Instrumental Development
Our group develops ultrahigh vacuum (UHV) measurement technology, such as high-resolution scanning tunneling microscopes (STMs). The STM’s are mostly fabricated within a diploma or master thesis. New developments include UHV-AFM/STMs operating at 0.4 K, high frequency STMs for time-resolved measurements down to 120 ps with atomic resolution, UHV-STMs with multiple contacts, tips for scanning tunneling microscopy in direct contact with GaAs wafers, as well as transport setups for UHV.
Recent Projects
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Left: Autocorrelation of the shortest accomplished voltage pulses at the tunnel junction. The inset shows the tunneling tip mounted on a high frequency coaxial jack. Right: The computer designed microscope with a diameter of 30mm.
Due to the narrow bandwidth of our tunneling current amplifier at high gain factors (109 V/A), transient phenomena can only be observed on time scale dwon to milliseconds.
However, using a stroboscopic measurement scheme (german information)Â the time resolution can be increased by a sequence of ultra-short voltage pulses aplied to the tunneling junction. Then, the time resolution is limited by the width of the voltage (probe) pulses.
We developed a microscope (see figure right) which uses high frequency coaxial wiring in order to apply voltage pulses as short as 120ps (figure left shows the autocorrelation of such a voltage pulse directly at the tunnel junction). The samples and high frequency tips can be exchanged in-situ without breaking the vacuum. Currently, the microscope is working in a ultra high vacuum at temperatures down to 4K where an additional 3D rotatable magnetic field of up to 7T can be applied to the tunnel junction.
Related Publications
- C. Saunus, J. Raphael Bindel, M. Pratzer, and M. Morgenstern Versatile scanning tunneling microscopy with 120ps time resolution Applied Physics Letters 102, 051601 (2013); doi:10.1063/1.4790180
[BibTeX] [Abstract] [Download PDF]
We describe a fully ultra-high vacuum compatible scanning tunneling microscope (STM) optimized for radio-frequency signals. It includes in-situ exchangeable tips adapted to high frequency cabling and a standard sample holder, which offer access to the whole range of samples typically investigated by STM. We demonstrate a time resolution of 120ps using the nonlinear I(V)-characteristic of the surface of highly oriented pyrolithic graphite. We provide atomically resolved images in pulse mode related to a spatially varying nonlinearity of the local density of states of the sample, thus, demonstrating the possible spatial resolution of the instrument in pulse mode. Analysis of the noise reveals that changes in the tunneling junction of 50pA are dynamically detectable at 120ps time resolution.
@article{:/content/aip/journal/apl/102/5/10.1063/1.4790180, abstract = {We describe a fully ultra-high vacuum compatible scanning tunneling microscope (STM) optimized for radio-frequency signals. It includes in-situ exchangeable tips adapted to high frequency cabling and a standard sample holder, which offer access to the whole range of samples typically investigated by STM. We demonstrate a time resolution of 120ps using the nonlinear I(V)-characteristic of the surface of highly oriented pyrolithic graphite. We provide atomically resolved images in pulse mode related to a spatially varying nonlinearity of the local density of states of the sample, thus, demonstrating the possible spatial resolution of the instrument in pulse mode. Analysis of the noise reveals that changes in the tunneling junction of 50pA are dynamically detectable at 120ps time resolution.}, added-at = {2015-03-17T00:20:01.000+0100}, author = {Saunus, Christian and Raphael Bindel, Jan and Pratzer, Marco and Morgenstern, Markus}, biburl = {http://www.bibsonomy.org/bibtex/2b1288f492f765ae277a07f424394a19a/institut2b}, description = {Versatile scanning tunneling microscopy with 120ps time resolution}, doi = {10.1063/1.4790180}, eid = {051601}, interhash = {42855c0861bdeef9a0cc9beb945cc8f4}, intrahash = {b1288f492f765ae277a07f424394a19a}, journal = {Applied Physics Letters}, keywords = {morgenstern}, number = 5, timestamp = {2015-03-17T00:20:01.000+0100}, title = {Versatile scanning tunneling microscopy with 120ps time resolution}, url = {http://scitation.aip.org/content/aip/journal/apl/102/5/10.1063/1.4790180}, volume = 102, year = 2013 } - M. Morgenstern STM Ready for the Time Domain Science 329, 1609-1610 (2010); doi:10.1126/science.1194918
[BibTeX] [Download PDF]@article{Morgenstern24092010, added-at = {2015-03-17T00:24:37.000+0100}, author = {Morgenstern, Markus}, biburl = {http://www.bibsonomy.org/bibtex/27a9b24225f19d1ca3d98ad3b3987f5ad/institut2b}, description = {STM Ready for the Time Domain}, doi = {10.1126/science.1194918}, eprint = {http://www.sciencemag.org/content/329/5999/1609.full.pdf}, interhash = {156746079e769878738197b21533868a}, intrahash = {7a9b24225f19d1ca3d98ad3b3987f5ad}, journal = {Science}, keywords = {morgenstern}, number = 5999, pages = {1609-1610}, timestamp = {2015-03-17T00:24:37.000+0100}, title = {STM Ready for the Time Domain}, url = {http://www.sciencemag.org/content/329/5999/1609.short}, volume = 329, year = 2010 }
NanoSense underneath the SEM.
