Wha-Tek Kim

Optical properties of SbSi: Co and SbSeI: Co single crystals

Abstract Single crystals of SbSI: Co and SbSeI: Co were prepared by the Bridgman technique. The optical absorption spectra of these single crystals were investigated in the wavelength range 500–2500 nm. We determined the optical energy gaps of these compounds, and identified the electric states of cobalt impurity in these single crystals. We observed the impurity absorption peaks at energies of 14000-4000 cm−1 range due to cobalt impurity. It can be explained that the impurity absorption peaks are attributed to the electronic transitions between the energy levels of cobalt ions, which are sited in tetrahedral (Td) symmetry of crystal lattice as Co3+ and Co2+ ions. The crystal field parameters Dq are given by 697 cm−1 for Co3+ and 454 cm−1 for Co2+ ions in SbSI: Co single crystals, and also 847 cm−1 for Co3+ and 452 cm−1 Co2+ ions in SbSeI: Co single crystals.

Optical properties of Ga2Se3:Co2+ single crystals

Abstract Optical absorption of Ga 2 Se 3 :CO 2+ single crystals grown by the Bridgman technique has been investigated in the region of the fundamental absorption edge and the near-infrared over the temperature range 20–300 K. The direct band gap of the undoped Ga 2 Se 3 single crystals is 1.92 eV at room temperature. The direct band gap of the Co-doped Ga 2 Se 3 is decreased with a increasing concentration of Co-dopant, and then that of the 3.5 mole % Co-doped Ga 2 Se 3 is given by 1.40 eV. The temperature dependence of the optical energy gaps is well explained by the Varshni equation. We observed three impurity absorption peaks at 4032, 6184, and 13831 cm −1 in the absorption spectrum of the Ga 2 Se 3 :Co 2+ (0.1 mole %) single crystals. We can see that these impurity absorption peaks are assigned to the allowed electron transitions from the ground state 4 A 2 ( 4 F ) to the excited states 4 T 2 ( 4 F ), 4 T 1 ( 4 F ), and 4 T 1 ( 4 P ) of Co 2+ ions in T d symmetry of Ga 2 Se 3 host lattice. The crystal field parameter Dq and the Racah parameter B are given by 403 and 503 cm −1 , respectively.

Structural and optical properties of CoxIn2S3+x thin films grown by spray pyrolysis method

CoxIn2S3+x thin films with various relative compositions were grown on thoroughly cleaned glass plates by the spray pyrolysis method. The substrate temperature during growth was maintained at 270 °C and the film then annealed in a vacuum chamber at 2×10−5 Torr at 500 °C for 30 min. The grown CoxIn2S3+x thin films, which had a composition of x=0.0–0.6, showed the tetragonal structure of In2S3, but above x=0.6 the films were amorphous. The energy gap for these films decreased with increasing x composition, and the impurity absorption spectra in near infrared region, ascribed to Co2+ ions, were observed at 13 333, 5970, and 4166 cm−1, which are the 4A2(F)−4T1(P), 4A2(F)−4T1(F), and 4A2(F)−4T2(F) transitions, respectively.

Optical absorption of MgGa2Se4:Ni2+ single crystals

Abstract Optical absorption of MgGa2Se4:Ni2+ single crystals have been investigated in the visible and near-infrared region. We observed optical absorption peaks at 4597, 5807, 8779, 10416, 11876 and 11936 cm−1 due to nickel impurity and at 9088 cm−1 due to phonon of MgGa2Se4. The three absorption bands at 4597, 8779 and 11876 cm−1 among them are attributed to the crystal field transitions between the ground state 3T1(F) and the excited states 3T2(F), 3A2(F) and 3T1(P) of Ni2+ ion with Td symmetry of MgGa2Se4 host. On the other hand, the two absorption bands at 5807 and 10416 cm−1 are assigned to the charge-transfer transitions of Ni 3+ + hv → Ni 2+ ( 3 T 1 (F)) + hole and Ni 3+ + hv → Ni 2+ ( 3 T 2 (F)) + hole , respectively. The shoulder structure at 11936 cm−1 is also ascribed to the charge-transfer transition of Ni 2+ ( 3 T 1 (F)) + hv → Ni 3+ + electron. From the optical absorption results, the energy levels of Ni2+ ion are deduced and illustrated schematically relative to the band edges of MgGa2Se4.

