Xiuying Zhang

Schottky Barriers in Bilayer Phosphorene Transistors

It is unreliable to evaluate the Schottky barrier height (SBH) in monolayer (ML) 2D material field effect transistors (FETs) with strongly interacted electrode from the work function approximation (WFA) because of existence of the Fermi-level pinning. Here, we report the first systematical study of bilayer (BL) phosphorene FETs in contact with a series of metals with a wide work function range (Al, Ag, Cu, Au, Cr, Ti, Ni, and Pd) by using both ab initio electronic band calculations and quantum transport simulation (QTS). Different from only one type of Schottky barrier (SB) identified in the ML phosphorene FETs, two types of SBs are identified in BL phosphorene FETs: the vertical SB between the metallized and the intact phosphorene layer, whose height is determined from the energy band analysis (EBA); the lateral SB between the metallized and the channel BL phosphorene, whose height is determined from the QTS. The vertical SBHs show a better consistency with the lateral SBHs of the ML phosphorene FETs from the QTS compared than that of the popular WFA. Therefore, we develop a better and more general method than the WFA to estimate the lateral SBHs of ML semiconductor transistors with strongly interacted electrodes based on the EBA for its BL counterpart. In terms of the QTS, n-type lateral Schottky contacts are formed between BL phosphorene and Cr, Al, and Cu electrodes with electron SBH of 0.27, 0.31, and 0.32 eV, respectively, while p-type lateral Schottky contacts are formed between BL phosphorene and Pd, Ti, Ni, Ag, and Au electrodes with hole SBH of 0.11, 0.18, 0.19, 0.20, and 0.21 eV, respectively. The theoretical polarity and SBHs are in good agreement with available experiments. Our study provides an insight into the BL phosphorene-metal interfaces that are crucial for designing the BL phosphorene device.

Electrical Contacts in Monolayer Arsenene Devices

Arsenene, arsenic analogue of graphene, as an emerging member of two-dimensional semiconductors (2DSCs), is quite promising in next-generation electronic and optoelectronic applications. The metal electrical contacts play a vital role in the charge transport and photoresponse processes of nanoscale 2DSC devices and even can mask the intrinsic properties of 2DSCs. Here, we present a first comprehensive study of the electrical contact properties of monolayer (ML) arsenene with different electrodes by using ab initio electronic calculations and quantum transport simulations. Schottky barrier is always formed with bulk metal contacts owing to the Fermi level pinning (pinning factor S = 0.33), with electron Schottky barrier height (SBH) of 0.12, 0.21, 0.25, 0.35, and 0.50 eV for Sc, Ti, Ag, Cu, and Au contacts and hole SBH of 0.75 and 0.78 eV for Pd and Pt contacts, respectively. However, by contact with 2D graphene, the Fermi level pinning effect can be reduced due to the suppression of metal-induced gap states. Remarkably, a barrier free hole injection is realized in ML arsenene device with graphene-Pt hybrid electrode, suggestive of a high device performance in such a ML arsenene device. Our study provides a theoretical foundation for the selection of favorable electrodes in future ML arsenene devices.

Three-layer phosphorene-metal interfaces

Phosphorene has attracted much attention recently as an alternative channel material in nanoscale electronic and optoelectronic devices due to its high carrier mobility and tunable direct bandgap. Compared with monolayer (ML) phosphorene, few-layer (FL) phosphorene is easier to prepare, is more stable in experiments, and is expected to form a smaller Schottky barrier height (SBH) at the phosphorene-metal interface. Using ab initio electronic structure calculations and quantum transport simulations, we perform a systematic study of the interfacial properties of three-layer (3L) phosphorene field effect transistors (FETs) contacted with several common metals (Al, Ag, Au, Cu, Ti, Cr, Ni, and Pd) for the first time. The SBHs obtained in the vertical direction from projecting the band structures of the 3L phosphorene-metal systems to the left bilayer (2L) phosphorenes are comparable with those obtained in the lateral direction from the quantum transport simulations for 2L phosphorene FETs. The quantum transport simulations for the 3L phosphorene FETs show that 3L phosphorene forms n-type Schottky contacts with electron SBHs of 0.16 and 0.28 eV in the lateral direction, when Ag and Cu are used as electrodes, respectively, and p-type Schottky contacts with hole SBHs of 0.05, 0.11, 0.20, 0.30, 0.30, and 0.31 eV in the lateral direction when Cr, Pd, Ni, Ti, Al, and Au are used as electrodes, respectively. The calculated polarity and SBHs of the 3L phosphorene FETs are generally in agreement with the available experiments.

