Swift Heavy ion irradiation

Prof. V. KrishnakumarProfessor and Head

Department of PhysicsPeriyar University

Salem – 636 011, India

High energy ion irradiation-ImportanceHigh energy ion irradiation-Importance Energetic ion beams play a vital role in the field of

research in Materials Science.

Energetic ions are suitable means for the modification of the surface and the bulk structure of solids.

All classes of materials can be modified and analyzed with ion beam in order to know about the improvements of their physico-chemical properties such as electrical, electronic, optical, mechanical, magnetic, catalytic structural and metallurgical etc.

When energetic ions passes through matter,it looses its energy in two ways

Electronic energy loss due to inelastic collision with electrons(Se)[Electronic stopping]

Dominant at higher energies (few tens of MeV and higher)-Swift heavy ion Irradiation(SHI)

Nuclear energy loss due to elastic collision with atoms of the solid(Sn)[Nuclear stopping]

Dominant at low energies (few tens of KeV to MeV)

• Low energy ions <2MeV – elastic collision – nuclear energy loss

• High energy ions > 2MeV – Inelastic collision – electronic energy loss - SHI.

Electronic stoppingElectronic stopping

Interaction of heavily charged ions with electrons of the target material through Coulomb forces, produce track of ionization and highly kinetic electrons along the path of the primary ion - latent track (Se>Sth) – Sth

depends on the material - Electronic energy loss.

When SHI passes through the materials Se increases with energy and mass of the ions. The effect of Sn is very small( range of the particle > sample thickness).

The desirable defects can be generated in materials by locking sufficient energy into the lattice - favors huge possibilities in tailoring of materials.

Energy loss can be varied by choosing proper ions and doses.

This remarkable flexibility coupled with new cluster beams provides new outlook in many fields.

Ion implantation is a crucial method for dopant incorporation in device fabrication which produces lattice disorder – detrimental for device performance.

Crystal defects due to electronic stopping

• If the heat conductivity is low enough (insulators), then the energy of the exited electrons is transferred to the target atoms in the vicinity of the ion trajectory. As a result crystal defects are formed.

• This swift heavy ion collision displacement damage manifests itself in the form of

1. Point defect (defect cluster) generation and2. dislocation loop formation at the periphery of the

ion trajectory.3. Disordered and even amorphous ion track cores.4. High energy heavy ion collisions (elastic and

inelastic) in a variety of solids create radiation damage on the target surface.

Nuclear stoppingNuclear stopping Causes damage and dislocation of nuclei from

their lattice sites due to elastic collisions

Always produce lattice defects

(Interstitial atoms, anionic or cationic vacancies)

Damage areas – modify material properties

Ex: change of color of diamonds produce interesting alloys

Nature of materials modification depends on Properties of the target material

o Electrical o Thermal o Structural

Mass of the projectile ion Irradiation parameters

o Ion energy o Fluence rate (ion concentration)o Ion species

Difference of materials modification by Difference of materials modification by energetic ionsenergetic ions

Low energy ionsLow energy ions High energy ionsHigh energy ions Embedded into the material Not embedded into the

material (large range) Modification due to cascade Modification due to

collision of impinging ions electronic excitation

Modification in the Modification in the

presence of embedded ion absence of embedded ion

Nuclear stopping Electronic stopping Produce point defects Columnar defects

• Ion beams play a significant role in engineering the properties of materials.

• To alter and tailor many materials properties like electronic and optical

• Energetic ions are suitable means for the modifications of the surface and the bulk structure of solids

• Possible to create optical wave guides in organic crystals.

Impacts of heavy ion irradiation

Ion induced effects on NLO materialsIon induced effects on NLO materialsIrradiation of heavy ions is expected to bring following changes Formation of gray tracks (coloration) on the

irradiated samples of high fluences irrespective of ion beam and its energy.

Efficient generation of harmonic frequencies requires a non-linear medium with following desirable properties

High Thermal stabilityLarge transparency windowHigh optical damage thresholdHigh mechanical hardness

Due to these facts, wave guide structures can be obtained

Light guidance demands adjacent regions of different refractive indices

Two methods to create wave guide structures

Heavy ion exchange- causes increased refractive indices

MeV irradiation of light elements forms a layer of reduced refractive index due to high nuclear energy deposition

This will increase the single mode spectral bandwidth for efficient SHG in wave guiding lasers.

Modifications in the refractive index of the materials on ion irradiation leads to the formation of wave guides.

Formation of wave guides will guide to modify the essential property of second harmonic generation, which widens their scope in photonic and opto-electronic applications.

Also, Post treatment after SHI irradiation into insulator leads to the nano-cluster formation and change of optical property.

Dielectric constant of a material is related to polarizability (ionic, electronic, oriental and space charge)of the material.

Disordering of the crystal lattice by ion irradiation causes increase in dielectric constant

Electro-optic co-efficient is directly proportional to dielectric constant of the material.

o Ion irradiation enhances the electro-optic co-efficient of NLO crystals

o Irradiated crystals can be a good EO modulator of light.

Ion irradiation also affects the transmittance properties of crystals, hence, it is also expected to influence the SHG property.