Improved Growth Behavior of Atomic-Layer-Deposited High‑<i>k</i> Dielectrics on Multilayer MoS₂ by Oxygen Plasma Pretreatment

We report on the effect of oxygen plasma treatment of two-dimensional multilayer MoS₂ crystals on the subsequent growth of Al₂O₃ and HfO₂ films, which were formed by atomic layer deposition (ALD) using trimethylaluminum and tetrakis-(ethylmethylamino)­hafnium metal precursors, respectively, with water oxidant. Due to the formation of an ultrathin Mo-oxide layer on the MoS₂ surface, the surface coverage of Al₂O₃ and HfO₂ films was significantly improved compared to those on pristine MoS₂, even at a high ALD temperature. These results indicate that the surface modification of MoS₂ by oxygen plasma treatment can have a major impact on the subsequent deposition of high-<i>k</i> thin films, with important implications on their integration in thin film transistors

Chemical Doping Effects in Multilayer MoS₂ and Its Application in Complementary Inverter

Multilayer MoS₂ has been gaining interest as a new semiconducting material for flexible displays, memory devices, chemical/biosensors, and photodetectors. However, conventional multilayer MoS₂ devices have exhibited limited performances due to the Schottky barrier and defects. Here, we demonstrate poly­(diketopyrrolopyrrole-terthiophene) (PDPP3T) doping effects in multilayer MoS₂, which results in improved electrical characteristics (∼4.6× higher on-current compared to the baseline and a high current on/off ratio of 10⁶). Synchrotron-based study using X-ray photoelectron spectroscopy and grazing incidence wide-angle X-ray diffraction provides mechanisms that align the edge-on crystallites (97.5%) of the PDPP3T as well as a larger interaction with MoS₂ that leads to dipole and charge transfer effects (at annealing temperature of 300 °C), which support the observed enhancement of the electrical characteristics. Furthermore, we demonstrate a complementary metal–oxide–semiconductor inverter that uses a p-type MoSe₂ and a PDPP3T-doped MoS₂ as charging and discharging channels, respectively

Tuning the Built-in Electric Field in Ferroelectric Pb(Zr_0.2Ti_0.8)O₃ Films for Long-Term Stability of Single-Digit Nanometer Inverted Domains

The emergence of new technologies, such as whole genome sequencing systems, which generate a large amount of data, is requiring ultrahigh storage capacities. Due to their compactness and low power consumption, probe-based memory devices using Pb­(Zr_0.2Ti_0.8)­O₃ (PZT) ferroelectric films are the ideal candidate for such applications where portability is desired. To achieve ultrahigh (>1 Tbit/in²) storage densities, sub-10 nm inverted domains are required. However, such domains remain unstable and can invert back to their original polarization due to the effects of an antiparallel built-in electric field in the PZT film, domain-wall, and depolarization energies. Here, we show that the built-in electric-field can be tuned and suppressed by repetitive hydrogen and oxygen plasma treatments. Such treatments trigger reversible Pb reduction/oxidation activity, which alters the electrochemistry of the Pb overlayer and compensates for charges induced by the Pb vacancies. This tuning mechanism is used to demonstrate the writing of stable and equal size sub-4 nm domains in both up- and down-polarized PZT films, corresponding to eight inverted unit-cells. The bit sizes recorded here are the smallest ever achieved, which correspond to potential 60 Tbit/in² data storage densities

Improving the Stability of High-Performance Multilayer MoS₂ Field-Effect Transistors

In this study, we propose a method for improving the stability of multilayer MoS₂ field-effect transistors (FETs) by O₂ plasma treatment and Al₂O₃ passivation while sustaining the high performance of bulk MoS₂ FET. The MoS₂ FETs were exposed to O₂ plasma for 30 s before Al₂O₃ encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoO<i>_x</i> layer formed during the plasma treatment was found between MoS₂ and the top passivation layer. The MoO<i>_x</i> interlayer prevents the generation of excess electron carriers in the channel, owing to Al₂O₃ passivation, thereby minimizing the shift in the threshold voltage (<i>V</i>_th) and increase of the off-current leakage. However, prolonged exposure of the MoS₂ surface to O₂ plasma (90 and 120 s) was found to introduce excess oxygen into the MoO<i>_x</i> interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS₂ FETs were also subjected to gate-bias stress tests under different conditions. The MoS₂ transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in <i>V</i>_th were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS₂ transistors are expected to open up new opportunities for the development of sophisticated electronic applications