Formation mechanism of low contact resistance PdZn-based ohmic contacts for p-type InP

Abstract

Recently, Sb(3 nm)/Zn(20 nm)/Pd(20 nm) ohmic contact materials to p-type InP which provides high reliable, low contact resistance after annealing at temperature around 375°C have been developed in our laboratories (where a slash (/) sign indicates the deposition sequence). The use of these contact materials for photodiodes made it possible to achieve simultaneous preparation of both p and n-type ohmic contacts, which resulted in significant reduction of the fabrication process steps. In the present paper in order to understand the mechanism of ohmic contact formation, the microstructural analysis of the interfaces between the Sb/Zn/Pd ohmic contacts and the Zn-doped p-type InP was carried out by X-ray diffraction, cross-sectional transmission electron microscopy, secondary ion mass spectroscopy, and electrochemical capacitance-voltage measurement. The present experiments proposed a new contact formation mechanism: reduction of the ohmic contact resistances was obtained primarily due to suppression of Zn (doped in the InP) outdiffusing from the InP substrate during annealing by the Pd2InP layers which contained Zn. Thus the direct contact of the Pd2InP layers to the InP substrate was found to be essential to prevent reduction of the net acceptor concentration near the InP surface and also to improve the thermal stability of ohmic contacts. The roles of Pd and Sb added to the contact materials also contributed to the reduction of contact resistance and their roles will be discussed.