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Bipolar metal oxide thin film diodes



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Khong, Yin Jou 


With the increasing application of active-matrix organic light-emitting diode (AMOLED) displays which are basically current-driven devices, a current-driven switch with larger current capability and higher frequency operation will prove to be beneficial. The growth of the Internet of Things (IoT) also results in increasing demand for transparent and conformable electronics, which requires materials that are highly uniform in electrical properties over large areas. All these factors have contributed to the prominent interest in metal oxides for semiconductor devices in the recent years, due to their high uniformity, high carrier mobility and low resistivity as a disordered material, when compared to the alternatives such as amorphous and polycrystalline silicon and organic semiconductors. In addition, metal oxides can be fabricated at much lower temperatures compared to high-temperature polycrystalline silicon (HTPS) and yet still achieve reasonable performance, enabling them to be easily deposited on flexible substrates like plastics and paper. Metal oxides generally with their larger band gaps are hard to be doped both n- and p-type, so heterojunctions are the feasible way to construct bipolar devices. Extensive literature review indicates that most research was focused on nickel oxide and oxides of copper for p-type materials, and variants of zinc oxide for n-type materials. This work identifies that, among these materials, cuprous oxide (Cu2O) and amorphous zinc-tin oxide (a-ZTO) are great materials with potential to form a good heterojunction, but this combination has been mostly unexplored. The simple mathematic model presented in this work shows that the charge carrier injection is possible with the proposed p-n heterojunction configuration, an important design requirement for heterojunction bipolar transistors which is the long-term goal of this work, but extreme care should be exercised in forming the junction as its quality heavily affects the injection efficiency. There is a need for a good quality thin film diode of Cu2O/a-ZTO p-n heterojunction as it is an essential component for the realization of flexible large-area electronics. However, this Cu2O/a-ZTO diode initially showed poor rectification characteristics. A systematic study of the origins of the poor performance is performed based on several experiments and measurements on the metal-semiconductor junctions and the p-n heterojunction. A good choice of metal contacts is crucial, and the contacts should be Ohmic so that diode rectification is only controlled by the p-n heterojunction. Experiments suggests that molybdenum and gold are good Ohmic contacts to a-ZTO and Cu2O, respectively. Further investigation and analysis are targeted on the properties of the p-n heterojunction. The results suggests that multiple carrier trapping and thermal release of carriers in defect states stemming from oxygen vacancies and oxygen related atomic coordination at the heterojunction interface is the primary cause of poor rectification. It is demonstrated that a plasma treatment is the simplest yet most effective way to optimize the population of oxygen vacancies at the heterojunction interface based on extensive material analyses, allowing a significant improvement in the diode performance with a much-enhanced rectification ratio from ~20 to 10 000, and a consequent facilitation of the next-generation of ubiquitous electronics. Finally, a more in-depth analysis on the p-n junction model, with a focus on capacitance-voltage (C-V) characteristics, is presented, with discussions on analytical derivations of the model and analyses of simulated junctions with various doping profiles. Some junction parameters in this thesis are obtained using the classical C-V model which was derived for more ideal situations. Hence, it is important to understand the accuracy of such a simple model when applied on real scenarios that differ significantly from the usual, basic assumptions. The analytical model of a p-n junction, even in only one dimension, is severely limited due to the complexity in mathematical representation. The results indicate that numerical simulation is the practical way to further investigate p-n junction properties. It is shown that the classical C-V analysis, which is simple, may be sufficient in extracting p-n junction parameters such as depletion depth and doping profiles, provided that the method’s limitations are understood. These findings allow readers further insight to the implementation challenges such as low rectification ratio of bipolar heterojunction thin film diodes made with Cu2O and a-ZTO, and future research direction for plausible solutions to these difficulties.





Flewitt, Andrew J


bipolar, metal oxide, diode, thin film, cuprous oxide, amorphous zinc tin oxide, high-target utilisation sputtering, HiTUS, p-n heterojunction


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Engineering and Physical Sciences Research Council (EPSRC) under grant no. EP/M013650/1 and EP/P027032/1; Cambridge Trust; Commonwealth Scholarship, funded by the UK government