# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 3 MSE Growth of GaN Thin Films Using Liquid Ga Target
### Doping and Fabrication of III-Nitride Devices Grown Using MSE  
In order for MSE to realize a functional GaN-based device, an efficient p-type and n-type doping scheme must be used. The growth of doped GaN using MSE has several advantages compared to using MOCVD. Due to the nonequilibrium growth condition and the relatively lower process temperature, very high carrier concentration can be achieved. As the MSE growth chamber is maintained under ultra-high vacuum environment, the concentration of carbon and hydrogen can be suppressed. Thus, p-type doping is only compensated lightly, eliminating the need for post-growth annealing for dopant activation [67]. Typically, Si and Ge are used as the dopant to achieve n-type conductivity within MSE-grown GaN [55; 57; 58]. [PERSON] et al. [55] has grown a degenerate n-GaN thin film with a carrier density as high as \(5.1\times 10^{20}\) cm\({}^{-3}\) by using Ge as the dopant using a PSD system. They have also demonstrated a Si-doped n-type GaN film which exhibits a room temperature resistivity value of as low as 0.16 m\(\Omega\) cm with electron concentration of \(3.9\times 10^{20}\) cm\({}^{-3}\) and mobility as high as 100 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\), which is almost as low as those of transparent conductive oxide (TCO) such as indium tin oxide (ITO). Under a nitrogen-rich condition, the compensation ratio is lower as the incorporation of Si or Ge at the N sites is suppressed. In addition, the concentration of Ga vacancies decreases at lower growth temperature due to the lack of energy for defect formation. P-type doping using Mg as the dopant has also been demonstrated by PSD [13; 56; 63]. [PERSON] et al. [56] obtained a highly tunable p-type carrier concentration over two orders of magnitude. A hole concentration from \(2.8\times 10^{16}\) cm\({}^{-3}\) to \(2.7\times 10^{18}\) cm\({}^{-3}\) was achieved, comparable to p-GaN grown using MOCVD. Optical characterization of the GaN shows negligible green luminescence and blue luminescence band associated with p-dopant compensation and hydrogen defects. The minimum room temperature resistivity achieved was as low as 0.6 \(\Omega\) cm, with compensation ratio as low as 5%. The superior quality is thought to arise from the suppression of nitrogen vacancies formation due to the dense nitrogen radicals contained within the plasma and the low growth temperature. In addition to MSE-growth of doped GaN using separate dopant sources, it is also possible to fabricate cermet sputtering target with the dopant directly incorporated within the target. Both GaN n-doping using Sn [68] and p-doping [69; 70; 71; 72; 73] using Mg and Zn have been demonstrated. The dopant concentration can be controlled either by varying the dopant composition inside the cermet target, or by adjusting the substrate temperature during deposition. Although this method is advantageous in  
Figure 5: Cross section TEM micrograph of a GaN sample grown by high-power impulse magnetron sputtering (HiPIMS), showing two different domains. The dashed line indicates the boundary between the less strained (**A**) and more strained (**B**) domain [10]. Reprinted with permission from AIP publishing. terms of cost, as it does not require the installation of additional effusion cell or magnetron to introduce dopants, its capability for dopant concentration control is not as flexible. This successful doping capability leads to the growth and fabrication of various electronic and optoelectronic devices grown using MSE. Devices including high electron mobility transistor (HEMT) [60], thin film transistor (TFT) [74; 75], field effect transistor (FET) [59], photovoltaic solar cell [76], and full-color LEDs [11; 12; 13; 14] has been successfully fabricated by using sputter-grown III-nitrides. The low process temperature in sputtering is also beneficial for the growth and fabrication of novel devices. It allows for the deposition of III-nitride on temperature-sensitive substrates, such as polyimide [74].