Graduation date: 1993

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

Includes bibliographical references.

published_or_final_version / abstract / Electrical and Electronic Engineering / Master / Master of Philosophy

No description available.

Graduation date: 2004

Graduation date: 1998

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

The goal of this thesis is to identify and to explore novel ACTFEL phosphor materials. Several important materials properties relevant to ACTFEL phosphor development are identified. All of these properties cannot be obtained simultaneously. Therefore, several key phosphor materials properties are identified as critical to the development of an ACTFEL phosphor. Then, using basic chemical trends, several classes of potential ACTFEL phosphors are identified. These materials systems include halides, nitrides, oxynitrides, oxides, sulfides, and inhibited concentration quenching systems. Representative materials from some of these proposed novel ACTFEL phosphor materials system are developed and evaluated as electroluminescence phosphors. Most of the ACTFEL devices made using these materials do not show any significant charge transfer. Detailed analysis indicates that the most probable cause of the lack of charge injection is that the phosphor threshold field is too large. This excessively large threshold field may be associated with the energy depth of the interface states, the low density of the interface states, or the large effective mass of the phosphor material explored. Several possible alternative solutions are presented to reduce the threshold field of the phosphor. These includes the use of thick-film insulator, the use of a charge injection layer, the use of a ceramic substrate coupled with a high temperature interface reaction, and the use of bulk doping of the phosphor. / Graduation date: 1997

Graduation date: 1997