Chapter Experimental Chapter Experimental 2.1 Introduction In this chapter, we will first explain how to clean substrates, deposit Ni thin films and anneal the samples in an ultra-high-vacuum (UHV) environment Then we will introduce the main characterization facilities used in this thesis as well as methods of data analysis 2.2 Sample handling and treatment 2.2.1 Growth method of Si0.8Ge0.2 virtual substrates The Si0.8Ge0.2(001) virtual substrates used in this project are grown by chemical vapor deposition (CVD) on Si(001) substrates at 800oC in Imperial College, UK Starting from a clean Si(001) surface, a buffer Si1-xGex layer of m thick is first grown with the Ge content (x) linearly increasing from 0% to 20% After that, a Si0.8Ge0.2 layer of m thick with constant Ge content (20%) is grown on the top with boron doping level of 1017/cm3 A schematic drawing of the structure is shown in Fig 2.1 (a) There is no chemical mechanical polishing (CMP) treatment after the growth Therefore, a cross-hatch pattern is present on the surfaces (Fig 2.1 (b)) The “cross-hatch” formation on Si1-xGex VS is believed to be associated with the strain relaxation process during the growth of VS However, its formation mechanism is still under investigation Gao et al attributed the mechanism to the stress40 Chapter Experimental induced morphological instability221 During growth, the strain caused by the lattice mismatch drives diffusional atomic flux along the film surface such that an initial flat film gradually evolves into an undulating profile, i.e., cross-hatch pattern However, using channeling-contrast-microcopy (CCM), Seng et al found evidence showing that the cross hatch structure is present in both compositionally graded structures and constant composition SiGe layer222 Seng believed that the pattern is associated with the presence of slight lattice tilt Constant Si0.8Ge0.2 m Graded Si1-xGex m Si substrate (a) (b) Fig 2.1 (a) Schematic drawing of Si0.8Ge0.2 virtual substrate structure and (b) the typical 10 m × 10 m AFM image of Si0.8Ge0.2 virtual substrate after growth 2.2.2 Cleaning methods for Si, Ge and Si0.8Ge0.2 virtual substrates Hydrogen terminated Si and Si0.8Ge0.2 samples are prepared by using a modified RCA wet chemical cleaning method, which is described in details in Table 2.1 and Fig 2.2 It is important to note that only Teflon© tweezers are used to handle samples throughout the cleaning process 41 Chapter Experimental The cut Si and Si0.8Ge0.2 substrates are firstly immersed in Solution A to remove the Si and Ge native oxides Following that, they are rinsed in de-ionized (DI) water to wash away the remaining hydrofluoric acid (HF) Note that DI water with less than parts per billion or with electrical resistivity not less than 18.2 M •cm should be used for rinsing Next, the substrates are introduced into Solution B, where the Si and Si0.8Ge0.2 surfaces are re-oxidized, before the substrates are re-immersed into Solution A again The “oxide strip-and-grow” process is repeated three times to enhance the removal of carbon contaminations Further removal of organic contaminants can be achieved by transferring the substrates into Solution C The solvating action of NH4OH and the powerful oxidizing action of H2O2 will eliminate stubborn and residual carbon species Finally, the Si and Si0.8Ge0.2 substrates are exposed to fresh Solution A, followed by a dip in DI water before mounting on the sample holder and introducing into the UHV chamber as quickly as possible to minimize dwell time of the sample in air To clean the as-received Ge substrates, a simpler cleaning sequence is adopted The Ge wafers are introduced in Solution A for minute followed by rinsing in DI water for minute This process is repeated for three times and thereafter the Ge samples are quickly loaded into the UHV chamber Both Solutions B and C significantly degrade Ge’s morphology and therefore are avoided Solution A 49% HF : H2O 1:4 Solution B 70% HNO3 Solution C 25% NH4OH : 30% H2O2 : H2O 15 : 40 : 100 Table 2.1 Chemical recipes in volume ratio for Solutions A, B & C used in modified RCA wafer cleaning method 42 Chapter Solution A (1 min) Experimental Rinse in Deionized Water (1 min) Solution B at 60°C (5 min) Rinse in Deionized Water (1 min) Repeat times Rinse in Deionized Water (1 min) Fresh Solution A (1 min) Solution C (10 min) Rinse in Deionized Water (1 min) Solution A (1 min) Rinse in Deionized Water (1 min) Fig 2.2 Flow chart summarizing the modified RCA cleaning procedures The final immersion of Si, Ge and Si0.8Ge0.2 substrates in HF solutions causes H-termination at the surfaces by bond formation between Si & Ge dangling bonds at the surfaces and hydrogen (H), thus passivating the dangling bonds of surface Si and Ge atoms with H and leaving surfaces with low defect density155 This results in the Si, Ge and Si0.8Ge0.2 surfaces being less susceptible to (1) airborne contaminations prior to introduction into the UHV chamber; (2) hydrocarbons from turbo-molecular pumps while substrate resides in the load-lock chamber; (3) residual oxygen and hydrocarbons in the UHV chamber156 The samples are thus stable in the UHV for extended periods