I have prepared a thin film nanocomposite of ZnO and analysing the results of UV spectroscopy I'm not sure about the meaning of an increment or reduction in the band tail parameter in a doped ZnO thin film?
Dear Víctor Fuertes
Hi my dear colleague
What you meant by the Band tail ? Is it the band tail energy? Or You mean something else?
Generally, along the curve of the absorption coefficient versus the photon energy and near the optical band edge there is an exponential part called Urbach tail. This exponential tail appears in the low crystalline, poor crystalline, the disordered and amorphous materials because these materials have localized states which extended in the band gap. But for the crystalline material this tail is not almost present and if it has a presence, its value will be very small value. Increasing the crystallinity of the material leads to the reduction of such tail. In the low photon energy range, the spectral dependence of the absorption coefficient (α) and photon energy (hν) is known as Urbach empirical rule, which is given by the following equation:
α = αo exp (hν/EU)
where αo is a constant and EU denotes the energy of the band tail or sometimes called Urbach energy, which is weakly dependent upon temperature and is often interpreted as the width of the band tail due to localized states in the normally band gap that is associated with the disordered or low crystalline materials. Taking the logarithm of the two sides of the last equation, hence one can get a straight line equation. It is given as follows:
ln α = ln αo + (hν/EU)
Therefore, the band tail energy or Urbach energy (EU) can be obtained from the slope of the straight line of plotting ln (α) against the incident photon energy (hν).
For more details, you can go to these links:
https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films?ev=prf_pub
https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films
Good luck and my best wishes
Yours,
Ahmed Saeed Hassanien
Article Influence of composition on optical and dispersion parameter...
Article Effect of Se addition on optical and electrical properties o...
Dear colleague Víctor Fuertes
Have a nice time and good day.
The structural properties may seriously affect the optical and electrical properties of materials depend on many parameters; material state (amorphous, poly crystalline and crystalline), type of materials (organic or inorganic), materials form (liquid, thin film, powder or bulk), preparation method, type of container, thermal history, aging, Structural relaxation.....e.t.c.
The thermal history of the prepared material is one of the most important parameters that may affect the properties of the produced material.
Materials that prepared by quenching in iced water or high cooling rate and thin films that prepared on substrate at room temperature or less had stressed; atoms are randomly arranged and have no time to satisfy its bond requirements because of sudden quenching where thermal energy is released out This issue produces strain, dislocations and defects or dangling bonds forming localized states (tail) or highly disordered materials characterized by a short range order or amorphous nature, it may also has nano structure or as amorphous; no crystallite size or certain arrangement. Such those materials have the tendency to structural relaxation.
These dangling bonds or localized states or defects (tail) are seriously affected by many factors Ex. Annealing, thickness and doping. Up certain point the increasing of the mentioned three parameters reduce the localized states (tail) or defects decreases.
The increasing of annealing, thickness and doping may
give enough time to the atoms to take its time to satisfy saturated bonds at the expense of unsaturated bonds which decreases the dangling bonds or disorder (tail) and consequently the medium and long range order may be introduced. As a resultant, nucleation of different shapes of crystallite size will be introduced.
Sometimes the increasing of these parameters broke the structural order and increases the localized states or tail. Such this issue depends on the DSC or DTA parameters like glass transition temperature, crystallization temperature and melting point. So the thermal literature is highly important.
XRD, SEM and TEM can be used as an indication of structural enhancement see the attached files. .
If you need reference you can use the next reference : J. Tauc (Ed.). Amorphous and Liquid Semiconductors, Plenum, New York, 1976].
I wish this is useful and help you.
Yours
A. El-Denglawey
Many thanks for all the information Ahmed Saeed Hassanien, your papers are brilliant and help me a lot. Thanks for you too,A. Eldenglawey , I have seen how the structural properties can affect the optical and electrical properties of materials in a clear way.
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