Using 3D nanoarchitectures to make ultrasmall LED microarrays

Nanomaterials show great promise as functional components in integrated electronic and optoelectronic devices, especially LED microarrays, because of their small size, 3D structure, and single crystallinity on a variety of substrates, including sapphire, silicon, glass, and plastic. High quality of the materials is essential to good performance in the final product. Top-down techniques, such as thin-film deposition, lithography, and etching have been used to produce thin film-based LEDs. But they require expensive single-crystal substrates, and high-quality structures can be difficult to achieve. On the other hand, bottom-up approaches based on nanometer-scale epitaxy offer high quality without problems in material compatibility. They also provide a solution to the challenges of conventional planar-type LEDs, such as high dislocation density and poor light extraction efficiency. Yet despite the successful demonstration of several different geometries of nanostructure LEDs employing these bottom-up methods,^1–3 practical application is still some way away because of difficulties in manipulating and positioning individual nanostructures. Here we describe nanoarchitectures that we recently fabricated using conventional lithography and catalyst-free metal-organic vapor phase epitaxy (MOVPE) to promote the selective growth of vertical nanotube arrays.⁴ Our results represent a significant advance toward the controlled fabrication of nanomaterials.

We propose a nanoarchitecture for LED microarrays based on coaxial nanorod and nanotube heterostructures that have quantum well layers on the cylindrical surfaces of the rods and tubes.⁵ These arrays enable us to accurately control the position, thickness, and composition of quantum structures embedded in the nanoarchitectures. In particular, position-controlled vertical arrays offer an ideal geometry for many LED applications, enhancing performance and enabling integration. Furthermore, coaxial p-n junction heterostructures have a significantly larger active area compared with thin-film heterostructures for high-brightness LEDs.

The nanoarchitecture LEDs consist of gallium nitride (GaN)-based p-n homojunction heterostructures embedding GaN/In_1−xGa_xN (indium gallium nitride) multi-quantum well (MQW) structures that are coaxially coated over the entire surface of zinc oxide (ZnO) nanotube arrays, as illustrated in Figure 1. As a template for fabricating the microarrays, we first prepared the ZnO nanotube arrays on a patterned substrate using MOVPE. That enabled us to easily regulate the dimensions of individual nanoarchitectures and the area density of the LED arrays by changing the geometric patterning parameters and growth conditions. The coaxial heterostructures clearly exhibited hexagonal facets, implying the heteroepitaxial growth of single-crystalline GaN/In_1−xGa_xN layers.

Figure 1. Schematic illustration and corresponding scanning electron microscopy (SEM) image for GaN/In_{1- x}Ga_xN/GaN/ZnO p-n homojunction coaxial nanoarchitecture heterostructure arrays. GaN: Gallium nitride. In_1-xGa_xN: Indium gallium nitride. MQW: Multi-quantum well. SiO₂: Silicon dioxide. ZnO: Zinc oxide.

We used these heterostructures to fabricate devices by forming ohmic contacts on both the p-GaN surface and the heavily doped n-GaN seed layer, where two metal contact layers were electrically isolated by filling the gaps between individual nanoarchitectures with an insulator. The individual structures of the LED microarrays readily emitted light, as shown in Figure 2, although a few did not, presumably due to failure of the contact layers. The electroluminescence (EL) color was mostly green and partly blue, and the light emission was so strong as to be visible to the unaided eye even under normal room illumination conditions.

Figure 2. Schematic illustration and photograph of light emission from nanoarchitecture LED microarrays. Ni: Nickel. Au: Gold. Ti: Titanium.

Figure 3 shows the EL spectra of nanoarchitecture LED microarrays at various applied current levels between 20 and 100mA. When an applied current was greater than 30mA, it exhibited a dominant peak centered at 2.45eV and a shoulder around 2.85eV. We tentatively attribute the dominant EL peak at 2.45eV, corresponding to the observed green emission, to the GaN/In_1−xGa_xN MQWs embedded in the nanoarchitecture heterostructures.

Figure 3. (a) Electroluminescence (EL) spectra of nanoarchitecture LED microarrays at various applied current levels of 20–100mA. T: Temperature in Kelvins.

In conclusion, we created 3D nanoarchitecture LED microarrays using position-controlled GaN/In_1−xGa_xN/GaN/ZnO coaxial heterostructures in series. The strong emissions from the LED microarrays originated from individual nanoarchitectures in the green and blue visible range. More generally, position-controlled nanoarchitecture heterostructures may be employed in making many other optoelectronic devices, including laser diodes and solar cells, as well as in integrated optoelectronic circuit applications. We are currently extending this work to microcavity LEDs for intrachip optical interconnect applications.

This work was financially supported by the National Creative Research Initiative Project (R16-2004-004-01001-0) of the Korea Science and Engineering Foundations.