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| Home > Publications database > High-k rare-earth oxides on GaN and ferroelectric properties of hafnia based thin films |
| Home > Publications database > High-k rare-earth oxides on GaN and ferroelectric properties of hafnia based thin films |
| Dissertation / PhD Thesis | FZJ-2019-02847 |
2019
Please use a persistent id in citations: http://hdl.handle.net/2128/22160
Abstract: The continuous improvement of the transistors is the basis for the increase of computing power. For enhanced metal-oxide-semiconductor (MOS) field-effect-transistors(FETs) and other devices suitable gate dielectrics are required, that exhibit a high relative permittivity, good insulating properties like a large band gap, and a goodlayer quality. Additionally, new device structures like negative capacitance FET (NCFET) are developed.In this thesis the ternary rare-earth oxides GdScO3 (GSO), LaLuO3 (LLO) and SmScO3 (SSO) are grown on GaN by pulsed laser deposition (PLD). Those oxidesreveal a hexagonal phase, that has been discovered few years ago. The oxides have a large lattice mismatch to the GaN and are not at thermodynamic equilibrium,according to ab-initio calculations. The hexagonal ternary rare-earth oxides reveal a permittivity of 30 (GSO) and of 32 (LLO) and a bandgap above 5 eV. To understandthe growth mechanism of the rare-earth oxides, the layer and especially the interface are investigated. First of all hexagonal GSO reveals a decomposition of Scand Gd at the interface, so that it exhibits twice as much Sc than Gd, which reduces the length of the in-plane axis of the GSO and subsequently the lattice mismatch.Secondly, the grains are tilted in-plane, revealing a fibre texture with the c-axis as fibre axis perpendicular to the interface. Thirdly, the distance of the lattice planesat the interface is enhanced, revealing a distance of 3.1 Å. The SSO also reveals a fibre texture and an enhanced interface distance, thought the decomposition of theSm and the Sc occurs in places. The hexagonal LLO reveals a fibre texture and a small decomposition.For a novel NCFET a ferroelectric is placed in between the gate metal and the oxide of a MOSFET. The ferroelectric should exhibit a negative capacitance and inthis way cause a voltage amplification, resulting in a subthreshold slope (SS) below 60 mV/dec. PLD grown ferroelectric HfO2 with 5% Gd and HfO2 with 5% Lu(HfLuO) reveal a remanent polarization of 12 μC/cm2, while the HfO2 with 5% Y, grown by atomic layer deposition, reveals one of 20 μC/cm2 and a steeper hysteresis.HfLuO exhibits a pinched hysteresis at the initial state, if the sample is annealed at 500 °C, which could be related to an antiferroelectric phase. At an annealingtemperature of 700 °C the HfLuO layer becomes ferroelectric.
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