| 1. |
- Bi, Zhaoxia, et al.
(författare)
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Bottom-up approaches to microLEDs emitting red, green and blue light based on GaN nanowires and relaxed InGaN platelets
- 2023
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Ingår i: Chinese Physics B. - : IOP Publishing. - 1674-1056. ; 32
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Tidskriftsartikel (refereegranskat)abstract
- Miniaturization of light-emitting diodes (LEDs) with sizes down to a few micrometers has become a hot topic in both academia and industry due to their attractive applications on self-emissive displays for high-definition televisions, augmented/mixed realities and head-up displays, and also on optogenetics, high-speed light communication, etc. The conventional top-down technology uses dry etching to define the LED size, leading to damage to the LED side walls. Since sizes of microLEDs approach the carrier diffusion length, the damaged side walls play an important role, reducing microLED performance significantly from that of large area LEDs. In this paper, we review our efforts on realization of microLEDs by direct bottom-up growth, based on selective area metal–organic vapor phase epitaxy. The individual LEDs based on either GaN nanowires or InGaN platelets are smaller than 1 μm in our approach. Such nano-LEDs can be used as building blocks in arrays to assemble microLEDs with different sizes, avoiding the side wall damage by dry etching encountered for the top-down approach. The technology of InGaN platelets is especially interesting since InGaN quantum wells emitting red, green and blue light can be grown on such platelets with a low-level of strain by changing the indium content in the InGaN platelets. This technology is therefore very attractive for highly efficient microLEDs of three primary colors for displays.
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| 2. |
- Bi, Zhaoxia, et al.
(författare)
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From nanoLEDs to the realization of RGB-emitting microLEDs
- 2021
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Ingår i: Semiconductors and Semimetals : Micro LEDs - Micro LEDs. - : Elsevier. - 0080-8784. ; 106, s. 223-251
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Bokkapitel (refereegranskat)abstract
- MicroLED technology is expected to be the technology of choice for next-generation displays. Its advantages in various applications are reviewed. The challenges of microLED manufacturing, including red microLED realization, adequate efficiency, mass transfer, and structure and process design from a system perspective, are discussed with a special emphasis on how nano-scale material science may open new avenues toward solutions to key issues. Technical solutions to the main challenges are discussed in detail, including epitaxial growth of GaN nanowires, relaxed InGaN templates and InGaN planar red LEDs, as well as mass transfer technologies. Two primary approaches are described, one with extreme down-scaling of well-developed planar LED technology, the other where an emphasis is on the realization of nanoLEDs, on submicrometer scale, used as building blocks for all pixel sizes.
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| 3. |
- Bi, Zhaoxia, et al.
(författare)
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Realization of Ultrahigh Quality InGaN Platelets to be Used as Relaxed Templates for Red Micro-LEDs
- 2020
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Ingår i: ACS Applied Materials and Interfaces. - : NLM (Medline). - 1944-8244 .- 1944-8252. ; 12:15, s. 17845-17851
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Tidskriftsartikel (refereegranskat)abstract
- In this work, arrays of predominantly relaxed InGaN platelets with indium contents of up to 18%, free from dislocations and offering a smooth top c-plane, are presented. The InGaN platelets are grown by metal-organic vapor phase epitaxy on a dome-like InGaN surface formed by chemical mechanical polishing of InGaN pyramids defined by 6 equivalent {101̅1} planes. The dome-like surface is flattened during growth, through the formation of bunched steps, which are terminated when reaching the inclined {101̅1} planes. The continued growth takes place on the flattened top c-plane with single bilayer surface steps initiated at the six corners between the c-plane and the inclined {101̅1} planes, leading to the formation of high-quality InGaN layers. The top c-plane of the as-formed InGaN platelets can be used as a high-quality template for red micro light-emitting diodes.
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| 4. |
- Chen, I. Ju, et al.
