GaN nanowires (NWs) are extensively studied due to their perfect structural characteristics in comparison with planar epitaxial layers. This allows creation of new electronic devices with improved parameters. In this work, the mechanism responsible for in-plane arrangement of self-induced GaN NWs grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(111) substrate was investigated.
We show that the process of substrate nitridation performed at various temperatures prior to the MBE growth leads to creation of thin Si-N film on Si(111) substrate. Our XRD data indicates that a coherency of in-plane orientation of NWs strongly depends on temperature Tni at which this Si-nitride film is grown.
First, the nitridation process of Si substrate was carried out at low temperature (LT). AFM measurements performed after nitridation process show that Si-N film is atomically smooth. However, GaN NWs grown on Si substrate with LT Si-N layer are twisted randomly. When the nitridation process of Si(111) substrate was carried out at high temperature (HT) post-growth AFM studies revealed formation of Si-N terraces with edges orientated along the <112> Si directions. In that case GaN NWs grown on HT Si-N film were ordered in-plane relative to Si substrate, i.e. the GaN basal <-1-120> directions were aligned with the Si <1-10> directions. This is the same epitaxial relationship between GaN and Si lattices as commonly observed in planar GaN layers grown on Si(111).
On the contrary, both SEM and XRD confirm that independently on substrate nitridation conditions NWs were grown with the c-axis being perpendicular to the substrate surface. As will be discussed, this indicates on quite different mechanisms responsible for in-plane and out-of-plane arrangements of the nanowires on the substrate.
This work was supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 NanoBiom).