Sunglae Cho

Ferromagnetic properties of Zn1−xMnxO epitaxial thin films

We report on ferromagnetic characteristics of Zn1−xMnxO (x=0.1 and 0.3) thin films grown on Al2O3(00⋅1) substrates using laser molecular-beam epitaxy. By increasing the Mn content, the films exhibited increases in both the c-axis lattice constant and fundamental band gap energy. The Curie temperature obtained from temperature-dependent magnetization curves was 45 K for the film with x=0.3, depending on the Mn composition in the films. The remanent magnetization and coercive field of Zn0.9Mn0.1O at 5 K were 0.9 emu/g and 300 Oe, respectively. For Zn0.7Mn0.3O, the remanent magnetization at 5 K increased to 3.4 emu/g.

Photoluminescence and ultraviolet lasing of polycrystalline ZnO thin films prepared by the oxidation of the metallic Zn

We report a simple method for preparing polycrystalline ZnO thin films with good luminescent properties: the oxidization of metallic Zn films. In photoluminescence (PL) studies at room temperature for wavelengths between 370 and 675 nm, we have observed a single exciton peak around 390 nm without any deep-level emission and a small PL full width at half maximum (23 meV), indicating that the concentrations of the defects responsible for the deep-level emissions are negligible. We have also observed optically pumped lasing action in these films. The threshold intensity for lasing was ∼9 MW/cm2.

Room-temperature ferromagnetism in Cr-doped GaN single crystals

We report on the discovery of a room-temperature ferromagnetism in Cr-doped GaN single crystals with a Tc=280 K. The addition of Cr into GaN single crystals grown by the flux method induces the lattice constant increase due to the larger Cr atomic radius. In x-ray photoelectron spectroscopy measurement, Cr 2p3/2 core-level exhibited spectra near 575.7 eV. This binding energy is similar to the reported value of CrN. The coercive field by magnetization–magnetic field (M–H) hysteresis curve at 250 K was 54 Oe. We verified the presence of ferromagnetic transition in the temperature dependence of the electrical resistance measurements. We discuss the ferromagnetic ordering in Cr-doped GaN bulk single crystals excluding the contribution of the substrate crystal structure.

ANTISITE DEFECTS OF BI2TE3 THIN FILMS

We have successfully grown Bi2Te3 thin films on CdTe(111)B using molecular-beam epitaxy. Structural and transport properties have been investigated using in situ reflection high-energy electron diffraction, θ–2θ x-ray diffraction analysis, thermopower, and Hall measurements. Both the crystallinity and the transport are found to be strongly affected by nonstoichiometry. The most stoichiometric sample had a high crystallinity, high thermopower, and high electron mobility. However, Bi2Te3 films with excess Te had a reduced lattice constant, poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility. All of these observations can be explained in terms of antisite defects in which excess Te occupies Bi lattice sites and behaves as a n-type dopant.

Structural and thermoelectric transport properties of Sb2Te3 thin films grown by molecular beam epitaxy

We have studied the structural and transport properties of Sb2Te3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition. Both the crystallinity and the transport properties are found to be strongly affected by nonstoichiometry. The most stoichiometric sample (prepared with a Te/Sb ratio of 3.6) had a high degree of crystallinity, high thermopower, and high carrier mobility. However, Sb2Te3 films with excess Sb or Te had poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility as a result of the formation of antisite defects. These antisite defects were able to be reduced by controlling the relative flow rate ratio of Te to Sb during growth.

Achieving ZT=2.2 with Bi-doped n-type SnSe single crystals

Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 1019 cm−3 at 773 K.

Large magnetoresistance in postannealed Bi thin films

We have observed a large increase in the magnetoresistance (MR) of molecular beam epitaxy grown Bi thin films, which were subjected to a postannealing procedure 3 °C below the Bi melting point. We have achieved an increase in the MR by a factor of 2560 at helium temperatures compared with of 343 for an as-grown film. The enhancement of the MR in the annealed films is due to higher electron and hole mobilities (μe≈1×106 cm2/V s at 5 K) relative to those of the as-grown films (μe≈9×104 cm2/V s at 5 K). The enhancement of the mobility in the annealed films is also supported by the observation of Shubnikov–de Haas oscillations.

Ferromagnetism in Cr-doped Ge

We have fabricated Cr-doped bulk Ge single crystal using the vertical gradient solidification method. The material shows ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements. A sample with x=0.01 was p type with nh=3×1017 cm−3 at 350 K. The measured magnetic moment per Cr was 0.83μB at 5 K.

Ferromagnetic properties in Cr, Fe-doped Ge single crystals

We have fabricated Cr and Fe-doped bulk Ge single crystal using the vertical gradient solidification method. The Cr-doped Ge single crystal showed ferromagnetic ordering at 126 K, as determined from temperature dependent magnetization and resistance measurements. The measured magnetic moment per Cr was 0.83 μB at 5 K. On the other hand, Fe0.05Ge0.95 showed ferromagnetic ordering at 233 K. The coersive field was 80 Oe at 5 K.

Substrate-modified ferrimagnetism in MnGa films

We report the substrate-modified magnetic properties of the CuAu type-I (L10) structure of MnxGa (1.2<x<1.5) films. The magnetic properties of the MnGa films differed greatly due to the influence of the substrate. The MnGa film is a hard ferrimagnet when grown on GaSb (111), becomes a soft ferrimagnet when grown on Al2O3 (0001), and exhibits an absence of a net magnetic moment when stabilized on a GaSb (100) substrate. This difference was attributed to the substrate, which forces MnGa film to be two-dimensionally stabilized in a different orientation and thus leads to the modified crystal symmetry and a change in the magnetic property. The results may be helpful for forming a comprehensive understanding of MnGa and for finding new applications in spintronic devices.