Mobility Anisotropy in Black Phosphorus MOSFETs With HfO2 Gate Dielectrics

• Published in: IEEE transactions on electron devices Vol. 65; no. 10; pp. 4093 - 4101
• Main Authors: Haratipour, Nazila, Liu, Yue, Wu, Ryan J., Namgung, Seon, Ruden, P. Paul, Mkhoyan, K. Andre, Oh, Sang-Hyun, Koester, Steven J.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-10-2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Anisotropic magnetoresistance; Anisotropy; Axes (reference lines); black phosphorus (BP); Crystal structure; Crystals; Dielectrics; Electron mobility; Fabrication; Hafnium oxide; Hole mobility; Logic gates; mobility; MOSFET; MOSFETs; phosphorene; Phosphorus; Temperature dependence; Transmission electron microscopy
• ISSN: 0018-9383; 1557-9646
• DOI: 10.1109/TED.2018.2865440

Precise measurements of the mobility anisotropy along high-symmetry crystal axes in black phosphorus (BP) MOSFETs are reported. Locally back-gated BP MOSFETs with 13-nm HfO 2 dielectric and channel length ranging from 0.3 to 0.7 <inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> are fabricated. A single BP flake of a uniform thickness is exfoliated and etched along armchair (AC) and zigzag (ZZ) crystal axes, and the orientations are confirmed using optical and transmission electron microscopy analyses. The hole and electron mobilities along each direction are extracted using the transfer length method. The AC-to-ZZ hole mobility ratio is found to increase from 1.4 (1.5) to 2.0 (2.9) as the sheet concentration increased from <inline-formula> <tex-math notation="LaTeX">5.1\times 10^{\textsf {11}} </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">1.9\times 10^{\textsf {12}} </tex-math></inline-formula> cm −2 at room temperature (77 K). The room-temperature electron mobility anisotropy is found to be similar to that for holes with an AC-to-ZZ mobility ratio increasing from 1.4 to 2.1 from <inline-formula> <tex-math notation="LaTeX">5.1\times 10^{\textsf {11}} </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">1.9\times 10^{\textsf {12}} </tex-math></inline-formula> cm −2 though electrons showed only a very weak temperature dependence. A Boltzmann transport model is used to explain the concentration- and temperature-dependent mobility anisotropies which can be well described using a charge center scattering model.