(författare)
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Hot-Carrier Extraction in Nanowire-Nanoantenna Photovoltaic Devices
- 2020
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:6, s. 4064-4072
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Tidskriftsartikel (refereegranskat)abstract
- Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley-Queisser limit was demonstrated in hot-carrier devices based on InAs-InP-InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this Letter, we combine plasmonic nanoantennas with InAs-InP-InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon-energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of nonthermalized hot electrons when the photoexcited electron energy is above the barrier and by photothermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5-1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures.
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| 5. |
- Chen, Yang, et al.
(författare)
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Semiconductor nanowire array for transparent photovoltaic applications
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:19
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Tidskriftsartikel (refereegranskat)abstract
- The surface area of a building that could potentially be used for Building Integrated Photovoltaics would increase dramatically with the availability of transparent solar cells that could replace windows. The challenge is to capture energy from outside the visible region (UV or IR) while simultaneously allowing a high-quality observation of the outside world and transmitting sufficient light in the visible region to satisfactorily illuminate the interior of the building. In this paper, we show both computationally and experimentally that InP nanowire arrays can have good transparency in the visible region and high absorption in the near-infrared region. We show experimentally that we can achieve mean transparencies in the visible region of 65% and the radiative limit of more than 10% based on measured absorption and calculated emission. Our results demonstrate that nanowire arrays hold promise as a method to achieve transparent solar cells, which would fulfill the requirements to function as windows. In addition, we show that by optical design and by designing the geometry of nanowire arrays, solar cells can be achieved that absorb/transmit at wavelengths that are not decided by the bandgap of the material and that can be tailored to specific requirements such as colorful windows.
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| 6. |
- Colvin, Jovana, et al.
(författare)
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Local defect-enhanced anodic oxidation of reformed GaN nanowires
- 2020
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Ingår i: Physical Review Materials. - 2475-9953. ; 4:7
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Tidskriftsartikel (refereegranskat)abstract
- Understanding formation and distribution of defects in GaN substrates and device layers is needed to improve device performance in rf and power electronics. Here we utilize conductive atomic force microscopy (c-AFM) for studying defect-related leakage paths in an unintentionally doped GaN film formed by nanowire reformation. A nanoscopic Schottky contact is formed between the c-AFM probe and the GaN surface, which, under reverse-bias conditions, reveals local leakage currents at the positions of the nanowires. Cathodoluminescence shows these areas to be dominated by yellow-band luminescence, in contrast to the surrounding GaN matrix, which mainly shows near-band-gap luminescence. These results are attributed to a high density of native and residual defects, confined to the nanowires. In addition, we use anodic oxidation to map defect-related conductive paths through locally induced growth of gallium oxide. The oxide yield, which is known to depend on the local electric field strength between the AFM tip and the sample, correlates well with the level of reverse-bias leakage current. Local irregularities in oxide height reveal extended oxidation attributed to defect-related deep-level states. Thisis confirmed by controlled dissolution of the oxide in NaOH, showing that a deeper oxide film is grown over areas where defect-related conductive paths are formed. Finally, we demonstrate how this approach can be used as a quick and easy diagnostic tool for evaluating the influence of specific growth conditions and process steps on defect-induced leakage current levels and defect distribution in GaN structures, demonstrating its potential for accelerated test of leakage degradation at critical positions in GaN-based devices.
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| 7. |
- Delgado Carrascon, Rosalia, et al.
