Surface physics is an important and rapidly developing field in solid state physics after the 1960s.Surface physics is that several atomic layers near the surface of a solid have many symmetrical properties different from those in the body.Surface physics research inUltrahigh vacuum(10~10Torr), the arrangement and electronic state of atoms in these atomic layers, the electronic state and other physical properties of foreign atoms or molecules adsorbed on the surface and foreign impurities in the atomic layers on the surface.Experimentallyelectron beam、ion beam, atomic beam, photon, heat, electric field and magnetic field, etcsurface structure , surface electronic state, type of adsorbate, binding type and bonding orientation.
idealCrystal surfacehaveTwo-dimensional periodicity, whose unit grid is composed ofFundamental vectora1 anda2. It is decided that according to the requirements of symmetry, there are five kinds of two-dimensional unit grids that may be formed, and these five kinds of grids are often called two-dimensional Braffian grids.Because the force on the surface atoms is different from that in the body, or because of the adsorption of foreign atomsSurface layerAtoms often have displacement perpendicular to or inclined to the surface, and several layers of atoms under the surface also have corresponding vertical or horizontal displacement, so the basis vector of the surface unit gridb1 andbDifferent from the ideal surface, this phenomenon is called surface reconstruction. If the surface atoms only move perpendicular to the surface, it is called surface relaxation.Surface crystallographyThe main research content ofb1、b2 anda1、a2.asb1=pa1,b2=qa2,pandqAll are integers, and the following symbols are often used to describe crystalssurface structure R(hkl) p×q, where R is the symbol of the element（hkl）RepresentativeMiller indexyeshklThe crystal face of.If the reconstruction is caused by the adsorbent A, the symbol R can be used（hkl)p×q- A or A/R（hkl)p×q。 If the base vector of the unit grid of the surface and the substrate is not parallel,b1 anda1、b2 anda2 have the same included angleα, the following symbols are often used to mark the surface reconstruction R（hkl)p×q－α。
To study the surface quantitatively, it is necessary to obtain thecoordinateInformation, for which the early experimental method isLow-energy electron diffraction(LEED)。The electron with energy in the range of 5~500eV is approximated toNormal incidenceDirection of fireCrystal surface, viaFluorescent screenInformation about the unit grid of the surface can be obtained from the diffraction points observed on the.Record the curve of spot intensity versus electron energy for several diffraction spots（I-VAnd fit the experimental results with the theoretical curve calculated according to a certain geometric configuration, so as to determine the position of atoms in the unit grid, which is LEEDCrystallographyResearchsurface structure Method.Using this method, a lot ofRelaxationAnd reconstruction, adsorption on metal surface, relaxation and reconstruction of semiconductor surface, etc.At present, the GaAs (110) surface structure that has been studied most clearly and understood more consistently.The arsenic atom on the surface relaxes outward, while the gallium atom moves inward. There is an included angle between the As Ga bond on the surface and the As Ga bond on the non relaxation surfaceω1.Because the electronCrystal surfaceOfMultiple scatteringAdded LEEDCrystallographyComplexity in theoretical analysis.In addition, medium energy can also be usedelectron diffraction(MEED) and high-energy electron diffraction（RHEED）To studysurface structure 。
Surface extensionX-rayabsorbfine structure (SEXAFS) is another method developed in recent years to study surface structure.When the atom A adsorbed on the substrate S absorbs the X-ray, the photoelectrons emitted from the core state can be scattered by the surrounding atoms, and there is interference between the outgoing electron wave and the scattered electron wave to form a undulating final state.This undulating final state makes the probability of X-ray absorption oscillate behind the absorption edge. The amplitude and period of the oscillation include the near number of adsorbed atom A and its formation with surrounding atomsKey lengthInformation for.The accuracy of key length determination reaches ± 0.03 extension.Utilize 60 meV energyHelium atomOn solid surfaceelastic scattering The substrate andAdsorption layerThe periodic structure of.Strong enough atomic beam and surfaceStrong interactionThis method has high sensitivity.Probing depthThe diffraction peak intensity depends mainly on the interaction potential between helium atoms and surface atoms. How to determine the potential closest to the actual situation is a difficult problem at present.
He or H with energy between 0.1~3MeVion beamCollimated alongCrystal axisWhen incident, because the ion beam is first scattered by the first atom of the crystal axis, the trajectory of the incident ion forms a shadow cone.Since the wave length of the incident beam is less than the lattice constant, the scattering process can be regarded asElastic collisionThe displacement of the first layer of atoms can be measured by measuring the scattering of atoms in the shadow cone.
