Here, we report the observation of a hitherto unreported optoelectronic effect, namely a light-induced diode-like response in multi-layered MoSe$_2$ field-effect transistors whose sense of current rectification is controllable through a gate voltage. We argue, through numerical simulations, that this behavior results from the difference in the size of the Schottky barriers between drain and source metal contacts. Each barrier can be modeled as a Schottky diode but with opposite senses of current rectification between them, with the diode response resulting from the light induced promotion of photo-generated carriers across the smaller barrier. The back gate voltage controls the sense of current rectification by modulating the relative amplitude between them. This effect, which gives rise to a novel type of optoelectronic switch, also yields a photovoltaic response. Hence, it could provide an alternative to PN-junctions when harvesting photovoltaic currents from transition metal dichalcogenides. We argue that the photovoltaic efficiency associated to this effect could be increased by just increasing the relative asymmetry between both Schottky barriers. We also suggest that this new electro-optical effect has potential for technological applications.
We demonstrate a vertical field-effect transistor based on a graphene/MoSe2 van der Waals (vdW) heterostructure. The vdW interface between the graphene and MoSe2 exhibits a Schottky barrier with an ideality factor of around 1.3, suggesting a high-quality interface. Owing to the low density of states in graphene, the position of the Fermi level in the graphene can be strongly modulated by an external electric field. Therefore, the Schottky barrier height at the graphene/MoSe2 vdW interface is also modulated. We demonstrate a large current ON-OFF ratio of 10^5. These results point to the potenti…
Schottky barrier field-effect transistors (SBFETs) based on few and mono layer phosphorene are simulated by the non-equilibrium Green's function formalism. It is shown that scaling down the gate oxide thickness results in pronounced ambipolar I-V characteristics and significant increase of the minimal leakage current. The problem of leakage is especially severe when the gate insulator is thin and the number of layer is large, but can be effectively suppressed by reducing phosphorene to mono or bilayer. Different from two-dimensional graphene and layered dichalcogenide materials, both the ON-cu…
Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With differe…
Engineering Schottky Barrier in Black Phosphorus field effect devices for spintronic applications
黑磷（BP）是近年来发现的一种很有前途的二维直接带隙半导体材料。在这里，我们报告的双极场效应晶体管的行为多层BP铁磁隧道接触。通过使用TiO ${_2}$ /Co接触，我们观察到肖特基势垒降低< 50 meV，这可以通过栅极电压进一步调节。突出地，在BP器件中实现了良好的晶体管性能，其中漏极电流调制在四到六个数量级上。在室温下，空穴和电子的载流子迁移率分别为$\sim$ 155和0.18 cm ${^2}$ V ${^{-1}}$ s ${^{-1}}$.此外，磁阻计算表明，与施加的栅极电压的BP设备的电阻是在适当的范围内注入和检测自旋极化的空穴。我们的研究结果表明，工程BP nanolayered器件的有效的纳米电子和自旋电子学应用的前景。
Circular photocurrents (CPC), namely circular photogalvanic (CPGE) and photon drag effects, have recently been reported both in monolayer and multilayer transition metal dichalcogenide (TMD) phototransistors. However, the underlying physics for the emergence of these effects are not yet fully understood. In particular, the emergence of CPGE is not compatible with the D3h crystal symmetry of two-dimensional TMDs, and should only be possible if the symmetry of the electronic states is reduced by influences such as an external electric field or mechanical strain. Schottky contacts, nearly ubiquit…
The Effect of Dielectric Capping on Few-Layer Phosphorene Transistors: Tuning the Schottky Barrier Heights
磷烯是一种独特的具有二维层状结构的单元素半导体。在这封信中，我们研究了机械剥离的少层磷烯顶栅晶体管的行为。我们实现了144 mA/mm的高导通电流和95.6 cm 2/Vs的空穴迁移率。我们沉积Al 2 O3原子层沉积（ALD）和研究的电介质覆盖的影响。我们观察到，晶体管的极性交替从p型到双极与Al 2 O 3生长在顶部。我们将这种转变归因于金属接触边缘处的电子和空穴的有效肖特基势垒高度的变化，这源于ALD电介质中的固定电荷。
