F7360 Characterization of thin films and surfaces Lenka Zajíčková Faculty of Science & CEITEC, Masaryk University, Brno lenkaz@physics.muni.cz 2. Chapter - Introduction to Surface Processes spring semester 2014 Central European Institute of Technology BRNO | CZECH REPUBLIC • 2. Introduction to Surface Processes • 2.1 Thermodynamics of Clean Surfaces • 2.2 Electronic Structure of Clean Surfaces • 2.3 Work Function • 2.4 Thermoemission • 2.5 Gas Adsorption to Surfaces • 2.6 Surface Relaxation & Reconstruction • 2.7 Methods for Preparation of Clean Surfaces F7360 Characterization of thin films and surfaces: 2.1 Thermodynamics of Clean Surfaces Lenka Zajíčková 3/46 We need to introduce the concept of surface (free) energy, surface tension and surface stress. The essential features of bulk thermodynamics (e.g. Callen, 1985) in equilibrium, a one-component system is characterized completely by internal energy, U *■ U is a unique function of extensive parameters, entropy S, volume V and particle number of the system N and extensive property can be written as U(\S, XV, AW) = \U(S, V, N) U=U{S,V,N) AU=^WtNAS+^\8iNAV+Q8iVANf It defines intensive parameters, the temperature T, pressure p and chemical potential fi dU = TdS - pdV + fidN. that are function of independent extensive parameters S, V and N. Differentiating the Euler equation U= TS-pV+^N. we can derive a relation among the intensive variables, the Gibbs-Duhem equation SdT - Vdp + A/d^ = 0 F7360 Characterization of thin films and surfaces: 2.1 Thermodynamics of Clean Surfaces Lenka Zajíčková 4/46 Let's create a surface of area A from the infinite solid by a cleavage process. The total energy of the system must increase by amount proportional to A. The constant of proportionality, 7, is called surface tension ([7] = J/m2) U= TS - pV + ßN + 7A In equilibrium, at any finite T and p, the semi-infinite solid coexists with its vapor. Gibbs ascribed definite amounts of the extensive variables to a given area of surface. There is nothing unique about the particular choice of the boundary positions V=Vi + V2-S = S, + S2 -N = A/, + N2- SB, ■A4. Once the surface volume is chosen, the other surface quantities are defined as excesses. Changes of surface quantities are completely determined by changes in the bulk quantities ASs = -AS) - AS2, AVS = -AV| - AV2, ANS = -AW, - AN2. F7360 Characterization of thin films and surfaces: 2.1 Thermodynamic :s of Clean Surfaces Lenka Zajíčková 5/46 Surface Stress <7,y and Strain e,y Tensors Consider the effect of small variations in the area of the system, e.g., by stretching. The energy change can be described by linear elasticity theory (Landau & Lifshitz, 1970). Accordingly, where de//<% — dA/A is the surface strain tensor. dU = TdS - PdV + fidN + A^2 Gibbs adsorption equation Ad-y + SsdT - Vsdp + Nsd^ + Aj2hsij - lifeij) = 0 F7360 Characterization of thin films and surfaces: 2.1 Thermodynamics of Clean Surfaces uation At first glance it seems that there 5 independent variables, 7, p, p, T and e in Ad-y + Ss T - Vsdp + Nsdp + A ]T(7<% - vii^ij) = 0 However, the two bulk phase Gibbs-Duhem relations reduce this number to 3. We define particle density p-,j and density of entropy s-,j for phase 1 and 2 as N, — p, Vj and Si = SjVj. Then, dp = -S< ~S2d7 dp = s,dr + p,dT S1_S2 Ad-y - P1 - P2 Ss - VS Si P2 - S2P1 P2 - P1 NS S, - S2 P2 - P1 P\ - P2 d7+/\^(7<5,y-CTj-)dej- = 0 It can be shown that the term in square brackets is independent of the arbitrary boundary positions defining Ns, Vs and Ss => Vs = Ns = 0 gives simple form of Gibbs adsorption equation Adj + SsdT + AY^h^ij - aij)deij = 0 distance =>■ surface tension and stress are => only if the system is free to rearrange in response to a perturbation, i. e. If df/dt < 0 atomic dislocations and elastic buckling of the surface can be expected. ■ energy cost/unit area to cleave a crystal, i. e. break bonds ... from bulk cohesive energy Ecoh S3 3 eV, fractional number of bonds broken Zs/Z S3 0.25, Energy 2yA areal density of surface atoms Ns S3 1015 crrn 7 = Ecoh^-Ns material 7 [J/m2] 1.2 J/m2 mica gold PTFE 4.5 S3 1 0.019 The surface tension i surfaces tend to as an excess /unit area. High energy of contaminants from environment. 