SPECIAL SECTION: CURRENT SCIENCE, VOL. 80, NO. 12, 25 JUNE 2001 1532 ERDA with swift heavy ions for materials characterization D. K. Avasthi* ,† and W. Assmann** *Nuclear Science Centre, Post Box 10502, Aruna Asaf Ali Marg, New Delhi 110 067, India **Sektion Physik, Universität München, 85748 Garching, Germany Elastic recoil detection analysis (ERDA) is a tech- nique specially suited for depth profiling of light elements, which overcomes the limitations of Ruther- ford backscattering (RBS). The developments in the technique enabled depth profiling of elements from hydrogen up to very heavy elements with single ele- ment resolution in the light mass region. The use of large area position sensitive telescope detectors made it a highly sensitive technique with which ion beam induced modifications such as interface mixing, elec- tronic sputtering and radiation damage studies can be performed during irradiation itself. 1. Introduction THE first nuclear particle accelerator was built in early 1930s with the aim to probe into the nucleus by nuclear reactions. With the passage of time, it was realized that accelerators can play an important role in other branches of science. Materials science gained a lot by accelerators. Ion implanters, which are basically low energy (typically 400 keV) accelerators are today essen- tial tools in semiconductor technology. Alpha particles of energy of a few MeV have been playing a major role in materials characterization by Rutherford backscat- tering (RBS) 1 and channeling 2 . RBS provides depth pro- filing of elements in the surface region up to a few microns. RBS channeling measurements allow 3 the quantification of crystallization, dopant atom location, determination of strain in superlattices, etc. RBS, how- ever, has poor sensitivity for the detection of light ele- ments (C, N, O, etc.) especially in the presence of a substrate of higher mass (such as Si, which is often the case). This is because of low Rutherford scattering cross section which is proportional to the product of the atomic numbers of the projectile and the scatterer. An- other limitation of RBS is its inability to detect hydro- gen because no projectile can get scattered back from this lightest element. The disadvantages of RBS are overcome by another technique called elastic recoil detection analysis (ERDA) 4 first demonstrated by L’Ecuyer et al. in 1976. The breakthrough of the ERDA technique came when heavy ion accelerators became † For correspondence. (e-mail: dka@nsc.ernet.in) available for materials research. Basically ERDA is a technique quite similar to RBS, but instead of scattered projectile detection at the back angle, the recoils are detected (resulting from elastic collision of the incident particle and the atoms in the sample) in forward direc- tion. Its use for hydrogen depth profiling was demon- strated 5 by Doyle and Peercey. ERDA technique further got strengthened in terms of its capabilities by the use of particle identifying techniques, which were com- monly used by experimental nuclear physicists. Salient features of ERDA with high energy heavy ions are: (i) Large recoil cross sections with heavy ions and hence good sensitivity. (ii) Almost same recoil cross section for a wide mass range of target atoms. (iii) Element depth profiling of a wide range of ele- ments from hydrogen to rare earth elements using particle identifying techniques. 2. Principles of ERDA In an elastic collision of the incident particle of mass m p (in atomic mass units) and energy E p , with the atom of mass m r (in atomic mass units) present in the sample, the atom in the sample which is at rest, recoils in for- ward direction after the collision. The energy of the recoiling atom can be derived from the basic principle of conservation of energy and momentum. The recoil energy E r of atom with mass m r at an angle φ with re- spect to the beam direction is given by E r = kE p , (1) where k is kinematic factor given by k = 4m p m r cos 2 φ/(m p + m r ) 2 . (2) The projectile mass, its energy and recoil angle remains fixed under a given experimental condition, therefore atoms of different masses in the sample come out with different recoil energies as governed by eq. (1). If m p > m r , projectiles can only be scattered in a limited angular range with a maximum angle θ defined by