Magic clusters on group IV surfaces 3

Figure 3.1: Plan view of OMICRON UHV schematic Fast Entry Chamber Analysis Chamber STM... Figure 3.2: Side view of OMICRON UHV schematic Turbo Pump Ion Pump Pneumatic Gate Valve Electro

Chapter 3: Experimental Procedures

3.1 The UHV System

In the study of surface structures in the nano-scale regime, effects of surface and interfacial energy become more pronounced as opposed to studies of bulk material characteristics This inevitably requires an atomically clean and flat substrate surface without the undesirable influence of contaminants on surface/interfacial energies in order

to accurately ascertain surface phenomenon As main sources of contaminants are typically oxygen, water or carbon-related species etc, which are found abundantly in the atmosphere, a UHV system is crucial towards providing a clean environment for in-situ sample preparation and material deposition It allows source materials to be maintained at elevated temperature in UHV over long period of time which tends to eliminate moisture and results in higher purity source material and higher quality monolayer film This environment also allows the sample surface to remain contaminant-free within the experimental time frame so as to ensure accuracy and consistency in observations

The reduction of contaminants such as moisture is important, as these impurities are known to affect the preparation of clean surfaces and prevent high quality layer growth due to creation of crystal defects and carrier traps This impedes surface migration

of adsorbates as the defects often act as nucleation centers for the formation of lattice defects, such as stacking faults and dislocations, which modify the adsorption and growth chemistry of materials

As the UHV environment does not suppress the occurrence of contaminants indefinitely, it is important to be able to appreciate the rate of contamination in order to plan the experiments By assuming that the rate of contamination is analogous to the rate

of arrival of gaseous molecules on a clean surface and from consideration of the kinetic

theory of gases, we can describe the rate of surface bombardment by molecules, Z, as

given by:

mkT

p Z

As the rate of surface contamination also depends on the sticking probability, S(θ),

we can, by assuming the worst case of S(θ) = 1, to estimate the coverage of CO (a typical

gaseous contaminant at 300K) at ambient pressures of 10-6 Torr and 10-10 Torr, respectively (1 Torr = 1.333 x 10-2 N.cm-2), using Eqn (3.1) At 10-6 Torr, Z is

determined to be 3.82 x 1014 cm-2.s-1 Assuming an atomic density of 1015 cm-2 (typical of most surfaces), this will imply a rate of contamination of 0.382 ML.s-1 Alternatively, the

ambient pressure is now 10-10 Torr, the time taken to saturate the clean surface will be

26178 seconds, which is about ~ 7 hours Hence these calculations demonstrate the importance of conducting experiments under UHV regimes where ambient pressures are

< 10-9 Torr, which will allow sufficient time for sample preparation, film growth and characterization before surface contamination

For this work, the samples were introduced into an UHV environment via a fast

entry lock and the experiments which involve in-situ sample preparation, XPS scanning

and STM imaging were performed in an OMICRON UHV System It comprises of 3 main chambers; a fast entry lock chamber, a preparation chamber and an analysis

chamber which are all interconnected to allow the experiments to be conducted in-situ

Figures 3.1, 3.2 and 3.3 show the OMICRON UHV system layout

Figure 3.1: Plan view of OMICRON UHV schematic

Fast Entry

Chamber

Analysis Chamber

STM

Figure 3.2: Side view of OMICRON UHV schematic

Turbo Pump Ion Pump

Pneumatic Gate Valve

Electron Beam

Sources

Manipulator

XPS Analyzer STM

Sample Transfer

Magnetic Probe

Arms

LEED STM Manipulator

The fast entry lock (FEL) chamber is pumped by a rotary pump and a molecular pump which makes it possible for the chamber to be pumped down in stages from atmospheric pressures until pressures equivalent to preparation chamber conditions

turbo-As the FEL is separated from the preparation and analysis chamber by a gate valve, this allows for the introduction of samples and tips without disrupting the vacuum conditions

in the preparation chamber The main function of the preparation chamber is to clean

sample surfaces and deposit thin films in-situ This chamber is pumped primarily by a

VARIAN turbo-molecular pump (useful for pumping light gases) with a pump speed of

