aymen-mahmoudi.ch

My research activity is located at the crossroads between condensed matter physics and materials science. More precisely, I am interested in low dimensional material and the transition metal dichalcogenides (TMD) in particular, which are nowadays considered with attention given future applications and more fundamental studies. I have developed a deep understanding of key techniques, allowing the investigation of their optical and electronic properties.

In this section I share the publication I done or being involved in:



Van der Waals epitaxial growth of few layers WSe2 on GaP(111)B

2D Mater. 11 035031

2D material epitaxy offers the promise of new 2D/2D and 2D/3D heterostructures with their own specific electronic and optical properties. In this work, we demonstrate the epitaxial growth of few layers WSe2 on GaP(111)B by molecular beam epitaxy. Using a combination of experimental techniques, we emphasize the role of the growth temperature and of a subsequent annealing of the grown layers under a selenium flux on the polytype formed and on its structural and morphological properties. We show that a low growth temperature promotes the formation of the 1T′ and 3R phases depending on the layer thickness whereas a higher growth temperature favours the stable 2H phase.

Stacking order and electronic band structure in MBE-grown trilayer WSe2 films

Phys. Rev. B 109, 115437
https://arxiv.org/abs/2310.05660

In continuation of previous work and after successfully growing and studying the bilayer structure of the 3R phase of WSe2 on a III-V GaP substrate, I continued to study the phase transition from 3R to 2H in the WSe2 by additional annealing at 600 °C. In this article, a new growth by molecular beam epitaxy is implemented with an adaptation of the growth parameters to result in a trilayer structure of WSe2 under the 2H-2H phase on the same GaP substrate. We thus show that a 2H-2H trilayer of WSe2 can be produced and stabilized by epitaxy on a gallium phosphide substrate. A sharp and high-quality WSe2-GaP interface was confirmed by high-resolution scanning transmission electron microscopy and X-ray photoemission spectroscopy. We present a combined experimental and theoretical study of the WSe2-GaP interface. We present a combined experimental and theoretical study of the valence band structure of trilayer WSe2. Nano-angle resolved photoemission spectroscopy and density functional theory calculation show that the bilayer electrons populate two distinct subbands associated with the K and Γ valleys.

Quasi van der Waals Epitaxy of Rhombohedral-Stacked Bilayer WSe2 on GaP(111) Heterostructure

ACS Nano 2023, 17, 21, 21307–21316
https://arxiv.org/abs/2310.05660

The growth of two-dimensional (2D) materials on conventional 3D semiconductors results in 2D/3D hybrid heterostructures, which offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Here, we demonstrate that a rhombohedral-stacked bilayer of tungsten diselenide (WSe2) can be easily obtained by molecular beam epitaxy growth on a gallium phosphide (GaP) substrate. We confirm the presence of a 3R-stacking bilayer using scanning transmission electron microscopy (STEM) and micro-Raman spectroscopy. We report also high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements on the grown WSe2 bilayer to probe its band structure and the effect of the GaP substrate. Indeed, our ARPES measurements reveal the expected valence band of WSe2 with the band maximum located at the Γ point of the Brillouin zone, and the combination with DFT calculation on free-standing bilayer WSe2 confirms the stack nature and the weak quasi-van der Waals interaction between the adjacent materials. The work was highlighted in the C2N/CNRS news page: https://www.c2n.universite-paris-saclay.fr/en/science-society/news/actu/306 and in the COST OPERA Scientific Highlights for years 2022/2023.

Electronic properties of rhombohedral-stacked bilayer WSe2 obtained by Chemical Vapor Deposition

Phys. Rev. B 108, 045417

In this work, we report an experimental and theoretical study of WSe2 homo-bilayers obtained in stable 3R configuration by chemical vapor synthesis. We investigate the electronic and structural properties using micro-Raman spectroscopy, angle-resolved photoemission nano-spectroscopy measurements (nano-ARPES) and Density Functional Theory (DFT) calculations. Our results demonstrate that the WSe2 bilayers with 3R crystal phase (AB stacking) show a significant valence-band splitting at the K point estimated to 550 ± 20 meV. Our work opens up new perspectives for the development of optoelectronic and spintronic devices based on 3R TMD homo-bilayers.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3

