Chapter Introduction Chapter Introduction 1.1 Introduction Silicon has been the dominant substrate material in the semiconductor industry for over 30 years This is because silicon is abundant in nature, non-toxic, strong, and a good conductor of heat In addition, very high purity wafers with very large diameter (300mm) can be grown at low cost compared to other substrates Another important advantage of Si is the superior thermal stabilities of silicon dioxide and silicon nitride, both of which act as necessary insulating films in the semiconductor device structures The advance of microelectronic technology has pushed the search for new materials and novel device structures Some III-V semiconductors, e.g GaAs, InP, demonstrate superior electronic properties compared to silicon However, they still cannot replace Si due to their high cost and lack of high quality oxides In addition, defect density in III-V crystal increases with wafer diameter, which makes fabrication of large diameter III-V wafers with high purity difficult Thus, it would be easier and cost-effective to integrate new materials or device structures with existing Si technology rather than to develop a new production line Among several interesting materials, Si1-xGex is a promising candidate due to its high hole mobility and its compatibility with the current Si technology Details on the structure of Si1-xGex and concerns with its applications in devices will be addressed in the next section Chapter 1.2 Introduction Literature Reviews 1.2.1 The basic structure and properties of Si, Ge, Si1-xGex, Ni and their compounds 1.2.1.1 Si, Ge and Si1-xGex Silicon (Si) and Germanium (Ge) are both Group IV elements They both have diamond structure with each atom tetrahedrally bonded to each other The lattice constants (a) for Si and Ge are 5.431 and 5.658 Å1, respectively, indicating a 4.2% lattice mismatch between them Compared to Si, Ge has a narrower bandgap and higher hole mobility1 Therefore, it is very promising to engineer the bandgap and carrier mobility of Si by alloying Ge with it to form the Si1-xGex compound In addition, it is possible to fabricate Si1-xGex to any desired x value because Si and Ge are completely miscible over the entire range of Si1-xGex composition Fig 1.1 The arrangement of atoms in a single crystal substrate for both Si and Ge There had been considerable interest by many groups to grow bulk, unstrained Si1-xGex in the 1960s and early 1970s However, they failed due to the difficulty to Chapter Introduction produce bulk Si1-xGex crystal with acceptable radial and axial homogeneity1,2 On the other hand, the development of low-temperature growth techniques, such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD), attracted a rapid increase of different groups dealing with thin Si1-xGex films since the 1980s2 Epitaxial Si1-xGex layers were first grown by MBE, but are now grown commercially by chemical vapor deposition (CVD) in the Si industry due to its lower cost From the studies on Si1-xGex thin films deposited on Si substrates, it was soon realized that Si1-xGex films are tetragonally distorted when the film is thinner than a critical thickness, due to the inherent lattice mismatch of ~4.2% between Si and Ge Such Si1-xGex films are strained and have the same lattice parameter as that of Si Strain is recognized later to be an important material property, because many parameters, such as band gaps, band offsets, effective mass, etc, depend largely on strain However, upon annealing at high temperatures, the strains in the Si1-xGex/Si heterostructures can be relieved by restoring to the intrinsic cubic lattice constant of Si1-xGex Such undesired strain relaxation degrades the performance of devices built on such substrates and poses a challenge to device stability When growing beyond the critical thickness, the Si1-xGex films relax and results in the appearance of misfit dislocations and significant degradation of the substrate quality To resolve this problem, a buffer layer in between the Si substrate and the desired Si1-xGex film has been proposed Ge concentration in the buffer layer gradually increases from 0% to the target value (x%) within a few microns The buffer layer serves to accommodate the misfit between Si substrate and the Si1-xGex layer On top of this buffer layer, a Si1-xGex layer with constant Ge% is grown This layer is strainrelaxed