Kwang Joo Kim

Optical constants and electronic interband transitions of disordered GaAs1−xPx alloys

We report dielectric function spectra of disordered GaAs1−xPx alloys measured using spectroscopic ellipsometry at room temperature in the 1.5–5.5 eV photon energy region. Energies of the interband-transition edges, E0, E1, E0′, and E2, of the alloys were determined by line-shape analyses on their dielectric functions. It is found that the E1, E0′, and E2 energies of the alloys increase linearly with the increase of the P composition between those of GaAs and GaP while the increase of their E0 energy shows finite positive bowing. The energy separation between the E1 and its spin-orbit-split E1+Δ1 edges of the alloys decreases linearly with the increase of the P composition with their lifetime broadenings becoming larger than those of the binary compounds by more than 20%. The effect of compositional disorder on the electronic structure of GaAs1−xPx alloy system was discussed based on the present optical data.

Effects of Cr doping on Magnetic Properties of Inverse Spinel CoFe₂O₄ Thin Films

Variation of magnetic properties through Cr substitution for Co in inverse-spinel CoFe₂O₄ has been investigated by vibrating-sample magnetometry (VSM) and conversion electron Mossbauer spectroscopy (CEMS). Cr x Co 1-x Fe₂O₄ samples were prepared as thin films by a sol-gel method. The lattice constant of the CrxCo1-x- Fe₂O₄ samples was found to remain unchanged, explainable in terms of a reduction of tetrahedral Fe 3+ ion to Fe 2+ due to substitution of Cr 3+ ion into octahedral Co 2+ site. The existence of the tetrahedral Fe 2+ ions in Cr x Co 1-x Fe₂O₄ was confirmed by CEMS analysis. Room-temperature magnetic hysteresis curves for the Cr x Co 1-x Fe₂O₄ films measured by VSM revealed that the saturation magnetization (M S ) increases by Cr doping. The MS is maximized when x = 0.1 and decreases for higher x but is still bigger than that of CoFe₂O₄. The increase of M S can be explained partly by the reduction of the tetrahedral Fe 3+ ion to Fe 2+ .

Strain effects in CdTe/Si heterostructures

Photoluminescence (PL) and spectroscopic ellipsometry measurements on CdTe/Si strained heterostructures grown by molecular beam epitaxy were carried out to investigate the effect of the strain and the dependence of the strain on the Si tilted substrates. The results of the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and the bound-exciton peak for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value. When rapid thermal annealing (RTA) was performed at 55u2009°C, the PL spectra showed that the relative intensity ratio between the peak at 1.452 eV and that at 1.574 eV for the CdTe epilayer grown on the Si (100) 8° tilted substrate had a minimum value and that the strain for the CdTe epilayer grown on the Si (100) 1° tilted substrate had a minimum value. Spectroscopic ellipsometry measurements showed that the spectrum of the dielectric constant of ...

Magnetic and Electronic Properties of Reduced Rutile Ti 1-x MnxO 2-δ Thin Films

Magnetic and electronic properties of reduced rutile titanium dioxide (TiO 2-δ ) thin films doped by Mn have been investigated. The present sol-gel-grown semiconducting TiO 2-δ :Mn films exhibit a ferromagnetic behavior at room temperature for a limited range of Mn content. The Mn-doped films have p-type electrical conductivity with the carrier concentration near 1019 cm ?3 . The observed room-temperature ferromagnetism is believed to be intrinsic but not related to free carriers such as holes. Oxygen vacancies are likely to contribute to the room-temperature ferromagnetism?trapped carriers in oxygen vacancies can mediate a ferromagnetic coupling between neighboring Mn 3+ ions. The energy band-gap change due to the Mn doping measured by spectroscopic ellipsometry exhibits a red-shift compared to that of the undoped sample at low Mn content. It is explainable in terms of strong spin-exchange interactions between Mn ion and the carrier.

Optical properties of Zn1−xMgxSeGaAs(0 0 1) epitaxial films studied by spectroscopic ellipsometry

Abstract Optical properties of zine-blende Zn 1− x Mg x Se ( x ≤ 0.21) alloys grown epitaxially on GaAs(0 0 1) by molecular beam epitaxy have been studied using spectroscopic ellipsometry at room temperature in the 1.5–5.5 eV photon energy region. Using the measured dielectric functions and their second-derivative spectra, the energies of the interband critical-point edges, E 0 , E 1 and their spin-orbit splittings, have been determined. These edges are found to shift linearly to higher energies as the Mg composition increases. The rates of increase of the E 0 and E 1 edges of the alloys are 10 and 4.7 meV/at.% respectively and their spin-orbit splittings remain at about the same as those of ZnSe. The increase rate of the E 0 edge of the present zinc-blende alloys is found to be smaller by about 20% than that of Bridgman-grown bulk sphalerite alloys [ J. Crystal Growth , 159 , 1996, 167].

Structural Phase Transition, Electronic Structure, and Magnetic Properties of Sol-gel-prepared Inverse-spinel Nickel-ferrites Thin Films

X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM) were used to investigate the influence of Ni ions on the structural, electronic, and magnetic properties of nickelferrites (NixFe 3-x O₄). Spinel NixFe 3- xO₄ (x ≤ 0.96) samples were prepared as polycrystalline thin films on Al₂O₃ (0001) substrates, using a sol-gel method. XRD patterns of the nickel-ferrites indicate that as the Ni composition increases (x > 0.3), a structural phase transition takes place from cubic to tetragonal lattice. The XPS results imply that the Ni ions in NixFe 3- xO₄ substitute for the octahedral sites of the spinel lattice, mostly with the ionic valence of +2. The minority-spin d-electrons of the Ni 2+ ions are mainly distributed below the Fermi level (E F ), at around 3 eV; while those of the Fe 2+ ions are distributed closer to E F (~1 eV below E F ). The magnetic hysteresis curves of the NixFe 3- xO₄ films measured by VSM show that as x increases, the saturation magnetization (M s ) linearly decreases. The decreasing trend is primarily attributable to the decrease in net spin magnetic moment, by the Ni 2+ (2 μB) substitution for octahedral Fe 2+ (4 μB) site.

Thickness-dependent strain relaxation in Si0.8Ge0.2Si(001) epitaxial films studied by spectroscopic ellipsometry

Abstract Variation of dielectric response function of Si 0.8 Ge 0.2 Si (001) epitaxial films for varying layer thickness was detected by spectroscopic ellipsometry measurements. As the epilayer thickness increases, interband-transition structures near 3.3 eV by E′0, E1 and E1 + Δ1 edges and near 4.2 eV by E2 edges of Si0.8Ge0.2 are found to shift gradually to lower energies indicating gradual relaxation of lattice-mismatch-induced strain in the epilayer. Result of a line-shape analysis on the dielectric functions of the alloys indicates that the rates of the strain-induced energy shifts of the E1 and E1 + Δ1 edges are larger than those of the E′0 and E2 and also the energy splitting between the E1 and E1 + Δ1 increases as the strain in the film increases. The strain dependence of the E1 and E1 + Δ1 energies of the alloys agrees fairly with the result of a theoretical estimation based on the deformation potential theory.

Spectroscopic ellipsometry study on strain relaxation of CdTeGaAs(0 0 1) epitaxial films

Abstract Energy shifts of interband-transition edges with strain relaxation of CdTe(1 1 1) epitaxial films grown on GaAs(0 0 1) substrates were detected using spectroscopic ellipsometry measurements at various layer thicknesses. The E 1 , E 1 + Δ 1 and E 2 edges for the films were found to shift to higher energies from those of bulk CdTe at the layer thicknesses below 1.3 μm. Result of a quantitative line-shape analysis on the transition edges indicates that the strain-induced energy shifts are proportional to the strength of the residual in-plane compressive strain in the film.

Evolution of magnetic and optical properties in spinel ferrite Fe/sub x/Co/sub 3-x/O/sub 4/ thin film

The spinel ferrite Fe/sub x/Co/sub 3-x/O/sub 4/ thin film was prepared on silicon substrate by sol-gel method. The evolution of structural, magnetic, and optical properties of Fe composition in the film was investigated by vibrating sample magnetometer (VSM), Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry.

Diluted ferromagnetic properties in Fe- and Co-doped TiO/sub 2-/spl delta// thin films

Magnetic and electronic properties of Fe- and Co-doped TiO/sub 2-/spl delta// thin films are investigated by vibrating sample magnetometer (VSM), Mossbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS). When the precursor films are annealed in vacuum, the resulting oxygen-deficient TiO/sub 2-/spl delta//:Fe and TiO/sub 2-/spl delta//:Co films are found to become semiconducting with p-type carriers in the 10/sup 18/ cm/sup -3/ range. The oxygen-deficient films go through conductivity transitions from n-type to p-type by Fe and Co doping. Results of VSM measurements show that ferromagnetism is exhibited at room temperature. XPS measurements on the thin films reveal that Fe ions have both Fe/sup 2+/ and Fe/sup 3+/ ionic valences while Co ions have Co/sup 2+/ mostly. Also, the density of Fe/sup 2+/ ions is found to decrease after annealing.