Quasi-2D Growth of Aluminum Nitride Film on Graphene for Boosting Deep Ultraviolet Light-Emitting Diodes

Quasi van der Waals Epitaxy of Single Crystalline GaN on Amorphous SiO2/Si(100) for Monolithic Optoelectronic Integration

The realization of high quality (0001) GaN on Si(100) is paramount importance for the monolithic integration of Si-based integrated circuits and GaN-enabled optoelectronic devices. Nevertheless, thorny issues including large thermal mismatch and distinct crystal symmetries typically bring about uncontrollable polycrystalline GaN formation with considerable surface roughness on standard Si(100). Here a breakthrough of high-quality single-crystalline GaN film on polycrystalline SiO2/Si(100) is presented by quasi van der Waals epitaxy and fabricate the monolithically integrated photonic chips. The in-plane orientation of epilayer is aligned throughout a slip and rotation of high density AlN nuclei due to weak interfacial forces, while the out-of-plane orientation of GaN can be guided by multi-step growth on transfer-free graphene. For the first time, the monolithic integration of light-emitting diode (LED) and photodetector (PD) devices are accomplished on CMOS-compatible SiO2/Si(100). Remarkably, the self-powered PD affords a rapid response below 250 µs under adjacent LED radiation, demonstrating the responsivity and detectivity of 2.01 × 105 A/W and 4.64 × 1013 Jones, respectively. This work breaks a bottleneck of synthesizing large area single-crystal GaN on Si(100), which is anticipated to motivate the disruptive developments in Si-integrated optoelectronic devices.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1 eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Lattice modulation strategies for 2D material assisted epitaxial growth

As an emerging single crystals growth technique, the 2D-material-assisted epitaxy shows excellent advantages in flexible and transferable structure fabrication, dissimilar materials integration, and matter assembly, which offers opportunities for novel optoelectronics and electronics development and opens a pathway for the next-generation integrated system fabrication. Studying and understanding the lattice modulation mechanism in 2D-material-assisted epitaxy could greatly benefit its practical application and further development. In this review, we overview the tremendous experimental and theoretical findings in varied 2D-material-assisted epitaxy. The lattice guidance mechanism and corresponding epitaxial relationship construction strategy in remote epitaxy, van der Waals epitaxy, and quasi van der Waals epitaxy are discussed, respectively. Besides, the possible application scenarios and future development directions of 2D-material-assisted epitaxy are also given. We believe the discussions and perspectives exhibited here could help to provide insight into the essence of the 2D-material-assisted epitaxy and motivate novel structure design and offer solutions to heterogeneous integration via the 2D-material-assisted epitaxy method.

Determination of the preferred epitaxy for III-nitride semiconductors on wet-transferred graphene

Transferred graphene provides a promising III-nitride semiconductor epitaxial platform for fabricating multifunctional devices beyond the limitation of conventional substrates. Despite its tremendous fundamental and technological importance, it remains an open question on which kind of epitaxy is preferred for single-crystal III-nitrides. Popular answers to this include the remote epitaxy where the III-nitride/graphene interface is coupled by nonchemical bonds, and the quasi-van der Waals epitaxy (quasi-vdWe) where the interface is mainly coupled by covalent bonds. Here, we show the preferred one on wet-transferred graphene is quasi-vdWe. Using aluminum nitride (AlN), a strong polar III-nitride, as an example, we demonstrate that the remote interaction from the graphene/AlN template can inhibit out-of-plane lattice inversion other than in-plane lattice twist of the nuclei, resulting in a polycrystalline AlN film. In contrast, quasi-vdWe always leads to single-crystal film. By answering this long-standing controversy, this work could facilitate the development of III-nitride semiconductor devices on two-dimensional materials such as graphene.

Plasmon-enhanced deep ultraviolet Micro-LED arrays for solar-blind communications

Localized surface plasmon resonance (LSPR)-enhanced deep ultraviolet (DUV) Micro-light emitting diodes (Micro-LEDs) using Al nanotriangle arrays (NTAs) are reported for improving the -3 dB modulation bandwidth. Through self-assembled nanospheres, the high-density Al NTAs arrays are transferred into the designated p-AlGaN region of the Micro-LEDs, realizing the effect of LSPR coupling. A 2.5-fold enhancement in photoluminescence (PL) intensity is demonstrated. Combined with the PL intensity ratio at 300 K and 10 K, internal quantum efficiency (IQE) may be increased about 15-20% by the plasmonic effect and the carrier lifetime decreases from 1.15 ns to 0.82 ns, suggesting that LSPR accelerates the spontaneous emission rate. Resulting from the improvement of the IQE, the electroluminescence intensity of Micro-LED arrays with LSPR is obviously increased. Meanwhile, the -3 dB bandwidth of 6 × 6 Micro-LED arrays is increased from 180 MHz to 300 MHz at a current density of 200 A/cm². A potential way is proposed to further increase both the IQE and the modulation bandwidth of DUV Micro-LEDs.

