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Development of p-type oxide semiconductors based on tin oxide and its alloys: application to thin film transistors
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In spite of the recent p-type oxide TFTs developments based on SnOx and CuxO, the results achieved so far refer to devices processed at high temperatures and are limited by a low hole mobility and a low On-Off ratio and still there is no report on p-type oxide TFTs with performance
similar to n-type, especially when comparing their field-effect mobility values, which are at least one order of magnitude higher on n-type oxide TFTs. Achieving high performance p-type oxide TFTs will definitely promote a new era for electronics in rigid and flexible substrates, away from silicon. None of the few reported p-channel oxide TFTs is suitable for practical applications, which demand significant improvements in the device engineering to meet the real-world electronic requirements, where low processing temperatures together with high mobility and high On-Off
ratio are required for TFT and CMOS applications.
The present thesis focuses on the study and optimization of p-type thin film transistors based on oxide semiconductors deposited by r.f. magnetron sputtering without intentional substrate heating.
In this work several p-type oxide semiconductors were studied and optimized based on undoped tin oxide, Cu-doped SnOx and In-doped SnO2. The influence of the deposition parameters, such as the percentage of oxygen and the deposition pressure and post deposition annealing treatments (up to 200 °C) parameters was investigated in order to optimize the properties of the p-type thin films. The detailed study of the material was accomplished through various techniques of characterization of their electrical and optical properties, crystal structure, chemical composition, topology and morphology.
The obtained undoped SnOx thin films showed p-type conduction for a narrow percentage of oxygen, between 2.5% and 4%, after an annealing treatment at 150 °C and 200 °C. The thin films have a mixture of both tetragonal β-Sn and α-SnO phases, mobilities between1.6 cm2/Vs and 2.6
cm2/Vs and a carrier concentration between 1016 and 1017 cm-3. TFTs produced with this material were optimized presenting very good electrical performances, with On-Off ratio ~104, µFE up to 3.5cm2/Vs and Vth between -0.41 V and 15 V. The influence of the dielectric was also studied and leading to new results. Depending on the gate dielectric used, n-, p-type and ambipolar devices were obtained for the same semiconductor deposition conditions.
Doping SnOx with Cu also results in transparent p-type oxide semiconductors with mobilities between 1.6 cm2/Vs and 2.6 cm2/Vs and a carrier concentration between 1016 and 1017 cm-3.
When applied as active layer, resulted in poor performance thin film transistors, with lower On-Off ratio and the higher Vth, despite µFE increased. When doping the SnO2 films with In, p-type conduction was achieved without the need of the annealing treatment. The obtained as deposited thin films are amorphous and show mobilities up to 27 cm2
/Vs and very low resistivities ~10-3 Ω cm, obtaining in this way the a p-type oxide transparent conductor with the lowest electrical resistivity so far reported in the literature.