ION-BOMBARDMENT OF NICKEL (110) AT ELEVATED TEMPERATURE

The goal of this thesis is to study the behavior of ion-induced defects at the Y point on the Ni (110) surface at elevated temperatures. The electronic structure of the surface is examined using inverse photoemission spectroscopy (IPES), and the geometric structure is observed using low energy electron diffraction (LEED). These measurements lead to a better understanding of the surface properties. The clean Ni (110) surface exhibits a peak ∼ 2.6 eV above the Fermi level, indicating an unoccupied surface state near the Y point of the surface Brillouin zone (SBZ). Defects are induced by low energy ion bombardment at various temperatures, which result in a decrease of the peak intensity. The surface state eventually disappears when bombarded for longer times. We also observed that the surface heals faster when the crystal is being simultaneously sputtered and annealed at higher versus lower temperature. Finally the data for annealing while sputtering versus annealing after sputtering does not seem to exhibit much difference.

I am grateful to my parents, and brother for their support. Special thanks to my dear wife Rita for her love, support and patience, especially during the last few weeks of the thesis editing. The love and support of our family on both sides is very much appreciated. Last but not least we can't forget Abby, our cat, who always kept us in good spirits. This paragraph is dedicated to all those people who iii I couldn't mention due to space constraints and to all the experimental researchers.  (a) Nickel band structure between L and X [10]. The E(k) parabola is for a free electron in a constant potential. (b) Nickel (110) surface brillouin zone (SBZ) projection to the bulk brillouin zone [11]. The notation follows the standard terminology, where Γ, W, L, U, corresponds to the bulk structure, and Γ, Y , X, S (with an overhead bar) corresponds to the surface. For an FCC (110) crystal Γ -origin, L -( ion-bombarded at different elevated temperatures, followed by the electronic and geometric structure study. The thesis is organized as follows: • Chapter 1 provides a brief overview on the Nickel (110) surface and the techniques used for the surface analysis.
• Chapter 2 provides a description of inverse photoemission spectroscopy (IPES) and low energy electron diffraction (LEED), two characterization tools used in the thesis. This chapter also discusses the experimental chamber setup.
• Chapter 3 presents results and discussion, followed by a brief discussion of future work.
The surface structure is typically the starting point which leads to a wide range of understanding of surface phenomena [1]. This research work is an extension of prior work in Dr. Heskett's lab [2,3,4]. Prior work in the lab also includes surface analysis on Copper, Nickel and adsorption on metals. In the Cu (110) study, it was found that copper didn't show any decrease in surface state peak intensity at room temperature, however the peak decreased in intensity at 170 K [4]. This was attributed to self-annealing of Copper at room temperature. From a more recent study on Ni (110), B. Young found that at room temperature (300 K) Ni needed 10 times the sputtering dosage to generate similar results as at 170 K [5]. It was concluded that the Ni was exhibiting some (but not complete) self-annealing at room temperature (300 K) when compared to a lower temperature (170 K). Some of these results are shown in the appendix ( Figure Figure 2 (a) highlights two such forbidden states between L 2 to L 1 and X 4 to X 1 . Unoccupied surface states are seen near the middle of these gaps [8]. These states are called Shockley surface states or crystal-induced states (where the gap is p-like at the bottom and s-like at the top; a detailed analysis is provided in [9,8] using the multi-reflection approach and near free electron analysis). The L 2 -L 1 gap extends from ∼0.9 eV to about + 6.5 eV. The surface state is around 2.8 eV.
The projected gap at Y is derived from the L 2 − L 1 gap, and the X corresponds to X 4 − X 1 gap. It is interesting to note that the L bandgap can be also sampled from X on the 100 plane and Γ on the 111 plane, as shown in Figure 3  The different techniques utilized in this thesis to study the Ni crystal are the following: • Surface cleaning is performed by sputtering/ion bombardment and annealing.
• Surface characterization is by inverse photoemission spectroscopy (IPES) and low energy electron diffraction (LEED).

