Interfacial structures and electrical properties in palladium-based metallizations to n-type indium phosphide

InP and InP-based compound semiconductors are becoming increasingly important materials in the electronics industry, because of their unique combination of electrical and optical properties. The fabrication of reliable ohmic contacts to InP has been a major obstacle in the development of InP-based semiconductor devices. Pd-based metallizations make up one type of ohmic contact scheme to InP.; Three Pd-based metallizations have been systematically studied, i.e., Pd/InP, Au/Ge/Pd/InP and Pd/Ge/Pd/InP. The Pd/InP metallization is studied in an effort to better understand the role of Pd in Pd-based metallizations, while the other two metallizations are actual ohmic contact designs. Pd reacts with InP to form an amorphous and an epitaxial layer at low temperatures, which provides good metallization adhesion to InP substrates and improves the contact morphology. Au reacts strongly with In to form Au-In compounds, which creates more In site vacancies in the InP substrate and enhances doping the InP surface. Ge is not only a dopant but also a reactant in the contacts. Changing the amount of Ge in the contact scheme can control the reaction products. Ohmic contacts to n-type InP have been fabricated in both the Au/Ge/Pd/InP and Pd/Ge/Pd/InP systems. The minimum specific resistances are 2.5 {dollar}times{dollar} 10{dollar}sp{lcub}-6{rcub}Omega{dollar}cm{dollar}sp2{dollar} in the Au/Ge/Pd/InP contacts and 1 {dollar}times{dollar} 10{dollar}sp{lcub}-5{rcub}Omega{dollar}cm{dollar}sp2{dollar} in the Pd/Ge/Pd/InP system. The annealing regime for ohmic contact formation is 300-375{dollar}spcirc{dollar}C for the Au/Ge/Pd/InP system and 400-425{dollar}spcirc{dollar}C for the Pd/Ge/Pd/InP system. The relationship between microstructural changes and electrical properties has been investigated. The onset of ohmic behavior in the contacts follows the decomposition of an epitaxial layer at the metallization/InP interface. The degradation of the contact resistance is due to the overreaction between metals and InP, which consumes more InP and destroys the heavily doped InP layer.