2.1 Thermodynamics of Clean Surfaces Annisotropy of 7 and Wulff theorem Lenka Zajíčková The tendency to minimize surface energy is a defining factor also in the morphology of surfaces and interfaces: Spherical equilibrium shape in an isotropic liquid (in the absence of gravity), In crystalline solids, the surface tension depends on the crystallographic orientation even if it implies a larger surface area. mimmiza il yim rum 1.060 1.0» 1.040 / \ • 2i»*c - 2»°c ,.-■> • 275 'c ''/ \ • »"c // \ % k\\ /// 1.0» Í f \ / 1.020 1.010 \/ 1.000 q 10 20 30 • 1 • » • » r (100} {uíí hid m step width ... n.a, ...p ension in direction [01].. .7(0) for large n... 9 ~ tan 9 — a/(n.a) — 1 /n tension in direction [1n] 7(0) = 7(0) + - 7(O)+4|0| Heyraud, Metois, Surf. Sci. 128, 334 (1983) F7360 Characterization of thin films and surfaces: 2.1 Thermodynamics of Clean Surfaces Lenka Zajíčková 9/46 Equilibrium shapes can be calculated but it is easier to use a graphics method, the Wulff construction. The surface tension is plotted in polar coordinates vs. the angle. The minimization: construction the surface from the inner envelope of planes perpendicular to the radius vector. Faceting is energetically favored for crystals, shown example is lead at 473 K Heyraud, Metois, Surf. Sci. 128, 334 (1983). \ It is important to note that the formation of the equilibrium shape requires sufficient mobility (or fast kinetics), not just thermodynamics. F7360 Characterization of thin films and surfaces: 2.1 Thermodynamics of Clean Surfaces Lenka Zajíčková 10/46 At finite T, the discussion must be supplemented to include entropy effects. ■J1-l_r fig. a At very low T any given facet is microscopically flat with only a few thermally excited surface vacancies. fig. b At higher T more and more energetic fluctuations in the local height of the surface can occur leading to delocalized interface with long wavelength variations in height (step free energy /? 4.). At certain roughening temperature, Tr, /3 < 0, i. e. the facets disappear and only a smoothly rounded macroscopic morphology remains (phase transition for T = Tr). F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces um Mechanical Calculations Knowing the positions of all the atoms in the semi-infinite crystal (described by the set of R) and ignoring ion motion (Born-Oppenheimer approx.), the Hamiltonian for N electrons is 2m 1 1 N Ze2 r '=1 ' '' - ¥i '' /=1 - ¥i The wave function describing the set of N electrons has the general form ^(^ i Szi 112, SZ2,..., 0> Sz/,... rN, Szn) where Sz; is electron spin in a chosen z-direction, with measurable values \h and — \h. Approximate methods for systems with many electrons: Hartree-Fock approximation - a particular case of the variational method SJ = S HvUdr = 0 in which the trial wave function is constructed from single-electron functions density functional theory (DFT) - application of variational method to the energy as functional of electron density (does not need to find a trial wave function) simple models: nearly-free electrons, tight-binding model F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková 12/46 Hartree-Fock Method of Self-Consistent Field The wave function in the ground state is constructed from a set of orthonormal single-electron functions if>i(x), each a product of a spatial orbital function fa(7) and a spin function ct(Sz). Single-electron functions are combined into multi-electron antisymmetric function by forming products in the form of N x N Slater determinant: 1pl(Xl) V>2(*l) ••• V>n(*i) V"l(^2) V>2(*2) ••• V>n(*2) V>i(*n) V>2(*n) ••• V>n(*n) The Hartree-Fock method needs to solve a set of coupled integro-differential equations that was obtained from the variational principle ["<+ E / ^ViifadTj-e^fa^O Km in which the potential energy ]C/(;y;) / * V\jATj is determined self-consistently starting from zero approximation of wave functions fa° of hydrogen-type atom. VU0 S3 vUHF = used in quantum chemistry but is quite awkward for use in extended