Effective Work Function Values Research Articles

Prediction of Thermionic Energy Conversion Performance and Parametric Effects Using Genetic Algorithms to Fit Physics-Inspired Model Equations to Prototype Test Data

Abstract Thermionic converters have potential as an energy conversion technology for high-temperature space and terrestrial applications using concentrated solar, nuclear reaction, and combustion processes as the heat source. Recent studies have generated experimental performance data for narrow-gap thermionic energy conversion devices. This investigation explores the use of genetic algorithms to fit existing data with physics-inspired model equations. The resulting model equations can be used for performance prediction for system design optimization or to explore parametric effects on performance. The model equations incorporate Richardson’s law for current density, including both the saturated and Boltzmann regimes, with appropriate relations for power delivered to the external load. The transition regime is characterized using two separate models, each accounting for nonuniformity in emission surfaces and irregularities in the manufacturing process. The trained models enable performance prediction of small-gap thermionic energy conversion devices. In this study, data were fitted for two different prototype designs. The prototype test data and postulated values for the work functions and a transition regime parameter are substituted into physics-inspired model equations, yielding performance models with three adjustable constants. Optimized values of these constants are determined using a genetic algorithm to best fit the experimentally determined performance data for prototype thermionic conversion devices tested in earlier studies. This approach is demonstrated to fit the performance data to within 9%. This methodology also allows the user to back-infer the effective work function values, which were found in this study to be consistent with independent measurement.

Investigation of dual work function metal (DWFM) gate stacks with ALD TaAlN and TaAlC for multi threshold voltages (VTHs) engineering in MOS device integration

This study investigates methods to modulate the effective work function (EWF) and control the multiple threshold voltages (VTHs) of metal-oxide-semiconductor (MOS) devices by applying metal electrodes with different work function materials (WFMs), including a single layer of n-type (TaAlC) and p-type (TaAlN) as well as their stacked structures, through atomic layer deposition (ALD). Dual-Work-Function-Metal (DWFM) gate is a crucial aspect of confirming the feasibility of metal gate stacks in MOS devices for a complementary metal-oxide-semiconductor (CMOS) integration scheme. The gate stack structure of p-Si/IL (Inter layer)/HfO2/DWFM (p/n-type compatible metal stack)/W exhibits good adhesion and even chemical composition distribution, as evidenced by transmission electron microscope (TEM) and energy dispersive spectrometer (EDS) analysis. By fabricating MOS capacitors using this WFM structure and extracting EWF from the measured capacitance-voltage (C–V) characteristics, EWF values of 4.62 eV and 4.99 eV were identified both before and after annealing using forming gas, indicating pWFM-like properties. These properties are attributed to the strengthened Al–O bonds and reduced CO peaks seen in x-ray photoelectron spectroscopy (XPS) analysis. The use of different Ta precursor from Tris(diethylamido)(tert-butylimido)tantalum(V) (TBTDET) as well as TaCl5 resulted in the increase of the EWF following the unique p-WFM like feature. As candidates for WFM electrode in metal-oxide-semiconductor field effect transistor (MOSFET) with the DWFMs (n/p stacks) were evaluated, resulting in a VTH of 0.84 V that places in the middle of the VTH shift range from 0.6 V (nWFM) to 1.16 V (pWFM), approximately. The DWFMs allow for modulation of the EWF, which in turn reduces ambiguity in addressing replacement metal gate (RMG) challenges and opens up new avenues for achieving multi-VTH solutions in CMOS integration schemes.

Impact of chemical bonding difference of ALD Mo on SiO2 and Al2O3 on the effective work function of the two gate stacks

