Bachelor und Master Arbeiten (A. Rizzi, J. Malindretos)

We are very happy about dynamic and curious students that are interested in our research work. Almost all project themes listed below can be adapted for Master purposes as well. Please contact us for more details and for a personal exchange.


1. Kelvin probe microscopy of GaN and InN layers and nanostructures

The electronic surface properties of nanostructures have a huge impact on their functionality in nano-devices, due to the high surface to volume ratio. The work function of a solid may be experimentally determined by using the Kelvin probe method, which measures the work-function difference of the surface under study and a vibrating metal probe taken as a reference. With Kelvin probe force microscopy local variations in the work function of the material can be measured.

The experimental Bachelor work will address the determination of the work function of GaN and InN epitaxial layers and nanostructures – in particular nanowires. A modern, commercial atomic force microscope will be used to perform the measurements. In the first period of the Bachelor work, the student should develop a deeper understanding of the physics and the technical aspects of Kelvin probe microscopy. Afterwards, the goal is to determine the local work function of different nanostructures prepared in our group and to assess whether this method yields reliable results when used with polar III-Nitride materials.


2. Temperature monitoring by optical spectroscopy

The real time measurement and control of the growing surface temperature is an important and critical issue for the epitaxial growth of high quality hetero- and nanostructures. For semiconductors, a method that has been recently applied is the determination of the gap energy through a spectral analysis of optical reflection. For a given material, this gap energy can be precisely correlated with an absolute temperature of the semiconductor layer.

The experimental Bachelor work will address the realization of a compact measurement set-up for the determination of the temperature of the growing surface during the molecular beam epitaxy of III-Nitride hetero- and nanostructures. This involves the selection of a suitable light source and the design of appropriate optics for illumination and light collection. Furthermore, the project could be extended to include also the construction of a suitable mount and housing and/or a software for automated analysis of the acquired spectra. A commercial solid-state spectrometer is available for the setup.



3. Surface passivation of GaN based heterostructures

The electronic properties of quantum wells can be tailored with high control via molecular beam epitaxy (MBE). In the case of surface quantum wells there is an enhanced probability of interaction and energy conversion processes with external beams, e.g. atom beams. To study these processes the sample surface must be prepared in a highly reproducible way. In particular, sample transfer processes in air can deteriorate the surface properties, so that an effective ultra-high vacuum compatible passivation is needed.

The experimental Bachelor work will address the preparation and analysis of GaN based heterostructures with surface quantum wells by MBE. The aim is to develop a procedure which involves the growth of a suitable cap layer that can be then removed after transfer in air from the ultra-high vaccum environment in the MBE growth system to the experimental setup for the study of energy conversion processes with atom beams. The surface properties of the heterostructures after exposure to air will be checked primarily with Auger electron spectroscopy, which is very sensitive to any chemical changes of the semiconductor surface.



4. Piezoelectric effects in GaN nanowires

Individual GaN nanowires exhibit strong piezoelectricity. By assembling GaN nanowires in ordered arrays several functions can be envisaged, e.g. the conversion of mechanical energy to electrical energy, or the controlled stimulation/sensing of biological cells.

The Bachelor work will first address the estimation of the piezoelectric properties based on the characteristics of available GaN nanowire arrays and the piezoelectric coefficients known from literature. Then an experimental configuration will be conceived to measure the piezoelectric properties of the nanowires.



5. Masks for selective area growth of GaN nanowires

GaN-based nanowires represent a very interesting system for studying and developing new functionalities for sensing applications, opto-electronics, nano-electronics and energy harvesting. The availability of ordered arrays of nanowires, in which single objects or uniformly distributed ensemble objects can be addressed, is crucial. Top-down or bottom-up approaches can be figured out for this scope. In our group we successfully apply a bottom-up approach by epitaxial growth of ordered nanowire arrays on a masked substrate.

The experimental Bachelor work will be concerned with the development of a new mask alternative to the thin Mo-mask that we are presently using. Both, SiOx and SiNx are promising candidates. Advanced plasma deposition as well as optical- and e-beam lithography will be applied for preparing the mask. Furthermore, the characterization of the mask itself and of the resulting structures after epitaxial growth of GaN nanowires will be in focus.



6. Doping of GaN and Hall effect characterization

The application of semiconductor layers and nanostructures in functional devices requires the capability of tailoring the electrical properties by doping. GaN can be effectively doped by statistical incorporation of Si atoms that preferentially occupy Ga sites and therefore provide a n-type doping. Carrier concentration and mobility of the charged carriers can be determined by Hall-effect experiments.

The experimental Bachelor work will be concerned with the growth of a series of epitaxial GaN layers and with their characterization by (temperature dependent) Hall-effect experiments. The feedback between growth and electrical characterization will provide the required high control of the doping process. The construction of a new sample holder for Hall measurements is a technical part of the Bachelor work and will allow quick routine measurements as compared to the more sophisticated system that is presently used.



(last update: Jan 2012)