Work Function Modulation Research Articles

Role of Oxygen in the Work Function Modification at Various Stages of Chemically Synthesized Graphene

Correlation between the work function and oxygen content at various stages during synthesis of graphene by chemical route is presented. The values of work function in terms of contact surface potential difference (CPD) were estimated using a scanning Kelvin probe method. Oxygen content was determined using energy dispersive X-ray absorption (EDAX), and the data was further supported using the Rutherford back scattering method in oxygen resonance mode. Results indicated a linear correlation between the oxygen content and measured values of CPD at various stages during the chemical synthesis of graphene.

Modulation of the effective work function of TiN metal gate for PMOS application

It is important to find a way to modulate the work function of TiN metal gate towards the valence band edge of Si, which can meet the lower threshold voltage requirement of p-type metal—oxide—semiconductor (MOS) transistor. In this work, effects of TiN thickness, post-deposition annealing (PDA), oxygen incorporation and N concentration variation on the work function of TiN metal gate in MOS structures are systematically investigated. It can be found that the work function positively shifts at the initial stage as the thickness of the TiN layer increases and stabilizes at such a thickness. PDA at N2 ambience with a trace of O2 can also cause a positive shift in the work function of TiN metal gate. The same tendency can be observed when oxygen is incorporated into TiN. Finally, increasing the N concentration in TiN can also positively shift the work function. All these measures are effective in modulating the TiN metal gate so that it is more suitable for PMOS application.

Role of point defects and HfO2/TiN interface stoichiometry on effective work function modulation in ultra-scaled complementary metal–oxide–semiconductor devices

We investigate the physical properties of a portion of the gate stack of an ultra-scaled complementary metal-oxide-semiconductor (CMOS) device. The effects of point defects, such as oxygen vacancy, oxygen, and aluminum interstitials at the HfO2/TiN interface, on the effective work function of TiN are explored using density functional theory. We compute the diffusion barriers of such point defects in the bulk TiN and across the HfO2/TiN interface. Diffusion of these point defects across the HfO2/TiN interface occurs during the device integration process. This results in variation of the effective work function and hence in the threshold voltage variation in the devices. Further, we simulate the effects of varying the HfO2/TiN interface stoichiometry on the effective work function modulation in these extremely-scaled CMOS devices. Our results show that the interface rich in nitrogen gives higher effective work function, whereas the interface rich in titanium gives lower effective work function, compared to a stoichiometric HfO2/TiN interface. This theoretical prediction is confirmed by the experiment, demonstrating over 700 meV modulation in the effective work function.

Work‐Function Modification of Au and Ag Surfaces upon Deposition of Self‐Assembled Monolayers: Influence of the Choice of the Theoretical Approach and the Thiol Decomposition Scheme

We have characterized theoretically the work-function modifications of the (111) surfaces of gold and silver upon deposition of self-assembled monolayers based on methanethiol and trifluoromethanethiol. A comparative analysis is made between the experimental results and those obtained from two widely used approaches based on density functional theory. The contributions to the total work-function modifications are estimated on the basis of two decomposition schemes of the thiol molecules that have been proposed in the literature. The contributions are found to differ significantly between the two approaches, as do the corresponding adsorption energies.

Work function modulation of bilayer MoS2 nanoflake by backgate electric field effect

We report the work function modulation of bilayer MoS2 nanoflake induced by backgate electric filed. The Fermi level of bilayer MoS2 increased by 115 meV with the backgate bias larger than the threshold voltage. Carrier doping induced by the electric filed was responsible for the variation of work function of MoS2 nanoflake. Meanwhile, asymmetric contact potential drops at electrical contacts were observed, which was consistent with the asymmetric electrical output characteristics. Thermonic field emission theory was used to explain the carrier transport mechanism between Pt and bilayer MoS2, and the larger contact barriers led to the lower carrier mobility.

Modulation of the effective work function of a TiN metal gate for NMOS requisition with Al incorporation

The effect of Al incorporation on the effective work function (EWF) of TiN metal gate was systematically investigated. Metal—oxide—semiconductor (MOS) capacitors with W/TiN/Al/TiN gate stacks were used to fulfill this purpose. Different thickness ratios of Al to TiN and different post metal annealing (PMA) conditions were employed. Significant shift of work function towards to Si conduction band was observed, which was suitable for NMOS and the magnitude of shift depends on the processing conditions.

