Dr. A.M. Lepadatu - Project Director

Dr. I. Stavarache

Dr. V.A. Maraloiu

Dr. A. Slav

Dr. C. Palade

Assessment of Project Implementation - SUMMARY

Project SIGESNANOMEM aimed to obtain nanostructured materials (3-layer structures) based on SiGeSn nanocrystals (NCs) embedded in oxide matrix with charge storage properties for non-volatile memory applications. 3-layer structures are MOS-like and have configuration of gate oxide/ floating gate of SiGeSn NCs embedded in oxide / tunnel oxide / p-Si substrate, the chosen oxide (Stage I / 2019) being HfO₂; SiGeSn NCs play the role of charge storage nodes, being positioned at tunnelable distance from the p-Si substrate in order to ensure charge injection. The charge storage properties were adjusted by modification of 3-layer structure morphology and SiGeSn NC composition by both modifying & adjusting deposition parameters (composition of SiGeSn-HfO₂ intermediate layer, layers thicknesses etc.) during Stage I / 2019 and Stage II / 2020 and varying & adjusting RTA annealing parameters for nanostructuring (temperature & duration) during whole project.

The project aim was achieved by following 5 specific objectives:

O1) obtaining 3-layer capacitors with floating gate of SiGeSn NCs embedded in oxide matrix of HfO₂ by magnetron sputtering deposition and nanostructuring by thermal annealing;

O2) morphology & structure characterisation of NCs-based 3-layers for optimizing technological parameters;

O3) investigation of electrical & charge storage properties and their correlation with structure & morphology;

O4) evaluation of memory parameters of SiGeSn NCs based capacitors in function of NCs morphology & composition;

O5) dissemination of project results.

Project implementation was carried out in 3 stages:

Stage I / 2019 Preparation of test memory structures based on SiGeSn NCs;

Stage II / 2020 Preparation & complex characterization of oxide/SiGeSn-oxide/oxide/Si substrate 3-layer structures;

Stage III/2021 Modelling of MOS capacitors with SiGeSn NCs based floating gate.

The expected results were obtained:

(i) Capacitor structures with intermediate layer of SiGeSn NCs in HfO₂: the optimal deposition configuration of gate HfO₂/ SiGeSn-HfO₂ / tunnel HfO₂ / p-Si substrate was found in which the intermediate layer / floating gate is obtained by co-sputtering SiGeSn-HfO₂ – Stage I / 2019; later, the technological parameters for preparation were adjusted, i.e. SiGeSn-HfO₂ composition of intermediate layer, thicknesses of the 3 component layers – Stage II / 2020 and RTA annealing temperature – Stage II / 2020, Stage III / 2021; SiGeSn NCs were evidenced by (HR)TEM, Raman spectroscopy and XRD, and their composition and diameter was evaluated – Stage I / 2019, Stage II / 2020.

(ii) Electrical characteristics: frequency dispersion of capacitance & resistance measured in accumulation regime. Memory characteristics: C – V hysteresis loops with counter clockwise direction were obtained for different frequencies, having memory windows up to 4 V that are frequency independent, meaning that the memory effect is due to charge storage in SiGeSn NCs, NCs being obtained by RTA nanostructuring at temperatures of 520 and 530 ^oC.

(iii) Modelling of gate oxide / SiGeSn- oxide / tunnel oxide / p-Si substrate capacitor structures in Stage III / 2021, the oxide being HfO₂ which is already a standard material in semiconductor processing in microelectronics: 3-layer capacitor was modelled with a complex equivalent RC circuit; electrical material parameters – resistivity ρ & dielectric constant κ of each component layer were evaluated; memory parameters – flatband voltage & stored carriers density were determined. The memory window (as the difference of flatband voltages corresponding to the C – V hysteresis branches) has high values from 3.1 V to 4 V for structures with floating gate of SiGeSn NCs storage nodes, and the stored carriers density reaches values on the order of 10¹³ electrons/cm². So, the best results were obtained on gate HfO₂ / floating gate of SiGeSn NCs embedded in HfO₂ / tunnel HfO₂ / p-Si substrate: 4 V memory window and 2×10¹³ electrons/cm² stored carriers density. For structures with amorphous SiGeSn clusters (for RTA temperatures smaller than or equal to 500 ^oC), memory window is smaller, reaching the maxim value of 2.3 V, and the stored carriers density has values on the order of 10¹² electrons/cm², reaching the highest value of ~9×10¹² electrons/cm². The dielectric constants of each component layer were determined: κ_g = 15 – 16 for gate oxide, κ_f = 14 for floating gate, and κ_t = 19 – 20 for tunnel oxide. The layers resistivities are on the order of 10 MΩ×cm. The dielectric constants determined from simulation were used for calculating the stored carriers density.

