Mn Thin-film Electroluminescent Devices Research Articles

Growth and Electroluminescence of CaF2:Mn Thin Films

We have prepared the CaF2:Mn thin films on several buffer layer-coated glass substrates by electron beam evaporation method and investigated their structural and electroluminescent characteristics. The experimental results show that the diffraction peaks in the X-ray patterns of CaF2:Mn thin films correspond to the cubic fluorite phase. In addition, the crystallinity and orientation plane of the films strongly depend on kinds of buffer layer. The CaF2:Mn thin film electroluminescent devices show the blue emission of the 495 nm peak, originating from the 4T1g(4G) → 6A1g(6S) dipole-dipole transition of Mn2+ ions in the Oh crystal field.

Red electroluminescent process excited by hot holes in SrGa 2S 4:Ce, Mn thin film

This paper reports the first observation of red electroluminescence (EL) in SrGa 2S 4:Ce, Mn thin film. The EL spectrum consists of single broad emission band having a peak wavelength of 665 nm. The dominant EL decay time was 31 μs. The relationship between the applied voltage and the EL waveform was measured in single insulating thin film electroluminescent (TFEL) devices. An asymmetric EL waveform was observed in SrGa 2S 4:Ce, Mn TFEL devices under a rectangular applied voltage. The polarity of the EL waveform in these devices was different from the waveform in manganese-activated zinc sulfide ZnS:Mn devices. This indicates that hot holes excite the Mn 2+ ions to cause the red EL.

Performance enhancement of ZnS:Mn thin film electroluminescent devices by combination of laser and thermal annealing

The combination of laser and thermal annealing was investigated as a post-deposition process for enhancing the luminescent properties of RF-magnetron sputtered ZnS:Mn thin film electroluminescent devices (TFEL). Laser annealing of the uncoated phosphor layer was performed using KrF excimer 248 nm laser pulses of 20 ns under an argon overpressure of 10 bar to limit laser ablation. Single, double and triple irradiation was applied at 1.4 J/cm 2. Thermal annealing was performed in vacuum at 500 °C and 550 °C for 1 h. In this paper we are reporting the brightness–voltage characteristics of devices that have been subjected to all combinations of the two annealing techniques (i.e. laser, thermal, laser + thermal, thermal + laser and finally non-annealed devices). Also, a simple lifetime comparison is made between the best performing device (laser + thermal) and the industrial standard (thermal). The lifetime (time to half brightness) and brightness of the best performing device is found to be more than double compared to the industrial standard.

Design and Optical Properties of ZnS:Mn Thin-Film Electroluminescent Devices on 2D SiO2 Corrugated Photonic Crystal Substrates

We studied the characteristics of ZnS:Mn thin-film electroluminescence (TFEL) display devices in which the glass substrate had been modified with a two-dimensional (2D) corrugated photonic crystal (PC) pattern. The finite-difference time-domain (FDTD) method was used to predict the behavior of 2D corrugated PC ZnS:Mn TFEL devices with varying PC structural parameters. Based on the optimal parameters derived from the FDTD calculations, a 2D corrugated PC layer was introduced onto the glass substrate of a typical ZnS:Mn TFEL structure by two-step irradiated hologram lithography and reactive ion etching. Experiments conducted under typical operating conditions showed that incorporation of the 2D corrugated structure improved the light extraction luminance at above the threshold voltage by over 3.1-fold compared to the conventional ZnS:Mn TFEL device, without noticeable degradation of the electrical characteristics. This improvement originates from the liberation of the photons trapped in the high-index guiding layers.

High-Luminance EL Devices Using Zn2Si1-XGeXO4:Mn Thin Films Prepared by Combinatorial Deposition by r.f. Magnetron Sputtering with Subdivided Powder Targets

A new technique incorporating combinatorial deposition to develop thin-film phosphors by r.f. magnetron sputtering is demonstrated using subdivided powder targets. In comparison with development using conventional r.f. magnetron sputtering, the atomic ratios of Si and Ge as well as the Mn content in Zn 2 Si 1-X Ge X O 4 :Mn thin film phosphors could be more efficiently optimized in order to obtain the highest intensity in electroluminescent and photoluminescent emissions. High luminances of 11800 and 1536 cd/m 2 were obtained in Zn 2 Si 0.6 Ge 0.4 O 4 :Mn thin-film electroluminescent devices fabricated under optimized conditions and driven at 1 kHz and 60 Hz, respectively.

