I try optimizing these parameters:
annealing temprature / gas
synthesis parameters(solvent,surfactant,dopant,,...)
coating method(spin, spray)
heating temp after every spin coating
Thickness of layers
.... but the min resistance was 25 M ohm!
Resistance depends on many factors. Please check the homogeneity of the film.
As mentioned in the previous answer homogeneity of your film may be responsible. Overall creating p-character ZnO films is an ongoing challenge due to the poor solubility of p-type dopants in ZnO, which is intrinsically n-type. you therefore need to consider your dopant as well. Even if you have a thick enough film to favor conductivity, your dopant may be effecting the erroneous resistance measurements when all the experimental variation you have employed thus far yields roughly the same results.
Porosity, which itself is affected by the processing parameters will also influence the conductivity.
Dear Dr. Baheri:
Taking in account a good film, firstly it is necessary to confirm the electrode configuration to define if measurements are monitoring surface conductivity/resistivity or bulk conductivity/resistivity. Surface measurements requer an apparatus more elaborate since both vapor and gas should be "eliminated" from surface prior to the electrical measurement be carried out.
Well, let's to consider that bulk properties are the hot point. After this, the great serie of procedures that have been adopted by you would affect in essence only electrical properties of grain boundary features. In this sense, structures with nanometric grains tend to reach grain boundary dominant phenomena. But if sub-micrometric or micrometric grain are generated, bulk properties are dominant.
ZnO is oxide of wide gap ~3.2 eV, a poor semiconductor. To dow size the electric resisitivity (increase of electric conductivity) is necessary a re-engineering of gap. A priori doping with cations of transition elements (orbital 3d with incomplete number of electrons) gives electric resistivity with minor values that undoped ZnO. A moderate decreasing of electric resistivity can be attained via doping with Co ions or Mn ions, it is necessary to consult a binary phase diagram, since there is solubility limit. Doping with Co, Mn Fe, Cr ... leads to color centers development, Co generated a palid green color and yellow to brown-redish to Mn doping. But if you desires a very low electric resistivity select dopants with high valence state. A great decreasing of electric resistivity is attained via doping with elemnts of 5A group of periodic Table, pentavalent cations type Nb5+(niobium) and V5+(vanadium), in fact this chemical elements exhibits several valences, prefer the thermal treatment of film in an oxidizing atmosphere type pure oxygen. See, due great difference of valence a decreasing of solubility limit of above elements is expected. But, the sufficient to reach further low electric resistivity.
The reduction of electrical resistance of p-ZnO thin film deposited by coated technique to have the potential for use in electrical device .
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