Grazing-incidence X-ray diffraction from free-standing InAs/GaAs nanorods
|Ullrich Pietsch 1, Andreas Biermanss 1, Felix Ntui Ayuk 1, Jörg Grenzer 2, Jens Bauer 3, Volker Gottschalch 3|
1. Universität Siegen, Adolf-Reichwein Str., Siegen 57068, Germany
Vertically aligned group IV and A(III)BV) nanorods (NR) typically grow by VLS mechanism onto <111>B oriented semiconductor substrates nearly independent from lattice mismatch between NR and substrate. In case of InAs NR onto GaAs this mismatch exceeds 7%. The mechanism of strain accomodation is not clear. Strain calculation in terms of a Valence-Force Field model suggest a strain energy which is 10 times smaller for epitaxy on  oriented support compared to epitaxy onto  material. However, the knowledge of strain accommodation is essential for the general understanding of the VLS growth process.
InAs NR were grown on GaAs [111B] by VLS technique using gold droplets as seed. Prior to x-ray measurements the NR were inspected by SEM showing pyramidal shape. First we measured reciprocal space maps in the vicinity of the (111) out-of plane Bragg peak of GaAs. It shows separate Bragg reflections of NR and substrate and appearance of out-of plane strain. Grazing-incidence in-plane X-ray diffraction experiments have been performed at ESRF beamline ID10b. Running in-plane longitudinal scans (Q - 2Q scans) at different ai close to the critical angle of total external reflection, ac, we measured the strain evolution as function of depth close to the interface between NR and substrate. Besides small peaks caused by scattering from Au seed we find strong zinc-blende type Bragg peaks for InAs NR and the GaAs substrate. Both indicate compressive and tensile strain components at NR and substrate, respectively. These features show the same depth dependence as the substrate reflection whereas the NR diffraction shows bulk behaviour. Our findings will be used to determine the nature of strain accommodation at the interface. In between the InAs NR and GaAs substrate peaks we find an additional peak of small intensity which can be interpreted by a thin layer made of solid solution of InAs and GaAs which has been grown prior to the NRs.
Presentation: Invited at E-MRS Fall Meeting 2007, Symposium H, by Ullrich Pietsch
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-21 17:14 Revised: 2009-06-07 00:44