XPS investigation of titanium contact formation to ZnO nanowires

Abstract

Ti is often used to form an initial Ohmic interface between ZnO and Au due to its low work function, and the TiO/ZnO heterojunction is also of great importance for many practical applications of nanoparticles. Here, Ti has been controllably deposited onto hydrothermally grown ZnO nanowires and the formation of metal-semiconductor contact has been investigated using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy and scanning electron microscopy. XPS results showed that that the Ti initially reacts with surface oxygen species to form TiO, and further deposition results in the formation of oxides with oxidation state numbers lower than four, and eventually metallic Ti on top of the TiO. The formation of TiC was also observed. XPS showed that the onset of metallic Ti coincided with a Zn 3p core level shift to lower binding energy, indicating upwards band bending and the formation of a rectifying contact. Annealing caused a near-complete conversion of the metallic Ti to TiO and caused the Zn 3p to shift back to its original higher binding energy, resulting in downwards band bending and a more Ohmic contact. PL measurements showed that the optical properties of the nanowires are not affected by the contact formation.

Investigation into the effects of surface stripping ZnO nanosheets

“…Before argon bombardment, the O 1s peak consisted of a main component centred at 530.2 eV, and a shoulder on the higher binding energy side consisting of two peaks offset from the main component by 1.35 eV and 2.3 eV, labelled as S1 and S2, respectively. These peaks are attributed to lattice oxygen, surface hydroxide ions and surface water vapour, respectively [18][19][20][21][22]. Curve fitting area analysis of these peaks was used to calculate the percentage water vapour component contribution to the O 1s peak and the results are show in figure 4(a).…”