Monolayer Bismuthene-Metal Contacts: A Theoretical Study

Bismuthene, a bismuth analogue of graphene, has a moderate band gap, has a high carrier mobility, has a topological nontriviality, has a high stability at room temperature, has an easy transferability, and is very attractive for electronics, optronics, and spintronics. The electrical contact plays a critical role in an actual device. The interfacial properties of monolayer (ML) bismuthene in contact with the metal electrodes spanning a wide work function range in a field-effect transistor configuration are systematically studied for the first time by using both first-principles electronic structure calculations and quantum transport simulations. The ML bismuthene always undergoes metallization upon contact with the six metal electrodes owing to a strong interaction. According to the quantum transport simulations, apparent metal-induced gap states (MIGSs) formed in the semiconductor-metal interface give rise to a strong Fermi-level pinning. As a result, the ML bismuthene forms an n-type Schottky contact with Ir/Ag/Ti electrodes with electron Schottky barrier heights (SBHs) of 0.17, 0.22, and 0.25 eV, respectively, and a p-type Schottky contact with Pt/Al/Au electrodes with hole SBHs of 0.09, 0.16, and 0.38 eV, respectively. The effective channel length of the ML bismuthene transistors is also significantly reduced by the MIGSs. However, the MIGSs are eliminated and the effective channel length is increased when ML graphene is used as an electrode, accompanied by a small hole SBH of 0.06 eV (quasi-Ohmic contact). Hence, an insight is provided into the interfacial properties of the ML bismuthene-metal composite systems and a guidance is provided for the choice of metal electrodes in ML bismuthene devices.

Electrical contacts in monolayer blue phosphorene devices

Semiconducting monolayer (ML) blue phosphorene (BlueP) shares similar stability with ML black phosphorene (BP), and it has recently been grown on an Au surface. Potential ML BlueP devices often require direct contact with metal to enable the injection of carriers. Using ab initio electronic structure calculations and quantum transport simulations, for the first time, we perform a systematic study of the interfacial properties of ML BlueP in contact with metals spanning a wide work function range in a field effect transistor (FET) configuration. ML BlueP has undergone metallization owing to strong interaction with five metals. There is a strong Fermi level pinning (FLP) in the ML BlueP FETs due to the metal-induced gap states (MIGS) with a pinning factor of 0.42. ML BlueP forms n-type Schottky contact with Sc, Ag, and Pt electrodes with electron Schottky barrier heights (SBHs) of 0.22, 0.22, and 0.80 eV, respectively, and p-type Schottky contact with Au and Pd electrodes with hole SBHs of 0.61 and 0.79 eV, respectively. The MIGS are eliminated by inserting graphene between ML BlueP and the metal electrode, accompanied by a transition from a strong FLP to a weak FLP. Our study not only provides insight into the ML BlueP–metal interfaces, but also helps in the design of ML BlueP devices.

Monolayer tellurene–metal contacts

Two-dimensional (2D) atomic crystals are promising channel materials for next generation electronics due to its outstanding gate electrostatics and few dangling bonds. Recently, tellurene, a new experimentally accessible Group-VI 2D tellurium, has drawn attention due to its large on/off ratios, high mobility and significant air stability. Herein, for the first time, we comprehensively examine the interfacial characteristics of monolayer (ML) tellurene field-effect transistors with a series of common bulk metals and 2D graphene as electrodes by using ab initio electronic structure calculations and quantum transport simulations. Furthermore, a lateral n-type Schottky contact is formed when contacting with Au in the a direction and Sc in both directions, while a lateral p-type Schottky contact is formed with Au in the b direction, and Cu, Ni, Ag, Pt, and Pd in both directions as a result of strong Fermi level pinning (FLP). The obtained FLP factor is 0.15 in the a direction and 0.09 in the b direction. Remarkably, a highly desirable lateral p-type Ohmic contact is formed with graphene in both directions. This investigation gives insight into the interfacial properties and guidance in electrode selection for ML tellurene devices.

