ZnO is a II-VI semiconductor with a large band gap (3.4 eV) and excitonic binding energy (60 meV), which make it particularly attractive for a great diversity of potential emerging applications in the field of nanotechnologies (UV light emitters, laser diodes…). Several synthesis routes of ZnO nanostructure arrays have been reported in the literature, essentially on sapphire substrates. However, very few studies have been reported on growth of ZnO nanostructures on Si(111) substrates due to the difficulty of matching materials with different spatial symmetry group.

In this study, ZnO nanostructures have been prepared on Si(111) by plasma assisted pulsed injection MOCVD (metallorganic chemical vapor deposition). This technique of elaboration offers the advantage of large area growth and is compatible with existing semiconductor processing technologies. The key role of the plasma during material preparation is to allow lower substrate temperature (as low as 350°C), thus providing lower activation energies. It also allows in situ thermal treatment of substrates and prepared nanostructures. Moreover, the active chemical species of the plasma can help force in situ doping of the semiconducting nanostructure, which opens up a large spectrum of metastable phase formation possibilities.

We have investigated the effect of several process parameters (precursor flow rate, O2/Ar partial pressure, plasma incident power) on the morphology of the produced nanostructures. Three growth regimes have been identified: 2 dimensional very fine grain thin films, ordered hexagonal shaped plates and oriented columnar grains with a very large density corresponding to nanowire growth. These results will be presented in the light of literature data [1, 2]. The transition between these three different growth regimes is induced by a careful adjustment of the substrate temperatureand Ar/O2 plasma flowrate. The role of these process parameters will be highlighted based on gas phase OES and SEM analyses, and the growth mechanism of densely packed 1D ZnO nanostructures will be presented.

 

[1] S. Agouram, J.A. Bastos-Segura and V. Munoz-Sanjos, [http://www.sciencedirect.com/science/journal/07496036] 42, 140 (2007).

[2] J. B. Baxter and E. S. Aydil, J. Electrochem. Soc. 156 (1) H52-H58 (2009).