Abstract
CdIn2O4 and CdIn2O4:Er3+ crystals were grown by solid-state sintering of CdO and In2O3 with ErCl3. X-ray diffraction analysis revealed that CdIn2O4 and CdIn2O4:Er3+ adopt a cubic structure with respective lattice constants of Å and Å. The optical absorption spectra obtained near the fundamental absorption edge showed that these compounds have a direct energy band gap. The photoluminescence (PL) spectra of CdIn2O4:Er3+ acquired in the wavelength range of 630–700 nm at 294 K showed peaks in the region of 651–686 nm and eight sharp emission peaks due to the Er3+ ions. These PL peaks were attributed to radiative transitions between the split electron energy levels of the Er3+ ions occupying the In3+ sites of the CdIn2O4 host lattice.
1. Introduction
CdIn2O4, a ternary semiconductor with the A-B-X composition, is an n-type semiconductor with a cubic structure that crystallizes in the Fd3m space group. CdIn2O4 has a wide optical energy gap and has been studied because of its high electrical conductivity and transparency in the visible light region. CdIn2O4 materials are applied in gas sensors and devices such as solar cells, flat panel displays, invisible security circuits, and windshield defrosters [1–5]. The energy gap, crystal structure, and photoluminescence (PL) of pure CdIn2O4 have been studied [4–6], but no research has been conducted on pure CdIn2O4 doped with 4f species. As a method of changing the electrical resistance of compound semiconductors and to widen the light energy region with photoelectric sensitivity, the effects of doping with 4f materials on the electrochemical characteristics should be evaluated. Although basic phenomena such as the characterized, photocatalytic activity, and evaluated photoelectrochemical water splitting for CdIn2O4 crystals [7, 8] doped with C and N, the energy gap and the photoluminescence spectrum of CdIn2O4:Er3+ have not been documented. In this study, the energy gap, photoluminescence spectrum, and crystal growth of CdIn2O4 and CdIn2O4:Er3+ materials crystallized at temperatures of 900, 1000, and 1050°C are investigated.
2. Materials and Methods
Growth of the CdIn2O4 and CdIn2O4:Er3+ crystalline crystals was achieved by employing high-purity (5 N) CdO and In2O3 powder; a mixture of these compounds based on weight was magnetically stirred for 2 h, and high-purity ethanol was added at a molar ratio of 1 : 1. To homogenize the mixed powder, the mixture was placed into a ball jar, mixed well with a mixer mill (Retsch, 200, Germany) and placed in an alumina crucible with a lid and the crucible which was then placed in a graphite crucible and filled with activated carbon. The graphite crucible was placed in the center of a low-temperature muffle furnace (Thermolyne, F48010-26, USA), and the temperature was first increased from room temperature to 800°C at a heating rate of 5°C/min, followed by sintering for 12 hours. The sintered powder sample was ground in an alumina crucible and powdered to achieve even mixing. The powder was mixed again in a mixer mill for about approximately 5 hours. The graphite crucible was then placed in the center of the furnace, where the temperature of the furnace (Thermolyne, F46120CM-33, USA), equipped with automatic thermostat could be controlled within the tolerance of 1°C at room temperature. The graphite crucible was then placed in the center of the furnace at 900~1050°C. The temperature was raised at a rate of 2°C/min and was maintained constant for 24 hours; the crystals were gradually cooled at a descending rate of 10°C/min. The amount of erbium added as an impurity was 1.00 mol%. The structure of the CdIn2O4 and CdIn2O4:Er3+ crystals was analyzed by X-ray diffractometer (D/MAX-3C, Rigaku Co., Japan). The intensity of the X-ray diffraction pattern peaks was recorded while varying the 2θ angle from 2° to 155° under irradiation with Cu-Kα rays at a wavelength of 1.5405 Å. The X-ray diffraction pattern obtained from the measurement was compared with the interval and peak intensity of the crystal planes provided in the JCPDS card (04-009-5478), and the crystal planes of the measured diffraction pattern were investigated. The lattice constant was calculated using the Nelson-Riley and Riley [9] relationships. The optical absorption properties of the crystals were evaluated in the range of 200~850 nm by using a UV-VIS-NIR spectrophotometer (U-3501, Hitachi, Japan). To determine the optical energy band gap, the optical absorption coefficient (α) corresponding to the energy () of the irradiation from the absorption spectrum was obtained and the optical energy interval was calculated. The PL characteristics were assessed using a double spectrometer (, Acton, Spectrograph 500i, USA) with a PL measurement apparatus comprising a charge coupled device (CCD, PI-MAX3, Princeton Inst. Co., IRY1024, USA). For the PL measurement, the 266 nm line of a diode pumped solid-state (DPSS) laser was used as the excitation source.