500 litres/sec and a VARIAN VacIon Plus 150 ion pump with a pump speed of 150 litres/sec (useful for maintaining oil free UHV conditions) The analysis chamber is linked to the preparation chamber via a gate valve and consists of the main analytical tools such as STM and XPS This chamber is pumped by a second VARIAN turbo-molecular pump with a pump speed of 500 litres/sec and a VARIAN VacIon Plus 300 ion pump with a pump speed of 300 litres/sec Both preparation and analysis chambers are also equipped with titanium sublimation pumps, which sublime titanium to getter chemically active gases if required

3.2 The sample holder and transfer system

The substrates are cut into 4mm by 9mm strips before ex-situ preparation and mounted onto Direct Heating (DH) sample holders provided by OMICRON The holders are made mainly from molybdenum and tantalum due to the high temperature stability of these materials as samples are expected to be heated to temperatures of up to 1200°C by passing a direct current through the sample Figure 3.4 shows the DH sample holder and

a sample mounting

Figure 3.4 Schematic of the OMICRON DH sample holder [1]

The sample holders held by a rotary grip mechanism as shown in Figure 3.5, and are transferred from one chamber to another via the transfer arms linearly using magnetic probe sliding mechanisms As both preparation and analysis chambers are equipped with

manipulators for high precision x, y, z positioning to allow fine control of sample

positions for film deposition, LEED and XPS analysis, the sample transfer between transfer arm to manipulator or manipulator to STM carousal is facilitated by both the transfer arms or wobble stick pincers as shown in Figure 3.6 The manipulators which are shown in Figure 3.7, are equipped with 2 modes of sample heating They are (1) direct current heating (DH), whereby current is passed through the sample via contact brushed and it is resistively-heated by the internal resistance of the sample itself; and (2) resistive heating (RH), whereby current is passed through rows of pyrolytic boron nitrite (PBN) wires located on the manipulator to heat up the back-end of the sample holder

Figure 3.5: Schematic of the head of the magnetic probe transfer arm, showing the rotary grip mechanism during gripping and releasing of sample holders Rotation of the transfer arm shaft opens and closes the gripping arm of the transfer head [1]

Gripping arm

Figure 3.6: Schematic diagram showing the sample transfer process between (A)

manipulator and transfer arm via transfer head and (B) manipulator and STM carousal via

wobble stick pincer [1]

Figure 3.7: Schematic diagram showing the manipulator head in (A) 3-D view (B) Plane

view and (C) Side view pincer [1]

RH heating

3.3 Scanning Tunneling Microscope

STM images are acquired by scanning an atomically sharp metal tip across a conducting surface at a distance of approximately 5 Å to 10 Å, such that the wave functions of the tip and sample overlap Consequently, quantum tunneling of electrons across these two materials can occur This situation is schematically illustrated in Figure 3.8

Figure 3.8: Overlapping wave functions of sample and tip

There is a finite probability that electrons can cross the gap from one end to the

other Fowler et al [2] derived an expression, which is shown in Eqn (3.2), for the

tunneling current based upon the tip – sample separation and the work functions of the materials involved

)exp( kd φ

where I is the tunneling current, U represents the applied bias across the gap, d is the separation of the gap, Φ is the average work function (U << Φ), and k is a constant

The tunneling current depends exponentially on the separation of the tip and

sample surface The constant k has a value of 1.025 Å-1 (eV)-1/2 for a vacuum gap Therefore, there is roughly an order of magnitude change in the tunneling current for a change in separation of only 1 Å This is the reason for the extremely high vertical resolution of STM

The basic setup of STM is given in Figure 3.9a, which shows the various key components of STM The feedback loop is essential for maintaining a constant tunneling current in the constant-current mode, which will be discussed later The current amplifier

is employed for amplifying the weak tunneling current There are 2 basic modes of STM operations, namely “constant-current” and “constant-height” mode, which are shown in Figure 3.9b In the constant-current mode, the vertical position of the tip is controlled by the feedback loop The value of the tunneling current measured at the tip position is compared with a value preset by the operator via the feedback loop If the measured value exceeds the present value (e.g smaller tip – sample separation), a voltage will be applied

to the z – piezotube, which then retracts the tip so as to maintain the current at the preset

value Similarly, if the current is too low (e.g smaller tip – sample separation), the feedback loop will drive the tip towards the surface The tip is raster scanned by means of

the x – and y – piezotubes across an area of up to several thousands of angstroms and the voltages applied to the z – piezotube will change accordingly in order to maintain a

constant tunneling current The voltages applied are then plotted as a function of position, which will then yield topographic or electronic image of the surface

