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We investigated curbing the defects and stress/strain in epitaxially grown crystalline GaN films on a metal-organic chemical vapor deposition-GaN/c-sapphire (MGcS) template by using plasma-assisted molecular beam epitaxy and demonstrated the impact of growth temperature on their structural, morphological, and optical properties. An in-plane compressive stress having a minimum value of 0.34 GPa has been investigated by vibrational spectroscopy. This alleviated stress was attributed to a less pitted and smoother surface morphology along with reduced threading dislocation densities. Moreover, photoluminescence measurements explicate reduced yellow band emissions relative to near-band edge emission for the film grown under optimum growth conditions. The stress-relaxed and defect-free crystalline GaN film can further be utilized for tremendous optoelectronic and photonic based applications. [ABSTRACT FROM AUTHOR]

Recent advancements in power electronics have been driven by Ga 2 O 3 -based ultrawide bandgap (UWBG) semiconductor devices, enabling efficient high-current switching. However, integrating Ga 2 O 3 power devices with essential silicon CMOS logic circuits for advanced control poses fabrication challenges. Researchers have introduced Ga 2 O 3 -based NMOS and pseudo-CMOS circuits for integration, but these circuits may either consume more power or increase the design complexity. Hence, this article proposes Ga 2 O 3 -based CMOS realized using heterogeneous 3D-stacked bilayer ambipolar transistors. These ambipolar transistors consist of HfO 2 /NiO/Ga 2 O 3 /NiO/HfO 2 heterostructures that are wrapped around by the Ti/Au gate electrode, resulting in record high electron and hole current on/off ratios of 10 9 and 10 7 . The threshold voltage, subthreshold swing, and current density measured from 100 ambipolar devices (across 5 batches) are around -7.99 ± 0.92 V (p-channel) and 7.81 ± 0.81 V (n-channel), 0.59 ± 0.07 V/dec (p-channel) and 0.61 ± 0.06 V/dec (n-channel), and 0.99 ± 0.26 mA/mm (p-channel) and 58.23 ± 12.99 mA/mm (n-channel), respectively. All the 100 ambipolar devices showed decent long-term stability over a period of 200 days, exhibiting reliable electrical performance. The threshold voltage shift (Δ V TH ) after negative bias stressing for a period of 3500 s is around 11.52 V (p-channel) and 10.21 V (n-channel), respectively. Notably, the n-channels exhibit ∼2 orders higher on/off ratio than the best Ga 2 O 3 unipolar transistors at 300 °C. Moreover, the polarities of ambipolar transistors are reconfigurable into p- or n-MOS, which are integrated to demonstrate CMOS inverter, NOR, and NAND logic gates. The switching periods from "0" to "1" and from "1" to "0" of NOR are 0.12 and 0.17 μs, and those of NAND are 0.16 and 0.13 μs. This work lays the foundation of oxide-semiconductor-based CMOS for future integrated electronics.

Transferable Ga 2 O 3 thin film membrane is desirable for vertical and flexible solar-blind photonics and high-power electronics applications. However, Ga 2 O 3 epitaxially grown on rigid substrates such as sapphire, Si, and SiC hinders its exfoliation due to the strong covalent bond between Ga 2 O 3 and substrates, determining its lateral device configuration and also hardly reaching the ever-increasing demand for wearable and foldable applications. Mica substrate, which has an atomic-level flat surface and high-temperature tolerance, could be a good candidate for the van der Waals (vdW) epitaxy of crystalline Ga 2 O 3 membrane. Beyond that, benefiting from the weak vdW bond between Ga 2 O 3 and mica substrate, in this work, the Ga 2 O 3 membrane is exfoliated and transferred to arbitrary flexible and adhesive tape, allowing for the vertical and flexible electronic configuration. This straightforward exfoliation method is verified to be consistent and reproducible by the transfer and characterization of thick (∼380 nm)/thin (∼95 nm) κ-phase Ga 2 O 3 and conductive n-type β-Ga 2 O 3 . Vertical photodetectors are fabricated based on the exfoliated Ga 2 O 3 membrane, denoting the peak response at ∼250 nm. Through the integration of Ti/Au Ohmic contact and Ni/Ag Schottky contact electrode, the vertical photodetector exhibits self-powered photodetection behavior with a responsivity of 17 mA/W under zero bias. The vdW-bond-assisted exfoliation of the Ga 2 O 3 membrane demonstrated here could provide enormous opportunities in the pursuit of vertical and flexible Ga 2 O 3 electronics.

