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

Photocurrent Switching of Monolayer MoS₂ Using a Metal–Insulator Transition

We achieve switching on/off the photocurrent of monolayer molybdenum disulfide (MoS₂) by controlling the metal–insulator transition (MIT). N-type semiconducting MoS₂ under a large negative gate bias generates a photocurrent attributed to the increase of excess carriers in the conduction band by optical excitation. However, under a large positive gate bias, a phase shift from semiconducting to metallic MoS₂ is caused, and the photocurrent by excess carriers in the conduction band induced by the laser disappears due to enhanced electron–electron scattering. Thus, no photocurrent is detected in metallic MoS₂. Our results indicate that the photocurrent of MoS₂ can be switched on/off by appropriately controlling the MIT transition by means of gate bias

Junction-Structure-Dependent Schottky Barrier Inhomogeneity and Device Ideality of Monolayer MoS₂ Field-Effect Transistors

Although monolayer transition metal dichalcogenides (TMDs) exhibit superior optical and electrical characteristics, their use in digital switching devices is limited by incomplete understanding of the metal contact. Comparative studies of Au top and edge contacts with monolayer MoS₂ reveal a temperature-dependent ideality factor and Schottky barrier height (SBH). The latter originates from inhomogeneities in MoS₂ caused by defects, charge puddles, and grain boundaries, which cause local variation in the work function at Au–MoS₂ junctions and thus different activation temperatures for thermionic emission. However, the effect of inhomogeneities due to impurities on the SBH varies with the junction structure. The weak Au–MoS₂ interaction in the top contact, which yields a higher SBH and ideality factor, is more affected by inhomogeneities than the strong interaction in the edge contact. Observed differences in the SBH and ideality factor in different junction structures clarify how the SBH and inhomogeneities can be controlled in devices containing TMD materials

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