16:00-16:30
Electron Holography Observation of AlGaN/GaN Heterointerfaces
Dr. M. Takeguchi (ANCC, Advanced Transmission Electron Microscopy Group)
- AlGaN/GaN heterostructures have attracted much attention because of powerful applications to high-power, high-frequency and high-temperature devices. In addition to a large conduction band offset at interfaces, an intense internal electric field caused by both spontaneous and piezoelectric polarizations in an AlGaN layer grown on a GaN layer induces the formation of a two-dimensional electron gas (2DEG) at the interfaces. With increasing Al content, the internal electric field is expected to become larger, leading to the increase of the 2DEG density. However, such higher Al content increases the lattice mismatch of AlGaN/GaN and consequently forms a larger number of defects at the interface, which results in the degradation of the device performance.
Recently, in order to reduce the unfaborable defects by minimizing the lattice mismatch, the incorporation of indium in AlGaN (namely AlInGaN) has been proposed, in where the electrical properties of AlInGaN/GaN were better than that of AlGaN/GaN. However, structural and compositional evaluations of the AlInGaN layer and the effects of them on the 2DEG have not been examine microscopically so far.
In the present work, we characterize an atomic structure, compositional uniformity and charge distribution of 2DEG at the AlInGaN/GaN quantitatively by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and electron holography. In situ electron holography observation of potential distribution change under applied bias onto a AlGaN top surface will also be introduced.
16:30-17:00
Study for multipole in rare-earth compound
Dr. H. Suzuki (QBC, Neutron scattering group)
- Electrons in solid have three freedoms of charge, spin and orbital. In f electron, the strong coupling of spin and the orbital results in that the freedom of felectron is described as multipoles, such as electric quadrupole, magnetic octupole, electric hexadecapole and more higher order tensors. Normally or until recently, only magnetic dipole is used in the analysis of the properties in the rare-earth compounds with f-electrons. Because strong magnetic dipole interaction obscures the nature of the multipole. However, some crystal field ground states have no-magnetic dipole but electric quadrupole and magnetic octupole. In such a case, multipole makes an appearance at low temperatures and plays an important role in anomalies of the low temperature properties. I will introduce such examples and present my studies focusing the multipole in rare-earth compounds.