7 Auger electron Spectroscopy – 1

Dr. Ajit K. Mahapatro

Learning Objectives

From this module students may get to know about the following

i. Introduction to Auger Electron Spectroscopy(AES)

ii. Basic Principle and Instrumentation of AES

iii. Qualitative Analysis of AES Spectra

1. Introduction

The Auger electron spectroscopy (AES) is an analytical, non-destructive, and efficient technique used to determine the elemental composition of the surface layers of a solid material. Secondary electrons emitted due to Auger process are analyzed and their kinetic energy is determined. Each element in a specimen would produce characteristic spectrum of peaks. The Auger effect was discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Auger electrons are emitted at discrete energies and helping to identify the origin of. The idea of using stimulated Auger signals for surface analysis was first suggested in 1953 by J. J. Lander. The technique became practical for surface analysis after Larry Harrisin in 1967 demonstrated the use of differentiation to enhance the Auger signals. Auger recognized that an atom in the excited state could return to the equilibrium state by releasing energy though ejection of an electron from the upper level. These electrons have characteristic energies depending on the electronic structure of the element, and hence the characteristic energies could uniquely identify the element from which the electron is ejected. The first commercial Auger electron spectrometer (AES) became available in 1969. This is becoming one of the most popular methods for surface analysis. AES is very powerful surface analytical technique that has found applications in many fields of solid-state physics and chemistry. It is a surface specific technique utilising the emission of low energy electrons in the Auger process and is one of the most commonly employed surface analytical techniques for determine the composition of the surface layers of a sample.

2. Principle of AES

Figure 1 illustrates the emission process of an auger electron. When a high-energy electron strikes an atom, with energy of the incident particle high enough to knock out the inner shell (K shell) electron of the atom, shifting the atom to ionize and in excited state. The atom will quickly return to its normal state after refilling the inner electron vacancy with an outer shell electron. During this transition process, the energy difference between the outer and inner shell electrons may cause emission of either a characteristic X-ray photon by radiating through a radioactive process or an Auger electron from an electron shell through non-radioactive process. Auger is a raditionless process, where an excited ion decays into two doubly charged ions by ejecting an electron. The kinetic energy of an Auger electron is approximately equal to the energy difference between binding energies in the electron shells involved in the Auger process. For example, the kinetic energy of an Auger electron in Figure 1 is approximated by the following equation.

^?KL₁L₂    ≈  ?_?K −  ?_{? L1}  − ?_{? L2,3}   (1)

The notation for kinetic energy of Auger electron describes its origin, but the nomenclature is rather complicated. For example, the kinetic energy of an Auger electron in Equation 1 is from a process illustrated in Figure 1 that is, an incident electron knocks out a K shell electron, a L1 shell electron refills the K shell vacancy, and a L2,  3  shell electron is ejected as the Auger electron. The subscript of EBX indicates the binding energy of electron shell X; for example, EBK is the binding energy of the K shell. Auger electron spectroscopy identifies the elements by measuring the kinetic energies of Auger electrons. In an AES spectrum, an individual kinetic energy peak from an Auger electron is marked with an elemental symbol and subscripts indicating the electron shells or sub shells involved.

Figure 1: Principle of AES

3. Instrumentation

A modern instrument for electron spectrometry contains both the XPS and AES in a single chamber as a multifunctional surface analysis system. A scanning electron microscope (SEM) system may also be included in order to image the microscopic area to be examined by electron spectroscopy. Figure 2 schematically illustrates the block diagram of AES and XPS, including an electron gun, an X-ray gun and a shared analyzer of electron energy. The electron beam for generating the Auger electron emission is focused and scanned over a sample surface to obtain two-dimensional mapping for AES analysis. The AES analysis is commonly used in scanning type, called scanning Auger microscopy.

The main components of AES are:

3.1 Ultra High Vacuum systems

The ultra-high vacuum is required to reduce the chances of low energy electrons being scattered by gas molecules on their way to reach at the detector, and to keep the samples surface free from contamination from the gas molecules. Low energy photoelectrons and Auger electrons are easily scattered by the gas molecules. Scattering reduces the signal intensity and increases background noise in the spectra.

The ultra- high vacuum chamber is commonly prepared using stainless steel, and joints of chamber parts are made from crushed copper gaskets. Ultra- high vacuum can be achieved using diffusion pumps, sputter ion pumps or turbo-molecular pumps. Magnetic shielding is also required for chamber, to change the trajectory of signal electrons, since the electrons are affected by the magnetic field.

3.2 Source Guns

(a) X-ray Gun:

An electron spectrometer system contains an X-ray gun for XPS analysis. The X-ray gun produces a characteristics X-ray line to excite atoms of the surface to be analyzed. The XPS uses monochromatic and non-monochromatic X-ray sources. The output from a non-monochromatic X-ray source consists of continuous energy distribution with high intensity of K alpha characteristics lines. The monochromatic source produces output by removing continuous X-rays from a radiation spectrum.

The energies of Al Kα and Mg kα are 1.48 and 1.25 kev, respectively which is lower than energies of Cu kα = 8.04 keV and Mo kα = 17.44 keV commonly used in X-rays diffractometry. The reason to choose lower width of X-rays is their narrow line width. The line width of characteristics X-rays refers to their range of energy. XPS requires a line width less than 1.0eV to ensure good energy resolution. Both Al Kα and Mg Kα exhibit line widths less than 1.0 eV and also have sufficient energies for photoelectron excitation.

