# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 3 MSE Growth of GaN Thin Films Using Liquid Ga Target  
### Effect of Growth Parameters on GaN Thin Films  
The deposition process of GaN thin film using sputtering relies on a complex interplay of various process conditions. In general, the parameters that affect the growth include substrate temperature, total working pressure, partial pressure of gas composition, substrate bias, and the amount of power delivered into the sputtering target. The effects of these parameters are highlighted in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Effects** \\ \hline \multirow{2}{*}{Substrate temperature} & At higher temperature, Ga desorbs from the substrate surface, which limits the film growth rate. Higher temperature leads to better adatom mobility and overall better crystal quality [9]. \\ \hline \multirow{2}{*}{Chamber pressure} & Changes the mean free path of reactive species and concentration of gas particle. Lower pressure results in more active nitrogen species. Affects the directionality of the metal flux [38]. \\ \hline \multirow{2}{*}{Partial pressure between Ar and N\({}_{2}\)} & Determines whether the growth occurs under Ga- or N-rich conditions. Determines the sputter yield of GaN and Ga within the gas [26; 37]. \\ \hline \multirow{2}{*}{Sputter target bias} & Changes ion current and sputter yield. Affects the amount of ion damage induced on the film [39]. \\ \hline \multirow{2}{*}{Substrate bias} & Changes the ion energy and affects the interaction between the ions and surface. May enhance the adatom mobility through ion-adatom momentum transfer, but may also induce ion damage. Can be employed to selectively attract metal ions for HiPIMS system [10]. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Effect of sputtering parameter on the growth of GaN. Figure 3: Schematics showing **(a)** liquid Ga on a flat stainless steel surface, **(b,c)** liquid Ga in cylindrical stainless steel trough, and **(d)** liquid Ga in concave stainless steel trough. Photographs of **(e)** a new Ga target on a stainless steel cup and **(f)** poisoned Ga target after several rounds of deposition in a mixture of Ar and N\({}_{2}\) process gases. Number 1 denotes a relatively cleaner Ga surface, while number 2 denotes a more poisoned Ga target area where most of the sputtering happens [21]. Reproduced with permission from Linkoping University Electronic Press. By taking advantage of the high kinetic energy of the sputtered atoms to stimulate the adatom mobility, MSE growth of GaN has been demonstrated at near room temperature. However, the film is generally polycrystalline with [0001] preferred orientation [38; 40]. An increase in growth temperature typically results in better film quality in terms of reduced domain boundaries and lattice defect density and smoother film surface [9; 36; 41; 42; 43; 44]. In general, increased substrate temperature helps to increase the surface adatom mobility, which in turn results in better crystal quality. As the substrate temperature is increased, the growth mode shifts from island growth to layer by layer growth mode [42]. However, a higher growth temperature also means a higher desorption rate of Ga adatom from the surface, which would put a limit on the deposition rate for the given temperature. The growth of GaN using MSE is typically done using a mixture of Ar and N\({}_{2}\) as the process gas. Parallel to the MBE process, changing the ratio of partial gas pressure between Ar and N\({}_{2}\) will directly change the ratio of Ga and nitrogen (N+ N\({}_{2}\)) arriving on the substrate surface [26; 37]. This would then determine whether the growth process occurs under metal-rich or nitrogen-rich condition. Under nitrogen-rich condition, Ga adatom mobility is reduced, resulting in three-dimensional growth and the formation of pyramidal structures [26; 37; 41; 45]. On the other hand, growth in a metal-rich condition results in a smoother surface morphology.