3.1 Multi-functional UHV systems
The thickness of the deposited layer we made is about only few mono layer, that any nano-scale unexpected deposition, or, pollution will contribute unknown effect to our measurement, or kill the weak micro-phenomenon we want to observe. For example, Lieberman dis-covered the pollution will result in magnetic dead layer in 1969 [37,38], and Hope found that only 0.08 mono layer CO falls on Co/Cu(110) will also change the easy axis [39]. For the purpose to avoid the sam-ple surface being polluted, an ultra high vacuum (< 10−10) is basically recommend. The other way, we can also sketch the pollution rate with respect to the base pressure by the dynamics of gas.
We know the frequency of the gas atoms collide the sample surface is
r = nva
4 (3.1)
where the root mean square velocity of a gas molecular of mass m under Kelvin degree T is
υrms2 = 3kBT
m (3.2)
where and the mean velocity of gas is υa = υrms
Combine these into ideal gas formulation
P = nkBT (3.5)
We can get
r = P
√2πkBmT (3.6)
Substituting, making this formula more readily useful by express-ing P in torr and mergexpress-ing all convention factors into a constant. The result is
r = 3.52 × 1022 P (torr)
pm(a.m.u)T (K)/cm2s (3.7) Using nitrogen of mass 28, room temperature is 300K and the nitrogen pressure is 1 × 10−6 torr to demonstrate the frequency of gas collide on the sample surface, we obtain r = 3.84×1014 bombardments /cm2s. Assume that 100% nitrogen molecules will stick on the sample surface, after 0.1 seconds, will have about 2 ML nitrogen atoms on sample surface of 0.5cm2.
In our UHV system, we can divide it into several areas roughly including the load lock, main chamber, MOKE chamber and the ”beer barrel” which is an important and interesting design that will be illus-trated later. In the pumping system, two sides of the main chamber link the load lock and MOKE chamber with valves respectively. The load lock and MOKE chamber also link the ”beer barrel” with valves respectively. There are two paths to discharge the gas out of the cham-ber. One of the paths is going from main chamber through the load
Figure 3.1: The UHV system in lab. C207 in NTNU
lock to the barrel and then into the air; and the other one is through the MOKE chamber. By these valves, we can control the pumping paths and lock the vacuum of the chamber optionally. The ”beer barrel” is used for the buffer between the UHV system and the air because of the difficulty for turbo to discharge the air into air directly.
In usually, we will open the valve and turn on the mechanic pump to let the pressure of the ”buffer” be about 10−2 10−3 torr four times a day to reduce the consumption of the turbo. Fig3.2 is the diagram of pumping process.
To reach UHV condition, the pumping system and process play an important roles. Usually we use a mechanical pump to fore-pumping
Figure 3.2: The diagram of the pumping process.
down to the pressure about 10−2 to 10−3 torr, then a turbo pump is capably on to help further pumping to 10−7 torr while the main cham-ber is also baked by about 120 0C for 24 hours and reach about 10−8 to 10−10 torr after baking stopped and cool down to room temperature.
When the base pressure is near 10−7 torr, we on and off the ion pomp to out gas, the ion pump will continuously on until it is clean enough. During the process with the base pressure is under 10−6 torr, TSP (titanium sublimation pump) is applied to help pumping and keep UHV getting better.
description
UHV chamber Clear environment
(Base pressure ∼ 1 × 10−10torr)
Scanning Tunneling Microscopy Morphology analysis (STM)
e-beam evaporator Fe-deposition
(home-made)
Auger electron spectroscopy Chemical element analysis (AES)
Low energy electron diffraction Structure analysis (LEED)
Magneto-Optical Kerr effect Magnetic property analysis
(MOKE) (in-plane & perpendicular)
Ion sputtering (Ar+2KeV ) Sample cleaning
Table 3.1: Description of UHV chamber in C207, NTNU
3.2 Scanning Tunneling Microscopy (STM)
Basic concept of STM
The basic idea of scanning tunneling microscopy (STM) is to mea-sure the tunneling current between STM tip and the conductive sam-ple. By applying a voltage between the tip and the sample, although the tip is indirect contact with the sample, a small electric current (0.01 nA - 50 nA) still flows from the sample to the tip or vice versa (depending on the polarity of the applied voltage). This phenomenon is called electrical tunneling, which can be explained by quantum me-chanics. The exponential dependence of the tunneling current on the tip to sample distance results in a high vertical resolution. By scanning the tip across the surface and detecting the tunneling current (one can also use the current as a vertical positioning signal for the tip) a map of the surface morphology can be generated with a resolution in the order of ˚A(depending on the wave function overlapping of atoms). It has to be mentioned that the image cannot just be interpreted as a topographic map because the tunneling current is influenced by the lateral (one can consider the shape limit of tip) and vertical variation of the electronic state density at the surface. The lateral resolution is about 1 ˚Awhereas a vertical resolution up to 0.01 ˚Acan be achieved.
The STM can be used in ultra high vacuum, air or other environments.