(författare)
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Optimization of GaN Nanowires Reformation Process by Metalorganic Chemical Vapor Deposition for Device-Quality GaN Templates
- 2020
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Ingår i: Physica Status Solidi (B) Basic Research. - : Wiley. - 0370-1972 .- 1521-3951. ; 257:4
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Tidskriftsartikel (refereegranskat)abstract
- Herein, the potential of reformed GaN nanowires (NWs) fabricated by metalorganic chemical vapor deposition (MOCVD) for device-quality low-defect density templates and low-cost alternative to bulk GaN substrates is demonstrated. The effects of epilayer thickness and NW reformation conditions on the crystalline quality and thermal conductivity of the subsequent GaN epilayers are investigated. Smooth surfaces with atomically step-like morphologies with no spirals are achieved for GaN epilayers on the reformed NW templates, indicating step-flow growth mode. It is further found that annealing of the NWs at a temperature of 1030 °C in the presence of NH3 and H2, followed by a coalescence done at the same temperature under planar growth conditions, leads to the most efficient screw dislocation density reduction by nearly an order of magnitude. At these optimized conditions, the growth takes place in a layer-by-layer fashion, producing a smooth surface with a root mean square (RMS) roughness of 0.12 nm. The highest thermal conductivity of k = 206 W m−1 K−1, approaching the respective value of bulk GaN, is obtained for the optimized 2 μm-thick GaN layer. The thermal conductivity results are further discussed in terms of the phonon-dislocation and the phonon-boundary scattering.
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| 8. |
- Deppert, Knut, et al.
(författare)
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Aerotaxy: High throughput gas-phase epitaxy of nanostructures
- 2020
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Konferensbidrag (refereegranskat)abstract
- Aerotaxy is an aerosol-based growth method for semiconductors and we present in detail how aerotaxy can be used to grow nanowires continuously with controlled nanoscale dimensions, with a high degree of crystallinity and remarkable throughput, including process details and our current understading of the growth processes. Catalytic size-selected Au aerosol particles travel through a heated flow-through reactor and mix with III–V precursor flux, which nucleates the growth of nanowires. We demonstrate that the method allows sensitive and reproducible control of the nanowire dimensions and shape. The reported continuous and potentially high-throughput method is expected to substantially reduce the cost of producing high-quality nanowires and may enable the low-cost fabrication of nanowire-based devices on an industrial scale.
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| 9. |
- Deppert, Knut, et al.
(författare)
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Pseudo-particle continuum modelling of nanowire growth in aerotaxy
- 2020
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Konferensbidrag (refereegranskat)abstract
- Aerotaxy growth of GaAs nanowires has shown to be capable of making GaAs wires, GaAsxP1-xwires (Metaferia, 2016) and even pn-junctions with phovovoltaic properties. With a goal of better understanding the aerotaxy process, we have developed a model for nanowire growth in Aerotaxy, based on a surface diffusion model for substrate-based growth. Our model considers the growth conditions, varying in time and space, as the seed particles and growing nanowires pass through the reactor. The growth is modeled as a chemical reaction, where gases but also particles are described as chemical species. By interpreting the continuous Au and GaAs species as wires with a length corresponding to the amount of material at that point, the particulate nature can be taken into account, without the arduous task of explicit particle modeling. Pseudo-particle modelling can potentially be used as a more general strategy for gas phase particle growth.
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| 10. |
- Duan, Xijian, et al.
(författare)
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Study of the Interfacial Oxidation of InP Quantum Dots Synthesized from Tris(dimethylamino)phosphine
- 2023
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 15:1, s. 1619-1628
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Tidskriftsartikel (refereegranskat)abstract
- InP quantum dots (QDs) are the most competitive in terms of environmentally friendly QDs. However, the synthesis of InP QDs requires breakthroughs in low-cost and safe phosphorus precursors such as tri(dimethylamino)phosphine [(DMA)3P]. It is found that even if the oxygen is completely avoided, there are still oxidation state defects at the core/shell interface of InP QDs. Herein, the record-breaking (DMA)3P-based red InP QDs were synthesized with the assist of HF processing to eliminate the InPOx defect and improve the fluorescence efficiency. The maximum photoluminescence quantum yield was 97.7%, which is the highest of the red InP QDs synthesized by the aminophosphine. The external quantum efficiency and brightness of the QD light-emitting diode device are also improved accordingly from 0.6% and 1276 cd·m-2 to 3.5% and 2355 cd·m-2, respectively.
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