STM: Basic principles of operation

In STM, whose basic principle of operation is illustrated graphically in figure 2.2, the two materials are represented by the sample of study, that needs to have conductive properties, and the probe, a very sharp metallic tip ideally terminated with a single atom. Semiconductors can also be investigated provided that a suitable bias voltage is applied for the valence band electrons to tunnel out. The sample area of interest is continuously scanned with the probe, and for each positional coordinate (x, y) the

tunneling current is recorded. This yields results in the form of two-dimensional graphs that are usually displayed as images with false-colour scale to represent the intensities in thez direction. Typically, dedicated software are used to process and analyse STM

images. The data presented in this thesis are analysed with the aid of the softwares Image SXM [15] and WSxM [16].

The nature of the acquired signal depends on the mode of operation of the instru- ment. STM can be used in two main modalities, namely in the constant height and constant current modes. In a constant height STM experiment, the tip is scanned

along the surface at a distance in z that is kept constant, while the tunneling current

intensity is collected for each sampled point. The advantage of this mode lies in not having to adjust the tip position along z, a fact that usually results in a higher scan

velocity. On the other hand, this mode of operation is not suitable for characterising areas that possess a high roughness or a non-negligible slope, cases in which the risk of a physical collision between the tip and the sample is considerable. In a constant current STM investigation, conversely, the tunneling current is preset to a value by the experimenter and the z position of the tip is acquired. In order to keep the tun-

neling current constant, and to adjust thezposition accordingly, a feedback system is

required. This procedure may result in an increased acquisition time, but renders the system safer and more reliable towards the risk of accidental crashes between the tip and the surface. All the STM images presented in this thesis are collected in a constant current mode, the most used method in the STM science community.

In any STM experiment, an extremely fine tuning of the (x, y, z) positions is re-

quired: this is particularly crucial for controlling the z position in a constant current

experiment, since a very small displacement in this direction results in a vast change in the total tunneling current (see section 2.3.2 for a quantitative estimation of this effect). The tip movements in the three dimensions are governed by tubular actuators made of piezoelectric materials [17], that show a mechanical response, in the form of expansion

Figure 2.2: STM working principle

or contraction, to electric stimuli. A common material used for the fabrication of a piezoelectric transducer is barium titanate (BaTiO3). Thus, if an electrical potential difference is applied across the piezotubes, these respond with a change in length that is proportional to the applied voltage. Typical values for the voltage sensitivity of piezoelectric actuators are in the order of 10−10 _mV−1_{. In the constant current mode,} thez voltage is recorded for each acquisition point, and from that the tip-sample dis-

tance is deduced by the software. The mechanical response of piezoelectric materials is typically non-linear, and that causes the frequent presence of a drift effect during STM acquisitions. This is mainly due to a creep-like response, with the material undergoing a strain even after the removal of the stress (applied voltage), and needs to be taken into account during STM experiments.

The quality of an STM image is strongly determined by the characteristics of the probe. These are usually unknown, and only approximations can be made on the parts of the tip contributing to the tunneling effect. Ideally only electrons from the very last atom of the tip tunnel towards the surface, and vice versa (see section 2.3.2), but this is not always guaranteed. For this reason, typically the tip can be conditioned via a sputtering treatment with Argon ions (Ar+_{), or via in-situ voltage pulses during} the scanning, with a trial-and-error approach. Two main tip fabrication materials are commonly used: tungsten (W) and platinum/iridium alloy (PtIr). W tips are formed via electrochemical etching using concentrated aqueous solutions of strong bases (NaOH or KOH). This method proves reliable in obtaining very sharp tip apices. The tungsten oxide layer that tends to be formed on the tip, which is not conductive and thus would prevent electron tunneling, is usually easily removed in vacuum via the usual tip conditioning methods (sputtering and voltage pulsing). The PtIr tips are simply made by a transverse cut of a PtIr wire, which is not prone to oxidation and is thus suitable for STM experiments in air or in rough vacuum.

Another part that is critical in an STM instrument is represented by the elec- tronic controls. High voltage power supplies are required to drive the movement of the piezotubes (x, y, z). Furthermore, a low voltage power supply is used to establish a

potential difference between the sample and the tip for a net tunneling current to be established: usually, a bias voltage is applied to the sample, while the tip is earthed. In the constant current mode, the tunneling current passes through an amplifier and is then converted to a voltage. On the basis of this reading, a feedback loop adjusts the high voltage to be sent to thezpiezodrive, in order to keep the tunneling current fixed

at the value set by the user. The z voltage signal is then recorded by a computer and

converted into dimensional units on the basis of thez piezoscanner voltage sensitivity.

This signal gives rise to a two-dimensional plot in the (x, y) plane, which is the final

output of the STM scan.

Finally, a damping system is always required, in order to remove any noise due to external mechanical sources. For this reason the STM scanner is typically mounted on a suspended stage held with springs, that is placed inside the UHV chamber. Additionally it is advisable to place the instrument on an anti-vibration table, in a room with sound- proof walls and at a low-level floor in a building.