Explain the mechanism of conduction in different structure.Ripunjay Tiwari
Ans. Electrical conductivity in thin metallic discontinuous granular films, is thermally activated type and much smaller than that of bulk material. Also the conductivity is field dependent- Ohmic at low field but non-linear at higher fields. The conduction process can be explained by various mechanisms such as thermionic and Schottky emission, tunneling thought gaps and traps, etc. Generally, it was found that in discontinuous metallic thin films the conduction mechanism is predominantly thermionic type or Schottky type at room temperature (- 300 K), if the particles (or grains) have a separation greater than 10 nm. But the cond·uction mechanism is tunneling type if the particle (or grain) separation is around 2-5 nm (at 300 K) for typical metal
work function 4-5 eV. Typically, thermionic emission is the flow of charged particles called thermions from a charged metal or a charged metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. For discontinuous thin films, the effect increases exponentially on the height of the barrier between grains of the film. At very low grain separation, the barrier height would be significantly low due to the overlap of image force potential, leading to higher conductivity. The conductivity in a discontinuous film may be expressed as –
where, A is a constant and depends on the characteristics of each film,
is the work function of the bulk metal, d is the distance between the particles,
(B =constant) represents the contribution of the image force. From the above equations, it is clear that at very low grain separation(- few nanometers), the is very low, leading to a higher cr value. Under an applied electric field E, the effective work function is further reduced as
and we get the so-called Schottky emission. Therefore, at low field we get
Ohmic behavior but at high field the conductivity is non-linear.
Jn insulating or wide band gap semiconducting thin film, with metal
electrodes connected across it as sandwich structure, shown in fig. 1.19, the
electron conduction can be explained by various mechanisms as described
from (i) to (vii) in fig. l.l9.
(i) Tunneling from one electrode to other through the thin film.
(ii) Carrier injection over metal/film barrier via then ionic/Schottky
(ill) Tunneling through barrier gap.
(iv) Transport in CB by scattering.
(v) Tunneling via traps.
(vi) Carrier. hoping from trap to trap.
(vii) Field-enhanced thermal excitation of trapped electrons to CB-Poole-
The above mentioned sandwich structure is essential for the measurement
of conductivity of thin films under various applied conditions. Under an applied
field across the electrodes, thermionic/Schottky emission is the dominant
conduction mechanism in the film and for that the net current density should
be a combination of thermionic currents in both directions due to the emission
over barriers of each electrode.
Under the applied field E between electrodes, the barrier heights of the
electrodes will be related as –
where, e is the electronic charge.
Now assuming the barrier to be symmetrical and applying parabolic image
force .correction factor for one barrier, the expression for the net current
density will be given as –
where, e is the dielectric constant of the film. Equation (v) is called as