Space charge generation in ZnS:Mn alternating-current thin-film electroluminescent devices

Abstract

The electrical characteristics of ZnS:Mn alternating-current thin-film electroluminescent (ACTFFL) devices grown by atomic layer epitaxy are assessed as a function of the thickness of the phosphor layer using capacitance-voltage (C-V) and internal charge-phosphor field (Q-F,) analysis. Deviations from the ideal in the measured CW and Q-F, characteristics are ascribed to the generation of space charge in the phosphor layer during ACTFEL device operation. C-V overshoot deviations are correlated to space charge generation via a simulation employing a single sheet of charge model which assumes that the centroid of the generated space charge is located at a discrete sheet within the phosphor. Space charge generation in these atomic layer epitaxy (ALE) ZnS:Mn ACTFEL devices is ascribed to impact ionization of the zinc vacancy portion of chlorine-zinc vacancy self-activated defect complexes. A thermodynamic argument is provided which suggests that zinc vacancies are created via self-compensation of ZnS when the ZnS is unintentionally doped with chlorine. It is contended that space charge generation could even be desirable in ALE ZnS:Mn ACTFEL devices because it leads to better aging stability and improved performance. 0 1995 American Institute of Physics.

Electrical Characterization of Thin-Film Electroluminescent Devices

“…We have observed two types of C-V deviations in which C cv i > C phys i (18). In the first case, the dynamic capacitance increases to a value of capacitance in excess of C phys i and then decreases to a lower value of capacitance but still larger than C phys i ; we refer to this as capacitance overshoot.…”

“…. This situation arises when dynamic space charge generation occurs within the phosphor (17,18). Additionally, C cv i is found to be greater than C phys i only if space charge is both created and trapped or localized within the bulk region of the phosphor (19).…”

“…Typically, we prefer to employ C phys i and C phys p for Q-F p analysis. However, sometimes it is advantageous when attempting to analyze ACTFEL devices that possess dynamic space charge generation to treat the C i and C p parameters in the Q-F p equations as freely adjustable parameters and to adjust these parameters until the most ideal Q-F p curve is obtained (ideal is defined as a Q-F p curve that has the most vertical and horizontal characteristics over the respective BC-GH and DE-IJ portions of the Q-F p curve) (18,(24)(25)(26). We refer to these adjusted parameters as C qfp i and C qfp p ; we believe that the deviation of these parameters compared with C phys i and C phys p is a measure of the amount and location of the dynamic space charge generated (18).…”

Electroluminescent characteristics of ZnGa2O4:Dy3+ thin film devices fabricated on glass substrates

“…The chromatic quality of the EL emissions was found to be independent of the variations in the applied voltage and the presence or absence of the interfacial ZnO layer. That is, the EL output is quite stable with variations in the applied voltage 39. It is noteworthy that the color stability is ensured over a wider range of the applied voltage when a ZnO layer is deposited between the active layer and the substrate.…”

Kinetics and aging in atomic layer epitaxy ZnS:Mn ac thin-film electroluminescent devices

“…Results from aging experiments on devices with a different phosphor layer thickness also confirm the importance of the electron energy in the aging rate. 24 Possibly aging is caused by a defect creation, which appears to be a combined process for which both thermal energy and the energy of accelerated electrons are needed. The latter can be several electron volts.…”