Influence of a two-dimensional SiO2 nanorod structure on the extraction efficiency of ZnS:Mn thin-film electroluminescent devices

To improve light extraction from thin-film electroluminescent (TFEL) devices, we have introduced a two-dimensional (2-D) SiO2 nanorod pattern into the glass substrate of a TFEL. This periodic modulation converts the guided waves in the high refractive index ZnS:Mn phosphor layers into leaked external waves. We used the finite-difference time-domain method to optimize the structural parameters of the 2-D nanorod pattern. Using a nanorod pattern with a depth of 300 nm, a lattice constant of 600 nm, and a radius of 180 nm, the extraction efficiency of the TFEL is theoretically predicted to increase by a factor of more than 4.8. We compare the calculated efficiency and that of the actual TFEL structure, with a view to decreasing diameter of nanorod.

Characterization of AlON-TiON Stacked Insulators For ZnS:Mn Thin Film Electroluminescent Devices

The use of TiON films by plasma-enhanced atomic layer deposition as insulators in thin film electroluminescent (TFEL) devices was examined and compared with that of TiO 2 by ALD. A ZnS:Mn TFEL device using AlON-TiON stacked insulator (ATON) exhibited the figure of merit of 12 μC/cm 2 . The luminance of the TFEL device with an ATON insulator was twice as high as that with a conventional Al 2 O 3 insulator. This high performance can be attributed not only to high dielectric constant of the ATON film, but the improved quality of the interface of the phosphor and insulator, as well.

Effects of photoassisted deposition and/or photoassisted annealing on characteristics of ZnS:Mn thin-film electroluminescent devices

Some essential photoinduced effects on the characteristics of ZnS:Mn electron-beam evaporated films and thin-film electroluminescent (TFEL) devices based on them have been revealed when Hg-lamp irradiation and quartz halogen lamp illumination are used during deposition and postannealing, respectively. The photoassisted deposition with the irradiation of the ZnS:Mn flux or a substrate results in some improvement of the crystal structure of the films, an enhancement of the intensity of photoluminescence and electroluminescence (EL), an increase of the threshold voltage of EL and the luminous efficiency, a lesser change and the quicker stabilization of EL characteristics during short-term forced aging. The use of the photoassisted rapid annealing instead of a usual long-term annealing in a vacuum furnace causes some enlargement of the crystallites and the improvement of performance of the TFEL devices. Both photoassisted processes used lead to a significant reduction of the impurity photocurrent in the photodepolarization spectra. The above results indicate that the following photoinduced changes take place in the ZnS:Mn film: a more densely packed lattice, an increase of the energy depth of the free-electron source, a decrease of the number of pores and defects, particularly, the defects responsible for the radiationless processes and the instability of the EL characteristics in the course of the device operation. Possible mechanisms of the photoinduced effects are discussed.

Oxide thin-film electroluminescent devices and materials

This report reviews newly developed oxide phosphors shown to be promising as the emitting layer of thin-film electroluminescent (TFEL) devices. Since the first report of a high-luminance TFEL device using a Mn-activated Zn 2SiO 4 (Zn 2SiO 4:Mn) phosphor, high-luminance multicolor-emitting TFEL devices have been fabricated using various oxide phosphors activated with Eu or Mn. In addition, many oxides that consist of binary and ternary compounds and multicomponent oxides have been developed and shown to be promising as a host material for TFEL phosphors. This report focuses on Mn-activated Y 2O 3-based oxide phosphors: a binary compound and various ternary compounds and multicomponent oxides, composed of Y 2O 3 in combination with another binary compound such as Ga 2O 3 or GeO 2. TFEL devices of which every constituent was an oxide material were fabricated using oxide phosphor thin films deposited by r.f. magnetron sputtering, pulsed laser deposition or a sol–gel process. High luminances and luminous efficiencies comparable to those of TFEL devices using ZnS:Mn sulfide phosphor were realized using Mn-activated Y 2O 3-based oxide phosphors. Luminances above 7000 cd/m 2 (1 kHz-driving voltage) and luminous efficiencies of approximately 10 lm/W (60 Hz driving voltage) were obtained in yellow emitting TFEL devices fabricated using a Y 2O 3:Mn, a ((Y 2O 3) 0.6–(GeO 2) 0.4):Mn or a ((Y 2O 3) 0.5–(Ga 2O 3) 0.5):Mn thin film and driven by an ac sinusoidal wave voltage. Also, a high luminance above 1000 cd/m 2 for green emission was obtained in a ((Y 2O 3) 0.3–(Ga 2O 3) 0.7):Mn TFEL device driven at 1 kHz.

Improved output coupling efficiency of a ZnS:Mn thin-film electroluminescent device with addition of a two-dimensional SiO2 corrugated substrate

We present a study of the effects of adding a two-dimensional (2-D) SiO2 corrugated substrate onto a ZnS:Mn thin-film electroluminescence (EL) device on its EL properties. Two-step irradiated hologram lithography and reactive ion etching were used to fabricate a 2-D square-lattice-dotted pattern of SiO2 on the glass substrate. The addition of this corrugation leads to a 220% enhancement in the EL efficiency at 40 V above the threshold voltage under a 500-Hz sinusoidal excitation. This improvement in the output coupling efficiency is due to the diffraction scattering produced by the 2-D SiO2 periodic corrugations.

