Novel ultra-violet/blue optoelectronic materials and devices based on copper halides (CuHa)
Cowley, Aidan (2012) Novel ultra-violet/blue optoelectronic materials and devices based on copper halides (CuHa). PhD thesis, Dublin City University.
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Considerable research is being carried out in the area of wide band gap semiconductor materials for light emission applications in the UV/Blue (300-400 nm) spectral range. This project explores the novel use of the Copper Halides (CuHa), specifically γ-CuCl and γ-CuBr, I–VII wide band gap mixed ionic–electronic semiconducting materials with light emitting properties suitable for novel UV/blue light applications. This project details novel research carried out towards achieving single crystal growth of γ-CuCl
from solution via Liquid Phase Epitaxy (LPE) based techniques. LPE growth runs are undertaken using an alkali halide flux compound (KCl) to depress the liquidus temperature of CuCl below its solid phase wurtzite-zincblende (β → γ) transition temperature for solution based epitaxy on lattice matched Si substrates (lattice constant of γ-CuCl (0.541 nm) is closely matched to that of Si (0.543 nm). Results show that the resulting KCl flux-driven deposition of CuCl onto the Si substrate has
yielded superior photoluminescence (PL) and X-ray excited optical luminescence (XEOL) behaviour relative to comparatively observed spectra for GaN or polycrystalline CuCl. The resulting deposited material is a textured CuCl/K2CuCl3 polycrystalline intermix, with strong broad
luminescence and novel luminescent characteristics not previously observed in CuCl. Difficulties inherent to LPE with CuCl/KCl melts, particularly with the CuCl/KCl eutectic system and the CuCl/Si surface reaction, are detailed.
The use of γ-CuBr for thin film based blue light emitting devices is investigated. Its structural and physical properties allow for vacuum deposition on a variety of substrates and herein we report on the deposition of γ-CuBr on Si, glass and indium tin oxide coated glass substrates via vacuum evaporation with controllable film thickness from 100 to 500 nm. Temperature dependent photoluminescence characteristics of these γ-CuBr films on Si substrates reveal familiar Zf and I1 excitonic features. Work towards the development of a thin filmelectroluminescent device using a γ-CuBr active layer is outlined.
Recently, dramatic improvements in the luminescent intensity of CuBr generated by the chemical interaction between CuCl films and KBr substrates have been demonstrated. The potential improvements in excitonic PL that can be gained from novel approaches to film preparation involving KBr and existing CuBr deposition techniques is promising. We report on the one such
novel approach, the vacuum deposition of KBr spots (~30 µm radius) onto similarly deposited γ-CuBr epitaxial layer on a Si substrate. Post-deposition annealing of the samples at 220 °C in conjunction with a small CuBr flux from a target source leads to the formation of intermixed CuBr/
KBr microdots. PL characterisation reveals enhanced UV-Blue excitonic emission centered on the Zf free exciton peak at ~418 nm, far superior to Zf emission from γ-CuBr films deposited previously.
An overview of the deposition process involving shadow masks to lay down an ordered array of KBr spots onto a γ-CuBr vacuum evaporated layer is presented, and the samples are characterised using XRD, EDX and spatially resolved room temperature PL.
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