Low Resistivity, Reliable and Efficient P-Type Doping of Zinc Oxide via Electronic Band Structure Engineering
DescriptionZinc Oxide (ZnO), a wide band gap semiconductor is an attractive material for various optoelectronic applications due to its high natural abundance, non-toxic and stable characteristics. In particular, it is potentially very suitable for optoelectronic devices such as light emitting diodes, laser diodes and photodetectors in the blue/UV spectral range. However, similar to most transition metal oxides, ZnO are predominantly n-type. This is primarily due to the electronic structures of oxide semiconductors. In most metal oxides, the valence band (VB) are derived from O 2p orbitals and consequently the position of their VB position is very low, making efficient p-type doping with shallow acceptors very difficult. After years of intense research in this area, researchers have identified several major difficulties in achieving p-type ZnO, e.g. low solubility of acceptor dopants, high ionization energy of acceptors, and favorable formations of compensating native donor defects. In the last two decades, many possible approaches have been proposed to overcome these difficulties. Although some successful attempts have been reported, to date, a reliable method to achieve low resistivity p-type ZnO has not been established, and hence limiting the advancement and growth of the ZnO electronic and optoelectronic industries.In this proposal, we exploit knowledge obtained from our previous work in electronic band engineering in semiconductors to modify the valence band of ZnO so as to lower the acceptor ionization energy and thus achieve reliable and efficient p-type ZnO by the incorporation of group Ib elements, acceptors which have been identified to have high solubility and low autocompensation. Specifically, we propose to modify the electronic band structures of ZnO by alloying with small mole fractions of ZnTe (forming ZnOTe highly mismatched alloys), CdO (forming wurtzite phase ZnCdO alloys), NiO (forming ZnNiO alloys). Significant valence band modifications have been demonstrated previously for these alloys. Synthesis and optimization of these alloys will be followed by doping with different concentration of Cu and Ag acceptors. Electrical, electronic and optical properties of the doped alloys will be systematically characterized and the results will be correlated to the electronic properties of the respective alloys. Finally, after optimizing the p-type ZnO, ZnO p-n junction structures will be fabricated and tested.The success of this work will transform the fields of oxide electronics, high power electronics, transparent electronics, displays and photovoltaics. It is also of fundamental importance in the understanding of defects in oxide semiconductors.
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