2-1 Principle of canning tunneling microscopy (STM) [227-231]
In quantum mechanics, Schrödinger’s equation describes an electron with energy E moving in a potential .
(2-1-1).
is the wavefunction, representing the state of the electron at a location z.
The solution of the Schrödinger’s equation is represented as follows when the energy of electron E is smaller than the potential U (see Fig. 2-1-1).
(2-1-2),
where
Fig. 2-1-1 A tunneling junction. The wave function of electron decays at the region where the potential is bigger than the energy of electron.
E
U(z)
Tip
Sample
(2-1-3).
Considering the entire sample states regarding the possible energy intervals (eV), the tunneling current is representing as
(2-1-4).
The local density of states (LDOS) at a location z with the energy E for a sufficiently small є is defined as
(2-1-5).
The LDOS shows the number of electrons per unit volume per unit energy. From Eq. (2-1-2), Eq. (2-1-4), and Eq. (2-1-5), tunneling current can also be expressed as
(2-1-6).
The LDOS can be obtained by calculating the derivative of the current
(2-1-7).
For transmission probability, it has been realized that some distance and energy dependency can be removed from the derivative of the current through dividing
by . The quantity
2-1 Principle of scanning tunneling microscopy (STM)
15
is thus being widely used to identify the density of states of each STM result.
There are a large number of STM studies being conducted to investigate the semiconductor surface with a clean and adsorbate covering. From which, one is indicated that the topic is highly interested by the field. It is also to suggest that STM is the reasonable approach to such system. In addition, one knew the fact that the electronic structure of a reconstructed semiconductor surface may exhibit differently according to its local condition.
Realizing the fact that the structure formed by any of the components ─ adatoms, dimers, or other complicates, is developed on the surface in order to minimize the number of dangling bonds in it, one confirms that STM is an outstanding technique used for studying the reconstructed semiconductor surface.
2-2 Principle of Auger electron microscopy (AES) [232]
Fig. 2-2-1 As electron A in K-shell being ejected by the electron beam, electron B in L1-shell transits to K-shell to fill the empty space. The energy released from this transition is desorbed by electron C in L3-shell. The ejected electron C is the Auger electron.
In AES, the sample is exposed to a primary electron beam with the energy of 3 keV. The electron beam ejects the electron A located in K-shell. To fill up the empty space, the electron B in L1-shell moves to K-shell. The electron C in L3-shell adsorbs the energy released due to the transition of electron B (see Fig. 2-2-1). Then, electron C departs from the atom. The corresponded kinetic energy
of electron C is given by
(2-2-1).
K A
B C primary electron
Auger electron
2-2 Principle of Auger electron microscopy (AES)
17
represents the binding energy of electron in K shell, whereas and stand for the binding energies in L1 and L3
shells respectively.
The ejected electron C is the Auger electron. Since the amount of kinetic energy of an Auger electron only relied on the binding energies of electrons in the atom, it is valued as the characteristic of an atom.
2-3 Principle of low energy electron diffraction (LEED) [233]
The wavelength of electron can be represented by de Broglie equation
(2-3-1).
where m symbolizes the mass of the electron, and indicates the velocity of the electron.
Constructive interference appears when the scattered waves contain the path differences of multiple wavelengths from the neighboring lattice. The represented equation is as follows,
(2-3-2),
or
(2-3-3).
stands for the incident angle of the primary wave, whereas is the directions of the backscattered waves. The distance a is measured between the periodically arranged scatters (see Fig.
2-3-1). Letter n symbolizes the integer denoting the order of the diffraction.
In two-dimensional space, the diffractive pattern is considered as the extension of de Broglie equation. It also corresponds to the reciprocal space of surface. On the other hands, the periodicity in
2-3 Principle of low energy electron diffraction (LEED)
19
Fig. 2-3-1 The filled balls represent the lattice points in the real space. The matter waves of electron interfere forming the pattern that corresponds to the reciprocal space.
2-4 The experimental setup and process
The experiment is performed in two separated UHV chambers.