This study investigates molybdenum deposited by atomic layer deposition (ALD) as a potential gate metallization for flash memory devices. Polycrystalline (110)-oriented, with low-resistivity (∼16 μΩ cm) ALD Mo films were deposited on SiO2 and Al2O3 using hydrogen reduction of Mo-oxychloride precursor. On SiO2, an effective work function (EWF) of 4.75 ± 0.1 eV was obtained for as-deposited samples, and its value increased up to 4.9 ± 0.05 eV upon annealing at 600 °C, whereas on Al2O3, a stable EWF value of 5.05 ± 0.05 eV was observed. The EWF variation is correlated with changes in the composition and chemical bonding at the metal/dielectric interface. The latter were investigated by energy dispersive x-ray spectroscopy and electron energy loss spectroscopy performed using scanning transmission electron microscopy and x-ray photoelectron spectroscopy. This analysis revealed that the presence of Mo oxide at the Al2O3/Mo interface stabilizes the EWF, and the EWF increase on SiO2 is attributed to Si enrichment at the SiO2/Mo interface upon annealing. A theoretical model is suggested to explain the chemical bonding difference on SiO2 and Al2O3, based on the Mo-precursor reactions with the surface groups of the dielectric. This study emphasizes the importance of the precursor/substrate reactions in determining the compositional and, therefore, electrical properties of the metal/dielectric interface, and demonstrates that ALD Mo deposited directly on SiO2 and Al2O3 is a promising candidate for gate metallization of flash devices due to its high EWF.

Chemically induced Fermi level pinning effects of high-k dielectrics on graphene

High-k materials such as Al2O3 and HfO2 are widely used as gate dielectrics in graphene devices. However, the effective work function values of metal gate in graphene FET are significantly deviated from their vacuum work function, which is similar to the Fermi level pinning effect observed in silicon MOSFETs with high-k dielectric. The degree of deviation represented by a pinning factor was much worse with HfO2 (pinning factor (S) = 0.19) than with Al2O3 (S = 0.69). We propose that the significant pinning-like behaviors induced by HfO2 are correlated with the oxygen exchange reactions occurred at the interface of graphene and HfO2.

Thermal stability of atomic layer deposited WCxNy electrodes for metal oxide semiconductor devices

This study is a thorough investigation of the chemical, structural, and electrical stability of W based organo-metallic films, grown by atomic layer deposition, for future use as gate electrodes in advanced metal oxide semiconductor structures. In an earlier work, we have shown that high effective work-function (4.7 eV) was produced by nitrogen enriched films (WCxNy) dominated by W-N chemical bonding, and low effective work-function (4.2 eV) was produced by hydrogen plasma resulting in WCx films dominated by W-C chemical bonding. In the current work, we observe, using x-ray diffraction analysis, phase transformation of the tungsten carbide and tungsten nitride phases after 900 °C annealing to the cubic tungsten phase. Nitrogen diffusion is also observed and is analyzed with time-of-flight secondary ion mass spectroscopy. After this 900 °C anneal, WCxNy effective work function tunability is lost and effective work-function values of 4.7–4.8 eV are measured, similar to stable effective work function values measured for PVD TiN up to 900 °C anneal. All the observed changes after annealing are discussed and correlated to the observed change in the effective work function.

Integration of Advanced MOSFET Device with Dual Effective Band Edge Work Function Metals Using Both HK and MG Last Scheme

The aggressive downscaling of CMOS requires both lateral and vertical shrinkage of dimensions. SiO2 served as gate dielectrics for more than four decades now is with severe leakage problem with quantum mechanical tunneling effect due to SiO2has been scaled down to only a few atomic layers to boost device performance. Thus high dielectric constant materials such as Hafnium oxide and Hafnium silicate are of great interests in recent years [1]-[3]. Meanwhile HK layers are not perfectly compatible with silicon channel surface as traditional silicon dioxide due to interface trap, dipole and defect are formed during processing which would be difficult for threshold voltage tuning. Gate stacks engineering like interfacial layer, post dielectric anneal and metal gate are needed to be carefully handled to alleviate these side effects. Due to excellent EOT scaling and short channel effect controllability, HKMG has been implemented into modern bulk silicon device to replace traditional Poly/SiON process. Different from Poly/SiON process which poly gate can be doped with N type and P type dopants respectively for NFET and PFET threshold voltage adjustment, Metal gate last process integration needs two kinds of metal to tune MOSFET work function in order to achieve suitable device performance [4, 5]. In this work, we have successfully fabricated devices with dual effective work function metals with 4.15eV and 5.04eV for NFET and PFET respectively. With EWF close to band edge, threshold voltage can be effectively achieved with 0.35V and -0.31V for NFET and PFET respectively. Process conditions and electrical results are also discussed.TiN served as PMOS work function metal and TiAl chosen as NMOS work function metal are integrated into CMOS. The detail integration flow of HKMG and TEM of cross section of gate structure are shown in Figure.1.In Fig. 2, threshold voltages with different Al% concentration are plotted. It can be shown that threshold voltage is strongly dependent on Al atomic concentration. With more percentage of concentration of Al, Vt would be lower due to work function Fermi level close to conduction band edge. Table.1 shows the effective work function values for NFET and PFET respectively. The EWFs are both close to band edges which are suitable for Vt tuning for devices. Fig.3 shows the NFET and PFET Id-Vg curves with low DIBL and sub-threshold swing performance. DIBL can be as low as 78mV/V and 82mV/V for NFET and PFET respectively. Swing can be as low as 62mV/dec and 64mV/dec for NFET and PFET respectively. It effectively demonstrates that controllability for metal gate to channel is much better comparing with Poly/SiON gate stackIn conclusion, we have successfully fabricated NFET and PFET devices with both HK and MG last integration scheme. By optimizing process conditions, Both NFET and PFET effective work functions can be tuned to be close to band edges with TiAl as NFET work function metal and TiN as PFET work function metal. Al concentration impaction on Vt is analyzed. Vt are well controlled with different channel length. DIBL and Swing are as low as to 78mV/V and 82mV/V and 62mV/dec and 64mV/dec for NFET and PFET respectively.

