Sey-Shing Sun

Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride

Electroluminescent ZnS:Mn thin films have been codoped with KCl via an ex situ deposition and diffusion process. The brightness, efficiency, and stability of sputter deposited ZnS:Mn alternating current thin film electroluminescent devices have been greatly improved over untreated devices. Potassium chloride doping is shown to have a modest fluxing effect on the grain size of ZnS:Mn films, particularly in the region of the film where nucleation occurs. However, improved electroluminescence is postulated to be strongly influenced by doping control of electric fields and injected charge.

Photoluminescence and electroluminescence characteristics of CaSiN2:Eu phosphor

Photoluminescenc eand electroluminescence of CaSiN2:Eu materials were investigted to develop a new phosphor for thin film electroluminescence (TFEL) device applications. Ca3N2 and Si3N4 powders were mixed to form CaSiN2 hostmaterials and Eu was added as the luminescent center. The mixed powermatrials were cold pressed under the pressure of 1 Kg/cm2 to make pellets, and fired at 1400 degrees Celsius for 2 hours under N2H2 envrionemtn. Th ex-ry diffraction(CRD) patterns of synthesizd materals wer well matched with CaSiN2 of joint committee for powder diffraction standards (JCPDS) csrad. When illuminated by ultravilet rays, th enew phosphors emitted very bright red ligh of peak wav lenegth centered at 620 nm. Th TFEL devices with CaSiN2:Eu phosphor layser swre grown by sputter depositonof CaSiN2:Eu target. Red light emission was observed when the peak amplitude of the applied voltge exceeded 116 V.l The luminance was shown to increase sharply withth increase of the applied voltage. The maximum luminance was 1.62 Cd/m2 at the applied peak voltage of 276 V. The red emission from CaSiN2:Eu TFEL device seems to result from electronic transition of Eu3+ ions.The emission spectra of TFEl devices matchwell withth ephotoluminescence spectra of CaSiN2:Ey powders. The new devices structure and fabrication processes for the iimprovement of emission intenityof CaSiN2:Eu TFEl devices ar under investigation.

Improved electroluminescence of ZnS:Mn thin films by codoping with potassium chloride

Alternating current thin film electroluminescent devices have been fabricated using sputter-deposited ZnS:Mn with and without codoped potassium chloride via both in situ and ex situ methods. In situ codoping proved to be difficult due to a memory effect in the deposition chamber. Samples codoped with potassium chloride via an ex situ diffusion method exhibited improvements in brightness of up to 70% (572 vs 337 cd/m2) and efficiency of up to 60% (1.95 vs 1.25 lm/W) over noncodoped samples. The threshold voltage increased by ≈5% (160 vs 168 V), and the brightness-versus-voltage curve stabilized more rapidly for the devices. Several possible mechanisms to explain these effects are discussed. While modest microstructural changes contribute to the improvements, changes in point defects which lead to modification of the space charge in the devices appears to be the dominant mechanism.

Static space charge in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices

The operation of alternating-current thin-film electroluminescent (ACTFEL) devices may be strongly affected by the presence of dynamic or static positive space charge within the phosphor layer during device operation. Dynamic space charge is a positive charge in the phosphor layer which is periodically created and annihilated during each period of the applied voltage waveform. In contrast, static space charge is a positive space charge in the phosphor layer whose charge state does not change appreciably during steady-state operation of the ACTFEL device. The static space charge density of evaporated ZnS:Mn ACTFEL devices is estimated to be ∼7×1016 cm−3 from measured trends in the phosphor clamping field as a function of phosphor thickness. This static space charge density estimate implies a cathode clamping field of ∼ 2.2 MV/cm and a clamping interface trap depth of ∼1.5 eV. Furthermore, from transferred charge trends as a function of the phosphor thickness it is concluded that the static space charge in ...

Microstructure and atomic effects on the electroluminescent efficiency of SrS:Ce thin film devices

Transmission electron microscopy and x-ray diffraction data show that rapid thermal anneals of SrS:Ce thin films enhance grain size and reduce crystalline defects. Electron paramagnetic resonance results suggest that these anneals lead to less variance in the crystal field environments at the nearly cubic Ce3+ sites along with the formation of another type of Ce3+ site believed to involve a nearby Sr vacancy. We suggest that the association of Ce3+ sites with VSr shifts the electroluminescence towards larger wavelengths as the symmetry of the activator site is lowered.