Electronic-Structure Theory of Organic Semiconductors: Charge-Transport Parameters and Metal/Organic Interfaces

We focus this review on the theoretical description, at the density functional theory level, of two key processes that are common to electronic devices based on organic semiconductors (such as organic light-emitting diodes, field-effect transistors, and solar cells), namely charge transport and charge injection from electrodes. By using representative examples of current interest, our main goal is to introduce some of the reliable theoretical methodologies that can best depict these processes. We first discuss the evaluation of the microscopic parameters that determine charge-carrier transport in organic molecular crystals, i.e., electronic couplings and electron-vibration couplings. We then examine the electronic structure at interfaces between an organic layer and a metal or conducting oxide electrode, with an emphasis on the work-function modifications induced by the organic layer and on the interfacial energy-level alignments.

Work-function modification of the (1 1 1) gold surface upon deposition of self-assembled monolayers based on alkanethiol derivatives

The modification in the work-function of the (111) gold surface upon deposition of self-assembled monolayers (SAMs) based on alkanethiol derivatives has been characterized at the Density Functional Theory level when introducing chemical derivatizations. Chemical derivatizations have been introduced by: (i) replacing thiol as anchoring group by selenol or isocyanide functionalities; and (ii) introducing terminal electroactive substituents varying by their electron donor versus acceptor character. Changes in the anchoring group have shown that substitution of S by Se has no significant impact on the total work-function shift. Consideration of various terminal substituents has also pointed to the key role played by internal restructuration of the SAM geometry on the work-function. These results contribute in establishing guidelines toward a fine control of interface engineering in opto-electronic devices.

Energy Level Alignment and Charge Carrier Mobility in Noncovalently Functionalized Graphene

Density functional theory calculations have been performed to assess the electronic structure of graphene overlaid with a monolayer of electro-active conjugated molecules, being either electron donors or electron acceptors. Such a noncovalent functionalization results in a work function modification that scales with the amount of electron transfer from or to graphene, in line with the formation of an interfacial dipole. The charge transfer is accompanied by a pinning of the donor HOMO/acceptor LUMO around the Fermi level and a shift in the vacuum level. The use of the Boltzmann transport equation combined with the deformation potential theory shows that large charge carrier mobilities are maintained upon noncovalent functionalization of graphene, thereby suggesting that molecular doping is an attractive approach to design conductive graphene electrodes with tailored work function.

The formation and stability of Rh nanostructures on TiO2(1 1 0) surface and TiOx encapsulation layers

Rh overlayers formed on slightly oxygen deficient TiO2(110) single crystals, as well as on ultrathin encapsulation titania layers prepared on Rh multilayers were characterized by AES, LEIS, XPS, TPD and work function (WF) measurements. Rh deposition on TiO2(110) below 0.1 monolayer (ML) Rh coverage led to electron transfer from the metal toward the TiO2(110) surface. Annealing of Rh multilayers up to 950K in UHV resulted in the surface diffusion of titanium and oxygen ions into a TiOx encapsulation layer of definite stoichiometry and a thickness of a few atomic layers. The accompanying 0.3–0.6eV WF enhancement at ΘRh=2–6ML can be attributed to the smoothing of the Rh overlayer and the formation of a continuous TiOx dipole layer consisting of positively charged titanium ions at the metal–oxide interface and negative oxygen ions in the topmost layer. De-encapsulation of Rh particles was observed on a TiO2 sample less reduced in its bulk, revealing the roles of bulk and surface substrate stoichiometry on the decoration process. Increasing the thickness of Rh multilayers supported on the TiO2(110) single crystal hampered ion diffusion and consequently, it led to an increase in the temperature characteristic of the completion of the encapsulation. Deposition of additional Rh on the TiOx encapsulation layers covering Rh multilayers resulted in the growth of Rh particles having a similar height up to 1ML. LEIS data indicated that the decoration of the second metal layer by titania was hindered. It occurred at a temperature more than 100K higher than that characteristic of the TiO2(110) surface at the same Rh coverage. Aspects of the produced structures in relation to the formation of protective oxide layers, modification of surface work function, catalysis and metal–insulator–metal (MIM) devices are considered.

