Scanning Tunneling Microscope (STM)

2.2.1 An introduction

In 1981, G. Binnig and H. Rohrer Invented STM and obtained the first successful operation with atomic resolution using the system the following year. Since then, STM technique has been widely used in various applications and fields. For example, it was used mostly in the fields such as condensed-matter physics, chemical and biological physics, and etc. Especially after it has resolved the Si(111)-(7x7) structure in real space, STM was then proved to be an extremely powerful and useful tool.

STM uses a sharp conducting tip and applies with a bias voltage between the tip and the sample. When the tip is brought close to the sample electrons can "tunnel" through the narrow gap either from the sample to the tip or vice versa, depending upon the induction of the bias voltage. This tunneling current changes with tip-to-sample distance, it decays exponentially with the distance, which gives STM its remarkably high precision in positioning the tip (sub-angstrom vertically and atomic resolution laterally). For the electron tunneling to take place, both the sample and the tip must be conductive, and thus STM cannot be used on insulating materials. Figure 2.3 shows a diagram with the essential elements of an STM. A detailed explanation will follow that.

2.2.2 The STM system

Fig. 2.3 A schematic diagram showing the essential elements of STM.

As shown in figure 2.3, a probe tip, usually made of tungsten (W) or Pt-Ir alloy, is attached to a piezoelectric scanner. By using the coarse positioner and the z-piezo, the tip and the sample are then brought to a distance within a few angstroms of each other. A bias voltage, applied between the tip and the sample, causes an electrical current to flow. This is a quantum-mechanical phenomenon, tunneling, which is the principle theory of the scanning tunneling microscopy. To achieve atomic resolution, vibration isolation is essential. A commonly used vibration isolation system consists of a set of suspension springs and a damping mechanism. This will help to reduce the amount of vibrations.

2.2.3 Principles of Electron Tunneling

The operating principle of the STM is based on the quantum mechanical phenomenon of tunneling. In this section, we will discuss the concept of the tunneling phenomenon through a one-dimensional model.

From classical mechanics, an electron with energy E moving in a potential U(z) can be described by

where m is the electron mass, 9.1×10^-28 g. In the regions where E > U(z), the electron has a nonzero momentum Pz. It means that the electron has the ability to be in those regions.

Whereas in regions with E < U(z), electrons cannot penetrate into those regions. In other words, electrons with energy E will be unlikely to be found in the regions where E < U(z).

In quantum mechanics, the state of the same electron is described by a wave function Ψ(z), which satisfies the Schrödinger’s equation, as

(z) where Ψ(z) is the wave function of the electron.

For an electron with E = U/2 incident upon in a square barrier from the left, as shown in Fig. 2.4, the Schrödinger’s equation of this electron thus becomes

0

=

Eq. 2.4. can be solved for the transmission coefficient T = |F/A|² by matching of the boundary conditions on Ψ and dΨ/dz at x = 0 and x = s. That is

Because the barrier width s is much thicker than the wave function decay length 1/K,Ks>>1, the transmission coefficient can then be approximated as

It is this exponential dependence of the transmission coefficient T on the barrier width s that enables the atomic resolution images in the tunneling microscopy. It provides a sufficient signal, the tunneling current, for the atomic scale feedback control of the gap width s along the z direction.

2.3 Tip preparation

All STM tips are prepared with the traditional DC drop-off method, as shown in Fig.

2.4. The tips are typically made from cut-to-size tungsten (W) wire with diameters about 0.38 mm. The tungsten wire is electrochemically etched to produce the STM tips. It is an easy way to produce the tip. A piece of the tungsten wire and a cylindrical stainless steel are then inserted into a solution of 2M NaOH. The depth of the tungsten wire is about 1.5 ~ 2 mm below the solution level. A positive voltage about 7 V is applies to the tungsten wire.

This wire acts as the anode while the cylindrical stainless steel acts as the cathode. This is shown in Fig. 2.5. At the anode and cathode the following reactions will take place:

2 2

The reaction etches the wire at the interface of air and the solution. This part then gets thinner and thinner, thereby forming a neck. The weight of the wire down below in the solution will eventually break the neck and causing the immersed portion of the tip to fall off. Therefore, a desired atomic tip is produced. Etching is usually stopped at this point by a feedback controller that senses the reduction in current. To remove the residual NaOH solution from the tip surface the tip are then been soaked in distilled water for 10 minutes and cleaned by pure methanol. The whole electrochemical etching process takes about 20 minutes.

Fig. 2.5 The tungsten wire is electrochemically etched to produce atomic tips. A tungsten wire is vertically inserted in a solution of NaOH as the anode. A cylindrical stainless steel is also inserted in this solution as the cathode. A positive bias is placed on the tungsten wire. This etching takes about 20 minutes.

2.4 Sample preparation

Various sample treatments will be conducted depending upon the type of the sample that will be required for the experiment. The Si(100) samples used in our experiment were sliced from a antimony(Sb)-doped wafers with a dopant concentration of approximately . The misalignment of the wafer is about 0.1 degrees. Before loading the samples into the vacuum chamber, we blow off the dust on the surface of the samples with pure nitrogen gas so we don’t have unwanted particles on the surface of the samples which could affect our measurements. After loading the samples to the UHV chamber, the samples are then being degassed for over 12 hours at ~900 K using a small AC current (300 mA). After degassing, the sample was flashed at ~1450 K for a few seconds in order to remove the oxide layer on the surface and form a dimerized clean Si(100)-(2x1) surface. After the direct heating, chlorine molecules were introduced through a leak valve and a stainless steel tube to the sample surface at room temperature to form the desired Cl-terminated Si(100)-(2x1) structure. A hot tungsten-spiral filament was used to produce atomic hydrogen. The filament was ~ 5 cm away from the Si(100) substrate and heated to ~1800 K when the chamber was backfilled for a period of time T with to a pressure P of about . From the geometry of the filament and the samples, it was estimated that the incident angles of the H atoms was less than ~25 from the normal.

The apparent exposure is presumably proportional to the actual dosage of hydrogen atoms on the surfaces.

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