Contact resistances between ZnO and Ti, Al, Zn, and Bi: correlation with the density of states at Fermi energies

Abstract

Zinc oxide is a seminal wide- and direct-bandgap semiconductor with ever-increasing device applications, and forming low contact-resistance (CR) ohmic contacts with ZnO is essential for making reliable internal and external connections in such devices. Among base metals, Al, Ti, and Zn have been used for contact formation because their work functions (WFs) match the ZnO electron affinity and all produce ohmic contacts, but the data available on the formed CRs are scattered and provide no comparative conclusion. Here, we report the fabrication of Ti/ZnO, Al/ZnO, Zn/ZnO and Bi/ZnO with similar geometrical specifications under identical conditions and compare their CRs under identical conditions, concluding for the first time that Bi, a semimetal with a WF equal to those of the other metals examined, results in the best contact by far. Zn is the second best, while Ti is the last in contact quality. The sequential order obtained for CRs is consistent with that of the charge conduction activation energies obtained for the respective contacts via Arrhenius plots, but contradicts the predictions of the simplified interface energy diagrams. The density of states (DOS) around the Fermi levels of the metals are estimated by ab initio quantum mechanical calculations. The results are strongly correlated with the experimental CR data and verify the opposing roles of the gap-state formation and gap-state saturation in determining CRs between metals and metal oxide thin films. Our findings are anticipated to be impactful in the broad field of electroceramics and initiate similar studies on other metal oxide semiconductors.