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.

Damping Properties of Selective Laser-Melted Medium Manganese Mn-xCu Alloy.

In this work, selective laser melting (SLM) technology was applied to directly realize the in situ synthesis of medium manganese Mn-xCu (x = 30-40 wt.%) alloys based on the blended elemental powders. The effects of heat treatment on the microstructural evolution and damping properties of the SLMed Mn-xCu alloys were investigated. The metastable miscibility gap was studied by thermodynamic modeling and microhardness measurement. The results showed that γ-(Mn, Cu) phase with dendritic arm spacing (DAS) of 0.9-1.2 µm was the main constituent phase in the as-SLMed alloys, which was one to two orders of magnitude finer than those of the as-cast samples. Aging at 400-480°C for the Mn-30%Cu or 430°C for Mn-40%Cu alloys can induce spinodal decomposition, martensitic transformation, and α-phase precipitation, whose direct evidence was provided for the first time by transmission electron microscopy and 3D atom probe tomography in the work. The miscibility gap obtained from thermodynamics calculation was basically consistent with the microhardness results for the SLMed Mn-xCu alloys. Solution and aging (SA) treatment can improve the microstructure, tensile and damping properties of the SLMed Mn-xCu alloys more obviously than aging treatment. A 2.3-2.8 and 4.3-4.5 times increase was produced in damping capacity in the aged SLMed and SLMed+SAed Mn-xCu samples, respectively.

Phosphor-free micro-pyramid InGaN-based white light-emitting diode with a high color rendering index on a ß-Ga2O3 substrate.

We demonstrate the InGaN/GaN-based monolithic micro-pyramid white (MPW) vertical LED (VLED) grown on (-201)-oriented ß-Ga2O3 substrate by selective area growth. The transmission electron microscopy (TEM) reveals an almost defect-free GaN pyramid structure on (10-11) sidewalls, including stacked dual-wavelength multi-quantum wells (MQWs). From the electroluminescence (EL) spectra of the fabricated MPW VLED, a white light emission with a high color rendering index (CRI) of 97.4 is achieved. Furthermore, the simulation shows that the light extraction efficiency (LEE) of the MPW VLED is at least 4 times higher compared with the conventional planar LED. These results show that the MPW VLED grown on ß-Ga2O3 has great potential for highly efficient phosphor-free white light emission.

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.

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.

A High-Reliability RF MEMS Metal-Contact Switch Based on Al-Sc Alloy.

This paper presents a new metal-contact RF MEMS switch based on an Al-Sc alloy. The use of an Al-Sc alloy is intended to replace the traditional Au-Au contact, which can greatly improve the hardness of the contact, and thus improve the reliability of the switch. The multi-layer stack structure is adopted to achieve the low switch line resistance and hard contact surface. The polyimide sacrificial layer process is developed and optimized, and the RF switches are fabricated and tested for pull-in voltage, S-parameters, and switching time. The switch shows high isolation of more than 24 dB and a low insertion loss of less than 0.9 dB in the frequency range of 0.1-6 GHz.

Quasi-van der Waals Epitaxy of a Stress-Released AlN Film on Thermally Annealed Hexagonal BN for Deep Ultraviolet Light-Emitting Diodes.

The heteroepitaxy of high-quality aluminum nitride (AlN) with low stress is essential for the development of energy-efficient deep ultraviolet light-emitting diodes (DUV-LEDs). In this work, we realize that quasi-van der Waals epitaxy growth of a stress-released AlN film with low dislocation density on hexagonal boron nitride (h-BN)/sapphire suffered from high-temperature annealing (HTA) treatment and demonstrate its application in a DUV-LED. It is revealed that HTA effectively improves the crystalline quality and surface morphology of monolayer h-BN. Guided by first-principles calculations, we demonstrate that h-BN can enhance lateral migration of Al atoms due to the ability to lower the surface migration barrier (less than 0.14 eV), resulting in the rapid coalescence of the AlN film. The HTA h-BN is also proved to be efficient in reducing the dislocation density and releasing the large strain in the AlN epilayer. Based on the low-stress and high-quality AlN film on HTA h-BN, the as-fabricated 290 nm DUV-LED exhibits 80% luminescence enhancement compared to that without h-BN, as well as good reliability with a negligible wavelength shift under high current. These findings broaden the applications of h-BN in favor of III-nitride and provide an opportunity for further developing DUV optoelectronic devices on large mismatched heterogeneous substrates.

