Atomic layer deposition of quantum-confined ZnO nanostructures

Abstract

The modulation of optoelectronic properties, such as the bandgap of apure-component semiconductor material, is a useful ability that can beachieved by few techniques. Atomic layer deposition (ALD) was used here toexperimentally demonstrate the ability to deposit films that exhibit quantumconfinement on three-dimensional surfaces. Polycrystalline ZnO films ranging from∼1.5 to 15 nm in thickness were deposited via ALD using diethylzinc and hydrogen peroxide at100 °C. Conformal, pinhole-free films were deposited on Si wafers and on nanosized sphericalSiO2 particles using an augmented central composite design strategy. Powderx-ray diffraction was used to measure the crystallite size of the films andmonitor size evolution on the basis of the number of ALD cycles and thermalannealing post-treatments. The absorbance of the ZnO films on Si wafers andSiO2 particles was measured using spectroscopic ellipsometry and diffuse transmittancetechniques, respectively. Post-deposition annealing steps increased the crystallitesize of the films, independently of the coating thickness. The ZnO bandgap wasincreasingly blue-shifted for films of decreasing crystallite size, approaching+0.3 eV at dimensions of 2–3 nm. The nonlinear bandgap response correlated well with the Brusmodel. This work represents an experimental demonstration of quantum confinement usingALD on two- and three-dimensional substrates.