3. Results and Discussion
3.1. Crystal Structure
Figure 1 shows the X-ray diffraction patterns of the CdIn2O4 and CdIn2O4:Er3+ crystals. Figure 1(a) shows the X-ray diffraction patterns of CdIn2O4 prepared at 900, 1000, and 1050°C. In the case of the pure CdIn2O4 crystals, distinct diffraction peaks of the (311), (511), and (440) planes were observed. When the temperature was increased from 900 to 1050°C, the peak of the (311) plane became dominant. Figure 1(b) shows the X-ray diffraction patterns of CdIn2O4:Er3+ prepared at 900, 1000, and 1050°C. The diffraction pattern of the CdIn2O4:Er3+ crystals doped with 1.00 mol% of erbium (Figure 1(b)) was the same as that of CdIn2O4. Figure 1(b) also shows that the main growth plane of the thin film was the (311) plane. Comparison of the experimental X-ray diffraction pattern with the JCPDS card showed that the CdIn2O4 and CdIn2O4:Er3+ crystals have a cubic crystal structure.
(a)
(b)
The lattice constants of the CdIn2O4 crystals were obtained using Nelson-Riley’s [9] correction formula, giving respective values of , 9.1661, and 9.1655 Å for the samples prepared at 900, 1000, and 1050°C, compared with the lattice constant of Å from the JCPDS card. When the lattice constant was determined after doping with 1.00 mol% Er3+, a slight decrease was observed according to the temperature used for crystal growth, where values of , 9.1664, and 9.1661 Å were obtained for the samples prepared at 900, 1000, and 1050°C, respectively.
3.2. Energy Gaps and Photoluminescence
Figure 2(a) shows the energy gap of the CdIn2O4 crystals prepared at temperatures of 900, 1000, and 1050°C, where = 2.779, 2.842, and 2.879 eV, respectively. Figure 2(b) shows the energy gap of the CdIn2O4:Er3+ crystals prepared at temperatures of 900, 1000, and 1050°C, where = 2.820, 2.852, and 2.866 eV, respectively. At the growth temperature of 1050°C, the energy gap of CdIn2O4:Er3+ decreased relatively to that of pure CdIn2O4, with a shift from 2.879 to 2.866 eV. The energy gap was reduced because Er3+, which replaces indium in the CdIn2O4 crystal, forms an impurity level between the valence band and the conduction band.
(a)
(b)
The optical energy gap, Eg, of a semiconductor with a direct band structure can be deduced from the following relation [10]: where α is the optical absorption coefficient near the fundamental absorption edge and is the incident photon energy. The direct energy band gap () can be obtained when . Figure 2(a) shows the CdIn2O4 crystal; the direct energy band gap was obtained from Figure 2(a) by extrapolation, giving a value of = 2.779~2.879 eV for the CdIn2O4 crystal at 294 K. Figure 2(b) is plot of versus hν for the CdIn2O4:Er3+ crystals near the fundamental optical absorption edge. The optical energy band gap () for the CdIn2O4:Er3+ crystals was 2.820~2.866 eV at 294 K, close to 3.0 eV [11].
Figure 3 shows the photoluminescence spectra of the CdIn2O4 and CdIn2O4:Er3+ crystals, measured at wavelengths of 250~950 nm at 294 K. For the CdIn2O4 crystals (Figure 3(a)), the photoluminescence peak position changed with increasing crystal growth temperature. For the CdIn2O4:Er3+ crystals (Figure 3(b)), when the crystal growth temperature increased, the position of the emission peak did not change and a narrow and sharp PL peak due to erbium impurities was observed in the 630~700 nm region. These PL peaks are interpreted as arising from electron transfer between the separated electron energy levels of the Er3+ ion. Figure 4 shows the photoluminescence spectra of the CdIn2O4:Er3+ crystals grown at 1050°C in the wavelength range of 630~700 nm.
(a) CdIn2O4
(b) CdIn2O4:Er3+
Figure 4 also shows the photoluminescence spectra of the CdIn2O4:Er3+ crystals, measured at wavelengths of 630~700 nm at 294 K. Eight narrow and sharp PL peaks due to erbium impurities were observed in the wavelength region of 651~686 nm. CdIn2O4 crystallized with a cubic structure in the Fd3m space group. The ionic radius of each component in the CdIn2O4 crystal is as follows: Cd2+: 0.97 Å, In3+: 0.81 Å, O2−: 0.14 Å. The ionic radius of Er3+ is 0.88 Å. When erbium is added to the CdIn2O4 crystals, the added erbium is most likely to enter the In3+ sites due to the similar ionic radius; thus, the incorporated erbium in the CdIn2O4:Er3+ crystal occupies a site of C3v symmetry [12].
4. Conclusion
CdIn2O4 and CdIn2O4:Er3+ crystals were grown in a high-temperature electric furnace, and their crystal structures and optical properties were analyzed. The crystal structure of the CdIn2O4 and CdIn2O4:Er3+ crystals, determined by X-ray diffraction, was cubic, and the crystals had a direct transition-type energy band structure. At the growth temperature of 1050°C, the energy gap of CdIn2O4:Er3+ decreased relatively to that of pure CdIn2O4, with a shift from 2.879 to 2.866 eV. Erbium substituted indium in the CdIn2O4:Er3+ crystals and narrow and sharp peaks appeared in the region of 651~686 nm due to this effect. These peaks are interpreted as electronic transitions between the energy levels of the Er3+ ions located at the site of C3v symmetry.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.