ZnO is a wide-band gap (3.37 eV) II–VI semiconductor with large exciton binding energy of 60 meV which is larger than other materials commonly used for light emission devices, such as ZnSe (22 meV) and GaN (25 meV). Especially ZnO nanorods due to their unique physical and chemical properties have many important applications for devices, such as transparent electronics, sensors, detectors, etc. It is well known that wurtzite ZnO in c-plane orientation has strong polarization which significantly reduces light-emitting efficiency. Though the polarization is eliminated for nonpolar ZnO, it is difficult to grow nonpolar ZnO nanorods as they are normally grown along c-axis normal to the substrate surface. Polarization can be greatly reduced for semipolar ZnO which is likely to be obtained if nanorods can be grown with c-axis in an inclined angle with substrate surface; however, well-aligned semipolar oriented ZnO nanorods have been rarely reported. Here, we report the first observations of growth of epi-ZnO nanorods with semipolar orientation on m-sapphire by microwave-assisted chemical bath deposition (CBD) with a fast growth rate.

ZnO nanorods have been widely grown by CBD which is a simple, low cost and low temperature process but with a low growth rate generally in the range of 5-10 nm/min. Microwave-assisted CBD is a newly developed technique because of microwave can directly transmit its energy to dielectric medium by radiation, such that quick and uniform heating without much energy loss caused by conduction is feasible for aqueous solution. In recent years, microwave heating has been demonstrated to have much higher ZnO nucleation and growth rates than with conventional heating. Furthermore, a several microns thick epitaxial c-plane ZnO film can be deposited on MgAl₂O₄ (111) substrate in just a few minutes by microwave assisted CBD.

In this study, 1 x 1 cm² m-sapphire substrates were seeded with semipolar ZnO at a temperature between 500-700 °C by using mist chemical vapor deposition. The seeded substrate was then placed in a 90 °C aqueous solution for microwave-assisted CBD of ZnO nanorods. The solution was pre-formulated at room temperature with 0.0375 mol/L zinc nitrate hexahydrate and 0.0375 mol/L hexamethylenetetramine. The microwave power can be automatically tuned to maintain the solution at a constant temperature during CBD.

Microstructural characterization was performed with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) to understand the growth process. Cross-sectional TEM specimens were prepared with focused ion beam. Photoluminescence (PL) spectroscopy was performed with He-Cd laser for measurements of optical properties.

For ZnO nanorods formed after growth for 10 minutes at 90°C, SEM in Fig. 1 shows that tilted hexagonal columns of ZnO with diameter about 100-200 nm are formed as a well-aligned and inclined array on the sapphire. Figure 2 shows a typical bright field TEM micrograph in cross-section view, illustrating that the ZnO nanorods grown on the 30 nm thick seed layer have an inclined angle about 30° with m-sapphire surface and an average length about 500 nm which gives a growth rate of 50 nm/min and an aspect ratio of 2.5-5 for the nanorods. Furthermore, high-resolution TEM and atomic-resolution annular dark field STEM micrographs with FFT patterns of sapphire and ZnO as shown in Fig. 2 reveal that the semipolar ZnO seed layer has an epitaxial relationship with m-sapphire as ZnO[1-100] || sapphire[11-20] and ZnO(0001)|| sapphire(1-10-4). Hence, the c-plane of ZnO seed has a 30° tilt away from the surface normal of the sapphire, resulting in the following growth of nanorods along the c-axis direction. As a result, the inclined nanorods has a semipolar orientation of (-1-122) || sapphire m-plane. It should be pointed out that ZnO (-1-122) reflection is hardly seen in XRD as its interplanar spacing is very close to that of sapphire (3-300). A typical PL spectrum of the ZnO nanorods measured at 10K is shown in Fig. 3 from which a strong narrow near band emission (NBE) peak centered at 3.31 eV is clearly seen with and a weak deep-level emission peak at around 2.17 eV, implying that the nanorods have a reasonably good quality.

Keywords: ZnO, nanorod, chemical bath deposition microwave