Jung-Hyun Jeong

White light generation from Dy3+-doped ZnO-B2O3-P2O5 glasses

Dysprosium doped ZnO–B2O3–P2O5 (ZBP) glasses were prepared by a conventional melt quenching technique in order to study the luminescent properties and their utility for white light emitting diodes (LEDs). X-ray diffraction spectra revealed the amorphous nature of the glass sample. The present glasses were characterized by infrared and Raman spectra to evaluate the vibrational features of the samples. The emission and excitation spectra were reported for the ZBP glasses. Strong blue (484 nm) and yellow (574 nm) emission bands were observed upon various excitations. These two emissions correspond to the F49/2→H615/2 and F49/2→H613/2 transitions of Dy3+ ions, respectively. Combination of these blue and yellow bands gives white light to the naked eye. First time, it was found that ZnO–B2O3–P2O5 glasses efficiently emit white light under 400 and 454 nm excitations, which are nearly match with the emissions of commercial GaN blue LEDs and InGaN LED, respectively. CIE chromaticity coordinates also calculated for...

Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition

Bi-excess SrBi2Ta2O9 (SBT) thin films on Pt/Ti/SiO2/Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P–E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400u200a°C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.

Host sensitized novel red phosphor CaZrSi2O7 : Eu3+ for near UV and blue LED-based white LEDs

A series of red phosphors Ca1?xZrSi2O7?:?Eux (x = 0.5,1,5,10,12?mol%) were prepared by a solid-state reaction technique at various temperatures and their structural and optical properties were investigated. The x-ray diffraction profiles showed that all peaks could be attributed to the monoclinic phase CaZrSi2O7 doped with Eu3+. SEM, FTIR, TG and DTA profiles have also been characterized to explore their structural properties. The luminescence properties of these resulting phosphors have been characterized by photoluminescence spectra. The host matrix itself has shown a strong blue emission which has its maximum intensity at 470?nm. The excitation spectra of CaZrSi2O7?:?Eu3+ revealed two excitation bands at 395 and 464?nm which correspond to the sharp 7F0?5L6 and 7F0?5D2 transitions of Eu3+ and matches well with the two popular emissions from n-UV/blue GaN-based LEDs. The prominent red emission was obtained at 615?nm by the excitation transitions 5L6, 5D2 of Eu3+ through the non-radiative energy transfer process from the host to the Eu3+ ion. The effects of charge compensation by monovalent ions on the luminescence behaviour of a red emitting phosphor CaZrSi2O7?:?Eu3+ were investigated. The high colour saturation and the low thermal quenching effect of these phosphors make it a potential red component for white light emitting diodes (w-LEDs).

Luminescence characteristics of ZnGa2O4 thin film phosphors grown by pulsed laser deposition

Abstract ZnGa 2 O 4 thin film phosphors have been deposited using a pulsed laser deposition technique on Si(100) substrates at a substrate temperature of 550 °C with various oxygen pressures and post-annealing temperatures. The structural characterization was carried out on a series of ZnGa 2 O 4 films grown at an oxygen pressure range of 50–300 mTorr, and subsequently post-annealed at 600 and 700 °C. The optimum oxygen pressure for luminescent characteristics was about 100 mTorr. The luminescent spectra show a broad band extending from 350 to 600 nm peaking at 460 nm. A post-annealing treatment of ZnGa 2 O 4 thin films led to the different shape of luminescent intensity and grain size. β-Ga 2 O 3 phase appeared at an annealing temperature of 600 °C and increased with increasing annealing temperature. We have also investigated the effect of the ligand field strength on the resultant energy levels for ZnGa 2 O 4 thin film phosphor. The annealing process at different temperatures may lead to different dominant emissions.

Conduction behavior of SrBi2Ta2O9 thin film grown by pulsed laser deposition

SrBi2Ta2O9 (SBT) thin films with different Sr and Bi concentrations were prepared on Pt/Ti/SiO2/Si substrate by the pulsed laser deposition technique. The structural feature and surface morphology were characterized by X-ray diffraction and SEM studies. The ferroelectricity was confirmed by polarization–electric field (P–E) hysteresis loops. The measured value of remanent polarization (2Pr) of Bi-excess SBT film was 14.3 µC/cm2 with a coercive field (2Ec) of 101 kV/cm at the applied voltage of 4 V. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range of 0.1 Hz–100 kHz and the temperature range of 25°C–400°C. From the slope of ac conductivity vs. 1/T plot, the activation energy was calculated to be 0.86 eV at the lowest frequency and high temperature range. Also electrical properties were investigated by current–voltage (I–V) measurements as a function of applied electric fields. To determine the influence of Sr and Bi concentrations on the SBT/Pt junction, the ferroelectric properties and conduction behavior of SBT thin films are discussed.

Impedance spectroscopy of Bi3.25La0.75Ti3O12 ceramics above and below ferroelectric transition temperatures

The complex impedance of Bi3.25La0.75Ti3O12 ceramic was measured over a temperature range 50–600 °C at several frequencies. The complex capacitance anomaly of the ferroelectric phase transitions was observed near Tc = 395 °C. Remarkable capacitance dispersion of the complex capacitance was found above 250 °C. The complex impedance spectra above 250 °C were fitted by the superposition of two Cole–Cole types of relaxations. The fast component in the higher frequency region may be due to the orientational grain–grain boundary, and the slow component in the lower frequency region is interpreted as the formation and migration of ions at the surface or electrode/crystal interfacial polarization. We found two different conduction mechanisms that have different activation energies: E ~ 0.33 eV above Tc and E ~ 0.44 eV below Tc in the high-frequency regions, and E ~ 0.32 eV above Tc and E ~ 0.57 eV below Tc in the low-frequency regions.

