Following a description of the purpose and participating members in the Phosphor Technology Center of Excellence, research on the growth and characterization of modulation doped ZnS:Mn and of Ca0.95Sr0.05Ga2S4:6%Ce are reported. ZnS:Mn has been grown using MOCVD and incorporation of Mn in 1 to 5 layers from 5 to 20 nm thick separated by layers of pure ZnS from 5 to 50 nm thick. This is shown to result in lower threshold voltages for ACTFELD displays. The luminescence spectra from sputter deposited, cerium-doped thiogallate thin films were measured and the diffusion of thin ZnS passivation layers versus temperature of heat treatment was discussed.

Electrophoretic deposition is a process used in producing display screens, in particular highresolution CRT screens. The deposition of phosphor particles from isopropanol suspensions is investigated. It is shown that two materials deposit during electrophoretic deposition on a cathode: the phosphor particles, and hydroxides formed by chemical and electrochemical reactions occurring at the cathode. A simple model to predict the deposition rates of these two materials, given the particle and ion concentrations and mobilities, is developed. Finally, the microstructure of the resulting deposited screen is investigated using scanning and transmission electron microscopy.

A novel Thin Film Light Emitting Diode (TFLED) flat panel display was developed. The TFLED is a carrier injection-type electroluminescence and made of hydrogenated amorphous silicon-based semiconductor p-i-n junctions. The amorphous layers employed in this work are for example, a-Si1−xCx:H and a-Si1−xNx:H. The TFLED has two basic structures; 1) glass substrate/ITO/amorphous p-i-n layers/Al and 2) metal sheet substrate amorphous n-i-p layers/ITO. The typical thicknesses of the amorphous p-i-n layers are 150 Å, 500 Å and 500 Å, respectively. The color of the emission can be changed from red to white-blue by increasing the optical energy gap (2.5−3.5 eV), that is the atomic fraction x, in the i-layer. The brightness of the TFLEDs are of the order of 1–10 cd/m2 with injection current density of 100–1000 mA/cm2 and applied voltage of 8–15 V.

We discuss the possible preparation techniques of the light emitting nanocrystalline silicon for large area electroluminescence (EL) displays and the control and optimization of the EL properties of the material. Some open problems are addressed and their possible solutions briefly discussed.

Laser ablation and post annealing was employed for the synthesis of silicon (Si) clusters on highly oriented pyrolytic graphite surfaces in an ultra high-vacuum environment. The size distribution of the clusters was determined as a function of annealing time and temperature using an in situ scanning tunneling microscope (STM). Pure Si clusters with sizes ranging from 1 to 10 nm showed no detectable photoluminescence (PL) in the visible range. Exposure of these clusters to oxygen at 10−6 Torr and for up to 8 hours showed adsorption of oxygen on the surface of the clusters without Si oxide formation and no detectable PL. Hydrogen termination of these clusters was accomplished by exposing them to atomic hydrogen beam but did not result in any detection of the PL spectra either. Prolonged exposure of these clusters to ambient air, however, resulted in strong PL spectra with color ranging from red to greenish-blue depending on average cluster size. Auger electron spectra (AES) revealed the existence of partially oxidized Si clusters for these samples. This PL could be due to either an oxide phase or to quantum confined Si inner cores.

Flat-panel displays based on cold-cathode field emission are actively being developed in a number of laboratories world-wide. Once fully developed, Field Emitter Displays (FEDs) will compete directly with Liquid Crystal Displays (LCDs). FEDs offer several significant advantages over LCDs including higher screen brightness, wider viewing angle, lower power consumption, and operation over a broader temperature range, making FEDs desirable for applications such as lap-top computers. Presently, several materials are being evaluated for use as coldcathode field emitters including molybdenum1, silicon2, and DLC3. At this time it is unclear which material (or materials) will ultimately be incorporated into commercial or military flatpanel display products. Each potential cathode material has its own set of advantages and disadvantages, and the ultimate choice will depend on the particular display requirements and architecture.

By the end of 1994, flat-panel displays for lap-top computers and televisions will exceed $4.0 billion in total sales. Currently, most such displays are based entirely upon an advanced liquid crystal technology requiring several precisely aligned LC layers. A new technology for flat-panel displays, based upon the solid-state luminescence of sol-gel derived microspheres has been proposed. Silica gel-derived microspheres can be prepared at near ambient temperatures, doped with either optically-active organic dye molecules or lanthanide ions. Microspheres doped to give red, blue, and green luminescence can be arranged in a pixel pattern to form the basis of a display screen. A single monochrome LC layer is used to modulate the excitation light, or pump source, that activates the fluorescence of each pixel.