Defects and doping in ZnO

ZnO occurs in a hexagon crystal phase, in which it has a wide band gap (3.4 eV) and a large exciton binding energy of 80 meV. This has lead to the use of ZnO in opto-electronic applications as well as an optically transparent conducting material.

[Fig 1: Schematic of a range of defects in ZnO]
(Hover over the defects for a close-up)
Bulk ZnO

A key problem with the application of ZnO in electronics relates to doping. Typical as-grown material contains hydrogen, and is strongly n-type. This has been thought to relate to simple interstitial H, but we have shown that this centre is rather mobile at temperatures where the conductivity persists. Other possibilities include the trapping of hydrogen at other sites, such as the common copper impurity. Complexes made up from CuZn and two interstitial H atoms yields a shallow donor. Indeed, transition metals such as Fe and Ni are common unintentional impurities in ZnO, many of which are theoretically electrically active.

At the other end of the band-gap, it has proven difficult to generate p-type material. This is in part due to the compensation of acceptors by the common donors, but also due to the difficulty in generating shallow acceptors. One candidate is lithium. However, we have shown that Li is readily forms stable complexes not only with hydrogen, but also with itself forming donor-acceptor complexes made up from LiZn-Lii.