# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 2 An Overview of MSE Growth of GaN
### Alternative Sputtering Target as Ga Source  
Using pure Ga as the sputtering target for the MSE process is challenging due to several conditions. The Ga target needs to have a proper container due the low melting point of metallic Ga (29 \({}^{\circ}\)C), and the formation of GaN layer on the target surface results in a drift of process condition over time. In magnetron sputter epitaxy for III-nitride growth, the sputtered group III metal normally interacts with the reactive gas particles, forming a compound material at the substrate surface. However, this reaction can occur at the target surface and deeper into the target depending on the ion energy [22]. The formation of a compound at the surface of the metal is referred to as target poisoning [23], and can be problematic if the formation rate is faster than the sputtering rate. The target poisoning can lead to a drift in the process condition due to the reduced sputtering yield and reduced yield of secondary electron. To avoid this problem, several alternative sputtering targets have been used as a Ga source.  
It is possible to grow GaN by using a GaAs target as the metal source. The incorporation of nitrogen within the process gas mixture is proven to be effective in suppressing the formation of GaAs [24]. [PERSON] et al. has grown single-phase GaN with a negligible incorporation of As in the film by using 100% nitrogen process condition at 550 \({}^{\circ}\)C. As the negative enthalpy of GaN formation is higher than that of GaAs, the formation of GaN is preferred even with GaAs as the sputtering target. [PERSON] et al. [25] controlled the formation of GaAs inside the GaN film by increasing the substrate temperature, where the film showed better crystallinity together with incorporation of As at growth above 700 \({}^{\circ}\)C.  
Al alloying the Ga metal with other group III-metal is shown to be another method to stabilize the Ga target and prevent melting during sputtering. [PERSON] et al. have demonstrated the growth of AlGaN and InGaN ternary compounds using Al\({}_{\text{x}}\)Ga\({}_{\text{1-x}}\) and In\({}_{\text{x}}\)Ga\({}_{\text{1-x}}\) metal alloy targets, respectively. By adjusting the substrate temperature, the composition of the ternary alloy can be tuned [26]. A \"pure\" GaN was claimed to be grown using an Al\({}_{\text{0.1}}\)Ga\({}_{\text{0.9}}\) alloy target, where the substrate temperature and partial gas pressure can be tweaked to modify the composition of the ternary compound in the direction of GaN. Alloyed target such as Al\({}_{\text{x}}\)Ga\({}_{\text{1-x}}\), however, shows strong phase separation at room temperature, making it challenging to control their composition.  
Cermet targets have also been shown to be suitable for growing ternary and quaternary III-nitride compounds with an adjustable composition [27; 28; 29; 30; 31]. For III-nitride cermet targets, the targets are typically fabricated by pressing together a mixture of group III metals and GaN. The amount of metals within the target mixture should be less than 50% to have a solid support within the target. By changing the composition of the cermet target, ternary films with varying composition can be obtained. In addition to modifying the target itself, the sputter power and substrate temperature can  
Figure 1: Simplified schematic of reactive sputtering process for the deposition of GaN [21]. Reproduced with permission from Linköping University Electronic Press.  
be used in order to tune the composition of the ternary and quaternary compounds [30]. However, the composition of the cermet target may vary over time after each sputtering process, which may affect the repeatability of experiments.