systems like solid surfaces The electron density is the central quantity in DFT. It is defined as the integral over the spin coordinates of all electrons and over all but one of the spatial variables (x = r, S) n(r) = N J |vU(x1,x2,...,xN,)|2dS1dS2...dSNdX2dx3...dxN ► The first Hohenberg-Kohn theorem: For any system of interacting particles in an external potential I4xt(7), n(T) uniquely determines the Hamiltonian operator (I4xt(?) is a unique functional of n(7)) and thus all the properties of the system. ► The second Hohenberg-Kohn theorem: The energy of the many-body problem can be written as functional of the density E[n]. The exact ground state is the global minimum value of this functional. A popular form of DFT functional was introduced by Nobel laureate W. Kohn and L. Sham E[n{7)\ = T[n{7)\ + ]TZe j dr^L j j drdr'^p^l + Exc [„(?)] *• T[n(T)] is the kinetic energy of a non-interacting inhomogeneous electron gas. ► The second term is ion-electron interaction. ► The third term is the average electrostatic interactions between the electrons. Exc[n(r)] is exchange-correlation term that represents all quantum mechanics many-body effects Exc[n(T)] = AEee + AT. F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková 14/46 The great advantage of this formulation is that the density that minimizes the energy is found by solution of a set of ordinary differential equations (Kohn-Sham) where n(r) = IV1/!2- Many-body interactions are hidden in the exchange-correlation potential VXC(T) = SExc[n(r)]/Sn(r) and practical implementation requires a good approximation for this quantity. The electron density appears in the effective potential which means that the Kohn-Sham equations needs to be solved self-consistently. The parameters e; and i[>j(7) that enter the Schrodinger-like equation formally have no physical meaning. Nevertheless, they are frequently interpreted as one-particle excitation energies and eigenfunctions, respectively. -V2+\/eff(F) Mr) = crti(r) 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková Density Functional Theory - Local Density Approximation It is necessary to introduce an approximation of Vxc(r) The exchange-correlation energy density of each infinitesimal region of the electron distribution, n(T), is taken to be precisely equal to the exchange-correlation energy density of a with the same density as the corresponding infinitesimal volume element. The LDA is easy to apply because Vxc(r) is known very precisely for the homogeneous electron gas at all densities of physical interest (Ceperley, Alder, Phys. Rev. Lett. 45(1980)566). F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková 16/46 Jellium Model The discrete ion cores are replaced by a uniform, positive background charge with density equal to the spatial average of the ion charge distribution. For the analogous surface problem, the semi-infinite ion lattice is smeared out as /\ P for rs - 5 Positive / background, - p* - \\ p for a) M7) = z < 0, z > 0 where n is expressed in terms of an inverse sphere volume Typical ra values are 2-5ao (ao being Bohr radius). The density variation perpendicular to the surface, n(z), reveals two features: 1. electrons spill out into vacuum region (z > 0) electrostatic dipole layer at the surface. 2. n(z) oscillates as it approaches an asymptotic value that exactly compensates the uniform (bulk) background charge. Distance z (-eini wavelenglhs) Electron oscillation arise because the electrons (with standing wave vectors between zero and kF, radius of Fermi sphere) try to screen out the positive background charge distribution which includes step at z = 0. Screening in metals is so effective that there are ripples in the response, corresponding to overscreening => Friedel oscillations with wavelength n/kp, where kF = (37r2n)V3 In 1993, electron density oscillations were observed in STM images of individual adsorbed atoms on surfaces (Eigler's IBM group). By assembling adatoms at low T into particular shapes, these 'Quantum Corrals' produce stationary waves of electron density on the surface. oo - do not satisfy the usual