This thesis describes the chemical modification and characterisation of SnO2 thin-films and nanostructures and single-crystal β-Ga2O3. Due to the native OH termination of the metal oxide surfaces, SnO2 and β-Ga2O3 exhibit surface band bending. Previous work in our research group has found that the OH coverage on metal oxide surfaces correlates with the amount of band bending at the surface. Generally, bare SnO2 and β-Ga2O3 surfaces exhibit downwards and upwards band bending, respectively, and the electron density at the surfaces can be increased by the formation of terminal OH groups on exposure to the atmosphere. Increased electron density results in greater conductivity, while a decrease in electron density makes the surface insulating. The OH coverage of metal oxides is highly susceptible to environmental changes, which can alter the conductivity at the surface and is undesirable for some transparent semiconducting devices. One approach to reducing the influence of the environment on OH coverage and changes in band bending is to coat the surface. Modification of SnO2 using covalently bound molecules has previously been investigated; however, no reports on the deliberate modification of β-Ga2O3 could be found. There has been very little investigation on chemically altering metal oxide band bending and none at all on β-Ga2O3. This work investigates the effects on band bending when aryldiazonium ions, aryliodonium ions, phosphonic acids, and Na2S are used to modify the (101) surface of SnO2 thin-films, (001) surfaces of SnO2 nanotubes, and the (201) and (010) surfaces of β-Ga2O3.

Two different aryldiazonium ions, two aryliodonium ions and octadecylphosphonic acid (ODPA), were used to modify SnO2 thin-film substrates, which were analysed using synchrotron X-ray photoelectron spectroscopy (XPS). Aryl layers ranging from 3– 6 nm thickness were grafted using electrochemical reduction of 4-nitrobenzenediazonium (NBD), 4-methoxybenzenediazonium (MeOBD), diphenyliodonium (DPI), and di-p-tolyl- iodonium (DTI) ions. The electrochemical behaviour of the ions was similar to carbon surfaces, although the reduction peak potential was dependent on the doping level of the SnO2 substrate. Reduction of the NO2 group on 4-nitrophenyl (NP)-modified samples resulted in 4-aminophenyl (AP) layers, with no significant loss in film thickness. ODPA modification resulted in a monolayer. All aryl layers induced an upwards shift in band bending, irrespective of the electron-donating/withdrawing ability of the aryl functional- ity, indicating the change in band bending comes from the presence of an aryl ring with the functionality exerting only a small influence. On the other hand, ODPA was the only modifier to induce a downwards shift in band bending. Samples were intentionally ex- posed to X-ray radiation for an extended time, and all samples saw an upwards shift in band bending, including the unmodified reference samples. The conversion of NP to AP layers resulted in the upwards shifts becoming more pronounced, which was unexpected due to the electron-donating ability of NH2. This indicates that shifts in the band bending due to X-ray exposure are partially a result of changes to the underlying substrate.

Using the methods developed to modify SnO2 thin-film samples, NBD, four different phosphonic acids (PAs), and Na2S were used to modify SnO2 nanostructures. The (001) surfaces of the nanotube was confirmed using transmission electron microscopy (TEM), while the NP and ODPA modifiers were detected seen using energy-dispersive X-ray spectroscopy (EDS). NP and ODPA modification resulted in upwards and downwards band bending shifts, respectively, similar to the SnO2 thin-film samples. (4-nitrobenzyl)phosphonic acid (NBPA), (2,3,4,5,6-pentafluorobenzyl)phosphonic acid (PFBPA), and (3,3,4,- 4,5,5,6,6,7,7,8,8,8,-tridecafluorooctyl)phosphonic acid (F13OPA) modification all resulted in an upwards band bending shift. The band bending shifts for the PA modification were predictable using the molecule’s dipole moment. Na2S modification of the SnO2 nanotubes was also explored. SOx was found to be the main S species on the surface of the nanotube and little to no shift in band bending was observed. All samples intentionally exposed to prolonged X-ray radiation exhibited a greater upwards band bending shift.

The (201) and (010) surfaces of β-Ga2O3 were modified with NP, 4-methylphenyl (MP), Ph, NBPA, F13OPA, ODPA, and SOx layers. These layers gave similar shifts in band bending to those observed on SnO2. This suggests that other metal oxides will behave similarly to SnO2 and β-Ga2O3. X-ray exposure of the samples induced a further upwards band bending shift. Pd|β-Ga2O3 Schottky contacts (SCs) were fabricated on NP- and ODPA-modified β-Ga2O3 and showed that the NP layers increase the rectification ratio of the diode with only a slight decrease in the forward current. ODPA SCs increased the rectifi- cation too; however, with respect to the reference samples, the changes were smaller than for the NP samples.

Overall, surface modification of SnO2 and β-Ga2O3 can tune the surface band bending over a −0.3–1 eV range using simple chemical methods. This work can be used to tune SnO2 and β-Ga2O3 devices and add additional functionality to the surfaces, which can be applied to a wide variety of optoelectronic applications.