Memory capacitors based on ZrO2 with Ge nanocrystals or traps as charge storage nodes
Project Director: Dr. Catalin Palade
Contract no.: PD11 /2020
Project type: Postdoctoral
Project Director: Dr. Catalin Palade
Funded by: Romanian National Authority for Scientific Research, UEFISCDI
Contractor: National Institute of Materials Physics
Project Status: In progress
Start Date: September 1st 2020
End Date: August 31st 2022
Project Abstract: The main goal of this project is to obtain 2 kinds of MOS-like capacitors for non-volatile memory applications. We will use 2 approaches, the first one is a trilayer (gate oxide ZrO2 / Ge NCs-ZrO2 floating gate / tunnel ZrO2 / Si substrate) in which the charge storage centres/nodes are Ge nanocrystals (NCs) embedded in ZrO2 – floating gate, and the other approach uses a ZrO2 layer only with high concentration of traps acting as charge storage nodes. The 2 kinds of capacitors will be deposited by magnetron sputtering (MS) followed by rapid thermal annealing (RTA) for Ge NCs formation (in the case of trilayers) or RTA under specific conditions for ferroelectric ZrO2 phase formation (ZrO2 only based capacitors). For project achieving we proposed the objectives: Ob1. Obtaining high performance trilayer memory capacitors with Ge NCs embedded in ZrO2 matrix acting as a floating gate; Ob2. Obtaining memory capacitors with ZrO2 only, having high traps concentration for charge storage; Ob3. Comparative analysis of memory characteristics and parameters of the 2 capacitors obtained by the 2 approaches for evidencing the highest performance capacitor; Ob4. Dissemination and exploitation of results and management activities. Ferroelectric ZrO2 is a hot topic together with proposed devices. Technological procedures and approaches prove the high level of novelty: *technological parameters used for obtaining high quality tunnel ZrO2, those for obtaining high density Ge NCs in ZrO2 floating gate in trilayers; *obtaining controlled high density of oxygen vacancies by controlling Zr concentration in the non-reactive MS deposition or by controlling O2/Ar ratio in reactive MS (ZrO2 only capacitors). The project results will be published in 3 ISI papers and disseminated to 2 international conferences. Both kinds of capacitors have direct application in micro- and nanoelectronics and the capacitor of ZrO2 only (contact/ZrO2/Si/ contact) can be used in artificial neuronal networks.
Project Objectives:The main goal of this project is to obtain 2 kinds of MOS-like capacitors for non-volatile memory applications. We will use 2 approaches, the first one is a trilayer (gate oxide ZrO2 / Ge NCs-ZrO2 floating gate / tunnel ZrO2 / Si substrate) in which the charge storage centres/nodes are Ge nanocrystals (NCs) embedded in ZrO2 – floating gate, and the other approach uses a ZrO2 layer only with high concentration of traps acting as charge storage nodes.
Project Director: Dr. Catalin Palade (U-1700-032W-4141)
Mentor: Dr. Valentin Serban Teodorescu (U-1700-027U-1242)
STAGE I SUMMARY
In this stage, Ge and ZrO2 layers were deposited by magnetron sputtering (MS) for finding the preparation conditions (deposition rate, post-deposition annealing temperature) with the aim to obtain the optimum morphology of films (density and roughness) for fabrication of (1) gate ZrO2 / NC Ge-ZrO2/ZrO2 tunnel/Si and (2) ZrO2/Si capacitors. Ge layers were MS deposited using powers of 5 – 25 W DC, while ZrO2 layers were deposited at powers ranging 25 – 35 W RF. The measurements of thicknesses, deposition rates and morphology of layers were performed by X-ray reflectometry (XRR). In the case of Ge it was observed that the films maximum density and minimum roughness is obtained in the 7 – 15 W power range. Test capacitor structures with (A) ZrO2/Ge-ZrO2/ZrO2/p-Si and (B) ZrO2/p-Si configurations were also deposited by MS. After deposition, the structures were annealed by rapid thermal annealing (RTA), at temperatures of 600 and 650 oC for 8 min in N2 atmosphere at atmospheric pressure. The X-ray diffraction analysis on structures (A) reveals that the film is highly crystallized, Ge does not present a separate phase, entering in the structure of ZrO2 forming a tetragonal Zr3GeO8 compound. Following the RTA annealing, structure (B) crystallizes in tetragonal phase.
As shown above, the expected results in the Work Plan of Stage I regarding test capacitors, technological parameters, diffractograms, web page setup and updated at: https://infim.ro/project/capacitori-de-memorie-pe-baza-de-zro2-cu-nanocristale-de-ge-sau-trape-ca-centri-de-stocare-de-sarcina/ and Stage I Report were fully achieved.
In conclusion, the proposed objectives and activities for Stage I/2020 were completely accomplished.
