Excitonic properties of high-quality iodide thin films grown by molecular beam epitaxy
Iodide semiconductors are quite promising materials for such optical devices as solar cells and light-emitting devices because of their strong optical absorption and high exciton stability. Moreover, iodides start to attract much attention as ideal platforms for exploring emergent quantum phenomena due to recently discovered topological magnetic structures and quantum transport. However, very few studies have been reported on the epitaxial films of iodides. We aim at establishing the growth method of high-quality iodide thin films employing molecular beam epitaxy (MBE), and exploring novel quantum physics and functionalities emerging at the interface of iodide heterostructures.
To begin with, we grew cuprous iodide (CuI) films on InAs substrates with excellent lattice matching, yielding in single-crystalline films having outstandingly high lattice coherence and atomically flat surfaces. The films exhibit extremely sharp free exciton emission at low temperatures that has never been observed in bulk single-crystals. This is a major step toward the development of iodide electronics.
Schematic of free exciton emission from epitaxially-grown CuI films
Quantum transport phenomena of spin polarized electrons in high mobility magnetic semiconductor EuTiO3
Magnetic semiconductors are a vital component in future spintronics. Conventional magnetic semiconductors contain a large amount of magnetic impurities as the dopant, resulting in the low mobility. Therefore, it is difficult to observe interference effects of spin polarized electrons such as Shubnikov- de Haas (SdH) oscillation. Here, we have developed a gas source molecular beam epitaxy system, yielding in high crystalline quality films of La doped EuTiO3. The maximum mobility reaches 3,200 cm2V-1s-1 at 2 K, showing clear quantum oscillations in magnetoresistance. Using first-principles calculations, we show that the observed SdH oscillations originate genuinely from Ti 3d-t2g states which are fully spin-polarized due to their energetical proximity to the in-gap Eu 4f bands. This suggests that EuTiO3 film is an ideal magnetic semiconductor, offering a fertile field to explore quantum phenomena.
Schematic of an interference effect of spin polarized electrons
Generation and nonlocal spreading of shift current under local photoexcitation
Noncentrosymmetric materials exhibit a generation of photocurrent without an external electric field, called the bulk photovoltaic effect. In this study, we have revealed the mechanism of the photocurrent generation and spreading under local photoexcitation by potentiometric measurements for a prototypical ferroelectric semiconductor SbSI.
We shined a focused laser light on a SbSI single crystal, and measured the voltage inside and outside of photoirradiated region simultaneously. The results indicate that the photocurrent emerging in photoirradiated region is a less dissipative current driven by the Berry phase of electron’s wavefunction, termed shift current, whereas current spread outside as a dissipative current driven by the internal field. On the basis of result, we determined the equivalent circuit model to simulate the bulk photovoltaic effect in various devise structures. We have also succeeded in fabricating thin films of SbSI with aligned polarization axis by molecular beam epitaxy.
Schematic of photocurrent generation under local photoexcitation in ferroelectric semiconductor SbSI
Control of conduction electrons by magnetic monopoles in a magnetic semiconductor
Magnetic semiconductor is one of the candidates for novel spintronics devices with low power consumption because both magnetic and transport properties can be controlled. Because the Hall effect of magnetic semiconductors can be electrically tuned, magnetic semiconductors have attracted considerable interest from the view point of the application.
Band crossing called as Weyl nodes which generate magnetic monopoles in momentum space is known to be one of the origin of anomalous Hall effect (AHE). We discovered a new phenomenon of AHE in a high electron mobility oxide magnetic semiconductor, EuTiO3. During the magnetization process of the spin moment on Europium (Eu) under applied magnetic field, it was found that the AHE is not proportional to the magnetization as usual magnets. We revealed that changing the energy position of Weyl nodes that create magnetic monopoles in momentum space by tiny change of Zeeman splitting in the magnetization process dramatically affect the trajectory of electron conductions.
Schematic of the trajectory of electron conduction modified by the monopole (red ball) in momentum space
Electron–electron and spin–orbit interactions compete to control the electron
