Dissertation committee:
1. Αθανάσιος Δημουλάς, Ερευνητής Α’ στο ΕΚΕΦΕ «Δημόκριτος»
2. Κωνσταντίνος Σιμσερίδης, Αναπληρωτής Καθηγητής Τμήματος Φυσικής ΕΚΠΑ
3. Βλάσιος Λυκοδήμος, Αναπληρωτής Καθηγητής Τμήματος Φυσικής ΕΚΠΑ
4. Σπυρίδων Γαρδέλης, Αναπληρωτής Καθηγητής Τμήματος Φυσικής ΕΚΠΑ
5. Αντώνιος Παπαθανασίου, Επίκουρος Καθηγητής Τμήματος Φυσικής ΕΚΠΑ
6. Δημήτριος Τσουκαλάς, Καθηγητής στη ΣΕΜΦΕ ΕΜΠ
7. Ιωάννης Ράπτης, Καθηγητής στη ΣΕΜΦΕ ΕΜΠ
Summary:
The dielectric material HfO2 has been extensively used in recent years and has entered industrial production since 2007 as a gate dielectric of CMOS technology transistors. HfO2 has recently been found to be ferroelectric when it is crystallized at the non-centrosymmetric orthorhombic phase. Alloying it with Zr or doping it with Si, Ge, Al, Gd and other elements stabilizes the ferroelectric phase or turns it into antiferroelectric (tetragonal phase). This opens new opportunities for many applications, including integrated ferroelectric non-volatile memories, since HfO2 and ZrO2 are compatible with silicon technology.
In the present dissertation, the optimal fabrication conditions of Hf1-xZrxO2 (HZO) to achieve a ferroelectric material were investigated. TiN/HZO/Ge metal-ferroelectric-semiconductor (MFS) capacitor structures were prepared by atomic oxygen/nitrogen plasma-assisted molecular beam deposition in an ultrahigh vacuum chamber at the Laboratory of Molecular Epitaxy and Surface Science of NCSR-Demokritos. Physical characterization of the capacitors confirms the predominance of the orthorhombic/ferroelectric phase of HZO and the clean HZO/Ge interfaces, leading to good ferroelectric characteristics.
Subsequently, the depletion region in the surface of the Ge semiconductor, formed at lower temperatures, and its effect on the ferroelectric characteristics of HZO were studied. From measurements of polarization, displacement current and capacitor capacitance on p, n and n+ type germanium substrates the depolarization field as a function of temperature was calculated.
Two main issues in the reliability of ferroelectric devices are “wake-up”, that is the broadening/improvement of the P-V loop by applying a series of electrical cycles, and polarization imprint, which is the partial stabilization of one of the two possible ferroelectric states over time. From time and temperature depended electrical measurements of the polarization, conclusions about the mechanism of the phenomenon were drawn.
The above study led to the fabrication of ferroelectric field-effect transistors (FeFETs) with TiN/HZO gate structure on a p-type Ge channel. By replacing the gate dielectric of conventional transistors with a ferroelectric material, the expected shift of the Ids-Vg characteristic curve with the polarization switching was observed, showing a memory window of MW = 0.55 V. Due to the possibility of partial polarization switching in HZO, FeFETs in addition to the ON and OFF states, also present intermediate states. This phenomenon makes them suitable for application in analog non-volatile memories, and due to the low oxide thickness (15 nm), they operate at low voltage/power.
Finally, some first results on capacitors with 5 nm HZO on Nb:SrTiO3 (NSTO) semiconductor substrates and metallic TiN or W as top electrode are presented, which show synaptic performance. The current of the capacitors is modulated by the polarization state in the ferroelectric, making it work as an analog memory. From current measurements at various temperatures and their analysis, the conductivity is attributed to thermionic Schottky emission and the change in current to the change in resistance in the surface of the semiconductor, due to modulation of the potential barrier. The devices show synaptic plasticity and are suitable for applications in neuromorphic networks.
Keywords:
ferroelectric Hf0.5Zr0.5O2, molecular beam deposition, low power, non-volatile memory, neuromorphics, Ge-MFS capacitors, FeFET, FTJ, ferroelectric synapses