Sputtering/ion bombardment
In any surface characterization study, it is desirable to start with a clean crystal surface. Sputtering or ion bombardment is an important technique used for sample cleaning. It involves bombarding the sample with inert gas ions. Typically, Figure 3. Bulk Brillouin zone projections for face centered cubic (FCC) structure [7].
Argon or Neon is used as they do not bind with the surface during the process.
High energy particles are used to remove the unwanted impurities from the surface, while low energy particles are used to induce defects in a controlled environment.
In our system, Argon (Ar+ ions) is used for this procedure.   The three simple defects produced by sputtering are vacancies or Schottky defects, interstitials and Frenkel defects, as shown in Figure 6. The first is missing atoms or vacancies, where atoms acquire energy from the incident energy particles and leave an interior lattice site to reach the crystal surface. These vacancies are also called Schottky defects. The second type is called interstitials: they are created when the atoms come to rest interstitially and perturb the lattice. The third one is referred as Frenkel defects, where the incident energy atoms generate vacancies and interstitials in the process. Hence these are also called Frenkel pairs [13,12]. Among these defects, surface vacancies are the most common defects generated by sputtering at low ion energies and small doses [14]. Figure 6. Different surface defects (a) Schottky (b) interstitial (c) Frenkel defects [12].

Annealing
After sputter cleaning, the sample is heated to a high temperature, approximately half the melting point, to remove the surface defects/vacancies. As illustrated in Figure 5, sputtering results in vacancies (surface defects) and an uneven surface. By annealing, most of these vacancies (surface defects) are healed by diffusion, thus resulting in a smooth, clean and healed surface. Figure 7 illustrates the surface after annealing.   IPES is also known as bremsstrahlung isochromat spectroscopy (BIS), and in-fact, it is still referred as BIS when working in the x-ray region [2]. IPES gained popularity when BIS was reformed to use low energy (≤ 1 keV) electrons and produce UV light, instead of using high energy electrons (on the order of 10 keV) and record X-rays. This modification enabled IPES to study surfaces [4]; in this sense BIS laid the foundation for IPES.
The key requirements of IPES are an electron source (also referred as an electron gun) and a photon detector. IPES can be operated in two modes: 1.
Isochromat mode: where the incident electron beam energy, E kin , is varied, and the emitted photons at a constant energy, ω, are detected. 2. Spectrograph mode: in this mode the incident electron energy, E kin , is fixed and the emitted photons are collected over a range of energies [1]. In this thesis, IPES is operated in the isochromat mode.
In our setup, we use a home built electron gun based on an Erdman-Zipf design [5] and a Geiger-Müller (GM) Tube, which is a popular choice as an isochromat photon detector. The GM counter is one of the oldest and a low cost radiation detector introduced by Geiger and Müller in 1928 [6]. Hans Geiger originally developed the principle; later Walther Müller collaborated to extend the detection to several types of radiation, such as alpha particles, X-rays, gamma rays to name a few. Figure 11 shows a typical GM construction and setup. It is a stainlesssteel tube typically (25 mm ∼ 1" in diameter) filled with Iodine and Helium. One end is sealed with a calcium fluoride (CaF 2 ) or strontium fluoride (SrF 2 ) window.
The CaF 2 (or SrF 2 ) window provides the high energy cutoff, while the Iodine gas gives the low energy cutoff [7]. The combination of CaF 2 window with Iodine provides detectable photon energy of 9.7 eV and a bandwidth of 0.8 eV. Using a SrF 2 window reduces the detectable photon energy to 9.5 eV with a bandwidth of 0.4 eV, as illustrated in figure 12, which is a plot of the detectable photon energy range for CaF 2 /SrF 2 windows.  The wall of the GM tube acts as a cathode and the wire in the center of the tube acts an anode. The GM tube uses Townsend avalanche to detect the radiation. The avalanche starts with a single electron, which gives rise to several excited gas molecules within a few nanoseconds. These excited molecules emit a photon when returning to the ground state, which generates free electrons by re-absorption or by hitting the cathode. The free electrons accelerated by the potential difference between the cathode and anode ionize the Iodine by collisions and the whole process repeats. The multiplication stops when a huge positive ion space charge is formed in the chamber, resulting in a reduced potential difference which cannot accelerate the free electrons. Another method to moderate these multiplications is to use a quencher, such as Helium [6]. The pressure of Helium should be moderated so it doesn't generate too many nor too low collisions.
Electrons of energy E kin from an electron gun are incident on the sample at an angle θ. The final energy state in terms of initial state energy is given by assuming bulk direct transitions [2,9], and using the principle of conversation. We can write E f as the following Where E f is the final energy state, , from which the momentum parallel to the surface can be obtained.
Where θ is the angle of the electron incidence (or, in other terms, it is the angle between electron beam and sample normal).

