Project Abstract:

Chalcogenide phase change materials are the most promising candidates for the next generation of non-volatile memories. The increasing need for storage capacity can be addressed using an innovative approach of recording multiple logical states in a single memory by stacking several different chalcogenide layers. In order to better understand the processes which take place in multiple states transitions and to further increase the number of states, we need to know the structure of the materials in those states and the relations between the structure and the electrical and optical properties. Single layer phase change memory materials based on GeTe, GaSb and SnSe and stacked structures of GeTe/SnSe, SnSe/GaSb and GeTe/GaSb will be prepared by pulsed laser deposition (PLD). Structural changes during phase transitions will be revealed by X-ray diffraction (XRD), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray reflectometry (XRR). Modifications of optical properties will be analyzed using in-situ thermal ellipsometry measurements. Electrical resistivity variation as a function of temperature will also be studied. Modeling of the phase change mechanism will be performed using Cellular Automata (CA) and the insights from the model will be experimentally implemented to optimize the number and stability of intermediary states. This investigation will open the way for innovative materials discovery suitable for designing denser, faster, cheaper and greener memories.

Project Objectives: 

 The main goal of this project is to investigate the switching process in stacked chalcogenide thin films which can be used for multistate memory devices. Preparation of the films will be carried out using Pulsed Laser Deposition (PLD). Changes in crystallinity as well as structural changes during phase transitions will be revealed by X-ray diffraction (XRD), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray reflectometry (XRR). Modifications of the optical properties will be analyzed using in-situ thermal ellipsometry measurements. Electrical resistivity variation as a function of the annealing temperature will be studied by four points R vs. T studies. Modeling of the phase change mechanism will be performed using Cellular Automata (CA) and the insights from the model will be implemented to optimize the number and the stability of intermediary states in chalcogenide bilayers.

In order to achieve the main goal, specific intermediate steps will be pursued:

1. Preparation of homogeneous and amorphous films of phase change materials by PLD;
2. In-situ crystallization studies of phase change in single films and investigations of the relation between structural changes and optical/electrical properties;
3. Investigations of the switching mechanism in chalcogenide bilayers by XRD, XRR and synchrotron techniques;
4. Studies of the optical properties of the bilayers by ellipsometry in order to reveal the changes in reflectivity during phase change;
5. Analysis of the electrical resistivity variation during switching by R vs. T measurements and the stability of intermediary states using different electrode geometries;
6. CA simulations of the multistate switching mechanism in stacked chalcogenide bilayers;
7. Optimization of the switching process in PCM bilayers by the insertion of a metallic buffer layer between the active phase change films.

Team: 

The project team consists of:

1. Dr. Alin Velea - project leader
2. Dr. Florinel Sava - postdoctoral researcher
3. Dr. Iosif-Daniel Șimandan - postdoctoral researcher
4. Dr. Oana-Claudia Mihai - postoctoral researcher
5. Dr. Aurelian-Cătălin Gâlcă - senior resercer
6. Dr. Gabriel Socol - senior researcher
7. Dr. Adam Lorinczi - senior researcher
8. Dr. Mihai Popescu - senior researcher
9. Drd. Adelina Matea - Phd. student
10. Drd. Mirela Ilie - Phd. student

Results

Stage 1: Investigation of crystallization in SnSe, GeTe and GaSb single films.

Deadline: 31.12.2015

Monolayers of GeTe, GaSb and SnSe were successfully prepared by pulsed laser deposition (PLD) method.

The prepared samples have a satisfactory stoichiometry whose influence on the properties of the obtained materials  will be studied in detail in the next stage of the project. Also, the deposition process will go through a second optimization phase in order  to improve the switching process.

XRD measurements showed that monolayers of GeTe and SnSe are amorphous in the as-deposited state. GaSb layers show some weak crystalline peaks due to PLD droplets formed during the deposition.

Stage 2: Study of phase change in bilayers of GeTe/SnSe, SnSe/GaSb and GeTe/GaSb. Modeling the phase transition in stacked chalcogenide films.

Deadline: 31.12.2016

Several sets of samples were made using pulsed laser deposition, with thicknesses between 12 and 500 nm, depending on the type of analysis performed. GeTe and SnSe monolayers are amorphous in the initial state as revealed by XRD measurements, whereas GaSb layers and some SnSe layers show some weak crystalline peaks which do not significantly influence the switching process as seen from the resistance vs. temperature measurements. XRD and EXAFS performed at Swiss Light Source synchrotron, showed that GeTe, GaSb and SnSe layers crystallized after the transition in rhombohedral, cubic and orthorhombic phases, respectively.

The cellular automata model developed and applied to GeTe, has reproduced very well the change in resistance with temperature, yielding a transition temperature very close to the experimental one.

Two terminal GeTe micrometric devices were built by electron beam lithography. They have a reproducible current-voltage characteristic for several switching cycles with a threshold voltage of about 1 V.

Finally, the deposition process has been optimized for bilayers, thus preparing the next phase of the project. The performance indicators have also been accomplished entirely. Two articles were submitted for publication in ISI indexed journals, one invited paper was presented at one of the most representative international conferences in the field of the project. The study has been published in-extenso in the proceeding of the conference. Also, this project contributed in maintaining two international collaborations, one with Paul Scherrer Institute in Switzerland, where synchrotron analysis was performed and the second with Imec institute in Belgium, where the two terminal devices were developed.

Stage 3: Optimization of the phase transition in the chalcogenide bilayers.

