the RT electron mobility in ZnO thin film grown by pulsed laser technique comes out to be 40-60 cm2/V-s. What is limiting this mobility; grain boundary scattering or other carrier scattering mechanisms?
Normally Textured or well oriented (c-axis) films can continue to have a lot of defects, and vacancies, when grown on a variety of substrate either by sputtering/PLD, and definitely in such film grain boundary scattering cannot be precluded.
since you have a PLD, you should try to grow an epitaxial ZnO film, on a matched substrate and check the mobility, and see if it is high.
K. Sreenivas
Dear prof. Raja,
you must check the energy gap, grain size, carrier concentration and other properties.
If there are more defects in the films, you must use lower thickness or you can use Thermal evaporation technique instead of PLD.
Agree with K. Sreenivas's suggestion, you can try an epitanial growth of ZnO.
In addition, thermal treatment may need to be performed to improve the crystallinity of ZnO.
Good luck.
Dear Prof. Raja,
In ZnO thin films electrical transport is limited by grain boundary barriers.
In this kind of thin films, a standard Hall effect measurement does not yield the electron mobility inside the grains, but an "effective mobility" that depends on the barrier height (E_B). Sometimes this effect is improperly referred to as "barrier limited mobility". This is not a good physical description of the effect. In highly crystalline thin films the grain size is far much larger than the electron mean free path. Then electron mobility inside the grains cannot be affected by the existance of barriers.
The Boltzmann factor associated to the grain boundary barrier actually appears in the effective conductivity of the thin film. If the barrier height is E_B, only a fraction of the free carriers inside the grains (those with kinetic energy larger than the barrier height) contributes to the macroscopic conductivity. This fraction would be proportional to the Boltzmann factor exp(-E_B/kT). The thin film macroscopic conductivity is much smaller than the conductivity inside the grain.
On the other hand, Hall effect measures the actual electron concentration inside the grains. As a consequence, the effective mobility (as measured through standard Hall effect and resistivity experiments) is the electron mobility multiplied by the Boltzman factor that detremines the macroscopic conductivity. The signature of this effect is an effective mobility exhibiting an activated temperature dependence. If you can perform Hall effect and resistivity measurements at high temperature, you could in principle determine the barrier height from an Arrhenius plot of the effective mobility as a function of temperature. Then you could estimate the actual mobility inside the grains.
With best regards, A. Segura
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