Final Report Summary - NANOWIREDEVICESTM (Combined structural and electronic characterization of semiconductor nanowire devices on the atomic scale using scanning tunnelling microscopy )
For all project sub-tasks as presented below, the recipient of the Marie Curie Fellowship (Rainer Timm) has taken a central role and responsibility for all parts of the project, collaborated with senior researchers, and supervised Doctor of Philosophy (PhD) and Master students. The success of the research program as described below is also testified by R. Timm being permanently employed as a researcher at Lund University after the end of the Marie Curie Fellowship.
Scientific progress following the points of the original work plan:
1. Preparation of nanowires and Scanning tunnelling microscopy (STM) on nanowire cleavage faces and surfaces
During the project we have successfully extend our experimental techniques to now enable STM on nanowires with a very large > 250 nm diameter and / or on several different crystal facets after each other. Here we could observe even the atomic scale structure while moving many micrometres along the long axis of the wire. We also managed to obtain atomically resolved imaging of all facets of clean GaAs and InAs nanowires on both wurtzite and zincblende crystal structures. For GaAs and InAs the wurtzite crystal structure only exists in nanowires. The work on cleavage surfaces to study embedded nanowires also lead to interesting work on the metallic or non-metallic properties of ErAs in GaAs published in the high profile journal Physical Review Letters. Work was now also begun on obtaining clean and crystalline surface for GaSb and InSb nanowires which are becoming of significant interest.
2. Scanning tunnelling spectroscopy (STS) on freely suspended nanowires
STS measurements with variable-gap-spectroscopy and dI/dV-spectroscopy using lock-in technique has been optimised and used for the investigations of especially the transition between the wurtzite and zincblende crystal structures in a single wire. Here a bandgap difference could be observed and spectra measured out on the clean surface in a consistent manner with nm resolution.
3. Under Ultrahigh vacuum (UHV) conductivity and transport measurements on nanowires
We started the first stage of the project, with measurements performed in the UHV chamber of the STM setup, but independent of the STM measurement itself. The UHV conditions have enabled us to clean the nanowire surfaces and begin study the effects of well-controlled surfaces. Here we have for example used nin-nanowires as test objects which were initially characterised using other synchrotron based methods (results published in: 99, 233113 (2011)). Results are very promising in that device breakdown is not observed until about 150 degrees of Celsius above the necessary cleaning temperatures. Finally we have been testing the effect of atomic layer deposition of high-k dielectrics on the surfaces of nanowires, which can potentially be very useful if defects leading to Fermi level pinning at the surfaces are to be avoided.
4. STM and STS on individually contacted nanowires under device operation
During the two years of the project we have developed a novel setup to enable combined measurements by performing STM / STS and atomic force microscopy (AFM) studies on individually contacted nanowires. Recently we obtained results for axial Esaki tunnel diode nanowires (consisting of InAs and GaSb segments), evaluating the position of the local Fermi level along the device for different external voltages by STM measurements while the nanowire was connected up in a device configuration and independent voltages could be applied across the device. It will be possible to correlate device performance to the abruptness of the interface with nm scale resolution. We also employed a different procedure we also demonstrated the use of the STM / AFM tip as a biased gate to locally influence the conductivity of the nanowire, while the resulting current through the entire device is measured in respect to the position of the gating tip.
5. Low temperatures We have extended our work to the use of a 4 Kelvin
STM situated at Brookhaven National Laboratory (the use of the instrument was awarded in open competition). This allowed us to demonstrate that STS was also possible on InAs nanowires at 4 K on several facets. These spectra show much sharper features and we can thus resolve peaks not observable at room temperature.
6. Additional characterisation methods, especially synchrotron based spectroscopy for characterising the surface cleanliness of the nanowires has turned out to be useful. But also SEM measurements on devices before/after measurements with our STM has been very useful to give a map for maneuvering with the scanning probe and evaluating afterwards if the STM inflicted damage on the device.
Training progress
R. Timm has had all the common resources of nmC@LU (the >70 permanent researcher Nano-consortium in Lund) made available to him including cleanroom facilities with Scanning electron microscopes (SEM), Atomic force microscopes (AFM) and a variety of other types of experimental equipment. R. Timm has used the three STMs available at Dep. of Physics, one of them has been upgraded with an AFM option and four in-vacuum electrical contacts for this project. Also the electrical measurement equipment used for evaluating nanowire semiconductor devices have been lent to R. Timm by researcher collaborators at the Dep. of Elec. Eng. R. Timm has used facilities at the synchrotron MAX-II in Lund and visited the synchrotron Bessy and the European Synchrotron Radiation Facility (ESRF) in Grenoble. R. Timm has been awarded the use of a 4-Kelvin STM at Brookhaven National Laboratory. R. Timm has presented his worked and discussed with the strongest experts in the field at a number of conferences both in Europa and the Americas during the two years of the project. In total he has participated in 11 events mostly giving oral presentations. R. Timm also participated in two events specially related to the MC programme. All the offices of the division including PhD students, postdocs and professor are gathered in two adjacent corridors and we have thus a very close integration with informal daily meetings and formal meetings in the STM group every Wednesday. On top of this there are weekly meeting with other researchers at nmC@LU on Fridays, and the big annual meeting of the nmC@LU. R. Timm participates in all of this. R. Timm was selected for an oral presentation at the annual meeting of the nmC@LU 2012, to present the results from this Marie Curie project. R. Timm has been involved in all parts of the work in the projects as described above he have been involved in taking responsibility for both planning, execution, data analysis and dissemination through publication. He has done this in major collaborations involving a number of different researchers on all levels from PhD to professor. Thus he has been allowed to train on all parts of the scientific research projects, while also using new equipment and totally new methods. Finally we are happy to report that after the conclusion of this Marie Curie project R. Timm has been permanently employed as a researcher at Lund University.
