AC Thin-film Electroluminescent Devices Research Articles

Current and light generation in UV-irradiated AC thin film electroluminescent devices

Free carriers in the active ZnS:Mn layer of AC-driven thin film electroluminescent devices are created by broadband UV irradiation and accelerated in an applied electric field F. Measurements of the additional dissipative current yield the carrier multiplication rate α( F). α( F) increases strongly close to the threshold for light emission without UV irradiation. The absolute values of α( F) are as expected by extrapolation of data obtained on other semiconductors. An analysis of field direction dependence of carrier multiplication and light generation emphasizes the importance of space charge effects for light generation, being more complex than assumed so far.

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 new low-voltage Si-compatible electroluminescent device

Thin film direct-current electroluminescent devices with ZnS:Mn as the active light-emitting layer have been fabricated on Si wafers. The devices incorporate a high-efficiency electron injector layer capable of passing high current densities (>10-2A cm-2dc) through the active layer. A useful luminance of 78 fL has been achieved at 48V, significantly lower voltage than for conventional ac thin film or dc powder electroluminescent devices. The structure also includes a potential energy step at the interface between SiO 2 and ZnS due to the difference in electron affinities which offers the possibility of hot electron injection into the active layer. The unique combination of efficient cathode emission and hot injection possible in these devices suggests that a solid state analogue of the cathode ray tube may be realizable.

Microstructure and light emission of ac thin-film electroluminescent devices

The microstructure of ac-thin film electroluminescent devices was studied by transmission electron microscopy (TEM) of cross-sectional specimens, and was correlated to electro-optical characteristics of the devices. The cross sections reveal the microstructure of the BaTiO3/ZnS:Mn/BaTiO3 layer structure as a function of film depth. The rf-sputtered BaTiO3 films are amorphous; the electron-beam evaporated ZnS films are polycrystalline. The first-to-grow region of the ZnS layers always exhibits a very small grain size. With increasing film thickness the growth of larger columnar grains is observed. At higher substrate temperatures the grains have a more conical shape. The Mn-concentration, measured by x-ray microanalysis on the cross-sectional specimens in a scanning TEM, shows an overall increase with growing ZnS film thickness, because the ZnS was evaporated from a single ZnS:Mn source. Annealing at 550 °C improves the electro-optic characteristics of the samples considerably, without changes in microstructure being observable. Annealing at 850 °C induces strong recrystallization. The initial fine-grained region disappears and the grains extend from the bottom to the top of the ZnS film.

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.