(b) Electron Gun

The electron guns used in AES analysis is similar to those used in electron microscopy, Lanthanum hexaboride (LaB6) and field emission guns are commonly used in electron spectrometers. LaB6 provides an electron beam of high brightness with spatial resolution of 200 nm. Field emission guns provides superior brightness and higher spatial resolution than LaB6 and also their emitting surface remains clean during operation without adsorption of gas molecules.

(c) Ion Gun

The function of ion gun is two fold in XPS. Firstly it provides high energy ion flux to clean sample surfaces before examination. Sample surfaces are commonly contaminated with adsorbed hydrocarbons, water vapors and oxides that need to be removed before surface analysis. The second function is to sputter out sample atom layer by layer so that an elemental depth profile can be revealed. The ion gun produces an argon ion beam by either electron impact or gaseous discharge.

Figure 2: Block diagram of AES.

3.3 Electron energy Analyzers

An AES spectrum is recorded using an electron energy analyzer as shown in Figure 3. The most commonly used analyzer is the concentric hemispherical analyzer (CHA) also called hemispherical sector analyzer (HSA).The analyzer is composed of two concentric hemisphere with radii R₁ and R₂ and its working principle can be understood from the figure. Negative potentials V₁ and V₂ are applied to inner and outer hemispheres, respectively. The applied potential generates a median equipotential surface with radius R0. The potential along the median surface (V₀) is called the pass energy of the CHA. A slit at one end of the CHA allows electrons from the sample to enter,and a slit at the other end of the CHA lets electrons pass through to an electron detector.

Auger analysis requires suppressing the electron signal at the low energy end of its spectrum. The CRR mode meets the Auger analysis requirement because the CHA mode exhibits a low transmission rate with low pass energy. When a constant retardation ratio is applied, a low Auger electron energy generates low CHA pass energy. For example, with a retardation ratio of 10, the pass energy is only 10 eV for E = 100 eV, and the pass energy is 100 eV for E = 1000 eV. In other words, electron transmission through a CHA is lower at a pass energy of 10 eV than at 100 eV. An Auger spectrum is commonly recorded by changing the CHA pass energy in the CRR mode.

Figure 3: Energy analyzer

4. AES Spectra

A typical AES spectrum is a plot of intensity versus kinetic energy. It is a plot of the first derivative of intensity versus the kinetic energy. The Auger peaks appear small against the background of the direct mode spectrum. This occurs because the signals from Auger electrons are relatively weak compared to the secondary electrons escaped from a solid surface. Electrons are either elastically or in elastically scattered when a primary beam strikes the material. Auger spectra can be expressed in two modes a direct mode and differential mode as shown in Figure 4. The direct mode presents the intensity distribution in a range of electron kinetic energies. The differential mode presents the derivative of intensity versus the kinetic energy. The differential mode is more widely used because the Auger peaks are more obvious than in the direct mode.

Figure 5 shows the Auger spectra of Pd metal as an example. The CRR mode of the CHA generates a constant relative resolution ΔE/E over the whole range of the spectrum. The direct mode spectrum required with the CRR mode expresses the energy distribution as the number of electrons multiplied by its kinetic energy, EN (E), as shown in Figure 5a. The differential mode spectrum is produced by taking the first derivative of the curve in the direct mode using computer software. The ordinate should be E dN (E) in the differential mode spectrum with the CRR acquisition as shown in Figure 5b. The differential mode effectively reduces the background and enhances the Auger peaks.

Figure 4: AES spectra of oxidized Copper surface (a) direct spectrum of intensity versus kinetic energy of electrons in direct mode. (b) Differential spectrum of intensity versus kinetic energy of electrons [http://in.bgu.ac.il/engn/mater/Documents/LaboratoryBriefings/4/XPSAESdoc.pdf].

Figure 4: Auger spectra of Pd metal (a) direct spectrum (b) differential spectrum [http://www6.cityu.edu.hk/appkchu/AP5301/Lecture-10-AES.pdf]

5. Application of AES

• AES is used to monitor the elemental composition of surfaces during physical property measurements.
• Several phenomena such as adsorption, desorption, surface segregation from the bulk, measurement of diffusion coefficients, and catalytic activity of surfaces can be investigated.
• It is used to study the compositional changes on the surface of alloys during ion sputtering. Generally, the chemical properties including corrosion, stress corrosion, oxidation and catalytic activity and mechanical properties such as fatigue, wear, adhesion, resistance to deformation processes, and surface cracking depend on surface properties.
• The grain boundary chemistry influences mechanical properties such as low and high temperature ductility and fatigue, chemical properties such as inter-granular corrosion and stress corrosion, and electrical properties.
• AES has been used to relate surface and grain boundary chemistry to properties of materials. AES has proved to be extremely valuable compared to most other techniques, which are limited by either large sampling depth or poor sensitivity.

Summary

• AES is an analytical and non-destructive technique and used to determine the elemental composition of the surface layer of the solid materials.
• The basic principle of AES is auger effect and Auger is a raditionless process, where an excited ion decayan excited ion decays into two doubly charged ions by ejecting an electron.
• The block diagram of AES includes an electron gun, an X-ray gun and a shared analyzer of electron energy. The electron beam for generating  the  Auger electron emission is focused and scanned over a sample surface to obtain two- dimensional mapping for AES analysis
• A typical AES spectrum is a plot of intensity versus kinetic energy. It is a plot of the first derivative of intensity versus the kinetic energy. The Auger  peaks appear small against the background of the direct mode spectrum.

References

1.Materials Characterization Book, Yang Leng, 2008 John Wiley and Sons (Asia) Pte Ltd.

2.T E GALLON and J A D MATTHEW, Physics Department, University of York, Heslington, Auger electron spectroscopy and its application to surface studies

3.P.K. Ghosh,  Introduction to photoelectron spectroscopy , Wiley, 1983