Comparison of the parameters between conventional WBG semiconductors and UWBG semiconductors[8, 14].

Material parameters WBG semiconductors UWBG semiconductors
SiC GaN Ga2O3 AlN Diamond
Bandgap (Eg, eV) 3.3 3.4 4.8 6.0 5.5
Breakdown electric field (EC, MV∙cm−1) 2.5 3.3 8 15.4 10
Mobility (μ, cm2∙V−1∙s−1) 1000 1250 200 30 2000
Thermal conductivity (k, W∙m−1∙K−1) 490 200 23 319 2200
Baliga FOM (BFOM, ε μ EC3) 317 846 3214 9.76 5.5
Tunneling effective mass (m0) − 0.15 0.31 − 0.69
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Table 2.   Epitaxial progress of ε/κ-Ga2O3 in the past 5 years.

MOCVD
Substrate Precursors Temperature Pressure FWHM RMS Highlights Ref
Al2O3TEGa: 350 sccm;
O2: 6000 sccm		420−490 °C25 torr8.316 nmtemperature controlled phase transition[100]
TEGa: 50−100 sccm;
O2: 200 sccm		Buffer: 600 °C;
Film: 700 °C		20 mbara Multistep Growth Process[64]
TEGa; O2;
VI/III ratio: 500−3500		500 °C50 mbar0.66° (002)VI/III ratio regulation[74]
TEGa: 600−1800 sccm;
TMGa: 35 sccm;
O2: 5−20 slpm		510−900 °C44−150 mbar0.963 nmsingle-phase control and wafer-scale growth[101]
TEGa: 160 sccm;
O2: 800 sccm		550−600 °C7.5−50 torr0.56° (002)1.23 nmphase boundaries investigation[102]
TEGa; H2O: 1000−3000 sccm;
N2O: 500−2000 sccm		600−640 °C100 torr2.2 nmH2O and N2O served as oxygen precursors[103]
TEGa; H2O600−640 °C404 arcsec (004)Epitaxial lateral overgrowth[104]
TEGa: 90 sccm;
H2O: 2000 sccm; N2O: 500 sccm		600 °C0.150° (004)8.99 nmsapphire substrates with large off-axis angles[105]
TEGa: 30 sccm;
O2: 4000 sccm
TDMASn		390 °C40 torr0.53° (004)1.77 nmpulse Sn-assisted growth[106]
SiliconTEGa: 160 sccm;
O2: 800 sccm		520−650 °C15 torr0.111° (004)0.143 nma TiN buffer is used[107]
4H-SiCTEGa: 70 sccm;
O2: 1000 sccm		500 °C15−60 torr0.95° (002)4.36 nmPhase engineering on the 4H-SiC substrate[108]
GaAs; BGaAsTEGa; H2O610 °C60 mbarFirstly grown on (001) p-type GaAs and BxGa(1-x)As /GaAs templates[99]
Mist-CVD
SubstratePrecursorsTransducerTemperatureFWHMRMSHighlightsRef
Al2O3Ga(acac)3; H2O; HCl;1.7 MHz560−640 °C0.073° (002);
0.101° (004)		1.83 nmPolycrystalline orientation competition[109]
Ga(acac)3; H2O; HCl;
O2: 0.005 SLPM;		three 2.4 MHz600 °C0.17° (004)1.56 nmcontrolling the initial mist flow[110]
Ga(acac)3; H2O; HCl;1.7 MHz580 °C0.43° (002)3.31 nmOptimization of oxygen flow rate[111]
Ga(acac)3; H2O; HCl;
SnCl4·5H2O;
O2: 1000 sccm;		2.4 MHz550°C0.08° (004)1.51 nmSn-assisted epitaxial growth[91]
Ga(acac)3; H2O; HCl;
SnCl2·2H2O; H2O2:		1.7 MHz450−700 °C0.86° (004)4.079 nmTin doped[112]
4H-SiCGa(acac)3; H2O1.7 MHzBuffer: 350−700 °C;
Film: 500 °C		1900 arcsec (004)1.1 nmTwo-step growth[86]
ε-GaFeO3GaCl3; H2O800 °C184 arcsec (004)0.45 nmε-GaFeO3 substrate; GaCl3 as Precursor[113]
HVPE
SubstratePrecursorsSource Zone TemperatureGrowth Zone TemperatureFWHMRMSHighlightsRef
Al2O3GaCl; HCl: 15 sccm;
O2: 2000 sccm		850 °C525−625 °C0.08 arcdeg (004)0.49 nmexhibiting multistep resembling a terrace morphology, a relatively low screw dislocation density[114]
Ga metal; O2800 °C700−1100 °C0.09° (002)The out-of-plane epitaxial relationship between the κ-Ga2O3 film and Al2O3 is given[74]
GaNGaCl; O2630 °C4.5′ (004)Thick κ-Ga2O3 film investigation[115]
PLD
SubstrateSn content in targetOxygen pressureTemperatureFWHMRMSHighlightsRef
BaSnO31 mol% Sn-doped Ga2O30.027 mbar700 °C2.32 nmε-Ga2O3/BaSnO3 epitaxial heterojunction is formed[116]
Diamond1 mol% Sn-doped Ga2O320 mtorr800 °Cestablished on the diamond substrate[117]
Al2O3high purity Ga2O3; 1.5 at% ZrO2
1 at% SnO2		0.004−0.015 mbar570 °C0.993 nmZrO2 doped[80]
ALD
SubstratePrecursorsTemperaturePlasma powerFWHMRMSHighlightsRef
Al2O3(0001)TEGa;
H2O, O3, O2 plasma		6.57 nmdifferent plasma oxygen source[81]
TEGa; O2 plasma250−400 °C2800 W0.1 °0.068 nmTemperature controlled[118]
MBE
SubstratePrecursorsTemperaturesMethodFWHMRMSHighlightsRef
AlNGa metal and Ga2O3 powder;
O3		700 °CMOCA0.4° (004)2 nmThree different MBE approaches are employed[43]
DownLoad: CSV

