Richard T. Tuenge

In Situ Pulsed Deposition Studies of Ce ( tmhd ) 4 on SrS Surfaces for Thin Film Electroluminescent Flat Panel Display Applications

In this paper, results are presented from a systematic investigation which aimed to evaluate the performance and efficiency of tetrakis(2,2,6,6-tetramethyl-3,5-heptadionato)cerium(IV), or Ce(tmhd) 4 , as the cerium dopant source in the atomic layer epitaxy (ALE) of strontium sulfide (SrS) thin films for thin film electroluminescent (TFEL) display applications. For this purpose, in situ growth and characterization studies of the adsorption and associated nucleation mechanisms of Ce(tmhd) 4 on SrS surfaces were performed in a clustered system under tightly controlled ultraclean conditions. The growth experiments were carried out in a specially designed processing chamber under pulsed deposition conditions that emulated the ALE process. The samples were then transferred in situ, without exposure to air, to various characterization chambers where compositional and chemical analyses were readily performed. In particular, chemical evaluation by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy showed incomplete decomposition of the Ce(tmhd) 4 molecule with the observation of high carbon levels corresponding to ligands from the precursor. It is believed that these ligands sterically hinder subsequent adsorption and nucleation of the Sr source by poisoning the substrate surface, and therefore require repeated cycles of exposure to pulses of the Sr source and associated reactants to ensure their complete removal. It is thus suggested that the incomplete decomposition of the Ce(tmhd) 4 molecule is the primary cause for its poor efficiency as Ce source precursor, an observation supported by experimental findings from actual ALE manufacturing of SrS:Ce films for TFEL applications.

A field‐sequential color VGA AMEL display

A 0.7-inch-diagonal full-color VGA active-matrix EL display using a liquid-crystal color shutter to provide field-sequential filtering of the broadband EL emission is reported. The shutters color-filter design improves the color gamut of AMEL displays using the SrS:Ce/ZnS:Mn EL phosphor. A digital gray-scale addressing scheme achieves 256 colors using temporal binary shading combined with error diffusion.

Electrical characterization of blue electroluminescent devices

A new class of blue thin-film electroluminescent (TFEL) devices based on thiogallate phosphors has been reported recently. The purpose of this work reported herein is to compare and contrast the electrical properties of CaGa2S4:Ce TFEL blue phosphor devices to those of conventional evaporated ZnS:Mn TFEL devices. Capacitance-voltage (C-V) and internal charge-phosphor field (Q-Fp) techniques are employed for electrical characterization.

In Situ Studies of the Nucleation Mechanisms of Tris(cyclopentadienyl)Cerium as Cerium Dopant Source in SrS:Ce Thin Films for Electroluminescent Displays

A systematie evaluation was made of the performance and efficiency of tris (cyclopentadienyl)cerium, Ce(CPD), as potential dopant source in atomic layer epitaxy (ALE) and chemical vapor deposition (CVD) of Ce-doped strontium sulfide (SrS:Ce) for thin-film electroluminescent display applications. In situ growth and characterization studies were carried out, without a vacuum break, of the adsorption and associated dissocation mechanisms of Ce(CPD) within the substrate temperature window typically employed in ALE and CVD SrS:Ce. Associated findings were compared to results from tetrakis (2,2,6,6-tetramethyl-3,5 heptadionato)cerium, Ce(tmhd) 4 , which was used as a comparative performance baseline. In this respect. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry studies indicated that, within the typical thermal budget used in ALE and CVD of SrS:Ce films, the Ce(CPD) source decomposed more efficiently than its Ce(tmhd) 4 counterpart, as supported by the observation of reduced hydrocarbon-based surface contamination and a cleaner Ce phase. It was concluded that Ce(CPD) might be a better candidate than Ce(tmhd) 4 for applications as Ce dopant source in ALE and CVD SrS:Ce films.

Metallorganic Chemical Vapor Deposition of SrS:Ce for Thin Film Electroluminescent Device Applications

Results are presented from a systematic study to develop and optimize a metallorganic chemical vapor deposition process for the growth of high brightness cerium-doped strontium sulfide (SrS:Ce) thin films for electroluminescent (EL) display applications. Growth of SrS:Ce was investigated in the temperature range from 400 to 530°C using strontium(2,2,6,6-tetramethyl-3,5-heptadionato)trimer (Sr(tmhd) 2 trimer), tetrakis(2,2,6,6-tetramethyl-3,5-heptadionato)cerium (Ce(tmhd) 4 ), and hydrogen sulfide (H 2 S) as reactants. Various Sr and HAS reactant flows and associated partial pressures were examined to explore corresponding effects on the films physical, chemical, and optical properties. Film structural and compositional properties were analyzed by Rutherford backscattering spectrometry, nuclear reaction analysis for hydrogen profiling, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy, EL measurements were carried out on SrS:Ce-based dielectric-sulfur-dielectric stacks. The results of these studies yielded key correlations between process parameters and film texture, grain size, and EL performance In particular, the highest EL performance was obtained for films with a predominantly (200) orientation, grain size larger than 1.0 μm, and Ce dopant level ∼0.14 atom %. A brightness of 51 cd/m 2 and efficiency of 0.22 lm/W were observed, as measured at 40 V above threshold voltage and 60 Hz frequency.

