Optoelectronic Memory Element with the Application of Photoconducting and Electroluminescent Thin Film Devices
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OPTOELECTRONIC MEMORY ELEMENT WITH THE APPLICATION OF PHOTOCONDUCTING AND ELECTROLUMINESCENT THIN FILM DEVICES Zbigniew W. PORADA Technical University, Institute of Electrical Engineering and Electronics, Warszawska 24, PL-31-155 Krak6w, Poland ABSTRACT An optoelectronic memory element was constructed from two photoconducting (PC) and two electroluminescent (EL) devices. Between one EL device and one PC device an optical feedback was provided, so that these two components constituted a bi-stable system. Experimental details of preparation of the PC-EL type thin film systems and results of investigations of the memory element are presented. INTRODUCTION Thin film systems composed of a photoconducting (PC) and an electroluminescent (EL) devices are interesting because of their possible applications in optoelectronic logical circuits. The optoelectronic memory element can be constructed with two photoconducting and two electroluminescent devices. Applied PC and EL devices within the optoelectronic memory element ought to perform the determined condition, namely the optical feedback between them must exist. A value of the feedback coefficient ought to be such as to perform the bi-stable circuit operation. This optoelectronic memory element was supplied with a sinusoidal voltage. The input signal was in the shape of recoutput The tangular light pulses illuminating the PC device. signal was in the form of the light emitted from the electroluminescent cell. EXPERIMENTAL DETAILS AND RESULTS A thin film PC-EL optoelectronic memory element was made by vacuum evaporating photoconductive and electroluminescent layers. The photoconducting device (Fig. 1) was prepared as a sandwich-type and gate-type systems on a Corning 7059 glass substrate. The first layer was a transparent electrode of tin-doped In 2 0 3 , obtained by the reactive cathode sputtering of a 90 % In 10 %Sn alloy. The second layer was a photoconductive CdS film [1,21 doped with copper and chlorine and evaporated under vacuum at a pressure of 0.007 Pa. Special purity cadmium sulphide undoped was used evaporated from an alundum crucible heated by a tungsten The temperature of the source was in the range wire resistor. of 900 0 C to 9500C. The distance of the substrate from the source was 12 cm. During evaporation the temperature of the substrate was held constant at 1500C. with Cu and C1, and CdS films were simultaneously doped, recrystallized by heating the films whilst embedded in a CdS Mat. Res. Soc. Symp. Proc. Vol. 228. (011992 Materials Research Society
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powder that had previously been doped with Cu and Cl. The recrystallization was carried out at 550 C in air for 30 min. The thickness of CdS(Cu,Cl) films was in the range 6-14 um. A metallic indium layer, deposited by vacuum evaporation, formed the upper electrode.
In 2 03 (Sn)
Fig.
1.
dCuC)IZ3S)
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The arrangement of thin film PC-EL system of an optoelectronic memory element
The electrolum~inescent cell (Fig. 1) was a thin film capacitor with an average thickness of 0.8 pm produced by the vacuu
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