Low-Energy Electron Diffraction (LEED) 1.4.5 In Low-Energy Electron Diffraction (LEED)a collimated monoenergetic beam of electrons in the energy range 10-1000 eV (=0.4-4.0 A)is diffracted by a speci- men surface.In this energy range,the mean free path of electrons is only a few A, leading to surface sensitivi .The diffraction patter n can be analyzed for the exist e or an ordered。 overla er stru tur Intensities of diffracted ace atoms relative to each other and to underlying layers.The shapes of diffracted beams in angle can be ana- lyzed to provide information about surface disorder.Various phenomena related to surface crystallography and microstructure can be investigated.This technique requires a vacuum Range of elements All elements,but not element specific Destructive No,except in special cases of electron-beam damage Depth probed 4-20A Detection limits 0.1 monolayer;any ordered phas n be de .1A 0.1 Resolving power Maxim solvable distance for detecting disorder: Lateral resolution Typical beam sizes,0.1 mm;best systems,-10um Imaging capability igvecnommeopi507mams(eg,lbm Typically,no with specialized ins crystals of conductors and semiconductors and stallin nde stanc ccial effort Main uses Cost 5$75,000;can be home built cheaply Size Generally part of other systems;if self-standing,-8 m2 20 INTRODUCTION AND SUMMARIES Chapter 1
Low-Energy Electron Diffraction (LEED) 1.4.5 In Low-Energy Electron Diffraction (LEED) a collimated monoenergetic beam of electrons in the energy range 10-1000 eV (A = 0.4-4.0 A) is diffracted by a specimen surface. In this energy range, the mean free path of electrons is only a few A, leading to surface sensitivity. The diffraction pattern can be analyzed for the existence of a clean surface or an ordered overlayer structure. Intensities of diffracted beams can be analyzed to determine the positions of surface atoms relative to each other and to underlying layers. The shapes of diffracted beams in angle can be analyzed to provide information about surface disorder. Various phenomena related to surface crystallography and microstructure can be investigated. This technique requires a vacuum. Range of elements Destructive Depth probed Detection limits Resolving power Lateral resolution Imaging capability All elements, but not element specific No, except in special cases of electron-beam damage 4-20 A 0.1 monolayer; any ordered phase can be detected; atomic positions to 0.1 step heights to 0.1 A; surface disorder down to - 10% of surface sites Maximum resolvable distance for detecting disorder: typically 200 A; best systems, 5 pm Typical beam sizes, 0.1 mm; best systems, - 10 pm Typically, no; with specialized instruments (e.g., lowenergy electron microscopy), 150 A Sample requirements Single crystals of conductors and semiconductors; insulators and polycrystalline samples under special circumstances; 0.25 cm2 or larger, smaller with special effort Main uses cost Size Analysis of surface crystallography and microstructure; surface cleanliness 1$75,000; can be home built cheaply Generally part of other systems; if self-standingY-8 m2 20 INTRODUCTION AND SUMMARIES Chapter 1
Reflection High-Energy Electron Diffraction(RHEED) 1.4.6 In Reflection High-Energy Electron Diffraction(RHEED),a beam of high-energy electrons(typically 5-50 kev),is accelrared roward the suffa conducti crystal,which is held ng o at gro potential.The prir ry beam strikes the sample at a grazing angle(-1)and is subsequently scattered.Some the electrons scatter elastically.Since their wavelengths are shorter than interatomic separations,these electrons can diffract off ordered rows of atoms on the surface, concentrating scattered electrons into particular directions,that depend on row separa whose traje screen placed opposite the n will excitet sphor.The light m the phosphor screen is called the RHEED pattern and can be recorded with a photo graph,television camera,or by some other method.The symmetry and spacing of the bright features in the RHEED pattern give information on the surface symme- try,lattice constant,and degree of perfection,i.e.the crystal structure. Range of elements All,but not chemical specific Destructive No,Except for electron-sensitive materials Depth probed 2-100A Depth profiling No Lateral resolution 200um×4mm,in special cases0.3nm×6nm tal structure parameters, Sample requirements Usually single crystal conductor or semiconductor surfaces Main use surface structu wth;can disti especially during thin nguish wo-and three. dimen Instrument cost $50,000-$200,000 Size -25 sq.ft.,larger if incorporated with an MBE chamber 21
Reflection High-Energy Electron Diffraction (RHEED) 1.4.6 In Reflection High-Energy Electron Diffraction (RHEED), a beam of high-energy electrons (typically 5-50 kev), is accelerated toward the surface of a conducting or semiconducting crystal, which is held at ground potential. The primary beam strikes the sample at a grazing angle (+ 1-5") and is subsequently scattered. Some of the electrons scatter elastically. Since their wavelengths are shorter than interatomic separations, these electrons can diffract off ordered rows of atoms on the surface, concentrating scattered electrons into particular directions, that depend on row separations. Beams of scattered electrons whose trajectories intersect a phosphor screen placed opposite the electron gun will excite the phosphor. The light from the phosphor screen is called the RHEED pattern and can be recorded with a