Aligned AlN Nanorods with Multi‐tipped Surfaces—Growth, Field‐Emission, and Cathodoluminescence Properties

Abstract

Aluminum nitride, an important member of the group III nitrides with the highest bandgap of about 6.2 eV, has excellent thermal conductivity, good electrical resistance, low dielectric loss, high piezoelectric response, and ideal thermal expansion, matching that of silicon. The interest in field-emission (FE) applications of AlN materials has grown because they exhibit a negative electron affinity. Exhibiting a negative electron affinity means that electrons excited into the conduction band can be freely emitted into vacuum. In addition, high-current emission at a relatively low field is most attractive for FE applications. As a result, significant effort is being devoted to reducing the tip size and increasing the density of the emitting sites by using hierarchical nanostructures. Therefore, the synthesis of AlN nanostructures, such as nanowires, nanotubes, nanocones, nanotips, hierarchical comb-like structures, and nanobelts, with controlled high-aspect-ratio shapes and sizes is an important topic worthy of exploration. The promise that one-dimensional (1D) nanostructures may dramatically improve the desired properties for many applications has stimulated great enthusiasm. For example, FE properties of various AlN nanostructures have been investigated. The turn-on fields of various 1D aluminum nitride nanostructures have been measured, such as nanowires (8.8 Vlm), nanocones (12 Vlm), nanotips (3.1–4.7 V lm), and hierarchical comb-like structures (2.45–3.76 Vlm). On the other hand, reports on the luminescence properties of AlN nanostructures are scarce. The AlN nanocones have been observed to have an emission band centered at 481 nm, referred to as a deeplevel or trap-level state. In the present study, we report the growth of well-aligned AlN nanorods with hairy surfaces by a vapor–solid (VS) process. The well-aligned AlN nanorods with hairy surfaces reported here not only provide a new hierarchical nanostructure, but also serve as a promising candidate for FE emitters because of their low electron affinity and the geometry of the multiple-nanotip surfaces. Compared with previous reports on hierarchal growth of AlN nanostructures, in this communication we report a higher density of smaller nanotips (∼ 3–15 nm) that were radially grown on the surfaces of AlN nanorods. Each nanotip may serve as an ultrasmall emitter. In addition, growing well-aligned AlN nanorods on Si substrates is amenable to current technology for the fabrication of Sibased microelectronics devices. The subsequent characterization of their cathodoluminescence (CL) reveals that these hierarchical AlN nanostructures possess an intense emission peak, further suggesting potential applications in optoelectronic nanodevices. The structure of the as-grown products has been determined by X-ray diffraction (XRD). As shown in Figure 1, all of the diffraction peaks in the XRD pattern can be identified; they correspond to a hexagonal wurtzite-structured AlN crys-