Understanding Ge impact on VT and VFB in Si1−xGex/Si pMOSFETs

The Ge impact on the threshold voltage (VT) and the flat band voltage (VFB) of SiGe pMOSFETs is evaluated through a comparison of experiments and simulations with various SiGe thicknesses and Ge contents. Increasing Ge content shifts linearly (VT–VFB) in full agreement with Poisson–Schrodinger simulation results accounting for adequate band discontinuity and strain. An additional VFB shift is needed to match the measured CV in accumulation, revealing a work function modification or the presence of dipole in the gate stack due to Ge diffusion.

Tunnel-field-effect-transistor based gas-sensor: Introducing gas detection with a quantum-mechanical transducer

A gas-sensor based on tunnel-field-effect-transistor (TFET) is proposed that leverages the unique current injection mechanism in the form of quantum-mechanical band-to-band tunneling to achieve substantially improved performance compared to conventional metal-oxide-semiconductor field-effect-transistors (MOSFETs) for detection of gas species under ambient conditions. While nonlocal phonon-assisted tunneling model is used for detailed device simulations, in order to provide better physical insights, analytical formula for sensitivity is derived for both metal as well as organic conducting polymer based sensing elements. Analytical derivations are also presented for capturing the effects of temperature on sensor performance. Combining the developed analytical and numerical models, intricate properties of the sensor such as gate bias dependence of sensitivity, relationship between the required work-function modulation and subthreshold swing, counter-intuitive increase in threshold voltage for MOSFETs and reduction in tunneling probability for TFETs with temperature are explained. It is shown that TFET gas-sensors can not only lead to more than 10 000× increase in sensitivity but also provide design flexibility and immunity against screening of work-function modulation through non-specific gases as well as ensure stable operation under temperature variations.

Effects of contact treatments on solution-processed n-type dicyano-ovalenediimide and its complementary circuits

The device performance of bottom-contact organic field-effect transistors (OFETs) was limited by contact resistance which originates from a non-optimal semiconductor growth morphology and limited charge injection area. Self-assembled monolayer (SAM) treatment has proved to be an effective approach to optimize thin film growth and align the energy levels. This SAM-induced work-function modification has different effects on the charge injection barrier as well as on the device performance in terms of threshold voltage. These results were analysed in detail from the perspectives of surface morphology and energy alignment at the Au–semiconductor interface in n-type ovalenediimide (ODI-CN)-based OFETs. Finally, complementary inverters were built using pentacene (p-type) and ODI-CN (n-type) OFETs, and these showed a voltage gain of 13.

Enhanced performance in organic photovoltaic devices with a KMnO4 solution treated indium tin oxide anode modification

The properties of poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic photovoltaic devices (OPVs) with an indium tin oxide (ITO) anode treated by a KMnO4 solution are investigated. The optimized KMnO4 solution has a concentration of 50 mg/L, and ITO is treated for 15 min. The modification of ITO anode results in an enhancement of the power conversion efficiency (PCE) of the device, which is responsible for the increase of the photocurrent. The performance enhancement is attributed to the work function modification of the ITO substrate through the strong oxygenation of KMnO4, and then the charge collection efficiency is improved.

A novel spin modulation of work function for C adsorbed Cr/Fe(001) metal gate

Work functions and magnetic moments of C adsorbed Cr/Fe(001) surfaces with different C coverages θ and magnetic alignments (parallel or antiparallel) between Cr and Fe atom moments are investigated using first-principles methods based on density functional theory. The calculated results reveal that the spin configuration plays a significant role in determining the work function of the systems. The work functions of the systems with parallel states are evidently larger than those with antiparallel states. Moreover, for θ≤0.5 ML, with increasing value of θ, the work function increases from 4.23 eV to 5.13 eV for antiparallel states and from 4.47 eV to 5.44 eV for parallel states. While for θ>0.5 ML, the work function decreases with increasing value of θ. It can be also found that, for θ≤0.5 ML, the smaller the Cr and Fe magnetic moments are, the lower the Fermi energy EF is and the larger the work functions of the systems are. Based on analysis and discussion, we conclude that the changes of the work functions and magnetic moments are mainly determined by those of Fermi level and density of state (DOS) induced by the spin polarization, the electron transfer and the surface structure. Our work strongly suggests that controlling the magnetic states is a promising way for modulating the work function of magnetic metal gate.

