MOCVD growth mechanism of ZnO nanorods for UV emission devices
Robin THIERRY1, Guillaume PERILLAT-MERCEROZ1, 2, Pierre FERRET1,
Matthieu LAFOSSAS1, François LEVY1 and Guy FEUILLET1
1. CEA-LETI/Minatec, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
CEA-INAC, 17 rue des Martyrs, 38054 Grenoble cedex 9, France
2.
Corresponding author e-mail: pierre.ferret@cea.fr
Zinc oxide is a very promising material for optoelectronic applications due to its direct band
gap of 3.37 eV and its high exciton biding energy (60meV). This material is considered as a
challenger to GaN based LEDs for solid state lighting applications. Furthermore, nanorods enable
the use of substrates with large lattice mismatch for the growth of defect free material, by relaxing
strain on free surfaces of nanorods.
In this study, we present results on ZnO nanorods and ZnO/ZnMgO heterostructures grown by
catalyst free MOCVD in a commercial Aixtron reactor using diethylzinc and nitrous oxide as
precursors grown onto c-sapphire. SEM images indicate well-aligned ZnO nanorods with uniform
diameters, length and density, grown perpendicularly to the substrate surface with c-orientation as
confirmed by X ray diffraction measurement.
Samples prepared with various deposition times (20s, 1, 2, 3, 5min) are used to investigate the
growth mechanism. It is found out that after 20s a thin wetting layer (few nm thick) and nanorods
(around dozens of nm long) are presents. In some cases nanorods are surrounded by small
pyramids. By pursuing the deposition, we mainly observe the growth of the nanorods. TEM
measurements are carried out to gain some insight into the growth as well as the strain relaxation
mechanisms Nanorods are of Zn polarity and the surrounding pyramids are of O-polarity. Lateral
facets of the hexagonal nanorods are {1-100}, suggesting that surface energies play a role in the
nanorod growth. Strain relaxation occurs first in the wetting layer (WL) with the creation of
numerous dislocations. These dislocations bend over at the WL / pyramid interface but we do not
observe any dislocation in the nanorod itself. We shall discuss the nanorod growth mechanisms in
the light of these observations.
We also report on the growth of ZnMgO/ZnO radial heterostructure nanowires using two different
temperatures for the ZnMgO growth. It turns out that a low temperature is needed to avoid Mg
diffusion in ZnO. Furthermore, SEM images and EDX measurement have shown rich Mg clusters at
the sidewall of nanowires. Photoluminescence spectra confirm the very good crystalline quality of
both ZnO and ZnMgO/ZnO nanorods, with high intensity of the UV near-band-edge peak. Green
luminescence band usually observed for ZnO nanorods is not observed on our spectra,
demonstrating the low deep defect density.