Influence of Oxygen Deficiency on the Rectifying Behavior of Transparent-Semiconducting-Oxide–Metal Interfaces

Transparent semiconducting oxides (TSO) are promising candidates for the fabrication of flexible and low-cost electronic devices, as they contain only abundant materials, are nontoxic, and exhibit high carrier mobilities. The formation of rectifying Schottky-barrier contacts is a prerequisite for devices, such as rectifiers, photodetectors, and metal-semiconductor field-effect transistors, and it was found that the presence of oxygen plays an essential role during the formation of the Schottky contacts. With electrical measurements on Pt/zinc-tin-oxide (ZTO) and ${\mathrm{PtO}}_x/\mathrm{ZTO}$ Schottky-barrier contacts and depth-resolved x-ray photoelectron spectroscopy measurements we demonstrate the important role of oxygen at the interface between TSOs and the metal contact for the rectifying behavior of diodes. In the vicinity of the interface, ${\mathrm{PtO}}_x$ is reduced to Pt in a two-step process. $\mathrm{Pt}{(\mathrm{OH})}_4$ is reduced within one day, whereas the reduction of PtO takes place over a time period of several weeks. The reduction results in improved rectification compared to $\mathrm{Pt}/\mathrm{ZTO}$, due to a filling of oxygen vacancies, which leads to a reduction of the free-carrier concentration in the vicinity of the ${\mathrm{PtO}}_x/\mathrm{ZTO}$ interface. This increases the depletion layer width and subsequently reduces the tunneling current, resulting in a higher rectification ratio. The time scale of the permanent performance improvement can be shortened significantly by applying a reverse bias to the diode. The described mechanism is most likely also present at other transparent-semiconducting-oxide–metal interfaces.

Figure 1

Current-density–voltage characteristics of the samples $A$ and $R$, showing rectifying and Ohmic behavior, respectively.

Figure 2

Depth-dependent elemental concentrations for (a) a ZTO film with a ${\mathrm{PtO}}_x$ contact deposited under 100% ${\mathrm{O}}_2$ atmosphere (sample $A$) (with a pure Pt cap on top) and (b) a ZTO film with a Pt contact deposited under 100% Ar atmosphere (sample $R$). Solid lines are guides to the eye. Dashed lines indicated the interface region.

Figure 3

(a) Pt $4f$, (b) O $1s$, and (c) Sn $3d_{5/2}$ core levels of sample $A$ (left panels) and sample $R$ (right panels). The sputter times correspond to the transition regions as indicated in Fig. 2. The fitted peaks correspond to Pt (red), PtO (blue), $\mathrm{Pt}{(\mathrm{OH})}_4$ (green), ZTO (pink), and oxygen vacancies (yellow).

Figure 4

Current density-voltage characteristics of one diode of sample $A$ (a) and sample $R$ (b) remeasured after 1, 3, 7, 14, and 28 days.

Figure 5

Change of the logarithm of the rectification ratio for samples $A$ and $R$ with storing time.

Figure 6

Change of the $\mathrm{Pt}{(\mathrm{OH})}_4$ (upper panel) and PtO (lower panel) component of the Pt $4f$ peak for an aged (30 days, red) and a nonaged (1 day, black) sample, as one measures from the bulk contact to the ZTO interface. The change is normalized to the bulk intensity. While the intensity of the $\mathrm{Pt}{(\mathrm{OH})}_4$ component drops to zero for both aged and nonaged contacts, the reduction of the PtO component is more pronounced for the aged sample.

Figure 7

Current density-voltage characteristics of a sample with Pt (a) and ${\mathrm{PtO}}_x$ (b) contact for negative bias application times as labeled. The evolution of the logarithm of the rectification ratio with increasing bias application time is shown in (c) for sample $A$ and sample $R$.