Effect of Plasma Treatment on Metal Oxide p–n Thin Film Diodes Fabricated at Room Temperature

Funder: Cambridge Trust; Id: http://dx.doi.org/10.13039/501100003343Abstract: There is a need for a good quality thin film diode using a metal oxide p–n heterojunction as it is an essential component for the realization of flexible large‐area electronics. However, metal oxide‐based diodes normally show poor rectification characteristics whose origin is still poorly understood; this is holding back their use in various applications. A systematic study of the origins of the poor performance is performed based on bias‐stress measurements using a cuprous oxide (Cu2O)/amorphous zinc‐tin oxide (a‐ZTO) heterojunction as an example. This suggests that multiple carrier trapping and thermal release of carriers in defect states stemming from oxygen vacancies at the heterojunction interface is the primary cause of poor rectification. It is demonstrated that a plasma treatment is an 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

Bipolar metal oxide thin film diodes

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.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 governmen

Doping density extraction of plasma treated metal oxide thin film diodes by capacitance–voltage analysis

Abstract High quality thin film p‐n junction diodes with high rectification ratios and low ideality factors have been fabricated from metal oxides, such as amorphous oxide semiconductors (AOSs), and characterized. Plasma treatment of interfaces has been demonstrated to improve devices made from AOSs, using current–voltage (I–V) measurements. However, capacitance–voltage (C–V) measurements of the devices have been scarcely reported in the literature. Therefore, the focus of this work is characterization of cuprous oxide (Cu2O)/amorphous zinc‐tin oxide (a‐ZTO) thin film heterojunction diodes using C–V analysis. Performance differences of plasma‐treated and untreated diodes that are difficult to observe in I–V analysis are more prominent in C–V analysis. Moreover, C–V analysis allows extraction of charge density profiles, which is a measure of the defect state density that led to intrinsic doping. The variation of doping densities of the untreated diode across the full range of applied reverse bias is shown to be up to 2 orders of magnitude, while those of the treated diodes are within a factor of 10 only. Junction charge profiles, interfacial charge depletion, and accumulation that are key features of rectifying diodes are shown to be clearly distinct between untreated, nitrogen‐treated, and oxygen‐treated diodes, thus explaining why oxygen‐treated diodes are superior

Effect of Plasma Treatment on Metal Oxide p–n Thin Film Diodes Fabricated at Room Temperature

Funder: Cambridge Trust; Id: http://dx.doi.org/10.13039/501100003343Abstract: There is a need for a good quality thin film diode using a metal oxide p–n heterojunction as it is an essential component for the realization of flexible large‐area electronics. However, metal oxide‐based diodes normally show poor rectification characteristics whose origin is still poorly understood; this is holding back their use in various applications. A systematic study of the origins of the poor performance is performed based on bias‐stress measurements using a cuprous oxide (Cu2O)/amorphous zinc‐tin oxide (a‐ZTO) heterojunction as an example. This suggests that multiple carrier trapping and thermal release of carriers in defect states stemming from oxygen vacancies at the heterojunction interface is the primary cause of poor rectification. It is demonstrated that a plasma treatment is an 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

Contribution of common and rare variants to Asian neovascular age-related macular degeneration subtypes

Abstract Neovascular age-related macular degeneration (nAMD), along with its clinical subtype known as polypoidal choroidal vasculopathy (PCV), are among the leading causes of vision loss in elderly Asians. In a genome-wide association study (GWAS) comprising 3,128 nAMD (1,555 PCV and 1,573 typical nAMD), and 5,493 controls of East Asian ancestry, we identify twelve loci, of which four are novel ( $$P \, < \, 1.19\times {10}^{-8}$$ P < 1.19 × 10 − 8 ). Substantial genetic sharing between PCV and typical nAMD is noted (r g = 0.666), whereas collagen extracellular matrix and fibrosis-related pathways are more pronounced for PCV. Whole-exome sequencing in 259 PCV patients revealed functional rare variants burden in collagen type I alpha 1 chain gene (COL1A1; $$P=1.05\times {10}^{-6}$$ P = 1.05 × 10 − 6 ) and potential enrichment of functional rare mutations at AMD-associated loci. At the GATA binding protein 5 (GATA5) locus, the most significant GWAS novel loci, the expressions of genes including laminin subunit alpha 5 (Lama5), mitochondrial ribosome associated GTPase 2 (Mtg2), and collagen type IX alpha 3 chain (Col9A3), are significantly induced during retinal angiogenesis and subretinal fibrosis in murine models. Furthermore, retinoic acid increased the expression of LAMA5 and MTG2 in vitro. Taken together, our data provide insights into the genetic basis of AMD pathogenesis in the Asian population

Recent progress in metal-organic complexes for optoelectronic applications

10.1039/c3cs60449gChemical Society Reviews43103259-3302CSRV