Graduate Thesis Or Dissertation
 

A study of ZnS:Mn electroluminescent phosphors grown by halide transport chemical vapor deposition

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  • A low pressure halide transport chemical vapor deposition (HTCVD) system to grow ZnS:Mn electroluminescent phosphors is characterized. Reactor parameters such as gas composition, gas flow rate, and source and substrate temperature are investigated. Crystal structure is investigated using x-ray diffraction, electron spin resonance, and transmission electron microscopy. Chemical characterization includes electron microprobe and Auger electron spectroscopy. Double-insulating alternating current thin film electroluminescent devices are constructed around the HTCVD phosphors. The devices are studied using electroluminescence (brightness-voltage), photoluminescence and electrical characterization. The luminescent properties of films with a (002) preferred orientation are studied. A maximum electroluminescent brightness of 1475 cd/m² is achieved. The photoluminescence (PL) of ZnS:Mn films grown at different substrate temperatures is compared. The intensity correlates to Mn concentration. Red emission is seen in films grown at lower substrate temperature which have low Mn concentration. Mechanisms proposed in the literature cannot explain the red emission. A blue PL ZnS film intentionally doped with chlorine is achieved. This blue emission is associated with self-activated (SA) emission. Hexagonal and cubic thin-film ZnS:Mn electroluminescent phosphors are grown by HTCVD. Processing conditions, most notably introduction of a H₂S ambient, lead to a change in the preferred orientation and phase of the polycrystalline thin film. In addition to the commonly reported growth along the closest packed plane [(111) for cubic crystal structure or (002) for hexagonal], thin films have been grown along the less dense cubic (311) direction. The electrical characterization of ZnS:Mn ACTFEL devices with phosphors having different structure and preferred orientation is studied. A comparison of different preferred orientations and structures on conduction charge, obtained by internal charge-phosphor field (Q-F[subscript]p), is performed. When grown in the (311) direction, the conduction charge of a ZnS:Mn ACTFEL device increases from 2.3 μC/cm² to 5.0 μC/cm². Moreover, the leakage charge, Q[subscript]l[subscript]e[subscript]a[subscript]k, of the (311) HTCVD films is small compared to other devices.
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