Characterization of the electro-optical behavior of Zn2Si0.5Ge0.5O4:Mn thin-film electroluminescent devices

Zn 2 Si 0.5 Ge 0.5 O 4 : Mn was investigated as a green-emitting oxide phosphor for thin-film electroluminescent (EL) display applications. Detailed electro-optical characteristics of thin-film EL devices with SrTiO3 dielectrics on a glass substrate (TFEL) and EL devices with a single ceramic insulator (CIEL) have been analyzed. The steeper brightness–voltage dependency of the TFEL device is ascribed to the capability to provide high current densities and efficient light generation for both voltage polarities. The CIEL device on the other hand suffered from asymmetrical behavior, related with the lack of a top dielectric. It was found that the use of high-permittivity dielectrics in the TFEL device results in a small voltage drop over the insulating layers, enhancing the amount of charge transferred through the phosphor. Transient charge–voltage (αQp–V) measurements on the TFEL device evidence that a high positive space charge density of ∼0.68 μC/cm2 is present in the oxide phosphor during steady-state operation. In addition, the transient behavior after ultraviolet illumination indicates the presence of shallow electron traps in the phosphor, giving rise to a conductive current flow at electric fields below the threshold.

A new thin-film phosphor using multicomponent oxides composed of Y 2O 3 and GeO 2

The luminescent characteristics of new multicomponent oxide (Y 2O 3–GeO 2):Mn thin films prepared by r.f. magnetron sputtering are described. High-luminance yellow emissions were obtained in photoluminescence (PL) and electroluminescence using (Y 2O 3–GeO 2):Mn thin films prepared with a GeO 2 content of 5–84 mol%. It was found that the GeO 2 content dependence of luminance obtained in (Y 2O 3–Ge 2O 7):Mn thin-film electroluminescent (TFEL) devices was similar to that of PL intensity as well as the crystallinity of the thin-film emitting layers used in the devices. A luminance of 7700 cd/m 2 was obtained in 1-kHz-driven TFEL devices using (Y 2O 3–Ge 2O 7):Mn thin films prepared with a GeO 2 content of 40 mol%.

The effect of space charge on the characteristics of ZnS thin-film electroluminescent devices

From experimental time dependences of the instantaneous brightness and the total current passing through a ZnS: Mn thin-film electroluminescent device, capacitance-voltage, charge-voltage, and current-voltage characteristics of the device are calculated. Conditions for negative differential resistance (NDR) of S and N types are found. An NDR mechanism that exploits the ionization and the recharge of deep donors and acceptors (zinc and sulfur vacancies) with the formation of space charge at the cathodic and anodic interfaces of the phosphor is suggested.

Oxide phosphor thin-film electroluminescent devices fabricated by magnetron sputtering with rapid thermal annealing

A procedure for producing high-luminance thin-film electroluminescent (TFEL) devices with an oxide phosphor thin-film emitting layer prepared without high-temperature postannealing is described. The TFEL devices were fabricated by depositing an oxide phosphor thin film onto a thick BaTiO3 ceramic sheet insulator using magnetron sputtering and in situ rapid thermal annealing (RTA). The oxide phosphor thin films were deposited by rf magnetron sputtering onto an insulating ceramic sheet mounted on a rotating platform; a thin film was deposited onto the ceramic sheet when it passed over the target, and subsequent in situ RTA was performed on the deposited film when it passed over the halogen lamps. High-luminance green emissions were obtained in TFEL devices using either a Ga2O3:Mn or a ZnGa2O4:Mn thin-film emitting layer prepared under optimized deposition conditions. A ZnGa2O4:Mn TFEL device driven at 1 kHz exhibited a luminance of 200 cd/m2.

On nature of centers responsible for inherent memory in ZnS : Mn thin-film electroluminescent devices

Some new experimental data concerning the characterization of ZnS:Mn thin-film electroluminescent devices with the inherent memory, which is due to a hysteresis of the voltage dependence of the luminance and transferred charge, are reported. Essential differences in the kind, number and energy depth of defects depending upon the magnitude of the hysteresis effect have been revealed by the study of the photodepolarization spectra. An estimation of a contribution of the impact ionization of the lattice and defects to the generation of a positive space charge responsible for the hysteresis has been carried out. Peculiarities in the temperature and frequency dependencies of the hysteresis width, threshold voltage and extinction voltage have been found in the devices, showing the different memory margin. The role of different defects in the memory effect is discussed with the allowance for the obtained results and published data.

