Epitaxy is an important step in making current microelectronic devices because it makes it possible to make single-crystalline thin films and nano- or phase-matched structured semiconductor materials. This process creates connections that work well together and have good quality electronics and optics. The high performance of epitaxial materials and structures comes from the “ordering” (epitaxialy) of the underlying electrical structures.
For halide perovskite-based electronics like photovoltaics, light-emitting diodes, photodetectors, and phototransistors, a material shape that looks like a thin film is commonly used. If you want to make a good thin film, it should have low dislocation counts, no grain borders, and coherent surfaces. In order for new nanostructured halide perovskite devices to have the desired physical qualities, there must be a chemically and electrically periodic contact between the epilayer and the substrate.
Chemical epitaxy, van der Waals epitaxy/quasi van der Waals epitaxy, and distant epitaxy are the three different ways to do epitaxy. In chemical epitaxy, a strong contact between the epilayer and the substrate lets you change the epilayer’s width and sharpness over a large growing window. But in van der Waals/quasi van der Waals epitaxy and remote epitaxy, problems caused by lattice mismatch can be fixed, which makes the crystals better.
It is possible to make III–V, Si/Ge, and new perovskite oxide devices with smaller sizes that let them handle elastic forces above 2%, which is much higher than the limit for their bulk versions. Elastic strains can be used to tune and improve the electronic and optical qualities. Since single-crystal halide perovskites are available in smaller sizes, it is possible to create large strains that may be similar to those seen in other semiconductors.
Most of the time, strains are caused in normal semiconductors by the contact between the thin film or nanostructure and the substrate. Halide perovskites are physically soft, and they might be able to be stretched or compressed by a contact that isn’t very strong. This chapter talks about the latest progress made in epitaxy and strain engineering of halide perovskites thin films and nanoparticles. It also talks about the possibilities and difficulties in these areas.
Epitaxy of Thin Film and Nanostructures
Epitaxial Substrates
When you expose the epilayer to a certain chemical or ion, epitaxy can be used to make thin films or nanostructures. To make epitaxy work, you need materials that have similar crystal structures and close lattice constants to the epilayer. There is a false-cube symmetry in 3D non-van der Waals halide perovskites, and their lattice constants are between 5.5 and 6.5 Å. As with 3D perovskites, the in-plane feature of 2D van der Waals halide perovskites has the same structure and characteristics. To do epitaxy of halide perovskite, you need to find surfaces that are cubic or pseudo-cubic in shape and have close lattice constants. Experiments have shown that a number of different surfaces can hold epitaxial halide perovskite thin films or nanostructures, and most of them have lattice constants that are similar to those of halide perovskites. Muscovite mica, sapphire, and CdS are some of these surfaces that don’t have cubic or pseudo-cubic symmetry or have very different lattice constants. Chemical epitaxy, van der Waals/quasi-van der Waals epitaxy, and distant epitaxy are some of the epitaxy methods that have been created so far. As a general rule, the lattice difference between the substrate and the epitaxial perovskites must be pretty small when chemical and remote epitaxy is used.
Epitaxial Growth and Defects Formation Mechanisms
It is very important to understand how halide perovskites grow and how defects form in the epitaxy process in order to create and control their electrical qualities. In chemical epitaxy, there is a lot of threading dislocations when the lattice difference between the epitaxial halide perovskite is big (e.g. >1%) and the epilayer is thick. Some of these are made to pull big misfit dislocations out from the surfaces. When the film thickness is above the critical thickness, plastic deformation (such as misfit dislocation) is added to reduce the elastic strain and thus strain energy. This situation should fit with the usual picture of how dislocations form in epitaxial films.
It was discovered that the number of threading dislocations could be many times lower in van der Waals/quasi-van der Waals epitaxy and remote epitaxy, even though the lattice difference values were the same. For instance, the threading dislocation density in epitaxial CsPbBr3 on NaCl (through ionic epitaxy) is many times higher than on graphene/NaCl (through remote epitaxy), even though the difference in the lattice is the same in both cases.
The low dislocation density in remote and van der Waals epitaxy may be caused by at least two things. First, if there is a small crystalline error between two nuclei (or grains), they will make some threaded dislocations at the junction where they join when they get bigger. Either kinetics or thermodynamics could cause this misalignment. Kinetic misalignment could happen if the process grows too quickly, leaving atoms without enough time to reach their stable states or places. Remote or van der Waals epitaxy does not make classical misfit dislocations from a thermodynamic point of view.
