Dipole engineering at HfO2/SiO2 interface by ultra-thin Al2O3 to modulate flat-band voltage for cryogenic temperatures

The behaviors of the flat-band voltage (Vfb) in Si/SiO2/HfO2/TiN metal-oxide-semiconductor capacitors (MOSCaps) were systematically investigated at temperatures ranging from 300 K to 77 K, in order to develop an effective threshold voltage (Vth) modulation strategy for gate-all-around field-effect transistors (GAAFETs) operating at cryogenic temperatures. A significant 226 mV negative shift in Vfb was observed as the temperature decreased from 300 K to 77 K. Both experimental and theoretical analyses indicated that, as the temperature approached cryogenic levels, the primary negative shift was attributed to the inherent increase in the Si Fermipotential (phi F), while a minor portion of that was associated with the reduction of dipole strength at the SiO2/ HfO2 interface. To compensate for the negative shift in Vfb and meet the requirement for a lower Vth at cryogenic temperatures, an effective Vfb modulation was demonstrated through the introduction of an ultra-thin Al2O3 layer into the SiO2/HfO2 interface, which enhanced the interfacial dipole strength. The experimental results demonstrated a maximum shift in Vfb of 240 mV at 77 K. Furthermore, X-ray photoelectron spectroscopy (XPS) characterization revealed that the interface dipole was linked to the aluminosilicate formed at the SiO2/HfO2 interface and the Hf-Al-O bonds formed at the Al2O3/HfO2 interface. The valence band offsets (VBO) were observed to increase with Al2O3 thickness, indicating that the strength of the dipole increased in proportion to the Al2O3 thickness. Consequently, dipole engineering was demonstrated to be a feasible and effective strategy for achieving Vth modulation in GAAFETs at cryogenic temperatures.