1000°C Thermal Stability and Low Trap Densities Achieved in HfO2/GaAs(001)
C. Y. Yang1*, H. W. Wan2, Y. H. Lin2, J. Kwo1, M. Hong2
1Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
2Graduate Institute of Applied Physics and Department of Physics, National Taiwan University, Taipei, Taiwan
* Presenter:C. Y. Yang
The SiO₂/Si interface, which is thermally and chemically stable, offers several advantages in the complicated complementary metal-oxide-semiconductor (CMOS) fabrication. However, to fulfill the aggressive scaling of CMOS, the thickness of SiO₂ layer is facing its physical limit. To overcome this issue, high-κ HfO₂-based dielectric has replaced SiO₂ since the 45nm node technology. But, there still exists a ~0.4nm SiO₂ interfacial passivation layers (IPLs) for Si, which is non-ideal for the EOT scaling of sub 10nm node. Also, high mobility channel is expected to replace Si as channel layer in near future. Hence, it is necessary to integrate HfO₂ directly on high mobility channel without IPLs. GaAs has high potential to replace Si as n-channel including several advantages such as: (I) high electron mobility, (II) a lattice constant of 5.65Å compatible with Si, and (III) a band gap of 1.42eV.
The hetero-structures of HfO₂/p-GaAs(001) were grown in our multi-chamber UHV system, which consists of a solid-source GaAs-based MBE chamber and an As-free oxide-MBE chamber. HfO₂ 1nm thick was e-beam evaporated directly onto GaAs(001), followed by an in-situ cap of ALD-Al₂O₃ 7nm thick to prevent HfO₂ from moisture absorption. The samples were rapid-thermal-annealed (RTA) to various high temperatures in He to examine the interfacial thermal stability. Ni and Ti/Au were deposited as metal gate and back ohmic contact for the fabrication of MOS capacitors (MOSCAPs). The samples were post-metallization annealed (PMA) in forming gas (H₂ 15%/N₂ 85%) at 400°C for duration of 5min.
For HfO₂/p-GaAs(001) MOSCAPs, as the post deposition annealing (PDA) temperature raised from 930°C to 1000°C, the frequency dispersion in accumulation region increased from lowest value of 7.1% to 13.9%. However, for HfO₂/n-GaAs(001) MOSCAPs, the lowest dispersion value ~16.9% was obtained with a lower thermal stability of 850°C. The J-E for the MOSCAPs, where the leakage current densities remain very similar in the order of 10-⁸A/cm-² at E < ±4 (MV/cm) for all the samples. The low leakage current density suggests that the HfO₂/GaAs interface and the bulk oxide remain intact even with the annealing temperature up to 1000°C. To realize the interfacial properties, the Dit spectrum was extracted from the QSCV measurement. A low Dit value below 10¹²eV-¹cm-² across the lower half band gap with a minimum value of 4.5 × 10¹¹eV-¹cm-² was obtained in 930°C annealed sample. Higher Dit value in the upper half of GaAs band-gap is extracted from QSCV.
In conclusion, record-high thermal stability at the interface of HfO₂/GaAs(001) hetero-structures was demonstrated in this work. The low interfacial trap densities at HfO₂/GaAs(001) interface even after 930 to 1000°C RTA is very promising for the future incorporation of GaAs-based nMOSFETs into post-Si CMOS.

Keywords: HfO₂, high-k, GaAs, high thermal stability