2.1.1 Ultra-high vacuum system
The STM and FIM experiments were conducted in an ultrahigh- vacuum (UHV) system. Our system needs to connect different working range of the pumping system. The pumping system is consisting by a dry pump, a turbo pump, a titanium sublimation pump (TSP), and an ion pump. The base pressure of this vacuum system is 1×10-10 torr.
The dry pump is used first to lower pressure in the vacuum chamber to ~10-3 torr. Then the turbo pump automatically starts to lower the pressure to the 10-7 torr range. At this lower pressure, the
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the chamber at about 120 °C for over 12 hours (FIM) or longer (STM). After the chamber cools down to RT, we gain the ultra-high vacuum about 1 × 10-10 torr.
2.1.2 UHV LT-STM system
All experiments completed in ultra-high vacuum (UHV). USM - 1400 system is a low-temperature STM in an UHV chamber. Fig.
2.1.1 (a) is the picture of our system, and (b) is STM part inner the main chamber. This STM system has three separated UHV chambers.
With the combination of liquid helium and nitrogen cryostat and three layers metal shielding outside the STM part, this cryostat is constructed as two tanks so that it can save liquid helium consumption. The outer tank is for liquid nitrogen, while the inner tank is for liquid helium. However, the operating minimum temperature can be lowered to 2 K. But, our all experiments conducted at 77 K. A prepare chamber isolated from the STM chamber, because it can let us prepare the sample and not interfere the main chamber. The preparation processes include cycles of sputtering and annealing for the substrates and deposition of target materials. Thus we have a sputtering system, an annealing system, and deposition systems in our prepare chamber. Finally, a load-lock
chamber isolated from prepare chamber can let us charge sample between atmosphere and UHV.
Fig. 2.1.1. (a) The photo image for USM- 1400S STM. Each arrow represents one of the components on the machine. (b) Major part of the STM.
2.1.3 Principle of scanning tunneling microscopy (STM)
Scanning Tunneling Microscope (STM) was invented by G.
Binnig and H. Rohrer in 1981 [1]; they shared the 1986 Nobel Prize in Physics for their invention. With the abilities of real-space surface image, atomic resolution and local density of states (LDOS) electrical analysis, STM has been widely used in many fields, such as condensed-matter physics, chemical and biology.
Fig. 2.1.2. Model is the tunneling effect in the STM. Rectangular potential barrier and particle wave function U(z). The energy of the tunneled particle is the same but the amplitude is decreased.
In classical physics an electron cannot penetrate into or across a potential barrier if its energy E is smaller than the potential Ψ(z) within the barrier. But, quantum mechanics treatment predicts an exponential decaying solution for the electron wave function in the barrier, the tunneling model shown in Fig. 2.1.2. For a rectangular barrier we get
In quantum mechanics, we know the Schrödinger’s equation [2]
describes an electron with energy E moving in a potential U(z).
− ħ 2𝑚
d2
d𝑧2𝛹(𝑧) + 𝑈(𝑧)𝛹(𝑧) = 𝐸𝛹(𝑧) (2.1.1) For a rectangular barrier we get
𝛹(𝑧) = 𝛹(0)𝑒−𝜅𝑧 (2.1.2) results in a tunneling current I. The height of the barrier can roughly
be approximated by the average workfunction of sample and tip [3].
U =1
2(𝑈𝑠𝑎𝑚𝑝𝑙𝑒 + 𝑈𝑡𝑖𝑝) (2.1.5)
If the voltage is much smaller than the workfunction eV << U, the inverse decay length for all tunneling electrons can be simplified to
𝜅 =√2m𝑈
ħ (2.1.6)
The current is proportional to the probability of electrons to tunnel through the barrier:
𝐼 ∝ ∑ |𝛹𝑛(0)|2
𝐸𝐹
𝐸𝑛=𝐸𝐹−𝑒𝑉
e−2ĸ𝑧 (2.1.7)
Therefore, the current markedly changes as the distance varies.
The STM can do measurements of surfaces up to atomic scale by utilizing this property. Besides, before the exponential component, there is another component which is also can modify the tunneling current. This component is related to the LDOS [3]. The LDOS at a location z with the energy E for a sufficiently small є → 0 is defined as:
The LDOS shows the number of electrons per unit volume per unit energy. From Eq. 2.1.2, Eq. 2.1.7, and Eq. 2.1.8, tunneling current can also be expressed as:
𝐼 ∝ 𝑉𝜌(𝑧, 𝐸) (2.1.9).
