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Gallium nitride (GaN), a promising alternative semiconductor to Si, is widely used in photoelectronic and electronic technologies. However, the vulnerability of the GaN surface is a critical restriction that hinders the development of GaN-based devices, especially in terms of device stability and reliability. In this study, this challenge is overcome by converting the GaN surface into a gallium oxynitride (GaON) epitaxial nanolayer through an in situ two-step "oxidation-reconfiguration" process. The O plasma treatment overcomes the chemical inertness of the GaN surface, and sequential thermal annealing manipulates the kinetic-thermodynamic reaction pathways to create a metastable GaON nanolayer with a wurtzite lattice. The GaN-derived GaON nanolayer is a tailored structure for surface reinforcement and possesses several advantages, including a wide bandgap, high thermodynamic stability, and large valence band offset with a GaN substrate. These physical properties can be further leveraged to enhance the performance of GaN-based devices in various applications, such as power systems, complementary logic integrated circuits, photoelectrochemical water splitting, and ultraviolet photoelectric conversion., (© 2023 Wiley-VCH GmbH.)

Two-dimensional van der Waals heterostructures with strong intrinsic ferroelectrics are highly promising for novel devices with designed electronic properties. The polarization reversal transition of the 2D ferroelectric Ga 2 O 3 monolayer offers a new approach to tune the photocatalytic and electrical properties of MoS 2 /Ga 2 O 3 heterogeneous bilayers. In this work, we study MoS 2 /Ga 2 O 3 heterogeneous bilayers with different intrinsic polarization using hybrid-functional calculations. We closely investigate the structural, electronic and optical properties of two stable stacking configurations with opposite polarization. The results reveal a distinct switch from type-I to type-II heterostructures owing to polarization reversal transition of the 2D ferroelectric Ga 2 O 3 monolayer. Biaxial strain engineering leads to type-I-to-II and type-II-to-III transitions in the two polarized models, respectively. Intriguingly, one of the MoS 2 /Ga 2 O 3 heterolayers has a larger spatial separation of the valence and conduction band edges and excellent optical absorption ranging from infrared to ultraviolet region under biaxial strain, thus ensuring promising novel applications such as flexible electrical and optical devices. Based on the highly tunable physical properties of the bilayer heterostructures, we further explore their potential applications, such as photocatalytic water splitting and field-controlled switch channel in MOSFET devices.

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS 2 , towards the quantum limit), the high field-effect mobility (up to 358,000 cm 2 V -1 s -1 in WSe 2 ), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design., (© 2022. The Author(s).)

A two-dimensional (2D) Ga 2 O 3 monolayer with an asymmetric quintuple-layer configuration was reported as a novel 2D material with excellent stability and strain tunability. This unusual asymmetrical structure opens up new possibilities for improving the selectivity and sensitivity of gas sensors by using selected surface orientations. In this study, the surface adsorptions of nine molecular gases, namely, O 2 , CO 2 , CO, SO 2 , NO 2 , H 2 S, NO, NH 3 , and H 2 O, on the 2D Ga 2 O 3 monolayer are systematically investigated through first-principles calculations. The intrinsic dipole of the system leads to different adsorption energies and changes in the electronic structures between the top- and bottom-surface adsorptions. Analyses of electronic structures and charge transport calculations indicate a potential application of the 2D Ga 2 O 3 monolayer as a room-temperature NO gas-sensing device with high sensitivity and tunable adsorption energy using plenary strain-induced lattice distortion.

A novel two-dimensional (2D) Ga 2 O 3 monolayer was constructed and systematically investigated by first-principles calculations. The 2D Ga 2 O 3 has an asymmetric configuration with a quintuple-layer atomic structure, the same as the well-studied α-In 2 Se 3 , and is expected to be experimentally synthesized. The dynamic and thermodynamic calculations show excellent stability properties of this monolayer material. The relaxed Ga 2 O 3 monolayer has an indirect band gap of 3.16 eV, smaller than that of β-Ga 2 O 3 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 Ga 2 O 3 monolayer, which helps to separate photon-generated electrons and holes within the quintuple-layer structure. By applying compressive strain, the Ga 2 O 3 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 cm 2 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 Ga 2 O 3 monolayer sheds light on low-dimensional electronic and optoelectronic device applications.

Using remote N 2 plasma treatment to promote dielectric deposition on the dangling-bond free MoS 2 is explored for the first time. The N 2 plasma induced damages are systematically studied by the defect-sensitive acoustic-phonon Raman of single-layer MoS 2 , with samples undergoing O 2 plasma treatment as a comparison. O 2 plasma treatment causes defects in MoS 2 mainly by oxidizing MoS 2 along the already defective sites (most likely the flake edges), which results in the layer oxidation of MoS 2 . In contrast, N 2 plasma causes defects in MoS 2 mainly by straining and mechanically distorting the MoS 2 layers first. Owing to the relatively strong MoS 2 -substrate interaction and chemical inertness of MoS 2 in N 2 plasma, single-layer MoS 2 shows great stability in N 2 plasma and only stable point defects are introduced after long-duration N 2 plasma exposure. Considering the enormous vulnerability of single-layer MoS 2 in O 2 plasma and the excellent stability of single-layer MoS 2 in N 2 plasma, the remote N 2 plasma treatment shows great advantage as surface functionalization to promote dielectric deposition on single-layer MoS 2 .

Transistors based on MoS2 and other TMDs have been widely studied. The dangling-bond free surface of MoS2 has made the deposition of high-quality high-k dielectrics on MoS2 a challenge. The resulted transistors often suffer from the threshold voltage instability induced by the high density traps near MoS2/dielectric interface or inside the gate dielectric, which is detrimental for the practical applications of MoS2 metal-oxide-semiconductor field-effect transistor (MOSFET). In this work, by using AlN deposited by plasma enhanced atomic layer deposition (PEALD) as an interfacial layer, top-gate dielectrics as thin as 6 nm for single-layer MoS2 transistors are demonstrated. The AlN interfacial layer not only promotes the conformal deposition of high-quality Al2O3 on the dangling-bond free MoS2, but also greatly enhances the electrical stability of the MoS2 transistors. Very small hysteresis (ΔVth) is observed even at large gate biases and high temperatures. The transistor also exhibits a low level of flicker noise, which clearly originates from the Hooge mobility fluctuation instead of the carrier number fluctuation. The observed superior electrical stability of MoS2 transistor is attributed to the low border trap density of the AlN interfacial layer, as well as the small gate leakage and high dielectric strength of AlN/Al2O3 dielectric stack.