Kunio Ichino

Carrier transport mechanism of PVCz-based multi-layered electroluminescent devices

Abstract The current-voltage ( I–V ) dependence of PVCz polymer films has been analyzed using transport models for insulating material. Space charge limited current, Schottky current, Poole-Frenkel current and field assisted tunnelling current have been examined for four types of device with different combinations of PVCz and PVCz:perylene layers; the devices with multi-layered structures are: (ITO/PVCz:perylene/ PVCz/Mg-Ag), (ITO/PVCz/PVCz:perylene/Mg-Ag), (ITO/PVCz/PVCz/Mg-Ag) and (ITO/PVCz:perylene/PVCz:perylene/Mg-Ag). Below the threshold voltage of electroluminescence (EL) operation, the current is mainly limited by space charge in both undoped PVCz and perylene doped PVCz emitting layers. Above the EL threshold, the I–V characteristic is controlled by field assisted tunnelling. EL is generated in the polymer layer near the metal (Mg-Ag) interface by carrier injection through the tunnelling limited current. It was made clear that the EL emission in the PVCz multi-layer devices took place at electric fields higher than 10 6 V/cm.

Multi-color emission of PVCz-based multi-layered electroluminescent devices

Electroluminescence (EL) of the multi-layered PVCz EL devices has been studied in order to reduce the energy transfer between dye molecules in the PVCz host simultaneously doped with three kinds of molecules. Single and triple layer devices have been fabricated, such as ITO/PVCz/PVCz:Nile Red,Coumarin 6,TPB/Mg-Ag and ITO/PVCz:Nile Red/PVCz:Coumarin 6/PVCz:TPB/Mg-Ag. Long-wavelength emission due to Nile Red dominated the spectrum in the case of the single layer device which was doped with three kinds of dye molecules. In contrast to this, the short-wavelength blue emission due to TPB was strongly enhanced in the three-layer structure in which only PVCz:TPB layer was coated next to the Mg-Ag electrode for electron injection. The emission from the molecules with the higher HOMO-LUMO energy separation could be effectively excited in the multi-layer structure. The effect of host excitation, impurity excitation and also the energy transfer between the molecules have been discussed for multi-color devices.

XPS study of ZnxMg1−xS:Mn ternary compound thin films

Abstract The chemical bonding of Zn, Mg, S and Mn atoms in Zn x Mg 1− x S:Mn thin films has been studied with X-ray photoelectron spectroscopy (XPS) in view to correlate the variation of electronic state with blue shift of Mn 2+ emission in Zn x Mg 1− x S. We have elucidated the clear dependence of binding energy, E B of core electron on the composition x for each element as follows: E B (Zn 2p 3/2 )=1022+0.6 x , E B (Mg 2p)=50+0.6 x , E B (S 2p 3/2 )=161+0.7 x , E B (Mn 2p 3/2 )=641+1.2 x (eV). The origin of the chemical shift of binding energies is discussed together with the effect of chemical shift on the photoluminescence.

Miscibility gap in ZnxSr1 − xS, ZnxCa1 − xS and ZnxSr1 − xSe thin films

Abstract The miscibility gap in Zn x Sr 1 − x S, Zn x Ca 1 − x S and Zn x Sr 1 − x Se thin film mixed systems has been studied from the results of film composition, lattice constant and absorption edge measurements. The phase transition in the immiscible range is discussed in terms of the fraction of ionic character based on the Phillips dielectric theory for binary compounds.

Growth and characterization of SPR-ZnS bulk crystal

The solid-phase recrystallization (SPR) method was used to grow ZnS bulk crystals. Scanning electron microscopy, polarized microscope and photoluminescence (PL) assessed the characterization of solid-phase recrystallized ZnS. A larger single grain has been grown by higher temperature annealing under sulfur atmosphere, and the grain growth mechanism was discussed based on both the annealing temperature and the atmosphere. The low-temperature PL indicated that the ZnS crystals grown by SPR was of high quality.