High-performance alkaline-earth thiogallate blue-emitting ACTFEL devices

Significant improvement in the luminescent performance of Sr x Ca 1-x Ga 2 S 4 :Ce (0 < x < 1) EL devices was achieved by optimizing the Sr/Ca ratio and oxygen doping. The key factor for improvement is oxygen doping of thiogallate thin films. Microstructural analysis from x-ray diffraction and transmission electron microscopy showed that oxygen doping increased crystal grain size and induced a preferred orientation along (602) planes. Photoluminescence studies showed the luminescence decay time increased with oxygen doping, indicating the suppression of non-radiative energy transitions at structural defects, e.g., sulfur vacancies, due to improved crystalline quality of the thiogallate thin films. The emission color blue shift in oxygen-doped Sr-Ca mixed thiogallate was attributed to the preferential substitution of Ce 3+ for Sr 2+ rather than Ca 2+ due to a better match of ion size and a possible ordering effect from lattice strain introduced by oxygen incorporation. The threshold-field reduction and steeper turn-on near threshold in oxygen-doped devices was considered to be a result of the introduction of shallow trapping states at the insulator/phosphor interface and the elimination of bulk trapping states by oxygen doping.

MOCVD OF SrS and SrS:Ce Thin Films for Eelctroluminescent Flat Panel displays

High quality SrS and SrS:Ce thin films were deposited from Sr(thd) 2 , Ce(thd) 4 and H 2 S via a low pressure MOCVD process. Film characteristics were found to be insensitive to the presence of the cerium dopant in the concentration range investigated. Depositions were carried out for a wide temperature range (250–550° C). Deposition rates were found to be relatively insensitive for the temperature range investigated. The films produced were found to be highly crystalline at all temperatures investigated. Deposited material showed texturing as a function of substrate material and temperature. FWHM of the reflections were found to have a 2Θ values of 0.15–0.18 deg. for all temperatures. RBS and AES shows stoichiometric 1:1 SrS with less than 2% carbon and oxygen contaminates. ERD indicates the films to have 1 – 2.5% hydrogen. Films doped with 0.019 – 0.043 atom – 250 volts.

L‐6: An Improved Analog AMEL Microdisplay

Color active matrix electroluminescent (AMEL) microdisplays with analog pixel design have demonstrated high brightness, low power consumption and over 256 gray levels per primary color. A new driving scheme for the analog pixel has been developed which eliminates luminance intensity variations and noise due to electronic coupling on the display. A new color pixel design has been implemented which produces a more saturated blue color and a higher white color temperature.

Subthreshold voltage‐induced transferred charge measurements of evaporated ZnS:Mn alternating‐current thin‐film electroluminescent devices

— Subthreshold voltage-induced transferred charge (VIQ) analysis is proposed as a novel method for assessing the nature of aging in alternating-current thin-film electroluminescent (ACTFEL) devices. VIQ analysis involves the application of 20,000 bipolar voltage pulses of variable amplitude and measurement of the subthreshold charge transferred in the phosphor for voltage amplitudes from 0 V to threshold. VIQ experiments provide information related to the physical location, density, energy depth, and capture cross-section of phosphor traps responsible for ACTFEL device aging. VIQ aging experiments were performed on ZnS:Mn ACTFEL devices whose phosphor layers were deposited by thermal evaporation, atomic layer epitaxy using chlorine as a constituent of the precursor gas [ALE (Cl)], and atomic layer epitaxy with diethyl zinc as a precursor gas [ALE (DEZ)]. The depth of the traps responsible for VIQ in evaporated, ALE (Cl), and ALE (DEZ) ZnS:Mn are estimated to be ∼1.0–1.2, ∼0.3, and ∼0.7–0.9 eV, respectively. It is speculated that the traps responsible for VIQ are due to sulfur vacancies, chlorine, and oxygen, for evaporated, ALE (Cl), and ALE (DEZ), respectively.

L2.4: Late‐News Paper: White Monochrome TFEL Displays with SrS:Cu,Ag/ZnS:Mn Stack Phosphor

Using a stack phosphor consisting of the newly developed SrS:Cu,Ag blue phosphor and ZnS:Mn, it is now possible to produce TFEL displays that emit true white color, e.g., CIE x∼ 0.40, y∼ 0.40. We have produced demo displays ranging from a fixed pattern segmented display to a 192 line mid-resolution graphic display with areal luminance between 100 and 1034 cd/m2. These white TFEL displays are expected to provide new market opportunities in applications where emitting color is a sensitive issue, e.g., automotive displays.