Spectroscopic ellipsometry study of optical transitions in Zn1−xCoxO alloys

Abstract

Zn 1−x Co x O (x⩽0.22) films were prepared on (0001)-oriented Al2O3 substrates by rf magnetron sputtering. The alloys show wurtzite crystal structure with the c-axis lattice constant increasing with increasing x. The optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. As x increases, the optical band gap absorption edge (E0) of the alloys shows a redshift from that of pure ZnO, reaching 350 meV for x=0.22. The excitonic character of the E0 edge is gradually reduced as x increases and is replaced by the three-dimensional critical-point shape. Optical absorption structures are also observed below the E0 edge near 2 eV and interpreted as due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ ions substituting Zn2+ ions.

Structural and optical properties of Zn1-xCoxO thin films prepared by RF reactive sputtering technique

“…In particular, for the Co content x = 0.02, three absorption bands at about 1.87, 2.03 and 2.19 eV have been revealed, which are in agreement with already reported absorption peaks [12,13] The optical band-gap energy (E g ) values for these films were evaluated by plotting (h) 2 versus h, where h is the photon energy, and  is the absorption coefficient that can be calculated from the film thickness d as well as transmittances T and T 0 , for the film and substrate, respectively, using equation…”

“…5). This result is in contrast with the reported data [13,15] where a blue shift of the absorption edge was observed. On the other side, a similar trend of decrease of the band edge in O Co Zn…”

Structural and optical properties of Co and Ni doped ZnO thin films prepared by RF magnetron sputtering

“…This result is in contrast with the reported data [16,18], where blue shift of the absorption edge was observed. On the other side, a similar trend of decrease of band edge in Zn 1-x Co x O films is reported by many workers [5,17,19]. This low energy shift of the E g as a function of the Co content can be explained due to the s,p-d exchange interactions between the band electrons and the localized d electrons of the Co 2+ ions substituting for Zn 2+ ions [20,21].…”

Search for Origin of Room Temperature Ferromagnetism Properties in Ni-Doped ZnO Nanostructure

“…The first peak (E 1 ) at 396 nm is related to NBE, which arises due to the recombination of free exciton existing in the ZnO nanostructure 48 . Ni doping shows the red shift in NBE for Ni 5% doping; this red shift can be feature of robust exchange interaction between s-p electrons and d-electrons of Ni 2+ ions of ZnO band 49 . E 2 peak at 401 nm is attributing to the first longitudinal optical phonon copy of free excitons 50 while an origin of rest of the peaks has been discussed in detail in our previous study 24,51 .…”