Preparation and characterization of ZnxSr1 − xS compound thin films

Abstract Zn x Sr 1 − x S thin films were prepared, for the first time, in the whole composition range by radio frequency (RF) sputtering as a thermally nonequilibrium deposition method. The possibility of existence of Zn x Sr 1 − x S solid solutions was systematically investigated from the results of thin film growth, in terms of structural and optical characteristics. Despite a general idea of insolubility between ZnS and SrS compounds, it has been found that for 0.86−0.91 ≤ x ≤ 1 Zn-rich Zn x Sr 1 − x S, solid solutions with a single-phased zincblende structure were formed and for 0 ≤ x ≤ 0.29 Sr-rich Zn x Sr 1 − x S, solid solutions with a single-phased rocksalt structure were formed. In the intermediate wide composition range 0.3 ≤ x ≤ 0.86−0.91, the miscibility gap including phase separation regions was observed. A deposition mechanism is proposed and the phase transition from rocksalt to zincblende structure is also discussed based on the fraction of ionic character f i for ZnS and SrS.

Optical band gap of ZnxMg1-xS thin films with composition x between 0.14 and 1.0

Solid solutions of crystalline Zn x Mg 1-x S thin films have been prepared in the composition range x between 0.13 and 1.0. The crystal structure and optical properties of Zn x Mg 1-x S solid solutions have been investigated by X-ray diffraction, optical absorption, photoluminescence and transmission electron microscope. It was revealed that a zinc-blende phase Zn x Mg 1-x S crystal could be formed and the lattice constant was determined to be a = 5.43x + 5.66(1 - x) (A). The optical band gap energy was evaluated to follow the equation E g = 3.81x + 5.02(1 - x) (eV). The relation between material parameters and crystallinity have been discussed.

Deposition and characterization of ZnxMg1−xS thin films on amorphous substrates

Abstract Zn x Mg 1− x S thin films are deposited on the ‘amorphous’ glass substrate in the whole composition range. The crystal structure of Zn x Mg 1− x S solid solutions has been determined from the results of film composition and from their structural and optical characteristics. It has been found that for 0.14 x ≤ 1, Zn x Mg 1− x S with a zincblende structure is formed whereas for 0 ≤ x x Mg 1− x S with a rocksalt structure is formed. In the intermediate range of 0.07 ≤ x ≤ 0.14, the miscibility gap has been found to exist. The miscibility behaviour is discussed on the basis of the ionicity fraction f i for ZnS and MgS.

Luminescent properties of ZnxMg1−xS : Mn thin film electroluminescent devices

Abstract Zn x Mg 1− x Sxa0:xa0Mn thin film electroluminescent (TFEL) devices have been developed in a wide range of compositions, x between 0.16 and 1.0, for use as yellow/green-light-emitting displays. Electroluminescence shows the blue shift alike photoluminescence with the increase in Mg composition. The Zn 0.98 Mg 0.02 Sxa0:xa0Mn TFEL device driven at 1xa0kHz has a high luminance of 1056xa0cd/m 2 and an efficiency of 1.2xa0lm/W. The CIE color coordinates varied from ( x , y )=(0.54, 0.45) at x =0.98 to (0.47, 0.52) at x =0.16 with the decrease in Zn composition x in consistence with the peak shift from 585xa0 to 565xa0nm.

Growth and luminescence of ZnxMg1−xS : Mn ternary compound crystal films

Abstract Solid solutions of crystalline Zn x Mg 1− x S thin films have been developed in the wide composition x between 0.13 and 1.0 for the use as short-wavelength-light-emitting devices. The crystal structure, optical and luminescent properties and device characteristics of Zn x Mg 1− x S thin films have been investigated by X-ray diffraction, optical absorption, transmission electron microscope, photoluminescence, and electroluminescence. It was revealed that a zincblende phase Zn x Mg 1− x S crystal could be formed and the lattice constant was determined and also the optical band gap energy was evaluated. Photo- and electro-luminescent properties of Zn x Mg 1− x Sxa0:xa0Mn have been investigated for the application as an electroluminescent thin film. The blue shift of the Mn emission band with the increase of Mg composition has been determined. Photoluminescent intensity has been also enhanced with the increase of Mg content. The origin of the blue shift of Mn emission in Zn x Mg 1− x S have been discussed in the framework of crystal field in relation with the composition dependence of the lattice parameter. The blue shift of Mn emission is ascribed to the change of lattice parameter which increase the bond length between Mn 2+ ion and ligand S atom with the increase of Mg content in Zn x Mg 1− x S. Present status of electroluminescent capability has been also described.