Sung-Hyu Choe

Optical properties of β-In2S3 and β-In2S3:Co2+ single crystals

β-In2S3 and β-In2S3:Co2+ single crystals were grown by the chemical transport reaction method using In2S3, S, and ZnS as starting materials and (ZnCl2 + I2) as a transport agent. The single crystals crystallized into a tetragonal structure. The indirect optical energy band gaps of the single crystals at 298 K were found to be 2.240 eV and 1.814 eV for β-In2S3 and β-In2S3:Co2+, respectively. The direct optical energy band gaps were found to be 2.639 eV and 2.175 eV for β-In2S3 and β-In2S3:Co2+, respectively. Impurity optical absorption peaks were observed for the β-In2S3:Co2+ single crystal. These impurity absorption peaks were assigned, based on the crystal field theory, to the electron transitions between the energy levels of the Co2+ ion sited in Td symmetry.

Optical properties of undoped and Co-DOPED CdIn2Se4 single crystals

Abstract CdIn 2 Se 4 and CdIn 2 Se 4 :Co 2+ single crystals were grown by the vertical Bridgman technique. The grown single crystals were pseudocubic in structure and their lattice parameters were a = 5.811 A for CdIn 2 Se 4 and a = 5.795 A for CdIn 2 Se 4 :Co 2+ . The direct and indirect energy gaps are found to be 1.67 and 1.55eV, respectively for, CdIn 2 Se 4 , and 1.58 and 1.27eV for CdIn 2 Se 4 :Co 2+ at 293 K. In CdIn 2 Se 4 :Co 2+ , nine impurity optical absorption peaks due to cobalt are observed. These peaks can be attributed to the electronic transitions between the split energy levels of Co 2+ ions located at the T d symmetry site of CdIn 2 Se 4 host lattice. The crystal field, Racah, and spin-orbit coupling parameters are given as Dq = 407cm −1 , B = 553 cm −1 and λ = −187cm −1 , respectively.

Optical properties of undoped and Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals

The optical energy band gaps of BaAl2S4 and BaAl2Se4 single crystals at 300 K were found to be 3.98 and 3.35 eV, respectively, and the optical energy band gaps of Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 single crystals were smaller than those of the undoped single crystals. Photoluminescence spectra peaked at 459 and 601 nm in the BaAl2S4 and at 486 and 652 nm in the BaAl2Se4. The photoluminescence emission peaks were attributed to donor–acceptor pair recombinations. Photoluminescence spectra of the Ho3+, Er3+, and Tm3+-doped BaAl2S4 and BaAl2Se4 at 5 K were measured in the wavelength range of 400–900 nm. Sharp emission peaks due to Ho3+, Er3+, and Tm3+ ions were observed and their transition mechanisms were proposed.

Optical properties of ZnAl2Se4, ZnAl2Se4:Co2+, and ZnAl2Se4:Er3+ single crystals

Single crystals of ZnAl2Se4, ZnAl2Se4:Co2+, and ZnAl2Se4:Er3+ were grown by the chemical transport reaction method using iodine as a transporting material. It has been shown that these single crystals have a defect chalcopyrite structure and a direct band gap. The direct band gap at 13 K has been found to be 3.525 eV for ZnAl2Se4, 2.952 eV for ZnAl2Se4:Co2+, and 3.283 eV for ZnAl2Se4:Er3+, respectively. Impurity optical absorption spectra of ZnAl2Se4:Co2+ showed absorption characteristics of Co2+ ions due to electron transitions between their split energy levels under a Td symmetry crystal field. Photoluminescence spectra at 13 K of ZnAl2Se4 showed two emission bands centered at 470 and 799 nm. For ZnAl2Se4:Er3+, we observed sharp photoluminescence peaks due to emission transitions between the energy levels of Er3+ ions with S4 symmetry sites.

