Peijiang Cao

Black Phosphorus Based Field Effect Transistors with Simultaneously Achieved Near Ideal Subthreshold Swing and High Hole Mobility at Room Temperature.

Black phosphorus (BP) has emerged as a promising two-dimensional (2D) material for next generation transistor applications due to its superior carrier transport properties. Among other issues, achieving reduced subthreshold swing and enhanced hole mobility simultaneously remains a challenge which requires careful optimization of the BP/gate oxide interface. Here, we report the realization of high performance BP transistors integrated with HfO2 high-k gate dielectric using a low temperature CMOS process. The fabricated devices were shown to demonstrate a near ideal subthreshold swing (SS) of ~69 mV/dec and a room temperature hole mobility of exceeding >400 cm2/Vs. These figure-of-merits are benchmarked to be the best-of-its-kind, which outperform previously reported BP transistors realized on traditional SiO2 gate dielectric. X-ray photoelectron spectroscopy (XPS) analysis further reveals the evidence of a more chemically stable BP when formed on HfO2 high-k as opposed to SiO2, which gives rise to a better interface quality that accounts for the SS and hole mobility improvement. These results unveil the potential of black phosphorus as an emerging channel material for future nanoelectronic device applications.

AlGaN/GaN Metal-Oxide-Semiconductor High-Electron-Mobility Transistor with Polarized P(VDF-TrFE) Ferroelectric Polymer Gating

Effect of a polarized P(VDF-TrFE) ferroelectric polymer gating on AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) was investigated. The P(VDF-TrFE) gating in the source/drain access regions of AlGaN/GaN MOS-HEMTs was positively polarized (i.e., partially positively charged hydrogen were aligned to the AlGaN surface) by an applied electric field, resulting in a shift-down of the conduction band at the AlGaN/GaN interface. This increases the 2-dimensional electron gas (2-DEG) density in the source/drain access region of the AlGaN/GaN heterostructure, and thereby reduces the source/drain series resistance. Detailed material characterization of the P(VDF-TrFE) ferroelectric film was also carried out using the atomic force microscopy (AFM), X-ray Diffraction (XRD), and ferroelectric hysteresis loop measurement.

Band alignment of HfO2/multilayer MoS2 interface determined by x-ray photoelectron spectroscopy: Effect of CHF3 treatment

The energy band alignment between HfO2/multilayer (ML)-MoS2 was characterized using high-resolution x-ray photoelectron spectroscopy. The HfO2 was deposited using an atomic layer deposition tool, and ML-MoS2 was grown by chemical vapor deposition. A valence band offset (VBO) of 1.98 eV and a conduction band offset (CBO) of 2.72 eV were obtained for the HfO2/ML-MoS2 interface without any treatment. With CHF3 plasma treatment, a VBO and a CBO across the HfO2/ML-MoS2 interface were found to be 2.47 eV and 2.23 eV, respectively. The band alignment difference is believed to be dominated by the down-shift in the core level of Hf 4d and up-shift in the core level of Mo 3d, or the interface dipoles, which caused by the interfacial layer in rich of F.

Influence of A-site size and disorder on metamagnetic transformation in A-site substituted Pr0.5Ca0.5MnO3

The influence of A-site size and disorder on metamagnetic transformation in polycrystalline Pr0.5−xLaxCa0.5MnO3 and Pr0.5Ca0.5−xBaxMnO3 at low temperature has been systematically investigated. The introduction of larger A-site cations such as La3+ or Ba2+ will locally suppress the lattice distortion. This “counterdistortion” effect becomes more pronounced with the increase in the A-site average ionic radius ⟨rA⟩, which is favorable to ferromagnetism. Thus the critical field HC1 corresponding to the first sharp metamagnetic step will decrease, and the field-induced magnetization M5 T will increase. However, the A-site ionic radii variance σ2 will dramatically increase when more barium is introduced and a spin glasslike state is induced, which is unfavorable to the stability of ferromagnetism state. Therefore, HC1 will increase and M5 T will decrease. Moreover, the evolution of resistivity at low temperature with σ2 exhibits strong correlation with that of magnetic properties.

Low temperature carrier transport study of monolayer MoS2 field effect transistors prepared by chemical vapor deposition under an atmospheric pressure

Large size monolayer Molybdenum disulphide (MoS2) was successfully grown by chemical vapor deposition method under an atmospheric pressure. The electrical transport properties of the fabricated back-gate monolayer MoS2 field effect transistors (FETs) were investigated under low temperatures; a peak field effect mobility of 59 cm2V−1s−1 was achieved. With the assist of Raman measurement under low temperature, this work identified the mobility limiting factor for the monolayer MoS2 FETs: homopolar phonon scattering under low temperature and electron-polar optical phonon scattering at room temperature.

C-Axis oriented crystalline IGZO thin-film transistors by magnetron sputtering

We demonstrate the direct formation of c-axis oriented crystalline IGZO thin films at room temperature by magnetron sputtering. The influence of processing parameters such as oxygen partial pressure, post-annealing temperature and channel thickness on the electrical performance of IGZO films and thin-film transistors (TFTs) was intensively investigated. The as-deposited crystalline IGZO TFTs exhibited a mobility of 4.49 cm2 V−1 s−1 and an on/off ratio of 2.08 × 107. For the annealed device, a high mobility of 10.51 cm2 V−1 s−1, a subthreshold swing of 0.672 V decade−1, a threshold voltage of 0.38 V, as well as an on/off current ratio of ∼108 are achieved with an annealing temperature of 400 °C. These results present a significant step towards the development of high-performance TFTs using oriented crystalline IGZO.