Two methods of electro-optically characterizing alternating-current thin-film electroluminescent
(ACTFEL) devices are investigated: photo-induced transferred charge
(PIQ) and luminescence (PIL), and subthreshold voltage-induced transferred charge (VIQ)
techniques. Both techniques provide information related to traps within the phosphor
layer. PIQ/PIL experiments monitor the transport of electrons and holes across the phosphor
layer which are photo-injected by a UV laser pulse. VIQ experiments monitor the
optical reset of traps ionized by bipolar subthreshold voltage pulses.
PIQ/PIL experiments are performed on three different SrS ACTFEL devices: ALE-deposited
SrS:Ce, sputter-deposited SrS:Cu, and undoped MOCVD-deposited SrS. From
the PIQ/PIL experiments, two distinct electron thresholds in the luminescent impurity
doped samples at ~0.8 (weak threshold) and ~1.2 MV/cm (strong threshold) are observed.
These thresholds are independent of the phosphor thickness, indicating that they
arise from a bulk property of the phosphor. The ~0.8 MV/cm weak threshold is attributed
to field emission of relatively shallow (~0.6 eV) electron-emitting bulk traps (e.g. cerium
or oxygen for SrS:Ce; a sulfur vacancy or oxygen for SrS:Cu). The ~1.2 MV/cm strong
threshold is ascribed to the onset of trap-to-band impact ionization. In contrast to electron
transport, PIQ/PIL studies reveal no hole transport in SrS doped with luminescent
impurities, although hole transport is observed for an undoped SrS ACTFEL device. The
lack of hole transport is attributed to the efficiency of hole capture in SrS doped with
luminescent impurities.
VIQ experiments are performed on the same SrS ACTFEL devices. VIQ trap energy
depths are estimated as ~0.1 eV for SrS:Ce; ~0.9 eV for SrS:Cu (with a capture cross-section
of ,~10�������cm��), and ~0.6 eV for undoped SrS. Tenative atomic identification of
traps responsible for these VIQ trends are: chlorine or a Ce shallow donor state for
SrS:Ce, a sulfur vacancy for SrS:Cu, and a sulfur vacancy or an oxygen isoelectronic trap
for undoped SrS. / Graduation date: 2000
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/34180 |
| Date | 30 April 1999 |
| Creators | Nevers, Corey A. |
| Contributors | Plant, Thomas K. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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