Luminescence characteristics of pulsed laser deposited ZnGa2O4 thin film phosphors grown on various substrates

Abstract ZnGa 2 O 4 thin film phosphors have been grown on Si (1xa00xa00), Al 2 O 3 (0xa00xa00xa01) and MgO (1xa00xa00) substrates using pulsed laser deposition. The structural characterization was carried out on a series of ZnGa 2 O 4 films grown on various substrates under fixed substrate temperature and oxygen pressure 550°C and 100xa0mTorr, respectively. The films grown on these substrates not only have different crystallinity and surface morphology, but also the Zn/Ga composition ratio. The photoluminescence properties of pulsed laser deposited ZnGa 2 O 4 thin films have indicated that Al 2 O 3 (0xa00xa00xa01) and MgO (1xa00xa00) are promising substrates for the growth of high quality ZnGa 2 O 4 thin films and luminescence brightness depends on the substrate. The luminescent spectra show a broad band extending from 350 to 600xa0nm and peaking at 460xa0nm.

Structural, optical, and magnetic properties of single-crystalline Mn3O4 nanowires

We report a simple and effective method to synthesize single-crystalline Mn3O4 nanowires using solvothermal technique. These nanowires with diameter in the range 15–40xa0nm grow uniformly along {2 0 0} direction and crystallize in tetragonal symmetry. A blue-shift in the absorption edge is observed when the synthesis temperature (TS) increases from 150 to 250xa0°C. The surface chemical analysis confirms the oxidation state of manganese as Mn3+. A relatively broad, intense, and symmetrical electron spin resonance (ESR) signal is detected at room temperature with the ‘g’ value 2. The relative intensity of ESR signal decreases and broadens with the rise of synthesis temperature due to the relaxation of surface anisotropy and induced magnetic field. The magnetization versus field measurements (M–H) at 300 and 5xa0K of Mn3O4 nanowires show hysteresis curves with large coercive field (HC) due to the large surface anisotropy. A step increase in the magnetization (ΔMxa0=xa01.7xa0emu/g) is noticed at H ~±1.1xa0kOe in the M–H curves. Possible origin of such field-induced transition is discussed. The magnitude of magnetic moment, remanence magnetization, and HC values drastically reduces with increasing the TS due to the disassembly of nanowire arrays.

Li-DOPING EFFECT ON THE CATHODOLUMINESCENT PROPERTIES OF Y2O3:Eu3+ PHOSPHORS

Influence of lithium doping on the crystallization, the surface morphology, the chemical states and the luminescent properties of Y2O3:Eu3+ phosphors was investigated. The structural, surface morphology characteristics and chemical states of the phosphors were analyzed by using X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS), respectively. The crystallinity, the surface morphology, and the cathodoluminescence (CL) of phosphors highly depended on the Li doping. The relationship between the crystalline and morphological structures and the luminescent properties was studied, and Li+ doping affected not only the crystallinity but also the luminescent brightness of Y2O3:Eu3+ phosphors. In particular, the incorporation of the Li+ ion into the Y2O3 lattice could induce remarkable increase in the CL intensity. The enhanced photoluminescence brightness with Li doping may result both from the improved crystallinity leading to higher oscillating strengths for the optical transitions, and the increased surface area due to the larger particle sizes. The strongest emission intensity was observed with Li doped Y2O3:Eu3+ ceramics whose brightness was increased by a factor of 1.8 in comparison with that of Y2O3:Eu3+ ceramics.

Influence of hydrogen patterning gas on electric and magnetic properties of perpendicular magnetic tunnel junctions

To identify the degradation mechanism in magnetic tunnel junctions (MTJs) using hydrogen, the properties of the MTJs were measured by applying an additional hydrogen etch process and a hydrogen plasma process to the patterned MTJs. In these studies, an additional 50 s hydrogen etch process caused the magnetoresistance (MR) to decrease from 103% to 14.7% and the resistance (R) to increase from 6.5 kΩ to 39 kΩ. Moreover, an additional 500 s hydrogen plasma process decreased the MR from 103% to 74% and increased R from 6.5 kΩ to 13.9 kΩ. These results show that MTJs can be damaged by the hydrogen plasma process as well as by the hydrogen etch process, as the atomic bonds in MgO may break and react with the exposed hydrogen gas. Compounds such as MgO hydrate very easily. We also calculated the damaged layer width (DLW) of the patterned MTJs after the hydrogen etching and plasma processes, to evaluate the downscaling limitations of spin-transfer-torque magnetic random-access memory (STT-MRAM) devices. With these calculations, the maximum DLWs at each side of the MTJ, generated by the etching and plasma processes, were 23.8 nm and 12.8 nm, respectively. This result validates that the hydrogen-based MTJ patterning processes cannot be used exclusively in STT-MRAMs beyond 20 nm.