Low-voltage driven ZnS:Mn MIS and MISIM thin film electroluminescent devices with Eu₂O₃insulator layer

Abstract

AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.

Effects of insulating layer on the performance of thin film electroluminescent devices

“…The brightness voltage (BV) characteristics of the device with MgF 2 and Eu203 insulating layers shows that the threshold voltage for the onset of emission increases with increase of excitation frequency (in the present experiments frequency-was varied from 50 Hz to 5 kHz). This anomalous behaviour is explained elsewhere (Pillai and Vallabhan 1986;Jayaraj and Vallabhan 1990a). Figure 2 shows the B/'V characteristics of the MIS devices with an active layer thickness 0.3/zm and insulator thickness 0-2 #m fabricated under similar conditions.…”

“…The optimal condition for MIS structure devices for low v~tage operation is when the thickness ratio between active layer and insulator layer lies between 1 and 2. A detailed discussion on this aspect is given elsewhere (Jayaraj and Vallabhan 1990a). The brightness of a typical EL cell is estimated in absolute units using a PMT having a spectral response closely matchihg the sensitivity of human eye and it is found to be 1450 ft-lamberts.…”

“…In order to fabricate legible low voltage and high brightness TFEL devices in MIS structure with ZnS:Mn, insulating layers of different figure of merit (Sm203, Eu20 3, NaaAIF6, MgF2, CeO2, and SiO) have been investigated here. For low voltage and high brightness the optimal thickness of ZnS layer and insulator (in the case of Eu20 3 and Sm203 *For correspondence, films) is found to be for a thickness ratio lying between 1 and 2 (Jayaraj and Vallabhan 1990a). This suggests the possibility of lowering the voltage using insulators of high figure of merit in MIS-structured devices.…”

AC Thin Film Electroluminescent Devices with Rare Earth Doped ZnS

“…8 for the MISIM structures and tional Y203 device which requires a driving voltage 150 (19). For MIS structure the driving voltage is below 50 V. _3 The brightest of the ZnS:Re, C1 devices studied, 1 O ZnS:Tb, C1, has a brightness of 500 fL, about one-third of a typical ZnS:Mn EL cell fabricated in the laboratory (11). The brightness of our ZnS:Tb, C1 devices, though the best among these, is only 35% of that of ZnS:Mn device.…”

“…In the case of devices with Sm203 as insulator, the thickness ratio (tits) of active layer (tz) to insulator (t,) was found to lie between 1 and 2 for optimal low-voltage operation of TFEL devices. The details of this aspect are given elsewhere (11). For maximum brightness and minimum threshold voltage, the present devices were prepared with an active layer thickness of 0.3 i~m and insulator thickness of 0.2 i~m.…”