photograph, television camera, or by some other method. The symmetry and spacing of the bright features in the RHEED pattern give information on the surface symmetry, lattice constant, and degree of perfection, i.e., the crystal structure. Range of elements Destructive All, but not chemical specific No, Except for electron-sensitive materials Depth probed 2-100 A Depth profiling No Lateral resolution Structural information sensitive to structural defects Sample requirements Usually single crystal conductor or semiconductor surfaces Main use Monitoring surface structures, especially during thinfilm epitaxial growth; can distinguish two- and threedimensional defects Instrument cost $50,000-$200,000 Size 200 pm x 4 mm, in special cases 0.3 nm x G nm Measures surface crystal structure parameters, +25 sq. ft., larger if incorporated with an MBE chamber 21
X-Ray Photoelectron Spectroscopy (XPS) 1.5.1 In X-Ray Photoelectron Spectroscopy (XPS)monoenergetic soft X rays bombard a sample material,causing electrons to be ejected.Identification of the elements present in the sample can be made directly from the kinetic energies of these ejected photoelectrons.On a finer scale it is also possible to identify the chemical state of the elements pres nt fron small variatic in the det relative concentrations of eleme hts can be e determined fr om the me ured photo electron intensities.For a solid,XPS probes 2-20 atomic layers deep,depending on the material,the energy of the photoelectron concerned,and the angle (with respect to the surface)of the measurement.The particular strengths of XPS are se niquantitative elemental analysis of surfaces without standards.and chemical tate analysis,for materials as 2hbnhes tallurgical.XPS also is Range of elements All except hydrogen and helium Destructive No,some beam damage to X-ray sensitive materials Elemental analysis Yes,semiquantitative without standards;quantitative with standards.Not a trace element method. Chemical state Yes information Depth probed 5-50A Depth profiling Yes,over the top 50 A;greater depths require sputter profiling Depth resolution A few to several tens of A,depending on conditions Lateral resolution 5 mm to 75 um;down to 5 um in special instruments Smplerprm Main uses Instrument cost $200,000-1,000,000,depending on capabilities Size 10f.×12丘. INTRODUCTION AND SUMMARIES Chapter 1
X-Ray Photoelectron Spectroscopy (XPS) 1.5.1 In X-Ray Photoelectron Spectroscopy (XPS) monoenergetic sofi X rays bombard a sample material, causing electrons to be ejected. Identification of the elements present in the sample can be made directly from the kinetic energies of these ejected photoelectrons. On a finer scale it is also possible to identify the chemical state of the elements present from small variations in the determined kinetic energies. The relative concentrations of elements can be determined from the measured photoelectron intensities. For a solid, XPS probes 2-20 atomic layers deep, depending on the material, the energy of the photoelectron concerned, and the angle (with respect to the surface) of the measurement. The particular strengths of XPS are semiquantitative elemental analysis of surfaces without standards, and chemical state analysis, for materials as diverse as biological to metallurgical. XPS also is known as electron spectroscopy for chemical analysis (ESCA). Range of elements Destructive Elemental analysis Chemical state Yes information Depth probed 5-50 A Depth profiling Yes, over the top 50 A; greater depths require sputter profiling Depth resolution A few to several tens ofA, depending on conditions Lateral resolution 5 mm to 75 pm; down to 5 pm in special instruments Sample requirements All vacuum-compatible materials; flat samples best; size accepted depends on particular instrument Main uses Determinations of elemental and chemical state wmpositions in the top 30 A $200,000-$1,000,000, depending on capabilities 10 fi. x 12 fi. All except hydrogen and helium No, some beam damage to X-ray sensitive materials Yes, semiquantitative without standards; quantitative with standards. Not a trace element method. Instrument cost Size 22 INTRODUCTION AND SUMMARIES Chapter 1
Ultraviolet Photoelectron Spectroscopy (UPS) 1.5.2 nergetic phorons in the 10-100eVenergy range strike asample material om the valence leves and low-ying cor eve( binding energy than the photon energy)are ejected.Measurement of the kinetic energy distribution of the ejected electrons is known as Ultraviolet Photoelectron Spectroscopy(UPS).The physics of the technique is the same as XPS,the only dif ferences be ing that much lower photon energies are used and the primary em is on ing the valence ron levels. rather than els.Owing to this emphasis. core le the primary use,when inv ngating solid surfaces,.isf relectronic struc ture studies in surface physics rather than for materials analysis.There are,however, a number of situations where UPS offers advantages over XPS for materials surface analysis. Elemental analysis Not usually,sometimes from available core levels Destructive No,some beam damage to radiation-sensitive material Chemical state Yes,but complicated using valence levels;for core information levels as for XPS Depth probed 2-100A Depth profiling Yes,over the depth probed;deeper profiling requires sputter profiling Lateral resolution Generally none(mm size),but Sample requirements Vacuum-compatible marerial;flat samples best;size accepted depe Main use Electronic structure studies of free molecules (gas hdinadofsndaotbasod Instrument cost No commercial instruments specifically for UPS:usu ally an add-on to XPS (increm ental cost-$30,000)or done using a synchrotron facility as the photon source Size 10丘.×l0 ft.for a stand-alone system 23
