|Title:||Lattice expansion in optically excited InAs/GaAs quantum dots|
|Abstract:||InAs quantum dots are crystalline inclusions on the scale of few nanometers that are buried in a GaAs embedding matrix representing zero-dimensional semiconductor heterostructures. Charge carriers residing in quantum dots are strongly confined giving rise to discrete electronic energy levels similar to that of atoms. InAs quantum dots residing in a GaAs matrix do not represent independent entities, but are coupled elastically to the surrounding lattice. As a consequence of this, charge carrier-phonon interactions taking place in quantum dots have an impact on the characteristics of the crystal lattice of both dots and embedding matrix. Indeed, several experimental works performed by high-resolution continuous wave and non-linear time-resolved optical spectroscopy e. g. four-wave mixing technique have shown evidence for a broadband in the optical excitation spectra. The origin of the broadband signal was proposed to be related to a charge carrier-acoustic phonon interaction that distorted the lattice resulting in a shift of the equilibrium lattice constant. Investigating optically excited quantum dots by means of a technique that probes the crystal lattice should provide direct evidence for this effect. Based on this idea, a high-resolution XRD study on optically excited InAs/GaAs quantum dots and, serving as a reference, bulk GaAs was realized in this work. For this purpose, a liquid helium continuous flow cryostat set-up for high-resolution XRD experiments on laser irradiated samples was developed. High-resolution XRD measurements were conducted by means of a laser shutter system, which allowed to measure XRD curves of the optical active and non-active sample within a single scan, permitting the detection of relative lattice deviations with a precision on the order of 2E-6. For bulk GaAs, isotropic lattice expansion was detected that was due to laser heating of the lattice, as confirmed by comparison with calculations based on the thermal expansion coefficient of GaAs. In contrast, the InAs quantum dot sample was characterized by an anisotropic lattice expansion that extended at least 4 μm into the GaAs lattice beneath the quantum dot structure. While the determined in-plane lattice expansion was comparable to the values found for bulk GaAs, the out-of-plane expansion was significantly enhanced. This result was attributed to a polaron induced tetragonal lattice distortion that superimposed the isotropic thermal expansion.|
|Subject Headings:||Electron phonon interaction|
|Appears in Collections:||Experimentelle Physik I|
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