Project web page https://infim.ro/en/project/sigesn-nanocrystals-with-charge-storage-properties-at-nanoscale-sigesnanomem/ was created & updated. 2 papers were published in Applied Surface Science and Nanotechnology, other 2 papers were submitted, being in reviewing stage at Acta Materialia and Scientific Reports, and other 2 papers are in writing stage, to be submitted to ISI-quoted journals (total of 6 papers – 5 papers being envisioned in Work Plan). 6 papers were presented at international conferences (3 planned): 1 invited, 1 oral, 4 posters.

In conclusion, the objectives & activities proposed in project “SiGeSn nanocrystals with charge storage properties at nanoscale” and results planned in Work Plan (Contract TE No. 19 /2018) were fully fulfilled.

Stage III / 2021 SUMMARY

In Stage III / 2021, the flatband voltage for each branch of C – V hysteresis and the memory window as the corresponding flatband voltage difference were determined. The flatband voltage was determined by 2 methods, namely by 1/C² plot or by evaluating the voltage position at 80% of maximum capacitance. This was made for memory structures similar with structures M1 and M2 with intermediate layer of co-sputtered SiGeSn-HfO₂, RTA annealed that were studied in Stage II / 2020. In Stage III / 2021, structures were RTA annealed at different temperatures and different times, on a large temperature range. For each kind of structure (M1/M2, RTA temperature and duration), C – V hysteresis loops were measured at different frequencies, and capacitance – frequency (C – f) and resistance − frequency (R – f) characteristics were recorded in accumulation regime. C – f and R – f curves were also simulated using an equivalent circuit model for the trilayer memory capacitor that consists in equivalent parallel RC circuit for each layer of the trilayer structure (gate oxide, floating gate, tunnel oxide, interfacial SiO_x layer) and a series resistance (includes contacts and p-Si substrate resistances). Based on this model, the intrinsic material parameters of each component layer, i.e. dielectric constants and resistivity were evaluated. Also, the stored carriers density was calculated using the simulation results, i.e. the dielectric constants of gate oxide and floating gate and by considering the capacitor geometry (thicknesses of gate oxide and floating gate layers; SiGeSn NCs diameter). The stored carriers density was calculated in function of structures morphology (case 1: floating gate with amorphous SiGeSn clusters, the memory effect is due to electronic states present in clusters, being in fact Si-Ge-Sn related states – deep energy levels; case 2 – SiGeSn NCs act as charge storage node in the NC floating gate morphology). Electrical and memory parameters were evaluated considering the morphology. Thus, the structures with floating gate formed of SiGeSn NCs in HfO₂ (gate HfO₂ / SiGeSn NCs - HfO₂ floating gate / tunnel HfO₂ / p-Si substrate) have high memory windows up to 4 V, the stored carriers density reaching the value of 2.0×10¹³ electrons/cm². Structures with amorphous SiGeSn show smaller memory windows, the highest value being 2.3 V; stored carriers density is on the order of 10¹² electrons/cm², reaching the maxim value of 8.9×10¹² electrons/cm². The obtained dielectric constants of each component layer are: 15 – 16 for gate oxide, 14 for floating gate, 19 – 20 for tunnel oxide; the resistivities are on the order of 10 M Ω×cm. The dielectric constants determined from simulation were used for calculating the stored carriers density. The values of dielectric constants obtained for HfO₂ layers are in good agreement with literature.

The results together with the objectives and activities considered in the Work Plan for Stage III related to modelling of MOS capacitors with 3 layers HfO₂ / SiGeSn - HfO₂ / HfO₂ / p-Si substrate and resulting parameters together with results dissemination (1 ISI paper submitted for publication) were fully fulfilled.

In Stage III / 2021, 2 papers were submitted for publication in ISI quoted journals, being in reviewing stage. Other 2 papers are in writing stage, to be submitted to ISI-quoted journals. We also participated with 2 papers at a prestigious conference in the field organized by APL Materials, namely Materials Challenges for Memory - MCFM 2021, virtual conference. The project web page https://infim.ro/en/project/sigesn-nanocrystals-with-charge-storage-properties-at-nanoscale-sigesnanomem/ was updated.

