Facile Formation of High-Quality InGaN/GaN Quantum-Disks-in-Nanowires on Bulk-Metal Substrates for High-Power Light-Emitters.

High-quality nitride materials grown on scalable and low-cost metallic substrates are considerably attractive for high-power light-emitters. We demonstrate here, for the first time, the high-power red (705 nm) InGaN/GaN quantum-disks (Qdisks)-in-nanowire light-emitting diodes (LEDs) self-assembled directly on metal-substrates. The LEDs exhibited a low turn-on voltage of ∼2 V without efficiency droop up to injection current of 500 mA (1.6 kA/cm(2)) at ∼5 V. This is achieved through the direct growth and optimization of high-quality nanowires on titanium (Ti) coated bulk polycrystalline-molybdenum (Mo) substrates. We performed extensive studies on the growth mechanisms, obtained high-crystal-quality nanowires, and confirmed the epitaxial relationship between the cubic titanium nitride (TiN) transition layer and the hexagonal nanowires. The growth of nanowires on all-metal stack of TiN/Ti/Mo enables simultaneous implementation of n-metal contact, reflector, and heat sink, which greatly simplifies the fabrication process of high-power light-emitters. Our work ushers in a practical platform for high-power nanowires light-emitters, providing versatile solutions for multiple cross-disciplinary applications that are greatly enhanced by leveraging on the chemical stability of nitride materials, large specific surface of nanowires, chemical lift-off ready layer structures, and reusable Mo substrates.

Droop-Free, Reliable, and High-Power InGaN/GaN Nanowire Light-Emitting Diodes for Monolithic Metal-Optoelectronics.

A droop-free nitride light-emitting diode (LED) with the capacity to operate beyond the "green gap" has been a subject of intense scientific and engineering interest. While several properties of nanowires on silicon make them promising for use in LED development, the high aspect ratio of individual nanowires and their laterally discontinuous features limit phonon transport and device performance. Here, we report on the monolithic integration of metal heat-sink and droop-free InGaN/GaN quantum-disks-in-nanowire LEDs emitting at ∼710 nm. The reliable operation of our uncooled nanowire-LEDs (NW-LEDs) epitaxially grown on molybdenum was evident in the constant-current soft burn-in performed on a 380 µm × 380 µm LED. The square LED sustained 600 mA electrical stress over an 8 h period, providing stable light output at maturity without catastrophic failure. The absence of carrier and phonon transport barriers in NW-LEDs was further inferred from current-dependent Raman measurements (up to 700 mA), which revealed the low self-heating. The radiative recombination rates of NW-LEDs between room temperature and 40 °C was not limited by Shockley-Read-Hall recombination, Auger recombination, or carrier leakage mechanisms, thus realizing droop-free operation. The discovery of reliable, droop-free devices constitutes significant progress toward the development of nanowires for practical applications. Our monolithic approach realized a high-performance device that will revolutionize the way high power, low-junction-temperature LED lamps are manufactured for solid-state lighting and for applications in high-temperature harsh environment.

Ultrabroad linewidth orange-emitting nanowires LED for high CRI laser-based white lighting and gigahertz communications.

Group-III-nitride laser diode (LD)-based solid-state lighting device has been demonstrated to be droop-free compared to light-emitting diodes (LEDs), and highly energy-efficient compared to that of the traditional incandescent and fluorescent white light systems. The YAG:Ce3+ phosphor used in LD-based solid-state lighting, however, is associated with rapid degradation issue. An alternate phosphor/LD architecture, which is capable of sustaining high temperature, high power density, while still intensity- and bandwidth-tunable for high color-quality remained unexplored. In this paper, we present for the first time, the proof-of-concept of the generation of high-quality white light using an InGaN-based orange nanowires (NWs) LED grown on silicon, in conjunction with a blue LD, and in place of the compound-phosphor. By changing the relative intensities of the ultrabroad linewidth orange and narrow-linewidth blue components, our LED/LD device architecture achieved correlated color temperature (CCT) ranging from 3000 K to above 6000K with color rendering index (CRI) values reaching 83.1, a value unsurpassed by the YAG-phosphor/blue-LD counterpart. The white-light wireless communications was implemented using the blue LD through on-off keying (OOK) modulation to obtain a data rate of 1.06 Gbps. We therefore achieved the best of both worlds when orange-emitting NWs LED are utilized as "active-phosphor", while blue LD is used for both color mixing and optical wireless communications.

Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates.