Low energy electron diffraction (LEED)
Low energy electron diffraction (LEED) is one of the most common techniques used to probe the geometric structure of a crystal surface. It involves using a collimated beam of low energy electrons whose wavelength is close to the inter-atomic spacing of the crystal. When these electrons hit the surface, some of them backscatter and form a Fraunhofer diffraction pattern, which is the Fourier transform of the surface atom arrangement [10,11]. A clean surface displays a good periodic pattern. Figure 13. Schematic of a low energy electron diffraction (LEED) system [11].
A typical LEED setup is shown in figure 13. Low energy electrons are emitted from the electron gun, and a small fraction of them back-scatter and reach grid G 1 . Only the elastically back-scattered electrons reach the second grid G 2 , which finally excites the fluorescent screen S. A detector, shown on the right, records this diffraction pattern. A popular LEED system manufactured by Omicron is shown in figure 14. The grids G 1 and G 2 can be seen on the top of the setup, where the fluorescent screen S is present, and the electron gun is located in the center.     More details of the setup are explained in D. Tang's thesis [13], and the IPES setup and the data collecting are further discussed in B. Young's thesis [14]. Figure 17. Sample mounted in the chamber using a sample holder, which is made with a tantalum foil folded around tungsten wires.

Experimental Setup
To generate a single inverse spectrum using IPES, the electron energy is varied from 6.5 to 10.5 eV in steps of 0.1 eV, and each step is held for 250 ms. 50 scans are performed to generate one spectrum. The resulting spectrum illustrates the normalized counts (normalized to the average current developed across the sample) as intensity on the y-axis versus the electron energy supplied by the electron gun on the x-axis.
The sample co-ordinates are selected to focus on the Y point on the SBZ, and the angle of the electron incidence, θ, can be obtained using equation (2).
The parallel, perpendicular co-ordinates can be obtained by observation and later refined.
Where the final energy state is E f , This chapter presents the experimental results, and discussion followed by a summary and future work. For this study three types of analysis were performed: 1. Room temperature investigation. 2. Annealing the sample while sputtering.
3. Comparing the effect of annealing the sample while sputtering to sputtering followed by annealing the sample (as two different steps).

Results and Discussion
In this thesis a typical experiment involves the following steps: • Step-1: 30 minute sputtering at 0 • relative to the sample normal with 500 eV Ar+ ions at a 5 ×10 −5 Torr pressure and an emission current of 20 mA. This step generates a clean yet disordered surface. For sputtering the chamber is back-filled with pure argon to the desired pressure. Once the sputtering is complete the argon is evacuated using the turbo-molecular pump and the chamber is allowed to recover back to ∼ 10 −10 Torr.
• Step-2: Sputtering is followed by a 5 minute anneal, where the Ni crystal is heated to approximately 1000 K to form a clean and smooth crystal surface.
• Step-3: LEED and IPES measurements are performed on the clean surface.
A LEED image is typically taken right away, whereas the sample is allowed to cool for a few minutes (by monitoring the thermocouple reading) before an IPES measurement.
• Step-4: Later surface defects are induced depending on the requirements, such as 10 minute sputter at 5 ×10 −6 Torr, followed by LEED and IPES measurements.

