Multilayered floating gate nonvolatile memory device with GeSi nanocrystals nodes in nanocrystallized high k HfO2 for high efficiency data storage (MultiGeSiNCmem)
Project Director: Dr. Magdalena Lidia Ciurea
The project goal is to fabricate a multilayered floating gate (FG) nonvolatile memory device (ML NVM) with charge storage nodes of GeSi nanocrystals (NCs) embedded in nanocrystallized high k HfO2 matrix (capacitor of top contact/ gate HfO2/ n layers of GeSi NCs in HfO2 as FG/ tunnel HfO2/ Si wafer/ bottom contact, n=1 to 5 for the 5 versions NVM1-NVM5). We target in project to obtain high performance ML NVMs (memory window >4 V, charge loss ratio <12% at 10000 s). The proposed solution (materials; NVM design) is completely new and benefits from advantage of using thermodynamically stable Ge-rich GeSi NCs with strong quantum confinement enabling efficient charge storage, being well separated by HfO2 NCs ensuring low charge leakage. We propose 5 objectives: O1) fabrication of capacitor test samples in NVM1-NVM5 versions by magnetron sputtering deposition and subsequent rapid thermal annealing, O2) their complex characterisation (morphology, crystalline structure, composition, memory properties), properties correlation, O3) simulation of test samples using O2 results, O4) ML NVM devices fully characterised, O5) evaluation of ML NVMs performances. The project has scientific impact (holistic understanding of electronic processes and operation of ML NVM); technological impact (processes and parameters); economic-social impact (industrial automation, portable devices, computer systems; young people formation). Project results: 6 ISI papers, 5 conference papers, 1 patent application.
The project goal is to fabricate a multilayered floating gate nonvolatile memory device with charge storage nodes of GexSi1–x nanocrystals embedded in nanocrystallized high k HfO2, that is a completely new solution.
RTAnnealed test samples and subsequently configured; technological parameters; photolithographic masks; morphology and structure characteristics, memory characteristics and parameters; completely characterized test samples; multilayered nonvolatile memory device fully characterized; project webpage, updated; 6 papers in ISI journals; 5 international conference communications; 1 patent application; progress reports and final report
Stage I / 2021 Fabrication of test samples - nonvolatile memories (NVMs) in different versions
Stage II / 2022 Complex characterization of test samples
Stage III / 2023 Fabrication of multilayered NVM device
CS I Dr. Magdalena Lidia Ciurea (https://www.brainmap.ro/lidia-magdalena-ciurea) - Project Coordinator
CS I Dr. Toma Stoica (https://www.brainmap.ro/toma-stoica) - Experienced Researcher
CS II Dr. Ana-Maria Lepadatu (https://www.brainmap.ro/ana-maria-lepadatu) - Experienced Researcher
CS III Dr. Adrian Slav (https://www.brainmap.ro/adrian-slav) - Experienced Researcher
CS II Dr. Ionel Stavarache (https://www.brainmap.ro/ionel-stavarache) - Experienced Researcher
CS Dr. Catalin Palade (https://www.brainmap.ro/catalin-palade) - Postdoc
CS I Dr. Valentin Serban Teodorescu (https://www.brainmap.ro/valentin-serban-teodorescu) - Experienced Researcher
CS II Dr. Valentin Adrian Maraloiu (https://www.brainmap.ro/valentin-serban-teodorescu) - Experienced Researcher
CS III Dr. Constantin Logofatu (https://www.brainmap.ro/constantin-logofatu) - Experienced Researcher
ACS Ioana Maria Dascalescu Avram (https://www.brainmap.ro/ioana-maria-dascalescu) - PhD Student (PhD Adviser Dr. M.L. Ciurea)
ACS Ovidiu Cojocaru (https://www.brainmap.ro/ovidiu-cojocaru) - PhD Student (PhD Adviser Dr. M.L. Ciurea)
ACS Marian Cosmin Istrate (https://www.brainmap.ro/marian-cosmin-istrate) - PhD Student (PhD Adviser Dr. Valentin Serban Teodorescu)
Summary of Stage 1: Fabrication of multilayer nonvolatile test samples – versions
The goal of the MultiGeSiNCmem project is to obtain multilayer non-volatile memories (ML-NVM) with multiple floating gates (FGs) consisting in GeSi nanocrystals (NCs) embedded in high-k nanocrystalline HfO2 dielectric. The memory structure is of metal-oxid-semiconductor (MOS) type with the following layer sequence: top metallic contact/ multi-floating gate obtained by n-time repetition of FG-tunnel double layer, i.e. n×(GeSi NCs in HfO2/ HfO2 tunnel)/ p-Si substrate/ bottom metallic contact. The multiple floating gates solution is adopted for improving the memory performances (increase of the memory window and retention time, possible multi-stage programming). By using GeSi alloy instead of pure Ge, the thermal stability is increased, due to reduced Ge diffusion in presence of Si.
