Field-effect transistor (FET) devices, such as metal-oxide-semiconductor FET (MOSFET) devices generally include a source region, a drain region, a channel region between the source and drain regions, and a gate electrode overlying the channel region and separated from the channel region by a dielectric material. A complimentary MOSFET (CMOS) device includes a p-type MOSFET device and an n-type MOSFET device. There are also three-dimensional transistor architectures like FinFET's. To operate as desired, a work function of the gate electrode of the n-type device and of the p-type device must differ by a certain amount. The difference in the work function is generally obtained by tuning the gate electrode material.
Traditionally, MOSFET devices are formed using silicon oxide as the dielectric material and polysilicon as the gate electrode material. Polysilicon has worked relatively well as a gate electrode material, because it allows relatively easy tuning of a work function of the devices and consequently a threshold voltage of the devices.
As MOSFET devices are scaled down to meet desired performance criteria, metal has generally replaced polysilicon as a gate electrode material and high dielectric constant material has generally replaced silicon oxide as the dielectric material for high performance devices. However, by replacing polysilicon with metal, a work function difference between the gate and the channel becomes more difficult to tune. As a result, modification of a threshold voltage of the device becomes more difficult.
To facilitate work function tuning of MOSFET devices including a metal gate electrode, gate structures can include an additional metal layer, i.e., a work function layer, to tune the work function and consequently the threshold voltage of the devices. Generally, the work-function layers are relatively less conductive than gate electrode metal, which can result in a loss of desired performance of the devices. Attempts to increase the conductivity of the work-function layers generally results in lower work function of the devices.
Accordingly, improved material layers suitable for tunable work function layers and structures and devices including such layers as well as methods of forming such layers, structures, and devices are desired.