Haruki Kozawaguchi

ZnS:Mn Electroluminescent Device Prepared by Metal-Organic Chemical Vapor Deposition

AC thin-film ZnS:Mn electroluminescent devices were fabricated by metal-organic chemical vapor deposition, using dimethylzinc and H2S as source gases and tri-carbonyl-methyl-cyclopentadienyl-Mn as the dopant gas. Tri-carbonyl-methyl-cyclopentadienyl-Mn was decomposed by heating at 580°C at the entrance of the reactor, enabling the ZnS film to be doped homogeneously with the optimum concentration of Mn. The brightness of metal/insulator/semiconductor electroluminescent devices made with the resulting ZnS:Mn film was more than 5000 cd/m2 (5 kHz, 130 V), while metal/insulator/semiconductor/insulator electroluminescent devices had a brightness of more than 6000 cd/m2 (5 kHz, 160 V) and exhibited hysteresis in their brightness-voltage characteristics.

Color Electroluminescent Devices Prepared by Metal Organic Chemical Vapor Deposition

This paper discusses the preparation of AC thin film ZnS:Tb (green), Sm (red) and Tm (blue) color electroluminescent devices by metal organic chemical vapor deposition, using dimethylzinc and H2S as source gases. A new doping method for luminescent centers was used. This was accomplished by evaporating TbF3, SmF3 SmCl3 and TmF3 after they had been heated to their melting points during the growth of ZnS in a MOCVD reactor. The maximum brightnesses of ZnS:TbF3, ZnS:SmCl3 and ZnS:TmF3 are 5000, 1000 and 10 cd/m2. The ZnS:SmCl3 EL devices showed a deeper red chromaticity than the ZnS:SmF3 EL ones.

Red Electroluminescence and Crystallinity of ZnS:SmF3 Thin Films

The dependence of the red emission characteristics (emission efficiency, brightness and emission decay time) and crystallinity of ZnS:SmF3 thin-film electroluminescent (TFEL) devices upon fabrication parameters such as the SmF3 concentration and substrate temperature during evaporation, has been investigated. The electroluminescent characteristics are considered to be influenced more by the SmF3 concentration and the grain height in the film thickness direction, than by the cubic ZnS structure {111} plane orientation and the film uniformity, or by the degree to which the cubic {111} plane is oriented parallel to the substrate. A bright red electroluminescence of 1000 cd/m2 under 5 kHz sinusoidal voltage excitation and an emission efficiency of 0.078 lm/W are obtained in these ZnS:SmF3 TFEL devices.

Thin-Film Electroluminescent Device Employing Ta2O5 RF Sputtered Insulating Film

In order to fabricate low-voltage-driven high-brightness thin-film electroluminescent devices, the emission characteristics of devices employing low dielectric loss Ta2O5 RF-sputtered insulating films have been studied in comparison with those of devices employing Y2O3, Sm2O3 and Al2O3 deposited insulators. It was found that the devices employing Ta2O5, RF-sputtered films exhibited higher brightness, lower driving voltage and higher stability than the other devices. In addition, no aging was observed in the Ta2O5 insulated devices. These results indicate that the Ta2O5 RF-sputtered film is suitable as the insulating film in thin-film electroluminescent devices, and that the properties of the insulating film are closely related to the emission characteristics in such devices.

Helical Twisting Power in Cholesteric Liquid Crystal Mixtures. I. Experimental Results

The composition dependence of the helical twisting power in various mixtures of cholesteric-cholesteric or nematic-cholesteric liquid crystals is investigated by measuring transmission or reflection spectra. In the binary cholesteric mixtures, the helical twisting power varies monotonously with the composition except in left-handed cholesteric-rich regions. In the namatic-cholesteric mixtures, the helical twisting power varies with the composition through a maximum value and the helical sense of these mixtures with a small amount of cholesterics is left-handed regardless of the helical sense of cholesteric components. Further, in the mixtures of nematics and right-handed cholesterics, cholesteric-to-nematic phase transition is observed.

MOCVD Preparation of Polycrystalline ZnS Films with Pronounced Columnar Grains

ZnS films having good crystallinity are grown on glass by MOCVD using dimethylzinc and diethylsulphide. These films consist of very pronounced columnar grains that extend from the bottom to the top of the film. The crystal structure of these films is hexagonal. Thin film electroluminescent devices are fabricated using these films and their characteristics are measured.

F/Tb ratio dependence of the photoluminescent and electroluminescent characteristics in MOCVD-prepared ZnS:TbFx green-emitting electroluminescent devices

The photoluminescent characteristics of MOCVD-prepared ZnS:TbFx films are highly dependent on x, while the eletroluminescent characteristics are independent of x. For photoluminescence excitation at 330 nm, which is indirect excitation from the ZnS host to the Tb center, the photoluminescent intensity increases rapidly with a decrease in x, while for the photoluminescence excitation at 377 nm, which is a direct excitation of the Tb center, the photoluminescent intensity is independent of x. On the other hand, the electroluminescent intensity of devices prepared using these films is almost independent of x. The difference in the site of the F ion between MOCVD- and sputtering-prepared films is discussed as well as the excitation mechanism for photoluminescence and electroluminescence.

Memory Effect in AC Thin Film ZnS:Mn Electroluminescent Devices Prepared by Metal Organic Chemical Vapor Deposition

AC thin film ZnS:Mn electroluminescent devices prepared by metal organic chemical vapor deposition have been found to exhibit a larger and more stable memory effect than devices prepared by EB-evaporation. The memory margin is about 15% of the threshold voltage for a device having an Mn concentration of 0.8 wt%. The memory margin is relatively stable. The results show that except for the first 300 hours, the brightness-voltage characteristic exhibits almost the same hysteresis shape during 2000 hours of aging at a constant brightness of 500 cd/m2, using a 5 kHz sinusoidal wave.

Helical Twisting Power in Cholesteric Liquid Crystal Mixtures. : II. Theoretical Treatment

The helical twisting power in cholesteric liquid crystal mixtures is investigated by means of the continuum theory. The calculated results are in good agreement with the experimental results. These results show that (a) the variation of the helical twisting power with the composition of binary cholesteric mixtures can be explained by considering the difference between the elastic constants of two components and that (b) the variation of the helical twisting power with the composition of nematic-cholesteric mixtures can be explained by the extended continuum theory with the induced helical twisting power of nematic molecules.

Photo-Assisted Metal Organic Chemical Vapor Deposition Preparation of Polycrystalline ZnS:Mn Films for Thin Film Electroluminescent Devices

Polycrystalline ZnS:Mn films with good crystallinity are grown by photo-assisted MOCVD using dimethylzinc and diethylsulphide. The growth temperature is decreased from 550 to 400°C by photo-irradiation (high-pressure mercury lamp) and no dead layer results. Patterned ZnS films can also be grown by this method. Thin film electroluminescent devices fabricated using these films have lower threshold voltages.