The STM experiment was conducted in an ultrahigh-vacuum (UHV) system. The main chamber is equipped with a variable-temperature scanning tunneling microscopy (VT-STM, Omicron), a manipulator, a pumping system, gas sources including H2, Cl2 and HCl, as shown in Fig. 2.1. The pumping system is consisting of 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 10-6 torr range. At this lower pressure, the ion pump turns on. As the pressure drops to ~10-7, we start to bake the chamber at about 120 °C for over 24 hours. After the chamber cools down to RT, we gain the ultra-high vacuum about 1×10-10 torr.
The core-level-photoemission experiment is carried out at the National Synchrotron Radiation Research Center (NSRRC) located in the Hsin-chu Science-based Industrial Park, Taiwan. Light from the 1.5-GeV storage ring was dispersed by a Dragon-type 6-m wide range spherical grating monochromator (SGM). This beamline has two energy range, i.e.
10-175 eV from a low energy branch and 120-1500 eV from a high energy branch. In our experiment, we use the high energy branch since the main photon energies used are 140, and 240 eV. All the adsorptions of H, Cl and HCl were prepared in situ in the ultra-high vacuum system, as shown in Fig. 2.2. In the photoemission experiment, the procedure to obtain the ultra-high vacuum is the same as the STM experiment.
Figure 2. 1 The UHV system of VT-STM.
Figure 2. 2 The vacuum system for core-level-photoemission spectroscopy.
2. 2 Scanning Tunneling Microscopy (STM)
Since Binnig et al. invented the Scanning Tunneling Microscopy (STM) and obtain the atomic resolution in 1982, the STM technique has been widely used in various fields, like condensed-matter physics, chemical, biology physics and etc. Especially, after resolving the structure of the Si(111)-7×7 in real space using STM , this instrument has proved to be an extremely powerful tool.
Figure 2.3. displays its essential elements. A probe tip, usually made of tungsten (W) or Pt-Ir alloy, is attached to a piezoelectric scanner. Using the coarse positioner and the z piezo, the tip and the sample are brought to 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.
Figure 2. 3 Schematic diagram displays the essential elements of STM
The operating principle of the STM is based on the quantum mechanical phenomenon of tunneling. In this section, we discuss the concept of the tunneling through one-dimensional model. First we consider the classical situation. In the classical mechanics, an electron with energy E moving in a potential U(z) is described by
2 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. Otherwise, in the regions where E < U(z), the electron can not penetrate into those regions. In other words, the electron with energy E has no possibility to be find in the regions with U(z) >E. Now we discuss the quantum effect.
In the quantum mechanics, the motion of the same electron is described by the Schrödinger’s equation, Ψ(z) is the wavefunction of the electron.
Figure 2. 4 Wave function Ψ(z) for an election with kinetic energy E = U/2 penetrating a potential barrier U.
For a electron with E = U/2 incident on a square barrier from the left, as shown in Fig. 2.4.
The Schrödinger’s equation of this electron 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 a barrier of width s that is much thicker than the wave function decay length of 1/K, KS >> 1, the transmission coefficient can be approximated as
e ks It is this exponential dependence of the transmission coefficient T on the barrier width s that enables atomic resolution images in tunneling microscopy. It provides a sufficient signal, the tunneling current, for atomic scale feedback control of the gap width s along the z direction.
Interestingly, use of 1° miscut Si(100) single-crystal wafers allows for highly rotationally oriented samples in which all the Si-Si dimers are pointed in the same direction, yielding anisotropic surfaces on a centimeter length scale. The high ordering of the dimers, showing both the filled and empty states, is shown in the stunning STM images of Fig. 2.5;
the filled and empty states were imaged by changing the tip bias.
Figure 2. 5 STM images of the Si-Si dimers, imaged with (a) Vs = -2.2 V and (b) Vs = +2.6 V. The filled and empty states of these highly ordered dimmers can be probed by biasing the surface in the opposite directions. The dimensions of the figure are 2.3 nm × 2.3 nm.
2. 3 Core Level Photoemission
The core level photoemission experiment is to collect the photoelectrons excited from core level near nucleus. Photoelectrons were collected and analyzed by a large hemispherical analyzer. By measuring the variation of the photoelectron kinetic energy, we can observe the species of the passivated atoms and chemical bonding etc.