One is equipped with Omicron variable temperature scanning tunneling microscopy (VT-STM) (see Fig. 2-4-1). The other is set up with the four-grid Retard Field Analyzer LEED, and AES (see Fig.
2-4-2). A concentric hemispherical analyzer (CLAM 2 VG) is used to inspect the data obtained from AES. For the research, we use p-type Ge(111) wafer (1 to 10 Ω-cm resistivity with 500 μm thickness) as the substrate. A K-cell dispenser and a well-collimated e-bean-bombardment type evaporator were for the depositions of Ag and Co respectively. In addition, all STM images presented in this paper are acquired at the temperature of 300 K.
Fig. 2-4-1 The chamber with STM.
2-4 The experimental setup and process
21
Fig. 2-4-2 The chamber with AES and LEED.
2-4-1 Preparation for clean - surface,
- surface, and the Ag/Ge(111)- surface
(i) Ge(111)-c surface: for the experiment, Ge(111)-c surface is cleaned in situ by repeating the procedure applying Ar+ bombardment (1.0 keV), followed by annealing the sample at the temperature between 920 K and 1070 K.
(ii) Ag/Ge(111)- surface: the Ag/Ge(111)- surface is prepared by exposing the Ge(111)-c substrate to Ag atoms of 1 ML coverage followed by annealing at temperature ranging from 720 K to 770 K.
(iii) Ag/Ge(111)- / surface: after annealing Ge(111)-c surface with the Ag submonolayer coverage at the
temperature of 670 K or 770 K, one prepares the region where - and Ag/Ge(111)- coexisted.
2-4-2 Processes for Co deposition and annealing
For Co deposition, as well as the corresponded annealing condition, three types of experimental processes are applied to the experiment in chap. 3-1.
Process Type 1: Co is deposited at the sample temperature of 300 K. The substrate is annealed after Co deposition in situ by direct annealing (Omicron VT-STM). The STM tip traces the same area during annealing the surface, as shown in Fig. 2-4-3. The method we used to determine the temperature is illustrated in chap. 2-4-3.
Fig. 2-4-3 The detected area was traced during annealing.
Process Type 2: Co is deposited at the sample temperature of 670 K. The substrate is annealed by the heating plate. The temperature of the substrate is measured with a K-type thermocouple.
Before Annealing During Annealing After Annealing The detected area STM tip
2-4 The experimental setup and process
23
Process Type 3: Co is deposited at the sample temperature of 300 K. The substrate is annealed by the heating plate after Co deposition. The temperature of the substrate is measured with a K-type thermocouple. Process Type 3 is applied for the experiment in chap. 3-2, chap. 3-3, and chap. 3-4 as well.
Two processes for Co deposition on Ag/Ge(111)- surface are developed. One is the substrate being annealed at 670 K during Co deposition (Process Type 2). The other is the substrate being annealed at the same temperature (670 K) after Co deposition (Process Type 3). The experimental results one obtained from the two processes are similar. That is, the structures of Co islands performed in the two processes are the same. In addition, the area covered by Co islands is limited although the amount of Co coverage applied is as much as 4.9 ML [224]. The percentage of Co covered area is saturated at 50% as annealing process happened during Co deposition (Process Type 2). In contrast, the percentage of Co covered area is saturated at 95% for annealing after Co deposition (Process Type 3). That is, the only difference is that the saturated area of Co islands is lager when annealing the sample during Co deposition (Process Type 2) than after Co deposition (Process Type 3).
2-4-3 Temperature determination for in situ direct annealing at VT-STM stage (Process Type 1)
The temperature of substrate is measured by infrared thermometer for the temperature higher than 850 K. For the temperature lower than 850 K, it is estimated by the linear relationship between the Napierian logarithm of the inverse of resistance and the inverse of temperature for semiconductor when the temperature is lower than 850 K [234-235], as shown in Fig. 2-4-4.
Fig. 2-4-4 The linear relationship between and is applied to extrapolate the temperature of the Ge substrate when it is lower than 850 K.
1.00 1.04 1.08 1.12 1.16 1.20 -0.4
-0.3 -0.2 -0.1 0.0