Li-Ping Feng

Effect of vacancies in monolayer MoS2 on electronic properties of Mo–MoS2 contacts

Revealing the influence of intrinsic defects in monolayer MoS2 on the electronic nature of metal–MoS2 contacts is particularly critical for their practical use as nanoelectronic devices. This work presents a systematic study toward electronic properties of Mo metal contacts to monolayer MoS2 with vacancies by using first-principles calculations based on density functional theory. Upon Mo- and S-vacancy formation in monolayer MoS2, both the height and the width of the tunnel barrier between Mo metal and monolayer MoS2 are decreased. Additionally, the Schottky barrier of 0.1 eV for the perfect Mo–MoS2 top contact is reduced to zero for defective ones. The partial density of states near the Fermi level of defective Mo–MoS2 top contacts is strengthened and electron densities at the interface of defective Mo–MoS2 top contacts are increased compared with those of the perfect one, suggesting that Mo- and S-vacancies in monolayer MoS2 have the possibility to improve the electron injection efficiency. Mo-vacancies in monolayer MoS2 are beneficial to get high quality p-type Mo–MoS2 contacts, whereas S-vacancies in monolayer MoS2 are favorable to achieve high quality n-type Mo–MoS2 contacts. Our findings provide important insights into future design and fabrication of nanoelectronic devices with monolayer MoS2.

Structural and electrical properties of thin SrHfON films for high-k gate dielectric

Thin SrHfON films were prepared by reactive cosputtering of Hf–O and Sr–O targets in Ar/N2 ambient environment. Structural and electrical properties of the as-deposited and annealed SrHfON films used as gate dielectrics have been investigated. The SrHfON films have crystallization temperature higher than 900 °C. After annealing at 900 °C, high dielectric constant of 19.3 and effective work function of 4.13 eV was obtained for the SrHfON films. It is worth mentioning that the leakage current density of Au/SrHfON/IL SiOx gate stack is two orders of magnitude lower than that of polycrystalline silicon/HfO2 structure.

Tuning the electronic properties of bondings in monolayer MoS2 through (Au, O) co-doping

Improving the electronic properties of Au–S bonding is the key to tuning the carrier transport of monolayer MoS2-based nanodevices. Herein, we systematically investigate the electronic properties for Au-doped, O-doped, and (Au, O) co-doped monolayer MoS2 to analysis the electronic properties of Au–S bondings using first-principles density functional calculations. Three gap states induced by Au–S bondings are observed at the band gap in Au-doped and (Au, O) co-doped monolayer MoS2, which are n-type semiconductors. Moreover, the n-type barriers between the Fermi level of Au-doped and (Au, O) co-doped systems and the CBM of un-doped monolayer MoS2 are 0.84 and 0.65 eV, respectively. In addition, low electron density and electron density difference are observed for Au–S bondings in Au-doped monolayer MoS2, suggesting weak covalent Au–S bondings with high resistance; this explains the observed low carrier mobility of monolayer MoS2 devices with an Au electrode. Upon introducing elemental O into Au-doped monolayer MoS2, electron density and electron density difference of Au–S bondings in (Au, O) co-doped monolayer MoS2 are increased to 0.58 and 0.15 eV A−3, respectively, showing that the covalent Au–S bondings are strengthened, and their resistance and electron injection efficiency are further improved by the elemental O dopant. Our findings may provide an effective way to improve the electronic properties of Au–S bondings in monolayer MoS2-based nanodevices with an Au electrode.

Role of vacancies in tuning the electronic properties of Au-MoS2 contact

Understanding the electronic properties between molybdenum disulfide (MoS2) and metal electrodes is vital for the designing and realization of nanoelectronic devices. In this work, influence of intrinsic vacancies in monolayer MoS2 on the electronic structure and electron properties of Au-MoS2 contacts is investigated using first-principles calculations. Upon formation of vacancies in monolayer MoS2, both tunnel barriers and Schottky Barriers between metal Au and monolayer MoS2 are decreased. Perfect Au-MoS2 top contact exhibits physisorption interface with rectifying character, whereas Au-MoS2 contact with Mo-vacancy shows chemisorption interface with Ohmic character. Partial density of states and electron density of defective Au-MoS2 top contacts are much higher than those of perfect one, indicating the lower contact resistance and higher electron injection efficiency of defective Au-MoS2 top contacts. Notably, Mo-vacancy in monolayer MoS2 is beneficial to get high quality p-type Au-MoS2 top contact, wh...

Density functional theory study of 3R– and 2H–CuAlO2 under pressure

We present a first-principles density-functional theory based study of the impact of pressure on the structural and elastic properties of bulk 3R– and 2H–CuAlO2. The ground state properties of 3R– and 2H–CuAlO2 are obtained, which are in good agreement with previous experimental and theoretical data. The analysis of enthalpy variation with pressure indicates the phase transition pressure between 3R and 2H is 15.4 GPa. The independent elastic constants of 3R– and 2H–CuAlO2 are calculated. As the applied pressure increases, the calculations show the presences of mechanical instability at 26.2 and 27.8 GPa for 3R– and 2H–CuAlO2, which are possibly related with the phase transitions.

