Epitaxy Growth Of Manganese Silicide Thin Films And Nanostructures On Silicon Surface

Metal silicides have low resistivity and high temperature stability. The epitaxygrowth of metal silicides on silicion surface are eminently compatible with traditionalintergrated circuit （IC） process and can be used as Ohmic or Schottky barrier contactsand as low resistive interconnections. The application of metal silicides caneffectively reduce the RC delay between the gates and the inner interconnections.Compared with other metal silicides, manganese silicides have more possiblechemical composition，such as Mn₆Si₁、Mn₉Si₂、Mn₃Si、Mn₅Si₂、Mn₅Si₃、MnSi andMn_Si₁.7. Mn₅Si₃and MnSi are metallic and can be used as low resistiveinterconnections; MnSi is a B20-type intermetallic compound, MnSi thin films werefound to be ferromagnetic and can epitaxally grow on silicon surface, its magneticproperties make it a promising candidate as spin injectors in future spintronic devices;On the other hand, MnSi_1.7is semiconducting with a direct band gap of about0.7eVand is a potential material for many optoelectronic applications such as silicon-basedinfrared detectors and light sources. In this dissertation, based on previous researches,the solid-phase （SP） epitaxial growth （manganese deposition at room temperaturefollowed by annealing） and the reactive epitaxial growth （deposition on a heatedsilicon substrate） of manganese silicide nanostructures on Si（111）-7×7surfaces were studied by ultra-high vacuum scanning tunneling microscope （UHV-STM），scanningelectronic microscope （SEM） and transmission electronic microscope （TEM）. Themajor results are summarized as follows:（1） Reactive epitaxy growth of Mn silicide nanostructures on Si （111）-7×7surfaces:below260℃, there is no reaction between Mn atoms and the substrate，small clusterswith congruent size which occupy single7×7unit cell halves formed; between260℃and500℃, Mn nanoclusters and various Mn_silicide islands coexist with bare Sisurface; above500℃, three kinds of large silicide islands （nanowires, tabular islandsand3-D irregular-shaped islands） have been distinguished. Besides, from390℃to610℃the nucleation density of silicide islands can be well described by conventionalnucleation theory.（2） Manganese silicide nanowires （NWs） with a large length/width ratio have beenpredominantly formed on Si₁₁₁-7×7surfaces with the reactive epitaxy method by adelicate control of growth parameters. The supply of free Si atoms per unit time playsa crucial role in the formation of the NWs. High growth temperature and low Mn_deposition rate are favorable for the growth of long NWs with a large length/widthratio and the formation of islands with other shapes can be greatly restrained underthese conditions. The formation of NWs is driven by the minimization of the strainenergy caused by the lattice mismatch between the silicide and substrate. Scanningtunneling spectroscopy measurements show that the NWs exhibit a semiconductingcharacter with a band gap of～0.8eV. TEM result shows that NWs are composed ofMnSi（1.7）.（3） Solid-phase epitaxial growth of manganese silicides on a Si （111）-7×7surface attemperatures between room temperature and⁷50°C has been studied using scanningtunneling microscopy. The as-deposited Mn film of⁰.6–1ML shows an orderedhoneycomb structure with each Mn cluster occupying a half of the7×7unit cell. TheMn clusters begin to react with the Si substrate to form silicides at²50°C. Twotypes of silicides, the three-dimensional （3D） and tabular islands, respectivelycorresponding to Mn-rich silicides and monosilicide MnSi, respectively, coexist onthe Si （111） surface at annealing temperatures between250and500°C. At500°C annealing, all3D islands convert into tabular islands and Mn_Si_is the only Mn silicidephase. Above600°C, the tabular islands convert into large3D islands that are likelyto be Si-rich manganese silicides. With increasing annealing temperature and time, thenumber density of silicide islands decreases, while the average size （area） of theremaining islands increases. The growth of large islands is a result of the dissolutionof small ones, which can be understood in the context of Ostwald ripeningmechanism.（4） Atomically flat MnSi thin films with few defects have been formed on Si （111）-7×7surfacewith the solid-phase epitaxy method by the codeposition of Mn and Si atoms. The thickness of thefilms is about⁰.7nm. The crystallographic structure of the films is studied by transmissionelectron microscopy （TEM）. The crystallographic orientation relationship between the silicionsubstrate and MnSi films can be （111）Si//（111）MnSi,[101]Si//[121]MnSi, with a lattice mismatchof-3.2%. Comparatively low annealing temperature （250-300℃） and long annealing timeare favorable for the growth of thin films, and the growth of Mn_Si_thin films isaccomplished by the lateral expansion of Mn_Si_tabular islands. Scanning tunnelingspectroscopy measurements show that the Mn_Si_films exhibit a metallic character.（5） The growth of Mn silicide nanostructures on Si（100）-2×1surfaces with thereactive epitaxy method was studied using scanning tunneling microscopy. Growthtemperature being set above³10℃, all the incident Mn_atoms react with thesubstrate and two kinds of silicide nanostructures are formed:3-D hut like islands andnanorods. With the increasing of growth temperature, nanostructures grow largelineally, but the nucleation density of silicide islands declined exponentially andfollows the conventional nucleation theory.（6） The growth of Mn silicide nanostructures on Si（100）-2×1surfaces with thesolid-phase epitaxy method between RT-⁷50℃was studied using scanning tunnelingmicroscopy. Mn amorphous structures are formed with RT deposition. When theannealing temperature is within the range of²80℃-305℃, silicide reaction takesplace and irregular-shaped silicide nanostructures form; after annealing at430℃, allthe irregular-shaped silicides convert into3-D islands; after annealing at600℃, only tabular islands form on the Si（100） surface. Scanning tunneling spectroscopymeasurements show that the silicides exhibit a semiconducting character with a bandgap of～0.8eV, which implies that they are composed of MnSi_1.7. The growth oflarge islands is a result of the dissolution of small ones, which can be understood inthe context of Ostwald ripening mechanism.