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 2/2022: Complex characterization of test samples
The goal of the MultiGeSiNCmem project is to obtain multilayer non-volatile memories (ML-NVM) with multiple floating gates (FGs) consisting in SiGe nanocrystals (NCs) embedded in high-k nanocrystalline HfO2 dielectric. The NVM structure is MOS capacitor type, i.e. metallic contact / HfO2 -gate oxide / (SiGe NCs or SiGe NCs in HfO2 as FG / HfO2 -tunnel)n / p-Si wafer -substrate / metallic contact with n =1,2...5 counting the FG / HfO2 -tunnel pairs and V1, V2, ...V5 proposed versions respectively.
The main objective of Stage 2 is to further investigate the effect of FG layers composition and number as well as the effect of FG layers morphology on the memory properties of ML capacitor test structures. This objective was achieved through the activities proposed in the Work Plan corresponding to Stage 2: ● Magnetron sputtering deposition of test samples in different versions - part II (Act. 2.1); ● RTA processing for formation of GexSi1–x NCs as charge storage nodes and nanocrystallized HfO2 matrix - part II (Act. 2.2); ● Deposition of metallic contacts for test NVMs (post-metallization annealing if necessary) - part II (Act. 2.3); ● Testing and characterization of test samples: crystalline structure, morphology, composition – feedback to preparation - part I (Act. 2.4); ● Investigation of electrical and memory properties – feedback to preparation - part I (Act. 2.5); ● Simulation of GexSi1–x NCs (DFT) and test ML NVMs - part I (Act. 2.6); ● Correlation of structure, morphology, electrical and memory properties with simulation results - part I (Act. 2.7); ● Analysis of test samples with optimal memory characteristics - part I (Act. 2.8); ● Updating of project web page (Act. 2.9); ● Dissemination: ISI papers and conferences (Act. 2.10).
Test structures were fabricated by using magnetron sputtering for deposition of n = 2 and 3 pairs of (SiGe / HfO2)n or (SiGe-HfO2 / HfO2)n layers on p-Si wafers with 7 – 14 Ωcm resistivity, and the SiGe (FG) composition was varied, as well as the SiGe : HfO2 composition for the case of co-deposited SiGe-HfO2 FG layer. The second fabrication step was the test samples nanostructuring by RTA at different temperatures (550 – 650 oC). The fabricated test structures are: „HfO2 -cap / (SiGe -FG / HfO2 -tunnel )n / Si -substrate” and „HfO2 -cap / (SiGe-HfO2 -FG / HfO2 -tunnel)n / Si -substrate” with n = 2 and 3, and HfO2 -cap being gate HfO2.
Advanced HRTEM studies were performed showing that the tunnel HfO2 layers are crystallized with orthorhombic phase for RTA annealing at temperatures over 600 oC in both structures with 2× FG / HfO2 -tunnel pairs and ones with 3× FG / HfO2 -tunnel pairs. The FG layers are in general partially crystallized, especially the top ones near the free sample surface.
Also, atomistic simulations were performed by DFT for investigating Si1–xGex NCs energy structure. The simulations were performed on spherical H-passivated SiGe NCs with diamond crystalline structure, with Ge atomic concentrations between 50% and 95%, and diameters in the 1.5 – 3.9 nm range. The asymptotic value for the band gap at large diameters E0NC deviates from the bulk value Egbulk up to 0.1 eV and does not reflect the influence of change in conduction band minimum from X to L as it happens in bulk Si1-xGex, x = 90%. This is explained by the quantum confinement effect in SiGe NCs.
The memory properties of test ML capacitors were evidenced by measurements of C – V loops and charge retention C – t characteristics. Most of the samples show C – V hysteresis loops, some of them having unusually-high memory windows (ΔV = 5 – 6 V) and consequently P – V polarization characteristics were measured, also showing hysteresis due to the ferroelectric orthorhombic crystalline structure of tunnel HfO2 layers. This explains the high ΔV memory window of the C – V loops and demonstrates that it is due to both charge storage in SiGe nodes and ferroelectric polarization of tunnel HfO2 layers. The retention C – t characteristics show that the best capacitors are the ones with 3 floating gates separated by 3 tunnel HfO2 layers, i.e. those structures with 3× SiGe / HfO2 pairs.