Deadline: 30.11.2017

In conclusion, the objective of this reporting phase and the objectives of the entire research project have been fully realized and exceeded in some aspects. Using the deposition parameters optimized in the previous step, bilayers of GeTe\GaSb, GeTe\SnSe and SnSe\GaSb were grpwth using the pulsed laser deposition method. XRD and EXAFS measurements performed at the synchrotron showed that the first bilayer is thermally stable up to 350 oC, and the other two form a single phase of either Ge-Sn-Te or Sn-Se-Ga-Sb due to layer mixing produced by thermal treatment. Further, the stable GeTe\GaSb bilayer was tested by temperature resistance measurements and current-voltage measurements and three logical memory states were obtained. Hafnium has proven to be a good barrier layer to block the diffusion and mixing of layers when deposited between the two monolayers of the bilayer.

The PCCA model developed within the project is a valuable tool that has the capacity to provide guidance in the construction and engineering decisions of future memory devices with multiple logic states based on complex structures and phase change chalcogenide materials.

The performance indicators of this stage were also fully achieved and even exceeded. Four articles were published  in ISI-ranked journals (compared to the three planned at this stage), an invited lecture, an oral paper and a poster were presented at two international conferences (as opposed to a single planned participation).

Within this project, collaboration were established with well-known research institutes in the field of materials science, such as the Paul Scherrer Institute in Switzerland where synchrotron analyzes were performed and Imec institute in Belgium, where the two terminals devices were prepared and measured.. The collaboration with them will be continued after the completion of this project.

Moreover, the objective of the program of creating an independent research teams was achieved. A strong team with strong work relations was formed, consisting of young researchers, but also of experienced researchers. Proof are all the articles that have been published within this project, in which it is observed that most of the authors are all members of the project. The team will continue to collaborate after the completion of this project for other research projects. We thank UEFISCDI for funding this project and the program management team for the contribution to the good conduct of the research activity.

Publications

The following scientific articles have been published:

1. Title: “Te-based chalcogenide materials for selector applications”, authors: A. Velea, K. Opsomer, W. Devulder, J. Dumortier, J. Fan, C. Detavernier, M. Jurczak și  B. Govoreanu, Scientific Reports 7, 8103 (2017), doi: 10.1038/s41598-017-08251-z.
2. Title: “Coarse-grained simulation of crystallization processes in phase change memories using a cellular automata model”, authors: C. Mihai și A. Velea, Modelling and Simulation in Materials Science and Engineering 26, 045006 (2018), doi: 10.1088/1361-651X/aab62f.
3. Title: "Thermal Stress Effect on the Structure and Properties of Single and Double Stacked Films of GeTe and SnSe", authors: F. Sava, C. N. Borca, A. C. Galca, G. Socol, D. Grolimund, C. Mihai, A. Velea, Physica Status Solidi B 255, 1700552 (2018), doi: 10.1002/pssb.201700552.
4. Title: "Thermal stability of phase change GaSb\GeTe, SnSe\GeTe and GaSb\SnSe double stacked films revealed by X-ray reflectometry and X-ray diffraction", authors: A. Velea, F. Sava, G. Socol, A. M. Vlaicu, C. Mihai, A. Lorinczi, I. D. Simandan,  Journal of Non-Crystalline Solids 492, 11 (2018), doi: 10.1016/j.jnoncrysol.2018.02.033.
5. Title: "Structural and optical properties of optimized amorphous GeTe films for memory applications", authors: A. C. Galca, F. Sava, I. D. Simandan, C. Bucur, V. Dumitru, C. Porosnicu, C. Mihai, A. Velea, Journal of Non-Crystalline Solids 499, 1 (2018), doi: 10.1016/j.jnoncrysol.2018.07.007.
6. Title: Structural characterisation and thermal stability of SnSe\GaSb stacked films, authors: F. Sava, C. N. Borca, A. C. Galca, G. Socol, D. Grolimund, C. Mihai, A. Velea, Philosophical Magazine 99, 55 (2019), doi: 10.1080/14786435.2018.1529442.
7. Title: “The interplay between material and device properties in Ge-Te and Si-Te binary systems as a guide for material selection in selector applications”, authors: A. Velea, K. Opsomer, W. Devulder, J. Dumortier, J. Loo, C. Detavernier, M. Jurczak și B. Govoreanu, E\PCOS Proceedings, 109 (2016).

Conference presentations:

1. Invited lecture: “The interplay between material and device properties in Ge-Te and Si-Te binary systems as a guide for material selection in selector applications”, authors: A. Velea,  European symposium on Phase-Change and Ovonic Sciences - E\PCOS2016, September 4-6, 2016, Trinity College, Cambridge, UK.
2. Invited lecture: “Chalcogenide materials for selectors and phase change memories”, authors: A. Velea, 8th International Conference on Amorphous and Nanostructured Chalcogenides - Fundamentals and Applications, July 2 - 5, 2017, Sinaia, Romania.
3. Oral presentation: “Spectroscopic ellipsometry, a useful tool to investigate phase transitions in thin films: case study on phase change materials”, authors: A. C. Galca, G. Socol, A. Velea, 4th Central and Eastern European Conference on Thermal Analysis and Calorimetry, August 28 - 31, 2017, Chișinău, Republic of Moldova.
4. Poster: “Thermal stability of heterojunction interfaces in GaSb/GeTe, GaSb/SnSe and SnSe/GeTe bilayers revealed by X-ray reflectometry”, authors: A. Velea, G. Socol, C. Mihai, A. Lőrinczi, I. D. Simandan, G. Schinteie, F. Sava, 8th International Conference on Amorphous and Nanostructured Chalcogenides - Fundamentals and Applications, July 2 - 5, 2017, Sinaia, Romania.

Patents:

1. Title: "Structure consisting of two superimposed thin layers of phase change materials with three logical memory states", authors: A. Velea, A. C. Galca, G. Socol, C. Mihai
   OSIM patent application, No. A/00964 of 22.11.2017.

Project Contact Person:

Dr. Alin Velea

alin.velea@infim.ro