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Figure 5.13 shows a schematic overview of an STM system consisting of the following basic components:
(1) sharp metal tip
(2) piezoelectric translator to move the tip relative to the sample
(3) control electronics for applying the bias voltage and measuring the tunneling current
(4) feedback system to keep the tunneling current constant by height readjustment ofthe tip
(5) imaging system to convert the single data points into an image
The tips used for STM experiments should be sharp and stable. Chemical stability can be achieved by using a noble metal. Mechanical rigidity can be reached by short wires. Alloys of Pt and Ir are frequently used for fabrication of STM tips. They can be produced in a surprisingly simple way just by cutting a metal wire with conventional cutting tools. Because of their high chemical stability, such Pt/Ir tips are well
5.2 Scanning Tunneling Microscopy (STM) 287
suited for atomic resolution experiments on flat samples. Because of their low aspect ratio near the apex, however, they fail to trace steep features and narrow trenches. Therefore, electrolytically etched tungsten tips with higher aspect ratios are sometimes used as an alternative, although they are less stable against oxidation.
Usually, in STM the position of the sample is fixed and the tip is raster-scanned. Like in AFM, after manual course approach with fine-thread screws, motion of the tip is performed with a piezo translator made of piezo ceramics like e. g. lead zirconate ti-tanate (PZT), which can again be either a piezo tripod or a single tube scanner.
Although physical studies of the electronic structure of surfaces have to be performed under UHV conditions to guarantee clean uncontaminated samples, the technique does not require vacuum for its operation. Thus, in-situ observation of processes at solid-gas and solid-liquid interfaces is possible as well. This has been utilized, for instance, to directly observe corrosion and electrode processes with atomic resolution [5.2, 5.37].
Lateral and Spectroscopic Information
STM images can only be interpreted in terms of surface topography for surface structures with dimensions well above the atomic scale. In general, images taken in constant current mode deliver contour maps of the local density of states. If the polarity of the sample is negative, then states in the valence band are imaged. For a positive polarity of the sample the distribution of electronic states in the conduction band can be recorded. In case different chemical species are present on the surface, the image contrast is further influenced by the varying effective barrier height (work function) at different positions. As an example Fig. 5.14 shows an STM image of a si-
288 I 5 Scanning Probe Microscopy
licon (111) surface with 1/3 of a silver monolayer on top of it [5.38]. The high contrast between the silicon surface and the silver islands does not represent the real height of the islands, but it appears exaggerated, because of the lower work function in the silver regions. Such local differences in the effective barrier height can be directly imaged by modulating the tip in vertical direction and recording dl/ds, which according to Eq. (5.1) is proportional to Ô1/2õ². STM can also yield spectroscopic information by recording dl/d U curves at fixed positions, which is called scanning tunneling spectroscopy (STS). This opens up the opportunity to perform electron spectroscopy with a resolution down to one single atom. An other option for obtaining spectroscopic information is simultaneous recording of STM images at various bias voltages. This can be accomplished by performing point spectroscopy at every image point or by modulating the bias voltage while scanning. In the literature simultaneous recording of images at various bias voltages has been called current imaging tunneling spectroscopy (CITS). As one example, the discrimination between different chemical species (Ga and As atoms on a GaAs (110) surface) in an atomically resolved STM image by simultaneously recording of two images at reversed bias voltage, the work of Feenstra et al. [5.39] should be cited.
Similar to the AFM, one advantage of STM is the fact that a wide range of scan sizes from more than 100 pm down to the atomic level can be covered in one experiment. Because the information is obtained in real space, local defects (e.g. mono-atomic defects, steps, dislocations) can be investigated. This clearly is a great advantage compared to diffraction methods relying on extended periodic structures, and thus showing averaged information. Moreover, the possibility to obtain spectroscopic information makes the STM a very valuable tool for studying surface processes on the atomic scale. An instructive example highlighting several figures of merit of STM has been introduced by the group of Avouris [5.40-5.42], which will be described in the following. Figure 5.15 shows an STM image of the unoccupied states of a Si (111)-7 x 7 surface exposed to 0.2 L 02 at a temperature of 300 K. Besides the charac-