Table 3.   Piezoelectric coefficients eij and dij of ε-Ga2O3 along with previous theoretical values shown in parentheses, and the unit is C∙m−2 and pm∙V−1[132, 133].

e31e32e33e24e15
0.011
(0.429)		−0.319
(−0.198)		0.941
(0.844)		0.194
(0.196)		0.595
(0.712)
d31d32d33d24d15
−0.489
(1.37)		−3.060
(−3.43)		4.858
(4.06)		2.345
(2.69)		9.622
(14.60)
DownLoad: CSV
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    Received: 26 May 2026 Revised: 24 June 2026 Online: Accepted Manuscript: 03 August 2026

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      Jiaqi Lu, Shan Li, Jingpeng Zhang, Jiaqing Yuan, Xueqiang Ji, Zeng Liu, Weihua Tang. A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics[J]. Journal of Semiconductors, 2026, In Press. doi: 10.1088/1674-4926/26050042 ****J Q Lu, S Li, J P Zhang, J Q Yuan, X Q Ji, Z Liu, and W H Tang, A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics[J]. J. Semicond., 2026, accepted doi: 10.1088/1674-4926/26050042
      Citation:
      Jiaqi Lu, Shan Li, Jingpeng Zhang, Jiaqing Yuan, Xueqiang Ji, Zeng Liu, Weihua Tang. A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics[J]. Journal of Semiconductors, 2026, In Press. doi: 10.1088/1674-4926/26050042 ****
      J Q Lu, S Li, J P Zhang, J Q Yuan, X Q Ji, Z Liu, and W H Tang, A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics[J]. J. Semicond., 2026, accepted doi: 10.1088/1674-4926/26050042
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      A review of metastable ε/κ-Ga2O3: Phase identification, epitaxy, and emerging device physics

      DOI: 10.1088/1674-4926/26050042
      CSTR: 32376.14.1674-4926.26050042
      • 1.

        College of Integrated Circuit Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China

      • 2.

        School of Electronic Information Engineering, Inner Mongolia University, Hohhot 010021, China

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      • Jiaqi Lu is a second-year Ph.D. student at Innovation Center for Gallium Oxide Semiconductor (IC-GAO), College of Integrated Circuit Science and Engineering, Nanjing University of Posts and Telecommunications. His major research focuses on the crystal growth and device applications of metastable gallium oxide
      • Shan Li is an associate professor at College of Integrated Circuit Science and Engineering, Nanjing University of Posts and Telecommunications. He received his Ph.D. degree from Beijing University of Posts and Telecommunications. His research mainly focuses on ultrawide bandgap semiconductor materials and information-sensing devices
      • Weihua Tang received his PhD degree from the Institute of Physics (IOP), CAS, Beijing. He is currently a full professor at Nanjing University of Posts and Telecommunications. He has been devoted to the research of ultrawide bandgap gallium oxide semiconductors for more than a decade
      • Corresponding author: shanli@njupt.edu.cn; whtang@njupt.edu.cn
      • Received Date: 2026-05-26
      • Revised Date: 2026-06-24
        • Available Online: 2026-08-03

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