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Saturday, December 26, 2015

What is Luminescence?



What is Luminescence?
The property of emission of light when a energetic particle impinges on  material (semiconductors)  leading to creation of electron hole pairs and excitation of carriers. When these carriers come to their equilibrium states they emit light.

Mechanism of Recombination

Radiative Recombination

When the excited excess carriers reach equilibrium positions by emission of photons it is said to be Radiative recombination  



Non-Radiative Recombination
When the excited excess carriers reach equilibrium positions by emission of phonons due to surface / bulk defects / other defects it is said to be non-radiative recombination.
 

What Is Activator?

Impurity atom occurring in relatively small concentrations in host material or a small stoichometric excess of one of constituents of material which exhibits the property of Luminescence. 

What Is Killer?

Presence of certain type of impurity may also inhibit Luminescence of other centers, in which case they are referred as killers.

After a particular level of concentration, the efficiency decreases as excess carrier returns to the ground state only if there is no other activator with in a sphere of
radius ‘R’ around central activator atom

Dependence of luminescence efficiency on Activator concentration




CHARACTERISTICS OF A PHOSPHOR

  1. High concentration of carrier traps.
  2. High emission efficiency.
  3. Large trap depth.
  4. Traps, Luminescence & lattice are not to be damaged by repeated irradiation & heating process.