Growing III-nitride nanowires on 2D materials is advantageous, as it effectively decouples the underlying growth substrate from the properties of the nanowires. As a relatively new family of 2D materials, MXenes are promising candidates as III-nitride nanowire nucleation layers capable of providing simultaneous transparency and conductivity. In this work, we demonstrate the direct epitaxial growth of GaN nanowires on Ti3C2 MXene films. The MXene films consist of nanoflakes spray coated onto an amorphous silica substrate. We observed an epitaxial relationship between the GaN nanowires and the MXene nanoflakes due to the compatibility between the triangular lattice of Ti3C2 MXene and the hexagonal structure of wurtzite GaN. The GaN nanowires on MXene show good material quality and partial transparency at visible wavelengths. Nanoscale electrical characterization using conductive atomic force microscopy reveals a Schottky barrier height of ∼330 meV between the GaN nanowire and the Ti3C2 MXene film. Our work highlights the potential of using MXene as a transparent and conductive preorienting nucleation layer for high-quality GaN growth on amorphous substrates.

Direct Growth of Single Crystalline GaN Nanowires on Indium Tin Oxide-Coated Silica.

In this work, we demonstrated the direct growth of GaN nanowires on indium tin oxide (ITO)-coated fused silica substrate. The nanowires were grown catalyst-free using plasma-assisted molecular beam epitaxy (PA-MBE). The effect of growth condition on the morphology and quality of the nanowires is systematically investigated. Structural characterization indicates that the nanowires grow in the (0001) direction directly on top of the ITO layer perpendicular to the substrate plane. Optical characterization of the nanowires shows that yellow luminescence is absent from the nanowire's photoluminescence response, attributed to the low number of defects. Conductive atomic force microscopy (C-AFM) measurement on n-doped GaN nanowires shows good conductivity for individual nanowires, which confirms the potential of using this platform for novel device applications. By using a relatively low-temperature growth process, we were able to successfully grow high-quality single-crystal GaN material without the degradation of the underlying ITO layer.

Direct Growth of III-Nitride Nanowire-Based Yellow Light-Emitting Diode on Amorphous Quartz Using Thin Ti Interlayer.

Consumer electronics have increasingly relied on ultra-thin glass screen due to its transparency, scalability, and cost. In particular, display technology relies on integrating light-emitting diodes with display panel as a source for backlighting. In this study, we undertook the challenge of integrating light emitters onto amorphous quartz by demonstrating the direct growth and fabrication of a III-nitride nanowire-based light-emitting diode. The proof-of-concept device exhibits a low turn-on voltage of 2.6 V, on an amorphous quartz substrate. We achieved ~ 40% transparency across the visible wavelength while maintaining electrical conductivity by employing a TiN/Ti interlayer on quartz as a translucent conducting layer. The nanowire-on-quartz LED emits a broad linewidth spectrum of light centered at true yellow color (~ 590 nm), an important wavelength bridging the green-gap in solid-state lighting technology, with significantly less strain and dislocations compared to conventional planar quantum well nitride structures. Our endeavor highlighted the feasibility of fabricating III-nitride optoelectronic device on a scalable amorphous substrate through facile growth and fabrication steps. For practical demonstration, we demonstrated tunable correlated color temperature white light, leveraging on the broadly tunable nanowire spectral characteristics across red-amber-yellow color regime.

An enhanced surface passivation effect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley-Read-Hall recombination.

We present a detailed study of the effects of dangling bond passivation and the comparison of different sulfide passivation processes on the properties of InGaN/GaN quantum-disk (Qdisk)-in-nanowire based light emitting diodes (NW-LEDs). Our results demonstrated the first organic sulfide passivation process for nitride nanowires (NWs). The results from Raman spectroscopy, photoluminescence (PL) measurements, and X-ray photoelectron spectroscopy (XPS) showed that octadecylthiol (ODT) effectively passivated the surface states, and altered the surface dynamic charge, and thereby recovered the band-edge emission. The effectiveness of the process with passivation duration was also studied. Moreover, we also compared the electro-optical performance of NW-LEDs emitting at green wavelength before and after ODT passivation. We have shown that the Shockley-Read-Hall (SRH) non-radiative recombination of NW-LEDs can be greatly reduced after passivation by ODT, which led to a much faster increasing trend of quantum efficiency and higher peak efficiency. Our results highlighted the possibility of employing this technique to further design and produce high performance NW-LEDs and NW-lasers.