STAGE II SUMMARY
In this stage, I prepared trilayer MOS capacitors with intermediate layer formed of:
- Ge nanocrystals - (1) gate ZrO2 / NC Ge-ZrO2/ZrO2 tunnel/Si
- high density traps in oxide - (2) ZrO2/Si
The depositions were made by magnetron sputtering, where I obtained MOS-like capacitors with different thicknesses of the component layers. The as-deposited structures were rapid thermally annealed in the N2 atmosphere, at different temperatures and different annealing times. The purpose of this step is to obtain Ge nanocrystals embedded in ZrO2 or to form a high-density traps layer in the intermediate layer.
In the case of the structures with an intermediate layer formed of Ge nanocrystals embedded in ZrO2, the best structures were those annealed at T = 550o C, t = 8 min.
The XRD analysis shows that the Ge atoms enter in ZrO2 lattice, forming a Zr3GeO8 compound, crystallized in a tetragonal phase, localized at the intermediate layer borders with tunnel oxide and gate oxide.
The TEM and HRTEM images show the existence of crystallization coherence from the tunneling ZrO2 layer to the intermediate layer of Ge-ZrO2, and partially in the gate ZrO2 layer. The HRTEM images present crystalline structures formed of an orthogonal and orthorhombic mix phase, and also regions of monoclinic deformation. The SAED analysis shows that ZrO2 is crystallized in the tetragonal phase.
The XPS spectra of Ge2p3/2 obtained at the free surface, clearly show the presence of oxidized Ge (GeO2) in the gate oxide. After 5 nm surface sputtering with Ar+ ions, the XPS spectra show the presence of oxidized Ge and also small quantities of metallic Ge with a concentration of 3%.
The annealed structures were metalized with different metals and the best result was obtained on MOS capacitors with Al electrodes.
The C-V characteristics present hysteresis effect with a memory window of ΔV = 1.1V which is frequency independent, meaning that the memory effect is due to only charge storage on the Ge nanocrystals in the intermediate layer.
The C-f dependence was measured at different voltages applied on the gate oxide between the accumulation regime and the inversion regime. The measurements were made between 10kHz and 1MHz. The results show that the capacitance variations in the accumulation regime are due to parasitic capacitance of the native SiO2 layer and the series resistance.
In the case of the structures with an intermediate layer formed of high-density traps in ZrO2, the best structures were those annealed at T = 550o C, t = 8 min.
The XRD analysis shows that after the annealing process, the ZrO2 crystallizes in the tetragonal phase, which is a high-pressure phase. The presence of metallic Zr, stresses the ZrO2 crystalline structure, shifting from monoclinic phase to tetragonal phase. The presence of metallic Zr is also evidenced by XPS measurements, where the concentration of metallic Zr in ZrO2 is 3.1%.
The C-V characteristics present a hysteresis loop with a memory window of ΔV = 0.6V. Shifting the C-V characteristics to a negative voltage, demonstrate the presence of charge traps in the oxide.
The C-V characteristics measured on the control structures present no memory effects.
In this stage, I published 1 ISI paper and 4 presentations at international conferences (2 proceedings papers in ISI indexed journals). The webpage of the project was updated at the address: https://infim.ro/project/capacitori-de-memorie-pe-baza-de-zro2-cu-nanocristale-de-ge-sau-trape-ca-centri-de-stocare-de-sarcina/
As shown above, the expected results in the Work Plan of Stage II regarding the characterized capacitors, diffractograms, electronic microscopy images, spectra, memory characteristics and parameters, 1 ISI paper, 1 conference presentation, project webpage, and Stage II Report were fully achieved.
In conclusion, the proposed objectives and activities for Stage II/2021 were completely accomplished.
Articles and proceedings:
- Palade, A.M. Lepadatu, A. Slav, O. Cojocaru, A. Iuga, V.A. Maraloiu, A. Moldovan, M. Dinescu, V.S. Teodorescu, T. Stoica, M.L. Ciurea, A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping, J. Mater. Chem. C 9, 12353–12366 (2021)
- Stavarache, C. Palade, P. Prepelita, V.S. Teodorescu, M.L. Ciurea In-situ magnetron sputtering co-deposition of Ge nanoparticles in Si3N4 films for near infrared detection IEEE CAS 2021 Proceedings; ISBN: 978-0-7381-2665-4 2021 International Semiconductor Conference, 6 – 8 Octombrie 2021 (IEEE 2021), p. 261-264 (2021).
- Palade, A. Slav, M.L. Ciurea Memory properties of GeZrO2 based trilayer structure IEEE CAS 2021 Proceedings; ISBN: 978-0-7381-2665-4 2021 International Semiconductor Conference, 6 – 8 Octombrie 2021 (IEEE 2021), p. 253-256 (2021).
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