Ion Implantation and Synthesis of Materials - Studium

50 5 Ion Stopping5.2 The Energy-Loss ProcessThe energy-loss rate, dE/dx, of an energetic ion moving through a solid is determinedby screened Coulomb interactions with the substrate atoms and electrons. Itis customary to distinguish two different mechanisms of energy-loss: (1) nuclearcollisions, in which energy is transmitted as translatory motion to a target-atom asa whole, and (2) electronic collisions, in which the moving particle excites orejects atomic electrons. For most purposes, this separation is a convenient oneand, although not strictly true, it is a good approximation. The energy-loss rate,dE/dx, can be expressed asdE dE dE= +dx dx dxne,(5.2)where the subscripts n and e denote nuclear and electronic collisions, respectively.Nuclear collisions can involve large discrete energy-losses and significant angulardeflection of the trajectory of the ion (Fig. 5.1). This process is responsiblefor the production of lattice disorder by the displacement of atoms from their positionsin the lattice. Electronic collisions involve much smaller energy-losses percollision, negligible deflection of the ion trajectory, and negligible lattice disorder.The relative importance of the two energy-loss mechanisms changes rapidly withthe energy E and atomic number Z 1 of the particle: nuclear stopping predominatesfor low E and high Z 1 , whereas electronic stopping takes over for high E and lowZ . Typical units for the energy-loss rate are electron-volt per nanometer or1IonLatticeElectronsAtomsNuclear CollisionsIonFig. 5.1. An ion incident on a crystal lattice is deflected in nuclear collisions with the latticeatoms and also loses energy in collisions with electrons