Surface physics I and II Lectures: Mon 12-14 ,Wed 12-14 D116 Excercises: TBA Lecturer: Antti Kuronen, [email protected] Exercise assistant: Ane Lasa, [email protected] Course homepage: http://www.physics.helsinki.fi/courses/s/pintafysiikka/ Objectives ● To study properties of surfaces of solid materials. ● The relationship between the composition and morphology of the surface and its mechanical, chemical and electronic properties will be dealt with. ● Technologically important field of surface and thin film growth will also be covered. Surface physics I 2012: 1. Introduction 1 Surface physics I and II ● Course in two parts ● Surface physics I (SPI) (530202) ● Period III, 5 ECTS points ● Basics of surface physics ● Surface physics II (SPII) (530169) ● Period IV, 5 ECTS points ● 'Special' topics in surface science ● Surface and thin film growth ● Nanosystems ● Computational methods in surface science ● You can take only SPI or both SPI and SPII Surface physics I 2012: 1. Introduction 2 How to pass ● Both courses: ● Final exam 50% ● Exercises 50% ● Exercises ● Return by ● email to [email protected] or ● on paper to course box on the 2nd floor of Physicum ● Return by (TBA) Surface physics I 2012: 1. Introduction 3 Table of contents ● Surface physics I ● Introduction: What is a surface? Why is it important? Basic concepts. ● Surface structure: Thermodynamics of surfaces. Atomic and electronic structure. ● Experimental methods for surface characterization: Composition, morphology, electronic properties. ● Surface physics II ● Theoretical and computational methods in surface science: Analytical models, Monte Carlo and molecular dynamics sumilations. ● Surface growth: Adsorption, desorption, surface diffusion. ● Thin film growth: Homoepitaxy, heteroepitaxy, nanostructures. Surface physics I 2012: 1. Introduction 4 Course material ● These lecture notes on course home page http://www.physics.helsinki.fi/courses/s/pintafysiikka/ ● There are useful links under 'Links and literature' ● Lecture notes username: XXXXXXXX, password: XXXXXX ● Textbooks ● M. Prutton: Introduction to Surface Physics, Oxford Science Publications ● A. Zangwill: Physics at surfaces, Cambridge University Press ● J. A. Venables: Introduction to Surface and Thin Film Processes, Cambridge University Press ● A. Pimpinelli, J. Villain: Physics of Crystal Growth, Cambridge University Press ● M. Manninen, R. Nieminen: Pintafysiikka (in Finnish), Suomen Fyysikkoseuran julkaisuja 1 ● Journals ● Surface Science: http://www.sciencedirect.com/science/journal/00396028 ● Surface Science Reports: http://www.sciencedirect.com/science/journal/01675729 ● Progress in Surface Science: http://www.sciencedirect.com/science//journal/00796816 ● Thin Solid Films: http://www.sciencedirect.com/science/journal/00406090 ● Physical Review B: http://prb.aps.org/ Surface physics I 2012: 1. Introduction 5 What is a surface? ● Atomic view: Top few atomic layers of a solid. ● Continuum view: Dividing surface vacuum, vapor dividing surface c1 c2 Surface physics I 2012: 1. Introduction 6 Why study surfaces? ● Technologically important ● Thin film growth: electronics, nanostructures Surfaces and ● Miniaturization → surface-to-volume ratio grows interfaces ● Catalysis, corrosion important ● Surface chemistry (2007 Nobel Prize in chemistry) ● Development of experimental methods ● Development of various microscopy and spectroscopy methods −7 ● Development of vacuum techniques (ultra high vacuum, UHV p 10 Pa ) ● Film growth methods (MBE, MOCVD, ALD): preparation of surfaces with controlled composition and morphology ● 'Real' surfaces vs. 'clean' surfaces ● Development of theoretical and computational methods ● Models for atomic level structure of surfaces ● Computational modeling of surface chemistry (ab initio methods) ● Modeling of crystal growth (Monte Carlo and molecular dynamics simulations) Surface physics I 2012: 1. Introduction 7 What kind of surfaces? ● In everyday life surfaces are not clean ● Surfaces of many materials react with oxygen and other gases in environment ● Formation of so called native oxide layer ● May be very thin: e.g. Si surface has a SiO layer of thickness ~1nm. 2 ● In the case of Si high temperatures and pressures needed to grow the layer thicker. ● Similarly for many metals (Al, Cu, ...) ● Often this layer passivates the surface: no further reactions (prevents corrosion). ● The green patina on copper objects ● Copper oxides, chlorides and carbonates. ● Takes years to form. http://en.wikipedia.org/wiki/File:Hancoin1large.jpg Surface physics I 2012: 1. Introduction 8 What kind of surfaces? ● Many covalent materials (e.g. semiconductors Si, Ge, etc. and diamond) may have so called dangling bonds on the surface: bonds not bound to any neighbors. ● These are