# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 2 An Overview of MSE Growth of GaN
### MSE System Configuration Based on Liquid Ga Target  
Although alternative targets as Ga sources exist and using a liquid Ga target is challenging, a liquid Ga-based sputtering system is still preferable due to its potential for a scalable and industrially relevant system. The use of liquid Ga target gives multiple advantages, including high growth rates, elimination of target erosion, and the possibility for continuous target supply [15]. In addition, precise control of the film composition can be achieved by using liquid Ga target. This can be done by co-sputtering and controlling the bias power delivered to each target, separately. In this section, most of the MSE chamber design considerations will be based on the MSE chamber used in the Thin Film Physics division of Linkoping University, although some design variations may exist. Variations may include the power supply choice (DC, RF, or pulsed), the magnetic configuration, the container material for liquid Ga, addition of effusion cell for doping, and chamber cooling system. A simplified diagram for a typical MSE system is shown in Figure 2a. To facilitate the liquid Ga target, the sputtering is done in a sputter-up configuration with the liquid Ga contained inside a 50 mm diameter water-cooled trough. The substrate is clamped by the substrate holder, facing downwards toward the target. During deposition, substrate rotation can be introduced to enhance uniformity across the substrate, and growth kinetics can be modified by heating the substrate and further enhanced by ion assistance through a negative substrate bias attracting ions from the sputtering plasma. The sputtering target is a very high purity liquid Ga (99.99999% pure) placed in a stainless steel cup clamped to the magnetron. The chamber pressure is controlled by adjusting a butterfly valve, which determines the total pressure during deposition. For MSE growth, ultra-pure process gas including Ar (99.999999% pure) and N\({}_{2}\) (99.99999% pure) are introduced during the sputtering process with the partial pressure of each gas controlled by mass flow controllers and a capacitance manometer. The ionization process during the sputtering is enhanced by using a combination of magnets on the back side of the target (called magnetron). The fast moving secondary electrons, generated during ion bombardment of the target, are trapped by magnetic field leading to an increase in electron density near the target surface. According to their magnetic field configuration, magnetrons can be classified as balanced, unbalanced type I, and unbalanced type II. In our system, we are using an unbalanced type II magnetron configuration, as shown in Figure 2b. In this configuration, the magnetic flux of the center pole is much weaker than that of the outer magnet, resulting in magnetic field lines extending from the magnetron toward the substrate. Secondary electrons, emitted from the sputtering target, are magnetically guided toward the substrate while ionizing the sputtering gas and, because of charge neutrality requirements they also pull the positive ions along with them. When the substrate is floating (without ground), a negative self-bias is formed due to the accumulation of electrons at the substrate surface. More ions can be attracted towards to the substrate, which can provide additional kinetic energy to the film growth through momentum transfer from the ions to the adatoms [32; 33]. By applying a proper external substrate bias (typically less than \(-100\) V), the as-grown film quality can be improved with the assistance of low-energy ions. An important aspect for sputtering, using liquid Ga target, is selecting the trough material and the design for containing liquid Ga. Due to the low melting point of Ga and the significant heat generated during the sputtering process, the Ga target will preferably be in a liquid phase during the deposition process, proper care must be taken to ensure a stable deposition condition and prevent the liquid Ga from spilling.