A scanhead (Nanofinger® or NanoSense) has been reconstructed and improved in order to vertically resolve nanostructures similar to a scanning force microscope. Due to its flat geometry it can be operated directly under the electron beam of the SEM. At a minimum working distance of 3 mm the imaging properties of the SEM are preserved.
In-situ structured samples (e.g. EBID, electron-beam induced deposition) or manipulations can be resolved vertically in addition to the good lateral resolution of the SEM, which allows the creation of a surface profile with a resolution of 1 nm.
left: SEM-image of a gold marker. The tip of the NanoSense points in from the upper right. right: Profile of the gold edge created by the NanoSense. Two measurements in each direction show the reproducibility. A shift in the linescans is caused by thermal drift.
The NanoSense is already implemented in the eLINE Plus system (Raith GmbH) and can be used to topographicaly analyze samples as well as scouting the surface for prober tips or other tools (Nanoworkbench of Klocke Nanotechnik GmbH), which can be precisely guided above the surface after it has been surveyed.
Left: Scanning electron microscopy image of an InAs nanowire (NW) placed at the edge of a GaAs wafer. The electric contacts at the nanowire are visible. Right: Atomically resolved STM image of a gold(111) surface taken by the InAs nanowire as a probe tip. The brighter parts of the image correspond to the surface reconstruction.
InAs nanowires are placed at cleaved edges of GaAs wafers and lithographically contacted subsequently. They are used as STM tips with a z-resolution of 3 pm. Integrated electronics in the GaAs wafers is currently developed.
Related Publications
- K. Flöhr, K. Sladek, H. Yusuf Günel, M. Ion Lepsa, H. Hardtdegen, M. Liebmann, T. Schäpers, and M. Morgenstern Scanning tunneling microscopy with InAs nanowire tips Applied Physics Letters 101, 243101 (2012); doi:10.1063/1.4769450
[BibTeX] [Abstract] [Download PDF]
Indium arsenide nanowiresgrown by selective-area vapor phase epitaxy are used as tips for scanning tunneling microscopy(STM). The STM tips are realized by positioning the wires manually on the corner of a double cleaved gallium arsenide wafer with sub-µm precision and contacting them lithographically, which is fully compatible with further integrated circuitry on the GaAs wafer. STM images show a z noise of 2pm and a lateral stability of, at least, 0.5nm on a Au(111) surface.I(z) spectroscopy reveals an exponential decay indicating tunneling through vacuum. Subsequent electron microscopy images of the tip demonstrate that the wires are barely modified during the STM imaging.
@article{:/content/aip/journal/apl/101/24/10.1063/1.4769450, abstract = {Indium arsenide nanowiresgrown by selective-area vapor phase epitaxy are used as tips for scanning tunneling microscopy(STM). The STM tips are realized by positioning the wires manually on the corner of a double cleaved gallium arsenide wafer with sub-µm precision and contacting them lithographically, which is fully compatible with further integrated circuitry on the GaAs wafer. STM images show a z noise of 2pm and a lateral stability of, at least, 0.5nm on a Au(111) surface.I(z) spectroscopy reveals an exponential decay indicating tunneling through vacuum. Subsequent electron microscopy images of the tip demonstrate that the wires are barely modified during the STM imaging.}, added-at = {2015-03-17T00:21:24.000+0100}, author = {Flöhr, Kilian and Sladek, Kamil and Yusuf Günel, H. and Ion Lepsa, Mihail and Hardtdegen, Hilde and Liebmann, Marcus and Schäpers, Thomas and Morgenstern, Markus}, biburl = {http://www.bibsonomy.org/bibtex/22b66d8eb206fb55826beff8015eff31e/institut2b}, description = {Scanning tunneling microscopy with InAs nanowire tips}, doi = {10.1063/1.4769450}, eid = {243101}, interhash = {f73d9d45cc3735c1550b890c1b68cc68}, intrahash = {2b66d8eb206fb55826beff8015eff31e}, journal = {Applied Physics Letters}, keywords = {morgenstern}, number = 24, timestamp = {2015-03-17T00:21:24.000+0100}, title = {Scanning tunneling microscopy with InAs nanowire tips}, url = {http://scitation.aip.org/content/aip/journal/apl/101/24/10.1063/1.4769450}, volume = 101, year = 2012 } - K. Flöhr, M. Liebmann, K. Sladek, Y. H. Günel, R. Frielinghaus, F. Haas, C. Meyer, H. Hardtdegen, T. Schäpers, D. Grützmacher, and M. Morgenstern Manipulating InAs nanowires with submicrometer precision Review of Scientific Instruments 82, 113705 (2011); doi:10.1063/1.3657135
[BibTeX] [Abstract] [Download PDF]
InAsnanowires are grown epitaxially by catalyst-free metal organic vapor phase epitaxy and are subsequently positioned with a lateral accuracy of less than 1 µm using simple adhesion forces between the nanowires and an indium tip. The technique, requiring only an optical microscope, is used to place individual nanowires onto the corner of a cleaved-edge wafer as well as across predefined holes in Si3N4 membranes. The precision of the method is limited by the stability of the micromanipulators and the precision of the optical microscope.
