Properties of the intermediately bound and -excitons in ZnO:Cu

Abstract

A microscopical model is proposed, describing the origin and properties of three closely spaced zero-phonon lines observed in the green Cu band in ZnO:Cu crystals labelled and . These excitations are known to be formed by a charge-transfer reaction with hole bound states. These lines are shown to originate from an intermediately bound exciton of acceptor type, . This sort of exciton, in which both carriers are captured at intermediate-radius orbitals, results from the wurzite-type symmetry of the ZnO:Cu system. The electronic structure obtained for these three intermediately bound excitons enables us to explain their magneto-optic behaviour and to calculate their g-values. Additionally, we determined the quantum efficiency of both intracentre and exciton transitions by using time-resolved and calorimetric absorption spectroscopy. While no luminescence is observed in ZnS, the exciton states in ZnO are purely radiative only to the ground state, . The picture of an intermediately bound exciton explains the recombination channels and also makes clear the difference between copper states in the ZnS and ZnO systems.

Photoluminescence and damage recovery studies in Fe-implanted ZnO single crystals

“…18 Besides the intrinsic nature of the defects responsible for the green and yellow/red emission bands, some authors argue that the recombination processes are due to extrinsic impurities, such as Cu and Li, respectively. [19][20][21][22][23][24][25][26] The Cu-related emission band has been known to be a LO-phonon-assisted transition with zero phonon lines ͑␣,␤,␥͒ near 2.86 eV and with maximum at ϳ2.43 eV. [19][20][21][22][23][24] This emission is also characterized by a fast exponential decay ͑ϳ440 ns at 1.6 K 19 ͒ and presents a nearly structured mirror-image PL excitation ͑PLE͒ spectrum at low temperatures with its excitation maximum near 3.12 eV.…”

“…[19][20][21][22][23][24][25][26] The Cu-related emission band has been known to be a LO-phonon-assisted transition with zero phonon lines ͑␣,␤,␥͒ near 2.86 eV and with maximum at ϳ2.43 eV. [19][20][21][22][23][24] This emission is also characterized by a fast exponential decay ͑ϳ440 ns at 1.6 K 19 ͒ and presents a nearly structured mirror-image PL excitation ͑PLE͒ spectrum at low temperatures with its excitation maximum near 3.12 eV. [20][21][22] The recombination model for this structured Cu-related emission corresponds to a transition of an acceptor-type bound exciton ͓Cu ϩ (d 9 ϩe͒,h͔ to the ground state ( 2 T 2 ) of Cu 2ϩ (d 9 ) ion.…”

“…[20][21][22] The recombination model for this structured Cu-related emission corresponds to a transition of an acceptor-type bound exciton ͓Cu ϩ (d 9 ϩe͒,h͔ to the ground state ( 2 T 2 ) of Cu 2ϩ (d 9 ) ion. [21][22][23] This state acts as an electron-trapping level located ϳ200 meV below the bottom of the conduction band. 21,23 The quantum yield of the green PL in ZnO is known to be of near 100%.…”

“…[21][22][23] This state acts as an electron-trapping level located ϳ200 meV below the bottom of the conduction band. 21,23 The quantum yield of the green PL in ZnO is known to be of near 100%. 23 Thus, it is concluded that the (Cu ϩ ,h) state in crystals with less than 250-ppm copper concentration decays purely radiatively to the ground state.…”

Lattice location and stability of implanted Cu in ZnO

“…[1][2][3][4] The main interest in Cu results from its possible role in the so-called ''structured'' green luminescence frequently found in ZnO. This role was first suggested by Dingle 1 from an apparent isotope split due to 63 Cu and 65 Cu in the two no-phonon lines of the structured green luminescence, which has since then often been attributed to Cu 2ϩ…”