Design and Characterization of ZnSe/SeO Heterojunction Devices رسالة ماجستير

Abstract

In this thesis we have designed and fabricated new class of heterojunction devices. n-ZnSe thin films are used as substrates to coat p-SeO . The formed pn junction is prepared by thermal evaporation technique under vacuum pressure of mbar. In accordance with x-ray diffraction studies the films shows stacking of polycrystalline ZnSe and amorphous SeO . To construct the energy band diagram of the ZnSe/SeO devices, the work function of SeO was determined by electrical measurements. The band gaps of ZnSe and SeO were also experimentally determined. In accordance with the designed energy band diagram ZnSe/SeO heterojunctions exhibit conduction and valence band offsets of and , respectively. The flat band build in potential is allowing formation of thin film transistor channels. When the ZnSe/SeO interfaces are coated onto Ag metal substrates they show microwave resonators characteristics. Particularly, the capacitance spectra which is studied in the frequency domain displayed negative capacitance effect accompanied with resonance- v antiresonance Phenominance. Modeling of the capacitance spectra using existing models have shown that SeO layer improves the geometrical capacitance of ZnSe by more than 4 times. In addition, deep analysis of the ac current conduction in the microwave frequency domain have shown the current is dominated by the correlated barrier hopping and by the quantum mechanical tunneling. The density of fermi levels at the interface region reaches . Moreover, the device is found suitable for ac signal filtering. It show band stop filter characteristics with acceptable return loss values and voltage standing wave ratios. The Ag/ZnSe/SeO devices can also be engineered by insertion of Ag nanosheets of 50 nm thickness between ZnSe and SeO . These nanosheets negatively affected the polycrystalline nature of the films but remove the negative capacitance effect of the devices. Ag nanosheets also increased the conductance in the spectral range of Ag nanosheets decrease the density of states near Fermi level by two orders of magnitude. In general with these properties ZnSe/SeO interfaces can be nominated as microwave resonators and negative capacitance sources.