Inverse photoemission spectroscopy (IPES) Results
Some of the inverse spectra are analyzed in IGOR and are presented below. Figure 18 illustrates two spectra after sputtering (without annealing, result of step-1 outline in the above procedure), and after annealing (step 2). In all the plots, the solid lines are the smoothed counterparts of the data shown by the markers alone.
The annealed spectrum (blue line), typically referred to as the clean spectrum, exhibits a peak around 2.6 eV. This corresponds to the surface state (S 1 ) shown in Figure 1. The red line (bottom line) corresponds to a spectrum after 30 minute sputtering at 5 ×10 −5 without annealing, which does not exhibit any significant peak as sputtering degraded the crystal, by creating surface vacancies. When the crystal is annealed, most or all of these defects are healed after annealing. The other peak ∼ 0.3 eV is primarily caused by the transition to the vacant d-band [1] (B1 as shown in Figure 1).  minute -2 1 2 minute -2 1 2 (a total of 4 -2 1 2 sputtering), followed by an additional Figure 18. The plot presents an inverse photoemission spectra after sputtering (with and without annealing). The x-axis is energy in eV, with 0 being the the Fermi edge E f and normalized counts (normalized to the average current developed across the sample) on the y-axis. The annealed crystal shows a clean peak around 2.6 eV, whereas the unannealed spectrum does not exhibit any such peak. The other peak ∼ 0.3 eV is primarily caused by transitions to the vacant d-band [1]. Also, the solid lines are the smoothed counterparts of the data shown by markers alone.
These results follow the observations made by B. Young which indicate that longer sputtering intervals decrease the surface peak intensity [2].
A plot summarizing data from Figures 19 and 20 is illustrated in Figure 21.
The plots shows the ratio of the surface area with respect to the clean surface on the y-axis; the area of the clean surface is normalized to 1. On the x-axis is the cumulative sputtering time. The data shows a clear decrease in the surface peak intensity with cumulative sputtering.
The cumulative time scale is computed as follows: all the sputter "times" are calculated assuming a 1 minute sputter at 5 ×10 −6 Torr. As an example, a 15 minute sputter at 2 ×10 −6 Torr can be thought as a 6 minute sputter at 5 ×10 −6 Torr.  Figure 21. The plots shows the ratio of the surface area with respect to the clean surface on the y-axis; the area of the clean surface is normalized to 1. On the x-axis is the cumulative sputtering time. The plot shows the decrease in the surface peak intensity as a function of cumulative sputter time.
As a next area of study, the sample was annealed while sputtering. Some of the data is illustrated in Figures 22 and 23, and a few other spectra are presented in the appendix (Figures A.7 and A.8). Figure 22 illustrates the spectra when the sample is being annealed while sputtering for 10 minutes at 5 ×10 −6 Torr. For the green (top) spectrum the sample is heated to 59 • C (332 K), whereas the blue (bottom) spectrum corresponds to 35 • C (308 K). It can be observed that the surface peak ∼ 2.6 eV is higher when the sample is annealed at a higher temperature, when compared to annealing at lower temperature.
The green (top) peak corresponds to 132 • C (405 K), followed by 71 • C (344 K) and the red (bottom) corresponds to 47 • C (320 K). Again the surface peak is higher in intensity when the sample is annealed at a higher temperature. Supplemental data corresponding to annealing at 296.15 K, 313.15 K, 344.15 K, 325.15 K is illustrated in the Appendix, and all the data follows the same trend. Figure 24 illustrates the effect of annealing while sputtering, by summarizing data from the Figures 22, 23, A.7 and A.8. The plot presents temperature on the x-axis, and the surface peak area as a ratio with the 405 K peak area. When the sample was sputtered and annealed at 132 • C (405 K), there is little change in the surface peak intensity. The surface peak is close to the same level as the clean spectrum. The increasing trend seems to indicate that the with annealing at higher temperature, a 10 minute sputter at 5 ×10 −6 Torr has little impact, in other words, annealing the sample at a higher temperature while sputtering has a healing effect. As anticipated, the sample heals faster at higher temperature for the same amount of sputtering. This is shown in the Figure 24 as an increase in Figure 22. Inverse spectra when the sample is annealed at 35 • C (308 K) and 59 • C (332 K) while sputtering for 10 minutes at 5 ×10 −6 Torr. the intensity of the surface peak as a function of increasing temperature. There are a few outliers, however the trend seems to agree with the statement. Figure 24. The plot presents temperature on the x-axis, and the ratio of the surface peak intensity with respect to 132 • C (405 K) for a 10 minute sputter at 5 ×10 −6 Torr.
As a third analysis, a comparison between (1) annealing the sample while sputtering for 10 minutes at 5 ×10 −6 Torr and (2) a 10 minute anneal after a 10 minute sputter at 5 ×10 −6 Torr was carried out. For this case, Figure 25 illustrates the inverse spectra at 35 • C (308 K). Similarly Figures 26 and 27 illustrate the inverse spectra for 23 • C (296 K) and 29 • C (302 K) temperatures respectively.
There seems to be some difference when the sample was prepared at 35 • C (308 K) as illustrated in Figure 25. The peak is higher for annealing while sputtering versus annealing after sputtering. However at 23 • C (296 K) and 29 • C (302 K) the Ni crystal did not show any difference in the surface peak intensity (area). Figure 25. Inverse spectra when the sample is annealed at 35 • C (308 K) during a 10 minute sputter at 5 ×10 −6 Torr, compared to a 10 minute sputter at 5 ×10 −6 Torr followed by a 10 minute anneal at 35 • C (308 K). Figure 26. Inverse spectra when the sample is annealed at 23 • C (296 K), during a 10 minute sputter at 5 ×10 −6 Torr, compared to a 10 minute sputter at 5 ×10 −6 Torr followed by a 10 minute anneal at 23 • C (296 K). Figure 27. Inverse spectra when the sample is annealed at 29 • C (302 K), during a 10 minute sputter at 5 ×10 −6 Torr, compared to a 10 minute sputter at 5 ×10 −6 Torr followed by a 10 minute anneal at 29 • C (302 K).