Surface composition
The determination of surface composition is another important topic in surface research.utilizeAtomic corestateenergy levelThe two characteristics of the position and the mass of the atom can confirm the category of the atom.
Surface physics
X-ray photoelectron spectroscopy(XPS) is measured byincidenceX-rayKinetic energy of photoelectrons that hit out of the surfaceEK to determine the position of the core state energy levelEb,So we can determine the type of atom and its bonding information with surrounding atoms.Core energyEB and incidencePhoton energy啚 ω, outgoing photoelectron kinetic energyEThe relationship between k is,
Fig. 3 Surface Physics
φ s iswork function(Figure 3).On solid surfaces,EThe value of b moves with the bonding with surrounding atoms“chemical shift”You can get information about bonding.
Figure 4 Surface Physics
Auger electron Spectrum（AES）Utilization involves threeenergy levelThe basic process is shown in Figure 4.Use energy of 3~5 keVelectron beameincidencereachCrystal surface, a core state electron in level A is excited to the outside body, and the electrons of higher level B can be filled through the non radiative recombination processholeAnd use the surplus energy to excite another electron at the C level.Elements can be identified by measuring the information that the number of these escaping electrons changes with energy.The process shown in Figure 4 is called Auger process, which involves three energy levels A, B and C.When elements form chemical bonds with other elementsSpectral lineMovement of（Auger electron Spectrum).
Appearance potential spectrum(APS) measurement is sufficient to produce core stateholeThe lowest energy of (see appearance potential spectrum).Since the core state energy varies with the element, the element can be identified by measuring this energy.The hole can be generated through the Auger process involved in filling the hole or the softX-rayTo detect.The former is called Auger appearance potential spectrum (AEAPS), and the latter is soft X-ray appearance potential spectrum (SXAPS).If measuredincidenceelectron beamThe reflection does not appear in the reflection due to the energy loss of the incident electron participating in the excitation of the core state electron, so the reflection is measuredBeam strengthThis method is called blanking appearance potential spectrum (DAPS) (see appearance potential spectrum).
Surface physics
When low energy (200~2000eV)inert gasWhen ions He, Ne, Ar, etc. are incident on the surface, they pass throughElastic collisionThe mass of surface atoms can be measured by scattering ions within a certain angle.By energy andLaw of conservation of momentum, energy isE0, the energy scattered to the angle θ of the laboratory reference system isEQuality isM1 ion beam,EandEThe relationship of 0 can be expressed as
This method is called ion scattering spectrum (ISS). The above formula is also applicable to ions with higher energy, but θ s ≈πBackscattering of.For low-energy ion scattering, θ s ≈π/2。
If argon with energy of 2~20keVnitrogen、oxygenOr cesium ions incident on the solid surface, secondary ions and ion clusters escape from the body through a series of collision processesMass spectrometerTo confirm the type of ion, this method is calledSecondary ion mass spectrometry(SIMS)。
Surface physics
For surfaces with adsorbentsDesorptionProcess to confirm the type of adsorbents and the relationship between adsorbents andsubstrateOfbinding energy。Desorption can be generated by heating, electron bombardment and light irradiation, respectively calledThermal desorption(TDS), electron induced desorption (ESD) or photon induced desorption (PSD).For thermal desorption, the activation energy of desorption is proportional to the absolute temperature at which the desorption peak is generated.When the surface is bombarded with electrons with energy between 10 and 1000 eV, the incident electrons can transfer the electrons in the atoms bonded to the substrate fromBonded stateFired toAntibonding stateThis excited state and the repulsive potential of the substrate can make atoms leave the surface as ions. This mechanism is often referred to as the Menzel Gomer Redhead model.The direction of ion escape forms discretecone。The taper shaft depends on theDesorptionFracturedMolecular bondThe orientation of.Escape by measuremention beamThis method is called electron stimulated desorption angular distribution (ESD IAD).For transition metal oxide M50. Nortek and PJ. Feibulman believes thatelectron beamAfter bombarding the surface, it can be generated in the core state of metal ionshole, oxygen ion can be excited by the Auger process between ionsAuger electron As neutral oxygen atoms or positively charged oxygen ions leave the surface, ESD can be used as a powerful tool to study the valence states of adsorbed atoms on the surface.