The LDOS can be obtained by calculating the derivative of the current:
d𝐼
d𝑉 ∝ 𝜌(𝑧, 𝐸) (2.1.10).
Therefore we can get the LDOS of the sample by doing the scanning tunneling spectroscopy (STS). Here is the measurement of change in current as a function of applied voltage with the tip held at a constant height. Since dI/dV is proportional to the density of states of the sample, p, the derivative of this curve corresponds to the LDOS.
STM has become a key tool for the investigation of superconducting materials by combining the high spectroscopic energy resolution possible in tunneling measurements with the high spatial resolution a scanning probe technique can provide.
2.1.4 UHV FIM system
Field ion microscope (FIM) was invented by Erwin E. Müeller in 1951 [4] at the Pennsylvania State University. The FIM invention was first time to observe atoms by human. It was developed from its forerunner, the field emission microscope (FEM) [5]. The instrument features is a sharp tip sample mounted on an electrically insulated stage, and that will be cooled to cryogenic temperatures (20 to 80K) in a ultrahigh vacuum chamber shows in Fig. 2.1.3 (a). The field ion image of the sample is formed on a micro-channel plate and phosphor screen assembly that is positioned approximately 10 cm in front of the sample, instrumentation Diagram shows in Fig.2.1.3 (b). To produce a field ion image, controlled amounts of image gas are admitted into the vacuum system. The type of image gas used depends on the material under investigation; common images gases are neon, helium, hydrogen.
The FIM imaging mechanism shows in Fig. 2.1.4. When high electric field is applied on a sharp surface of a tip, and imaging gas atoms are ionized at the protruding sites on the sample surface [6, 7].
These ions hit screen along the electric field line, forming brightly imaging spots. Hence, the FIM is a projection type microscope of an atomic resolution with an approximate magnification of a few million
Fig. 2.1.3 (a) Schematic diagram of detail of the sample holder, sample loop and cooled cryostat contact. (b) The photo image for FIM. Each arrow represents one of the components on the machine.
times. By applying higher electric filed, surface atoms of the specimen are also ionized. This high electric field produced evaporation phenomenon is usually called field evaporation if the surface atoms are lattice atoms, and is called field desorption if they
a
b
desorption layer by layer of atoms on surface. So, the field evaporation mechanism is easily to clean the shape surface of a tip.
Field evaporation is a field induced process which involves the removal of atoms from the surface itself at very high field strengths and typically occurs in the range 2-5 V/Å . Atoms always evaporate from the surface, so the special resolution in the depth direction is a mono-atomic layer. A unique feature of the atom probe compared with the other analytical instrument is its extremely high spatial resolution and the equal detection efficiency for light elements.
Fig. 2.1.4 Schematic diagrams the imaging mechanism of FIM.
2.1.5 Principle of FIM system
In 1928, Oppenheimer observes the field ionization phenomena formed by quantum tunneling effect [8]. However, this inference was confirmed in FIM experiments by E.W. Müller in 1951.
In free atoms, the electrons are trapped in the potential wells in an atom shows in Fig. 2.1.5 (a).The atoms ionized needs us to give the energy greater than “I” of the inner electron. But the potential will be changed when the atoms placed in an electric field shows in Fig.
2.1.5 (b). Therefore, we can use the Wentzel-Kramer-Brillouin (WKB) to calculate the probability of tunneling out of barrier:
When this atom gas is close to the metal surface, the Eq. 2.1.11 with an electron potential approximate formula:
Figure 2.1.5 (c) show the barrier width narrows as the atom gas close to metal surface which critical value xc. This value determines whether occur the atomic electron tunneling. However, the energy of the electron from the gas atom must coincide with, or be higher than, the lowest available conduction level in the metal, which is close to the Fermi level. If this condition is not fulfilled, there are no vacant energy levels in the metal available for the tunneling electron [6, 9, 10, 11–16]. After considering the relevant conditions can be obtained the formula:
Fig. 2.1.5 The principle of field ionization.
a b
c
2.2 Experimental procedure