This thesis describes the method development and characterisation of the chemical modification of polar and non-polar ZnO surfaces. Due to the native OH termination on ZnO, the material exhibits downwards band bending at the surface, and an accompanying surface electron accumulation layer. This gives the ZnO surface a conducting nature which is undesirable for the development of efficient transparent semiconductor devices. The use of covalently-bound organic modifiers to tune the work function and stability of ZnO surfaces has previously been investigated, but there is little systematic data on the changes in band bending produced by these modifiers on the three main crystal faces of ZnO: Zn-polar, O-polar, and m-plane ZnO. This work investigates the effects on band bending when phosphonic acids, thiols, and aryldiazonium ions are used to modify the surface of ZnO.

Three different phosphonic acid (PA) modifiers on ZnO were studied using synchrotron X-ray photoelectron spectroscopy. Octadecylphosphonic acid (ODPA) significantly increased the downwards band bending at the surface of all three ZnO faces. Surprisingly, a fluorinated analogue (1H,1H,2H,2H-perfluorooctyl phosphonic acid, F13OPA) also increased the surface band bending despite the electron withdrawing properties of fluorine. This was attributed to X-ray-induced decomposition of the molecules and subsequent F-Zn interactions. A fluorinated aromatic PA, 2,3,4,5,6-pentafluorobenzyl phosphonic acid (PFBPA), was analysed using a lower X-ray flux, and was found to decrease the downwards band bending on ZnO. The PA modification layers were stable when stored in air and darkness, but UV light caused degradation of all three modifiers. Despite this, ODPA was able to prolong the effect of persistent photoconductivity on O-polar ZnO.

Polar and non-polar ZnO samples were also modified with alkane- and arylthiols. In contrast to some previous studies, the main binding mode of thiols on ZnO was determined to be via sulfonate bonds, which increased downwards band bending slightly. O-polar ZnO with unoxidised octadecanethiol (ODT) on the surface showed a decrease in surface band bending. Zn-polar and m-plane ZnO appeared to oxidise thiols more readily than the O-polar face, due to the relative differences in catalytic reactivity. A large increase in downwards band bending was seen for all ZnO faces that had been modified with an isothiocyanate derivative (3,5-bis(trifluoromethyl)benzyl mercaptan), although the chemical nature of the bound modifier and its binding mode are unclear.

Aryl layers 1 – 8 nm thick were grafted to ZnO via electrochemical reduction of nitrobenzene diazonium ions. The electrochemical behaviour was similar to that on carbon substrates. The modification reduced the downwards band bending on all three ZnO faces, and the magnitude of the change appeared dependent on the thickness of the modification layer. Electrochemical reduction of nitro to amino groups was attempted on the nitrophenyl films but was only successful on the O-polar and m-plane faces, and the band bending did not significantly change relative to unreduced samples for any ZnO face. In contrast, large upwards band bending was observed when nitrophenyl films were treated with Na2S in attempt to chemically reduce the nitrophenyl groups. This is a very interesting phenomenon that is tentatively attributed to Na adsorption on ZnO and possible incorporation into the lattice.

Sb-doped ZnO thin films (MBE-ZnO) with high charge carrier concentrations were electrochemically modified with nitrophenyl and trifluoromethyl layers from the corresponding aryldiazonium salts, giving good reproducibility across samples and removing the downwards band bending of ZnO as shown with synchrotron XPS. These changes were attributed to the electron withdrawing nature of the modifiers, however when synchrotron XPS was used to reduce the nitrophenyl groups to aminophenyl moieties, upwards surface band bending became more pronounced despite amino groups being electron donating. This was attributed to ZnO valence band electrons participating in the reduction mechanism and leaving an electron-depleted surface. Both modifiers were stable when stored in the dark but desorbed from the surface when exposed to UV light.

Overall, this thesis work shows that ZnO remains a promising transparent conducting oxide for electronic applications because of the ability to tune the surface band bending over a range of ~1 eV with relatively stable covalently-bound surface modifiers.