ACS Nano 2023, 17, 19, 18924–18931
https://arxiv.org/abs/2308.04864

As a 2D ferroelectric (FE) material, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, which is a distinguished behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field.
In this study, we use angle-resolved photoemission spectroscopy (ARPES) to probe the band structure of the phase 2H of α-In2Se3 single crystals and reveal a highly metallic two-dimensional electron gas (2DEG) due to a quantum confinement of electrons at the surface. Combining the experimental scans and density functional theory (DFT) calculations gives access to more physical properties as the band gap nature and value and the doping value. The paper was featured on the October 2023 cover of ACS Nano.
The paper was highlighted by CNRS in the C2N news page: https://www.c2n.universite-paris-saclay.fr/en/science-society/news/actu/305

Direct observation of highly anisotropic electronic and optical nature in indium telluride

Phys. Rev. Materials 7, 074601

In this study, by combining angle-resolved photoemission spectroscopy and density functional theory calculations, we demonstrate the stability of InTe in the tetragonal crystal structure, with a semiconducting character and an intrinsic p-type doping. InTe material has a one-dimensional chain structure, from which intriguing properties arise. Indeed, we found that the effective mass of the hole carriers is about ten times larger along the chain direction compared to the perpendicular one. This in-plane anisotropy of effective mass from the experiment fit the theoretical calculations. These observations indicate a highly anisotropic character of the electronic band structure, making InTe of interest for electronic and thermoelectric applications.

Intrinsic defects and mid-gap states in quasi-one-dimensional Indium Telluride

Phys. Rev. Research 5, 033152
https://arxiv.org/abs/2308.08975

The anisotropic semiconductors in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry can have essential electronic and structural properties and provide a better understanding of device applications.
In this work, we use scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to image the in-plane structural anisotropy directly in tetragonal InTe. As a result, we demonstrate that InTe exhibits a bandgap of about 400 meV located at the M point of the Brillouin zone, and we report a direct observation of one-dimensional In1+ chains in InTe. These findings claim a high anisotropic character of the material, which we confirmed through ARPES scans of the band structure.

Evidence for highly p-type doping and type II band alignment in large scale monolayer WSe2/Se-terminated GaAs heterojunction grown by molecular beam epitaxy.

Nanoscale, 2022, 14, 5859-5868

In this work, we studied a mixed vdW 2D/3D heterostructure made by a substrate of single layer WSe2 grown on Se-terminated GaAs(111) via MBE. ARPES measurements revealed that the WSe2 layer exhibits a high p-type doping due to a charge transfer to GaAs across the interface. The findings of the work are related to the tuning of the electronic properties of WSe2 and the successful growth of large area of the 2D on 3D is likely applicable to other TMDs.

Photoferroelectric All-van-der-Waals Heterostructure for Multimode Neuromorphic Ferroelectric Transistors

ACS Appl. Mater. Interfaces 2023, 15, 12, 15732–15744

Interface-driven effects in ferroelectric van der Waals (vdW) heterostructures provide fresh opportunities in the search for alternative device architectures with better electronic properties. Here, we demonstrate the electrical and optical control of the ferroelectric polarization states of ferroelectric field effect transistor (FeFET) fully made of ReS2/hBN/CuInP2S6 (CIPS) vdW materials. I have performed and analyzed the PL/Raman response of different thickness of CIPS and ReS2 layers embedded in the transistor.

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

ACS Nano 2023, 17, 21, 21865–21877
https://arxiv.org/abs/2310.14648

To overcome some of the limitations of CMOS technology, the emerging approach of reconfigurable FETs (ReFET) offers a promising solution by providing the ability to adjust the transistor functionality. In fact, in a typical ReFET which consists of two gate electrodes, that independently control the energy band profile within the semiconductor channel. In this work, we demonstrate the suitability of reconfigurable ferroelectric field-effect transistors for designing nonvolatile reconfigurable logic-in-memory circuits with multifunctional capabilities. 2D tungsten diselenide (WSe2) layer is used for the modulation of the energy landscape by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP2S6 or CIPS) layer. Although the CIPS is a good insulator, a layer of hBN is added to protect the gates. I have performed and analyzed the different PL/Raman response from WSe2, hBN and CIPS as well as the thickness of each layer via AFM.
The paper was highlighted by CNRS Physique: https://www.inp.cnrs.fr/fr/cnrsinfo/des-transistors-qui-ont-de-la-memoire-une-revolution-dans-la-topologie-des-circuits