Towards Two-Dimensional Oxides for Optoelectronic Applications
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.219
Towards Two-Dimensional Oxides for Optoelectronic Applications Jonathan C. Rackham1*, Bin Zou1 and Michelle A. Moram1,2 *Corresponding Author: [email protected]
1
Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ U.K.
2
Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE U.K.
ABSTRACT
Preliminary thin films of barium zirconate-titanate (BZT) are grown by pulsed laser deposition from bulk ceramic targets on magnesium oxide (001) substrates. Growth conditions are optimized for film crystallinity and thick (100 nm) films are deposited. These films are seen to be growing with a cube-on-cube epitaxial relation. The lattice constant of the alloy follows Vegard’s law as expected but shows a reasonable amount of non-linearity in the relation between band gap and composition.
INTRODUCTION The perovskite oxide barium zirconate-titanate (BaZrxTi1-xO3, BZT) has wide interest as a high dielectric constant material and exhibits both ferroelectric and relaxortype behaviour for varying zirconium content. However, with its band gap lying in the range between barium titanate (BTO, 3.2 eV) and barium zirconate (BZO, 5.3 eV) [1] BZT also has promise as a wide band gap material. This band gap range is of particular value as it spans the UV-B (280-315 nm, 3.94–4.43 eV) range and part of the UV-C (100-280 nm, 4.43–12.4 eV) range. Investigation of materials with band gaps in this range is crucial for the advancement of germicidal LEDs where emission is optimal at 4.6-4.8 eV. Bulk BZT has a wide band gap spanning this range but its indirect character makes it impractical for optoelectronic devices. Following the discovery of two-dimensional materials such as graphene and molybdenum disulphide and their dramatically different properties, it is worthwhile considering the effects of constrained thickness on a material’s properties. MoS2 in particular exhibits a change in the character of its primary band gap as atomic layer count is reduced [2]. This paper outlines the preparation and initial characterisation of BZT material for further investigation into the presence of similar effects.
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EXPERIMENTAL DETAILS Bulk ceramic preparation Bulk ceramics were prepared from high purity oxide powders sourced from Sigma-Aldrich. The precursors used were 99.995% purity BaCO 3, 99.99% ZrO2 and 99.999% TiO2. These were dried and weighed individually to ensure the correct atomic percentage (resulting in BaZrxTi1-xO3, x = 0, 0.25, 0.5, 0.75 and 1.0) and mixed overnight using a high-speed rotary ball milling machine with zirconia balls. The resulting powder was calcined in an alumina crucible at 1200–1450 °C for 2–6 hours. After calcination the powders were groun
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