Scientific progress following the points of the original work plan:
1. Preparation of nanowires and Scanning tunnelling microscopy (STM) on nanowire cleavage faces and surfaces
During the project we have successfully extend our experimental techniques to now enable STM on nanowires with a very large > 250 nm diameter and / or on several different crystal facets after each other. Here we could observe even the atomic scale structure while moving many micrometres along the long axis of the wire. We also managed to obtain atomically resolved imaging of all facets of clean GaAs and InAs nanowires on both wurtzite and zincblende crystal structures. For GaAs and InAs the wurtzite crystal structure only exists in nanowires. The work on cleavage surfaces to study embedded nanowires also lead to interesting work on the metallic or non-metallic properties of ErAs in GaAs published in the high profile journal Physical Review Letters. Work was now also begun on obtaining clean and crystalline surface for GaSb and InSb nanowires which are becoming of significant interest.
2. Scanning tunnelling spectroscopy (STS) on freely suspended nanowires
STS measurements with variable-gap-spectroscopy and dI/dV-spectroscopy using lock-in technique has been optimised and used for the investigations of especially the transition between the wurtzite and zincblende crystal structures in a single wire. Here a bandgap difference could be observed and spectra measured out on the clean surface in a consistent manner with nm resolution.
3. Under Ultrahigh vacuum (UHV) conductivity and transport measurements on nanowires
We started the first stage of the project, with measurements performed in the UHV chamber of the STM setup, but independent of the STM measurement itself. The UHV conditions have enabled us to clean the nanowire surfaces and begin study the effects of well-controlled surfaces. Here we have for example used nin-nanowires as test objects which were initially characterised using other synchrotron based methods (results published in: 99, 233113 (2011)). Results are very promising in that device breakdown is not observed until about 150 degrees of Celsius above the necessary cleaning temperatures. Finally we have been testing the effect of atomic layer deposition of high-k dielectrics on the surfaces of nanowires, which can potentially be very useful if defects leading to Fermi level pinning at the surfaces are to be avoided.
4. STM and STS on individually contacted nanowires under device operation
During the two years of the project we have developed a novel setup to enable combined measurements by performing STM / STS and atomic force microscopy (AFM) studies on individually contacted nanowires. Recently we obtained results for axial Esaki tunnel diode nanowires (consisting of InAs and GaSb segments), evaluating the position of the local Fermi level along the device for different external voltages by STM measurements while the nanowire was connected up in a device configuration and independent voltages could be applied across the device. It will be possible to correlate device performance to the abruptness of the interface with nm scale resolution. We also employed a different procedure we also demonstrated the use of the STM / AFM tip as a biased gate to locally influence the conductivity of the nanowire, while the resulting current through the entire device is measured in respect to the position of the gating tip.
5. Low temperatures We have extended our work to the use of a 4 Kelvin
STM situated at Brookhaven National Laboratory (the use of the instrument was awarded in open competition). This allowed us to demonstrate that STS was also possible on InAs nanowires at 4 K on several facets. These spectra show much sharper features and we can thus resolve peaks not observable at room temperature.
6. Additional characterisation methods, especially synchrotron based spectroscopy for characterising the surface cleanliness of the nanowires has turned out to be useful. But also SEM measurements on devices before/after measurements with our STM has been very useful to give a map for maneuvering with the scanning probe and evaluating afterwards if the STM inflicted damage on the device.
Training progress
R. Timm has had all the common resources of nmC@LU (the >70 permanent researcher Nano-consortium in Lund) made available to him including cleanroom facilities with Scanning electron microscopes (SEM), Atomic force microscopes (AFM) and a variety of other types of experimental equipment. R. Timm has used the three STMs available at Dep. of Physics, one of them has been upgraded with an AFM option and four in-vacuum electrical contacts for this project. Also the electrical measurement equipment used for evaluating nanowire semiconductor devices have been lent to R. Timm by researcher collaborators at the Dep. of Elec. Eng. R. Timm has used facilities at the synchrotron MAX-II in Lund and visited the synchrotron Bessy and the European Synchrotron Radiation Facility (ESRF) in Grenoble. R. Timm has been awarded the use of a 4-Kelvin STM at Brookhaven National Laboratory. R. Timm has presented his worked and discussed with the strongest experts in the field at a number of conferences both in Europa and the Americas during the two years of the project. In total he has participated in 11 events mostly giving oral presentations. R. Timm also participated in two events specially related to the MC programme. All the offices of the division including PhD students, postdocs and professor are gathered in two adjacent corridors and we have thus a very close integration with informal daily meetings and formal meetings in the STM group every Wednesday. On top of this there are weekly meeting with other researchers at nmC@LU on Fridays, and the big annual meeting of the nmC@LU. R. Timm participates in all of this. R. Timm was selected for an oral presentation at the annual meeting of the nmC@LU 2012, to present the results from this Marie Curie project. R. Timm has been involved in all parts of the work in the projects as described above he have been involved in taking responsibility for both planning, execution, data analysis and dissemination through publication. He has done this in major collaborations involving a number of different researchers on all levels from PhD to professor. Thus he has been allowed to train on all parts of the scientific research projects, while also using new equipment and totally new methods. Finally we are happy to report that after the conclusion of this Marie Curie project R. Timm has been permanently employed as a researcher at Lund University.