Sputter deposition of ZnS:Mn/SrS:Ce multilayered thin film white phosphor

Abstract A full color thin film electroluminescent (TFEL) display can be fabricated by using color filters in combination with a high efficiency ‘white’ phosphor, such as a thin film multilayered stack of ZnS:Mn and SrS:Ce (denoted ZnS:Mn/SrS:Ce). To date, deposition of these multilayers has been limited to vacuum evaporation techniques and atomic layer epitaxy, both of which require different substrate temperatures for growth of high quality ZnS:Mn and SrS:Ce. This repeated thermal cycling during multilayer deposition can adversely affect electroluminescent (EL) performance and manufacturability. Sputter deposition of ZnS:Mn and SrS:Ce produces high quality phosphors for a wider range of substrate temperatures. We have determined a common set of radio frequency (rf) sputter deposition parameters for ZnS:Mn and SrS:Ce that result in high performance, multilayered white phosphors for use in TFEL devices. The EL performance of our samples is comparable to the best performance reported for evaporated multilayered samples. The major improvement is that the rf sputtered ZnS:Mn and SrS:Ce layers were deposited at the same substrate temperature. We report on the effects of sputter deposition parameters on the resultant composition and morphology of ZnS:Mn and SrS:Ce thin films and multilayers. Their EL performance was evaluated and correlated to composition and morphology.

CHROMATICITY AND ELECTROLUMINESCENT EFFICIENCY OF ATOMIC LAYER EPITAXY SRS:CE THIN FILMS

We have studied various factors that control the luminous properties of atomic layer epitaxy (ALE) SrS:Ce electroluminescent (EL) devices by using electron paramagnetic resonance, photoluminescent decay, x-ray diffraction, and charge-voltage characterization. EL and PL light is green shifted in ALE SrS:Ce films, compared to powders and thin films prepared with some other techniques. The origin of the green shift is discussed. Moderate heat treatments of as-deposited SrS:Ce devices were found to dramatically improve the EL efficiency. This is due to the removal of defects leading to nonradiative Ce3+ decay and to reduction of Ce4+ to radiative Ce3+. Different heat treatments were found to influence the transferred charge, so that there is an optimal heat treatment temperature between 500 and 750 °C. Heat treatments also induce macroscopic stress in these SrS:Ce films. This was seen to induce a local stress around Ce3+ sites. However, the stresses were not found to directly correlate with the luminous effic...

Modification of vapor‐deposited WO3 electrochromic films by oxygen backfilling

The effects of oxygen backfilling during WO3 vapor deposition have been studied. Rutherford backscattering and secondary ion mass spectrometry were used to show that the O/W ratio and alkali/W ratio were increased by oxygen backfilling. The durability of the deposited films in H2SO4 was shown to be limited by two mechanisms—a general uniform film dissolution and an interfacial attack resulting in delamination. The amount of degradation was reduced as the O/W ratio increased. the electrochromic coloration speed remained unchanged, the bleaching speed and self‐erasure rates were faster, and the maximum optical density was lowered for oxygen‐enriched films. The mechanisms by which these modifications were achieved are discussed.

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.

High-luminance SrS: Ce, Ga, F thin-film-electroluminescent devices

Gallium codoping has been identified as an effective flux agent for SrS:Ce phosphor thin films. When rapid-thermal-annealed at temperatures above 750°C, the microstructure of sputtered SrS:Ce,Ga,F thin films underwent a transformation from columnar to spherulitic morphology. In addition, the crystal grain size increased by tenfold from 50 to 500 nm. Time-resolved photoluminescent measurements on 810°C-annealed SrS:Ce,Ga,F films showed decay times as long as 25 ns, indicating effective incorporation of Ce ions. As a result of crystallinity improvement, excellent electroluminescent performance, e.g., 60-Hz L 40 of 155 cd/m 2 , has been achieved in EL devices with 1.5-μm-thick SrS:Ce,Ga,F films. In addition, high luminous efficacy, e.g., e 40 = 1.15 Im/W, was achieved in devices with a thin SiON buffer layer added between the SrS:Ce and a BaTa 2 O 6 top insulator.