In constant-height mode, the feedback loop is deactivated The tip, moving at constant height, experiences a tunneling current, which varies with the atomic corrugation of the surface (see Figure 3.9c) This mode has the advantage that the tip can

be scanned rapidly across the surface However, it is restricted to samples that are very flat, since sudden large changes in surface topography can result in tip crashes

(a)

Figure 3.9 Schematic illustration of (a) the STM apparatus, (b) “constant-current” mode and (c) “constant-height” mode of operations [3]

Eqn (3.2) shows that the tunneling current includes contributions from both

topographic and electronic features (i.e d and Φ) Figure 3.10 illustrates the principle of

sampling different electronic states in the region of the bandgap using STM By choosing

a particular bias for scanning, different electronic states can be probed Applying a negative sample bias allows occupied-states of the sample to be imaged (Figure 3.10a), while positive sample bias yields empty-state information (Figure 3.10b) All states within the energy window defined by the bias voltage may contribute to tunneling, but major contribution comes from those close to the Fermi energy as they have the largest decay lengths

Figure 3.10: (a) Occupied-state imaging, and (b) empty-state imaging

STM studies were performed using an OMICRON STM A carousel, which resides within the analysis chamber, provides storage space for up to 6 samples or tip holders The holders are transferred to and from the STM by means of a wobble stick Figure 3.11 illustrates the STM scanning stage

Figure 3.11: Schematic diagram of the STM scanning stage [3]

E F eV

Sample (+ve)

Tip (-ve)

E F

The sample holder is inserted into the STM scanning stage and it will rest on the 3-contact bearing, which is shown in Figure 3.12 All STM images were taken with tunneling currents from 0.1 to 1.2 nA and bias from –2 V to +2 V applied to the sample The OMICRON VT STM uses a single tube scanner with a maximum scan range of

about 10 x 10 µm with a z – travel of about 1.5 µm A z – resolution of better than 0.1 Å

can be achieved The STM has been configured such that voltage biases are measured with respect to the substrate, this would imply that a negative bias voltage produces a filled state image, where the electrons would be tunneling from the substrate surface to the tip All the images were recorded using the constant current mode Dark features on the images correspond to depressions and bright to elevations of the surface Height analysis, length and the surface reconstruction periods of the STM were carried out with the use of a single scan line evaluation

Figure 3.12: Schematic of loading of sample plate into STM stage [3]

For eddy current damping during STM scanning, a ring of Cu plates is mounted

on the STM stage A ring of magnets is fixed at the columns of the spring suspension as a counterpart successfully damping excursions in all directions The resonance frequency

of the system is about 2Hz The STM stage can be locked by using the push-pull motion drive, in order to enable sample and tip exchange by means of a wobble stick The eddy current and vibration damping mechanisms are shown in Figure 3.13

Figure 3.13: Side-view schematic of the VT STM [3]

3.4 X-Ray Photoelectron Spectroscopy (XPS)

XPS was developed in the mid 1960s by K Siegbahn and his co-workers who were later awarded the Nobel Prize for Physics in 1981 The phenomenon is based on the photoelectric effect, where the electrons were ejected from a surface when bombarded with photons The photoemission principle states that if an atom absorbs a photon of

known energy, an electron from the core shell of the atom, where binding energy, E B, of

that electron is lower than the excitation photon, hυ, will be ejected out from the atom This photoelectron will possess a kinetic energy of (hυ – EB) However, this

photoelectron has to leave the surface and into vacuum, it must overcome the work

function, φ, of the specimen Hence, the resultant kinetic energy, EK, possesses by the

photoelectron will be given in Eqn (3.3) Figure 3.14 shows the photoemission process

2p 2s 1s

The XPS technique is highly surface specific due to the short inelastic path of photoelectrons that are excited from the solid Typical excitation X-ray sources

mean-free-for XPS are Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) Other X-ray lines can also be chosen such as Ti Kα (2040 eV) The energy of the photoelectrons leaving the sample is

determined using a concentric hemispherical analyzer (CHA) and this gives a spectrum with a series of photoelectron peaks Figure 3.15 shows the schematic diagram of the CHA

Figure 3.15: Schematic diagram of a concentric hemispherical analyzer (CHA)