often saturated by e.g. hydrogen. ● Clean crystalline surface can be produced by various crystal growth methods ● Particularly clean surfaces are needed in manufacturing electronic components ● Many layers of thin films. ● Interface (surface) structure more and more important as length scale becomes smaller. http://navier.engr.colostate.edu/whatische/ChEL04Body.html Surface physics I 2012: 1. Introduction 9 Some basic concepts ● Classification of crystal surfaces ● Surfaces of crystalline material (grown or cleavage) consist of single crystal plane ● Crystal structure (lattice points) determined by vectors a, b, c. ● Crystal direction: ● Set up a vector of arbitrary length in the direction of interest. ● Decompose the vector into its components along the principal axes. ● Using an appropriate multiplier, convert the component values into the smallest possible whole-number set. ● Crystal planes: example ● Plane intercepts crystal axes a, b, c at 3a, 2b, 2c ● Take the reciprocals of the numbers: ⅓, ½, ½ ● The smallest integers having the same ratio are 2, 3, 3 ● The Miller indices of the plane are (233) ● For cubic crystals, a plane and the direction normal to the plane have precisely the same indices (except for possible scaling) Surface physics I 2012: 1. Introduction 10 Some basic concepts ● Classification of crystal surfaces ● Miller indices of a lattice plane = coordinates of the shortest reciprocal lattice vector normal to that plane ● Reminder: reciprocal lattice A , B , C of lattice a , b , c is defined as b× c c×a a× b A=2 a⋅b ×c B=2 a⋅b ×c C=2 a⋅b×c ● Reciprocal lattice of a simple cubic crystal = simple cubic ● For diamond, fcc and bcc lattices we almost always use the cubic unit cell (and not the primitive cells) → for these lattices Miller indices tell the normal of the lattice plane. ● For non-cubic lattices (e.g. hcp) one must remember that this is not the case. Surface physics I 2012: 1. Introduction 11 Some basic concepts ● Crystal structure (unit cell) determined by vectors a, b, c. ● And possibly the basis: locations of atoms in the unit cell. ● Crystal plane determined by its Miller indices ● Notation: crystal direction: [hkl] = ha+kb+lc family of directions: <hkl> (crystal symmetry) crystal plane: (hkl) family of planes: {hkl} (crystal symmetry) ● Example: face centered cubic (fcc) lattice (001) (110) (111) Surface physics I 2012: 1. Introduction 12 Some basic concepts ● Corresponding surfaces look like below (001) (110) (111) ● We immediately(?) see that atoms on certain surfaces are more tightly bound than on others. ● This has effect on the surface energy of a particular crystal surface. Surface physics I 2012: 1. Introduction 13 Some basic concepts ● Surface energy ● Creating surface from a bulk material costs energy (you must do work in order to create surface) ● You must break atomic bonds Energy needed = Area A 2A E1 E −E 2 1 E 2 = 2A ● Surface energy has the unit of J/m2 of eV/Å2 etc. ● Difficult to determine experimentally; theoretical calculations needed. ● We will delve into this subject in more detail in the following chapters. Surface physics I 2012: 1. Introduction 14 Some basic concepts ● The effect of surface energy is nicely manifested in the equilibrium shape of nanoclusters (sizes from few nanometers). ● For small clusters the surface energy is minimized by the expense of introducing defects inside the cluster. ● When cluster size increases defects cost more energy and a lower-energy shape has also non-optimal surface facets. ● This is confirmed by experiments for many metals. (001) (111) (111) Surface physics I 2012: 1. Introduction 15 Some basic concepts ● Surface energies of the three fcc surfaces (from semiempirical atomistic simulations): (001) (110) (111) eV/Å2 0.0802 0.124 0.0735 J/m2 1.29 1.99 1.18 ● Literature values ab initio calculations: 2.17 (001), 2.24 (110), 1.95 (111) experimental: 1.79, 1.83 (111) Surface physics I 2012: 1. Introduction 16 Some basic concepts ● Surfaces of other crystal structures bcc (001) diamond (001) diamond (001) (reconstructed) bcc (110) diamond (111) bcc (111) Surface physics I 2012: 1. Introduction 17 Some basic concepts ● Surfaces of crystalline materials most often observed are the low-index ones. ● They have the highest atomic densities and consequently the lowest surface energies. ● High-index surfaces may often viewed as containing facets and steps. facet, terrace step ● These surfaces slightly miscut from a low-index direction are also called vicinal. ● In growing smooth surfaces vicinal surfaces are often used: step flow growth. ● The exact structure of a crystal surface is in most cases not bulk-terminated (or bulk-exposed). ● There may be relaxation and reconstruction of the outermost atomic layers bulk-terminated