Current conduction mechanisms in HfO2 and SrHfON thin films prepared by magnetron sputtering

Metal–oxide–semiconductor (MOS) capacitors incorporating HfO2 and SrHfON gate dielectrics were fabricated by magnetron sputtering. The interface quality, thermal stability, and electrical properties of the MOS capacitors have been investigated. Compared to HfO2 dielectric film, SrHfON dielectric film has thin interface layer with Si substrate, good thermal stability, and low leakage current densities. The dominant current conduction mechanisms (CCMs) of HfO2 film are Schottky emission or Poole–Frenkel emission at low and high electric fields. The main CCMs of SrHfON film are Schottky emission or Poole–Frenkel emission at low electric field, whereas, the CCMs are replaced by space charge limited current at high electric field.

Designing high performance metal–mMoS2 interfaces by two-dimensional insertions with suitable thickness

Thickness has been proved to have significant influence on the physical properties of two-dimensional (2D) materials and their corresponding devices. Understanding the effect of the thickness of 2D insertions on the contact properties of metal-monolayer MoS2 interfaces (viz. metal-mMoS2 interfaces) is vital to designing high performance mMoS2 devices. In this work, the electronic structures, Schottky barriers, contact resistance, and tunneling barriers of scandium-mMoS2 (Sc-mMoS2) interfaces with BN and graphene insertions have been comparatively studied by density functional theory. No Schottky barriers are found at Sc-mMoS2 interfaces with monolayer 2D insertions. Although the contact resistance and charge injection efficiency of Sc-mMoS2 interfaces with monolayer insertions deteriorate relatively to those of the Sc-mMoS2 interface, they are still sufficient to realize high-performance mMoS2-based devices. Note that, upon increasing the number of layers of 2D insertions, these contact properties are further deteriorated with the increasing number of layers of insertions. Moreover, additional significant Schottky barriers are introduced into Sc-mMoS2 interfaces with multilayer BN; the nature Dirac points of graphene insertions are opened, suggesting low performances of Sc-mMoS2 interfaces with multilayer BN and graphene insertions. These variations can be understood on the basis of the orbital hybridization and charge redistribution between the Sc slab and mMoS2 layer. In addition, these characteristics are expected to occur in other metal-mMoS2 interfaces with two-dimensional material insertions. Overall, monolayer rather than multilayer two-dimensional insertions can be used to improve the transport properties of mMoS2-based devices.

The dielectric properties enhancement due to Yb incorporation into HfO2

The effects of Yb concentration and annealing temperature on the dielectric constant change of Yb-doped HfO2 thin film were investigated. The results show that the dielectric constant of Yb-doped HfO2 increased when doping with Yb after annealing. Compared with the undoped HfO2, the dielectric constant enhancement may result from the crystallographic change from monoclinic phase to the cubic phase. The Yb-doped HfO2 exhibited a lower leakage current than that of undoped HfO2 thin film. The electrical characteristics of Yb-doped HfO2 thin film illustrated that it is a promising gate dielectric layer for future high dielectric constant (high-k) gate dielectric applications.

Heterostructure consists of monolayer MoS2 and arsenene with novel electronic and optical conductivity

Two-dimensional (2D) materials receive a lot of attention due to their novel properties and wide applications. Here, we study the electronic structure and optical conductivity of new heterostructures made of two monolayers of molybdenum disulfide (MoS2) and arsenene (As). Our first-principles density functional calculations show that only the valence band of As–MoS2 heterostructures is significantly affected by the interlayer distance. Upon shortening the interlayer distance, the valence-band maximum transfer from the Γ point to Λ and Σ point, and the holes effective masses decline monotonically and lower than the electrons effective masses. Tuning the interlayer distance may change the conduction character of As–MoS2 heterostructures. In addition, the valence-band maximum and conduction-band minimum of As–MoS2 heterostructures are localized on opposite monolayers, suppressing the hole–electron recombination. Unlike transition metal dichalcogenides–transition metal dichalcogenides heterostructures, the optical conductivity of As–MoS2 heterostructure is different to its monolayers, and a new optical conductivity peak is observed in visible light region. Moreover, its intensity and position are enhanced and shifted-red with the shortening interlayer distance, respectively, due to the enlarging Fermi level with the decreasing interlayer distance. Our findings suggest that engineering As–MoS2 heterostructure is beneficial for improve the application of monolayer MoS2 in the photoelectric device.

Influence of cerium-doping on the structural and electrical properties of hafnium oxide gate dielectric

The undoped and cerium-doped hafnium oxide (HfO2) thin films have been deposited on p-type single crystal Si(100) substrates using radio frequency magnetron sputtering method. The structure and electrical properties have been investigated as a function of doping concentration. The results show that cerium serves effectively as a dopant to induce the crystallographic change from the monoclinic to the cubic phase. The ceium-doped HfO2 shows higher dielectric constant than undoped HfO2. The dielectric constant enhancement can be explained by the shrinkage of molar volume and the increase of molar polarizability. Compared with undoped HfO2, the cerium-doped HfO2 exhibits a lower leakage current due to the decrease of the oxygen vacancies number.