To find outsurface structure After surface composition, one of the main research contents of surface physics is surface electronic states and related physical properties.Photoelectron spectroscopyIt is one of the important methods to study surface electronic states.Vacuum ultravioletThe emitted photons can convert the electrons in the valence state of the solid body intoSurface stateThe electrons ofdistribution curve(EDC）Relevant possession is availableState densityInformation for.Since surface state electrons and internal electrons obey different selection rulesPhoton energyFor different energy distribution curves, the peak that does not move with the change of photon energy is the peak corresponding to the surface state.In recent years, due tosynchrotron radiationWith the development of, light sources with continuously variable energy can be obtained.Choosing different photon energies can make photoelectrons have the minimum escape depth, thus improving the surface sensitivityPhotoelectron spectrum, the energy of the occupied state in the surface electronic state can be obtaineddispersion relation。
Measure total lightNumber of electronsThe spectrum changing with photon energy is called yield spectrum, which was first used to detectenergy gapinSurface stateWhen the electron is excited from the occupied state to slightly higher than the vacuumenergy levelThis electron can be excited by Auger process, and can also escape from the body after multiple collisions.The states in the energy gap can be detected sensitively by measuring the total yield as a function of photon energy.With synchrotron radiation, photons canPrice bandThe electrons in are excited toconduction bandOr empty surface state, the surface state can be detected by controlling the excitation of photoelectrons with escape depth of 5~30.When the photon is absorbed, the excited core state electrons can be de excited through the Auger process, or through theexcitonOfDirect recombination, or direct recombination with the surface empty state, the resulting fast electrons can again generate slowerSecondary electron。In the total yield spectrum, both fast and slow electrons are collected. If only secondary electrons with energy below 5eV are collected, this analysis method is called partial yield spectrum (PYS).If the yield spectrum with energy higher than 5eV is measured in a certain range, it is called constant final state spectrum (CFS). Through this mode, the influence of initial state and exciton can be studied;If the incident photon and the energy of the electronic analyzer are changed synchronously, a constant initial state spectrum (CIS) is obtained.If appropriateE, makePrice bandThe probability of emitting photoelectrons is small, and increasing the yield of core state Auger decay can greatly enhance the core state toSurface stateOftransition。Use lightelectron emissionOfDiffraction phenomenonCan also be studiedsurface structure 。
Figure 7 Surface Physics
For example, the low energy of 50~200eVelectron beamincidenceTo the solid surface, measure the energy loss of reflected electrons as shown in Figure 7Energy loss spectrumVolume plasmonsSurface plasmonAnd other information.Energy loss can also be used to excite transitions between bands or transitions between core levels.
The surface electronic states can also be detected by the tunneling process of electrons.When the ions are close to the solid surface, the electrons in the surface valence state can be neutralized by tunneling effect and ions, and the released energy can be used to excite the electrons in the solid valence state to the outside world. The method of detecting the surface electronic state by using this process is called ion neutralization spectrum (INS).Because only the electrons very close to the surface can be combined with the empty state of ions through the tunnel effect, and only the electrons excited at the surfaceAuger electron Therefore, the ion neutralization spectrum is a very sensitive detection method for the surface.If the ions are excited during neutralizationexcited stateThis process is called metastable depolarization spectroscopy (MDS).
Field emission microscope(FEM) is based oncold cathodeLaunch principlecathodeThe corrosion becomes a tip with a radius of 1~2000 extension. After applying negative voltage, electrons can penetrate the solid surface through the tunnel effect and hit the fluorescent screen in front of the cathode.Because the probability of electron penetrating the tunnel is related to the external electric field and thework functionIt is related, so it is marked inFluorescent screenThe number of electrons on the tip is a copy of the work function of the tip material.The adsorption, decomposition and diffusion of gas atoms on the surface of the needle tip can be understood through the change of the graph.
Surface physics
In order to improve the resolution, the field emission microscope has developedField ion microscope(FIM)。Make the metal sample into a needle shape, then apply positive voltage and charge low voltage around the needleinert gas, electrons of gas can enter Fermi of sample through tunnel effectenergy levelIn the above empty state, the positively charged ions are repelled by the needle tip, hit on the fluorescent screen and display a certain pattern, which can provide information about surface molecular ionization, chemical reaction, decomposition and evaporation.stayField ion microscopeMake a small hole on the fluorescent screen of and connect it withTime of flight mass spectrometerWhen combined, an atomic detection beam is formed.