GaN-based parallel micro-light-emitting diode arrays with dual-wavelength InxGa1-xN/GaN MQWs for visible light communication.

The dual-wavelength InxGa1-xN/GaN micro light emitting diode (Micro-LED) arrays are fabricated by flip-chip parallel connection. It is noted that the Micro-LED arrays with smaller diameter present considerably bigger light output power density (LOPD). For all Micro-LEDs, the LOPD increases continuously with increasing injection current density until it "turns over". It also can be observed that the maximum value of LOPD is determined by the blue quantum well (QW) for the broad area LED. In comparison, the green peak intensity dominates the change of LOPD in the Micro-LEDs. In addition, the enhancement of the green peak intensity value for the Micro-LEDs are considered as a consequence of the combined effects of the reduction in the quantum-confined Stark effect (QCSE) and the crowding effect, high LEE as well as geometric shape. Moreover, -3dB modulation bandwidths of the four different kinds of Micro-LEDs increase with the decrease of the device diameter in the same injected current density, higher than that of the broad area LED. The -3dB modulation bandwidth of the 60 µm Micro-LED shows 1.4 times enhancement compared to that of the broad area LED under the current density of 300 mA/cm2. Evidently, the dual-wavelength InxGa1-xN/GaN Micro-LEDs have great potential in both solid-state lighting (SSL) and the visible light communication (VLC) in the future fabrication.

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.

Three dimensional truncated-hexagonal-pyramid vertical InGaN-based white light emitting diodes based on ß-Ga2O3.

In this Letter, we describe the fabrication of three dimensional (3D) truncated-hexagonal-pyramid (THP) vertical light emitting diodes (VLEDs) with white emission grown on ß-Ga2O3 substrate. In the 3D n-GaN layer, it is noted that the longitudinal growth rate of the 3D n-GaN layer increases as the flow rate of N2 decreases and H2 increases. Moreover, the 3D THP VLED can effectively suppress the quantum-confined Stark effect (QCSE) compared with planar VLEDs due to the semipolar facets and strain relaxation. Thus, the internal quantum efficiency (IQE) of the 3D THP VLED has been doubled and the V-shaped pits have been greatly reduced. In particular, the 3D THP VLED enables multi-wavelength emission (448.0 nm and 498.5 nm) and also shows better light extraction efficiency (LEE), which presents an effective way for the realization of phosphor-free white LED devices.

Semipolar (112Ì 2) AlGaN-Based Solar-Blind Ultraviolet Photodetectors with Fast Response.

The high-quality semipolar (112̅2) AlGaN epitaxial films have been obtained on m-plane sapphire by metal-organic chemical vapor deposition. X-ray rocking curve measurements show the full-width at half-maximums of semipolar (112̅2)-oriented AlGaN films are 0.357° and 0.531° along [112̅3̅]AlGaN and [11̅00]AlGaN, respectively. The fabricated semipolar AlGaN metal-semiconductor-metal solar-blind ultraviolet (UV) photodetector (PD) exhibits a high responsivity of 1842 A/W. The fast response and reliability of the UV PD are ensured via fast switching with a rise and decay time of 90 ms and 53(720) ms, respectively. The UV PD exhibits a significant reduction in the dark current, that is, from 100 µA to 780 fA at 10 V, using a simple wet chemical etching to modify the surface properties of materials. The photo-to-dark-current ratio value of the etched UV PD reaches 4 orders of magnitude higher than the unetched UV PD under 270 nm illumination. These are attributed to the fact that KOH wet etching assists in eliminating the surface states and reconstructing the surface oxides. This work might provide a new potential for the development of solar-blind UV PDs with high performance.

