Moon-Seog Jin

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.

PHOTOLUMINESCENCE PROPERTIES OF MGXZN1-XSE SINGLE CRYSTALS

MgxZn1−xSe single crystals were grown by the closed tube sublimation method. The MgxZn1−xSe single crystals crystallized into zincblende and wurtzite structures in the composition ranges of x=0.0–0.1 and x=0.2–0.6, respectively. Blue and violet emissions with LO phonon replica and self-activated emissions in the MgxZn1−xSe single crystals were observed at 10 K.

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.

Optical properties of CuAlSe2:Er3+ single crystal

Abstract CuAlSe 2 :Er 3+ single crystals were grown by the chemical transport reaction method. The single crystal crystallized in the chalcopyrite structure. The optical energy gap at 10 K was found to be 2.65 eV. The sharp impurity optical absorption peaks due to Er 3+ ion were observed at 544 nm, 657 nm, 980 nm, 990 nm, 1006 nm, 1014 nm and 1544 nm. The sharp photoluminescence peaks due to Er 3+ ion were observed at 544 nm, 654 nm, 661 nm and 667 nm, and the broad photoluminescence peak due to a donor-acceptor pair recombination was observed at 674 nm.

Blue photoluminescence of CdAl2S4 single crystal

The CdAl2S4 single crystal grown by the chemical transport reaction method in an excess sulfur atmosphere showed the intensive blue photoluminescence (PL) peaked at 462 nm at 280 K. The CdAl2S4 single crystal had the defect chalcopyrite structure, and the optical energy gap of the single crystal with the direct band structure was found to be 3.398 eV at 280 K, and to be 3.577 eV at 17 K.

Photoluminescence spectra of Zn1−xCdxAl2S4 (0.0⩽x⩽0.2;0.8⩽x⩽1.0) single crystals

The photoluminescence spectra as well as the lattice constants and band gaps for the mixed single crystals Zn1−xCdxAl2S4 with 0.0⩽x⩽0.2 and 0.8⩽x⩽1.0 grown by the chemical transport reaction method were investigated. The Zn1−xCdxAl2S4 crystals were a cubic spinel phase α in the range of 0.0⩽x⩽0.2 and a defect chalcopyrite in the range of 0.8⩽x⩽1.0, and showed a miscibility range from x=0.2 to x=0.8 in the composition dependence of the lattice constants and band gaps. We observed two emission bands consisting of a strong blue emission band and a weak broad emission band due to donor-acceptor pair recombinations in the crystals with a cubic spinel and a defect chalcopyrite structure. These emission bands showed a different behavior in their temperature and composition dependence. An energy band scheme for the radiative mechanism of the Zn1−xCdxAl2S4 was proposed on the basis of our experimental results along with the measurements of photoinduced current transient spectroscopy and thermoluminescence.

Photoluminescence spectra of Er3+-doped MgAl2S4 and CaAl2S4 single crystals

MgAl2S4, CaAl2S4, MgAl2S4:Er3+ and CaAl2S4:Er3+ single crystals were grown by the chemical transport reaction method. The single crystals crystallized in an orthorhombic structure. The single crystals had the direct energy band structure and their optical energy gaps were 4.334 eV, 4.213 eV, 4.835 eV and 4.716 eV for the MgAl2S4, MgAl2S4:Er3+, CaAl2S4, and CaAl2S4:Er3+ single crystals, respectively, at 13 K. Sharp emission peaks appeared in the MgAl2S4:Er3+ and CaAl2S4:Er3+ single crystals, and they were interpreted to originate from the Er3+ ion substituted Mg2+ and Ca2+ ions in the MgAl2S4:Er3+ and CaAl2S4:Er3+ single crystals.

Photoconductivity of TlGa0.8Sb0.2S2 single crystals

TlGa0.8Sb0.2S2 single crystals were grown by the Bridgman–Stockbarger method. The phtotoconductivity spectrum of the single crystal was measured at 20 K. Four peaks at 504 nm (2.460 eV), 525 nm (2.361 eV), 571 nm (2.171 eV), and 584 nm (2.123 eV) were observed in the spectrum. The high intensity-principal peak was observed at 525 nm and was described to be due to its indirect energy band gap. The peak at 504 nm was described to correspond to the direct energy band gap. The peaks at 571 and 584 nm could be attributed to the electron transition from the valence band to the donor levels with the activation energy of 0.186 and 0.234 eV, respectively.

Impurity optical absorption of Co2+-doped MgAl2Se4 and CaAl2Se4 single crystals

MgAl2Se4, MgAl2Se4:Co2+, CaAl2Se4 and CaAl2Se4:Co2+ single crystals were grown by the chemical transport reaction method in a closed system using iodine as a transport agent. The MgAl2Se4 and MgAl2Se4:Co2+ single crystals crystallized into a hexagonal (rhombohedral) structure, and the CaAl2Se4 and CaAl2Se4:Co2+ single crystals crystallized into an orthorhombic structure. The optical energy gaps of the MgAl2Se4, MgAl2Se4:Co2+, CaAl2Se4 and CaAl2Se4:Co2+ single crystals at 10 K were found to be 3.286 eV, 2.596 eV, 3.823 eV and 3.082 eV, respectively. The impurity optical absorption peaks were observed in the MgAl2Se4:Co2+ and CaAl2Se4:Co2+ single crystals and analysed to appear as a result of the electron transitions between energy levels of the Co2+ ion in sites of Td symmetry.

Photoluminescence Spectra of Zn 1- x Cd x Al 2 Se 4 Single Crystals

We investigated the photoluminescence spectra as well as the crystal structure and optical energy gaps of the Zn 1- x Cd x Al 2 Se 4 single crystals grown by the chemical transport reaction method. It was shown from the analysis of the observed x-ray diffraction patterns that these crystals have a defect chalcopyrite structure for a whole composition. The lattice constant a increases from 5.5561 A for x = 0.0 (ZnAl 2 Se 4 ) to 5.6361 A for x = 1.0 (CdAl 2 Se 4 ) with increasing x, whereas the lattice constant c decreases from 10.8890 A for x = 0.0 to 10.7194 A for x = 1.0. The optical energy gaps at 13 K were found to range from 3.082 eV ( x = 1.0) to 3.525 eV ( x = 0.0). The temperature dependence of the optical energy gaps was well fitted with the Varshni equation. We observed two emission bands consisting of a strong blue emission band and a weak broad emission band due to donor–acceptor pair recombination in the Zn 1- x Cd x Al 2 Se 4 for 0.0 ⩽ x ⩽ 1.0. These emission bands showed a red shift with increasing x. The energy band scheme for the radiative mechanism of the Zn 1- x Cd x Al 2 Se 4 was proposed on the basis of the photoluminescence thermal quenching analysis along with the measurements of photo-induced current transient spectroscopy. The proposed energy band model permits us to assign the observed emission bands.