Metal gate work function tuning by Al incorporation in TiN

Titanium nitride (TiN) films have been used as gate electrode on metal-oxide-semiconductor (MOS) devices. TiN effective work function (EWF) values have been often reported as suitable for pMOS. For nMOS devices, a gate electrode with sufficient low EWF value with a similar robustness as TiN is a challenge. Thus, in this work, aluminum (Al) is incorporated into the TiN layer to reduce the EWF values, which allows the use of this electrode in nMOS devices. Titanium aluminum (TiAl), Al, and aluminum nitride (AlN) layers were introduced between the high-k (HfO2) dielectric and TiN electrode as Al diffusion sources. Pt/TiN (with Al diffusion) and Pt/TiN/TiAl/TiN structures were obtained and TiN EWF values were reduced of 0.37 eV and 1.09 eV, respectively. The study of TiN/AlN/HfO2/SiO2/Si/Al structures demonstrated that AlN layer can be used as an alternative film for TiN EWF tuning. A decrease of 0.26 eV and 0.45 eV on TiN EWF values were extracted from AlN/TiN stack and AlN/TiN laminate stack, respectively. AlN/TiN laminate structures have been shown to be more effective to reduce the TiN work function than just increasing the AlN thickness.

Composition and crystallography dependence of the work function: Experiment and calculations of Pt-Al alloys

The work function (WF) of several phases of Pt-Al alloys is investigated. A first-principles study is performed to determine the dependence of the vacuum WF (VWF) on the phase, crystallographic orientation, and atomic termination. In parallel, the effective WF (EWF) of these alloys is experimentally measured using metal-oxide semiconductor devices. A detailed microstructural characterization based on x-ray diffraction and transmission electron microscopy allows the comparison between experiment and calculations. It is found that despite the formation of a complex microstructure, the calculated VWF values are in good agreement with the experimental EWF values. The possible VWF range for each phase has a relatively small (\ensuremath{\sim}0.5 eV) span for different orientations and atomic terminations, and it is found that Pt atoms in the terminating plane are more dominant than Al atoms. The results demonstrate that first-principles calculations of the WF of alloys can be used to successfully predict the experimental WF and that knowledge of the microstructure is important for the accuracy of these calculations.

Effective Metal Work Function of Pt Gate Electrode in Ge Metal Oxide Semiconductor Device

We fabricated Ge and Si metal oxide semiconductor devices with gate stacks and investigated their structural and electrical properties. Postmetallization annealing in ambient reduced the accumulation capacitance more significantly in Si devices than in Ge devices due to the increase in the thickness of a low- interfacial layer in between the film and Si substrate. Ge devices exhibited lower effective work function values for a Pt gate electrode than Si devices owing to the presence of a large number of positively charged dipoles caused by a strong Fermi-level pinning at the Ge surface.