Optical properties of Ho3+-, Er3+-, and Tm3+-doped BaIn2S4 and BaIn2Se4 single crystals

BaIn 2 S 4 , BaIn 2 S 4 :Ho 3 + , BaIn 2 S 4 :Er 3 + , BaIn 2 S 4 :Tm 3 + , BaIn 2 Se 4 , BaIn 2 Se 4 :Ho 3 + , BaIn 2 Se 4 :Er 3 + , and BaIn 2 Se 4 :Tm 3 + single crystals were grown by the chemical transport reaction method. The optical energy gap of the single crystals was found to be 3.057, 2.987, 2.967, 2.907, 2.625, 2.545, 2.515, and 2.415 eV, respectively, at 11 K. The temperature dependence of the optical energy gap was well fitted by the Varshni equation. Broad emission peaks were observed in the photoluminescence spectra of the single crystals. They were assigned to donor-acceptor pair recombination. Sharp emission peaks were observed in the doped single crystals. They were attributed to be due to radiation recombination between the Stark levels of the Ho 3 + , Er 3 + , and Tm 3 + ions sited in C 1 symmetry.

Growth and characterization of MgxCd1 − xSe single crystals

Single crystals of MgxCd1 − xSe over the range 0 ⩽ x ⩽ 0.46 were grown by the modified chemical transport reaction technique. The structural and optical properties of the MgxCd1 − xSe single crystals were investigated at room temperature from X-ray powder diffraction and optical absorption measurements. It has been found that the MgxCd1 − xSe has a wurtzite structure over the investigated composition range. The lattice constants of a and c decrease linearly with increasing a composition x. The band gap increases linearly with increasing a composition x, which is represented by a linear relationship Eg(x) = 1.742 + 1.813 x.

Optical properties of MnAl2S4 and MnAl2Se4 single crystals

MnAl2S4 and MnAl2Se4 single crystals were grown by the chemical transport reaction method. Optical energy gaps of the MnAl2S4 and MnAl2Se4 single crystals were 3.75 and 3.21 eV, respectively, at 300 K. Emission peaks due to donor-acceptor pair recombinations were observed at 450 and 603 nm in the MnAl2S4 single crystal and at 488 and 655 nm in the MnAl2Se4 single crystal. Optical absorption peaks and emission peaks described as appearing due to Mn2+ ion sited in Td symmetry were observed at 414, 450, 482, and 527 nm in the MnAl2S4 single crystal and at 416, 455, 488, and 532 nm in the MnAl2Se4 single crystal.

Photoluminescence spectra of ZnAl2Se4-4xS4x single crystals

Single crystals of ZnAl2Se4-4xS4x quaternary solid solution were grown by the chemical transport reaction method. The ZnAl2Se4-4xS4x single crystals were found to have a defect chalcopyrite structure in the region of 0.0≤x≤0.2 and a cubic spinel in the region of 0.8≤x≤1.0. It was also shown in these regions that the lattice constants and optical energy gaps depend on a composition x. A miscibility region existed in the region of 0.2<x<0.8. The photoluminescence spectra for the crystals with 0.0≤x≤0.2 and 0.8≤x≤1.0 showed a strong blue emission band and a weak broad emission band due to donor-acceptor pair recombination at low temperatures. A simple energy band scheme for the radiative mechanism in the ZnAl2Se4-4xS4x was proposed on the basis of our experimental results along with the measurements of thermoluminescence and photo-induced current transient spectroscopy.

Electronic energy levels of Er3+ in ZnInGaS4:Er3+ single crystal and its site symmetry

ZnInGaS4 and ZnInGaS4:Er3+ single crystals were grown using the CTR technique. The single crystals grown have a layered crystal structure. These compounds have both a direct and an indirect energy gap. Eight distinctive emission peaks of the ZnInGaS4:Er3+ single crystal were observed at 10 K. These photoluminescence peaks were attributed to the radioactive decay among the 4f split electron energy levels of the Er3+ ions that occupy the C2v site symmetry of the ZnInGaS4 single crystal host lattice.

Photoluminescence spectra of undoped and Er3+-doped MgAl2Se4 and CaAl2Se4 single crystals

MgAl2Se4, MgAl2Se4:Er3+, CaAl2Se4, and CaAl2Se4:Er3+ single crystals were grown by the chemical transport reaction method. The single crystals had the direct energy band gap and their optical energy gaps were 3.286, 2.855, 3.823, and 3.525 eV for the MgAl2Se4, MgAl2Se4:Er3+, CaAl2Se4, and CaAl2Se4:Er3+ single crystals, respectively, at 13 K. Broad photoluminescence spectra peaked at 477 and 672 nm for the MgAl2Se4 single crystal and at 459 and 633 nm for the CaAl2Se4 single crystal were obtained. Sharp emission peaks due to the Er3+ ion in the MgAl2Se4:Er3+ and CaAl2Se4:Er3+ single crystals were observed.