periodic boundary conditions For the semi-infinite problem, the solution that grows for positive z is acceptable since it will be matched at z = a/2 onto a function that describes the decay of the wave function in the vacuum: vU(z) = eKZ cos(gz/2 + 8) z< a/2 vU(z) = e-qz z > a/2 where q2 — VQ - E. If the logarithmic derivative of vf(z) is continuous at z = a/2 existence of electronic state localized at the surface of the lattice chain. The energy of this surface state lies in the bulk energy gap. This solution is often called a Shockley state. ... here, bottom figure (for Vg > 0), curve 2 F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková 22/46 1D tight-binding model - wave functions are constructed from atomic-like orbitals (appropriate for semiconductor surface). The lattice potential is constructed from a superposition of N free atom potentials, 14(7), arranged on a chain with lattice constant a: N VU~r) = J2 va(r- na) n=\ where [-V2 + 14(7) - Ea](7) = 0 The non-self-consistent Schrodinger equation for the bands is {-V2 + 14(7) + [VL(7) - \4(7)]}vU(7) = EvU(7) The simplest trial function is a superposition of s-like Wannier orbitals - one on each site: N vu(?) = ]Tcn0(7- na) n=\ Tamm surface states F7360 Characterization of thin films and surfaces: 2.2 Electronic Structure of Clean Surfaces Lenka Zajíčková 23 / 46 Surface states can be donors - they are occupied (neutral) for EDs < EF and unoccupied (positive) for EDa > EF acceptor - they are occupied (negative) for Eas < EF and unoccupied (neutral) for EAa > EF Energy levels are straight up to the surface in case of no electrical field and no surface states. Appearance of acceptor surface states below EF: electrons from conductive band will occupy them and surface will be negatively charged sub-surface positive space-charge layer - bending of energy bands Work Function, Electron Affinity Difference between Fermi level and chemical potential is neglected (temperature below 1000 K) for metals: The is the difference between the vacuum level E0, and the bottom of the conduction band Ec- tor semiconductors: ork functi photoelectric work functi Ev difference between E0 (ionization potential) - F7360 Characterization of thin films and surfaces: 2.3 Work Function Lenka Zajíčková 25 / 46 Contact Potential - Metal/Metal Before Contact .......... After Contact metals A and B are electrically isolated (xa < Xb) => an arbitrary potential difference may exist metals A and B are brought into contact => electrons flow from the metal B to the metal A until the electrochemical potentials (Fermi energies) are equal. The actual numbers of electrons that passes between the two phases is small, and the occupancy of the Fermi levels is practically unaffected. Metal A is charged positively and metal B negatively, i. e. work functions does not change but contact potential appears. eVcont. = xb - xa Measurement of Work Function from Contact Potential Experimental methods for determination of work function measurement of contact potential difference eVcorit. — xb - Xa in which the work function of one material has to be known; measurement of characteristics of various electron emission processes condensator method ( ): between two surfaces creating a condensator with capacity C. If C is changed, a current / will flow dt where U is the voltage difference between the condensator plates. This voltage is equal to the contact potential difference if there is no external voltage applied. If we the field between the plates can be => external voltage is equal to the contact potential differen The changes of capacity are realized by vibration of one electrode ^^mm SKP470 Scanning Kelvin Probe http://www.bio-logic.info/instruments/skp470-2/ For metals the work function has a linear relation with the temperature change: X(T) = x(7~o) + a(T - T0), where a has values between 10~4-10~5 eV/K. For semiconductors and insulators the chemical potential varies strongly with temperature: ^^ + f + ¥<' Eg - band gap, Eaf - electron affinity, Nc and Nv - densities of the conduction and valence bands. For n-type semiconductors at