# 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
Selectivity between TiN\({}_{\mathrm{x}}\) and the native oxide was achieved by performing the growth at 950 \({}^{\circ}\)C with an N\({}_{2}\) partial pressure of 20 mTorr. By performing a series of growth periods with varying deposition time, a time lapse of MSE-SAG nanorod growth mechanism is obtained, as shown in Figure 11. The growth stage of MSE SAG-nanorods is as follows. (1) The formation of a polycrystalline GaN wetting layer within the nanohole; (2) formation of 3D GaN nuclei inside the nanohole; (3) coarsening and coalescence of the nuclei into a single nanorod; (4) growth of a single nanorod driven by directly impinging adatoms and surface diffusion under catalyst-free conditions; and (5) quasi-equilibrium growth, where adatom diffusion is suppressed due to self-shadowing effects. By taking advantage of the well-formed facets of the nanorods during the SAG process, [PERSON] et al. has also demonstrated lasing characteristics from the nanorods, shown in Figure 12. The sample was excited using a \(\upmu\)-PL setup with a 50 \(\upmu\)W 266 nm Nd:YAG laser as the excitation source. The lasing cavity was confirmed to be based on a Fabry-Perot type, with a Q factor of 548. Calculation based on wavelength-dependent refractive index and Fabry-Perot cavity in the bottom part of Figure 12 confirms that the peak of the lasing mechanism indeed comes from the Fabry-Perot lasing mode instead of random lasing or whispering gallery mode. Despite the advantages of NSL for fabricating a large array of nanoholes with uniform diameter, the method is unable to fabricate nanoholes with precise geometric placement for applications such as photonic crystals. FIBL is an alternative nanoscale lithography technique that is faster and less expensive compared to EBL, but is capable of forming patterns with accurate control over nanoholes opening size and positions. [PERSON] et al. [108] demonstrated the first use of FIBL for MSE growth of nanorods on a Si(001) substrate with a TiN\({}_{\mathrm{x}}\) mask. An example of a \(14\times 14\) GaN nanorod array grown using a combination of MSE and FIBL-based SAG process is shown in Figure 13.  
Figure 11: Plan-view (**a–f**) and elevated-view (**g.h**) SEM micrographs showing the evolution of nanorod morphology grown using MSE on patterned substrate. (**a**) Patterned substrate with TiN\({}_{\text{x}}\) mask. (**b**) Sample after 3 min of growth, showing the formation of wetting layer. (**c**) Sample after 5 min of growth, showing transition of growth mode from 2D to 3D. (**d**) Sample after 10 min of growth, with individual nuclei forming. (**e**) Sample after 30 min of growth, showing the coalescence of nuclei. (**f,g**) Sample after 1 h of growth, showing coalescence of nuclei into a single nanorod. (**h**) Single nanorod after 2 h of growth, exhibiting specific axial growth and pencil shaped termination. Panels (**a–f**) and (**g,h**) have the same scale, respectively. (**i**) Elemental mapping of the sample using STEM-EDX after 5 min of growth [17]. Reproduced with permission from Nature Publishing Group. Figure 12: PL spectrum of the SAG GaN nanorod upon pulse laser excitation showing lasing behavior. Bottom plot shows the dispersion of wavelength dependent refractive index [17]. Reproduced with permission from Nature Publishing Group. In order to achieve a proper nanorod growth using FIBL, the patterning parameters, including ion beam milling current and milling time, must be optimized to minimize substrate damage. The effect of milling current dose and duration on the nanorod growth is summarized in Figure 14. It is found that high milling current generally results in the removal of the mask layer, resulting in a self-assembled growth mode of nanorods directly on the substrate [108]. The high milling current also results in the roughening of the substrate surface which creates a high density of tilted nanorods. Ion beam-induced damage will result in the growth of multiple and tilted nanorods inside a single nanohole opening.