Shallow Donor States Induced by In-Diffused Cu in ZnO: A Combined HREELS and Hybrid DFT Study

Abstract

A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnOð000 " 1Þ surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study.

Unambiguous identification of the role of a single Cu atom in the ZnO structured green band

“…A great deal of ZnO material contains Cu as an unintentional dopant in varying concentrations and probably in a range of different charge states, on different lattice sites and also in complexes with other Cu atoms and perhaps also with native defects and impurities. [16][17][18][19][20][21] Reports in the literature indicate that some fraction of this intrinsic Cu may be activated by annealing at high temperatures (~ 900 0 C), rendering the defect configuration responsible for the SGB optically active. 16 The authors of reference 16 specifically attribute this activation to a change in Cu charge state from Cu 1+ to Cu…”

“…22 While the exact defect structure remains unknown our studies, and other reports in the literature, suggest that the defect chemistry of Cu in ZnO is quite complicated and that Cu may exist in ZnO in a variety of charge states, lattice sites and complexes with other defects and impurities. [16][17][18][19][20][21] For example, we have grown ZnO nanorods using a carbothermal reduction-based vapour phase transport (VPT) process and have doped the deposited material by introducing Cu into the source powder prior to VPT growth by a modified Pechini process. Directly after growth there is only a very weak SGB emission.…”

Copper impurities in bulk ZnO: A hybrid density functional study

“…The thermodynamic transition level ε therm (+1/0) for Cu i defect center is computed to be 0.13 eV from the CB minimum (see Table I). This computed value can be directly compared with the donor ionization energy of 0.088 eV, 20 experimentally derived from a high resolution electron energy loss (HREEL) study. The agreement is extraordinary good, as discussed in a previous work.…”

“…20 Both tetrahedral and octahedral interstitial voids exist in bulk ZnO. Interstitial Zn species were computed to be 0.9 eV more stable in the larger octahedral cavity, 52 thus we assume that the same interstice is also favored for Cu i species [see Fig.…”

“…Copper impurities are first investigated as wellestablished electron acceptors 19 but secondly also as potential electron donors, in line with recent experimental findings. 20 Acceptor and donor properties are discussed in terms of thermodynamic and optical transition levels from a recently proposed model. 21 Then the controversial relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) phases [22][23][24][25][26][27] for substitutional Cu to lattice Zn atoms is analyzed.…”