lower ionizations: r- AED kT , Nc \(T) = Eaf H--- H--n —, AED - activation energy, ND - density of the donor states. At high temperatures it is chang( In case of p-type semiconductors At high temperatures it is change to: kT Nc x(T) — E,f H--In — ; 2 A/, D X(r) = Eaf + Eg- —-^In- and for high T x(V) = Eaf + Eg-f ,„£. F7360 Characterization of thin films and surfaces: 2.3 Work Function Lenka Zajíčková 29/46 If we apply el. field E close to the surface of metal there will be two types of forces exerting to the electrons attractive image force (between the electron and its mirror image inside metal) 167T£0X2 el. force accelerating electrons out of the metal F(x) = F0(x) - eE. In certain distance xk from the surface, the final force F(x) will be equal to zero nad for x > xk the electron will be accelerated from the surface. Work function will be then equal x = *k oo oo A/f J(F0(x) - eE)dx = J F0dx - J F0dx - J eEdx e2 eE = xo - --eExk = xq- ew--. 107reoX/( y 47reo This dependence of work function on external el. field is called Schottky effect. F7360 Characterization of thin films and surfaces: 2.4 Thermoemission Lenka Zajíčková 30/46 Addition of heat => increased energy of lattice vibration and energy of electrons => some electrons obtain energy required to pass surface potential barrier and are emitted from the surface Process of thermoemission can be described 1. by thermodynamics - electrons are the evaporated material, terms like heat of evaporation, Clausius-Clapeyron equation, equation of state 2. by statistical physics - known statistical distribution of electron velocities is taken to calculate those electrons that have enough energy to overcome work function. This approach was originally suggested by Richardson for metals (1901) but using classical statistics. Statistical description of thermoemission using Fermi-Dirac statistics For a system of identical fermions, the average number of fermions in a single-particle state ;', is given by the Fermi-Dirac (F-D) distribution where g, is the state degeneracy (the number of states with energy e-,) In solids, the states are characterized by a quasi-continuum energies with defined density of states g(t) (the number of states per unit energy range per unit volume) and the average number of electrons per unit energy range per unit volume is for metals where from Heisenberg principle of uncertainity g(e) — 2/h3 and for reasonable T we assume ß — EF Number of electrons having momentum in the range from (px,py,pz) to (px + Qpx,Py + dpy,pz + dpz): The axis z will be perpendicular to the surface and we look for the number of electrons with the energy from (pz,Pz + dpz). After integration in polar coordinates (/ ^rpr<*f — x - /n(1 + ex)) anci substitution e = p2/2/n we have N(p)dpxdpydpz g0 dpxdpydpz ft3 exp(fi^pi) + l' F7360 Characterization of thin films and surfaces: 2.4 Thermoemission Lenka Zajíčková 32/46 Number of electrons impinging on unit surface area per unit time with energy (e, e + de) is obtained by multiplication with vz — y/2e/m u(e)de=^^kT\n(^+e-^)de. Emitted electron have to fulfil the condition e > Eaf but integration of the flux term is not possible in general. Assuming (e - EF)/kT > 1 the flux can be simplified as v(e)de = —^-e~ kt and density of emission current; is obtained by integration considering a certain probability of electron reflection at the surface barrier R(t) /' = e y [1 - R(e)]v(e)de R(e) is for simplicity approximated by an averaged value R — 1 - D and then i = D-,— 72e~ kt = DA0T2e^x/KT. F7360 Characterization of thin films and surfaces: 2.4 Thermoemission Lenka Zajíčková 33/46 From previous slide we have /= DAoT2exp in which the constant D should not differ for different metals but it was found out that DA0 is quite different for different metals => we need to consider temperature dependece of work function x(T) = x(To) + a(T - T0) and obtain Richardson-Dushman equation i=DAQT*exP(-«/k)exP(-X{T0}kTaT0) = ^2exp(-g): where Richardson constant A is not an universal constant but it is characteristics for given material and eip is reduced or Richardson work