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Title: Atomic layer deposition zinc oxide devices for transparent electronics
Author: Shaw, Andrew
ISNI:       0000 0003 9624 9347
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Zinc oxide (ZnO) films deposited using atomic layer deposition (ALD) and plasma enhanced (PE)-ALD for transparent electronics have been explored in this thesis by characterising the films electrically and physically. Thin-film transistors (TFTs) and Schottky diodes have been successfully demonstrated using ALD based thin-films as active layers. The challenge of reducing the intrinsically high conductivity is addressed through two approaches namely the use of substitutional dopants via ALD and tuning of the plasma conditions during PEALD deposition. Initial characterisation established that using Mg as a substitutional alloy, reduced the films conductivity. TFTs were fabricated using lithography, on highly doped Si wafers with thermally grown SiO2 as the gate oxide. The effect of using Mg was to reduce the off-current by a factor of 105. An optimum ratio between the Mg and Zn precursor of 12.5 % was established, defined by a maximum saturation mobility (μsat) of 4 cm2/Vs. In addition, the band gap of the Mg doped ZnO film increased from 3.3 eV to 3.44 eV, through the formation of MgO states within the film. The first instance of Nb doped ZnO for TFTs applications are reported in this thesis. TFTs were fabricated in the same manner as for Mg doped ZnO, however, a capping layer of 5 nm of Al2O3 prior to NbZnO deposition was required to mitigate gate oxide leakage and improve the interface quality. Optimal characteristics were achieved with a lower ratio of 3.8 % between the Nb and Zn precursor. A maximum μsat of 8 cm2/Vs was achieved. The higher mobility and lower precursor percentages for Nb originate from the higher oxidation state of the dopant. A lower sub-threshold swing of 220 mV/dec was achieved for Nb doped films compared to 900 mV/dec for Mg doped films, supporting the advantage of using Nb to control the conductivity of ZnO ALD thin-films for TFT applications. In addition, the sub-band gap states in ZnO film was modelled from current-voltage and capacitance-voltage measurements, where good correlation between both techniques was achieved. Optimisation of PEALD ZnO for Schottky diode applications was established with a deposition temperature of 80 oC and plasma time of 50 s. The use of oxidised metals for the Schottky contact, namely AgxO and Pt¬O¬x was required to obtain good Schottky parameters. These contact metals aim to mitigate the oxygen deficiencies at surface of ZnO. Relatively low ideality factors of < 1.4 are achieved using both Schottky contacts. Thermionic emission was established as the dominant conduction mechanism. The relatively large ideality factors are thought to originate from the polycrystalline nature of the ZnO film.
Supervisor: Hall, Steve Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral