Myoung Hee Lee

Comparative magneto-optical investigation of d–d charge–transfer transitions in Fe3O4, CoFe2O4, and NiFe2O4

Magneto-optical transitions in CoFe2O4 and NiFe2O4 spinel ferrites are investigated in comparison with those in isostructural Fe3O4. The Kerr angle and ellipticity of the ferrites have been measured by magneto-optic Kerr effect (MOKE) spectroscopy in the 1.5–5 eV region. The results indicate that the d–d intervalence charge–transfer (IVCT) transitions, Co2+(t2g)→Fe3+(eg), and Ni2+(t2g)→Fe3+(eg), in the octahedral sites in CoFe2O4 and NiFe2O4 are located at about 3.6 and 4.5 eV, respectively, shifted to higher energies by about 1.5 and 2.4 eV, respectively, from the corresponding IVCT transition in Fe3O4, Fe2+(t2g)→Fe3+(eg). The MOKE spectra of CoFe2O4 and NiFe2O4 also indicate that the IVCT transitions, Co2+(t2g)→Fe3+(t2g) and Ni2+(t2g)→Fe3+(t2g), exist at about 2.2 and 3.1 eV, respectively. On the other hand, the intersublattice charge-transfer (ISCT) transitions between the tetrahedral and the octahedral Fe3+ sites in CoFe2O4 and NiFe2O4 are observed at about 2.6 and 4.0 eV, same as the corresponding IS...

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.

Investigation of quantum-confinement effect and Stokes shift in strained Ga1−xInxN/GaN double quantum wells by spectroscopic ellipsometry and photoluminescence

The evolution of the optical absorption and emission properties of strained Ga1−xInxN/GaN double quantum wells grown on (0001)-oriented sapphire substrates with varying well width has been investigated by spectroscopic ellipsometry (SE) and photoluminescence (PL). The SE result shows that the band-gap absorption energy of the wells shifts to higher energies as the well width decreases, indicating a quantum-confinement effect. The decreasing trend agrees with the result of one-dimensional square-well potential calculations. The PL result shows a Stokes shift of the emission edge from the corresponding absorption edge, attributable to the combined effects of the strain-induced piezoelectric potential and the In-fluctuation potential in the well. The blueshift of the emission edge with increasing PL excitation density further supports the existence of a strong piezoelectric field in the well.

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 55 °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 ...

Spectroscopic ellipsometry investigation of Ga1-xInxN/GaN single and double quantum well structures

Abstract We present the results of spectroscopic ellipsometry (SE) measurements of Ga 1− x In x N/GaN single and double quantum well samples grown on (0001)-oriented sapphire substrates with varying well and barrier widths. The band-gap energies of Ga 1− x In x N wells of the samples were estimated by an optical multilayer model analysis on the observed pseudodielectric constant spectra. The change of the band-gap energy with the well and barrier width can be explained by the quantum-confinement effect of the carriers in the well. The variation of the band-gap energy of the well with the barrier width is in good agreement with the result of theoretical calculations based on the periodic square-well potential model.

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].

Cathodoluminescence study of InxGa1−xN quantum wells

We studied the cathodoluminescence spectra of InxGa1−xN, focusing on the spatial variation of the spectra. Strong inhomogeneity of cathodoluminescence spectra was observed on 22 nm thick InxGa1−xN layers, where the peak energy varied up to 400 meV. In a double quantum well with a well width of 10 A, the luminescence peak in a broad area spectrum was at 3.18 eV, but on some areas the peak was at 3.07 eV, the size of the area being about 1 μm. The variation of the cathodoluminescence spectra clearly indicates the presence of in-plane potential fluctuation in some InxGa1−xN samples, although the cathodoluminescence spectra of most of our quantum wells optimized for devices were found to be uniform at least within the spatial resolution limit.

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.

Optical properties of Fe1−xSix alloys (x⩽0.1) studied by spectroscopic ellipsometry

Optical properties of dilute Fe1−xSix (x⩽0.1) alloys were studied by spectroscopic ellipsometry in the 1.5–6 eV photon energy region. The absorption strength of the alloys is found to decrease in the entire energy region as the Si composition increases. The interband-transition edges of Fe at about 2.5 eV in the minority-spin bands and at about 5 eV in the majority-spin bands shift to lower energies as the Si composition increases. The decrease in the absorption strength is interpreted as due to a decrease in the density of states of the Fe host and also due to a reduction of conduction-electron concentration in the alloys. The shifts of the 2.5 and 5 eV interband-transition edges can be explained in terms of a perturbative variation in the band structure of Fe induced by its alloying with Si.