Stage II / 2020 SUMMARY

In Stage II / 2020, capacitor-like trilayer memory structures of gate HfO₂ / SiGeSn NCs - HfO₂ floating gate / tunnel HfO₂ / p-Si substrate with floating gate formed of SiGeSn nanocrystals (NCs) acting as charge storage nodes, embedded in HfO₂ were obtained and complexly characterized. The layers were deposited by magnetron sputtering deposition and subsequently nanostructured by rapid thermal annealing (RTA). Complex characterization of layers composition, morphology and structure was carried out by means of cross-section transmission electron microscopy (XTEM), Raman spectroscopy and X-ray diffraction (XRD). The electrical and memory properties of capacitors were investigated and correlated with the morphology and structure results.

Thus, in this stage we prepared different types of structures based on the Stage I / 2019 results, meaning that in Stage II / 2020, the technological parameters for structures preparation were refined, namely the SiGeSn-HfO₂ intermediate layer composition (SiGeSn : HfO₂ vol. ratio), the thicknesses of the 3 layers from the stack and the RTA temperature. For deposition, the gate HfO₂ / SiGeSn - HfO₂ / tunnel HfO₂ / p-Si substrate configuration was chosen considering the best results from the first stage regarding the memory properties. 2 types of structures were deposited, one type M1 similarly with the one from the first stage and the second type M2 that has thicker intermediate and gate oxide layers and thinner tunnel oxide layer.

The XTEM investigations revealed the formation of Si_xGe_1-x-ySn_y NCs in the floating gate, the NCs being rich in Ge, having very low Si content (x = 5% Si), diameters of ~ 5 nm and diamond crystalline structure (M1-RTA structures). 2 types of Si_xGe_1-x-ySn_y NCs were evidenced in function of their position in the intermediate layer. First type NCs are practically of Ge (low content y ~ 2% Sn) and are positioned in the intermediate layer, while the second type of NCs are of GeSn with y ~ 15% Sn content and are positioned at the edge of intermediate layer. Raman spectroscopy showed also the formation of both GeSn NCs (y ~ 11 – 12% Sn, x = 5% Si) and Ge NCs in the M1 and M2 structures with RTA at 520 and 530 ^oC. It was shown that the annealing at 600 ^oC leads to the decrease of Sn content in the GeSn NCs due to the Sn diffusion to the surface. The GeSn NCs formation was also confirmed by XRD.

From the point of view of memory properties (correlated with the structure and morphology ones obtained from the complex characterization), in this stage, the best M1 and M2 structures are the ones RTA annealed at 520 and 530 ^oC. The SiGeSn NCs evidenced by XTEM, Raman and XRD measurements produce the counter clockwise C – V hysteresis with high memory window ΔV = 3 – 4 V as they act as charge storage nodes. The RTA annealing at higher temperatures (550 – 600 ^oC) leads to the deformation of hysteresis and decrease of its memory window to ~ 1 V.

The results together with the objectives and activities considered in the Work Plan for Stage II related to obtaining capacitor structures with NC SiGeSn-based floating gate and their complex characterization together with results dissemination (3 ISI papers; 2 papers presented at international conferences) were fully fulfilled.

In Stage II / 2020, 1 paper was accepted for publication in Applied Surface Science (and published - Applied Surface Science 542, 148702 (2021)) and 2 manuscripts are under finalization stage to be send to prestigious ISI-quoted journals. The project web page https://infim.ro/en/project/sigesn-nanocrystals-with-charge-storage-properties-at-nanoscale-sigesnanomem/ was updated and it is permanently updated.

Stage I / 2019 SUMMARY

In Stage I / 2019 we prepared trilayer memory capacitors of gate HfO₂ / SiGeSn NCs - HfO₂ floating gate / tunnel HfO₂ / p-Si substrate in which the floating gate is formed of SiGeSn nanocrystals (NCs) acting as charge storage nodes, embedded in HfO₂. The capacitors were prepared by magnetron sputtering and subsequently annealed by rapid thermal annealing for nanostructuring. Preliminary studies were carried out with the aim to optimize the technological conditions for obtaining structures with good memory properties.

Two types of structures were tested, i.e. M1 structures in which trilayers of gate HfO₂ / SiGeSn / tunnel HfO₂ were deposited on p-Si wafers and M2 structures with deposited trilayer configuration of gate HfO₂ / SiGeSn-HfO₂ / tunnel HfO₂ on p-Si.

The formation of SiGeSn NCs in the floating gate was evidenced by X-ray diffraction (M1 structures). The capacitance-voltage characteristics measured on all structures (both M1 and M2 types) present counter-clockwise hysteresis loop with frequency-independent memory window demonstrating that the charge storage takes place only in SiGeSn NCs. The best memory characteristics were achieved on M2 structures, i.e. large memory window of 3.9 V.