Flexible Ga 2 O 3 photodetectors have attracted considerable interest owing to their potential use in the development of implantable, foldable, and wearable optoelectronics. In particular, β-phase Ga 2 O 3 has been most widely investigated due to the highest thermodynamic stability. However, high-quality β-phase Ga 2 O 3 relies on the ultrahigh crystallization temperature (usually ≥750 °C), beyond the thermal tolerance of most flexible substrates. In this work, we epitaxially grow a high-quality metastable κ-phase Ga 2 O 3 (002) thin film on a flexible mica (001) substrate under 680 °C and develop a flexible κ-Ga 2 O 3 thin film photodetector with ultrahigh performance. Epitaxial κ-Ga 2 O 3 and the mica substrate are maintained to be thermally stable up to 750 °C, suggesting their potential for harsh environment applications. The responsivity, on/off ratio, detectivity, and external quantum efficiency of the fabricated photodetector are 703 A/W, 1.66 × 10 7 , 4.08 × 10 14 Jones, and 3.49 × 10 5 %, respectively, for 250 nm incident light and a 20 V bias voltage. These values are record-high values reported to date for flexible Ga 2 O 3 photodetectors. Furthermore, the flexible photodetector shows robust flexibility for bending radii of 1, 2, and 3 cm. More importantly, it shows strong mechanical stability against 10,000 bending test cycles. These results reveal the significance of high-quality κ-phase Ga 2 O 3 grown heteroepitaxially on a flexible mica substrate, especially its potential for use in future flexible solar-blind detection systems.

The nanoplasmonic impact of chemically synthesized Au nanoparticles (Au NPs) on the performance of GaN nanostructure-based ultraviolet (UV) photodetectors is analyzed. The devices with uniformly distributed Au NPs on GaN nanostructures (nanoislands and nanoflowers) prominently respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoflower-based device yields the highest photoresponsivity of ∼ 380 mA/W, detectivity of ∼ 10 10 jones, reduced noise equivalent power of ∼ 5.5 × 10 -13 W Hz -1/2 , quantum efficiency of ∼ 145%, and fast response/recovery time of ∼40 ms. The report illustrates the mechanism where light interacts with the chemically synthesized nanoparticles guided by the surface plasmon to effectively enhance the device performance. It is observed that the Au NP-stimulated local surface plasmon resonance effect and reduced channel resistance contribute to the augmented performance of the devices. Further, the decoration of low-dimensional Au NPs on GaN nanostructures acts as a detection enhancer with a fast recovery time and paves the way toward the realization of energy-efficient optoelectronic device applications., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Surface-engineered nanostructured nonpolar (112̅0) gallium nitride (GaN)-based high-performance ultraviolet (UV) photodetectors (PDs) have been fabricated. The surface morphology of a nonpolar GaN film was modified from pyramidal shape to flat and trigonal nanorods displaying facets along different crystallographic planes. We report the ease of enhancing the photocurrent (5.5-fold) and responsivity (6-fold) of the PDs using a simple and convenient wet chemical-etching-induced surface engineering. The fabricated metal-semiconductor-metal structure-based surface-engineered UV PD exhibited a significant increment in detectivity, that is, from 0.43 to 2.83 (×10 8 ) Jones, and showed a very low noise-equivalent power (∼10 -10 W Hz -1/2 ). The reliability of the nanostructured PD was ensured via fast switching with a response and decay time of 332 and 995 ms, which were more than five times faster with respect to the unetched pyramidal structure-based UV PD. The improvement in device performance was attributed to increased light absorption, efficient transport of photogenerated carriers, and enhancement in conduction cross section via elimination of recombination/trap centers related to defect states. Thus, the proposed method could be a promising approach to enhance the performance of GaN-based PD technology., Competing Interests: The authors declare no competing financial interest.