LEED results
For all LEED measurements, a 120 eV electron beam energy and emission current of 1 mA was used. The phosphor screen voltage was set to 5 keV. LEED images were acquired with a home-built CCD camera located on the opposite side of the LEED phosphor screen.

Summary and Future work
The behavior of ion-induced defects on the Ni(110) surface at elevated temperatures was examined using inverse photoemission spectroscopy and low energy electron diffraction.
The clean Ni (110) surface exhibits a peak ∼ 2.6 eV above the Fermi level, indicating an unoccupied surface state near the Y point of the surface Brillouin zone (SBZ). Defects are induced by low energy ion bombardment at various temperatures, which result in a decrease of the peak intensity. The surface state eventually disappears when bombarded for longer times. We also observed that the surface heals faster when the crystal is being simultaneously sputtered and annealed at higher versus lower temperature. Finally the data for annealing while sputtering versus annealing after sputtering does not seem to exhibit much difference.
The next step would be to model a Monte Carlo based simulation of the IPES data to draw further observations. From the data presented for annealing while sputtering and annealing after sputtering for the same conditions doesn't seem to have a major effect. However more data is required.
The spots from the LEED data show some qualitative differences due to annealing after ion bombardment, however to get a better idea of temperature dependence a quantitative study is necessary. This includes digitizing the LEED spots and determine their intensities.