Charge transport through Nanostructures
A current challenge in nanoscience is to measure the charge transport through nanostructures grown by self-assembly. The controlled fabrication of such self-organized nanostructures with dimensions in the single-digit nanometer range has become possible. For instance semiconductor nanowires and nanorings with a width down to a few atoms have been fabricated in a controlled way. Quantum effects are expected in the charge transport through self-organized nanostructures. Besides the fabrication aspects, the ability to provide contacts to the nanostructures in order to characterize them is a major challenge, since, in contrast to nanostructures grown by lithographic methods, the location of the self-organized structures is not predefined. The scanning tunneling microscope (STM) is an appropriate tool for imaging these nanostructures down to the atomic range and characterizing them by spectroscopic methods. However, only one probe alone is not sufficient to measure the charge transport properties of laterally grown nanostructures. An additional probe is needed to provide the second contact.
Combination of STM and SEM
Scanning tunneling microscopy (STM) is an ideal method for imaging surfaces on an atomic scale. But if the samples are structured on a larger scale, as for example integrated circuits, the analysis is limited by the STM's scan range of typically less than 20 ?m. It is in this case difficult to locate characteristic features of the sample with the STM. We have therefore combined a beetle-type STM with a scanning electron microscope (SEM).
With the SEM it is possible to measure overview scans at various magnifications and to image relatively large areas of the sample (up to about 2 x 2 mm²). The SEM images are then used to select certain parts of the sample for the STM-analysis. All this is done under ultrahigh vacuum conditions. The electron gun and the secondary electron detector (SED) of the SEM are arranged perpendicular to each other so that the STM-tip causes a shading of secondary electrons coming from the part of the sample which is opposite to the SED. This tip shadow is important for the positioning of the tip relative to the sample. Thus the SEM is very helpful for the tip approach to the sample and for the selection of the sample area to be analyzed with STM. Beyond that it is possible to investigate the quality of the tip and the sample preparation with the SEM.
More details can be found here: Rev. Sci. Instrum. 72, 3546 (2001).
Double-tip STM design
In order to allow a fast and well-defined approach of the STM tips, an ad-on scanning electron microscope (SEM) is used which allows imaging over a broader range of resolution and allows to monitor the STM tips during approach. To make the STM as small and rigid as possible, our construction is based on the Besocke beetle-type STM invented in Jülich. This type of STM shows unique properties in rigidity, thermal drift compensation and simplicity of use. Especially the possibility of integrating the main components (coarse approach, coarse movement, sample holder and scanner) in a small space makes this concept promising for a multi-tip STM.
In order to prevent the coupling between vertical and lateral movement in the original beetle scheme (using a helical ramp ring), we introduced a second ring which is exclusively devoted to transversal movement (see figure). Since this flat ring is planar any motion of the flat ring will not affect the tip-sample distance. The scanning tube piezo, which provides the fine movement of the STM tip, is mounted to the upper ramp ring, so that it performs both motions: the lateral motion of the flat ring and the vertical motion of the ramp ring.
Performance of the Double-tip STM
In order to obtain well-resolved SEM images the STM must not be decoupled from the chamber (during SEM operation) since a rigid connection between the SEM column and the STM is needed.
Following figure shows an SEM image of the two STM tips brought close together (2 ?m) on a Si surface. On the right side of the figure a higher magnification SEM image is shown with Si step bunches visible as dark horizontal lines. The minimum practical separation of both STM tips in tunnel contact is limited by the radius of curvature of the tip apex. If the tips come too close they touch each other. With chemically etched tungsten tips a radius of curvature of about 10-50 nm can be obtained. This would lead to a minimum tip-tip distance of 20-100 nm.
The same area of a sample can be accessed and scanned with each tip. The next figure shows two STM images of the overlapping area on a Si(111) surface obtained with both tips. The area was first scanned with the inner scanner and after parking the tip at a save distance, the outer scanner was used to image the same region.
To demonstrate that it is the very same area, four identical objects in the images are highlighted by ovals. The difference in the step orientation of the two images originates in the different orientation of the inner and outer scanner. This result shows that the double-tip STM is able to image the same objects of nanometer size with both STM tips. This ability will be essential for contacting and electrically characterizing nanostructures. To image and study nanostructures consisting of only a few atoms, which are most promising for quantum electronics, atomic resolution is indispensable for the instrument. Therefore atomic resolution is the most desirable, but also the most delicate feature of a multi-tip STM. If single atoms can be resolved, the position of the tip with respect to the surface is very stable. This is particularly important for spectroscopic measurement with the STM.
To demonstrate the ability of the beetle-type double-tip STM to resolve single atoms, the prominent (7x7) reconstruction of the Si(111) surface was chosen. The following figure shows a constant-current-mode scan with positive sample voltage. The adatoms of the (7x7) unit cell can be clearly recognized.
Measuring the I/V characteristics of Au nanocontacts on low temperature GaAs
The I/V characteristic of Au nanocontacts on low temperature GaAs has been measured with our STM/ SEM combination. The measurements were performed directly on the nanocontacts with the help of the STM tip. The SEM was used to guide the STM tip toward the nanocontacts. I-V characteristics for two nanocontacts with lateral dimensions of 100 nm and 200 nm, respectively, are shown. The tip was pushed into the contact until a saturation of the I/V curve was observed.
Measuring the I/V characteristics of GaAs resonant tunnelling diodes
Resonant tunnelling diodes are contacted using one STM tip and I/V characteristics with the peak structure usual for resonant tunnelling diodes are measured on diodes down to 40 nm size. When a gate voltage was applied using the second STM tip small shifts in the peak structure of the resonant tunnelling diodes are observed.
We have presented a new concept of a multiprobe STM. By using the reliable beetle-type design two STM stages were arranged coaxially. Furthermore, the original Besocke design was extended by an additional flat ring to provide the probes with full freedom of movement. An add-on SEM provides safe navigation of the two STM tips down to the nm scale. The instrument shows all the features which make it a promising tool for measuring charge transport through nanostructures. It is straightforward to extend the concept of the described beetle-type double-tip STM towards a four-tip STM with the ability to perform four-point probe measurements on the nanoscale.
P. Jaschinsky, P. Coenen, G. Pirug, and B. Voigtländer, Rev. Sci. Instr. 77, 093701 (2006).
J. Wensorra, M. I. Lepsa, K. M. Indlekofer, A. Förster, P. Jaschinsky, B. Voigtländer, G. Pirug, and H. Lüth, phys. stat. sol. (a) 203, 3559 (2006).
Philipp Jaschinsky, Jakob Wensorra, Mihail Ion Lepsa, Josef Myslive?ek, and Bert Voigtländer J. Appl. Phys. 104 094307 (2008).