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Start of funding 01.07.2010

Atomic-scale composition, dopant and interface segregation analysis of single semiconductor nanowires by laser-induced atom probe tomography

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Semiconductor nanowires show great promise as multifunctional components in a number of emerging device technologies, such as opto- and nanoelectronics, photovoltaics, sensing and thermoelectrics. The continued advancement of nanoscale devices based on nanowires depends critically on knowledge of their atomic-scale structure, because compositional fluctuations as small as a single dopant atom can effect device performance. In this project, we focus on three-dimensional composition and dopant mapping of III–V compound semiconductor nanowires with single-atom sensitivity and subnanometer spatial resolution using the unique atom probe tomography technique. The semiconductor nanowires of interest are currently grown with high precision by ultra-high purity MBE in the group of Dr. Gregor Koblmüller at the Walter Schottky Institute, TU Munich and will be investigated by the laser-induced local electrode atom probe (LEAP) microscope (group of Prof. Jim Speck) at the shared research facilities between the California NanoSystems Institute (CNSI) and the Materials Department at UC Santa Barbara.

Final report:
The core challenges of this project were mainly related to preparing specific specimen suitable for subsequent atom probe tomography analysis. The free-standing nanowires as grown by molecular beam epitaxy were first transferred onto appropriate substrates (Silicon posts) by use of a so-called “focused ion beam” (FIB)-based technique. We recognized that the transfer of single nanowires had to be performed with the utmost precision for vertical alignment, such that together with the underlying Si posts they could be further milled to very sharp needles suitable for analysis. It was found that this transfer process is quite difficult and time-consuming – a bottle-neck for the entire experiments – such that only a few specimen could be prepared successfully. Consecutively, the milled needle-shaped specimen consisting of InAs/InGaAs nanowire heterostructures were tested for the first time in the atom probe tomography setup. In the first runs the specimen experienced massive evaporation rates probably due to the high heat transfer from the laser-induced operation and non-optimized nanowire/Si-post bonding, such that no realistic mass spectrometric data and reconstruction of the atomically resolved composition could be gained at this stage. Further optimization of the FIB-transfer process and the process parameter window for local electrode-based and laser-induced field evaporation in the atom probe tomography are necessary.