Continuous Single-Crystalline GaN Film Grown on WS2 -Glass Wafer

Use of 2D materials as buffer layers has prospects in nitride epitaxy on symmetry mismatched substrates. However, the control of lattice arrangement via 2D materials at the heterointerface presents certain challenges. In this study, the epitaxy of single-crystalline GaN film on WS₂ -glass wafer is successfully performed by using the strong polarity of WS₂ buffer layer and its perfectly matching lattice geometry with GaN. Furthermore, this study reveals that the first interfacial nitrogen layer plays a crucial role in the well-constructed interface by sharing electrons with both Ga and S atoms, enabling the single-crystalline stress-free GaN, as well as a violet light-emitting diode. This study paves a way for the heterogeneous integration of semiconductors and creates opportunities to break through the design and performance limitations, which are induced by substrate restriction, of the devices.

Atomic Mechanism of Strain Alleviation and Dislocation Reduction in Highly Mismatched Remote Heteroepitaxy Using a Graphene Interlayer

Remote heteroepitaxy is known to yield semiconductor films with better quality. However, the atomic mechanisms in systems with large mismatches are still unclear. Herein, low-strain single-crystalline nitride films are achieved on highly mismatched (∼16.3%) sapphire via graphene-assisted remote heteroepitaxy. Because of a weaker interface potential, the in-plane compressive strain at the interface releases by 30%, and dislocations are prevented. Meanwhile, the lattice distortions in the epilayer disappear when the structure climbs over the atomic steps on substrates because graphene renders the steps smooth. In this way, the density of edge dislocations in as-grown nitride films reduces to the same level as that of the screw dislocations, which is rarely observed in heteroepitaxy. Further, the indium composition in In_xGa_1-xN/GaN multiquantum wells increases to ∼32%, enabling the fabrication of a yellow light-emitting diode. This study demonstrates the advantages of remote heteroepitaxy for bandgap tuning and opens opportunities for photoelectronic and electronic applications.

Graphene-driving strain engineering to enable strain-free epitaxy of AlN film for deep ultraviolet light-emitting diode

The energy-efficient deep ultraviolet (DUV) optoelectronic devices suffer from critical issues associated with the poor quality and large strain of nitride material system caused by the inherent mismatch of heteroepitaxy. In this work, we have prepared the strain-free AlN film with low dislocation density (DD) by graphene (Gr)-driving strain-pre-store engineering and a unique mechanism of strain-relaxation in quasi-van der Waals (QvdW) epitaxy is presented. The DD in AlN epilayer with Gr exhibits an anomalous sawtooth-like evolution during the whole epitaxy process. Gr can help to enable the annihilation of the dislocations originated from the interface between AlN and Gr/sapphire by impelling a lateral two-dimensional growth mode. Remarkably, it can induce AlN epilayer to pre-store sufficient tensile strain during the early growth stage and thus compensate the compressive strain caused by hetero-mismatch. Therefore, the low-strain state of the DUV light-emitting diode (DUV-LED) epitaxial structure is realized on the strain-free AlN template with Gr. Furthermore, the DUV-LED with Gr demonstrate 2.1 times enhancement of light output power and a better stability of luminous wavelength compared to that on bare sapphire. An in-depth understanding of this work reveals diverse beneficial impacts of Gr on nitride growth and provides a novel strategy of relaxing the vital requirements of hetero-mismatch in conventional heteroepitaxy.

Transfer-Enabled Fabrication of Graphene Wrinkle Arrays for Epitaxial Growth of AlN Films

Formation of graphene wrinkle arrays can periodically alter the electrical properties and chemical reactivity of graphene, which is promising for numerous applications. However, large-area fabrication of graphene wrinkle arrays remains unachievable with a high density and defined orientations, especially on rigid substrates. Herein, relying on the understanding of the formation mechanism of transfer-related graphene wrinkles, the graphene wrinkle arrays are fabricated without altering the crystalline orientation of entire graphene films. The choice of the transfer medium that has poor wettability on the corrugated surface of graphene is proven to be the key for the formation of wrinkles. This work provides a deep understanding of formation process of transfer-related graphene wrinkles and opens up a new way for periodically modifying the surface properties of graphene for potential applications, including direct growth of AlN epilayers and deep ultraviolet light emitting diodes.

