Type of presentation: Poster

MS-1-P-3092 Surface dependent structure of GaN nanowires spontaneously grown on Si

Koukoula T.¹, Kehagias T.¹, Kioseoglou J.¹, Eftychis S.², Kruse J.², Georgakilas A.², Komninou P.¹

¹Physics Department, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece, ²Microelectronics Research Group, IESL, FORTH, P.O. Box 1385, GR-71110 Heraklion, and Physics Department, University of Crete, P.O. Box 2208, GR-71003 Heraklion, Greece

tkouk@auth.gr

Self-assembled GaN nanowires (NWs) were grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(111) substrates. Treatment of the substrate surface is critical for NWs growth, as well as their morphological features and crystal quality, and hence their optical properties. To this end, a transmission electron microscopy (TEM) study was performed, to compare the spontaneous nucleation of GaN NWs, when they grow on bare Si, with or without nitridation of the surface, and when they grow on top of an AlN nucleation layer (NL) of varying thickness.
Direct GaN growth on bare Si without nitridation resulted in a high density of tilted GaN NWs, grown on a thin amorphous Si_xN_y layer due to the inevitable interaction of the active N species with the Si surface. NW tilting is attributed to the roughness of the Si_xN_y layer, following the roughness of a stepped Si surface. Indeed, high-resolution TEM (HRTEM) images revealed that tilted NWs nucleated on Si_xN_y just over Si surface steps. When the surface was intentionally nitridated, prior to NWs growth, axial alignment of NWs substantially improved, owing to the formation of a thicker Si_xN_y at the GaN/Si interface, which accommodated any Si surface steps (Fig. 1). Moreover, wurtzite GaN crystalline remnants detected on Si_xN_y might have functioned as potential NW nuclei at the onset of NW growth. Besides the axial inclination of GaN NWs from the growth direction, plan-view observations showed, occasionally, a ~3^o in-plane rotation between GaN and Si. We constructed the interfacial atomistic models of a GaN NW epitaxially grown on Si, and a NW where the (0001) planes of GaN were rotated about 3^o relative to the (111) planes of Si. In both cases, the GaN/Si superlattice unit cell exhibits a hexagonal shape.
In order to optimize the Si surface treatment, an AlN NL with thickness ranging from 2 nm to 20 nm was used, either as-grown or annealed under active N, i.e., nitridated. In contrast to the previous cases, the amorphous Si_xN_y layer was eliminated from the interface allowing improved alignment and crystal quality of the GaN NWs. When using a 2 nm thick AlN nitridated NL, GaN islands appeared along with GaN NWs (Fig. 2). Conversely, a compact faceted GaN layer with sparse GaN NWs was observed over a 20 nm thick AlN NL. The latter also emerged when the 2 nm AlN NL was not nitridated, however in this case a significantly higher density of NWs was observed (Fig. 3). Therefore, at the initial stages of NL growth, AlN forms 3D islands, which during annealing evolve into a compact 2D AlN NL affecting the morphology of the NWs and the GaN faceted domains.

Research co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES, project “NanoWire”.

Fig. 1: (a) TEM image, near the [11-20]_GaN zone axis (z.a.), showing the morphology of GaN NWs grown on nitridated Si. (b) HRTEM image of the GaN/Si interface, along the [ 11-20]_GaN z.a., with GaN crystalline remnants (black arrows) on top of the amorphous Si_xN_y layer.	Fig. 2: (a) TEM image, off the [11-20]_GaN z.a., depicting the improved alignment of NWs, when they grow on top of 2 nm AlN nitridated NL. (b) HRTEM image of the GaN/AlN/Si interface, along the [11-20]_GaN z. a., showing the solid AlN NL.
Fig. 3: (a) TEM image, near the [11-20]_GaN z.a., depicting a compact faceted GaN layer when the 2 nm AlN NL was not nitridated. (b) HRTEM image, along the [11-20]_GaN z.a., showing the formation of AlN 3D islands.