PART III. Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) has become an extremely powerful method for surface topography and stmcture. This was the first technique capa- 

For STM, both electrodes, the probe and the sample, must be conductive. A sharp tip is used as a probe, and it is usually produced by electrochemical etching of a tungsten wire in KOH or NaOH solution (see Section 5.1.1). The tip is brought close to the sample surface by piezoelements until a tunnel current in the nA-range flows through the vacuum or air gap. 

A gap in between two conductive materials represents an energetic barrier for electron waves. In the media the wave can propagate quasifreely and in the barrier it is damped exponentially with the penetration depth. 

The maximal possible current flowing through an atomically sharp tip is of the order /q = 17/Rk with gap voltage U and Klitzing constant Rr = hle 25 kJ2. Actual tunnel currents are of the order 1 nA. The saturation of preamplifiers used in STM is typically reached with 50 nA. 

For steady electron tunneling conditions a small bias U must be applied between sample and tip. A tunnel current of a few nA indicates that the distance between probe and sample is of the order of some 10 A, that is, typical wave lengths of the valence and conduction electrons close to the Fermi level. With crystalline materials the wave lengths of the electrons contributing to the tunnel current depend on the effective mass m and on the relative energy with respect to the bottom of the bulk or surface band Eq from which the tunnel electrons originate  

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