# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 3 MSE Growth of GaN Thin Films Using Liquid Ga Target
### MSE Growth of Single-Crystal GaN Films Using Liquid Ga Target  
Although the growth of GaN film using sputtering has been reported from as early as 1972 [48], early works typically report the growth of polycrystalline GaN films [49; 50]. In order for MSE to compete with established epitaxial growth techniques such as MOCVD and MBE, the GaN film must have high crystallinity, an acceptable level of impurities, and contain a low number of threading dislocations. This section describes the progress for the epitaxial growth of single crystal GaN films using MSE. [PERSON] et al. [51] has epitaxially grown single-crystal GaN on (0001) sapphire substrate using DC MSE with Ar and 99.9995% pure ammonia as the working gas. Under this configuration, the growth at higher temperature (>850 \({}^{\circ}\)C) is limited by the Ga flux due to additional thermal cracking of ammonia. The growth was performed at 4 mTorr with two growth steps consisting of the initial GaN buffer layer growth and the GaN epilayer at 900 \({}^{\circ}\)C. X-ray diffraction (XRD) measurement using rocking curve and \(\omega\)-20 scan gives an FWHM of 620 and 32 arcsec, respectively, indicating high degree of crystal quality, but with low angle tilt between the crystal grains. Low-temperature PL measurement on a 1.7 \(\upmu\)m thick GaN epilayer gives a strong band-edge emission at 3.48 eV, with an FWHM of 8 meV. In this ammonia-based technique, the supply of active nitrogen species is sensitive to the substrate temperature as the temperature may affect the thermal cracking of ammonia. [PERSON] et al. [41] utilized RF MSE to produce single crystalline c-plane GaN film on (0001) sapphire. In this work, only pure N\({}_{2}\) gas is used as the working gas. By adjusting the growth temperature and total working pressure, the crystalline quality of the GaN can be improved. By using an optimized growth condition, 1.5 \(\upmu\)m thick GaN film was grown at 800 \({}^{\circ}\)C substrate temperature, 100 W RF power, and 2 mTorr total pressure. XRD rocking curve measurement gives an FWHM of 2106 arcsec while Raman spectroscopy measurement gives a E\({}_{2}\) (high) phonon mode peak width that is close to bulk single crystal GaN (3.5 cm\({}^{-1}\)). The film surface shows a faceted pyramid structure with six [10; 11; 11], which can be attributed to the limited Ga adatom diffusion under N-rich growth conditions. [PERSON] et al. [8] have demonstrated the possibility of using MSE to produce an electronic grade GaN thin film grown by DC power source and liquid Ga metal source. By using an ultra-high vacuum chamber condition coupled with ultra-pure materials, thin film growth with a growth rate of \(\sim\)1.5 A/s is obtained. The growth was performed at a substrate temperature of 700 \({}^{\circ}\)C with the substrate at floating potential (-20 V) under 2.5 mTorr Ar partial pressure (P\({}_{\rm Ar}\)) and 2 mTorr N\({}_{2}\) partial pressure (P\({}_{\rm N_{2}}\)). Ga was sputtered at 20 W for 2 min before the power was reduced to 10 W. Figure 4a shows the time of flight elastic recoil detection (ToF-ERDA) analysis of the grown GaN. From the relative amount of Ga and N, the film grows under a stoichiometric condition. Selective-area electron diffraction (SAED) pattern result in Figure 4b gives an epitaxial relationship of [11-20]GaN/[1-100]Al\({}_{2}\)O\({}_{3}\) and (0001)GaN//(0001)Al\({}_{2}\)O\({}_{3}\). The rocking curve FWHM of GaN(0002) peak measured using high-resolution XRD is 1054 arcsec, comparable to GaN layer grown directly on sapphire by HVPE and MBE [52; 53]. The low-temperature \(\upmu\)-PL result shown in Figure 4c gives a near band-edge emission at 3.474 eV with an FWHM of 6.3 meV, indicating good crystal quality. The threading dislocation density of the GaN film is estimated to be \(\sim\)\(8\times 10^{9}\) cm\({}^{-2}\). KOH etching shows that the film grows in the 0001 direction with nitrogen polarity. Figure 4: (a) Time of flight elastic recoil detection (ToF-ERDA) measurement showing the elemental profile of a \(\sim\)200 nm thick MSE-grown GaN film.