a-Axis GaN/AlN/AlGaN Core-Shell Heterojunction Microwires as Normally Off High Electron Mobility Transistors

Two-In-One: End-Emitting Blue LED and Self-Powered UV Photodetector based on Single Trapezoidal PIN GaN Microwire for Ambient Light UV Monitoring and Feedback

With the continuous miniaturization and integration of the semiconductor industry, micro/nanoscale integrated photonics has received extensive attention as a key technology for optical communication, optical storage, and optical interconnection. Here, a two-in-one device is reported with both unidirectional blue light emission and UV photodetection functions based on single trapezoidal PIN GaN microwire. By constructing a Fabry-Perot resonator cavity structure, the end-emitting blue light-emitting diode with a low turn-on voltage (≈0.97 V) and high color purity (full width at half maximum ≈22 nm) is implemented. Furthermore, benefiting from the slow growth rate of the semipolar planes on both sides of the trapezoidal microwire and the high diffuse reflectivity of the patterned substrate, the trapezoidal microwire sides can be used as a high-performance UV photodetector. In self-driven mode, the device exhibits a large responsivity (0.218 A W^-1 ), high external quantum efficiency (83.31%) and fast response speed (rise/decay time of 0.48/0.98 ms). Finally, the prepared two-in-one device is successfully integrated into ambient light UV monitoring and feedback system and tested. This work provides a novel strategy to combine luminescence with photodetection, demonstrating high potential for applications, such as on-chip photonic integration, energy-saving communication and ambient light monitoring and feedback system.

Tuning interfacial thermal conductance of GaN/AlN heterostructure nanowires by constructing core/shell structure

The ability to tune the interfacial thermal conductance of GaN/AlN heterojunction nanowires (NWs) with a core/shell structure is shown using molecular dynamics and non-equilibrium Green's functions method. In particular, an increase in the shell thickness leads to a significant improvement of interfacial thermal conductance of GaN/AlN core/shell NWs. At room temperature (300 K), the interfacial thermal conductance of NWs with specific core/shell ratio can reach 0.608 nW K^-1, which is about twice that of GaN/AlN heterojunction NWs due to the weak phonon scattering and phonon localization. Moreover, changing the core/shell type enables one to vary interfacial thermal conductance relative to that of GaN/AlN heterojunction NWs. The results of the study provide an important guidance for solving the thermal management problems of GaN-based devices.

Nanofabrication Techniques: Challenges and Future Prospects

Nanofabrication of functional micro/nano-features is becoming increasingly relevant in various electronic, photonic, energy, and biological devices globally. The development of these devices with special characteristics originates from the integration of low-cost and high-quality micro/nano-features into 3D-designs. Great progress has been achieved in recent years for the fabrication of micro/nanostructured based devices by using different imprinting techniques. The key problems are designing techniques/approaches with adequate resolution and consistency with specific materials. By considering optical device fabrication on the large-scale as a context, we discussed the considerations involved in product fabrication processes compatibility, the feature's functionality, and capability of bottom-up and top-down processes. This review summarizes the recent developments in these areas with an emphasis on established techniques for the micro/nano-fabrication of 3-dimensional structured devices on large-scale. Moreover, numerous potential applications and innovative products based on the large-scale are also demonstrated. Finally, prospects, challenges, and future directions for device fabrication are addressed precisely.

Epitaxial Lift-Off of Flexible GaN-Based HEMT Arrays with Performances Optimization by the Piezotronic Effect

High-electron-mobility transistors (HEMTs) are a promising device in the field of radio frequency and wireless communication. However, to unlock the full potential of HEMTs, the fabrication of large-size flexible HEMTs is required. Herein, a large-sized (> 2 cm²) of AlGaN/AlN/GaN heterostructure-based HEMTs were successfully stripped from sapphire substrate to a flexible polyethylene terephthalate substrate by an electrochemical lift-off technique. The piezotronic effect was then induced to optimize the electron transport performance by modulating/tuning the physical properties of two-dimensional electron gas (2DEG) and phonons. The saturation current of the flexible HEMT is enhanced by 3.15% under the 0.547% tensile condition, and the thermal degradation of the HEMT was also obviously suppressed under compressive straining. The corresponding electrical performance changes and energy diagrams systematically illustrate the intrinsic mechanism. This work not only provides in-depth understanding of the piezotronic effect in tuning 2DEG and phonon properties in GaN HEMTs, but also demonstrates a low-cost method to optimize its electronic and thermal properties.

Three-dimensional hierarchical semi-polar GaN/InGaN MQW coaxial nanowires on a patterned Si nanowire template

We have demonstrated for the first time the hybrid development of next-generation 3-D hierarchical GaN/InGaN multiple-quantum-well nanowires on a patterned Si nanowire-template. The patterned Si nanowire-template is fabricated using metal-assisted chemical-etching, and the conformal growth of the GaN/InGaN multiple-quantum-well (MQW) coaxial nanowires is conducted using metal-organic-chemical-vapor-deposition by the two-step growth approach of vapor-liquid-solid for the GaN core and vapor-solid for the GaN/InGaN MQW shells. The growth directions of the GaN nanowires are confirmed by transmission electron microscopy and selected area electron diffraction patterns. The emission of the GaN/InGaN MQW nanowire is tuned from 440 nm to 505 nm by increasing the InGaN quantum-well thickness. The carrier dynamics were evaluated by performing temperature-dependent time-resolved photoluminescence measurement, and the radiative lifetime of photogenerated electron-hole pairs was found to range from 30 to 35 ps. A very high IQE of 56% was measured due to the suppressed quantum-confined Stark effect which was enabled by the semi-polar growth facet of the GaN/InGaN MQWs. The demonstration of the growth of the hybrid 3-D hierarchical GaN/InGaN MQW nanowires provides a seamless platform for a broad range of multifunctional optical and electronic applications.

In Situ Conformal Coating of Polyaniline on GaN Microwires for Ultrafast, Self-Driven Heterojunction Ultraviolet Photodetectors

Independent and zero-maintenance systems would be in urgent need in the near future internet of things. Here, we present high-performance, self-driven organic/inorganic heterojunction ultraviolet (UV) photodetectors (PDs) by in situ polymerization of polyaniline (PANI) on Gallium nitride microwires. The GaN microwires with a high crystalline quality are grown on patterned Si substrates by metal organic chemical vapor deposition. Using a facile in situ chemical polymerization method, PANI is conformally coated on the surface of GaN microwires. The constructed GaN/PANI hybrid microwire PD exhibits a high responsivity of 178 mA/W, a remarkable detectivity of 4.67 × 10¹⁴ jones, and an ultrafast UV photoresponse speed (rise time of 0.2 ms and fall time of 0.3 ms) under zero bias. The intimate heterojunction in the form of N-Ga-N bonds between GaN and PANI may account for the observed high performances. The presented self-driven microwire UV PDs featuring ultrahigh-speed (sub-millisecond) response to UV light may find applications in future nano/micro-photosensor networks.