Morphological and optical studies of self-forming ZnO nanocolumn and nanocone arrays grown by PLD on various substrates

Abstract

ZnO nanostructures were grown by pulsed laser deposition on c-sapphire, Si (111) (n-type and p-type) and analysed using morphological and optical techniques. Under optimized growth conditions, self-forming arrays of vertically aligned nanostructures were obtained. Scanning electron microscopy studies revealed two main structures: nanocolumns and a ‘moth-eye type’ array of nanocones, which gave a graded effective refractive index. X-ray diffraction measurements indicate that both types of structures are highly oriented along the c-axis. The optical properties were investigated using low temperature photoluminescence (PL) and room temperature (RT) reflection measurements. Low temperature PL spectra for nanocolumns grown on sapphire and n-type Si showed an emission which is similar to that observed for bulk ZnO with a spectrum dominated by donor bound exciton recombination at ∼3.36 eV and a structured green band. The PL intensity was particularly enhanced for nanocone structures grown on cubic Si substrates. For these samples, additional features, which are not observed for bulk ZnO were observed at 3.12, 3.02 and 2.92 eV following a vibronic progression of 100 meV. RT angular-dependent specular reflection measurements indicated that all the nanostructures act as highly effective broadband antireflection coatings.