Flexible High-Stability Self-Variable-Voltage Monolithic Integrated System Achieved by High-Brightness LED for Information Transmission

The emergence of visible light information transmission systems is profoundly affecting the future of the Internet of Things (IoT) technology. The complex sensing and driving circuits of the IoT have become the key factor to hinder signal conversion and processing. Herein, a high-performance self-variable-voltage light information transmission integrated system (SVV-LTS) is reported and its application potential in low-power, self-powered optical communication transmission systems is demonstrated. Diffusion-adsorption regulation growth method and laser induction technology are innovatively used to realize high-brightness light-emitting diode (LED) and flexible micro-supercapacitor (MSC) on graphene. Meanwhile, MSC realizes the dual functions of supplying power to the system, realizing pressure signal response, and converting pressure signals into electrical signals. Finally, the MSC as power, sensor and LED as signal transmitter are integrated into an SVV-LTS. The response time of SVV-LTS is 80 ms and the luminous wavelength fluctuation of the LED is stable at 1.2 nm. This study will provide a new approach to realize low-power optical communication transmission systems affecting the IoT technology.

Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall

In this Letter, we perform a comprehensive investigation on the optical characterization of micro-sized deep-ultraviolet (DUV) LEDs (micro-LEDs) emitting below 280 nm, highlighting the light extraction behavior in relation to the design of chip sidewall angle. We found that the micro-LEDs with a smaller inclined chip sidewall angle (∼33^∘) have improved external quantum efficiency (EQE) performance 19% more than that of the micro-LEDs with a larger angle (∼75^∘). Most importantly, the EQE improvement by adopting an inclined sidewall can be more outstanding as the diameter of the LED chip reduces from 40 to 20 μm. The enhanced EQE of the micro-LEDs with smaller inclined chip sidewall angles can be attributed to the stronger reflection of the inclined sidewall, leading to enhanced light extraction efficiency (LEE). In the end, the numerical optical modeling further reveals and verifies the impact of the sidewall angles on the LEE of the micro-LEDs, corroborating our experiment results. This Letter provides a fundamental understanding of the light extraction behavior with optimized chip geometry to design and fabricate highly efficient micro-LEDs in a DUV spectrum of the future.

High-Quality Transferred GaN-Based Light-Emitting Diodes through Oxygen-Assisted Plasma Patterning of Graphene

Two-dimensional (2D) release layers are commonly used to realize flexible nitride films. Here, high-quality, large-area, and transferable nitride films can be precisely controlled grown on O₂-plasma-assisted patterned graphene. The first-principles calculation indicates that the patterned graphene introduced by O₂ plasma changes the original wettability of sapphire and the growth behavior of Al atoms is related with layer number of graphene, which is consistent with experimental results. The as-fabricated violet GaN-based light-emitting diodes (LEDs) show high stability and high light output power (LOP). This work provides a general rule for the growth of high-quality and transferable III-nitride films on graphene from the atomic scale and provide actual demonstration in LED. The advantages of the proposed new growth method can supply new ways for electronic and optoelectronic flexible devices of group III nitride semiconductors.

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200-280 nm), such as AlGaN-based light-emitting diodes (LEDs) show excellence in preventing virus transmission, which further reveals their wide applications from biological, environmental, industrial to medical. However, the relatively low external quantum efficiencies (mostly lower than 10%) strongly restrict their wider or even potential applications, which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures. Here, we review recent progress in the development of novel concepts and techniques in AlGaN-based LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures. In addition, we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.

Full wafer scale electroluminescence properties of AlGaN-based deep ultraviolet LEDs with different well widths

Deep ultraviolet (DUV) LEDs have great potential in sterilization, water, air purification, and other fields. In this work, DUV LED wafers with different quantum well (QW) widths were grown by metal-organic chemical vapor deposition. It is found that the light output power (LOP) and peak wavelength of all chips are not only related to the QW thickness but also affected by warpage. For the first time, to the best of our knowledge, a positive correlation between the LOP and peak wavelength of DUV LED chips on the same wafer was observed, which is very important for improving the yield of DUV LEDs and reducing costs. Furthermore, the influence of QW thickness on the external quantum efficiency (EQE) of DUV LED has also been investigated. As the thickness of the QW increases, the exciton localization effect decreases and the quantum confinement Stark effect increases. Consequently, DUV LED wafers with a QW thickness of 2 nm have the highest EQE and yield. These findings not only help to improve the efficiency of DUV LEDs but also provide new insights for evaluating the performance of DUV LED wafers.

Growth mechanism on graphene-regulated high-quality epitaxy of flexible AlN film

We report a novel diffusion–adsorption regulation growth method in the epitaxy of AlN on graphene for the high-quality and transferable large-size AlN film.

Quality improvement mechanism of sputtered AlN films on sapphire substrates with high-miscut-angles along different directions

We studied the annealing mechanism of the films with high-miscut-angles at low cost and high efficiency and revealed the essence of annealing to improve the film quality lies in the annihilation of grain boundaries during the recrystallization.