Berlin 2012 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 5: Quantum Dots and Wires: Preparation and Characterization I

HL 5.1: Talk

Monday, March 26, 2012, 09:30–09:45, EW 202

Phonon frequency shift in strain-engineered nanowire multi-quantum wells — •Martin Wölz, Manfred Ramsteiner, Vladimir M. Kaganer, Oliver Brandt, Lutz Geelhaar, and Henning Riechert — Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5--7, 10117 Berlin, Germany

Technologically relevant epitaxial semiconductor films are biaxially strained if the substrate has a different lattice constant. In planar layers, strain is maximum for coherent growth and can relax only through the formation of crystal defects. In contrast, axial nanowire (NW) heterostructures allow for elastic strain relaxation at the free sidewalls. By choosing appropriate heights of the NW segments, strained insertions can be grown free of defects with arbitrary strain state.

We grew self-induced GaN nanowires (NWs) containing axial (In,Ga)N heterostructures by plasma-assisted molecular beam epitaxy. By varying the heigth of the GaN barriers, we obtained (In,Ga)N insertions with different strain states between the pseudomorphic case and full elastic relaxation.

Linear elastic theory reveals a progressive relaxation from the center of the NW to the sidewall. The resulting broadening of phonon spectra is quantified based on published deformation potentials. We show that resonant Raman spectroscopy can be used to determine the variation of the (In,Ga)N strain state in NW segments.

Finally, we discuss the impact of such strain relaxation in the active region of (In,Ga)N NW light-emitting diodes, where a benefit over conventional planar devices is expected.