The second case is when there isn’t any misalignment between the two grains that are merging. In this case, misfit dislocations and threading dislocations could still form because of the lattice mismatch and the need to release the large strain energy when the epilayer is above the critical thickness. But this problem doesn’t show up in remote or van der Waals epitaxy. In distant or van der Waals epitaxy, the relationship between the epilayer and the substrate can be unmatched and free of misfit dislocations, as shown by experiments. New ideas about how halide perovskites are formed show that this kind of situation can be created through the following mechanism:
In distant or van der Waals epitaxy, unlike chemical epitaxy, the weak-film interaction could let some small grains move randomly. These grains get more energy when they join with nearby big grains than when they stay on the local substrate lattice. By using random walk and “random rotation,” remote and van der Waals epitaxy can make an interface that doesn’t fit together properly and doesn’t move. However, when the grains are too big, they stop the random walk and “random rotation” processes. Dislocations should still form, but they will be less dense.
Experimental Progresses
Halide perovskites have been used in a lot more epitaxy experiments in the past few years. This part talks about some new experiments that have been done in this area. These include ionic epitaxy by vapour phase deposition, van der Waals/quasi-van der Waals epitaxy by vapour phase deposition, remote epitaxy by vapour phase deposition, and ionic epitaxy by solution method.
Ionic epitaxy of halide perovskites has been shown to work on different surfaces and with different shapes. With the help of ions, Wang et al. were able to grow thin films of CsSnBr3 and CsPbBr3 on NaCl, a metal alkali halide. The films had an epitaxial relationship of perovskite [100] parallel to NaCl [100] or perovskite [110] parallel to NaCl [100]. It was possible to see the hot photoluminescence (PL) and photo-Dember effect in halide perovskite through these high-quality protective films.
Shift ionic epitaxy of halide perovskites has also been done on oxide bases. Chen et al. showed that CsPbBr3 could grow on SrTiO3, with both CsPbBr3 [001] || SrTiO3 [100] and CsPbBr3 [010] || SrTiO3 [110] relationships being observed. Optical and electrical features of epitaxial CsPbBr3 thin films on SrTiO3 were similar to those of halide perovskite single crystals.
Oksenberg et al. used surface-guided CVD to make CsPbBr3 nanowires that are growing on c-plane sapphire. The spacing between the CsPbBr3 (110) wires is the same as the spacing between the sapphire (1100) wires. To make epitaxial halide perovskites, Van der Waals epitaxy was also used. Wang et al. used muscovite mica as a van der Waals medium to grow CsPbX3 (X = Cl, Br, I, or mixes of these) nanowires on top of each other. They used pole figure XRD to find that CsPbX3 (011) was parallel to mica (001) and that CsPbX3 (100) was 30° away from mica (020). PL studies that changed with temperature and looked at the lines’ shape showed unique photon transport behaviours.
Van der Waals epitaxy was also used to make other shapes, such as epitaxial flakes or thin films. Wang et al. also used CVD to make CsPbBr3 thin films that grew on graphene/NaCl and graphene/CaF2 surfaces over large areas. Scientists found that remote epitaxial CsPbBr3 films could hold charges for longer.
Besides ionic epitaxy, remote epitaxy was also achieved. Graphene and other 2D materials were put between the base and the flakes to act as buffer layers. With CVD, Jiang et al. made it possible for CsPbBr3 to grow on graphene/NaCl and graphene/CaF2 surfaces. Through reciprocal space mapping, we found that the epitaxial relationship is CsPbBr3 [100] || CaF2 [0].
Finally, ionic epitaxy of halide perovskites has been shown to work on a variety of surfaces and with different shapes. Halide perovskites have come a long way thanks to ionic epitaxy by vapour phase deposition, van der Waals/quasi-van der Waals epitaxy by vapour phase deposition, remote epitaxy by vapour phase deposition, and ionic epitaxy by solution method.
Strain Engineering
In electrical and electronic devices, strain engineering is a method that focusses on strain caused by epitaxial and thermal mismatch. Lattice imperfection and mismatch of thermal expansion factors can be used to create this method. Halide perovskites work in a similar way, but their shear moduli are much smaller than those of many common semiconductors, being in the range of several GPa. This means that the energy loss from deforming a ductile material is small, which means that halide perovskites are easily bendable by outside forces. This chapter talks about how strain engineering has changed halide perovskites over time.
Theoretical Progresses
A lot of different materials, like halide perovskite CsGeI3, Cs2AgInX6, and CsPbI1.5Br1.5, have physical qualities that are affected by strain. We used DFT calculations to look into how strain affects these materials. We found that the band gap can change from 0.73 eV to 2.30 eV when there is external strain. Materials with mild strain (−1%) are likely to have a bandgap of around 1.36 eV and better absorption.