function. metal melting point (K) 4(Acm~2K2) \ (eV) W 3640 80 4.6 Ta 3270 60 4.1 Pt 2050 170 5.6 dip/dT is of the order of 10~4 to 10~3 eV/K, with both positive and negative signs. When data is taken over a limited range of T, this temperature dependence will not show up on such a plot, but will modify the pre-exponential constant. -24 -26 -28 £ -30 ^ -32 -34 -36 -38 Q =0.13eV 10 12 14 16 18 20 22 1000/T (K') Using Richardson line: a plot of log(//72) versus 1/7 yields a straight line whose negative slope gives the work function 4>. The constant, A, can be measured in principle, but is complicated in practice because we need to know the emitting area independently, since what is usually measured is the emission current / rather than the current density,;'. Je třeba zajistit, aby se měření proudu uskutečňovalo v režimu nasyceného proudu, tj. aby v měřícím systému nehráli roli prostorové náboje. To znamená, že mezi emitující katodu a anodu musí být vloženo dostatečně velké napětí. Při větších napětích se pak ovšem uplatňuje Schottkyho jev, takže naměřené hodnoty by měly být správně extrapolovány na nulové vnější pole: ; = 472exp(-^): 472exp(-^)exp Musíme měřit dostatečně přesně teplotu katody (pyrometrická metoda nebo pomocí změn odporu žhaveného vlákna). F7360 Characterization of thin films and surfaces: 2.4 Thermoemission Lenka Zajíčková 35/46 ectnc Met h o Metoda kalorimetrická: emitované elektrony s sebou odnášejí určitou energii, tj. katoda se ochlazuje a chceme-li, aby její teplota zůstala konstantní, musíme zvětšit příkon. Energie spotřebovaná na jeden elektron je e = x + 2kT — e4> + 2kT a spotřebovaný výkon pro N elektronů za čas ŕ je Ne 2kTl 2kTl - approaching molecule is being attracted to the potential well (like for condensation that is a special case of adsorption in which the substrate composition is the same as that of adsorbant) molecule physisorption - trapping in the potential well because enough of the molecule perpendicular momentum was dissipated trapping probability S S is different from thermal accomodation coefficient 7 introduced previously, molecule is at least partially accomodated thermally to the surface temperature 7; even when it is reflected vapor molecule reflection desorption a » ,_. „_„ incorporation physisorption chemisorption F7360 Characterization of thin films and surfaces: 2.5 Gas Adsorption to Surfaces Lenka Zajíčková 37/46 The physisorbed molecule is mobile on the surface except at cryogenic T - hopping (diffusing) between surface atomic sites. It may desorb after a while by gaining enough energy in the tail of the thermal energy distribution. It may undergo a further interaction consisting of the formation of chemical bonds with the surface atoms, i.e. chemisorption. The condensation coefficient ac is not used in the case of chemisorption on a foreign substrate, use chemisorption reaction probability £. Some of adsorbed species eventually escape back into the vapor phase => sticking coefficient Sc - fraction of the arriving vapor that remains adsorbed for the duration of the experiment. vapor molecule reflection desorption a » .,_. „_, incorporation physisorption chemisorption Consider hypothetical diatomic gas-phase molecules chemisorbing as two Y atoms: 2Y(g) dissociative chemisorption Y2(g) adsorbing and then dissociative Lifting atomic Y out of its potential well along curve c results in much higher molar potential energy Ep in the gas phase - roughly the heat ol formation, AfH, of 2Y(g) from v2(g). The curve b represents activated chemisorption - there is an activation energy Ea to be overcome for Y2(g) to become dissociatively chemisorbed. For deeper precursor well, b chemisorption is not activated though there is still a barrier. F7360 Characterization of thin films and surfaces: 2.6 Surface Relaxation & Reconstruction Lenka Zajíčková 39/46 Uspořádané povrchy lze rozdělit do dvou skupin ► povrchy které mají jednotkovou buňku mřížky stejnou jako průmět objemové jednotkové buňky do roviny povrchu ► povrchy které jsou charakterizované jednotkovými buňkami, jejichž rozměry jsou celistvým násobkem