Hiroshi Sasakura

The dependences of electroluminescent characteristics of ZnS:Mn thin films upon their device parameters

The dependences of brightness, emission efficiency η, average electric field EA, conduction current JA, and emission lifetime τ upon the device parameters such as film thickness, substrate temperature during evaporation, and Mn concentration have been systematically investigated in ZnS:Mn thin‐film electroluminescent devices. The value of η increases rapidly with film thicknesses below 3000 A but EA decreases slowly. These results can be explained by the increase of the crystallinity of the ZnS:Mn films. The value of η increases with the Mn concentration and reaches its maximum at about 0.45 wt %. At above this Mn concentration, η and τ decrease rapidly, EA increases, and JA decreases slowly. These results may be attributed to a decrease of hot electron energy and/or an increase of the nonradiative transition probability of the excited Mn centers. The brightness‐voltage (B‐V) hysteresis characteristic is observed in this Mn concentration region. This memory effect is also discussed.

Evidence for the direct impact excitation of Mn centers in electroluminescent ZnS:Mn films

In order to clarify the excitation mechanism of Mn luminescent centers in ZnS:Mn electroluminescent thin films, time‐resolved emission spectra in photoluminescence and electroluminescence were measured. In photoluminescence, both emissions arising from ZnS host and Mn centers were observed. For the emission arising from Mn centers, the time delay showing energy transfer from ZnS host to Mn centers was recognized. In electroluminescence, contrary to this, the emission from the ZnS host was hardly observed, and the emission from Mn centers was observed immediately after excitation. These experimental results strongly suggest that the excitation mechanism of Mn centers in electroluminescence is due to the direct impact excitation of Mn centers by hot electrons accelerated in the high electric field in the Zns host. These results are also supported by efficiency measurements.

Electroluminescence in thin‐film CaS:Ce

We report a double insulated CaS:Ce thin‐film electroluminescent (EL) device which emits a bright green EL due to Ce3+ luminescent centers, being characteristic of parity allowed 5d–4f transitions. A brightness level of 500 cd/m2 and emission efficiency of 0.11 lm/W have been obtained under 5‐kHz sinusoidal voltage excitation. The CaS:Ce thin film has been fabricated by coevaporation of CaS and sulfur.

Excitation Mechanism of Electroluminescent ZnS Thin Films Doped with Rare-Earth Ions

Electroluminescence(EL) and photoluminescence(PL) experiments have been carried out on the ZnS: Nd, ZnS: Cu, Cl and ZnS: Cu, Cl, Nd films. The EL emission spectra show two peaks due to Cu-Cl and Nd centers, while the PL emission spectra show only one peak due to Cu-Cl centers. By comparing the efficiencies and the emission spectra of the EL with the PL measurements, it has been clearly shown that the excitation mechanism of the Nd ions in the electroluminescent ZnS: Nd film is via direct impact excitation by hot electrons.

Voltage Dependence of Brightness in Rare-Earth Doped Electroluminescent ZnS Thin Film Devices

Variations of the brightness and efficiency of Ta-Ta2O5-ZnS: Ln3+-Au (M-I-S-M structure) thin film devices with the applied voltage across the ZnS layer doped with rare-earth ions have been investigated precisely. From the analysis of the experimental data, the operating regions of the devices can be classified into the follwing three separate regions: (1) the low voltage region where no light emission is observed, (2) the intermediate voltage region where the light emission occurs and the brightness B is given by the form B∝exp (-V0/Vp) and (3) the high voltage region where the electric field is clamped at a certain constant value and the brightness is expressed as B∝(Vp-Vb). The field clamping is attributed to the avalanche of the ZnS lattice electrons.

The Electron Injection Mechanism of the Electroluminescent ZnS: Tb3+ Films

The role of interfacial states in the electroluminescence process for Ta-Ta2O5-ZnS: Tb3+-Au and Ta-Ta2O5-ZnS: Tb3+-SiO2-Au structures has been studied. A similar electroluminescence has been observed for both types of cells. This indicates that electron injection via tunneling from the Au contact is not necessarily required to produce electroluminescence. The relaxation times of electrons related to the injection process from interfacial states are measured to be 10–100 ms. These values are the same for three interfacial states of interest (ZnS-Au, ZnS-SiO2 and ZnS-Ta2O5). Their temperature and electric field dependences are also presented. It can be concluded that field emission from the interfacial states is essential to the electron injection process for these types of cells.

Heat Treatment Effects of ZnO:Al Thin Films Prepared by Facing-Target-Type Sputtering Method

ZnO:Al films were deposited by a facing-target-type method on a glass substrate, which is held at 300°C. The minimum resistivity was obtained with an Al concentration of 1.3%. The decrease of resistivity after the heat treatment of ZnO:Al (1.3%; calculated from area ratio of target) film seems to be mainly attributed to the increase of Hall mobility. The improvement of Hall mobility is not due to the improvement of crystallinity, but it may be due to the decrease of oxygen impurities at interstitial sites.

Thin-film DC electroluminescent devices of an In 2 O 3 -ZnS : Mn-ZnSe-Al structure

Thin-film electroluminescent (EL) devices having a conductor-semiconductor-resistor-metal (CSRM) structure (In2O3-ZnS: Mn-ZnSe-Al), that are capable of dc excitation, have been prepared. The brightness is 50 fL and the external quantum efficiency is 5 × 10-5. To clarify the effect of the ZnSe layer,B-VandI-Vcharacteristics for dc excitation were studied at room temperature. The experimental results show that the ZnSe layer acts as a distributed resistive layer that protects the ZnS:Mn emitting layer against breakdown due to avalanche current.

Photoquenching of electroluminescence in ZnS:TbF3 thin films

The photoquenching effect of electroluminescence (EL) has been observed for the first time in thin‐film ZnS:TbF3 EL devices. The luminescent intensity and the conductive charge are quenched, when the device is exposed to a visible light of energy between 1.75–2.75 eV. The maximum photoquenching rate of 45% is observed at a voltage slightly higher than the EL threshold voltage. The recovering time from the photoquenching is of the order of several seconds. This photoquenching effect is likely to be due to deep hole traps.

Electroluminescent mechanisms of ZnS:Mn and ZnS:TbF3 ac thin film devices

Abstract Electroluminescent mechanisms of ZnS:Mn and ZnS:TbF 3 devices have been investigated systematically by measuring the concentration dependences of the brightness, the emission efficiency, the emission lifetime, the electric field and the conduction current. From these dependences, the mechanisms, which optimize Mn and Tb concentrations, are proposed to obtain the highest brightness. The properties of the Tb emission centers and memory mechanism of ZnS:Mn are also discussed.