Question

What are the orbital electron transitions in ZnO? What about Co²⁺:ZnO?

Answer 1

ZnO is a wide band gap semiconductor. It possesses inherent n-type conductivity even in the absence of any doping. Observation of n-type character in ZnO is due to the presence of oxygen vacancies or Zn interstitials.

When an electron in an orbit drops to a lower energy state by giving up energy through the creation of a photon. This process is known as going through an electron transition. This results in the glow or luminescence that we generally observe in materials. To create the glow or the luminescence in materials we need to create excited atoms or electrons and these excited particles created depends significantly on the amount of energy required to produce the transition.

The transition to the ground state typically is a multistep process. In general, the transitions that go from shell to another shell are allowed. The band gap of ZnO is 3.44 eV which results in the strong emission peak in the UV region originating from the band to band transition. Besides, there also exists weak defect emission peaks within falling within the band gap, with the deep level transitions in the visible region. Thus in ZnO, there exist UV excitonic emissions corresponding to the band to band transitions and weak visible emissions due to the deep level transions in ZnO. The visible emissions suggest orange-red emission and green emission could be related to interstitial (Zn_i) shallow donors or oxygen vacancy (Vo) deep donors.

The cobalt doping induces a decrease of band gap energy and fluorescence quenching of the emission bands. The spectra is related to the transition within the tetrahedral Co²⁺ ions in the ZnO host crystal. Due to the doping of Co there exists a variation compared to pure ZnO resulting in a shift in the PL emission and UV-Vis absorption peaks to red due to the Co incorporation.