rozměrů objemové jednotkové buňky povrchy, které vykazují jiné pravidelné uspořádání Povrchové struktury patřící do druhé a třetí skupiny se vytvářejí buď při adsorpci cizích atomů na povrchu, nebo v důsledku rekonstrukce povrchu. http://www.chem.qmul.ac.uk/surfaces/scc/scat1_6.htm F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 40/46 Dodáme-li adsorbované látce dostatečnou energii ve formě tepla, neudrží se adsorbované částice na povrchu a dojde k desorpci. Rychlost desorpce, tj. počet částic opouštějící jednotku povrchu za jednu sekundu je väeB = nB-exp(-°^), (1) kde na je povrchová koncentrace adsorbovaných částic,r0 je doba kmitu oscilací částic vázaných na povrchu, Qada je adsorpční energie, T je teplota. Pro každý systém je možné stanovit teplotu nutnou pro dokonalé vyčištění povrchu (vakuum!) od dané adsorbované látky. Žhavení se provádí přímým průchodem proudu 2. radiací 3. bombardováním elektrony (z opačné strany, jinak změny na povrchu) Výhody: experimentální jednoduchost; Nevýhody: Protože teploty, které musíme použít, jsou většinou dost vysoké (> 1000 K), hodí se tato metoda pouze pro látky s dostatečně vysokým bodem tání a pro látky, které při použitých teplotách nedissociují. Při pomalém zahřívání a udržování vzorku na vysoké teplotě nastává difúze nečistot z objemu. Díky tepelné desorpci může dojít k porušení stechiometrie a naleptávání povrchu krystalu. Nelze odstranit libovolnou nečistotu. F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 41 /46 Kov je kladným pólem. Je-li el. pole dost silné (řádově 108 V/cm), může se hladina valenčního elektronu adsorbované látky vyrovnat s Fermiho hladinou kovu, resp. se dostat těsně nad ni. V tomto případě je umožněno protunelování elektronu do kovu. Z atomu se stává kladný iont, který je elektrostatickými silami odmrštěn od kladného povrchu kovu. Nejsnadněji lze realizovat desorpci elektropozitivnívh adsorbátů, je však možné desorbovat i látky elektronegativní, ovšem potřebná pole jsou větší a může dojít i k vytrhávání vlastních atomů (tzv. vypařování v poli). Tento způsob čištění je usnadněn při zahřátí (větší migrace). Nevýhody: omezeno na materiály, ze kterých umíme a chceme vyrobit velmi ostrý hrot (pod 1 /^m), a na kovy. F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 42/46 Přímé ostřelování zkoumaného povrchu elektrony relativně nízkých energií (50-200 eV), takže zahřátí je nepatrné. Jedná se pravděpodobně o přechod adsorbované částice do excitovaného stavu, který může být k povrchu vázán slaběji nebo vůbec. F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 43/46 ivani Používají se těžší ionty, většinou Ar nebo Xe. Díky své hmotnosti předávají ionty účinně energii povrchové částici. Je důležitá čistota pracovního plynu a správné soustředění svazku (pozor na rozprašování okolních materiálů!). Výhody: univerzální metoda pro libovolnou látku umožňuje postupné odprašovaní. Nevýhody: u dielektrika se musí neutralizovat náboj iontů, jsou vytvářeny poruchy v bombardovaném materiálu kombinace bombardu a vyhřátí, F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 44 / 46 Čištění pomocí laserového paprsku Moderní modifikace čištění tepelnou desorpcí. Laserový svazek dopadá na čištěný povrch skrz okénko. Výhody: vakuum, žádné cizí částice ► pro krátké pulzy dojde k ohřevu jen povrchu a nikoliv objemu Nevýhody: lokální tavení materiálu (jako pro ostatní tepelné metody) nelze odstranit libovolnou nečistotu vysoká cena a prostorové nároky vhodných laserů F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 45 / 46 Štípání nebo lámání v ultravysokém vakuu Vhodné pro některé monokrystaly. Čistota povrchu je dokonalá. štípání břitem zatlačovaným do vrypu na povrchu krystalu => povrch má většinou hodně nepravidelností lámání krystalu ohybem povrch lepší F7360 Characterization of thin films and surfaces: 2.7 Methods for Preparation of Clean Surfaces Lenka Zajíčková 46/46 Vhodné v určitých speciálních případech. Organické nečistoty lze odoxidovat v kyslíku, některé nečistoty lze převést na plynné sloučeniny zahřátím ve vodíku atd. Většinou se vzorek v dané atmosféře žíhá.