Hikari Shinya

First principles studies of GeTe based dilute magnetic semiconductors

The electronic structure and magnetic properties of GeTe-based dilute magnetic semiconductors (DMS) are investigated by the Korringa-Kohn-Rostoker Greens function method and the projector augmented wave method. Our calculations for the formation energies of transition metal impurities (TM) in GeTe indicate that the solubilities of TM are quite high compared to typical III-V and II-VI based DMS and that the TM doped GeTe has a possibility of room temperature ferromagnetism with high impurity concentrations. The high solubilities originate from the fact that the top of the valence bands of GeTe consists of the Te-5p anti-bonding states which are favorable to acceptor doping. (Ge, Cr)Te system shows strong ferromagnetic interaction by the double exchange mechanism and is a good candidate for DMS with high Curie temperature. Additionally, in the case of (Ge, Mn)Te with the d(5) configuration, by introducing the Ge vacancies the p-d exchange interaction is activated and it dominates the antiferromagnetic superexchange, resulting in ferromagnetic exchange interactions between Mn. This explains recent experimental results reasonably. Based on the accurate estimation of the Curie temperatures by Monte Carlo simulation for the classical Heisenberg model with the calculated exchange coupling constants, we discuss the relevance of the TM doped GeTe for semiconductor spintronics.

First-principles investigations of defect and phase stabilities in thermoelectric (GeTe)x(AgSbTe2)1−x

We focus on the defect and phase stabilities in the pseudo binary alloy (GeTe)x(AgSbTe2)1−x (TAGS; tellurium antimony germanium silver). TAGS is expected to be a high effective thermoelectric material because its thermal conductivity shows anomalous behavior around the concentration of x = 0.8. The origin of the anomalous thermal conductivity and the stable structure in TAGS have not been well understood. To clarify the stable structure, we calculate the formation energies of the point and complex defects. It is found that the chain structure of Ag–Te–Sb has a lower formation energy in GeTe, and the system becomes more stable by assembling the Ag–Te–Sb chain structures. Moreover, the calculated mixing energy also shows that the TAGS system tends to undergo phase separation. In such structures, the grain boundary plays an important role in inducing large phonon scattering, leading to the thermal conductivity reduction.

Inherent instability by antibonding coupling in AgSbTe2

In the present paper, an inherent instability in the ternary chalcogenide compound AgSbTe2 is described from the electronic structure viewpoint. Our calculations, which are based on the cluster expansion method, suggest nine stable crystal structures involving the most stable structure with symmetry. The effective pair interactions calculated by the generalized perturbation method point out that the stability of these structures originates from the number of linear arrangements of the Ag–Te–Sb atomic bonds. Moreover, it is found that AgSbTe2 has a special electronic structure, where the dominant components of the top of the valence band are the Te-5p antibonding states. Such an antibonding contribution leads to an inherent instability, such that the system spontaneously forms various mutation phases caused by charge-compensated defect complexes. We propose that these mutation phases play an important role in the thermal conductivity and thermoelectric efficiency in AgSbTe2.

First-principles prediction of the control of magnetic properties in Fe-doped GaSb and InSb

Recently, Fe-doped semiconductors have been attracting much attention as ferromagnetic semiconductors due to the possibility that they may exhibit high Curie temperatures and low power consumption and that they may be useful for high-speed spin devices. High Curie temperature ferromagnetism has been observed in Fe-doped InAs, from which both n- and p-type ferromagnetic semiconductors can be fabricated. In order to obtain a higher Curie temperature than that of (In, Fe)As, we have focused on GaSb and InSb as host semiconductors. We have investigated their electronic structures, magnetic properties, and structural stability by using the Korringa-Kohn-Rostoker Greens function method within density functional theory. We have found that (Ga, Fe)Sb and (In, Fe)Sb show complex magnetic properties, which are determined by the correlation between magnetic exchange coupling constants and chemical pair interactions. Isoelectronic Fe-doped GaSb and InSb have strong antiferromagnetic interactions due to the super-exchange mechanism. By shifting the Fermi level–i.e., by n- or p-type doping–(Ga, Fe)Sb and (In, Fe)Sb can be made to undergo a magnetic transition from antiferromagnetic to ferromagnetic ordering. This transition can be well understood in terms of the Alexander-Anderson-Moriya mechanism. Our calculations indicate the possibility of manipulating (Ga, Fe)Sb and (In, Fe)Sb to achieve high Curie temperatures.Recently, Fe-doped semiconductors have been attracting much attention as ferromagnetic semiconductors due to the possibility that they may exhibit high Curie temperatures and low power consumption and that they may be useful for high-speed spin devices. High Curie temperature ferromagnetism has been observed in Fe-doped InAs, from which both n- and p-type ferromagnetic semiconductors can be fabricated. In order to obtain a higher Curie temperature than that of (In, Fe)As, we have focused on GaSb and InSb as host semiconductors. We have investigated their electronic structures, magnetic properties, and structural stability by using the Korringa-Kohn-Rostoker Greens function method within density functional theory. We have found that (Ga, Fe)Sb and (In, Fe)Sb show complex magnetic properties, which are determined by the correlation between magnetic exchange coupling constants and chemical pair interactions. Isoelectronic Fe-doped GaSb and InSb have strong antiferromagnetic interactions due to the super-exc...

Theoretical investigation of phase separation in thermoelectric AgSbTe2

The pseudo-binary alloy (GeTe) x (AgSbTe2)1− x , which is one of the highly efficient thermoelectric materials, shows the anomalous lattice thermal conductivity and figure of merit ZT, when the GeTe concentration (x) is around 0.8. These singularities are considered to be due to the inherent instability of AgSbTe2, derived from the antibonding contributions at the valence band maximum. Here, we investigate the defect physics in AgSbTe2 by first-principles electronic structure calculations. It is found, from our calculations, that one must carefully control the atomic chemical potentials of Ag and Sb to grow AgSbTe2 in thermal equilibrium, and that a defect complex 2VAg+SbAg, which has a low formation energy, is easily generated under the Ag-poor crystal growth condition. Additionally, we propose spinodal nanodecomposition between AgSbTe2 and ordered defect compounds using 2VAg+SbAg, which is a crucial rule for the experimentally observed unusual thermoelectric properties.