Abstract

Thin film electroluminescence (TFEL), based on the ZnS:Mn system, is an active area of study for a flat-panel display technology. Although AC driven devices have been widely investigated and are currently in the market place, DC TFEL has many attractions from a commercial viewpoint. Unfortunately considerable difficulties exist in obtaining stable operation in DC structures and hence progress has been slow. The present authors have recently reported DC TFEL in a novel I.T.O./ZnS:Mn/p-n⁺ Si diode structure capable of limiting the device current during luminescence¹. In contrast with the very strong dependence of current (I) on voltage (V) observed during light emission in conventional structures, the new devices display a pronounced "knee" or reduction in the rate of growth of current with voltage at a voltage V_c corresponding to the onset of electroluminescence. A tentative model describing the I-V characteristics was proposed in reference 1.

Summary Thin film electroluminescence (TFEL), based on the ZnS:Mn system, is an active area of study for a flat-panel display technology. Although AC driven devices have been widely investigated and are currently in the market place, DC TFEL has many attractions from a commercial viewpoint. Unfortunately considerable difficulties exist in obtaining stable operation in DC structures and hence progress has been slow. The present authors have recently reported DC TFEL in a novel I.T.O./ZnS:Mn/p-n⁺ Si diode structure capable of limiting the device current during luminescence¹. In contrast with the very strong dependence of current (I) on voltage (V) observed during light emission in conventional structures, the new devices display a pronounced "knee" or reduction in the rate of growth of current with voltage at a voltage V_c corresponding to the onset of electroluminescence. A tentative model describing the I-V characteristics was proposed in reference 1. In the present paper, in addition to further I-V and brightness-voltage data, we present capacitance (C) versus voltage characteristics for a variety of devices. The C-V data also exhibit a 'knee' or 'step' at V^ under forward bias conditions (ZnS electrode positive). The observations are consistent with the previous model in which the device current undergoes a transition from primarily ZnS-limited to semiconductor-limited in the vicinity of V . Below this voltage the ZnS layer is essentially insulating. An inversion layer of electrons thus builds up at the ZnS/Si interface with increasing forward bias. The voltage across the Si surface depletion region remains approximately constant whilst that across the ZnS increases. Eventually, at V , the latter becomes 'leaky' due to high field effects and further buildup of the inversion charge is prevented. A non-equilibrium mode ensues in which more and more of the applied voltage is dropped across the Si surface depletion region. The latter widens to satisfy current continuity and the device capacitance therefore falls below its inversion value and displays the observed 'knee'. The device goes into deep-depletion and its j behaviour becomes governed primarily by the supply of electrons from the depletion region. The I-V characteristics thus ’flatten-out' due to the current-j limiting action of the depletion layer. Data will be presented on the critical field for appreciable charge leakage through the ZnS films as a function of film thickness. Preliminary results i on the behaviour of devices equipped with a third 'control' electrode to the p epi-layer region will also be presented.

© 1984 Optical Society of America