As elemental main group materials (i.e., silicon and germanium) have dominated the field of modern electronics, their monolayer 2D analogues have shown great promise for next-generation electronic materials as well as potential game-changing properties for optoelectronics, energy, and beyond. These atomically thin materials composed of single atomic variants of group III through group VI elements on the periodic table have already demonstrated exciting properties such as near-room-temperature topological insulation in bismuthene, extremely high electron mobilities in phosphorene and silicone, and substantial Li-ion storage capability in borophene. Isolation of these materials within the postgraphene era began with silicene in 2010 and quickly progressed to the experimental identification or theoretical prediction of 15 of the 18 main group elements existing as solids at standard pressure and temperatures. This review first focuses on the significance of defects/functionalization, discussion of different allotropes, and overarching structure-property relationships of 2D main group elemental materials. Then, a complete review of emerging applications in electronics, sensing, spintronics, plasmonics, photodetectors, ultrafast lasers, batteries, supercapacitors, and thermoelectrics is presented by application type, including detailed descriptions of how the material properties may be tailored toward each specific application.

Emerging Applications of Elemental 2D Materials

Two-dimensional (2D) gallium nitride (GaN) has been highly anticipated because its quantum confinement effect enables desirable deep-ultraviolet emission, excitonic effect and electronic transport properties. However, the currently obtained 2D GaN can only exist as intercalated layers of atomically thin quantum wells or nanometer-scale islands, limiting further exploration of its intrinsic characteristics. Here, we report, for the first time, the growth of micrometer-sized 2D GaN single crystals on liquid metals via a surface-confined nitridation reaction and demonstrate that the 2D GaN shows uniformly incremental lattice, unique phonon modes, blue-shifted photoluminescence emission and improved internal quantum efficiency, providing direct evidence to the previous theoretical predictions. The as-grown 2D GaN exhibits an electronic mobility of 160 cm2·V–1·s–1. These findings pave the way to potential optoelectronic applications of 2D GaN single crystals.

Growth of 2D GaN Single Crystals on Liquid Metals

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp2 -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

Biocompatible and sustainable power supply for self-powered wearable and implantable electronics using III-nitride thin-film-based flexible piezoelectric generator

Here we introduce a simple, superfast, and scalable strategy that obtains graphene quantum dots (GQDs) within 3 min under microwave irradiation (MA-GQDs). The MA-GQDs exhibit excellent fluorescence quantum yields up to 35% in the optimum reaction condition. The MA-GQDs with single-crystalline and few-layer structure can reach the visible region with the longest absorption wavelength at 700 nm. Moreover, these ultrabright-fluorescence and stable MA-GQDs as a phosphor and fluorescence probe could be efficiently applied in white light-emitting diodes and cell-imaging fields. The developed pathway to GQDs can provide unambiguous and remarkable insights into the design of high-fluorescence and few-defect GQDs, and expedite the applications of GQDs.

Three Minute Ultrarapid Microwave-Assisted Synthesis of Bright Fluorescent Graphene Quantum Dots for Live Cell Staining and White LEDs

Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm2 V-1 s-1 and sheet carrier density above 1.07 × 1013 cm-2 . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.

Flexible Gallium Nitride for High‐Performance, Strainable Radio‐Frequency Devices

In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

We have used the gate end-to-end (GEE) resistance method to measure channel temperatures in GaN HEMTs. This method is appealing for its simplicity and sensitivity to temperature immediately adjacent to the base of the gate, where several important degradation mechanisms occur. This region is not normally accessible with optical measurement techniques, due to shadowing by the gate and field plate overhangs; yet, it is considerably hotter than the regions that can be reached with optics. We found agreement with a finite-difference model, with reasonable inputs for the thermal barrier resistance at the SiC–GaN interface and temperature coefficients. We repeated this successfully for a second GaN HEMT technology with a very different gate geometry. Finally, we conducted micro-Raman measurements on FETs from the first technology and found excellent agreement with the GEE measurement, once it had been adapted to the more-distant location of the micro-Raman laser spot, by means of our finite-difference model. We conclude that GEE is a valuable tool, complementary to micro-Raman, for calibrating thermal models.

Measurement of Temperature in GaN HEMTs by Gate End-to-End Resistance

AlGaN/GaN high-electron-mobility transistors (HEMTs) were characterized in cross-section by Kelvin probe force microscopy (KPFM) during in situ biasing. The HEMTs used in this study were specially designed to maintain full and representative transistor functionality after cross-sectioning perpendicular to the gate width dimension to expose the active channel from source to drain. A cross-sectioning procedure was established that produces samples with high-quality surfaces and minimal degradation in initial transistor performance. A detailed description of the cross-sectioning procedure is provided. Samples were characterized by KPFM, effectively mapping the surface potential of the device in two-dimensional cross-section, including under metallization layers (i.e., gate, field plates, and ohmic contacts). Under the gate and field plate layers are where electric field, temperature, and temperature gradients are all most commonly predicted to have peak values, and where degradation and failure are most likely, and so this is where direct measurements are most critical. In this work, the surface potential of the operating device was mapped in cross-section by KPFM. Charge redistribution was observed during and after biasing, and the surface potential was seen to decay with time back to the prebias condition. This work is a first step toward directly mapping and localizing the steady-state and transient charge distribution due to point defects (traps) before, during, and after device operation, including normally inaccessible regions such as under metallization layers. Such measurements have not previously been demonstrated for GaN HEMT technology.

Elizabeth A. Moore

Papers

Flexible Gallium Nitride for High‐Performance, Strainable Radio‐Frequency Devices

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

Measurement of Temperature in GaN HEMTs by Gate End-to-End Resistance

Characterization of Cross-Sectioned Gallium Nitride High-Electron-Mobility Transistors with In Situ Biasing

Flexible Gallium Nitride: Flexible Gallium Nitride for High-Performance, Strainable Radio-Frequency Devices (Adv. Mater. 47/2017)