ZnO hexagonal micro- and nano-rods are regarded as important building blocks for nanoscale optoelectronic devices because they provide high quality (Q) factors leading to strong optical feedback and sharp whisperings gallery modes (WGM) within micro- and nanocavities [1]. Whispering gallery modes have potential applications in ultra-small mode volume devices wherein the laser threshold comes down drastically and the lowest reported value of laser threshold ~0.45kW/cm2[2]. Lipoxin Sun et.al [3] reported on the direct observation of the exciton-photon coupling in a ZnO tapered whispering gallery explained by the relation (1.1)
(1.1)
Where, R: Cavity radius, n: Refractive index, β: Polarization factor, and N: Interference order of the resonance.
We fabricated ZnO and Zn0.9Mg0.1O microrods using nanoparticle assisted pulsed laser deposition (NAPLD) [4]. Micro-PL studies reveal that Zn0.9Mg0.1O microrods exhibited a significant blue shift in the UV emission that was attributed to an increase in bandgap due to Mg doping [5] and clear whispering gallery modes were observed for both ZnO and Zn0.9Mg0.1O. We also observed that the size dependence of whispering gallery modes in Au coated ZnO microrods with hexagonal cross-section within the near band edge region. The WGMs exhibited significant splitting for the Au coated ZnO microrods and exhibited a blue shift with a decrease in rod size. Also, Au coated ZnO microrods offer several advantages in the fabrication of WGM lasers because of enhanced WGM emission and stability at the ambient.
We also fabricated ZnO/p-GaN based high responsivity ultraviolet (UV) photodetector. Hydrothermal technique has been followed to grow ZnO nanorods. Interestingly, it was found that the ZnO nanorods grown using nitrate precursor are less prone to defect emission in comparison to the chloride-based precursor which resulted in low dark current levels in ZnO nanorod based devices grown using nitrate precursor. The photo-responsivity and photo-detectivity values of the as-fabricated device were calculated to be 350 mA/W and 3.5x1011 cm√Hz/W (Jones), respectively at 360 nm excitation wavelength and ~1.79 µW/cm2 excitation intensity. The demonstration of high responsivity UV detectors (at low excitation intensity values) using ZnO nanorods/GaN can pave the way towards the development of high-performance next generation UV photodetectors.
References:
1.Thomas Nobis et.al Phys. Rev. Lett. 93,103903 (2004).
2. H.M Dong et.al Sci. Rep. 5, 8776 (2014).
3. Liaoxin Sun et.al Phys. Rev. Lett. 100,156403 (2008).
4. Bellarmine…MSR Rao, J. Lumin.210,404 (2019).
5. Bellarmine… MSR Rao, Scr. Mater. 146, 1969 (2018).