# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 4 Growth of GaN Nanorods Using MSE
### Selective-Area Growth of Nanorods Using MSE  
As self-assembled nanorods nucleate and grow in a random manner, each nanorod can have variations in properties such as of length, diameter, alignment, composition, and doping concentration. This can lead to undesirable effects, such as the dispersion of electrical and optical properties. Furthermore, the variation in size would lead to reduced injection efficiency when the nanorods are fabricated into a single device [91; 92]. Selective-area growth (SAG) of GaN nanorods addresses this issue by controlling the diameter and spatial location of the nanorods, resulting in better spatial uniformity. This opens up various novel applications, such as the fabrication of photonics crystal [93], monolithic integration of multicolor LED [94; 95; 96], and controlled coalescence of nanorods [97]. In SAG mode, the growth substrate is covered with a mask material (typically Ti, SiN, or SiO\({}_{2}\)) with an array of nanoholes opening. In order to achieve successful SAG mode, there must be a good selectivity between the mask and the growth substrate, i.e., the nanorods only nucleate within the mask opening. This selectivity is driven by various factors, including the sticking coefficient, mask material, Ga adatom diffusion length, desorption rates, and the geometry of the nanoholes [98]. The  
Figure 10: (**a**) Cross-sectional high-resolution transmission electron microscope (HRTEM) images from a self-assembled GaN nanorod grown by MSE showing BSFs. Near band-edge PL spectra measured at various temperatures for a single GaN NR with (**b**) and without (**c**) BSFs. The measurement geometry is shown in the inset. Polarization angle vs. PL intensity for a single NR measured at 5 K (**d**) and at room temperature (**e**). Inset shows the excitation and detection geometry. The BSF emission (3.43 eV) and donor bound exciton (3.48 eV) are shown by circles and squares, respectively. The PL intensity at room temperature is taken at 3.42 eV [90]. Reprinted with permission from AIP publishing. selectivity between the mask and the substrate can be improved by increasing the growth process temperature, with Ga desorption rate setting the upper temperature limit. One of the most commonly used techniques for the fabrication of nanohole arrays is e-beam lithography (EBL) [99]. The technique has achieved sub-10 nm resolution for resist patterning, which is enough for most nanoscale lithography feature demands. EBL has been used for SAG patterning of III-nitride nanorods array, due to its capability to fully control the geometric features in the nanometer scale [94; 97; 100; 101; 102; 103]. However, EBL is not suitable for large-scale production due the very low throughput compared to conventional photolithography techniques. As an alternative to EBL, we have demonstrated SAG patterning and GaN nanorod growth using nanosphere lithography (NSL) and focused ion beam lithography (FIBL). NSL is a technique where a monolayer of nanospheres with diameters ranging from sub-100 nm to 1 \(\upmu\)m is deposited on a substrate surface. The nanospheres can be made of silica or polystyrene, and nanosphere deposition techniques may include spin-coating [104] or scooping [105]. After nanosphere deposition, a mask material is deposited on the substrate. Because of the shadowing effect by the nanospheres, an array of nanohole is formed. The final step is the removal of the nanospheres, either by chemical etching or by physical removal. NSL-based SAG growth of III-nitride nanorods has been demonstrated by using MOCVD, showing very high uniformity and density of nanorods [106; 107]. The first MSE-based SAG growth of GaN nanorods was demonstrated by [PERSON] et al. using a combination of 150 nm wide nanoholes array and TiN\({}_{\mathrm{x}}\) mask formed using NSL [17]. As no oxide removal was performed prior to GaN deposition, the nanorods grow directly on a layer of native oxide.