Invited: Ohmic Contacts to Semiconductor Compounds: From III-V Semiconductors to Layered Transition Metal Dichalcogenides

In this presentation, I will review work on low-resistance ohmic contacts to III-V semiconductors, particularly our group’s most recent work on contacts to InGaAs. Then I will discuss contacts to layered transition metal dichalcogenide semiconductors and how some of the lessons learned from III-V semiconductors may apply to contacts to the transition metal dichalcogenides. I will also highlight important differences between the families of materials and their ramifications for forming low-resistance ohmic contacts to layered semiconductor compounds.

To realize low-resistance ohmic contacts to semiconductors, it is necessary to understand and control many factors. The Schottky barrier between the metal and semiconductor should be minimized but is not always strongly affected by the choice of metal due to Fermi level pinning. The Schottky barrier can also be affected by the preparation of the semiconductor surface prior to metallization and the presence of intentional or unintentional interfacial oxide layers. I will discuss this effect in contacts to heavily doped n-type InGaAs, particularly Ni contacts of interest for self-aligned “salicide-like” processes. I will also discuss a possible explanation for the vastly different values of specific contact resistance reported by different groups that have studied these Ni contacts.

The interfacial reactions between a metal and semiconductor can be used in many ways to engineer current transport through contacts, so it is beneficial to understand the contact metallurgy­–both phase equilibria and reaction kinetics. This topic has been studied extensively for contacts III-V semiconductors. As a recent application, I will highlight the introduction of co-dopants (Si and S) through solid-state reactions to form a shallow ohmic contact to n-type InGaAs, which resulted in an exceptionally low contact resistance on lightly doped n-type InGaAs.

Finally, I will discuss our newest studies on contacts to transition metal dichalcogenide semiconductors. I will present information on phase equilibria and reaction kinetics in metal-Mo-S systems and their implications for contacts to MoS₂. Then I will compare these systems to the metal-III-V systems and predict how reactions and surface preparation will be important for engineering contacts to layered transition metal dichalcogenide semiconductors.