Optical properties of Cd2AgGaSe4 and Cd2AgGaSe4:Co2+ crystals

Abstract Structural and optical properties ofCd 2 AgGaSe 4 and Cd 2 AgGaSe 4 :Co 2+ crystals are investigated. It is determined by the X-ray diffraction analysis that these crystals have a wurtzite structure with lattice parameters a = 4.245 A and c = 7.230 A for undoped Cd 2 AgGaSe 4 and a = 4.234 A and c = 7.343 A for Cd 2 AgGaSe 4 :Co 2+ . At 293 K the direct band gap is found to be 1.51 eV for undoped Cd 2 AgGaSe 4 and 1.46 eV for the Co-doped one, respectively. We observed absorption bands ofCo 2+ ions at 12,854, 11,990, 5705 and 3759 cm −1 in the absorpton spectra of the Cd 2 AgGaSe 4 :Co 2+ crystals. These absorption bands are assigned to the electronic transitions between the split energy levels of Co 2+ ions located at T d symmetry of the Cd 2 AgGaSe 4 host lattice.

Optical properties of AgGaSe2 and AgGaSe2:Co2+ single crystals

The temperature dependence of the optical energy gaps in AgGaSe2 and AgGaSe2:Co2+ single crystals has been investigated. The impurity optical absorption of AgGaSe2:Co2+ has also been studied. In the temperature dependence of the energy gaps, the temperature coefficients (dEg/dT) are measured to be 1.8×10−4 eV/K (10–60 K) and −3.2×10−4 eV/K (95–300 K) for the AgGaSe2, and 8.9×10−5 eV/K (10–60 K) and −1.0×10−4 eV/K(80–300 K) for the AgGaSe2:Co2+, respectively. In the optical‐absorption spectrum of the AgGaSe2:Co2+, three absorption bands of 1.54, 0.72, and 0.51 eV are observed. It is identified that these impurity absorption peaks are attributed to the electronic transitions between the split energy levels of Co2+ ion with Td symmetry sites of the AgGaSe2 host lattice.

Optical absorption of ZnGa2Se4: Cr2+ single crystals

Abstract Single crystals of ZnGa 2 Se 4 : Cr 2+ were grown by the chemical transport reaction method. To identify the energy band gap and origin of the impurity optical absorption, the optical absorption spectra of the single crystals were investigated in the temperature region 20–300 K. The temperature dependence of the energy band gap had good agreement with the Varshni equation. The broad band absorption peaks observed at 6317 and 12315 cm −1 in the impurity optical absorption spectrum are attributed to the electron transition 5 T 2 ( 5 D ) → 5 E ( 5 D ) and 5 T 2 ( 5 D ) → 3 T 2 ( 3 H ) of the Cr 2+ ions sited with T d symmetry in the doping ZnGa 2 Se 4 : Cr 2+ single crystal with chromium as the impurity. The crystal field parameter is given by Dq = 632 cm −1 in case of the above.

Impurity optical absorption spectra of ZnGa2Se4:Ni2+ single crystals

Abstract The optical absorption of single crystals of ZnGa2Se4:Ni2+ grown by the chemical transport reaction method was investigated in the temperature region 20–300 K. In the single crystals the impurity optical absorption peaks due to the transitions 3T1(3F) → 3T2(3F), 3T1(3F) → 3A2(3F) and 3T1(3F) → 3T1(3P) of the Ni2+ ions sited in the host lattice of the ZnGa2Se4 single crystal with Td symmetry appeared at 4444, 7874 and 11 600 cm−1, respectively. The crystal-field parameter and the Racah parameter were given by Dq = 340 cm−1 and B = 615 cm−1, respectively. The peak due to the transition 3T1(3F) → 3T1(3P) split into four levels by first order spin-orbit-coupling effects of Ni2+ ions in the lower temperature below 150 K. The spin-orbit-coupling parameter was found to be λ = −400 cm−1.

Impurity optical absorption of SbSI: Ni single crystals

Abstract Single crystals of SbSI and SbSI: Ni were grown by the Bridgman technique. The optical absorption spectra of these single crystals were investigated, and also the electronic states of nickel impurity in the SbSI: Ni single crystal were identified. We observed the impurity absorption peaks at 4502, 7770, 8904, 9090, 12 180 and 14 598 cm−1 due to the electron transitions between the energy levels of Ni2+ ions sited at Td symmetry of SbSI host lattice. The crystal field parameter and the Racah parameter are given by Dq = 327 and B = 663 cm−1, respectively.

Impurity optical absorption of SbSeI:Ni single crystals

Single crystals of SbSeI:Ni were grown by the Bridgman technique. The impurity optical absorption spectra of these single crystals were investigated at room temperature. We observed impurity optical absorption peaks at 4334, 7763, 8857, 8952, and 11 587 cm−1 due to nickel impurity. These impurity optical absorption peaks are assigned to the electronic transitions from the ground state 3T1(3F) to the excited states 3T2(3F), 3A2(3F), 1T2(1D), 1E(1D), and 3T1(3P) of Ni2+ ions sited at Td symmetry of the SbSeI host lattice.