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 InxGa1-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.

Atomic-Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD.

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal-organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Strain modulated luminescence in green InGaN/GaN multiple quantum wells with microwire array by piezo-phototronic effect.

We have demonstrated piezo-phototronic enhanced modulation in green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a microwire array (MWA) structure. It is found that an a-axis oriented MWA structure induces more c-axis compressive strain than a flat structure when a convex bending strain is applied. Moreover, the photoluminescence (PL) intensity exhibits a tendency to increase first and then decrease under the enhanced compressive strain. Specifically, light intensity reaches a maximum of about 123% accompanied by 1.1-nm blueshift, and the carrier lifetime comes to the minimum simultaneously. The enhanced luminescence characteristics are attributed to strain-induced interface polarized charges, which modulate the built-in field in InGaN/GaN MQWs and could promote the radiative recombination of carriers. This work opens a pathway to drastically improve InGaN-based long-wavelength micro-LEDs with highly efficient piezo-phototronic modulation.

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.

Van der Waals epitaxy of nearly single-crystalline nitride films on amorphous graphene-glass wafer.

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interfacial layer. The epitaxial GaN-based light-emitting diode structures, with a record internal quantum efficiency, can be readily lifted off, becoming large-size flexible devices. Without the effects of the potential field from a single-crystalline substrate, we expect this approach to be equally applicable for high-quality growth of nitrides on arbitrary substrates. Our work provides a revolutionary technology for the growth of high-quality semiconductors, thus enabling the hetero-integration of highly mismatched material systems.

Graphene-Nanorod Enhanced Quasi-Van Der Waals Epitaxy for High Indium Composition Nitride Films.

The nitride films with high indium (In) composition play a crucial role in the fabrication of In-rich InGaN-based optoelectronic devices. However, a major limitation is In incorporation requiring a low temperature during growth at the expense of nitride dissociation. Here, to overcome this limitation, a strain-modulated growth method, namely the graphene (Gr)-nanorod (NR) enhanced quasi-van der Waals epitaxy, is proposed to increase the In composition in InGaN alloy. The lattice transparency of Gr enables constraint of in-plane orientation of nitride film and epitaxial relationships at the heterointerface. The Gr interlayer together with NRs buffer layer substantially reduces the stress of the GaN film by 74.4%, from 0.9 to 0.23 GPa, and thus increases the In incorporation by 30.7%. The first principles calculations confirm that the release of strain accounts for the dramatic improvement. The photoluminescence peak of multiple quantum wells shifts from 461 to 497 nm and the functionally small-sized cyan light-emitting diodes of 7 × 9 mil2 are demonstrated. These findings provide an efficient approach for the growth of In-rich InGaN film and extend the applications of nitrides in advanced optoelectronic, photovoltaic, and thermoelectric devices.

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

Efficient and low-cost production of high-quality aluminum nitride (AlN) films during heteroepitaxy is the key for the development of deep ultraviolet light-emitting diodes (DUV-LEDs). Here, the quasi-2D growth of high-quality AlN film with low strain and low dislocation density on graphene (Gr) is presented and a high-performance 272 nm DUV-LED is demonstrated. Guided by first-principles calculations, it is found that AlN grown on Gr prefers lateral growth both energetically and kinetically, thereby resulting in a Gr-driven quasi-2D growth mode. The strong lateral growth mode enables most of dislocations to annihilate each other at the AlN/Gr interface, and therefore the AlN epilayer can quickly coalesce and flatten the nanopatterned sapphire substrate. Based on the high quality and low strain of AlN film grown on Gr, the as-fabricated 272 nm DUV-LED shows a 22% enhancement of output power than that with low-temperature AlN buffer, following a negligible wavelength shift under high current. This facile strategy opens a pathway to drastically improve the performance of DUV-LEDs.