High-performance sub-10-nm monolayer black phosphorene tunneling transistors

Moore’s law is approaching its physical limit. Tunneling field-effect transistors (TFETs) based on 2D materials provide a possible scheme to extend Moore’s lawdown to the sub-10-nm region owing to the electrostatic integrity and absence of dangling bonds in 2D materials. We report an ab initio quantum transport study on the device performance of monolayer (ML) black phosphorene (BP)TFETs in the sub-10-nm scale (6–10 nm). Under the optimal schemes, the ML BP TFETs show excellent device performance along the armchair transport direction.The on-state current, delay time, and power dissipation of the optimal sub-10-nm ML BP TFETs significantly surpass the latest International Technology Roadmap for Semiconductors (ITRS) requirements for high-performance devices. The subthreshold swings are 56–100 mV/dec, which are much lower than those of their Schottky barrier and metal oxide semiconductor field-effect transistor counterparts.

Many-Body Effect and Device Performance Limit of Monolayer InSe

Due to a higher environmental stability than few-layer black phosphorus and a higher carrier mobility than few-layer dichalcogenides, two-dimensional (2D) semiconductor InSe has become quite a promising channel material for the next-generation field-effect transistors (FETs). Here, we provide the investigation of the many-body effect and transistor performance scaling of monolayer (ML) InSe based on ab initio GW-Bethe-Salpeter equation approaches and quantum transport simulations, respectively. The fundamental band gap of ML InSe is indirect and 2.60 eV. The optical band gap of ML InSe is 2.50 eV for the in-plane polarized light, with the corresponding exciton binding energy of 0.58 eV. The ML InSe metal oxide semiconductor FETs (MOSFETs) show excellent performances with reduced short-channel effects. The on-current, delay time, and dynamic power indicator of the optimized n- and p-type ML InSe MOSFETs can satisfy the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors 2013 both down to 3-5 nm gate length in the ballistic limit. Therefore, a new avenue is opened to continue Moores law down to 3 nm by utilizing 2D InSe.

Gate-tunable interfacial properties of in-plane ML MX2 1T′–2H heterojunctions

The two-dimensional (2D) transition-metal dichalcogenides (TMDs) field effect transistor (FET) with in-plane heterojunction contacts between the semiconducting 2H phase (as channel) and the metallic 1T or semi-metallic 1T′ phase (as electrode) has received much recent attention because it has significantly reduced contact resistance and enhanced gate tunability and thus improved device performance. However, the underlying mechanism of its good conductivity remains open. We systematically explore for the first time the contact properties of the monolayer (ML) 2H MX2 (MoS2, WS2, MoSe2, WSe2, MoTe2) FET with their 1T′ phase as electrode by using ab initio quantum transport simulations. We find that the metal induced gap states (MIGS) at the interface penetrate the Schottky barrier and bridge the electrodes and the conduction/valence band of the channel, thereby forming a lower and tunable effective Schottky barrier height (ESBH) and causing an equivalent Ohmic contact under the appropriate gate voltage. Our study provides a new insight into the observed reduced contact resistance with the 1T′ phase as electrode and is instructive for further experiment.

Correction: Gate-tunable interfacial properties of in-plane ML MX2 1T′–2H heterojunctions

Correction for ‘Gate-tunable interfacial properties of in-plane ML MX2 1T′–2H heterojunctions’ by Shiqi Liu et al., J. Mater. Chem. C, 2018, DOI: 10.1039/c8tc01106k.