Gert Stuyven

Atomic layer deposition of ZnS thin films based on diethyl zinc and hydrogen sulfide.

Abstract The atomic layer deposition (ALD) of ZnS based on diethyl zinc (DEZn) and hydrogen sulfide (H 2 S) was investigated. ZnS thin films were grown between 200°C and 350°C and the effect of other processing conditions was examined. The growth temperature, the DEZn dosing and the purge times were found to be decisive parameters, which indicate that the film growth is strongly affected by the limited stability of the film surface after the DEZn pulse. ZnS:Mn TFEL samples with a phosphor layer based on the DEZn process were shown to exhibit efficient light emission and improved stability and symmetry with aging compared with traditional chloride-based TFEL samples.

Characterization of the electro-optical behavior of Zn2Si0.5Ge0.5O4:Mn thin-film electroluminescent devices

Zn2Si0.5Ge0.5O4:Mn was investigated as a green-emitting oxide phosphor for thin-film electroluminescent (EL) display applications. Detailed electro-optical characteristics of thin-film EL devices with SrTiO3 dielectrics on a glass substrate (TFEL) and EL devices with a single ceramic insulator (CIEL) have been analyzed. The steeper brightness–voltage dependency of the TFEL device is ascribed to the capability to provide high current densities and efficient light generation for both voltage polarities. The CIEL device on the other hand suffered from asymmetrical behavior, related with the lack of a top dielectric. It was found that the use of high-permittivity dielectrics in the TFEL device results in a small voltage drop over the insulating layers, enhancing the amount of charge transferred through the phosphor. Transient charge–voltage (αQp–V) measurements on the TFEL device evidence that a high positive space charge density of ∼0.68 μC/cm2 is present in the oxide phosphor during steady-state operation. ...

LOW-FIELD RECOMBINATION IN SRS:CU THIN-FILM ELECTROLUMINESCENT DEVICES

Thin-film electroluminescent devices with a SrS:Cu phosphor layer emit bright saturated blue light, useful for display applications. Whereas SrS:Ce-based devices normally emit two light pulses for each voltage polarity, the studied SrS:Cu devices yield an additional light peak at lower voltages. Optoelectronic measurements combined with numerical simulations indicate that the third light peak is due to low-field recombination of electrons, which are released from shallow interface states at low fields.

Hybrid Electroluminescent Devices with Atomic Layer Deposited Thin Films on a Screen Printed Dielectric

A hybrid electroluminescent (EL) device structure was developed, consisting of a screen printed Pt/Ag conductor and high-k BaTiO3 dielectric, on which a phosphor, a thin insulator and a transparent conductor were deposited by atomic layer deposition (ALD). The use of the hybrid EL structure results in more than a doubling of brightness compared with the thin-film EL alternative structure. While hybrid EL devices with ALD-grown thin-film stack deposited directly on top of a rough dielectric yield a uniform light emission, the aging stability is determined largely by the occurrence of local breakdown events due to electric field inhomogeneities originating from the combination of the rough BaTiO3/ZnS:Mn interface and the high dielectric constant of BaTiO3. Nevertheless, lifetimes of more than 600 h at 1 kHz could be obtained.

Electrical modeling of interface roughness in thin film electroluminescent devices

If two dielectric materials with different permittivities are in contact with each other and the interface between them is rough, then the electric field near this interface will be very inhomogeneous. In thin film electroluminescent devices, light is generated when electrons move back and forth in the phosphor layer under the influence of a strong ac electric field. At high electric fields, the electrons trapped in deep states at the interface between phosphor and insulator layer tunnel into the conduction band of the phosphor. This tunnel process is very sensitive to the electric field at the interface, so for a rough interface the electron flow will be very inhomogeneous. The relation between the interface roughness and the inhomogeneous charge transfer in thin film electroluminescent devices is investigated, based on an analytical flux tube model. The importance of the inhomogeneous current for the use of gray levels and aging is discussed.

Photoluminescent ZnS:Cu phosphor films made with atomic-layer chemical vapor deposition and thermal evaporation

— Photoluminescent (PL) ZnS:Cu thin films were prepared by atomic-layer chemical vapor deposition (ALCVD) and by thermal evaporation. The deposition, structure, and PL properties of the films were investigated, as well as the effect of heat treatment on photoluminescence.

The study of the low-field behavior of optically generated charges in thin-film electroluminescent devices

— The measurement of photo-stimulated currents (PSC) or thermally stimulated currents (TSC) in ACTFEL devices is sometimes used as a method to identify energy levels of trap centers in the phosphor layer. These methods are based on the fact that optically or thermally generated charges are able to move in the polarization field in the phosphor layer of an EL device, which remains after charging the device with a voltage pulse. Similar information about space charge can be obtained by the measurement of loss factor and capacitance of uncharged EL devices under illumination during a low ac-voltage excitation. This new method avoids one of the major disadvantages of the other methods, namely, the continuous change of the field situation throughout the measurement. The measurement of low-voltage photo-stimulated currents (LVPSC) creates an identical and reproducible field situation at each wavelength, which makes interpretation more reliable. Moreover, this new method allows the extraction of information about the low-field conduction behavior of EL thin films.

Experimental Determination of the Space Charge in AC Thin-Film Electroluminescent Devices

We developed equations for the center of mass and the amount of space charge in the phosphor layer of AC thin-film electroluminescent devices. The equations are function of the mean electric field in the phosphor layer and the electric field at both interfaces of the phosphor layer. We describe an electrical measurement method to determine these electric fields, assuming a constant space charge during steady state operation. Results are given for an undoped device based on ZnS.