Investigation and comparison of GaN nanowire nucleation and growth by the catalyst-assisted and self-induced approaches

This work focuses on the nucleation and growth mechanisms of GaN nanowires (NWs) by molecular beam epitaxy (MBE). The two main novelties of this study are the intensive employment of in-situ techniques and the direct comparison of selfinduced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE without the use of any external catalyst seed. On sapphire, in contrast, NWs grow under identical conditions only in the presence of Ni seeds. NW nucleation was studied in situ by reflection high-energy electron diffraction (RHEED) in correlation with line-of-sight quadrupole mass spectrometry (QMS). The latter technique allows to monitor the incorporated amount of Ga. For the catalyst-assisted approach, three nucleation stages were identified: first incorporation of Ga into the Ni seeds, second transformation of the seed crystal structure due to Ga accumulation, and last GaN growth under the seeds. The crystalline structure of the seeds during the first two stages is in accord with the Ni-Ga binary phase diagram and evidenced that only Ga incorporates into the Ni particles. GaN forms only after the Ga concentration is larger than the one of Ni, which is in agreement with the Ni-Ga-N ternary phase diagram. The observation of diffraction patterns generated by the Ni-Ga seed particles during the whole nucleation evidences the solid state of the seeds. Therefore nucleation is ruled by the vapor-solid-solid mechanism. Moreover, the QMS study showed that it is not Ga incorporation into Ni but GaN nucleation itself that limits the growth processes. For the self-induced NWs, QMS and RHEED investigations indicate very similar nucleation processes on Si(001) and Si(111) and two nucleation stages were identified. Transmission electron microscopy on samples grown on Si(001) revealed that the first stage is characterized by the competition between the nucleation of crystalline SixNy and GaN. During this stage, the Si surface strongly roughens by the formation of pits and Si mounds. At the same time, very few GaN islands nucleate. During the second stage, the amorphization of the SixNy layer leads to the massive nucleation of GaN islands that are free of the substrate lattice constraint and therefore form in the wurtzite (WZ) structure. The processes leading to NW nucleation are fundamentally different for both approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst Ni particles whose crystal structure and phases are decisive for the NW growth. In the catalyst-free approach, N forms an interfacial layer with Si before the intense nucleation of GaN starts, and the lattice-mismatch to the substrate plays the most important role. Both approaches are viable to produce NWs within the same range of substrate temperatures and V/III ratios, provided the latter is larger than one (N-excess). Both yield monocrystalline GaN NWs of WZ structure, which grow in the Ga-polar direction. However, strong differences are also observed. First, the catalyst-assisted NWs are longer than the catalyst-free ones after growth under identical conditions (duration, substrate temperature and V/III ratio), and the former grow at the rate of the supplied N. This observation can be explained by the local Ga-excess established at the Ni-particle position. Therefore, this result is in good agreement with the catalyst-assisted nucleation model described above. In contrast, the selfinduced NWs grow with an intermediate rate between the supplied Gaand Nrates. Second, the catalyst-assisted approach provides GaN NWs that contain many