In this Stage 1 of the project, memory structures with FGs of Ge1-xSix NCs in HfO2 were fabricated and tested. Two values of the Ge1-xSix alloy concentration, x = 5% and 10% and two NVM versions with one and two FGs (NVM1 and NVM2) have been investigate. The reduction of Ge diffusion by increasing the Si concentration to 10% was tested by HRTEM-EDX measurements on NVM1 type structure of active layers with the layer sequence of: „HfO2 – gate oxide/ Ge0.90Si0.10 -HfO2 floating gate (FG)/ HfO2 – tunnel oxide 1/ Si – substrate”. Such NVM1 memory structure was compared with the version of two FGs, i.e. NVM2 with the following layer sequence: „HfO2 – gate oxide/ Ge0.90Si0.10 -HfO2 FG2/ HfO2 – tunel 2/ Ge0.90Si0.10 -HfO2 FG1/ HfO2 – tunel 1/ Si – suport”. The layers are deposited by magnetron sputtering from separate targets of Ge1-xSix (DC plasma) and HfO2 (RF plasma), by independently controlling the plasma intensities. Nanocrystallisation was obtained by ex-situ rapid thermal annealing RTA in an inert atmosphere.
Advanced HRTEM and GI-XRD measurements revealed the nanocrystallization of the structure after RTA at 600oC for 8 min. The formation of monoclinic (M) and orthorhombic (O) HfO2 NCs was detected for all investigated NVM1 and NVM2 structures, but also very often the M and O phases are distorted due to internal stress induced by nanocrystallization and upper layers. In some cases, the coherence of the nanocrystallization is largely extended in the film plane of NVM2 structures up to 50 nm. The evolution of the orthorhombic phase is especially annualized because of possible ferroelectric contribution of this phase to the memory effect, in addition to the charge injection in GeSi NCs of floating gates. High orthorhombic/monoclinic ratio of about 60% was found in NVM1 and NVM2 using Ge0.90Si0.10 -HfO2 FG. This ration is strongly decreased to 30% by increasing the RTA temperature to 700oC, by transforming the distorted orthorhombic phase to monoclinic. The HfO2 nanocrystallization was not detected by µ-Raman measurements due to low light scattering efficiency in thin films HfO2, but nanocrystallization of GeSi was clearly revealed in all investigated samples by using highly absorbed in GeSi laser light of 325 nm wavelength, and with 633 nm in NVM2. µ-Raman measurements have also shown strong crystallization of GeSi nanoparticles after RTA at 700oC.
Hysteresis measurements have been performed by measuring capacitance – voltage (C–V) cycles on NVM1 and NVM2 memory capacitors with top and bottom Al electrodes. The comparative testing of NVM1 and NVM2 memory performances were performed by measuring the C–V cycles, varying the bias voltage within a protocol of successive repetition of voltage cycles with 3 branches: 0V-Up-Un-Up, where 0 V, Up and Un are the ends of the cycle branches. In a series of repeated cycles, the “writing” voltage Up was varied from 1.0 V to 5.0 V with steps of 0.5 V, while the “erasing” voltage Un was caped to -2.0 V. Thus, it was demonstrated the superior performances of the double FGs memory NVM2 in respect to NVM1 for which at Up = 5.0 V, the dynamic memory windows were found 3.32 V for MVM2 and 0.99 V for NVM1.
The web page of the project was created and updated: https://infim.ro/en/project/multilayered-floating-gate-nonvolatile-memory-device-with-gesi-nanocrystals-nodes-in-nanocrystallized-high-k-hfo2-for-high-efficiency-data-storage-multigesincmem/.
In this Stage 1/2021, the project dissemination consists of: 2 ISI papers were published and 2 conference presentation (1 invited, 1 oral).
- PhD Student Ovidiu Cojocaru - Assistant Researcher in SiGeSn Group in NIMP (https://infim.ro/en/@ovidiu-cojocaru/): Thesis related to photoelectrical properties of SiGeSn NCs with different compositions in correlation with their energy structure determined by DFT, PhD Adviser Prof. M.L. Ciurea. Results published in Scientific Reports 11, 13582 (2021), „Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals”, authored by O. Cojocaru, A.M. Lepadatu, G.A. Nemnes, T. Stoica and M.L. Ciurea.
Papers in ISI-quoted journals
- „SiGeSn quantum dots in HfO2 for floating gate memory capacitors”, C. Palade, A. Slav, O. Cojocaru, V.S. Teodorescu, T. Stoica, M.L. Ciurea, A.M. Lepadatu, Coatings 12, 348 (2022)
- „Bandgap atomistic calculations on hydrogen-passivated GeSi nanocrystals”, O. Cojocaru, A.M. Lepadatu, G.A. Nemnes, T. Stoica, M.L. Ciurea, Scientific Reports 11, 13582 (2021)
- „Nanocrystallized Ge-rich SiGe-HfO2 highly photosensitive in short-wave infrared”, C. Palade, A.M. Lepadatu, A. Slav, V.S. Teodorescu, T. Stoica, M.L. Ciurea, D. Ursutiu, C. Samoila, Materials 14, 7040 (2021)
- Invited presentation: „From GeSi to SiGeSn alloy nanocrystals with benefits in SWIR detection”, M.L. Ciurea, T. Stoica, I. Stavarache, A.-M. Lepadatu, C. Palade, A. Slav, I. Dascalescu, O. Cojocaru, 13th International Conference on Physics of Advanced Materials (ICPAM-13), September 24-30, 2021, hybrid conference, Sant Feliu de Guixols, Costa Brava, Spain
- Oral presentation: „SiGeSn quantum dots for non-volatile memories”, A.M. Lepadatu, C. Palade, I. Dascalescu, A. Slav, I. Stavarache, A.V. Maraloiu, V.S. Teodorescu, T. Stoica, ML Ciurea, EMRS 2021 Fall Meeting, September 20-23, 2021, virtual conference
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