and can be treated as a virtual “bulk” substrates (VS) for further device Chapter Introduction fabrication In this work, we will mainly focus on the strain-relaxed Si1-xGex virtual substrates (VS) with a constant Ge content of 20% 1.2.1.2 Ni Nickel is a transition metal with a face-centered cubic (FCC) crystal structure at room temperature (RT) The lattice parameter of the unit cell as shown in Fig 2.2 is a = 3.524 Å2 Fig 1.2 FCC unit cells of Ni crystal 1.2.1.3 Ni silicides There are common Ni-Si phases They are nickel rich silicide (Ni2Si), nickel monosilicide (NiSi) and nickel disilicide (NiSi2) Ni2Si has a orthorhombic structure with lattice parameters of a=4.99Å, b=3.72Å and c=7.01Å2,3 NiSi belongs to the orthorhombic MnP crystal system (a=5.233 Å, b=3.258 Å and c=5.659 Å)2,3 NiSi2 belongs to CaF2 structure, where Ni atoms take up the “Ca” positions and the Si atoms Chapter Introduction occupy the “F” positions in the CaF2 unit cell2,3 NiSi2’s lattice constant is 5.406 Å, only 0.4% smaller than that of Si Hence, epitaxial growth of NiSi2 on Si is not unexpected and has been observed on both Si(111) and Si(001) surfaces by using various growth methods, e.g solid phase reaction, molecular beam epitaxy and template methods4 Ni Si (a) (b) Fig 1.3 Crystal structures of (a) NiSi and (b) NiSi2 Ni silicides are usually grown through a two-step process Firstly, Ni thin films are deposited on Si surfaces at room temperature (RT) Thereafter, the thin films are annealed to high temperatures to promote the reaction between Ni and Si With a thick Ni film (~ 1000Å) on Si, annealing at 250oC first leads to the growth of Ni2Si phase NiSi phase starts to form when the entire Ni film has been transferred into Ni2Si5 Eventually the NiSi2 is formed in between 700oC and 800oC Among three types of Ni silicides, NiSi is the one with most technological importance because it is able to replace CoSi2 as a contact material Compared to CoSi2, NiSi is produced at lower temperature by one-step annealing and its low sheet resistance (14-20 Ω cm) remains unchanged even for linewidths below 0.1 m6 In Chapter Introduction addition, Si consumption is also relatively low during NiSi formation This is very crucial for the use of thin silicon on insulator (SOI) substrates and shallow junctions2 1.2.1.4 Ni germanides There are mainly common Ni germanides: Ni2Ge and NiGe They share similar crystallographic structures and have close lattice parameters with their counterparts of silicides: Ni2Si and NiSi, respectively7,8 Contrary to the appearance of NiSi2 at high temperature (>700oC), NiGe2 is not believed to exist in any system even after annealing to 700oC (either thin film or bulk)9,10, which makes NiGe the terminal phase in Ni-Ge system Among the different phases of Ni germanides, NiGe has the most technological importance because it has the lowest resistivity (14-20 Ω cm)10 and is stable over widest temperature window11 Similar to NiSi, NiGe also possesses an orthorhombic crystal structure as shown in Fig 1.3 (a) with a=5.381 Å, b=3.428 Å & c=5.811 Å12 For thick Ni layers (100-150nm) grown on hydrogen-terminated Ge(001) (HGe) surface, it is generally agreed that Ni2Ge is the first phase to form after annealing between 150oC and 250oC13, although there is a controversy in the literature on the first phase obtained by reacting Ni thin film with Ge For example, others observed that Ni5Ge3 is the first formation phase149 Between 250oC and 600oC, NiGe is formed with a flat NiGe-Ge interface as seen by XTEM The best epitaxy of NiGe on Ge(001) is observed at 500oC9 Above 600oC, NiGe starts to agglomerate and form islands9 Further increasing the annealing temperature leads to a bigger grain size and a rough surface Chapter Introduction 1.2.1.5 Ni germanosilicides There have been intensive studies on the reactions between thick Ni films and Si1-xGex substrates in the context of interfacial phase formation sequence, thermal and morphological stability For Ni layers (>10nm) deposited on Si1-xGex substrates, it has been found from XRD results that a Ni-rich phase (predominantly Ni2(Si1-xGex) with NiSi1-xGex) is formed when annealed below 325oC14,15 When the annealing temperature increases further (x), there is a controversy in the literature about the second phase Some researchers observed the second phase to be a Ge-deficient Ni mono-germanosilicide (NiSi1-yGey,y