# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 1 Introduction  
For three decades, III-nitride semiconductors including GaN, InN, AlN, and their alloys have been a subject of intensive research and development activity, resulting in the maturing of the material system and widespread adoption of III-nitride-based devices. III-nitrides can thus be found in a wide range of devices, including high-power and high-frequency field-effect transistors (FET) [1], visible and ultraviolet (UV) light-emitting diodes (LEDs) [2], and laser diodes (LDs) [3]. The tunable band-gap of ternary III-nitride enables the material system to cover a wide range of spectrum, from the UV regime to the near infrared. For commercial large-scale application, the metal-organic chemical vapor deposition (MOCVD) method is still the most widely used method to grow GaN layers due to its high throughput capabilities. However, MOCVD requires high growth temperature in order to enable the pyrolysis of the ammonia gas into active nitrogen species. This requirement results in limited substrate selection, an upper limit for material doping, and low indium incorporation due to the phase separation in ternary and quaternary systems [4]. Furthermore, the chemical vapor deposition (CVD) system requires the use of toxic gases which can be harmful to the environment and require careful handling procedures. In contrast to the MOCVD reactor, molecular-beam epitaxy (MBE) is a physical vapor deposition (PVD) process where growth proceeds in a layer-by-layer manner. The ultra-high vacuum (UHV) condition promotes the growth of high-purity GaN and the low working pressure of \(\sim\)10\({}^{-5}\) Torr enables long mean free path for the atoms inside the chamber. The metal atoms are supplied through thermal effusion using Knudsen cells, while the nitrogen plasma source in plasma-assisted MBE allows N\({}_{2}\) bonds to be broken, producing the reactive nitrogen species without the need for high growth temperature. However, the high operation cost and difficulty in scaling up the growth process makes MBE impractical for large-scale industrial application, limiting it to lab scale research. Magnetron sputter epitaxy (MSE) is a more recent addition to the PVD method that is based on the reactive sputtering process widely adopted in both engineering and semiconductor industries. The growth process during MSE takes place in a non-equilibrium condition, and the kinetic diffusion of surface adatom plays an important role during growth [5; 6]. The main advantages of MSE include mature technology, low equipment cost, and the possibility for large area deposition. Using MSE, high flux and low energy (\(\sim\)20 eV) ion bombardment can be employed to increase the crystallinity of the growing film even at low growth temperatures. In terms of environmental impact, the MSE is more environmentally friendly compared to the MOCVD as the process uses non-toxic gases, such as argon, nitrogen, and oxygen. The comparison between MOCVD, MBE, and MSE for the growth of GaN film is given in Table 1.  
Over the past decade, significant progress has been made in the growth of semiconductor-grade GaN and its compounds using MSE [7]. GaN thin films with material quality comparable to films grown using alternative methods such as hydride vapor phase epitaxy (HVPE) and MBE has been demonstrated [8; 9]. Furthermore, a novel growth GaN film method using high-power impulse magnetron sputtering (HiPIMS) has been demonstrated, with the potential to improve GaN material quality [10]. Beyond material growth, electronic and optoelectronic devices grown using MSE have been demonstrated [11; 12; 13; 14]. A more recent progress shows the possibility of using MSE to produce high-quality GaN nanorods on various cost-effective and functional substrates [15; 16; 17], allowing for the fabrication of novel devices.