Investigation and Integration of Polycrystalline Silicon/TiN/SiON Gate Stack in Silicon on Thin Buried Oxide Complementary Metal Oxide Semiconductor Field Effect Transistors

We developed fully depleted silicon on thin buried oxide (SOTB) complementary metal oxide semiconductor field effect transistors (CMOSFETs) with a polycrystalline-silicon (poly-Si)/TiN/SiON gate stack. We investigated the flat-band voltage (Vfb) shift of the gate stack for the threshold voltage (Vth) symmetry of SOTB CMOSFETs. We found that the Vfb shift depended on both TiN thickness and thermal load. Thicker TiN above 15 nm is preferable for obtaining the midgap value of Vfb with considering the thermal budget of the SOTB process. We also integrated the gate stack into SOTB CMOSFETs, which showed that the Vth roll-off characteristics corresponded to the proper control of the effective work function by considering how the impurity-related work function modulation affects. Narrow channel characteristics of the TiN-gate SOTB CMOSFETs were also shown to be superior to fully silicided gate SOTB devices due to the less silicidation.

Blue fluorescent organic light emitting diodes with multilayered graphene anode

As an innovative anode for organic light emitting devices (OLEDs), we have investigated graphene films. Graphene has importance due to its huge potential in flexible OLED applications. In this work, graphene films have been catalytically grown and transferred to the glass substrate for OLED fabrications. We have successfully fabricated 2mm×2mm device area blue fluorescent OLEDs with graphene anodes which showed 2.1% of external quantum efficiency at 1000cd/m2. This is the highest value reported among fluorescent OLEDs using graphene anodes. Oxygen plasma treatment on graphene has been found to improve hole injections in low voltage regime, which has been interpreted as oxygen plasma induced work function modification. However, plasma treatment also increases the sheet resistance of graphene, limiting the maximum luminance. In summary, our works demonstrate the practical possibility of graphene as an anode material for OLEDs and suggest a processing route which can be applied to various graphene related devices.

The Role of Rare Earth Metals on Effective Work Function Modulation of Nickel Fully-Silicided Gate/High-k Dielectric Stacks for n-Channel Metal Oxide Semiconductor Device Applications

It was found that the structural properties with gadolinium (Gd) and europium (Eu) incorporation into nickel (Ni) fully silicided (FUSI) gate electrodes are markedly different and resulted in different degrees of effective work function modulation. It was found that Ni–Gd alloys tend to form stable compounds during silicidation and produced a Si-rich layer with amorphous/nanocystalline structure near the FUSI gate electrode/high-k dielectric interface. This compositional and structural change is the main mechanism responsible for effective work function modulation with Gd incorporation. However, in the case of Europium, Eu atoms tend to segregate outside the Ni-FUSI layer during silicidation and resulted in a uniform NixSiy layer with Eu pile-up layer at the FUSI gate electrode/high-k dielectric interface. This pile-up is believed to be the main cause of effective work function modulation with Eu incorporation. It was also found that the incorporation of Gd and Eu metals into Ni-FUSI gate can remotely scavenge the interfacial oxide layer resulting in lower equivalent oxide thickness (EOT) of the device.

The Role of Rare Earth Metals on Effective Work Function Modulation of Nickel Fully-Silicided Gate/High-kDielectric Stacks for n-Channel Metal Oxide Semiconductor Device Applications

The Role of Rare Earth Metals on Effective Work Function Modulation of Nickel Fully-Silicided Gate/High-<i>k</i>Dielectric Stacks for n-Channel Metal Oxide Semiconductor Device Applications

Impact of Thermally Induced Structural Changes on the Electrical Properties of TiN/HfLaSiO Gate Stacks

Thermally induced structural changes in TiN/Hf(La)SiO gate stacks were investigated by back-side X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). A distinct correlation between bottom oxide growth and an increase in equivalent oxide thickness (EOT) was confirmed under high-temperature annealing at over 850 °C regardless of La content. Back-side XPS also revealed that oxygen and nitrogen diffusion occurs, forming partially oxidized TiON layers at a metal/high-k interface under moderate annealing temperatures of approximately 600 °C, and that annealing at over 750 °C leads to the reduction of the oxide phase and produces a thinner inter-layer with a clear Ti–N bond feature. Moreover, with an increase in annealing temperature, a change in the local atomic configuration in the HfLaSiO dielectric layer was identified from oxygen K-edge spectra. This structural change induced by thermal reaction can be considered as a possible cause of the Vth instability of La-incorporated high-k gate stacks. On the basis of these findings on structural changes, the physical origins of the effective work function modulation of the gate stacks are discussed in detail.