Effect of Temperature on I-V Characteristic for ZnO/CuO

Research on nonmaterials has become increasingly popular because of their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Therefore, many efforts have been made to synthesize multidimensional nanostructures for new and efficient nanodevices. Among those materials, zinc oxide (ZnO) has gained substantial attention owing to many outstanding properties. ZnO besides its wide band gap of 3.34 eV exhibits a relatively large excitons binding energy (60 meV) at room temperature which is attractive for optoelectronic applications. Likewise, cupric oxide (CuO) has a narrow band gap of 1.2 eV and a variety of chemo-physical properties that are attractive in many fields. Moreover, composite nanostructures of these two oxides (CuO/ZnO) may pave the way for various new applications. So in this thesis, eight samples of CuO/ZnO junction were synthesized and exposed to temperatures 60, 70, 80, 90, 100, 110, 120 and 130. The electrical properties of Schottky diode junctions were analyzed by I-V measurements under the influence of direct solar radiation and, lag of radiation (darkness) which shows the semi-logarithmic I-V characteristic curve of the fabricated photodiodes. Also energy band gap was estimated and the morphology and particle sizes of the as-prepared sample were determined by SEM. The SEM images of ZnO + CuO sample films were annealed at 60 ̊C to 130 ̊C step 10.


Introduction
During the last few decades, nanomaterials have been the subject of extensive World Journal of Nuclear Science and Technology interest because of their potential use in a wide range of fields like, optoelectronics, catalysis and sensing applications. The physical and chemical properties of nanomaterials can differ significantly from their bulk counterpart because of their small size. In general, nanomaterials comprised novel properties that are typically not observed in their conventional, bulk counterparts. Nanomaterials have a much larger surface area to volume ratio than their bulk counterparts, which is one of the bases of their novel physical and/or chemical properties.
In addition, metal oxide nanomaterials have drawn a particular attention because of their excellent structural flexibility combined with other attractive properties. These metal oxides nanostructures not only inherit the fascinating properties from their bulk form such as piezoelectricity, chemical sensing, and photo detection, but also possess unique properties associated with their highly anisotropic geometry and size confinement [1]. Furthermore, a direct wide band gap ~3.37 eV and relatively large excitonic binding energy ~60 meV of ZnO along with many radiative deep level defects, make ZnO attractive for its emission tendency in blue/ultraviolet and full colour lighting [8] [9]. To utilize theses properties of ZnO in LEDs application, another p-type material is necessary as ZnO NRs is unintentionally n-type material.
Since mostly polymers are p-type and their special properties, like low cost, low power consumption, flexible and easy manufacturing, all make polymers a better choice to use with ZnO NRs to fabricate a flexible device that utilizes the properties of both materials for large area lighting and display application [10] [11].
On the other hand, natural abundance of copper (II) oxide (CuO) as well as its low production cost, good electrochemical and catalytic properties makes the copper oxide to be one of the best materials for various applications. CuO also World Journal of Nuclear Science and Technology has a variety of nanostructures and can be grown using the low temperature aqueous chemical method. It is one of the most important catalysts and is widely used in environmental catalyst.

Growth of CuO Thin Films
Copper oxide (CuO) thin films were prepared by dissolving 0.2 molar copper acetate and monoethanolamine in a 1:1 Molar ratio in 20 ml of 2-methoxyethanol solvent. Acetic acid was added drop wise to achieve a homogeneous solution.
The above stock solution was vigorously stirred at 80˚C for 120 min. The Cu aqueous solution was filtered through a 0.2 μm poly-tetrafluoroethylene membrane and was aged for 24 h. The colour of the solvent became dark green. The precursor solution was uniformly deposited on cleaned ITO glass substrates by spin coating technique at a spin speed of 2000 rpm for 60 s. The coating process was repeated to attain the desired thickness. The films were annealed at 90˚C for 5 min after each layer deposition.

Growth of ZnO Thin Films
The precursor solution for fabricating zinc oxide thin films were prepared by

Characterization Studies
Scanning Electron Microscopy (SEM) The morphology and particle sizes of the as-prepared sample were determined by SEM ((SEM, Tuscan Vega LMU).. The SEM images of ZnO + CuO sample films were annealed at 60˚C temperatures are shown in Figure 1. These indicate that sphere-like ZnO + CuO sample films were annealed at 60˚C temperatures nanostructures obtained by this method are uniform in both morphology and particle size, but have agglomeration to some extent. The average size was calculated to be 1.5 μm from the measurements on the SEM micrographs. Corresponding histograms, showing the particle size distribution, are also presented in

Discussion
In this work the ZnO/CuO junction V-I characteristics was studied in two cases firstly exposed to light directly secondly when it was no light (in darkness). When no light is exposed (in darkness), it was observed that upon increasing the temperature from 60 to 130 in steps of 10˚C, the current increases with temperature when the voltage is fixed. This may be attributed to the fact that the increase of temperature gives more electrons to gain thermal energy to move from the valance band to conduction band thus increases the current. It is also interesting to note that the current is nearly vanishes at a negative voltage equal to about −1.8 volt. This reflects the existence of reverse bias voltage and energy gap   For reverse bias the voltage is negative, thus the photon generates current dominance, thus this current is assumed to be generated by invisible infra red photons in darkness. These infra red photons generated by human surrounding bodies and the building that exists near the ZnO/CuO diodes. These photon generate currents are less than that generated in light as we will see later The V-I characteristics in Figure 4 of ZnO/CuO unction in light shows again increase in current when temperature increases. This result again confirm the fact that, temperature increase, increases thermal energy, which in turn increases the number of electrons that absorb this energy and transfer to the conduction band. This causes electric current to increase. It is also very interesting to note that the energy gap g E , which correspond to zero current, increases with temperature, which agrees with theoretical relations, when light is about 0.7 mA. This is relates to the fact that reverse current ~p I I . Thus in light current generated by visible photons is considerately large than that generated in dark by only free infra red photons.

Conclusion
The ZnO/CuO diode energy gap and V-I characteristics are sensitive to temperature as well as light. This sensitivity can be theoretically explained. Also it was found that for different temperature (60 to 130), the average Particle diameter varied from 1.5 micrometer to 92 nm which indicates that the particle size decreases with raising annealing temperature.