Webster Eugene Howard

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.

One‐dimensional numerical simulation of ac discharges in a high‐pressure mixture of Ne+0.1% Ar confined to a narrow gap between insulated metal electrodes

The present work is concerned with a one‐dimensional dynamic simulation, incorporting a self‐consistent treatment of space‐charge effects, of the buildup and decay of ac discharges in a high‐ (≳100 Torr) pressure mixture of Ne+0.1% Ar confined to a narrow gap (∼10−2 cm) between insulator (MgO) covered metal electrodes. Numerical modeling has been done of the dynamics of successive avalanches triggered by the electric‐field‐dependent secondary emission of electrons at the cathode due to ionic species, metastables, and photons. The basic continuity equations of the problem deal with the following collisional phenomena in the gas volume: (i) electron impact excitation and ionization of neon atoms, (ii) collisional ionization involving excited neon states, (iii) two‐body and three‐body collisional processes involving energy transfer, and (iv) absorption and reemission of imprisoned resonant radiation. The computed results show a satisfactory agreement with measured parameters on a single cell of a specially c...

The importance of insulator properties in a thin-film electroluminescent device

AC-coupled thin-film electroluminescent devices employ insulating layers to limit the current density through the active layers. The requirements and limitations in driving such devices are then dependent upon the properties of the dielectric layers. We define an external efficiency for an ac thin-film electroluminescent device and consider two limiting cases, current response 1) fast, and 2) slow, compared to an applied voltage change. Relationships are then derived among external efficiency, relative luminance, and the two principal characteristics of the insulating layer, specifically charge density at breakdown and thickness. It is shown, for example, that charge density at breakdown for the insulators should be at least three times the corresponding quantity for the active layer.

Effect of Crystal Orientation on Ge‐GaAs Heterojunctions

The electrical characteristics of Ge‐GaAs heterojunctions, in particular n‐n junctions, have been studied experimentally. Barrier voltages, determined from the temperature dependence of reverse current and from capacitance vs voltage, depend upon the crystal orientation of the interface. This is seen to imply a corresponding variation of the conduction band discontinuities; namely ΔEc[111]Ga > ΔEc[111]As >ΔEc[110]. The voltage dependence of the reverse current is interpreted in terms of the shift of the Fermi level with respect to the conduction band edge in the accumulation layer. The transient behavior of n‐n heterojunctions yields a switching time for 20 mA less than 10−9 sec, and no observed storage time.

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.

Memory in thin-film electroluminescent devices

Abstract The characteristics of thin-film electroluminescent devices made from ZnS:Mn and exhibiting hysteretic behavior are reviewed. The nature of the hysteresis (which leads to display devices with inherent memory), the switching characteristics, and some of the empirical requirements for observing the phenomenon are described and discussed. This background is used as a basis for a discussion of device operation, leading to a description of a phenomenological model which has been developed to explain the hysteretic behavior. Some technological difficulties associated with the phenomenon are also described.

Thin film electroluminescent displays

Abstract Thin film electroluminescence (EL) as exploited in display devices is discussed from the standpoint of a display user and a display technologist or engineer. An attempt is made to relate the characteristics of importance for displays to the electrical properties of the device and to the underlying physical processes or mechanisms. Particular emphasis is given to the widely used case of ZnS:Mn in a double-insulated sandwich structure. Some recent innovations relevant to display application are also described.

Performance characteristics of electrochromic displays

Several different types of electrochromic display devices are discussed. Their electrooptical properties and device characteristics are investigated and compared. Performance parameters for these devices are discussed in terms of their applicability to information display. It is concluded that electrochromism is best suited for direct-addressed displays.

Thin-film-transistor/liquid crystal display technology: an introduction

Liquid crystais are simpie and very efficient eiectro-optic transducers, or iiglit valves. Thinfiim transistors are simpie electronic control devices which can be fabricated on large transparent substrates. These two technologies, when combined, allow the fabrication of electronic displays which challenge the dominance of the cathode ray tube (CRT). This paper reviews the history of this important development, presents the current status in comparison to the color CRT, and describes the remaining challenges to be overcome if the color CRT is truly to be displaced.

Photoconductivity and optical properties of amorphous GeTe

Abstract Unlike crystalline GeTe, amorphous GeTe has a relatively wide energy gap of 0.7 eV at 295°K, and 0.77 eV at 77°K. Its resistivity is about 10 3 ohm-cm at room temperature, and increases exponentially with T −1 , giving an activation energy of 0.35 eV. The crystallization temperature for the amorphous GeTe films is about 390°K. The measured absorption coefficient is used to calculate a density of states which behaves very much as one would expect on the basis of the Mott model. The photocurrent for low photon energies has a long lifetime of tens of milliseconds, however, at higher photon energies, above 1 eV, the lifetime is less than 30 nsec. At low temperatures the fast lifetime is slowed to a few μsec. Near room temperatures where the thermally generated carrier density is approximately 3 × 10 16 cm −3 , obtained from our density of states, the dependence of the photocurrent on the light intensity is linear; below 150°K, the intensity dependence is a square root law, indicating bimolecular recombination.