Time-Resolved Kelvin Probe Force Microscopy of Nanostructured Devices

Research output: Types of thesisDoctoral thesis

Contributors

Abstract

Since its inception a quarter of a century ago, Kelvin probe force microscopy (KPFM) has enabled studying contact potential differences (CPDs) on the nanometre scale. However, current KPFM investigations are limited by the bandwidth of its constituent electronic loops to the millisecond regime. To overcome this limitation, pump-probe-driven Kelvin probe force microscopy (pp-KPFM) is introduced\nthat exploits the non-linear electric interaction between tip and sample. The time resolution surpasses the electronic bandwidth and is limited by the length of the probe pulse. In this work, probe pulse lengths as small as 4.5 ns have been realized.\nThese probe pulses can be synchronized to any kind of pump pulses. The first system\ninvestigated with pp-KPFM is an electrically-driven organic field-effect transistor\n(OFET). Here, charge carrier propagation in the OFET channel upon switching the drain-source voltage is directly observed and compared to simulations based on a transmission line model. Varying the charge carrier density reveals the impeding influence of Schottky barriers on the maximum switching frequency.\nThe second system is an optically-modulated silicon homojunction. Here, the speed of surface photovoltage (SPV) build-up is accessed and compared to timeaveraged results. Due to slow trap states, the time-averaged method is found to lack comprehensiveness. In contrast, pp-KPFM exposes two intensity-dependent\nrecombination times on the same timescale — high-level Shockley-Read-Hall recombination in the bulk and heat-dominated recombination in the surface layer — and a delay of the SPV decay with rising frequency, which is attributed to charge carrier retention at nanocrystals.\nThe third system is a DCV5T-Me:C60 bulk heterojunction. The SPV dynamics is probed and compared to measurements via open-circuit corrected transient charge carrier extraction by linearly increasing voltage. Both methods reveal an exponential onset of the band bending reduction that is attributed to the charge carrier diffusion time in DCV5T-Me, and a double exponential decay, hinting at different recombination paths in the studied organic solar cell.\nThe above-mentioned experiments demonstrate that pp-KPFM surpasses conventional KPFM when it comes to extracting dynamic device parameters such as charge carrier retention and recombination times, and prove that pp-KPFM is a versatile and reliable tool for studying electrodynamics on nanosurfaces.

Details

Original languageEnglish
Qualification levelDr. rer. nat.
Awarding Institution
Supervisors/Advisors
  • Eng, Lukas, Reviewer
  • Milde, Peter, Main supervisor
  • Lüssem, Björn, Reviewer
Defense Date (Date of certificate)7 Jul 2016
Publication statusPublished - 2016
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Keywords

DFG Classification of Subject Areas according to Review Boards

Keywords

  • Kelvin Probe Force Microscopy, KPFM, nanostructuring