In this stage, one article was published in Nanotechnology and 4 papers were presented at international conferences (ICASS – Italy, EMRS – France, IBWAP & EuroNanoForum – Romania). The project web page https://infim.ro/en/project/sigesn-nanocrystals-with-charge-storage-properties-at-nanoscale-sigesnanomem/ was created and updated.

Papers in ISI-quoted journals

1. „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)
2. „Nanoscale continuous transition from monoclinic to ferroelectric orthorhombic inside HfO₂ nanocrystals stabilized by HfO₂ capping and self-controlled Ge doping”, C. 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, Journal of Materials Chemistry C 9, 12353 (2021)
3. „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)
4. „Effects of Ge-related storage centers formation in Al₂O₃ enhancing the performance of floating gate memories”, I. Stavarache, O. Cojocaru, V.A. Maraloiu, V.S. Teodorescu, T. Stoica, M.L. Ciurea, Applied Surface Science 542, 148702 (2021)
5. „Orthorhombic HfO₂ with embedded Ge nanoparticles in nonvolatile memories used for the detection of ionizing radiation”, C. Palade, A. Slav, A.M. Lepadatu, I. Stavarache, I. Dascalescu, A.V. Maraloiu, C. Negrila, C. Logofatu, T. Stoica, V.S. Teodorescu, M.L. Ciurea, S. Lazanu, Nanotechnology 30, 445501 (2019)
6. „Ferroelectric orthorhombic HfO₂ phase formation in HfO₂/Hf-HfO₂/HfO₂ 3-layer stacks”, manuscript  in writing stage, to be submitted to ISI-quoted journal

Papers at international conferences

1. „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 2022), Constanta, July 12-15, 2022 (oral presentation)
2. „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, M.L. Ciurea, EMRS 2021 Fall Meeting, virtual conference, September 20-23, 2021 (oral presentation)
3. „Trilayer non-volatile memory structures with floating gate of SiGeSn nanocrystals embedded in HfO₂”, C. Palade, A.M. Lepadatu, A. Slav, I. Stavarache, O. Cojocaru, V.A. Maraloiu, V.S. Teodorescu, T. Stoica, M.L. Ciurea, Materials Challenges for Memory - MCFM 2021, virtual conference, April 11-13, 2021, https://horizons.aip.org/materials-challenges/ (poster
4. „Ferroelectric orthorhombic HfO₂ phase in 3-layer memory structures of control HfO₂ /floating gate of Ge nanoparticles in HfO₂ /tunnel HfO₂ on Si wafers”, C. 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, MCFM 2021, April 11-13, 2021 (poster)
5. „Advances in Ge nanocrystals-based structures for SWIR sensors and non-volatile memories”, C. Palade, A. Slav, A.M. Lepadatu, I. Stavarache, I. Dascalescu, O. Cojocaru, I. Lalau, S. Lazanu, C. Logofatu, T. Stoica, V.S. Teodorescu, M.L. Ciurea, 19th International Balkan Workshop on Applied Physics and Materials Science (IBWAP 2019), July 16-19, 2019, Constanta (invited presentation)
6. „Germanium nanocrystals in oxide matrix for non-volatile memories and ionizing irradiation sensors”, M.L. Ciurea, I. Stavarache, A. Slav, C. Palade, A.-M. Lepadatu, I. Dascalescu, I. Lalau, O. Cojocaru, V.S. Teodorescu, A.V. Maraloiu, S. Lazanu, T. Stoica, 3^rd International Conference on Applied Surface Science (ICASS 2019), June 17-20, 2019, Pisa (poster)
7. „New advanced materials based on SiGeSn nanocrystals in oxides for SWIR phototodetectors and non-volatile memory devices”, C. Palade, I. Stavarache, A.M. Lepadatu, A. Slav, S. Lazanu, T. Stoica, V.S. Teodorescu, M.L. Ciurea, F. Comanescu, A. Dinescu, R. Muller, G. Stan, A. Enuica, M.T. Sultan, A. Manolescu, H.G. Svavarsson, EuroNanoForum 2019 (Nanotechnology and Advanced Materials Progress Under Horizon2020 and Beyond), June 12-14, 2019, Bucharest (poster)
8. „GeSnSiO₂ layers with embedded GeSn nanocrystals for sensing in SWIR”, A. Slav, C. Palade, C. Logofatu, I. Dascalescu, A.M. Lepadatu, I. Stavarache, S. Iftimie, V. Braic, S. Antohe, S. Lazanu, V.S. Teodorescu, D. Buca, M.L. Ciurea, T. Stoica, M. Braic, EMRS 2019 Spring Meeting, May 27-31, 2019, Nice (oral presentation)

lepadatu@infim.ro