of time As hydrogen desorbs from Si and Ge surface around 300oC157-158, in the present study, clean Si, Ge and Si0.8Ge0.2 surfaces are prepared by annealing the H-terminated Si, Ge and Si0.8Ge0.2 surfaces surface at 500oC-600oC for 10 minutes to completely desorb the surface hydrogen 43 Chapter Experimental Prior to Ni deposition, the cleanliness of the sample’s surface is examined by XPS Figure 2.3 represents typical survey spectra taken from the as-received, Hterminated and clean Si(001) surfaces, respectively Compared to the as-received Si surface, the XPS peaks belonging to C 1s, O 1s and O KLL are clearly not detected on the H-terminated and clean Si surfaces, which indicate that this modified RCA cleaning method is effective in removing the native oxide and carbonaceous contaminants Similar cleaning effects have also been observed on Ge and Si0.8Ge0.2 surfaces and hence are not shown here Alternatively, clean Si, Ge and Si0.8Ge0.2 surfaces can be prepared by heating the as-received Si, Ge and Si0.8Ge0.2 surfaces to 650oC, 400oC and 650oC inside the UHV chamber, respectively The corresponding native oxides (SiO2 or GeO2) on the surfaces can be thermally decomposed and then desorbed from surface However, this method is not adopted to prepare the clean Si, Ge and Si0.8Ge0.2 surfaces because it roughens the surface morphology and leads to carbide formation on the surfaces O 1s Si 2s Si 2p Intensity (a.u.) As-received Si(001) surface O KLL C 1s O 2s Si 2s Si 2p H-terminated Si(001) surface Si 2s Si 2p Clean Si(001) Surface 1100 1000 900 800 700 600 500 400 300 200 100 Binding Energy (eV) Fig 2.3 XPS survey spectra of as-received, H-terminated and clean Si (001) surfaces 44 Chapter Experimental 2.2.3 Ni deposition method In this dissertation work, Ni deposition on Si, Ge and Si0.8Ge0.2 surface is achieved by using an OMICRON EFM UHV electron-beam evaporator (Fig 2.4(a)) Evaporant materials, i.e., Ni, are in the form of rods The bombarding electron beam, from a heated tungsten (W) filament, induces a temperature rise of the evaporant, causing Ni evaporation The evaporator is equipped with a flux monitor (Fig 2.4(b)), which is actually an ion collector and is located in the evaporant exit column At a given electron emission current, IE, and e-beam energy, the ion flux measured is directly proportional to the flux of evaporated atoms Once calibrated, the flux monitor can replace a quartz thickness monitor by continuously monitoring the evaporation rate The interior components in the front part of the evaporator can be seen more clearly in Fig 2.4(b) 45 Chapter Experimental (a) (b) Fig 2.4 shows (a) the entire OMICRON EFM structure and (b) the cross-section view of front part of e-beam evaporator (Source: OMICRON EFM3 user manual) Before evaporation, the sample is positioned on the same axis as the evaporant exit column Filament current, If, will be increased to about 1.95 A to supply the bombardment electrons A positive bias ranging from 850 V to 950 V is applied to the evaporant rod This is to attract and to focus the electrons towards the end of the rod At this stage, IE would have increased and the end of the evaporant rod should be heated up The parameters mentioned can be adjusted until a suitable and stable flux is achieved The Ni deposition can then be started by opening the cell shutter To 46 Chapter Experimental terminate the deposition process, the cell shutter is closed and the control parameters decrease slowly to cool the evaporant rods 2.2.4 Annealing method In-situ sample annealing inside XPS system was achieved through resistive heating in the UHV Analysis chamber During annealing the pressure was kept at ~10-9 mbar region The sample temperature was monitored by a thermocouple in direct contact with the sample surface A picture of two annealing sample holders is shown in Fig 2.5 Thermal Couples Fig 2.5 Pictures of two annealing sample holders 47 Chapter 2.3 Experimental Ultrahigh Vacuum (UHV) System 2.3.1 Why UHV is needed There are mainly two reasons for the necessity of an UHV system for surface science studies using photoelectron spectroscopy (PES): Firstly, there is the probability that an ejected photoelectron will interact with a gas molecule of residual gas This is related to the typical distance traveled by the electron before such an interaction will take place, which is known as the mean free path of the electron The equation for the mean free path of a particle (λ) is given by: λ (nm) = RT kT = 2π d N A P 2π d P (2.1) where R is the universal gas constant, T is the temperature, d is the diameter of the gas particles, NA is Avogadro's number, P is the pressure and k is the Boltzmann constant For a UHV system having a pressure of 10-7-10-12mbar, the mean free path is in the range of 1km to 105km, which is long enough to for the photoelectrons to travel from sample surface to the detector without colliding with the residual gas molecules inside the chambers The second reason is related to the requirement of photoelectron spectroscopy (PES) analysis PES techniques, i.e., X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), typically analyze only the few top atomic layers (