Paul Matthew Alt

A simple model for the hysteretic behavior of ZnS:Mn thin film electroluminescent devices

A model is proposed for the observed hysteretic behavior of ac‐coupled ZnS:Mn thin‐film electroluminescent devices. The following mechanisms are invoked: (1) tunnel injection from ZnS‐dielectric interfaces (E4106 V/cm), (2) electron‐hole pair generation, (3) deep trapping of holes, leading to space‐charge formation, (4) charge storage at the ZnS‐dielectric interfaces, and (5) direct recombination of injected electrons and trapped holes. When these mechanisms are combined in a self‐consistent numerical simulation model, a bistability of charge transfer versus applied voltage is obtained which exhibits many of the characteristics of the observed device behavior. Experimental evidence in support of the individual assumptions is also discussed.

Experimental results on the stability of ac thin‐film electroluminescent devices

Experimental results on the stability of ac‐coupled thin‐film electroluminescent devices are presented. The luminance‐voltage characteristics of ZnS:Mn devices, including both memory and nonmemory types, were measured throughout constant luminance aging experiments. Deposition conditions were varied, and post‐deposition treatments by ion implantation and thermal annealings were investigated. We find a pervasive shift in the threshold voltage, and a corresponding loss of hysteresis in memory devices. We postulate that memory loss results from the creation of shallow interface states. The responsible physical mechanism is very sensitive to both hydrogen and oxygen incorporation in the ZnS film; in similar fashions, H or O increases the rate of memory loss. By comparison, the devices were insensitive to common contaminants such as Na, Cl, and F. We have found that hydrogen is generated during the deposition process. Even so, some of our devices exhibited significant memory for several thousand hours.

A gray-scale addressing technique for thin-film-transistor/liquid crystal displays

An addressing technique aliowing continuoustone color images to be rendered on thin-fllm-transistor/ilquid crystal displays having bilevel drivers Is described. The technique uses multiple subflelds per frame, with driver voltages changed synchronously with the field data. By using N bits of data per pixel, excitation Is applied to the display one bit-plane per field for N consecutive fields. The technique Is analyzed, Its benefits and limitations discussed, and experimental results presented. Up to 16 gray levels have been demonstrated with good Image quality.

A one-megapixel reflective spatial light modulator system for holographic storage

A prototype reflective spatial light modulator (SLM) system has been developed for writing megabit pages of data into a holographic medium at a rate of 12 pages per minute. The SLM is based upon a crystalline-silicon reflective active-matrix array with integrated data drivers, using liquid crystal (LC) electrooptics and a personal computer system with an interface to provide data. The LC has been optimized for high contrast and efficiency with coherent illumination. The resolution-limited contrast was measured at 4:1, which was high enough to provide bit-error-free charge-coupled-device images using modulation and error-correction codes.

Design and fabrication of a prototype projection data monitor with high information content

A prototype 28-in.-diagonal desktop data monitor capable of displaying 2048 × 2048- pixel images has been designed, built, and evaluated. The monitor uses optical projection technology. A reflective, crystalline silicon active-matrix light valve using liquid crystal electro-optics and a digital electronic interface architecture is described. This rear-projection monitor has four million resolvable pixels, uses three light valves to achieve color, has a low-gain surface diffuser screen, and functions as a fully interactive, color personal computer monitor with motion video capability. The monitor is 20 in. deep.

A 10.5-in.-diagonal SXGA active-matrix display

A 157-dot-per-inch, 262K-color, 10.5-in.- diagonal, 1280 × 1024 (SXGA) display has been fabricated using a six-mask process with Cu or Al-alloy thin-film gates. The combination of high resolution and gray-scale accuracy has been shown to render color images and text with paperlike legibility. The low-resistivity gate metallization and trilayer-type TFTs with a channel length of 6-8 µm were fabricated with a six-mask process which is extendible to larger, higher-resolution displays. A combination of double-sided driving and active line repair was used so that open gate lines or data lines did not result in visible line defects. A flexible drive-electronics system was developed to address the display and characterize its performance under different drive conditions.

Eliminating crosstalk in thin film transistor/liquid crystal displays

One of the problems inherent in active matrix displays, which can prevent them from achieving gray-scale images of CRT quality, is crosstalk, whereby information is coupled along columns of the display. While alternating data voltages line by line can reduce such crosstalk, it requires a sacrifice of resolution in the displayed image. The authors have developed a scheme for reducing crosstalk by a cancellation technique based also on a change of drive waveforms, but producing no loss of resolution. This approach does, however, require a factor of two increase in bandwidth or switching speeds. The technique can be applied to all types of active matrix displays, including thin-film-transistor, MIM, and diode types.<<ETX>>

Optical switching in thin film electroluminescent devices with inherent memory characteristics

Quantitative data is presented concerning the optical process of writing and erasing of memory electroluminescent (EL) devices. The switching process is dependent on the energy of the exciting photons and the energy required varies from approximately 10 µJ/cm2at 325 nm (associated with bandgap photoconductivity)to about 75 mJ/cm2at 515 nm (associated with impurity photoconductivity). Experimental evidence is presented to show that necessary and sufficient conditions for EL cell erasure are the photoconductive discharging of the insulator-ZnS charge and the decay of the bulk excitation of the ZnS:Mn film. The technique of photoinduced detrapping of the stored space charge is shown to lead to the determination of midgap as the approximate location in energy of the deep traps in one of our sample films.

Device characterization of an electron-beam-switched thin-film ZnS:Mn electroluminescent faceplate

The concept of a storage CRT display using a thin-film electroluminescent (EL) faceplate is discussed. The storage is based on the hysteretic behavior of some thin-film EL devices made from ZnS:Mn. Results are presented of experiments in which the writing and erasing of such devices with electron beams were investigated in detail. Since realization of the concept involves large-area hysteretic thin-film devices having stable characteristics, the constraints and problems arising from these requirements and the current status of efforts to meet them are discussed.

Increasing electronic display information content: an introduction

This paper is an introduction to a group of eighteen papers on electronic display research and development within IBM. Displays in the computer industry and some of the directions display research is taking are discussed. A motivation for the IBM work on increasing the spatial resolution and image content of various liquid crystal displays is offered. Finally, the significant progress in IBMs display technology over the last decade is discussed.