Since the surface can be seen as a defect that destroys the periodic lattice, the atoms on the surface have different vibration modes from those in the body.When there are molecules on the surfaceoverburdenBy studying the vibration mode of these coatings, the structure of adsorbed molecules can be determined and the adsorption position of molecules on the surface can be determined.The orientation of adsorbed molecules relative to the substrate can be obtained by observing the excitation of some vibration modes, and the change of frequency with coverage can be studied to understand the lateral interaction of the coating.Infrared reflectance spectrum (IRAS), high resolutionElectron energy loss spectrum(HREELS) and inelastic electron tunneling spectroscopy (IETS) were used to study surface vibration.The advantage of infrared reflectance spectrum is high resolution, which can simulate the real catalytic action under pressure in the surrounding environment, but the disadvantage is low sensitivity.High resolution electron energy loss spectrum has high sensitivity, but low resolution.The principle of this method is the same as that described previouslyEnergy loss spectrumSame, but because the phonon energy is only tens of millionthsElectron voltTherefore, it requires specially designed high-resolution detectors and highly monochromaticelectron gun。The inelastic tunneling spectroscopy (IETS) is based on the use of metal insulator metal（Superconductive state）Tunnel process in sandwich structure (seeJosephson effect）To study the vibration spectrum of the system adsorbed on the insulation layer, which can determine the molecular structure of adsorbed molecules, determine the surface concentration of adsorbed molecules, the orientation of adsorbents, the interaction between adsorbents, etc.
Research direction
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The main research topics of surface physics are:
①surface structure , i.eSurface layerThe arrangement of atoms, including the type of atoms, their relative positions (key length andBond angleAnd various defects deviating from the two-dimensional periodic structure (such as vacancies, interstitial atoms, domain boundaries, etc.).
③ Adsorption of foreign atoms or molecules on the surface andDesorptionProcess, and the resulting changes in chemical composition and structure.
④ Transverse transport of surface atoms.
⑤ Surface electronic states andPhonon state。
The study of surface physics includes both experimental and theoretical aspects.In the experimentParticle beamOr the ray beam incident on the solid surface, collect and analyze the products of the interaction between the incoming beam and the surface to obtain information about various aspects of the surface area.Theoretical research is based on the traditional solidQuantum TheoryAnd quantum chemistry theory applied to the surface areaMicroparticleThe theoretical calculation of the motion state and the interaction between them are carried out and compared with the experimental results.In the research of surface physics, experimental and theoretical methods complement each other.
theoretical research
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Surface physics
In addition to using various experimental means to study the surface, theoretical research is also an important aspect of surface physics.
Main purpose: try to understand the behavior of electrons near the surface, and compare with the experimental results. The most ideal situation is to determine the location of surface atoms by calculating the total energy and finding the minimum value of energy, but the main difficulty encountered in the calculation process is near the surfacechargeThe distribution and the position of the atom are different from those in the body, so the potential field is also different from that in the body.Because of the relationship between potential field and charge, complex self consistent calculation must be used.At present, the sheet model, which is similar to the traditional energy band calculation method, or the molecular cluster model, which is commonly used in quantum chemistry, is often used.The latter uses a finite number of atoms to simulate a semi infinite crystal. This method can easily calculate the total energy of the group. After calculating the total energy of the group composed of atoms with different geometric configurations, relevant physical properties can be given from the configuration corresponding to the minimum total energy, such as the adsorption position of atoms on the surfaceKey lengthEtc.Tight binding methodPseudopotentialMethod, affixationplane waveLinear combination (LAPW), etc.In recent years, there is a tendency to develop the way of calculating total energy by self consistent calculation.By comparing lightelectron emissionSpectrum and electronic structure calculated according to a certain model are used to confirmsurface structure A possible approach to.For metals,work functionThe calculation of the self consistent surface potential can be used to check the accuracy of the self consistent surface potential.Surface energyThe quality of the calculation results depends on how to account for the interaction between electrons, which are still under in-depth study.
becauseCatalysisMetal corrosion is a process occurring on the surfacelarge-scale integrated circuitDevelopment, especiallyIntegrationSurface physics is a subject with a strong application background, and has received widespread attention.
research meaning
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(1) The study of solid surfaces, such as metals andAlloy materialThe corrosion, wear and fracture of the surface are directly related to the chemical composition of the surface;
（2）semiconductor device Its performance is greatly affected by the surface condition;Heterogeneous catalysisThe mechanism, aging and poisoning of materials are all related to the surface condition;
（3）Controlled thermonuclear reactionThe interaction mechanism between plasma and wall surface in the device must consider the surface characteristics, etc.
Surface physics is the same asMetallurgy、material science , semiconductor physics, catalysis, vacuum physics and other fields are closely integrated, and the methods and equipment involved in the experimental research of surface physics are more related to a wide range of theoretical and technical achievements, so surface physics is a highly comprehensive and increasingly important discipline.
