# Review of GaN Thin Film and Nanorod Growth Using Magnetron Sputter Epitaxy
## 4 Growth of GaN Nanorods Using MSE
### Self-Assembled GaN Nanorods Grown Using MSE
At increased pressure, several lines associated with free A exciton recombination (X\({}_{\text{A}}\)), free B exciton recombination (X\({}_{\text{B}}\)), and neutral donor bound exciton transitions (D\({}^{0}\)X\({}_{\text{A}}\)) and (D\({}^{0}\)X\({}_{\text{B}}\)) can be seen. The excitonic lines become more visible with increasing N\({}_{2}\) pressure, indicating an increase in crystal quality and enhanced optical properties. The full-width at half maximum (FWHM) of the D\({}^{0}\)X\({}_{\text{A}}\) line of the GaN nanorods grown at 20 mTorr is as narrow as \(\sim\)1.7 meV, which is comparable with a high-quality CVD-grown 2 \(\mu\)m thick GaN layer (\(\sim\)0.5 meV) [86], further demonstrating the high crystal quality of MSE-grown nanorods. [PERSON] et al. [85] has also shown that the dilution of the N\({}_{2}\) gas with Ar affects the growth condition. In their experiment, nanorods are grown on a Si(111) substrate at 1000 \({}^{\circ}\)C while P\({}_{\text{N}_{2}}\) and P\({}_{\text{Ar}}\) is varied from P\({}_{\text{N}_{2}}\) = 5 mTorr and P\({}_{\text{Ar}}\) = 0 mTorr to P\({}_{\text{N}_{2}}\) = 1 mTorr and P\({}_{\text{Ar}}\) = 4 mTorr. As shown in Figure 9, the introduction of Ar (P\({}_{\text{N}_{2}}\) = 4.5 mTorr and P\({}_{\text{Ar}}\) = 0.5 mTorr) leads to an increase of nanorod nucleation density and increased growth rate. This is caused by improved metal sputtering yield by enabling momentum transfer from the sputter gas and also by preventing target poisoning. However, further dilution by Ar will result in the transition from nitrogen-rich to metal-rich condition, reducing the diffusion driven growth mechanism. This is indicated by the reduction of average rod length and increase of average rod diameter. Increasing P\({}_{\text{Ar}}\) beyond 2.5 mTorr will result in the formation of continuous GaN film with large columnar grains instead of well-separated nanorods. Figure 8: PL spectra from MSE-grown GaN nanorods on Si(111) under purely N\({}_{2}\) ambient with varying chamber pressures [15]. Reproduced with permission from Elsevier Publishing. The ratio of \(\mathrm{P_{N_{2}}}\) and \(\mathrm{P_{Ar}}\) also affects the formation of structural imperfections within the nanorods. Under different partial pressures, a PL peak at \(\sim\)3.43 eV associated with basal plane stacking faults (BSFs) can be observed [85; 87]. BSFs can be understood as the insertion of cubic zincblende structure inside the wurtzite matrix of GaN nanorods. Currently, the mechanism that results in the formation of BSFs is still debatable. [PERSON] et al. [88] attributed their presence in MBE-grown nanorods to process instabilities during the growth process. BSFs were also observed in CVD-grown GaN nanowires using Au as catalyst, where their formation is attributed to variations in the supersaturation during growth [89]. Generally, the formation of BSFs is not desirable, as it reduces the quantum efficiency by introducing type II multiple quantum well (MQW)-like structure within the nanorod structure. The BSF-related PL emission spectra are shown in Figure 10. This emission shows linear polarization, with almost 50% degree of polarization [90]. By using an optimized growth condition (\(\mathrm{P_{N_{2}}}\) = 2.5 mTorr and \(\mathrm{P_{Ar}}\) = 2.5 mTorr), [PERSON] et al. has grown nanorods free of BSFs, which confirms that BSFs can be eliminated under proper growth condition. Figure 9: SEM micrographs showing cross section (left) and plan views (right) from self-assembled GaN nanorod samples grown by MSE at **(a,b)**\(\mathrm{P_{N_{2}}}\) = 5 mTorr, **(c,d)**\(\mathrm{P_{N_{2}}}\) = 4.5 mTorr, **(e,f)**\(\mathrm{P_{N_{2}}}\) = 3.5 mTorr, **(g,h)**\(\mathrm{P_{N_{2}}}\) = 2.5 mTorr, and **(i,j)**\(\mathrm{P_{N_{2}}}\) = 1 mTorr pressure on Si(111) substrates [85]. Reproduced with permission from MDPI publishing. Taking advantage of the capability of GaN nanorods to relief strain, [PERSON] et al.