Yunki Kim

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.

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.

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.

Scanning plasmon optical microscope

A new scanning near‐field optical microscopy based on the surface plasmon resonance is presented. Enhanced fields are localized at individual surface irregularities by the scattering of plasmons and the scattered plasmons produce a conical radiation. Variations in the conical radiation intensity caused by interactions between a raster‐scanned probe tip and the enhanced fields are recorded and related to the surface topography, providing an optical image with lateral resolution exceeding the diffraction limit. This technique is compared to complementary atomic force microscopy and scanning tunneling microscopy measurements and potential advantages are discussed.

Surface plasmon scanning near-field optical microscopy

We have constructed a surface plasmon scanning near-field optical microscope. Surface plasmons (SP) are excited by laser light at the silver–air interface in the Kretschmann attenuated total reflection geometry. A scanning tunneling microscope or atomic force microscope tip penetrates the evanescent electromagnetic field and serves as a scatterer of the SP’s which, in turn, radiate conical electromagnetic waves. The conical radiation is collected on a photodiode by an annular mirror or a fiber-optic bundle. It is shown that the SP’s are predominantly scattered in the forward direction. The conical radiation intensity is recorded as a function of the tip position, creating an optical image of the silver surface. It is demonstrated that the technique’s spatial resolution is determined by the tip size, rather than the wavelength of light, though the interference of the scattered SP’s creates interference patterns that may present a problem.

Scanning plasmon optical microscope operation in atomic force microscope mode

A scanning plasmon optical microscope has been operated with an atomic force microscope rather than a scanning tunnel microscope to scan a surface. The instrument produced a nanoscale optical image of dyemolecule crystals adsorbed on a silver film. Images of dye-molecule crystal islands are presented.

Large magnetoresistance in post-annealed polycrystalline and epitaxial Bi thin films

We have observed a large increase in the magnetoresistance (MR) of polycrystalline and epitaxial thin Bi films, which were subjected to a post-annealing procedure at 3°C below the Bi melting point. We have achieved an increase in the MR by a factor of 2560 at 5 K as compared with 343 for an as-grown epitaxial film due to enhanced carrier mobilities.