The growth of single‐crystal III‐nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN‐derived deep‐ultraviolet light‐emitting diodes ...(DUV‐LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi‐van der Waals epitaxial (QvdWE) growth of high‐quality AlN films on graphene/sapphire substrates is reported and their application in high‐performance DUV‐LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror‐smooth single‐crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as‐fabricated DUV‐LED shows a low turn‐on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.
Guided by density functional theory (DFT) calculations, a N2‐plasma‐treated graphene buffer layer is developed to grow high‐quality AlN for deep‐ultraviolet light‐emitting diode (DUV‐LED) applications. The plasma treatment of graphene can facilitate AlN nucleation, thus increasing the growth rate of the AlN film and improving the film quality. The as‐fabricated LED shows a low turn‐on voltage, good reliability, and high output power.
For III‐nitride‐based devices, such as high‐brightness light‐emitting diodes (LEDs), the poor heat dissipation of the sapphire substrate is deleterious to the energy efficiency and restricts many of ...their applications. Herein, the role of vertically oriented graphene (VG) nanowalls as a buffer layer for improving the heat dissipation in AlN films on sapphire substrates is studied. It is found that VG nanowalls can effectively enhance the heat dissipation between an AlN film and a sapphire substrate in the longitudinal direction because of their unique vertical structure and good thermal conductivity. Thus, an LED fabricated on a VG‐sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction. Moreover, the introduction of VG nanowalls does not degrade the quality of the AlN film, but instead promotes AlN nucleation and significantly reduces the epilayer strain that is generated during the cooling process. These findings suggest that the VG nanowalls can be a good buffer layer candidate in III‐nitride semiconductor devices, especially for improving the heat dissipation in high‐brightness LEDs.
The use of vertically oriented graphene (VG) nanowalls as a buffer layer for the growth of single‐crystal AlN film on a sapphire substrate to improve heat dissipation is proposed. Thus, a light‐emitting diode fabricated on a VG‐sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction.
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.
Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and ...fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and InxGa1−xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.
A high‐quality GaN layer is directly grown on chemical‐vapor‐deposition‐derived graphene/sapphire substrate. The graphene/sapphire substrate greatly releases biaxial stress in the GaN film, leading to a reduction of the dislocation density. The thus‐obtained GaN film enables much higher light output power of as‐fabricated light‐emitting diode devices which brings several disruptive technologies for III‐nitride epitaxy growth and graphene applications.
We study the roles of graphene acting as a buffer layer for growth of an AlN film on a sapphire substrate. Graphene can reduce the density of AlN nuclei but increase the growth rate for an individual ...nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AlN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in AlN and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AlN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.
In this work, we fabricated and characterized 4 × 4 parallel flip-chip AlGaN-based micro-LED arrays with varied mesa diameters of 120 µm, 100 µm, 80 µm, and 60 µm. The reported micro-LED arrays have ...a maximum bandwidth of 380 MHz and a peak wavelength of ∼275 nm. It is found that the electrical and optical characteristics of AlGaN-based micro-LED arrays show strong size dependence for ultraviolet communication (UVC). The differential resistance increases from 28.8 Ω to 112 Ω, the external quantum efficiency (EQE) is increased by ∼30%, and the bandwidth doubles as the diameter of individual micro-LED decreases from 120 µm to 60 µm. Our research proves that tailoring the mesa size of parallel flip-chip AlGaN-based micro-LED arrays can further enhance its bandwidth and promote its application in UVC.
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.
For the first time, the growth of nitride on sapphire by metal–organic chemical vapor deposition can be monitored layer‐by‐layer, starting at the substrate, to observe the inverse domain boudary formation and the evolution of the polarity inversion. This work offers a breakthrough in the understanding of the underlying mechanism that dictates the polarity of the nitride epilayer.
In this work, the three-dimensional transient temperature field model of two typical damping alloys (i.e., AZ31 Mg alloy and Mn-Cu alloy) were developed during selective laser melting (SLM) using ...ANSYS finite element analysis software, considering thermophysical properties and phase transformation latent heat of the materials. The simulated molten pool width values of the Mn-Cu alloy show a 6 % error with the experimental SLMed Mn-Cu sample, verifying that the developed model agrees well with the experimental results. Based on the developed numerical model, the temperature distribution, temperature gradients, and cooling rates of the two alloys were investigated during horizontal deposition and vertical deposition of SLM at different scanning speeds (300–700 mm/s). The results showed that the simulated peak temperatures of the AZ31 Mg alloy during the SLM process show a small fluctuation and are well below its boiling point (1107 °C) at the scanning speed of 500–700 mm/s. The temperature gradient increases from 8.5 × 106 °C/m to 20.2 × 106 °C/m with the laser scanning speed. The cooling rate increases from 1.7 × 106 °C/s to 4.1 × 106 °C/s, then reaches stable at 500–600 mm/s. As for Mn-Cu alloy, the cooling rate fluctuates between 7.5 and 8.8 × 106 °C/s during the SLM process at a scanning speed of 300–700 mm/s. By comparing the thermal behavior of the two alloys, the peak temperatures, temperature gradients, and cooling rate of both alloys are closely associated with their thermophysical properties and the laser energy input.
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