Epitaxial Metal Halide Perovskites by Pulsed Laser Deposition
Junia Solomon Sathiaraj is a PhD student in the Department of Inorganic Materials Science. Promotors are prof.dr. M. Morales Masis and prof.dr.ing. A.J.H.M. Rijnders from the Faculty of Science & Technology.
The revolution of the semiconductor industry was driven by the ability to precisely control structure and interfaces at the atomic scale. Epitaxial growth played a central role in enabling this control, giving rise to novel physical phenomena and device architectures. A similar approach has been widely applied in the field of oxide perovskites, where epitaxial films allowed access material properties such as exotic electronic, magnetic, and ferroelectric properties not observable in bulk or polycrystalline forms.
In many ways, research is often about being inspired by nature or from parallel fields, and applying them in new contexts. In this spirit, we extend the concept of epitaxy to metal halide perovskites (MHPs), materials already known for their exceptional optoelectronic properties but mostly explored in polycrystalline form. As with silicon, where the transition from amorphous to crystalline silicon led to major leaps in performance, we believe that epitaxial halide perovskites could unlock entirely new functionalities and device applications.
Epitaxial growth where ‘epi’ means ‘upon’ and ‘taxy’ implies ‘arrangement’ refers to the ordered growth of a crystalline film on a crystalline substrate, allowing for precise lattice matching and interface control. The epitaxial growth of MHPs presents a promising path for phase stability, tuning film strain, and enabling heterostructure development. In this thesis, the epitaxial growth of MHPs and growth of layered metal halide perovskites via pulsed laser deposition (PLD) was explored, with an emphasis placed on growth control, crystallographic orientation studies, and optoelectronic characterization.
Chapter 2 introduces PLD technique used throughout this thesis and highlights the potential for achieving epitaxial growth. Also, the key characterisation methods, including X-ray diffraction (XRD), reciprocal space mapping, XRD pole figure analysis, atomic force microscopy, and photoluminescence, are outlined as tools to evaluate crystallinity, texture, and optical quality.
In Chapter 3, the epitaxial growth of CH₃NH₃PbI₃ (MAPbI₃) was explored. The epitaxial growth was achieved on lattice-matched KCl substrates at room temperature. RSM and XRD pole figure measurements confirmed the stabilisation of the cubic α-phase. Futher, the electron backscatter diffraction measurement revealed the smooth, single-oriented films with one in-plane orientation. The stabilised cubic phase of MAPbI3 remained stable for over 300 days, with a bandgap of 1.66 eV as confirmed from photoluminescence (PL) measurements. Having estabilished the epitaxial growth on closely lattice-matched KCl, we extended this approach to other substrates with different strucutral motifs and larger lattice mismatch. The results highlighted the necessity of closely matched crystalline substrates to grow phase-pure, single-oriented halide perovksites thin films. First-principles density functional theory (DFT) calculations corroborated the role of strain in stabilizing the α-phase. This work establishes PLD as a viable method for vapor-phase heteroepitaxy of hybrid perovskites and opens the door to further strain-engineering studies.
While epitaxial growth had been demonstrated in chapter 3, the resulting morphology indicated island growth mode. In Chapter 4, we examined the feasibility of transitioning to layer-by-layer (LBL) growth of MAPbI₃ on KCl. We studied the influence of growth temperature and laser spot size on the transiiton from island to LBL growth. The results revealed improved surface diffusion at elevated temperatures, but also degradation due
to loss of methylammonium (MA) and PbI₂ formation. A factorial design was applied to visualize the experimental trends of growth. Finally, we assessed the limitations of KCl as a substrate and proposed the use of alternative single-crystal perovskite substrates as a potential strategy to enable LBL growth.
Building on the α-MAPbI₃/KCl platform, Chapter 5, presents the oriented growth of n = 1 Ruddlesden-Popper (PEA)₂PbI₄ films by PLD. In situ PL confirmed early-stage formation of the RP phase. Structural analysis via XRD and GIWAXS verfied single-phase, oriented films. Interestingly, the oriented growth was found to be independent of the underlying substrate. Conformal surface coverage of the grown layer was confirmed by AFM and spatially resolved PL. Notably, (PEA)₂PbI₄ RP films grown on α-MAPbI₃/KCl remained stable for over 184 days without evidence of cation exchange, while those grown on MAPbI3 single crystal showed evidence of cation exchange. This chapter demonstrated the capability of PLD for direct synthesis of 2D perovskites and room-temperature 2D/3D heterostructures.
In Chapter 6, we turned to the lead-free perovskite CsSnI₃. Epitaxial growth on KCl was achieved by tuning deposition rate and substrate temperature, resulting in the orthorhombic phase of CsSnI₃. In-plane RSM and pole figures analysis revealed the formation of in-plane domains, likely resulting from strain relaxation due to lattice mismatch. The films exhibited nanorod-like features, indicating a seed-mediated growth mechanism. Additionally, the presence of Cs-rich particulates on the films pointed to the need for improved target fabrication. These findings highlight the potential for extending PLD-based epitaxial growth to other MHPs.
The thesis concludes with a perspective on future recommendations for epitaxial MHPs by PLD, focusing on growth optimization towards thin-film single crystals of MHPs and substrate engineering to advance the integration of epitaxial MHPs into optoelectronic applications.
Overall, this thesis highlights how gaining control over the growth of MHPs thin films enables direct tuning of key material properties. For instance, in Chapter 3, we showed how strain-induced phase stabilization influences the bandgap and optical quality of MAPbI₃. Such control opens pathways for designing materials with tailored functionalities-whether for stable light emission, efficient charge transport, or integration into complex heterostructures. By advancing our understanding of epitaxial growth in these soft ionic semiconductors, this work lays the foundation for next-generation optoelectronic materials engineered at the atomic level.
