Scanning Transport Spectroscopy on Nanostructures

InAs self assembled quantum dots are promising candidates for novel optoelectronic devices and are also considered for spintronics and quantum computation. To characterize such dots, a wide range of optical and electrical techniques is used. With the upcoming interest in single dot "devices", however, Scanning Probe Microscopy (SPM) techniques are the most promising tools to study the electronic properties of single InAs dots and dot superlattices. In this project we propose to realize a "scanning nanostructure parameter analyzer" employing our new cryogenic SPM system. Using conducting tips, contacts to single dots can be established and all kinds of electrical measurements are possible in the whole temperature range between 300 K and cryogenic temperatures. The cryo SPM can yield spatially resolved information on the dots and their surrounding so that e.g. inhomogeneities which cause artifacts in the measurements can be identified. Moreover, one can study the physics of tunneling processes between electron systems of different dimensionality, where the spatial position of current injection can be controlled precisely. Cross sectional measurements will also be interesting in order to determine MBE growth parameters such as barrier heights (by temperature dependent measurements e.g.). Compared to measurements on dot ensembles, single dot current spectroscopy has a larger spectral resolution since the typical size distribution of the different dots cannot average out the observed effects. In this way coulomb blockade and even spin blockade effects can be studied with high resolution. Using capacitance measurements, charging effects shall be studied. Employing an ultrahigh resolution capacitance bridge it is possible to measure the carrier concentration in single dots. As our setup can also be operated as scanning capacitance microscope, even the lateral charge distribution can be measured on a dot. We also have optical access to the sample during the measurements in our setup and therefore, electroluminescence and photoconductivity measurements are possible, too. By varying the contact force between the SPM tip and the sample, spreading resistance, pressure and strain studies can be carried out on single quantum dots.

InAs self assembled quantum dots are promising candidates for novel optoelectronic devices and are also considered for spintronics and quantum computation. To characterize such dots, a wide range of optical and electrical techniques is used. With the upcoming interest in single dot "devices", however, Scanning Probe Microscopy (SPM) techniques are the most promising tools to study the electronic properties of single InAs dots and dot superlattices. In this project we propose to realize a "scanning nanostructure parameter analyzer" employing our new cryogenic SPM system. Using conducting tips, contacts to single dots can be established and all kinds of electrical measurements are possible in the whole temperature range between 300 K and cryogenic temperatures. The cryo SPM can yield spatially resolved information on the dots and their surrounding so that e.g. inhomogeneities which cause artifacts in the measurements can be identified. Moreover, one can study the physics of tunneling processes between electron systems of different dimensionality, where the spatial position of current injection can be controlled precisely. Cross sectional measurements will also be interesting in order to determine MBE growth parameters such as barrier heights (by temperature dependent measurements e.g.). Compared to measurements on dot ensembles, single dot current spectroscopy has a larger spectral resolution since the typical size distribution of the different dots cannot average out the observed effects. In this way coulomb blockade and even spin blockade effects can be studied with high resolution. Using capacitance measurements, charging effects shall be studied. Employing an ultrahigh resolution capacitance bridge it is possible to measure the carrier concentration in single dots. As our setup can also be operated as scanning capacitance microscope, even the lateral charge distribution can be measured on a dot. We also have optical access to the sample during the measurements in our setup and therefore, electroluminescence and photoconductivity measurements are possible, too. By varying the contact force between the SPM tip and the sample, spreading resistance, pressure and strain studies can be carried out on single quantum dots.