@article{:/content/aip/journal/rsi/82/11/10.1063/1.3657135, abstract = {InAsnanowires are grown epitaxially by catalyst-free metal organic vapor phase epitaxy and are subsequently positioned with a lateral accuracy of less than 1 µm using simple adhesion forces between the nanowires and an indium tip. The technique, requiring only an optical microscope, is used to place individual nanowires onto the corner of a cleaved-edge wafer as well as across predefined holes in Si3N4 membranes. The precision of the method is limited by the stability of the micromanipulators and the precision of the optical microscope.}, added-at = {2015-03-17T00:24:12.000+0100}, author = {Flöhr, Kilian and Liebmann, Marcus and Sladek, Kamil and Günel, H. Yusuf and Frielinghaus, Robert and Haas, Fabian and Meyer, Carola and Hardtdegen, Hilde and Schäpers, Thomas and Grützmacher, Detlev and Morgenstern, Markus}, biburl = {http://www.bibsonomy.org/bibtex/21772892de8682a3b33d1f46543586b59/institut2b}, description = {Manipulating InAs nanowires with submicrometer precision}, doi = {10.1063/1.3657135}, eid = {113705}, interhash = {3ed5557bf542886617cd9b0d24d8af7d}, intrahash = {1772892de8682a3b33d1f46543586b59}, journal = {Review of Scientific Instruments}, keywords = {morgenstern}, number = 11, timestamp = {2015-03-17T00:24:12.000+0100}, title = {Manipulating InAs nanowires with submicrometer precision}, url = {http://scitation.aip.org/content/aip/journal/rsi/82/11/10.1063/1.3657135}, volume = 82, year = 2011 }
The ultra high vacuum STM system allows measurements at 380mK and within magnetic fields up to 14T. The field can be applied perpendicular to the sample surface. For this purpose, a home-built combination of a scanning tunneling microsope and a scanning force microscope (AFM/STM) equipped with a qplus sensor is placed in the center of a superconducting coil within a cryostat. The coils are cooled by liquid helium (4.2K). The microscope is cooled by helium-3, a rare helium isotope, down to 0.38K.
Schematic drawing of the low temperature AFM/STM system
Samples and tips for the scanning tunneling microscope can be prepared and analysed in two separated ultra-high vacuum chambers. The system ist equipped with an MBE evaporator, an ion gun, a LEED/Auger system and a sample heater. As the scanning tunneling microscope is extremely sensitive to mechanical vibrations, the whole equipment ist located in an acoustically insulated room. The weight of the whole system amounts to 2.5 tons. It is additionally placed onto four air damping legs.
Photos showing the new developed carrier systems for samples, qPlus sensors and STM-tips as well as the two stages for 2- and 4-point measurements including a gate contact.
The combined scanning force/scanning tunneling microscope (AFM/STM) equipped with a quartz tuning fork has been built in our group. The qPlus sensor is mounted on a x/y-positioning stage with a range of 2×2 mm. The sensor can be exchanged in-situ. The sample holder is mounted on the scanner piezo and addionally features a separate gate contact. The combination of AFM and STM allows a navigation towards nano-structured samples prepared on insulating substrates (like, e.g., graphene on SiO2). Another special feature of the microscope is the built-in stage for transport measurements, which in addition allows a 4-point conductivity measurement of the samples.
Combined low temperature scanning force and scanning tunneling microscope.
Home built scanning tunneling microscopes. Left: Low temperature ultra high vacuum AFM/STM with Q-Plus sensor. Right: Low temperature high frequency STM designed for UHV and high magnetic fields. The time resolution is 120 ps. The microscope enables an in situ tip exchange.