Self-catalyzed GaAs nanowires

In the 21st century, one of the central concerns of technology is the production and application of efficient and miniature electronic devices. As one example, gallium-arsenide nanowires (NWs) are superior candidates for several (opto)electronic applications, e.g., solar cells. However, we are currently a few steps away from bringing the NWs into our daily lives due to incomplete developments in their engineering as well as basic science. In this thesis, the author makes contributions to necessary advancements in the synthesis of gallium-arsenide NWs on economical Si substrates. These contributions are: i) engineering of the NW growth with controlled size and position, which are required in future devices and ii) developing fundamental understandings in the integration of the NWs on Si platforms by in-situ monitoring of the NWs during synthesis.
The author demonstrates his successful attempts in controlling the growth of vertically aligned self-catalyzed gallium-arsenide NWs on Si(111) substrates. In particular, the NWs are grown by the vapor-liquid-solid mechanism within a molecular beam epitaxy system after gallium droplet positioning. On the native oxide of silicon substrates, the author could control the size and homogeneity of the NW arrays. On the thermal oxide of silicon substrates, the NW growth position could be precisely defined using a focused ion beam. The author could also offer a significant decrease in nucleation of undesired parasitic objects on the both native and thermal oxide of Si substrates. To further enhance the quality of growth, aftergrowth annealing processes were studied, which could entirely remove the parasitic growth and decrease the diameter of the NWs.
For the first time, the author could in-situ monitor the NW growth of individual NWs utilizing a unique in-situ X-ray diffraction experiment. In particular, the NWs were illuminated by a micrometer-sized X-ray beam within the synthesis chamber during growth and reverse-growth (annealing). In this way, the dynamics of NW condensation and evaporation were extracted. The author could demonstrate analysis strategies in order to identify, and in-situ characterize individual nano-objects of different dimensions (0D, 1D), sizes, crystal orientations, and structures. Accordingly, a few original findings were realized. It was found that the NW growth divides into two stages: i) axial growth at the initial stage along with angular unstablities of the NW; ii) radial growth at the second stage along with angular stabilization. The in-situ monitoring during reverse reaction growth revealed that the NWs undergo vibration/bending at decreased diameters. Eventually, the NWs can fall flat on the substrate after prolonged evaporation of the facets. Interestingly, when exposed to the initial growth conditions, the vibrations/bendings are suppressed, and the tilting is reversed. The author could identify a periodic oscillation of the NWs during both growth and reverse growth within a few 0.01°, which we hope that will be studied further in the future.