Tunable Mobility in Double-Gated MoTe₂ Field-Effect Transistor: Effect of Coulomb Screening and Trap Sites

There is a general consensus that the carrier mobility in a field-effect transistor (FET) made of semiconducting transition-metal dichalcogenides (s-TMDs) is severely degraded by the trapping/detrapping and Coulomb scattering of carriers by ionic charges in the gate oxides. Using a double-gated (DG) MoTe₂ FET, we modulated and enhanced the carrier mobility by adjusting the top- and bottom-gate biases. The relevant mechanism for mobility tuning in this device was explored using static DC and low-frequency (LF) noise characterizations. In the investigations, LF-noise analysis revealed that for a strong back-gate bias the Coulomb scattering of carriers by ionized traps in the gate dielectrics is strongly screened by accumulation charges. This significantly reduces the electrostatic scattering of channel carriers by the interface trap sites, resulting in increased mobility. The reduction of the number of effective trap sites also depends on the gate bias, implying that owing to the gate bias, the carriers are shifted inside the channel. Thus, the number of active trap sites decreases as the carriers are repelled from the interface by the gate bias. The gate-controlled Coulomb-scattering parameter and the trap-site density provide new handles for improving the carrier mobility in TMDs, in a fundamentally different way from dielectric screening observed in previous studies

Suppression of Interfacial Current Fluctuation in MoTe₂ Transistors with Different Dielectrics

For transition metal dichalcogenides, the fluctuation of the channel current due to charged impurities is attributed to a large surface area and a thickness of a few nanometers. To investigate current variance at the interface of transistors, we obtain the low-frequency (LF) noise features of MoTe₂ multilayer field-effect transistors with different dielectric environments. The LF noise properties are analyzed using the combined carrier mobility and carrier number fluctuation model which is additionally parametrized with an interfacial Coulomb-scattering parameter (α) that varies as a function of the accumulated carrier density (<i>N</i>_acc) and the location of the active channel layer of MoTe₂. Our model shows good agreement with the current power spectral density (PSD) of MoTe₂ devices from a low to high current range and indicates that the parameter α exhibits a stronger dependence on <i>N</i>_acc with an exponent −γ of −1.18 to approximately −1.64 for MoTe₂ devices, compared with −0.5 for Si devices. The raised Coulomb scattering of the carriers, particularly for a low-current regime, is considered to be caused by the unique traits of layered semiconductors such as interlayer coupling and the charge distribution strongly affected by the device structure under a gate bias, which completely change the charge screening effect in MoTe₂ multilayer. Comprehensive static and LF noise analyses of MoTe₂ devices with our combined model reveal that a chemical-vapor deposited <i>h</i>-BN monolayer underneath MoTe₂ channel and the Al₂O₃ passivation layer have a dissimilar contribution to the reduction of current fluctuation. The three-fold enhanced carrier mobility due to the <i>h</i>-BN is from the weakened carrier scattering at the gate dielectric interface and the additional 30% increase in carrier mobility by Al₂O₃ passivation is due to the reduced interface traps

Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS₂/<i>h</i>‑BN Heterostructure

Layered hexagonal boron nitride (<i>h</i>-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the <i>h</i>-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of <i>h</i>-BN thin film as a substrate for monolayer MoS₂ can induce ∼6.5 × 10¹¹ cm^–2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS₂ on <i>h</i>-BN was found to be ∼5 × 10¹³ cm^–2 at high temperature and was significantly reduced at low temperature. Further, the inserted <i>h</i>-BN enables us to reduce the Coulombic charge scattering in MoS₂/<i>h</i>-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal–insulator transition at a much lower charge density of ∼1.0 × 10¹² cm^–2 (<i>T</i> = 25 K). The reduced effective Schottky barrier height in MoS₂/<i>h</i>-BN is attributed to the decreased effective work function of MoS₂ arisen from <i>h</i>-BN induced <i>n</i>-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO₂

Understanding Coulomb Scattering Mechanism in Monolayer MoS₂ Channel in the Presence of <i>h</i>‑BN Buffer Layer

As the thickness becomes thinner, the importance of Coulomb scattering in two-dimensional layered materials increases because of the close proximity between channel and interfacial layer and the reduced screening effects. The Coulomb scattering in the channel is usually obscured mainly by the Schottky barrier at the contact in the noise measurements. Here, we report low-temperature (<i>T</i>) noise measurements to understand the Coulomb scattering mechanism in the MoS₂ channel in the presence of <i>h</i>-BN buffer layer on the silicon dioxide (SiO₂) insulating layer. One essential measure in the noise analysis is the Coulomb scattering parameter (α_SC) which is different for channel materials and electron excess doping concentrations. This was extracted exclusively from a 4-probe method by eliminating the Schottky contact effect. We found that the presence of <i>h</i>-BN on SiO₂ provides the suppression of α_SC twice, the reduction of interfacial traps density by 100 times, and the lowered Schottky barrier noise by 50 times compared to those on SiO₂ at <i>T</i> = 25 K. These improvements enable us to successfully identify the main noise source in the channel, which is the trapping–detrapping process at gate dielectrics rather than the charged impurities localized at the channel, as confirmed by fitting the noise features to the carrier number and correlated mobility fluctuation model. Further, the reduction in contact noise at low temperature in our system is attributed to inhomogeneous distributed Schottky barrier height distribution in the metal–MoS₂ contact region

Brush-Shaped ZnO Heteronanorods Synthesized Using Thermal-Assisted Pulsed Laser Deposition

Brush-shaped ZnO heteronanostructures were synthesized using a newly designed thermal-assisted pulsed laser deposition (T-PLD) system that combines the advantages of pulsed laser deposition (PLD) and a hot furnace system. Branched ZnO nanostructures were successfully grown onto CVD-grown backbone nanowires by T-PLD. Although ZnO growth at 300 °C resulted in core–shell structures, brush-shaped hierarchical nanostructures were formed at 500–600 °C. Materials properties were studied via photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations. The enhanced photocurrent of a SnO₂–ZnO heterostructures device by irradiation with 365 nm wavelength ultraviolet (UV) light was also investigated by the current–voltage characteristics