In Stage 2, PhD Student Ovidiu Cojocaru (PhD Supervisor - Prof. M.L. Ciurea), Assistant Researcher (https://infim.ro/en/@ovidiu-cojocaru/) has ended the research activity within the PhD scientific research programme (established jointly with the PhD supervisor), and his PhD thesis is under finalization stage. Additionally, he is co-author of 2 papers.
The project web page 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/ was updated.
Results were disseminated in 4 scientific papers (compared to 2 planned papers), i.e. 1 published in ISI-quoted journal, 1- in finalization stage and 1-revised version under review for publication in ISI-quoted journals and 1- with DOI published in journal indexed in Google Scholar, and presented (1- invited, 2- oral) at 3 international conferences (2 planned presentations).
In conclusion, the proposed objectives and activities and planned results for Stage 2/2022 were fully achieved.
Summary of Stage 1/2021: 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).
- 2022: PhD Student Ovidiu Cojocaru (PhD Supervisor - Prof. M.L. Ciurea), Assistant Researcher (https://infim.ro/en/@ovidiu-cojocaru/) has ended the research activity within the PhD scientific research programme (established jointly with the PhD supervisor). His PhD thesis is under finalization stage. Additionally, PhD Student O. Cojocaru is co-author of 2 papers (Coatings 12, 348 (2022) si Annals of the Academy of Romanian Scientists Series on Physics and Chemistry Sciences 7, 53 (2022)).
- 2021: 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.
Articles 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)
- „Modulating SiGe-SiO2 VIS-SWIR photoresponse by rapid-like modified furnace annealing versus rapid thermal annealing”, M.T. Sultan, I. Stavarache, A. Manolescu, U.B. Arnalds, V.S. Teodorescu, H.G. Svavarsson, S.T. Ingvarsson, M.L. Ciurea, revised version under review at Surfaces and Interfaces
- „SiGe alloy nanocrystals in oxides obtained by magnetron sputtering for VIS-SWIR light sensing and detection: A review”, M. Lepadatu, I. Stavarache, C. Palade, A. Slav, I. Dascalescu, O. Cojocaru, V.A. Maraloiu, V.S. Teodorescu, T. Stoica, M.L. Ciurea, under completion to be submitted to Materials
Paper with DOI in journal indexed in Google Scholar
- „From Si nanowires to Ge nanocrystals for VIS-NIR-SWIR sensors and non-volatile memories: A review”, M. Lepadatu, I. Stavarache, C. Palade, A. Slav, V.A. Maraloiu, I. Dascalescu, O. Cojocaru, V.S. Teodorescu, T. Stoica, M.L. Ciurea, Annals of the Academy of Romanian Scientists Series on Physics and Chemistry Sciences 7, 53-87 (2022),
- Invited presentation: „Continuous change from monoclinic to ferroelectric orthorhombic HfO2 by a martensitic-like transition – challenge for nonvolatile memories”, M.L. Ciurea, C. Palade, A.M. Lepadatu, A. Slav, O. Cojocaru, A. Iuga, V.A. Maraloiu, V.S. Teodorescu, T. Stoica, International Colloquium "Physics of Materials" – PM 7, November 10-11, 2022, Bucharest
- Oral presentation: „Strained Ge-rich SiGe nanocrystals in nanocrystallized HfO2 layers for extended VIS-SWIR photoelectric sensitivity”, A.M. Lepadatu, C. Palade, A. Slav, V.S. Teodorescu, T. Stoica, M.L. Ciurea, 2022 Spring Meeting of the European Materials Research Society (E-MRS), virtual Conference, May 30 – June 3, 2022
- Oral presentation: „Rapid thermal annealing temperature effects on charge storage behavior of SiGeSn quantum dots embedded in the high-k CMOS-compatible HfO2 in floating gate non-volatile memories”, C. Palade, A. Slav, O. Cojocaru, V.S. Teodorescu, T. Stoica, M.L. Ciurea, A.M. Lepadatu, 20th International Balkan Workshop on Applied Physics and Materials Science (IBWAP), July 12 – 15, 2022, Constanta
- 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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