CHAPTER Chapter 3: 3.1 Experimental Procedures 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 103 CHAPTER 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: Z= p cm-2.s-1 2πmkT Eqn. (3.1) where p is the ambient pressure (in N.cm-2), m is molecular mass (in Kg.molecule-1), T is absolute temperature (in K), and k is Boltzmann constant (in J.K-1). 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 time taken for a clean surface to be saturated with ML of CO will be 2.6 seconds. If the 104 CHAPTER ambient pressure is now 10-10 Torr, the time taken to saturate the clean surface will be 26178 seconds, which is about ~ 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 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. Fast Entry Load Lock Preparation Chamber STM Analysis Chamber Figure 3.1: Plan view of OMICRON UHV schematic 105 CHAPTER XPS Analyzer STM Manipulator Electron Beam Sources Pneumatic Gate Valve Turbo Pump Ion Pump Figure 3.2: Side view of OMICRON UHV schematic Manipulator STM Sample Transfer Magnetic Probe Arms LEED Figure 3.3: 3-D view of OMICRON UHV schematic. 106 CHAPTER The fast entry lock (FEL) chamber is pumped by a rotary pump and a turbomolecular pump which makes it possible for the chamber to be pumped down in stages from atmospheric pressures until pressures equivalent to preparation chamber conditions. 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 turbomolecular 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. 107 CHAPTER 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]. 108 CHAPTER 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 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. Gripping arm 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]. 109 CHAPTER (B) Transfer Arm (A) Wobble Stick Pincer Manipulator Transfer Arm inserts sample into manipulator groove Sample holder Rotary Grip Mechanism – Head rotates to release grip Transfer Arm retracts leaving sample in groove 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]. PBN wires for RH heating Contact brush for DH heating (B) Plane view Sample holder (a) 3-D view (C) Side view Figure 3.7: Schematic diagram showing the manipulator head in (A) 3-D view (B) Plane view and (C) Side view pincer [1]. 110 CHAPTER 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 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. Sample Vacuum Tip Separation ~ Å to 10 Å 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. I∝ U exp(−kd φ ) d Eqn. (3.2) 111 CHAPTER 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 [...]... Heating 30 0 200 Cooling 100 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Power (W) Figure 3. 20: Typical “Temperature vs Heating power” calibration plot for Si substrates [electrical resistivity: < 0.1 /square; dimensions: 12 mm (L) x 5 mm (W) x 280 µm (T)] 130 CHAPTER 3 3.6 .3 STM Tip Etching It is well-known that STMs has the capability of imaging sample surfaces with atomic resolution Such... 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 Flux Monitor Feedthrough HV Feedthrough... crashes (a) 1 13 CHAPTER 3 (b) (c) 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... 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. 13 117 CHAPTER 3 Figure 3. 13: Side-view schematic of the VT STM [3] 118... 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... tripod will, in turn, be mounted onto a tip carrier and then loaded into the UHV chamber for degassing The tip is typically degassed at about 30 0°C for 3 hours before it is being used 136 CHAPTER 3 3.6.4 Preparation of Clean Surfaces: Ex-situ and In-situ Methods For experimental work that involves epitaxial growth and nanostructure formations on Si surfaces, the atomic perfection of the initial growth interface... filaments (e.g LEED, evaporators, ion gauges, etc.) and the ion pumps if pressures exceed 10-6 mbar Finally, the pressure is then brought down to the 10-10 mbar range using the titanium sublimation pump Once the base pressures are obtained, constant monitoring of the pressures in both chambers is done through the ion gauges 128 CHAPTER 3 3.6.2 Temperature Calibration Temperature readings of the sample... 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 1 23 CHAPTER 3 3.5 UHV Electron-Beam Evaporator In this dissertation work, all materials deposition is done using... 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 Concentric Hemispheres Entrance Slits Exit Slits Electron Detector Electron Path Electrostatic Lens Sample Figure 3. 15: Schematic diagram of a concentric hemispherical analyzer (CHA) 120 CHAPTER 3 A... hυ − E B − ϕ Eqn (3. 3) vacuum vacuum hυ φ φ EK valence 2p valence 2p 2s 2s 1s 1s Core Level Electron Photoelectron Figure 3. 14: A typical photoemission process 119 CHAPTER 3 The XPS technique is highly surface specific due to the short inelastic mean-freepath of photoelectrons that are excited from the solid Typical excitation X-ray sources for XPS are Al Kα (1486.6 eV) and Mg Kα (12 53. 6 eV) Other X-ray . Chamber Analysis Chamber STM C C H H A A P P T T E E R R 3 3 106 Figure 3. 2: Side view of OMICRON UHV schematic Figure 3. 3: 3- D view of OMICRON UHV schematic. Turbo Pump Ion Pump Pneumatic. C C H H A A P P T T E E R R 3 3 114 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. C C H H A A P P T T E E R R 3 3 118 Figure 3. 13: Side-view schematic of the VT STM [3] C C H H A A P P T T E E R R 3 3 119 3. 4 X-Ray Photoelectron Spectroscopy (XPS) XPS