Theory
In classical physics, an electron cannot penetrate into or across a potential barrier if its energy E is smaller than the potential within the barrier. A quantum mechanical treatment predicts an exponential decaying solution for the electron wave function in the barrier. For a rectangular barrier we get
Ψ(d) = Ψ(0)e−k·d (3.8)
k = p2m(Φ − E)
~
(3.9) Scanning Tunneling Microscope (STM) is a potent tool for us to investigate the physics of surface at atomic level including the action of particle and surface morphology. For the STM, its resolution can reach to be 0.1 nm lateral resolution and 0.01 nm depth resolution.
With the resolution, we can routinely catch the image of the atoms.
The STM can be used not only in ultra high vacuum but also in air and other liquid or gas ambient, and at temperatures ranging from near 0 K to a few hundred degree Celsius.
The basic concept of STM is based on the quantum tunneling effect. When we apply a bias (a voltage differential value) between the conducting tip and the surface, which the tip is very close to the surface, the electrons will tunnel through the vacuum between them.
The resulting tunneling current will be the function of tip position, applied voltage (bias) and the local density of states (LDOS) of the
sample. How we get the information is monitoring the current as the tip scans through the surface and it will form an image. Since STM is such a micro-probing machine, a clean surface, a sharp tip and a stable ambient are highly required.
Classically, an object hitting an impenetrable barrier will not pass through. In contrast, objects with a very small mass, such as electrons, have wavelike characteristics which permit an event, referred to as tunneling. Assuming the 1-D case, the electrons behave as beams of energy and in the presence of potential U (z). Then the energy levels ψn(z) are given by the solutions to Schr¨odinger’s equation,
~2 2m
∂2ψn(z)
∂z2 + U (z)ψn(z) = Eψn(z) (3.10) where ~ is the reduced Plank’s constant, z is the position, and m is the mass of an electron. When E > U (z), the wave function is : For E < U (z), inside the barrier, the wave function is :
ψn(z) = ψn(0)e±κz (3.13) where
κ = p2m(U − E)
~
(3.14) In the case of tunneling, the wave functions of the tip and sample will overlap when we apply a bias and it will have the probability to find the electron in the barrier region and even on the other side of the barrier. The probability P is :
P ∝ |ψn(0)|2e−2κW (3.15) where W is the barrier width. If the bias is small, we can let U − E ≈ ϕM in the expression for κ, where ϕM , the work function, gives the minimum energy needed to bring an electron from an occupied level, the highest of which is at the Fermi level, to vacuum level.
When a small bias V is applied to the system, only electronic states very near the Fermi level are excited and these excited electrons can tunnel across the barrier.
In other words, the tunneling current is from the electrons near the Fermi level, and is proportional to the probability of the decaying wave function.
One can sum the probability over energies between Ef − eV and eV to get the number of states available in this energy range per unit volume, thereby finding the local density of states (LDOS) near the
Figure 3.3: Schematic diagram of electron tunneling
Fermi level. The LDOS near some energy E in an interval ε is given by
ρs(z, E) = 1
E
X
E−
|ψn(z)|2 (3.17)
The tunneling current applied a small bias is proportional to the LDOS near the Fermi level. So
I ∝ V ρs(0, Ef)e−2κW (3.18) where ρs(0, Ef) is the LDOS near the Fermi level of the sample surface.
Now, Fermi’s Golden Rule gives the rate for electron transfer across the barrier :
w = 2π
~
|M |2δ(Eψ − Eχ) (3.19) where Eψ − Eχ restricts tunneling to occur only between electron levels with the same energy. The tunnel matrix element, given by
M = ~
is a description of the lower energy associated with the interaction of wave functions at the overlap, also called the resonance energy.
Summing over all the states gives the tunneling current as
I = 4πe where f is the Fermi function, ρs and ρT are the density of states in the sample and tip, respectively.
Operation mode
As I know, the operation of STM can divide into two modes roughly.
1. Constant current mode(Fig3.5): By using a feedback loop, the tip is vertically adjusted in such a way that always stays con-stant. The software will record the vertical position of the tip during scanning. Because of the current is proportional to the
Figure 3.4: The loop of a working STM(Text and graphics by Michael Schmid, TU Wien)
local density of states, it can transfer to the image of surface morphology.
2. Constant height mode(Fig3.6): By contrast with the constant current mode, this mode will fix the vertical height of the tip and record the variation of tunneling current to get the surface morphology.
Figure 3.5: STM constant current mode Figure 3.6: STM constant height mode
3.3 AES and LEED
An atom which has been ionized in a core level may come back to its ground state by the following way. One is that an electron in higher state jumps into the core level and simultaneously releases the energy by x-ray radiation (Fig. 3.7(a)). The other one is that an elec-tron in higher state jumps into the core level and simultaneously the energy is transmitted to another electron, which may leaves the atom with a characterizing kinetic energy (Fig. 3.7(b)). The second process is called Auger process and the emitted electron is Auger electron.
Therefore, the Auger electron has a characterizing kinetic energy de-noting the kind of atom where it comes from. By detecting the Auger electron, the composition of the sample can be characterized. In gen-eral, the kinetic energy of the Auger electrons are less than 1000 eV, so their mean free path are almost less than 20 ˚A. That is why the Auger electron spectroscopy (AES) which can only detect the region within the mean free path of Auger electron is used as a standard tool
in surface science.