Atomically thin materials, like semiconducting transition metal dichalcogenides (S-TMDs), are highly sensitive to the environment. This opens up an opportunity to externally control their properties by changing their surroundings. We investigate the effect of several metallic substrates on the optical properties of MoSe$_2$ monolayer (ML) deposited on top of them with photoluminescence and reflectance contrast techniques. The optical spectra of MoSe$_{2}$ MLs deposited on Pt, Au, Mo and Zr have distinctive metal-related lineshapes. In particular, a substantial variation in the intensity ratio …
Two-dimensional material-based field effect transistors (2DM-FETs) are playing a revolutionary role in electronic devices. However, after years of development, no device model can match the Pao-Sah model for standard silicon-based transistors in terms of physical accuracy and computational efficiency to support large-scale integrated circuit design. One remaining critical obstacle is the contacts of 2DM-FETs. In order to self-consistently include the contact effect in the current model, it is necessary to perform self-consistent calculations, which is a fatal flaw for applications that priorit…
The performance of electronic and spintronic devices based on two-dimensional semiconductors (2D SC) is largely dependent on the quality and resistance of the metal/SC electrical contacts, as well as preservation of the intrinsic properties of the SC channel. Direct Metal/SC interaction results in highly resistive contacts due to formation of large Schottky barriers and considerably affects the properties of the 2D SC. In this work, we address these two important issues in monolayer $\mathrm{MoSe_2}$ Field-Effect transistors (FETs). We encapsulate the $\mathrm{MoSe_2}$ channel with hexagonal B…
Due to Fermi level pinning (FLP), metal-semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal - an essential requirement for several applications, including light-emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky-Mott limit of de-pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this work, we pr…
Field-Effect Transistors Based on Few-Layered alpha-MoTe_2
本文报道了在SiO_2上用机械剥离法生长的几层化学气相传输法生长的α- MoTe_2晶体的场效应晶体管的特性。我们进行了场效应和霍尔迁移率测量，以及拉曼散射和透射电子显微镜。与MoS_2和MoSe_2相比，我们的MoTe_2场效应晶体管（FET）被观察到是空穴掺杂的，显示超过106的开/关比和典型的亚阈值摆幅为每十倍约140 mV。场效应迁移率和霍尔迁移率的最大值接近或超过10 cm ^2/Vs，这与以前报道的室温下单层或双层MoS_2和/或MoSe_2在SiO_2上剥离的数据相当。拉曼散射揭示了尖锐的模式与以前的报告，其频率被发现显示很少或没有依赖于层数。由于两种MoS_2都是电子掺杂的，MoTe_2在MoS_2上的堆叠可以产生双极场效应晶体管和间隙调制。尽管MoTe_2的总体电子性能与MoS_2和MoSe_2相当，但较重的元素Te应该会导致更强的自旋轨道耦合，并可能伴随着更长的激子谷和自旋指数的退相干时间。
The metal contacts on 2D black phosphorus field-effect transistor and photodetectors are studied. The metal work functions can significantly impact the Schottky barrier at the metal-semiconductor contact in black phosphorus devices. Higher metal work functions lead to larger output hole currents in p-type transistors, while ambipolar characteristics can be observed with lower work function metals. Photodetectors with record high photoresponsivity (223 mA/W) are demonstrated on black phosphorus through contact-engineering.
Electric Field-Dependent Charge-Carrier Velocity in Semiconducting Carbon Nanotubes
研究了高偏压下具有肖特基势垒接触的半导体单壁纳米管（SWNTs）中的电荷输运。我们观察到几乎对称的双极传输与电子和空穴电流显着超过25微安，在金属单壁碳纳米管由于光学声子发射报告的电流极限。在单极状态下，研究了四种简单的场相关速度模型（弹道、电流饱和、速度饱和和恒定迁移率）;饱和速度为2 x 10^7 cm/s的速度饱和模型可以最好地解释高偏压行为。
Atomically-thin 2D semiconducting materials integrated into van der Waals heterostructures have enabled architectures that hold great promise for next generation nanoelectronics. However, challenges still remain to enable their full acceptance as compliant materials for integration in logic devices. Two key-components to master are the barriers at metal/semiconductor interfaces and the mobility of the semiconducting channel, which endow the building-blocks of ${pn}$ diode and field effect transistor. Here, we have devised a reverted stacking technique to intercalate a wrinkle-free h-BN tunnel …