Researchers also looked at Cs2AgInX6, and found that when it was compressed, it changed from having an indirect bandgap to a direct one. This meant that charge carriers could move around more easily, which made it a possible material for solar devices. Lin et al. looked into how biaxial strain affected the thermal and thermoelectric qualities of CsPbI1.5Br1.5. They discovered that the Seebeck coefficient could go up with external biaxial compressive/tensile strain, but the thermal conductivity would go down a lot with big strains (>6%).
It was also thought that stressed halide perovskites would have phase shifts and new phases. Zhang et al. used DFT calculations to guess what strain-stabilized phases would form in MAPbI3. The results showed that new phases would form when artificial frames bend and organic molecules rotate. Lee et al. said that MAPbI3 would go through a metallisation process when it was under atmospheric pressure. This would cause its space group to change from Fmmm to Im3. Jin et al. did first-principle studies and tight binding on the halide perovskites CsPbI3 and CsSnI3. They predicted that external pressure would allow a topological insulator phase to form. Leppert et al. looked into the Rashba effect caused by strain in APbX3. They found that the polarisation and size of the Rashba effect could be changed by applying strain from the outside. They also expected that MAPbI3 and CsPbI3 would have a Rashba-active polar phase with about 1% strain.
Experimental Progresses
Several techniques, such as epitaxy, heat stress, using phase transition substrates, electric field, optical doping, and chemical strains, have been shown to make halide perovskites stretch. Epitaxy and heat stress methods could be used to create strain on a chip level in halide perovskites. MAPbI3 and CsPbBrxI3−x quantum dots have been put under pressure using mechanical bends and an atomic force microscope (AFM) tip. These quantum dots may be good for quick science demonstrations in the lab, but they can’t be made bigger.
Chen et al. used ionic epitaxy to make strains in halide perovskite films at the epitaxial interface, which shows that strain engineering is possible. When they changed the x number, they could tune the strain value from 0% to 2.4%. It was discovered that the epitaxial strain can change the bandgap and make holes move around more easily. This makes the chosen halide perovskite phase more stable, which isn’t possible without the epitaxial interface. This project also shows that this epitaxial method can be used on a larger scale.
You can also make strain with Van der Waals epitaxy. Wang et al. demonstrated that the junction between MAPbBr3 and van der Waals mica could cause stress in the perovskite epilayer. The PL study showed that the bandgap of MAPbBr3 is changed by van der Waals epitaxial strain. This is likely because halide perovskites are mechanically soft.
To make molecular strain in halide perovskite, Zhu et al. were able to use differences in makeup. Theoretically, strained halide perovskites should have a smaller bandgap when they are compressed, higher charge carrier mobility when they are compressed, a larger Seebeck coefficient when they are compressed and stretched in two directions at the same time, a semiconductor-metal phase transition when the hydrostatic pressure is about 100 GPa, and the Rashba effect and polarisation in APbX3 should change because of the strain.
Researchers have shown that mechanically bending MAPbI3 can cause strain, that an AFM tip can cause local strain on CsPbBrxI3−x quantum dots, and that phase separation and the resulting composition gradient of halide perovskites (FAPbI3)0.85(MAPbBr3)0.15 can cause a strain gradient. Grazing impact X-ray diffraction (GIXRD) and TEM were used to measure strain.
Light exposure and an external electric field are two other ways that strain can be introduced into halide perovskite. In their study, Kim et al. found that halide perovskites (FAPbI3)85(MAPbBr3)15 would stretch when exposed to light or an electric field. This was linked to the formation of domains in these materials when they were lit up or pushed against something. Finally, Wang et al. used VO2±´’s metal-insulator phase shift to make CsPbBr3 flakes ∼1% more stretched.
Opportunities and Challenges
People are now more interested in halide perovskite for uses like light-emitting diodes, photodetectors, lasing, and tunnelling diodes after the success of III–V, Si/Ge, and perovskite oxide heterostructures and superlattices. Researchers can improve the optical performance of halide perovskites-based devices by creating a halide perovskite superlattice and carefully controlling the electronic levels of each layer. They can do this by using design principles that have been created for III–V quantum well and superlattice systems. But problems like not having enough electrical protective surfaces and halide perovskites that change a lot slow down the growth of these kinds of devices. To solve these problems, scientists need to learn more about how epitaxy works on an atomic level in halide perovskite and come up with better methods.
One way to make the strain engineering approach bigger is to choose the right supports and design the strain state of the halide perovskite films. But there aren’t many epitaxial substrates for halide perovskites right now. To make better theoretical and computer predictions and to test them in the real world, we may need to think about more than just the strain states and the different surfaces. The spin-orbit coupling in halide perovskites is very strong, so any mixed contact would cause big changes in their band structures. This would make the strain-tuned electronic structure even more complicated. Getting these problems solved would open up creation and production of new and useful perovskite devices in ways that have never been seen before.
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