The photoelectrons are excited from inner energy levels (binding energy >20 eV), of which the orbital radius is less than 0.3 Å. In solid state, the core level wave functions are independent such that the binding energies of the atoms in bulk are the same. However, the potential of the atoms near surface becomes different because the local atomic environment changes. The potential difference of surface atoms results in chemical shift of the core level binding energy.
We can explain the relationship between the kinetic energy (KE) of excited photoelectrons and energy of incident photons by the energy conservation law as Eq. 2.7.
The relation of the energies is shown in Fig. 2.6.
KE=hν-B-Φ
………(2.7) KE : kinetic energy of excited photoelectron
hν: photon energy B: binding energy Φ: work function.
In this formula, the binding energy B is the difference between the core level and Fermi level. The work function Φ is the difference between the Fermi level and vacuum level. This formula is based on the ideal situations; however, we have to consider other factors like secondary electrons and escape depth etc. The escape depth of the excited photoelectron is dependent on the kinetic energy, in other word, the higher kinetic energy, the larger escape depth. Therefore, the escape depth of photoelectrons of kinetic energy 20 eV ~ 110 eV is less than 10 Å. The spectra obtained by analyzing these photoelectrons provide us the message of the surface.
Figure 2. 6 Schematic for the energy levels in the core-level photoemission.
After electrons excited from core level, left holes will be occupied by other electrons.
The reaction of occupation can occur in two processes. First, the electrons in the higher energy level occupy the left holes and release the photons of energy equivalent to the difference between two levels. Next, the electrons in the higher energy level occupy the electron holes and release energy. The released energy is not carried by photons but directly excites electrons to leave surface. The excited electrons in the second process are so-called Auger electrons. The Si 2p and Ge 3d core level photoemission is mainly contributed from Auger electrons.
The lifetime of the electron holes yields Lorentzian broadening. The other factor to result in broadening spectra is the resolution of the analyzer, which produces a Gaussian width of the spectra. The convolution of the Lorentzian width and Gaussian width yields a Voigt lineshape for the spectra.
2. 4 Sample Preparation and Temperature Measurement
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 up into pieces of size 1×8 mm2 from a antimony (Sb) doped wafers with a dopant concentration of approximately 1.5×1015 cm-3. 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. 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)-2×1 surface.
The substrate was heated by passing a controlled dc current directly through the sample. The sample temperature that corresponds to each current was obtained using an infrared optical pyrometer and calibrated by gluing a tiny type-K thermocouple to the center of the sample following the final last STM run, as shown in Fig. 2.7. The uncertainty in the temperature measurement was estimated to be approximately 5 K.
Figure 2. 7 A chart of the sample current vs. corresponding temperature.
2. 5 The Tip of STM
The size, shape and cleanliness of a scanning tunneling microscope (STM) tip are very important for the resolution of a STM. In the UHV system, the W tip is often used. A W tip has to be prepared by electrochemical etching.
For etching we used a tungsten wire as the anode electrode in a special design screw, a circle stainless steel was taken as the cathode, and a 2 M NaOH solution is used as the electrolyte, as shown in Fig.2. The following reactions take place.
2 2(g)
The tip was cut from straight W (purity is 99.99%) wire of 0.5mm diameter and the 6mm length (Goodfellow Ltd, England) because the diameter of the tip holder is also 0.5mm.
In order to remove the oxide layer of the surface W wires, the wires has to be slightly (about 5 seconds) electropolished over a large part of the wire in the NaOH solution with constant voltage of 15V.
The depth of wire under the surface of NaOH is about 2mm.When the tip start to etch, etching reaction is happened at the interface of the air and the solution. The power supply has constant voltage with an automatic switch-off control. During etching the current through the tip will decrease linearly with time, because the tip below the solution surface decrease and the resistance of the tip (anode) increase. After a long time, the tip forms a neck shape and become thinner and thinner. Eventually the part of the neck is cut down by the weight of the wire under the solution surface dropped down. To remove the residual NaOH solution from the tip surface the tip are then been soaked in de-ioned water for 30 minutes and cleaned by pure methanol. Finally, the tip is set in the tip holder, and backing with 120℃ in the transport chamber at P=10-8 torr, after 16 hours the tip is complete.
Figure 2. 8 The sketch of the etching procedure for the tungsten tip. 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.
