Nickel silicide has long been used in the various microelectronic devices as ohmic contacts with p-type semiconductors, Schottky barrier contacts, gate electrodes, local interconnects and diffusion barriers. In addition, nickel silicide has also been utilized in the photovoltaic industry mainly as adhesion promoter and metallization layer for nickel/copper plated electrodes on silicon solar cells (SCs) and as a metallic component in silicide-on-Si Schottky-junction SCs.1 However, its lack of bandgap has prohibited it from being used in applications that require a bandgap as FETs, optoelectronics, thermoelectrics etc. unlike other silicides such as germanium silicide, magnesium silicide, ruthenium silicide, iron disilicide and so on.2,3
Herein, we demonstrate a modification of the electronic band structure of nickel-based silicides via producing wafer scale single crystal-like films of only a few nanometers thick. Uniform thick (~ 100nm) and ultra-thin (< 6nm) nickel-based silicide films were prepared by RF magnetron sputtering of nickel on an intrinsic silicon substrate followed by rapid thermal annealing (RTA) in an inert environment in the temperature range of 300–650 °C. The Seebeck coefficient (S) and Hall measurements at room temperature for the thick films reveal S value and sheet resistance as low as 1.3 μV/K and 2.2 μΩ.cm, respectively with carrier concentration in the order of 1023 cm-3 indicating their metallic behavior. On the other hand, the ultra-thin films demonstrate a monolithic enhancement of room temperature S values versus RTA temperature up to 52 μV/K for the film annealed at 500 °C, indicating its p-type semiconducting behavior with the sheet resistance of 45 μΩ.cm and carrier concentration of 4×1022 cm-3. Interestingly, a transition to n-type was obtained by further increasing the RTA temperature with a peak S value of -148 μV/K, sheet resistance of 37 μΩ.cm and carrier concentration of -2.74×1021 cm-3 for the films annealed at 600 °C. This drastic change in the electronic transport properties is attributed to the altering the electronic band structure due to quantum confinement effect in the film with the thickness close to the electron wavelength. Also, the ease of fabrication and remarkable enhancement in the Seebeck coefficient along with the obtained high power factor (P = σS2) highlight the ultra-thin nickel-based silicide film as a good thermoelectric material among the thermoelectric silicides. The electron transport properties were further studied through performing temperature dependent Hall and Seebeck measurements, and correlated to the structural information obtained by transition electron microscopy (TEM) and X-ray crystallography. Our obtained results offer a new paradigm for the exploration of the nickel-based silicide applications in the modern solid-state devices.
1. J. Phys. D: Appl. Phys. 2017, 50, 035102.
2. Jpn. J. Appl. Phys. 2017, 56, 05DA04.
3. Microelectron. Eng. 2000, 50, 223.