Sanjay Kumar Mohanta

A comparative analysis of deep level emission in ZnO layers deposited by various methods

This study examined the origin of visible luminescence from ZnO layers deposited on p-Si substrates by various growth methods using temperature dependent photoluminescence measurements. The deep level emissions of ZnO layers are found to be strongly dependent on the growth conditions and growth methods used. For the samples grown by sputtering, the visible emission consisted of violet, green, and orange-red regions, which corresponded to zinc interstitial (Zni), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. In contrast, the deep level emissions of metal organic chemical vapor deposition grown samples consisted of blue and green emissions and blue and orange-red emissions at low and high oxygen flow rates, respectively. The ZnO nanorods synthesized by thermal evaporation showed a dominant deep level emission at the green region, which is associated with oxygen vacancies (VO).

Enhanced exciton-phonon interactions in photoluminescence of ZnO nanopencils

We report enhanced exciton-phonon interactions in the photoluminescence (PL) of ZnO nanopencils compared with ZnO nanorods grown on ZnO/Si templates by thermal evaporation. Although the low temperature ( 100 K) showed dominant contributions from the free exciton emissions and phonon-replicas of free excitons for nanorods and nanopencils, respectively. This discrepancy in the behaviors of excitonic emissions of the ZnO nanorods and nanopencils was related to surface defects causing different strengths of exciton-phonon coupling. The different excitonic emissions of the nanorods and nanopencils revealed a 52 meV redshift in the room temperature PL of nanopencils.

Enhancement of band-edge emission of ZnO from one-dimensional ZnO/MgZnO core/shell nanostructures

One-dimensional MgZnO nanostructures were grown directly on p-Si substrates by thermal evaporation at a variety of synthesis temperatures. Transmission electron microscopy and energy dispersive x-ray spectroscopy analysis revealed the formation of slim ZnO nanowires with twin boundaries on low Mg content nanosheets at lower synthesis temperatures, where the slim nanowires on the nanosheet did not have any detectable Mg content. The MgZnO nanostructures at elevated synthesis temperatures showed core/shell structures consisting of h-ZnO/h-MgZnO/c-MgZnO, where the h-MgZnO layer has a Mg content up to ~9 at% and a further increase in Mg content in the outer shell induced the formation of the c-MgZnO phase. The ZnO nanorods covered with MgZnO layers showed enhanced band-edge emission due to the existence of a h-MgZnO barrier and a c-MgZnO dielectric layer.

Defects in interfacial layers and their role in the growth of ZnO nanorods by metallorganic chemical vapor deposition

We report the evolution of ZnO nanorods by metalorganic chemical vapor deposition on sapphire substrates and an investigation of their microstructure. Well-aligned ZnO nanorods with a high aspect ratio were grown on an interfacial layer with several types of defects at a lower reactor pressure. Planar defects such as stacking mismatch boundaries and inversion domain boundaries were formed in the interfacial layer during the coalescence of the islands, and finally constituted the side facets of the nanorods. Based on the microstructural changes and origin of the defects in the interfacial layers, we propose a model to explain the growth evolution of ZnO nanorods on sapphire substrates.

Exciton recombination in ZnO nanorods grown on GaN/sapphire template

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes.

Structural transition from MgZnO nanowires to ultrathin nanowalls by surface separation: growth evolution and gas sensing properties.

This paper reports a spontaneous method of controlling the growth mode from vertically arrayed ultra-slim MgZnO nanowires to nanowalls through the in-plane random motion of the seed crystals formed by surface phase separation. Seed crystals with a relatively Zn-rich phase were formed by the simultaneous injection of Mg and Zn and became strongly networked when the Zn/Mg flux ratio was increased at high temperatures, leading to the formation of MgZnO nanowalls on various conducting substrates. The hydrogen sensing performance of the MgZnO nanowalls with a two-dimensional network structure was superior to that of the one-dimensional MgZnO nanowires. Based on the microstructural characterizations, the growth procedure for the structural transition from MgZnO nanowires to nanowalls on the Si substrates was proposed.

Synthesis and characterization of ZnO/MgZnO heterostructure nanorods by simple two-step evaporation.

ZnO-core/MgZnO-shell heterostructure nanorods with high aspect ratio were synthesized using a two-step thermal evaporation procedure, in which the core and the shell layers were formed separately at different temperatures. Microstructural characterization revealed a position dependence of the crystal structure and composition in the shell layer. The shell layer in the upper region consisted of MgO with quantum dot-like structure having cubic phases embedded in an amorphous oxide layer, while a Mg(0.35)Zn(0.65)O shell layer with a self-assembled superlattice structure of triple periodicity was formed in the middle region.

Emission characteristics of ZnO nanorods on nanosilicon-on-insulator: competition between exciton–phonon coupling and surface resonance effect

We investigated the optical properties of ZnO nanorods on nanosilicon-on-insulator using variable temperature photoluminescence (PL) spectroscopy, and explored the contribution of exciton–phonon coupling and surface resonance effect on the emission characteristics of the nanorods. The low-temperature (<100 K) PL spectra revealed different strengths of exciton–phonon interaction for nanorods of different surface structures. The exciton–phonon coupling strength was stronger for nanorods of rougher surfaces with enhanced contribution of longitudinal optical phonon replicas of free exciton. Despite exhibiting different coupling strengths of exciton–phonon interactions, the room-temperature PL showed an unchanged energy position at 3.28 eV for nanorods of different surface structures. The unchanged energy position of band-edge emission was caused by the competitive effect of the surface defects induced exciton–phonon interaction and the surface resonance effect in faceted nanorods.

Density and aspect ratio controlled MgZnO nanowire arrays by spontaneous phase separation effect

This study reports a simple method of controlling the density and aspect ratio of vertically arrayed ultra-slim MgZnO nanowires developed through the formation of selective crystalline seeds by metal–organic chemical vapour deposition at a high growth temperature. The thin multilayer structures with an amorphous MgO layer and amorphous and single crystal MgZnO layers developed by self-phase separation between the nanowires and substrates played a vital role in the growth of the MgZnO nanowires. The dimensional competition between the Zn-rich seed crystals and Mg-rich amorphous layers induced by spontaneous phase separation at different Mg fluxes determined the density and aspect ratio of the nanowires. Based on the thermodynamical calculations and microstructural characterizations, the growth procedure and models for the evolution of the ultra-slim nanowires on the Si substrates are proposed for different Mg flow rates.

Influence of Buffer Layer on Structural and Optical Properties of ZnO Nanorods on Glass Substrates

In this article, we report on the structural and optical properties of ZnO nanorods grown at a relatively low temperature on glass substrates with and without a ZnO buffer by metallorganic chemical vapor deposition for transparent optoelectronic application. The thickness of the buffer layer strongly influences the aspect ratio and alignment of the ZnO nanorods. The nanorod growth rate becomes fast when the optimized stress-free buffer thickness is used. Photoluminescence measurements show strong bandedge excitonic features. The visible and UV resonant Raman scattering measurements suggest that the structural and optical properties of the nanorods on amorphous substrates are improved.