Understanding the Potential of 2D Ga 2 O 3 in Flexible Optoelectronic Devices: Impact of Uniaxial Strain and Electric Field

Predicting the Raman spectra of ferroelectric phases in two-dimensional Ga2O3 monolayer

We investigate the vibrational properties and Raman spectra of the two-dimensional Ga₂O₃ monolayer, using density functional theory. Two ferroelectric (FE) phases of the Ga₂O₃ monolayer with wurtzite (WZ) and zinc blende (ZB) structures (FE-WZ and FE-ZB, respectively) are considered. The Raman tensor and angle-dependent Raman intensities of two major Raman peaks (A11 and A21) in both FE-WZ (497, and 779 cm^-1) and FE-ZB (481, and 772 cm^-1) Ga₂O₃ monolayers, are calculated for the polarization of scattered light, parallel and perpendicular to that of the incident light. The characteristics of angle-dependent Raman intensities are analyzed. The average non-resonant Raman spectra of the minor peaks in FE-WZ (E¹) and FE-BZ (E¹ and E²) are compared with those of major peaks A11 and A21. These predictions of the Raman spectra of the Ga₂O₃ monolayer may guide the rational design of two-dimensional optical devices.

A novel 2D material with intrinsically low thermal conductivity of Ga2O3(100): first-principles investigations

The discovery of new semiconducting materials with low thermal conductivity is of vital importance in promoting thermal energy conversion and management. Herein, lattice dynamical and thermal transport mechanism of new energetically stable 2D Ga₂O₃(100) is presented using density functional theory. The results show that 2D Ga₂O₃(100) possesses an extremely low lattice thermal conductivity of ∼0.71 W mK^-1 at 300 K. We find that 2D Ga₂O₃(100) possesses two intrinsic features that decrease the lattice thermal conductivity: (1) the existence of interspersed distorted tetrahedral and pentahedral coordination geometries, which improves the phonon anharmonicity of the system; (2) compared to bulk β-Ga₂O₃, the reduced dimensionality suppresses heat transfer by introducing interfacial scattering in 2D Ga₂O₃(100). Additionally, the strong Ga-O covalent bond results in a low speed of sound, high phonon-phonon scattering rates, and thus low lattice thermal conductivity. Our finding is remarkable because ultralow thermal conductivity can be realized in a simple 2D oxide, which provides replaceable materials for further applications in the field of thermal management.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Quasi-Epitaxial Growth of β-Ga2O3-Coated Wide Band Gap Semiconductor Tape for Flexible UV Photodetectors

The epitaxial growth of technically important β-Ga₂O₃ semiconductor thin films has not been realized on flexible substrates due to the limitations of high-temperature crystallization conditions and lattice-matching requirements. We demonstrate the epitaxial growth of β-Ga₂O₃(-201) thin films on flexible CeO₂(001)-buffered Hastelloy tape. The results indicate that CeO₂(001) has a small bi-axial lattice mismatch with β-Ga₂O₃(-201), inducing simultaneous double-domain epitaxial growth. Flexible photodetectors are fabricated on the epitaxial β-Ga₂O₃-coated tape. Measurements reveal that the photodetectors have a responsivity of 4 × 10⁴ mA/W, with an on/off ratio reaching 1000 under 254 nm incident light and 5 V bias voltage. Such a photoelectrical performance is within the mainstream level of β-Ga₂O₃-based photodetectors using conventional rigid single-crystal substrates. More importantly, it remained robust against more than 20,000 bending test cycles. Moreover, the technique paves the way for the direct in situ epitaxial growth of other flexible oxide semiconductor devices in the future.

Microstructures and electronic characters of β-Ga2O3 on different substrates: exploring the role of surface chemistry and structures

Unveiling the microstructural and electronic properties of β-Ga₂O₃ on different substrates is vital to realize the high quality and performance of β-Ga₂O₃. Here, the microstructure disorder, defect characters and orbital structures of β-Ga₂O₃ on the Al₂O₃, MgO, and SiC substrates with different terminations are studied. Although several growth mechanisms for β-Ga₂O₃ are observed on the same substrate, β-Ga₂O₃ prefers to deposit the octahedral Ga atom firstly on the Al₂O₃ and MgO substrates, and the latter can restrain the oxygen-vacancy formation and migration. The structural disorder, band offsets and gap states can be improved upon depositing β-Ga₂O₃ on a substrate with metal terminations under the oxygen-poor conditions. Compared to the Al₂O₃ substrate, β-Ga₂O₃ on the SiC substrate shows a smaller structure disorder and a higher defect formation energy, in particular under the oxygen-rich conditions, since β-Ga₂O₃ prefers to deposit the tetrahedral Ga atom firstly on the SiC substrate to form a SiC-Ga₂O₃ interface with less dangling bonds. The type-II band alignment of the SiC-Ga₂O₃ interface can be changed into the type-I character with larger band offsets when β-Ga₂O₃ is deposited under the oxygen-rich conditions, irrespective of the termination of the SiC substrate. These results provide a useful understanding of the effect of substrates on the quality and performance of β-Ga₂O₃ and a scientific basis for the application of substrate-Ga₂O₃ interfaces.

Tunable Properties of Novel Ga2O3 Monolayer for Electronic and Optoelectronic Applications

A novel two-dimensional (2D) Ga₂O₃ monolayer was constructed and systematically investigated by first-principles calculations. The 2D Ga₂O₃ has an asymmetric configuration with a quintuple-layer atomic structure, the same as the well-studied α-In₂Se₃, and is expected to be experimentally synthesized. The dynamic and thermodynamic calculations show excellent stability properties of this monolayer material. The relaxed Ga₂O₃ monolayer has an indirect band gap of 3.16 eV, smaller than that of β-Ga₂O₃ bulk, and shows tunable electronic and optoelectronic properties with biaxial strain engineering. An attractive feature is that the asymmetric configuration spontaneously introduces an intrinsic dipole and thus the electrostatic potential difference between the top and bottom surfaces of the Ga₂O₃ monolayer, which helps to separate photon-generated electrons and holes within the quintuple-layer structure. By applying compressive strain, the Ga₂O₃ monolayer can be converted to a direct band gap semiconductor with a wider gap reaching 3.5 eV. Also, enhancement of hybridization between orbitals leads to an increase of electron mobility, from the initial 5000 to 7000 cm² V^-1 s^-1. Excellent optical absorption ability is confirmed, which can be effectively tuned by strain engineering. With superior stability, as well as strain-tunable electronic properties, carrier mobility, and optical absorption, the studied novel Ga₂O₃ monolayer sheds light on low-dimensional electronic and optoelectronic device applications.