Q1 Q2 Excitonic properties of layer-by-layer CVD grown ZnO hexagonal microdisks Q3 Q4 Mrinal K Sikdar Q5 , Q6 Q7 Bhabesh Sarangi and Pratap K Sahoo School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar, HBNI, Jatni, Odisha-752050, India E-mail: pratap.sahoo@niser.ac.in Received 5 May 2021, revised 14 June 2021 Accepted for publication 1 July 2021 Published DD MM 2021 Abstract We have investigated the excitonic properties of highly crystalline ZnO hexagonal microdisks grown by the chemical vapour deposition technique. It was observed that a suitable negative catalyst like chlorine suppresses the crystal growth along the (0001) direction. We propose a qualitative model for the experimentally observed layer-by-layer growth mechanism of the microdisks. Room temperature photoluminescence of the microdisks manifests a very high near- band-edge (NBE) emission peak in the UV region and a minor defect peak in the visible region. The excitonic emission of the microdisks was studied using the low-temperature photoluminescence down to 83 K, which reveals a surface exciton peak in the NBE region and well fitted higher-order phonon replicas. Keywords: ZnO, photoluminescence, surface exciton, hexagonal, microdisks, CVD (Some figures may appear in colour only in the online journal) 1. Introduction ZnO is a direct wide bandgap (3.37 eV) semiconductor having a high exciton binding energy (60meV ) at room temperature (RT). It is one of the few materials having abundant self-assembled nanostructurecon figurations. The micro and nanostructures are usually grown using various bottom-upsynthesis techniques like hydrothermal, sol –gel, vapour phase transport (VPT) and chemical vapour deposition (CVD) [1, 2]. Over the years, various ZnO micro and nanostructures have been grown [1, 3], such as rods [4, 5], rings [6, 7], tubes [8, 9], spheres [10, 11], wires [12, 13], combs [14, 15], belts [16, 17] and so on. Unique structures have their advantages in photonics, chemical sensing, photo- catalytic, and optoelectronic applications depending on their morphology [18–20]. Also, various optically and electrically active defects in ZnO can be helpful in ZnO based optoe- lectronic devices [21, 22]. ZnO structures with a large sur- face-to-volume ratio can naturally play a significant role in electronic and excitonic coupling with other materials [23, 24]. Photoluminescence studyin semiconductors is a non-destructive probe to understand the different excitons and their phonon replicas. Typically, the free exciton energies follow the bandgap as a function of temperature. There are reports that the electron–phonon interaction and lattice dila- tion dominantly contributes to the redshift in the bandgap of the semiconductors with an increase in temperature [25]. The temperature-dependent PL is more suitable for characterizing the excitonic properties of ZnO because of its direct bandgap and large exciton oscillator strength [26]. Depending on the geometry of the ZnO nanostructure, crystalline quality and growth methods, different excitonic features have been reported [27]. Among all the architectures, the hexagonal disk shape is a lesser-explored morphology of ZnO, and it can have pos- sible optoelectronic applications in sensors, information sto- rage and transducers [20]. ZnO is highly luminescent at RT because of its high exciton bindingenergy. This property makes ZnO useful in optoelectronic applications, such as in the construction of blue and UV excitonic lasers. Based on the type of micro and nanostructures used as the light amplifying optical cavity, there are three kinds of lasers: random, Fabry– Perot and whispering-gallery-mode (WGM) resonator-based lasers. Random lasers can be constructed if we use the usual randomly shaped ZnO nanopowder, both as the light scatterer and the luminescent material [28]. The light ampli fication occurs by multiple random scattering phenomena in the dis- ordered gain medium. Fabry–Perot resonance-enhanced Nanotechnology Nanotechnology 00 (2021) 000000 (9pp) APP Template V1.70 Article id: nanoac1096 Typesetter: MPS Date received by MPS: 06/07/2021 PE: MAC009405 CE: LE: UNCORRECTED PROOF 0957-4484/21/000000+09$33.00 © 2021 IOPPublishing Ltd Printed in the UK 1