by Dr. Mahitosh Biswas, CNRS/Université Paris-Saclay, Palaiseau, France

Abstract:

Molecular beam epitaxy (MBE) is a highly controlled thin-film growth technique widely used
in quantum materials research. In MBE, atomic or molecular beams are deposited onto a crystalline
substrate under ultra-high vacuum, allowing control of thickness, composition, and heterostructure at the
atomic scale. This precision is crucial for fabricating quantum materials that exhibit emergent quantum
phenomena such as superconductivity, topological states, magnetism, ferroelectricity, and strongly
correlated electron behavior. It also makes it possible to integrate these materials into devices, including
spintronic components and quantum information platforms.
The first part of this talk will focus on engineering topological HgTe phases. By precisely controlling strain
through ZnTe/CdTe superlattices (SLs), it is possible to realize two-dimensional topological insulators and
Dirac/ Weyl semimetals. Conventional growth on GaAs:Si substrates, however, suffers from a large lattice
mismatch, leading to surface islands that hinder sub-micrometer device fabrication. We show that S-doped
InAs substrates, with much smaller mismatch, facilitate high crystalline quality, homogeneous ZnTe/CdTe
SLs that serve as near-ideal virtual substrates for HgTe. Compared to GaAs-based structures, unstrained
HgTe quantum wells (QWs) and compressively strained bulk HgTe grown on InAs exhibit significantly
smoother surfaces. Furthermore, magnetotransport measurements confirm high electronic quality of HgTe
QWs and demonstrate the back-gating feasibility of the S-doped InAs, paving the way for micro- and
nanoscale devices based on topological materials [1].
The second part of the talk will explore metastable polar phases in fluorite oxides, specifically HfO2/ZrO2
compounds. These materials are promising for ferroelectric applications, but films grown by ALD, CVD,
or PLD are usually polycrystalline and/or nanocrystalline, limiting access to their intrinsic properties. Using
a hybrid MBE system equipped with zirconium tert-butoxide metal-organic precursor that provides
excellent control over stoichiometry, we have stabilized single-crystalline, single-phase rhombohedral ZrO2
thin films [2], a polar polymorph that is otherwise metastable. This epitaxial breakthrough provides access
to intrinsic ferroelectricity and strong nonlinear optical responses in a simple binary oxide, opening
pathways for next-generation non-volatile memory and integrated quantum photonics.
Together, these studies demonstrate how epitaxial engineering enables topological phases in HgTe and

stabilizes ferroelectric phases in ZrO2, paving the way for multifunctional heterostructures for next-
generation quantum devices.

References
[1] Mahitosh Biswas*, Lena Fürst, Martin Zift, Steffen Schreyeck, Hartmut Buhmann, and Laurens W.
Molenkamp, Molecular beam Epitaxy of homogeneous topological HgTe on S-doped InAs substrate,
Advanced Functional Materials, 35, 2413229 (2024).
[2] Mahitosh Biswas*, Findling Nathaniel, Largeau Ludovic, Thomas Maroutian, and Vivien Laurent,
Stabilizing single-crystalline, single-phase, rhombohedral phase in ZrO2 thin films using hybrid
molecular beam epitaxy, In preparation.