Figure 3.7: Process for demagnetization of atomic core holes. (a) Emission of X-ray radiation. (b) Emission of an AES electron.
Because many properties of ultrathin film, such as mechanical, electronic, and magnetic properties are strongly correlated to the ar-rangement of atoms, i.e. the geometric structure of the surface, the exact knowledge of this structure is an important information for the quantitative understanding. Well-defined and periodic structures like the surfaces of single crystals are most suitable for structural inves-tigations. In most cases the bulk structure is known through X-ray diffraction. However, the surface structure within only a few atomic layers, usually is not identical to that of the bulk. From the de Broglie relation λ = h/p, with electron energy ranging from 25 eV to 600 eV, the wavelength varies from 2.5 ˚A to 0.5 ˚A which is comparable to the lattice constant. Therefore the most ideal and reliable tool we can utilize to investigate those structures is the low energy electron diffraction (LEED). By the analysis of LEED patterns, the intensities,
the size and the shape of the surface unit cell, the degree of order and the atomic structure can be determined with high precision. Further-more, LEED measurement with a CCD camera allows fast and reliable acquisition of data.
Figure 3.8: Schematic display of a LEED structure
3.4 Magneto-Optical Kerr effect(MOKE)
If a linear polarized light is incident into a ferromagnetic sample, since of the different reflection coefficients of right and left circular polarization components, the reflection beam will become elliptical polarized. This phenomenon is so called magneto-optical Kerr effect.
The angle between the primary axis of the elliptical polarization and the linear polarization is called Kerr rotation, and the ellipticity of the elliptical polarization is called Kerr elliptical, as shown in Fig. 3.9.
Figure 3.9: Schematic illustration of magneto optical Kerr effect. After reflected from the ferromagnetic sample, the linear polarized laser beam becomes elliptical polarized.
Let r+eiθ+ and r−eiθ− stand for the reflection coefficients of right and left circular polarization, respectively. The Kerr rotation and Kerr ellipticity can be illustrated as ϕk = −θ+−θ2 − and εk = ab =
r+−r−
r++r−, respectively. Both of them are proven to be proportional to the magnetization of sample. Thus by measuring ϕk and εk with cyclic
applied magnetic field, we can get the hysteresis loop. In general, there are three types of MOKE measurement. Each of them has different geometry of the magnetization and the light path, as shown in Fig.
3.10. In the polar Kerr effect, the magnetization lies in the plane of incidence and is perpendicular to the surface. In the longitudinal Kerr effect, the magnetization lies in the plane of incidence and is parallel to the surface. In the transverse geometry, the magnetization is perpendicular to the plane of incidence and on the surface.
Figure 3.10: Different geometry for MOKE measurement
In magnetic ultrathin films, the Kerr signal is so small that the noise may result in significant effect. Therefore, in our experiment, a modulator is added between the polarizer and the sample such that the modulated signal can be taken by lock-in technique with a larger ratio of signal to noise. The schematic illustration is shown in Fig.
3.11. Due to the difference with the DC MOKE shown in Fig. 3.9, this method is called AC MOKE.
Fig. 3.12 is a MOKE chamber in our system. Part A is a combi-nation of ”tilt” and ”rotator”. It can adjust the direction and angle of the sample to let the angle between incident laser and sample be
Figure 3.11: Schematic illustration of AC MOKE
450. Part B has four electromagnets. We can control the direction of magnetism by changing the direction of current passed through the electromagnets. As a result of the above two equipments, we can mea-sure the magnetism in longitudinal and polar direction at the same time without moving the sample.
In our system, we can measure the magnetism at low temperature.
It’a s simple concept that let the cold trap be full of liquid nitrogen.
The bottom of cold trap sticks to sample holder tightly by using a specific gasket which is done with copper. Because of the heat trans-mitted by the contact between sample, sample holder and clod trap, the gap of screw reduces the efficiency of thermal conduction. So we add some cooper wire to help the sample cooling down to 100 K in ten minutes, just like Fig. 3.14. And the process of measurement is shown in Fig. 3.15.
characteristics
Magnetic field We have four electromagnets to form the magnetic field which is varied as the direc-tion of passing current being changed. As a result, we can measure the magnetism in in-plane and perpendicular directions at the same time without moving the sam-ple.
Temperature We can do the MOKE measurement at
room temperature(RT) & low tempera-ture(LT:100K)
Modulator In our system, it’s used in a different way with the general AC MOKE. A modula-tor is added on the laser source and mod-ulated signal can be taken by lock-in tech-nique with a larger ratio of signal to noise.
Parameter of SR810 DSP Lock-in frequency : 10KHz
Amplifier Amplitude : 0.4V
Phase : −88.020
Table 3.2: Description of MOKE chamber in C207, NTNU
Figure 3.12: MOKE chamber in NTNU C207 Lab
Figure 3.13: Specific gasket for LT-MOKE measurement
Figure 3.14: Cu wire for cooling sample
Figure 3.15: Schematic display of a DC-MOKE loop in C207, NNU