Author:

Keywords:

Ohmic contacts, High mobility semiconductors, CMOS

Abstract:

Ohmic contacts, essential components for any electronic device, are difficult to obtain on III-As and Ge due to intrinsic obstacles such as Fermi level pinning. In addition, Si-integration imposes the use of specific processes and materials that limits the options and adds complexity to the problem. The focus in this PhD work is to contribute in finding integration-ready solutions to produce low resistive, thermally stable ohmic contacts on III-As and Ge that would effectively enable these materials for MOSFET devices in the sub-20 nm CMOS technology nodes. Ohmic contacts are metal-semiconductor systems which display linear I-V characteristics. The figure of merit to evaluate the quality of an ohmic contact is its specific contact resistivity, which expresses the property of the interface between the contact and the semiconductor material. Typically ohmic contacts are formed by the deposition of a metal on a semiconductor surface followed by a thermal treatment which triggers reactions to form alloys between the two materials.For III-As materials (GaAs and InGaAs), the most commonly used metal schemes for ohmic contacts are Au-based, which is not allowed for integration in a Si platform. Therefore Au-free schemes have been researched, developing integration-ready solutions by characterizing thermal stability of contacts and defining optimal metal stacks to match ITRS required values of contact resistance for the very scaled nodes of the future. A self-aligned integration-ready solution, using the combination of selective Ge epitaxial growth in contact areas and selective germanidation process, was also demonstrated to lead to ohmic contacts.This PhD also offers solutions to obtain very low contact resistance for germanide-based ohmic contacts on n-type Ge. Two approaches are tested: in one case laser annealing dopant activation is used, while the other is exploiting the effect of dopant segregating during the low temperature reactions of metals with Ge (germanidation). GeSn alloy was also explored since it is very interesting to use it as embedded stressors in Ge-based MOSFETs, contributing in boosting the device performances. Promising results have been achieved, able to encourage the work towards integration of GeSn in source/drain area of MOSFETs using Ge as channel material.The current at the metal-semiconductor interface was modeled for the high mobility semiconductors of interests. This allowed to indicate what values of technological parameters (like doping concentration) or physical properties (like Schottky barrier height) should be targeted to reach the low values of contact resistance for very scaled devices.