Luminescent properties of Zn xMg 1− xS : Mn thin film electroluminescent devices

Zn x Mg 1− x S : Mn 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 0.98Mg 0.02S : Mn TFEL device driven at 1 kHz has a high luminance of 1056 cd/m 2 and an efficiency of 1.2 lm/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 585 to 565 nm.

Thin-Film Oxide Phosphors as Electroluminescent Materials

ABSTRACTIn this paper, we introduce rare earth- or transition metal-activated oxide phosphor thin films prepared by magnetron sputtering and dip-coated solution deposition. Thin-film electroluminescent (TFEL) devices using these oxide phosphor thin films as the emitting layer were investigated with a single insulating layer device structure using a thick ceramic sheet insulating layer. High-luminance multicolor emissions and high luminous efficiencies were obtained in TFEL devices using various oxide phosphors: binary compound, Ga2O3; ternary compound, Zn2SiO4, CaGa2O4 and ZnGa2O4; and multicomponent oxide, Zn2SiO4- Zn2GeO4 system. Luminances above 2000 cd/m2 were obtained in CaGa2O4:Mn and Zn2Si1−XGeXO4:Mn TFEL devices driven at 1 kHz. Maximum luminous efficiencies of 2.53, 1.7 and 1.2 lm/W were obtained in Zn2Si0.7Ge0.3O4:Mn, Ga2O3:Mn and ZnGa2O4:Mn TFEL devices, respectively. Stable long-term operation was obtained in dip-coated and sputtered Ga2O3:Mn phosphor TFEL devices even when driven at 10 kHz. Various thin-film oxide phosphors activated with transition metals are suitable for the emitting layer of full-color electroluminescent displays (ELD) without the use of color filters. In particular, Ga2O3 is very promising as a phosphor host material for ELDs.

Thin-Film Oxide Phosphors as Electroluminescent Materials

ABSTRACTIn this paper, we introduce rare earth- or transition metal-activated oxide phosphor thin films prepared by magnetron sputtering and dip-coated solution deposition. Thin-film electroluminescent (TFEL) devices using these oxide phosphor thin films as the emitting layer were investigated with a single insulating layer device structure using a thick ceramic sheet insulating layer. High-luminance multicolor emissions and high luminous efficiencies were obtained in TFEL devices using various oxide phosphors: binary compound, Ga2O3; ternary compound, Zn2SiO4, CaGa2O4 and ZnGa2O4; and multicomponent oxide, Zn2SiO4-Zn2GeO4 system. Luminances above 2000 cd/m2were obtained in CaGa2O4:Mn and Zn2Si1-XGeXO4:Mn TFEL devices driven at 1 kHz. Maximum luminous efficiencies of 2.53, 1.7 and 1.2 Im/W were obtained in Zn2Si0.7Ge0.3O4:Mn, Ga2O3:Mn and ZnGa2O4:Mn TFEL devices, respectively. Stable long-term operation was obtained in dip-coated and sputtered Ga2O3:Mn phosphor TFEL devices even when driven at 10 kHz. Various thin-film oxide phosphors activated with transition metals are suitable for the emitting layer of full-color electroluminescent displays (ELD) without the use of color filters. In particular, Ga2O3 is very promising as a phosphor host material for ELDs.

Fundamental properties and application of Zn1−XMgXS:Mn thin film electroluminescent devices

By Mg doping of ZnS, the lattice constants and the band gap of Zn1−XMgXS host material were increased. The emission spectrum of Mn in Zn1−XMgXS shifted to shorter wavelengths than that of ZnS:Mn. The electroluminescent (EL) devices fabricated with the source of X=0.32 show 552 nm peak yellowish–green emission. Deposition conditions for the Zn1−XMgXS:Mn phosphor layer were optimized to obtain bright emission. As a result, the Zn1−XMgXS:Mn EL devices have a luminance of 282–333 cd/m2 which is as high as that of the conventional ZnS:Mn EL devices.

Characteristics of ZnS:Mn thin film electroluminescent device using layered BaTiO 3 thin film structures

A multilayered and a double-layered BaTiO 3 thin film structure were applied to the preparation of ZnS:Mn thin film electroluminescent (TFEL) devices. From the characterization of the TFEL devices, it was confirmed that the multilayered BaTiO 3 thin film structure prepared by a new stacking method, i.e. deposition of an amorphous layer during continuous cooling of the substrate after the deposition of a polycrystalline layer at higher temperature, had very suitable electrical properties for the insulating layer of the TFEL device. The ZnS:Mn TFEL device using the multilayered BaTiO 3 thin film structure showed stable efficiency characteristics with operating time as well as a low turn-on voltage of ∼50 V and a high saturated brightness of ∼3000 cd m −2.