Heteroepitaxy of high-quality AlN film is the key to advance the prosperity of deep-ultraviolet (DUV) devices when a large-size and low-cost native substrate is unavailable. Here, we proposed a strategy to obtain high-quality AlN film by combining growth-mode modification with sputtered AlN buffer using metal− organic chemical vapor deposition (MOCVD). Compared with the MOCVD AlN buffer, the sputtered AlN buffer consists of smaller and more uniform grains with better c-axis orientation, leading to a better growth-mode modification in the subsequent growth process. On one hand, the better c-axis orientation is inherited by the upper AlN epilayer, resulting in a lower screw dislocation density. On the other hand, the better growth-mode modification significantly suppresses edge dislocations by producing high-density nanoscale voids and many 90°bent dislocations. Therefore, the total threading dislocation density of the AlN film grown on the sputtered AlN buffer is dramatically reduced to an extremely low value of 4.7 × 10 7 cm −2 , which is 81.2% less than that of the AlN film grown on the MOCVD AlN buffer. This very simple yet effective technique demonstrates great potential for the massfabrication of low-cost and high-performance DUV devices.
It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm. It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 10 cm, which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.
In this paper, the self-consistent solution of Schrödinger-Poisson equations was realized to estimate the radiative recombination coefficient and the lifetime of a single blue light InGaN/GaN quantum well (QW). The results revealed that the recombination rate was not in proportion to the total injected carriers, and thus the Bnp item was not an accurate method to analyze the recombination process. Carrier screening and band filling effects were also investigated, and an extended Shockley-Read-Hall coefficient A(kt) with a statistical weight factor due to the carrier distributions in real and phase space of the QW was proposed to estimate the total nonradative current loss including carrier nonradiative recombination, leakage and spillover to explain the efficiency droop behaviors. Without consideration of the Auger recombination, the blue shift of the electroluminescence spectrum, light output power and efficiency droop curves as a function of injected current were all investigated and compared with the experimental data of a high brightness blue light InGaN/GaN multiple QWs light emitting diode to confirm the reliability of our theoretical hypothesis.
technology because of its higher brightness, lower power consumption, and faster response than OLED and LCD technologies. [1-5] They are expected to be suited for many applications in the brightnesssensitive or power-sensitive environments such as wearable devices, optogenetics, outdoor displays, and AR/VR. [6-13] Despite these attractive advantages, fabricating high-resolution micro-LED displays is proven to be very challenging because accurately assembling millions of micro-LEDs onto a driving circuit requires complicated transfer and bonding process. [3,14] A scalable active-driven micro-LED display device is primarily composed of two major parts: a micro-LED array, and a driver backplane. [14-20] In order to generate any display pattern, the micro-LED array must be electrically connected to the pads on the driver backplane. This is most commonly achieved by flip-chip bonding technologies such as bump bonding and anisotropic conductive film (ACF) bonding, [13] whereas in some cases it can be done using via filling or metal wiring technology. [3,4,21] The latter technology is not preferred, because for high-resolution display, both metal wires and vias occupy extra space. In the course of bonding, the small chip size may incur serious registration/alignment errors, resulting in forming display defects. Therefore, the fabrication of a micro-LED display device involves two challenging processes: i) assembling millions of micro-LED chips in a fast, accurate, and low-cost manner, and ii) reliably bonding the micro-LEDs onto the driver circuit with minimized displacement. Micro-LED arrays can be made in a monolithic manner, without using tedious pick-and-place technology. [12] In a monolithic approach, an array of micropixels is formed simultaneously on the same native substrate using only a lithography process, and all these pixels on the same substrate are then integrated onto a driver backplane via one-time flip-chip bonding. Optionally, the sapphire can be taken off by laser liftoff (LLO), [21,22] in order to suppress the optical crosstalk and beam divergence induced by the thick substrate. [23,24] Flip-chip bonding is a proven technology which is fast and compatible with wafer-level bonding. Furthermore, it can improve the light-emitting efficiency because of less light absorption caused The development of micro-sized light emitting diode (LED) displays has driven the research of micro-LED mass-transfer technology. To date, various transfer technologies are proposed, but ample room for improvements in the transfer yield and transfer accuracy still remains. Furthermore, whether these techniques are suited for the subsequent bonding process is not well investigated, which is essential for achieving a good electric connection between micro-LEDs and driver electronics. Here a systematical solution, termed as "tape-assisted laser transfer," which is not only suited for high-yield micro-LED transfer but also well compatible with subsequent bonding process, is developed. Using a low-cost adhesive tape as the support s...
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