Abstract
High-quality crystalline nanostructured ZnO thin films were grown on sapphire substrates by reactive sputtering. As-grown and post-annealed films (in air) with various grain sizes (2 to 29 nm) were investigated by scanning electron microscopy, X-ray diffraction, and Raman scattering. The electron–phonon coupling (EPC) strength, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminished by reducing the ZnO grain size, which mainly relates to the Fröhlich interactions. Our finding suggests that in the spatially quantum-confined system the low polar nature leads to weak EPC. The outcome of this study is important for the development of nanoscale high-performance optoelectronic devices.
Highlights
Bulk ZnO is an n-type semiconductor material with a direct wide band gap energy (3.37 eV) and large exciton binding energy (60 meV) at room temperature
We report on the effect of post-annealing on the structural and the optical properties of the magnetron sputtering deposited ZnO thin films
The main focus of this study is to examine the influence of the grain size on the evolution of phonon confinement and the strength of electron–phonon coupling (EPC) using Raman spectroscopy
Summary
Bulk ZnO is an n-type semiconductor material with a direct wide band gap energy (3.37 eV) and large exciton binding energy (60 meV) at room temperature. These properties make ZnO one of the most promising oxide semiconductors for high-performance optoelectronic devices, such as touch screens, liquid crystal displays, light-emitting diodes, chemical/biological sensors, dye-sensitized solar cells, and piezoelectric devices [1,2,3]. The electric field within a material correlates to Coulomb interactions with the exciton, and the strength of EPC will be enhanced if the wavelength of the phonon vibration is comparable to the spatial extent of the exciton [8,9,10,11,12,13] Such a quantum-confined spatial system of semiconductors is possible to prepare. The optical and the electrical properties of such system differ significantly from their bulk counterparts
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