In recent years, GaN nanorods are emerging as a very promising novel route toward devices for nano-optoelectronics and nano-photonics. In particular, core-shell light emitting devices are thought to be a breakthrough development in solid state lighting, nanorod based LEDs have many potential advantages as compared to their 2 D thin film counterparts. In this paper, we review the recent developments of GaN nanorod growth, characterization, and related device applications based on GaN nanorods. The initial work on GaN nanorod growth focused on catalyst-assisted and catalyst-free statistical growth. The growth condition and growth mechanisms were extensively investigated and discussed. Doping of GaN nanorods, especially p-doping, was found to significantly influence the morphology of GaN nanorods. The large surface of 3 D GaN nanorods induces new optical and electrical properties, which normally can be neglected in layered structures. Recently, more controlled selective area growth of GaN nanorods was realized using patterned substrates both by metalorganic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Advanced structures, for example, photonic crystals and DBRs are meanwhile integrated in GaN nanorod structures. Based on the work of growth and characterization of GaN nanorods, GaN nanoLEDs were reported by several groups with different growth and processing methods. Core/shell nanoLED structures were also demonstrated, which could be potentially useful for future high efficient LED structures. In this paper, we will discuss recent developments in GaN nanorod technology, focusing on the potential advantages, but also discussing problems and open questions, which may impose obstacles during the future development of a GaN nanorod based LED technology.

GaN based nanorods for solid state lighting

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {1100} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1–8 nm) and In-composition in the wells (∼16%). The core–shell MQW light emission characteristics (390–420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.

M-Plane Core-Shell InGaN/GaN Multiple-Quantum-Wells on GaN Wires for Electroluminescent Devices

III-Nitride nanowire optoelectronics

III–V nanowires (NWs) have been envisioned as nanoscale materials for next-generation technology with good functionality, superior performance, high integration ability and low cost, because of their special growth modes and unique 1D structure. In this review, we summarize the main challenges and important progress of the fabrication and applications of III–V NWs. We start with the III–V NW growth, that significantly influences the NW morphology and crystal quality. Attention is then given to the fabrication of some advanced III–V structures composed of axial and radial junctions. After that, we review the advantages, challenges, and major breakthroughs of using III–V NWs as solar energy harvesters and light emitters. Finally, we attempt to give a perspective look on the future development trends and the remaining challenges in the research field of III–V NWs.

https://iopscience.iop.org/article/10.1088/0022-3727/48/46/463001/pdf

III–V nanowires and nanowire optoelectronic devices

Uniform GaN nanorod arrays are grown vertically by selective area growth on (left angle bracket 0001 right angle bracket) substrates. The GaN nanorods present six nonpolar {1⁻100} facets, which serve as growth surfaces for InGaN-based light-emitting diode quantum well active regions. Compared to growth on the polar {0001} plane, the piezoelectric fields in the multiple quantum wells (MQWs) can be eliminated when they are grown on nonpolar planes. The capability of growing ordered GaN nanorod arrays with different rod densities is demonstrated. Light emission from InGaN/GaN MQWs grown on the nonpolar facets is investigated by photoluminescence. Local emission from MQWs grown on different regions of GaN nanorods is studied by cathodoluminescence (CL). The core-shell structure of MQWs grown on GaN nanorods is investigated by cross-sectional transmission electron microscopy in both axial and radial directions. The results show that the active MQWs are predominantly grown on nonpolar planes of GaN nanorods, consistent with the observations from CL. The results suggest that GaN nanorod arrays are suitable growth templates for efficient light-emitting diodes.

InGaN/GaN Multiple Quantum Wells Grown on Nonpolar Facets of Vertical GaN Nanorod Arrays

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN From the macroscopic point of view, the devices emit light in the green spectral range (around 550?nm) under electrical injection At 100?mA DC current, a 1?mm2 chip that integrates around 107 nanowires emits an output power on the order of 10??W However, the emission of the nanowire-based LED shows a spotty and polychromatic emission By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire Detailed ?-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation By combining both ?-electroluminescent and ?-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs

http://iopscience.iop.org/article/10.1088/0957-4484/22/34/345705/pdf

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

A report is presented on the fabrication of light emitting diodes (LEDs) based on GaN core/shell wires on conductive substrates by metal organic vapour phase epitaxy. Catalyst-free GaN-based wires are grown spontaneously on 2-inch n-doped silicon substrates without any thick buffer layer. The LED wire heterostructure consists of an n-type GaN:Si core covered radially by five InGaN/GaN quantum wells and a p-type GaN:Mg shell. Macroscopic devices that integrate around 10 6 wire-LEDs have been obtained thanks to a simple, direct and full-wafer scale contacting process. For the first time, continuous-wave electrical injection at room temperature through the Si substrate into a cm 2 -chip of GaN-based core/shell wire-LEDs is successfully demonstrated, producing blue electroluminescent emission at 450 nm.

Light emitting diodes based on GaN core/ shell wires grown by MOVPE on n-type Si substrate

We studied and improved gallium nitride (GaN) nanowire (NW) based light emitting diodes (LEDs). PIN nanodiodes with and without InGaN/GaN multiple quantum wells (MQWs) were grown by molecular beam epitaxy (MBE) under N-rich conditions on n-doped Si(111) substrates. Thanks to the coalescence of the p-type region of the NWs grown at low temperature, an autoplanarization process has been performed to obtain LEDs. Ni/Au top contacts have been deposited and patterned in order to bias the devices. A multiple-scale characterization approach has been carried out through the comparison of localized cathodoluminescence (CL) and macroscopic electroluminescence (EL) spectra. It shows that the EL emission of PIN-based LED at room temperature is related to defects in the p-type region of the NWs. In order to enhance the radiative recombinations of NW-based LEDs, we have first added InGaN/GaN MQWs, and secondly an electron blocking layer (EBL) has been inserted between the MQWs and the p-type zone of the NWs. The LED with EBL exhibited an emission band at 420 nm. The blue-shift of this emission band with increasing injected current is attributed to quantum confined Stark effect (QCSE) and evidences the radiative emission of InGaN/GaN MQWs. At 50 mA dc current, this improved NW-based LED emits about 500 times more light than the heterostructure without EBL. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

B. André

Papers

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

Light emitting diodes based on GaN core/ shell wires grown by MOVPE on n-type Si substrate

GaN-based nanowires: From nanometric-scale characterization to light emitting diodes