Electrical and luminescent properties of ZnO:Bi,Er ceramics sintered at different temperatures

Abstract

The electrical and luminescent properties of a series of polycrystalline sintered ZnO pellets containing small amounts of Bi and Er oxides have been studied as a function of the sintering temperature, between 950°C and 1100°C. The current–voltage curves of the pellets are non-linear; the conductivity of the pellets increases with the applied voltage in good agreement with the results of the literature reported for ZnO:Bi composites. They indicate the presence of electrically active grain boundaries, in which an excess charge of majority carriers is trapped at the interface. All the pellets are photoluminescent and electroluminescent, and in addition the pellets sintered at 1050°C and 1100°C are triboluminescent. The photoluminescence spectra under ZnO band-to-band excitation do not depend on the sintering conditions and show only the broad pattern characteristic of ZnO. The electroluminescence spectra are a mixture of the broad pattern attributable to ZnO and of sharp lines characteristic of the Er 3+ ion. The relative intensity of the signals arising from ZnO and the rare earth depends on the sintering temperature. For a low sintering temperature, the broad ZnO spectrum predominates, whereas for a high sintering temperature, only the Er 3+ sharp lines can be observed. The abrasion of pellets sintered at 1050°C and 1100°C results in an emission of green light. The triboluminescence spectrum of the pellet sintered at 1100°C is identical to its electroluminescence spectrum and shows only the emission of the Er 3+ ion. The comparison between the electro- and the photoluminescence spectra shows that electroluminescence arising either from ZnO or Er 3+ ions is due to direct hot electron impact excitation. Comparison between the tribo- and electroluminescence spectra shows that triboluminescence of the Er 3+ ions has an electrical origin. The triboluminescence of Er 3+ ions is proposed to result from a wrenching of the ZnO grains inducing intense electric fields across the grain boundaries cleavage in Er 3+-rich regions in good accordance with the presence of electrically active grain boundaries. The Er 3+ electroluminescence evidences the presence of hot electrons for applied voltages far below the breakdown voltage. The grain boundary barrier model usually put forward to account for the electric properties of ZnO-based varistors is unable to explain the electroluminescent properties of the ZnO:Bi,Er sintered pellets. Consequently, the electroluminescence of the Er 3+ ions is proposed to result from an electric conduction along the grain boundaries parallel to the field, involving hot electrons in Er 2O 3-rich regions.