# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 5 Conclusions and Future Outlook  
In this paper, we have reviewed the progress made in MSE growth of GaN thin film and nanorods. By using a liquid Ga target combined with strict vacuum condition, MSE is able to produce high-quality, semiconductor-grade GaN films. The capability of the sputtering system for large-scale  
Figure 14: **(a)** Plan-view SEM images of GaN nanorods deposited by MSE at 950 \({}^{\circ}\)C on Si(001) substrates with TiN\({}_{\mathrm{x}}\) mask pre-patterned using 50 s milling time and various milling current. **(b)** Plan-view SEM images of GaN NRs grown at 980 \({}^{\circ}\)C on 2 pA pre-patterned substrates using varying milling time [108]. Reprinted with permission from Elsevier Publishing. Figure 13: An array of GaN nanorods grown by MSE using FIBL-based SAG process on Si(001) substrate and TiN\({}_{\mathrm{x}}\) mask. Author’s work, unpublished. production makes MSE highly relevant for the semiconductor industry. By taking advantage of high-flux low-energy ion bombardment, adatom mobility on the substrate can be enhanced, leading to improved crystallinity. A GaN film with x-ray rocking curve (0002) reflection FWHM of 1054 arcsec and threading dislocation density of the order of \(\leq 10^{10}\) cm\({}^{-2}\) grown on sapphire using DC-MSE has been demonstrated. The material quality can be improved further by using a GaN template as an MSE growth substrate, resulting in a GaN film with threading dislocation density of \(\sim\)\(1.7\times 10^{8}\) cm\({}^{-2}\). Furthermore, effective material doping using PSD has been demonstrated, with electron concentration as high as \(5.1\times 10^{20}\) cm\({}^{-3}\) for n-GaN and hole concentration of \(2.7\times 10^{18}\) cm\({}^{-3}\) for p-GaN. This doping capability enables the fabrication of various functioning devices, including LEDs and transistors. The use of HiPIMS system was also discussed. Compared to PSD, the high energy pulse used in HiPIMS enables the ionization of metal ions, resulting in momentum transfer to the adatoms without the formation of interstitials within the film. To date, the research of GaN material growth using HiPIMS is still in its infancy. The high energy ions produced during the process is shown to induce damage in the growing GaN film, resulting in the formation of GaN domains with high residual stress. Future work is needed to improve the growth condition, for example, by applying a substrate bias to act as a retardant to the ions, or adjusting the timing of the substrate bias as a filter to select which ions are allowed to reach the substrate. Beyond the growth of thin films, the growth of self-assembled and SAG GaN nanorods on various functional substrates using MSE has also been demonstrated. A range of process conditions, including substrate temperature, process gas composition, and total chamber pressure, and their effects on the growth of the nanorods, have been investigated. Under an optimized growth condition (T\({}_{\text{s}}\) = 1000 \({}^{\circ}\)C, P\({}_{\text{N}_{2}}\) = 2.5 mTorr, and P\({}_{\text{Ar}}\) = 2.5 mTorr), the nanorods can grow on Si(111) free from structural defects. In order to achieve structural uniformity between the nanorods, we employed multiple SAG methods using Si(001) substrate and TiN\({}_{\text{x}}\) mask. The first method, based on NSL, enables the growth of identical nanorods within a large area. It is found that there is a minimum substrate temperature required in order to achieve selective growth between the mask and the substrate. The second method, based on FIBL, enables the exact geometrical arrangement of nanorods. However, the ion beam current and exposure time must be tuned in order to prevent substrate and mask damage. Despite the progress in MSE-grown GaN, several issues need to be addressed. The shift of target condition from pure metal to the poisoned mode under high nitrogen partial pressure can change the deposition condition over time. Therefore, a pre-sputtering would be beneficial in order to stabilize the target condition before deposition begins.