Concentric Hemispheres

Entrance Slits

A CHA consists of two metal hemispheres One hemisphere is concave in shape, while the other is convex They are arranged such that their centers of curvature are coincident Different voltages are applied on each hemisphere such that there is an electric field between the two hemispheres Photoelectrons are injected into the gap between the hemispheres If the electrons are travelling very fast, they will impinge on the outer hemisphere If they are travelling very slow, they will be attracted to the inner hemisphere Hence only electrons in a narrow energy region, called the pass energy, succeed in getting all the way round the hemispheres to the detector A series of lenses are placed before the CHA The lenses enable two operating modes - Constant Retard Ratio (CRR), or Constant Analysis Energy (CAE) With CRR mode, the electrons are slowed down in order to maintain a constant “actual:detected” energy ratio of the electron

to be analyzed That is if the retard ratio is 10, and 1000eV electrons are to be detected, then the electrons will be slowed down to 100eV, and the pass energy will be set to 100

eV In the CAE mode, the pass energy is fixed Hence if the pass energy is 50 eV, then electrons of 1000eV will have to be slowed down by 950 eV in order to be detected The CRR mode gives constant resolving power and the CAE mode gives constant energy resolution

The binding energy of the peaks is characteristic of each element The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface The shape of each peak and the binding energy will be slightly altered

by the chemical state of the emitting atom Hence, XPS can provide chemical bonding

information as well XPS is not sensitive to hydrogen or helium, but can detect all other

elements

The XPS system comprises of a XR3E2 twin-anode X-ray source, an EA 125

hemispherical analyzer and associated electronic racks together with an output link to a

PC-based DAT 125 software system to capture and analyze XPS spectra The schematic

diagram of the twin-anode X-ray source is given in Figure 3.16

Figure 3.16: Schematic diagram of the XR3E2 twin-anode X-ray source [4]

The XR3E2 X-ray source is equipped with a dual anode, magnesium coated on

one side and aluminum on the other, which provides an X-ray line of 1253.6 or 1486.6

eV, dependent on the anode selected The anode is seated on an annealed copper gasket

mounted through the source-mounting flange to project through the inner filament shield

Three ceramic bushes centralize the position of the anode within the filament inner

shield The filament is mounted concentrically with the inner shield enclosed by an outer

shield, in which a thin aluminum foil window is fitted The electrical connections to the

Filament Filament

Outer Shield

Window

Filament Inner Shield

CU gasket

Anode Assembly Mounting

Flange Insulated Tube

Anode Cooling Coupling Flange

Source cover

Cutaway Side

double filament are made through three filament supports mounted on the three-way feed through connectors in the source-mounting flange The high voltage connection to the anode is made through the conduit supplying cooling water to the anode region of the source

The X-ray source operated at a potential of up to 15 KV and is controlled by the

8025 X-ray power supply Electrons emitted by the coated filament, which is near earth potential, are rapidly accelerated toward the high voltage potential of the anode The rapidly moving electrons impinge on the anode causing the emission of the X-rays A thin aluminum window mounted in the filament outer shield isolates the anode field and prevents stray electrons from affecting experiments The window also acts as a partial filter for unwanted x-ray lines When used in a hostile analysis chamber environment, the window enables the filament section of the source to be differentially pumped

All wide scans were carried out in step sizes of 1 eV; narrow scans were carried

out in step sizes of 0.05 eV Binding energy calibrations were carried out using Si 2p3/2 as the reference peak

3.5 UHV Electron-Beam Evaporator

In this dissertation work, all materials deposition is done using a FOCUS EFM3T UHV electron-beam evaporator This is a 3-in-1 evaporator whereby 3 different materials can be evaporated onto a substrate simultaneously It consists of 3 separated evaporation cells to avoid cross-contaminations The 3 cells will be water-cooled during the deposition process Figure 3.17 below shows the exterior-view of the FOCUS EFM3T

Figure 3.17: Exterior-view of the FOCUS EFM3T [5]

Evaporant materials, in this case, Si and Co, are in the form of rods The bombarding electron beam, from a heated filament, induces a temperature rise of the evaporant, causing evaporation The evaporator is equipped with a flux monitor Once calibrated, the flux monitor can replace the necessity of a quartz thickness monitor by continuously monitoring the evaporation rate Flux is measured directly, which allows a