Ultraviolet Photoelectron Spectroscopy (UPS) 1.5.2 If monoenergetic photons in the 10-100 eV energy range strike a sample material, photoelectrons from the valence levels and low-lying core levels (i.e., having lower binding energy than the photon energy) are ejected. Measurement of the kinetic energy distribution of the ejected electrons is known as Ultraviolet Photoelectron Spectroscopy (UPS). The physics of the technique is the same as XPS, the only differences being that much lower photon energies are used and the primary emphasis is on examining the valence electron levels, rather than core levels. Owing to this emphasis, the primary use, when investigating solid surfaces, is for electronic structure studies in surface physics rather than for materials analysis. There are, however, a number of situations where UPS offers advantages over XPS for materials surface analysis. Elemental analysis Destructive Chemical state information levels as for XPS Not usually, sometimes from available core levels No, some beam damage to radiation-sensitive material Yes, but complicated using valence levels; for core Depth probed 2-1 00 A Depth profiling Yes, over the depth probed; deeper profiling requires sputter profiling Lateral resolution Generally none (mm size), but photoelectron microscopes with capabilities down to the 1-pm range exist Sample requirements Vacuum-compatible material; flat samples best; size accepted depends on instrumentation Main use Electronic structure studies of free molecules (gas phase), well-defined solid surfaces, and adsorbates on solid surfaces No commercial instruments specifically for UPS; usually an add-on to XPS (incremental cost +$30,000) or done using a synchrotron ficility as the photon source 10 ft. x 10 ft. for a stand-alone system Instrument cost Size 23
Auger Electron Spectroscopy(AES) 1.5.3 Auger Electron Spectroscopy(AES)uses a focused electron beam to create second- ary electrons near the surface of a solid sample.Some of these(the Auger electrons) have characteristic of the elements and,in many of the chem bond energ ng o atom m which they are rela ed.Becau of their characteristi energies and the shallow depth from which they escape without energy loss,Auge electrons are able to characterize the elemental composition and,at times,the chemistry of the surfaces of samples.When used in combination with ion sputter- ing to gradually remove the surface,Auger spectroscopy can similarly characterize th mpe in depth.The high spa acial r solution of the electron bea and the cess allows is of three- ional s of solid samples.AES has the attributes of high lateral resolution,relatively high sensitivity,standardless semi- quantitative analysis,and chemical bonding information in some cases. Range of elements All except H and He Destructive No,except to electron beam-sensitive materials and during depth profiling Elemental Analysis Yes,semiquantitative without standards;quantitative with standards Absolute sensitivity 100 ppm for most elements,depending on the matrix Chemical state Yes,in many materials information? Depth probed 5-100A Depth profiling Yes,in combination with ion-beam sputtering Lateral resolution 300 A for Auger analysis,even less for imaging Imaging/mapping Yes,called Scanning Auger Microscopy,SAM Sample requirements Vacuum-compatible materials Main use Elemental composition of inorganic materials Instrumentcost $100,000-$800,000 Size 10f丘.×15ft INTRODUCTION AND SUMMARIES Chapter 1
Auger Electron Spectroscopy (AES) 1.5.3 Auger Electron Spectroscopy (AES) uses a focused electron beam to create secondary electrons near the surface of a solid sample. Some of these (the Auger electrons) have energies characteristic of the elements and, in many cases, of the chemical bonding of the atoms from which they are released. Because of their characteristic energies and the shallow depth from which they escape without energy loss, Auger electrons are able to characterize the elemental composition and, at times, the chemistry of the surfaces of samples. When used in combination with ion sputtering to gradually remove the surface, Auger spectroscopy can similarly characterize the sample in depth. The high spacial resolution of the electron beam and the process allows microanalysis of three-dimensional regions of solid samples. AES has the attributes of high lateral resolution, relatively high sensitivity, standardless semiquantitative analysis, and chemical bonding information in some cases. Range of elements Destructive All except H and He No, except to electron beam-sensitive materials and during depth profiling Yes, semiquantitative without standards; quantitative with standards 100 ppm for most elements, depending on the matrix Yes, in many materials Elemental Analysis Absolute sensitivity Chemical state information? Depth probed 5-1 00 a Depth profiling Lateral resolution Imaging/mapping Sample requirements Vacuum-compatible materials Main use Instrument cost $100,000-$800,000 Size Yes, in combination with ion-beam sputtering 300 A for Auger analysis, even less for imaging Yes, called Scanning Auger Microscopy, SAM Elemental composition of inorganic materials 10 ft. x 15 ft. 24 INTRODUCTION AND SUMMARIES Chapter 1