# 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  
The self-assembled growth of GaN nanorods is attractive as it is capable of producing a large number of nanorods with high crystal quality without the need of complex growth preparation. Interfacial studies of GaN nanorods grown on silicon shows that the nanorods grow on a thin layer of amorphous silicon nitride (SiN\({}_{\text{x}}\)), indicating that the nanorods can grow even with no epitaxial relationship with the substrate [81; 82]. The main driving force for spontaneous formation of GaN nanorods is the anisotropy of sticking coefficient and chemical potential between the various GaN facets during the nanorod formation. This results in the self-assembled nanorods growing in the 0001 direction. The model for spontaneous nanorod growth under nitrogen-rich condition has been proposed by [PERSON] et al. and [PERSON] et al. [83; 84] for MBE-grown GaN nanorods. The spontaneous growth of nanorods follows the Volmer-Weber growth mechanism, where the nanorods grow spontaneously without the formation of wetting layer prior to 3D growth. The nanorod growth begins with the nucleation of GaN islands by the impinging adatoms. During the nucleation process, the number of islands on the substrate surface increases until the saturation of nucleation sites occurs. To be energetically stable, the dimension of the islands must be bigger than a certain critical size. Islands with dimension below the critical size will decay either through desorption or diffusion to other islands. On the other hand, island above the critical size are considered stable and will continue growing by incorporating diffusion atoms. Under heavily nitrogen-rich condition, the coalescence of the islands is blocked due to the preferential incorporation of metal atoms on the top side of the island, leading to vertical growth. The 3D growth of nanorod then occurs through two main mechanisms: (i) direct incorporation of Ga atoms impinging on the apex and (ii) diffusion of Ga atoms arriving on either the substrate surface or the nanorod's sidewalls towards the top. As the (0001) top surface is more energetically favorable compared to the nonpolar sidewalls, growth in the axial direction is faster than in the lateral direction. [PERSON] et al. [15] have demonstrated the first growth of self-assembled GaN nanorods using MSE. The GaN nanorods were grown on a Si(111) substrate. Under MSE growth condition, the N\({}_{2}\)-containing plasma is enough to supply the nitrogen-rich condition required to drive vertical nanorod growth without the need of other nitrogen sources such as ammonia or atomic nitrogen. From the XRD pole figure measurement, a GaN (0001)//Si(111) and GaN[11-20]//Si[110] epitaxial relationship can be observed. The density, length, and diameter of MSE-grown self-assembled nanorods are controlled by the growth conditions, including the substrate temperature, chamber pressure, and partial gas pressures P\({}_{\text{Ar}}\) and P\({}_{\text{N}_{2}}\)[15; 85]. The effect of the MSE working pressure on the nanorod growth is investigated by changing the pressure from 5 mTorr to 20 mTorr under a purely N\({}_{2}\) gas ambient. When operating at a low pressureof 5 mTorr, the Ga flux is relatively high and unidirectional. The sputtered Ga atoms have longer mean free path, allowing them to retain their kinetic energy, which is typically a few eV. This leads to growth of nanorods with a relatively larger diameter from the nucleation stage. As the pressure is increased, gas phase scattering results in a more diffuse Ga flux, with part of the Ga atoms lost to the chamber walls. The reduced Ga flux and increased nitrogen saturation results in smaller nucleation size, and eventually smaller nanorod diameters. The low-temperature micro-photoluminescence (\(\mu\)-PL) spectra of the nanorods grown at different pressure are shown in Figure 8. The sample grown at 5 mTorr shows a broad spectrum with a band edge luminescence at 3.475 eV and a line width of \(\sim\)35 meV.