Thesis defense by Hongun LIU

keywords :

chemical method, electronic, new material 

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Abstract

In recent years, metal oxide electronics has attracted more and more attention in research, mainly thanks to their potential lower cost and the possibility they offer to develop transparent electronics. There are several potential applications concerned with metal oxides including photovoltaics, thin film transistors and photo-electrochemistry. There are several n-type metal oxides with excellent electronic properties being well developed, such as Al doped zinc oxide. But the fabrication of devices fully made with metal oxides is largely impeded by the poor electronic properties of the p-type oxides so far studied. Therefore, there is the need for developing p-type metal oxide semiconducting materials with better electrical properties.

Among several families of p-type metal oxides, copper based p-type metal oxides have a great potential due to the abundance of copper, its low toxicity and the possibility to process copper oxides by chemical approaches. As an example, for photovoltaic applications, Cu₂O based solar could reach a maximum theoretical efficiency 20%. But the maximum efficiency reported so far is merely about 6 %. Pure Cu₂O is a semi-transparent p-type material with direct bandgap of about 2.1 eV. The electronic properties of Cu₂O can vary a lot, heavily depending on the synthesis techniques used. The low quality of Cu₂O thin films is one of the major reasons responsible for low device performance. Cu₂O layers with the best electrical properties have been synthesized through thermal oxidation at temperatures above 1000 °C, an approach having thermal budget too high for industrial application. Thus, optimization of the quality of Cu₂O via more viable and low cost synthesis techniques is needed. Besides the interests on optimization of pure phase Cu₂O, developing new copper based compounds as thin films with better electrical properties is also fascinating. Currently, there are several ternary Cu based p-type oxides reported, including CuAlO₂, SrCuO₂, CuCrO₂, with promising properties but still lagging behind n-type oxides in terms of performance. Recently, AgCuO₂ has drawn much attention from researchers since it presents high conductivity values. AgCuO₂ is a p-type oxide with delafossite crystal structure and has theoretically been predicted to have a small bandgap, about 1 eV. Although until now a suitable low-cost technique has not been reported yet for the deposition of AgCuO₂ thin films, while it presents great potential in photovoltaics and also for thin film transistors. Therefore, in order to confirm the optical and electrical properties predicted and for further device fabrication, finding a suitable and low-cost technique for the deposition of high quality AgCuO₂ coating would be of high interest.

In this thesis, the optimization of pure Cu₂O thin film deposition was conducted using Aerosol Assisted MOCVD (AA-MOCVD). As a result, homogenous Cu₂O films were deposited at low temperature (about 335 °C) without detectable amount of carbon contamination with high crystallinity. In addition, by incorporation of humidity during the deposition, particle size and the orientation of the Cu₂O films could be tuned, thus Cu₂O films with (111) textured large grain sizes (> 300 nm) were achieved. For optimized Cu₂O films, the mobility can reach a maximum of 15 cm²/V.s with carrier concentration in the order of 10¹⁵ cm^-3. Lastly, an excellent diode behaviour was observed by combining the optimized Cu₂O films with ZnO, obtaining an on-off ratio exceeding 10⁴.

Besides the Cu₂O optimization, the deposition of AgCuO₂ by MOCVD was also tackled. In order to do so, the deposition of silver and silver oxide thin films was previously optimized.  For that, two new silver precursors, namely, Ag(hfac)phenanthroline  and  Ag(hfac)triglyme were synthesized and fully characterized. High quality Ag coatings could be obtained with both precursors. Silver oxide films were obtained through electrochemical oxidation and oxygen plasma treatment of pre-deposited Ag coatings. 

Due to the incompatibility between the thermal AgCuO₂ stability window and the temperatures needed to deposit Ag and Cu compounds by CVD with the precursors used, the direct deposition of AgCuO₂ could not be obtained. Thus, solution based thin film coating techniques were adopted for AgCuO₂ film deposition. In particular, Successive Ionic Layer Adsorption and Reaction (SILAR) allowed the deposition of AgCuO₂ thin films. Using a proper seed layer on glass, dense and continuous AgCuO₂ films were coated, with minimum RMS value of 8 nm. The deposited AgCuO₂ films had almost pure phase. The optical and transport properties of AgCuO₂ thin films were thus carried out for the first time. Transmittance measurements confirmed the predicted low bandgap of AgCuO₂ (1.2 eV), while by using the Tauc formula, we found that this material is more likely to have a direct bandgap, in agreement with published DFT calculations. Thanks to Hall Effect measurements, the deposited AgCuO₂ films were confirmed to be p-type. The lowest resistivity achieved was 0.2 Ω.cm. In addition, those films had carrier density in the order of 10¹⁷ cm^-3 and the best mobility achieved was 24 cm²/V.s. Comparing with the previously reported CuMO₂ (M= Al, Cr, Ga etc) delafossite p-type compounds, this material has shown the lowest bandgap (appropriate for photovoltaic application) and rather high conductivity. The most interesting characteristic is that the general problem of low carrier mobility in those delafossite compounds has been solved in this AgCuO₂, thanks to its mixed-valence electronic structure and charges delocalization. Thus, those unprecedented characterization results pave the way for using AgCuO₂ films in functional devices.

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