Figure 2. 9 SEM images of tungsten tip. (a) Macrostructure of a 0.5 mm diameter wire after electrochemical etching with DC current. (b) A 0.5 mm diameter wire after electrochemical etching with AC current.
Figure 2. 10 Omicron STM tip holder.
2. 6 The evaporant of KCl and NaCl in EFM 3
In the UHV evaporator EFM3 (Evaporator with integral flux Monitor) is made from Omicron Vakuumphysik GMBH. The NaCl evaporate is evaporated from a crucible which is made of Al2O3 (see Fig 2.). This is achieved by electron bombardment heating. The bombarding electron beam induces a temperature rise at the top of the evaporant, causing evaporation.
From appreciate crucibles low melting point, low vapor pressure or reactive material can be evaporated. The appreciate material of crucible can be made of Al2O3, pyrolytic boron nitride (pBN), grapite, tantalum, and etc. In my experiment the Al2O3 crucible is chosen for setting the NaCl of evaporant. The instrument is designed for high precision sub-mono-layer up to multi-layer deposition of a wide variety of evaporants including highly refractory materials. The fig 2. is the dimensions of Al2O3 crucibles.
An important feature of the EFM3 is the integrated flux monitor. In order to obtain the precise rate, once calibrated the flux monitor replaces the necessity of a quartz thickness monitor by continuously monitoring the evaporation rate. Flux is measured directly, which allows a much more precise rate adjustment and much faster rate control than an indirect.
The beam exit column contains an ion collector which serves as a flux monitor. At a given electron emission current (IEM) and e-beam voltage the ion flux measured there is directly proportional to the flux of evaporated atoms. The ion flux is displayed on the left indicator of the electronics unit. The flux monitor also operates with the shutter closed thus allowing to preset the evaporation rate.
The EFM3 comes with an shutter at its outlet which can be opened and closed by a rotary drive. This allows precise flux adjustment prior to exposure, and exact control of the evaporation time.
The evaporation cell is contained in a water-cooled copper cylinder (cooling shroud).
When NaCl is evaporated, the filament is heating and causing the background pressure increased. So the cooling shroud usefully prevents the background pressure increase too high. But this also depends on the material of evaporant and the pumping speed of the vacuum system.
In my experiment of KCl/Si(100) and NaCl/Si(100), the HV = 800 V, flux = 200 nA ,
IEM = 6 mA, and Ifil = 2A, was used. Eventually the molecular bean was giving about 1ML/minute. The ML is referred to the surface density of the unreconstructed Si(100) surface, i.e. 1ML = 6.8×1014/cm2.
Figure 2. 11 The evaporator EFM instrument of (a) outside view and (b) mounting a crucible.
Material Measure
Ta/Mo mm
pBN mm
C mm
Al2O3
mm
SSteel mm
ID 4 4.5 5.3 6 5
OD 5 or 8 8 10 10 7
L 6 7 10 14 15
Tmax (℃) 2000 1600 1400 1400 800
Figure 2. 12 The dimension of crucibles.
2. 7 ICl and IBr
Vapors of IClwere introduced into the vacuum chamber through a precision leak valve and a tube facing the sample while tips were retracted for hundreds of nanometers.
The process of Freeze-Pump-Thaw Degassing for ICl(or IBr)
(1) Place the solution of ICl(99.999%) in a glass tube shown in Figure 2. 13. Make sure the leak valve is closed. Be careful not to use more than 50% of the volume of the glass tube because overfilled the glass tube frequently shatter during this process.
(2) Hook it up to the oil pump and freeze the liquid of ICl. Liquid nitrogen is usually best for this. Before freezing make sure that the environment in the glass tube is free of oxygen to prevent condensing liquid oxygen upon freezing.
(3) When the solution is frozen, only open the valve B to vacuum and pump off the atmosphere for 10-30 minutes.
(4) Close the valve B.
(5) Thaw the solution until it just melts using a tepid water bath. You will see gas bubbles evolve from the solution of ICl. Try not to disturb the liquid. Note: Letting the frozen solution thaw by itself, or using a container of water that melts only the bottom of the frozen solution of ICl may cause the vessel to break.
(6) Replace the water bath with the cooling bath and refreeze the solution of ICl.
(7) Repeat steps (3) – (7) until you no longer see the evolution of gas as the solution thaws. The solution should be put through a minimum of three cycles.
(8) The solution of ICl is ready to use.
Figure 2. 13 experimental setup for the ICl.