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This thesis describes time-resolved terahertz spectroscopy measurements on various semiconductor nanostructures. The aim is to study the carrier dynamics in these nanostructures on a picosecond timescale. In a typical experiment carriers are excited with a visible or near-infrared pulse and by measuring the transmission of a terahertz probe pulse, the photoconductivity of the excited sample can be obtained. By changing the relative arrival time at the sample between the pump and the probe pulse, the photoconductivity dynamics can be studied on a picosecond timescale. The rst studied semiconductor nanostructures are InGaAs/GaAs quantum dots. By exciting carriers into the GaAs barrier, a fast decay of the photoconductivity after optical excitation is observed, due to trapping of charge carriers into the quantum dots. If the carriers were excited into the ground state of the quantum dots, a slow rise of the photoconductivity is observed, due the release of carriers from the quantum dots into the conducting barrier states. Secondly, the carrier dynamics in InGaN/GaN quantum wells subject to a built-in piezoelectric eld is described. An initial fast decay of the photoconductivity as the piezoelectric eld is screened by the dipole eld of the excited electron-hole pairs is observed. The decay of the photoconductivity slows down as the carriers recombine and the piezoelectric eld is restored. Finally, the photoconductivity dynamics in black silicon is described. Black silicon is microstructured silicon, which is in this case produced by laser annealing of amorphous silicon lms. The amplitude and the decay time of the photoconductivity depends strongly on the method and annealing uence used in the production process. Furthermore, it is shown that by adding copper to the black silicon, the decay time of the photoconductivity can be signicantly reduced. Besides time-resolved terahertz spectroscopy measurement, optical transmission, Raman spectroscopy, scanning electron microscope, energy dispersive X-ray, and X-ray diffraction spectroscopy experiments on black silicon are presented.
|Place of Publication||Kgs. Lyngby, Denmark|
|Publisher||Technical University of Denmark|
|Number of pages||136|
|Publication status||Published - Apr 2011|