Impact of Oxygen on Work Function of Ru Oxide Metal Gate

A mechanism of effective work function change of the Ru oxide metal gate under various anneal conditions has been studied. High temperature annealing was found to result in changing the tetragonal phase of RuO 2 to the hexagonal phase characteristic for Ru metal. By extracting the electrode-dielectric band offset magnitude, it was confirmed that such a change in the stoichiometry of Ru oxide leads to the lower effective work function values of the postanneal Ru oxide/high-k gate stacks. The low temperature oxygen anneal treatment of the high temperature annealed Ru oxide electrode stacks recovers the high work function value by supplying oxygen into the decomposed Ru oxide.

Energy level alignment at organic semiconductor/metal interfaces: Effect of polar self-assembled monolayers at the interface

We determined the shifts in the energy levels of approximately 15 nm thick poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] films deposited on various substrates including self-assembled monolayer (SAM) modified Au surfaces using photoelectron spectroscopy. As the unmodified substrates included Au, indium tin oxide, Si (with native oxide), and Al (with native oxide), a systematic shift in the detected energy levels of the organic semiconductor was observed to follow the work function values of the substrates. Furthermore, we used polar SAMs to alter the work function of the Au substrates. This suggests the opportunity to control the energy level positions of the organic semiconductor with respect to the electrode Fermi level. Photoelectron spectroscopy results showed that, by introducing SAMs on the Au surface, we successfully increased and decreased the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. Our study showed that when a substrate is modified by SAMs (or similarly by any adsorbed molecules), a new effective work function value is achieved; however, it does not necessarily imply that the new modified surface will behave similar to a different metal where the work function is equal to the effective work function of the modified surface. Various models and their possible contribution to this result are discussed.

Characteristics of Ni/Gd FUSI for NMOS Gate Electrode Applications

This letter investigates the work function tuning of nickel/gadolinium (Ni/Gd) fully silicided (FUSI) gate electrodes on HfSiOx dielectrics. It was found that as the percentage of Gd in the Ni/Gd increased from 10% to 30%, the effective work function value after a one-step 450-degC FUSI anneal decreased from 4.75 to 4.35 eV. In addition, the presence of Gd also resulted in lowering of equivalent oxide thickness (EOT) values. The mechanism for a decreased EOT is attributed to the reduction of low-kappa interfacial layers by the presence of Gd in the gate stack. The decrease in work function is attributed to the creation of oxygen vacancies within the high-kappa layer created by the presence of Gd layer.

Investigations of Work Function Shift in Lanthanum Silicate High-K Dielectric MIS Capacitors

The effective work function for lanthanum silicate high-k dielectric MIS capacitors with TaN electrodes is reported in comparison to devices with thin lanthanum oxide capping layers on HfSiO and HfSiON dielectric layers. For the hafnium-based dielectrics, La2O3 capping results in a Vfb shift of up to -184 mV after a 1000 {degree sign}C, 5 sec anneal in nitrogen. LaSiOx films were formed by reacting La2O3 and a chemical oxide. Annealing of these films resulted in a negative Vfb shift of higher magnitude. Ozone or ammonia pretreatment of the chemical oxide prior to silicate formation reduces the magnitude of the shift, allowing for effective work function values between 3.75 and 4.25 eV after annealing. After a 700 {degree sign} C, 10 sec anneal, work function tuning of +/- 150 mV was a achieved while maintaining EOT (0.9 nm) and leakage (0.6 A/cm2 at Vfb + 1 V) for all samples with and without pretreatments.

Patchwork field emission properties of lanthanum monosulfide thin films

The field emission properties of lanthanum monosulfide (LaS) films, deposited on Si substrates by pulsed laser deposition, have been thoroughly analyzed via the scanning anode field emission microscopy technique. Using the conventional Fowler-Nordheim relation, the work function of LaS thin films has been extracted from the slope of the plot ln(J∕F2) vs 1∕F, where J is the field emission current density and F is the local applied electric field. The threshold for an emission current density of 1mA∕cm2 occurs around a 230V∕μm electric field applied across the vacuum gap. This leads to an outstanding, reproducible effective work function value of ∼1eV across a 1cm2 sample area.

Comparison of effective work function extraction methods using capacitance and current measurement techniques

The effective work function (EWF) extracted on terraced oxide structures by capacitance-voltage-based techniques was compared with the work function calculated from the barrier height extracted by current-voltage measurements. The results show a reasonable correlation-within /spl plusmn/ 0.1 eV-in the EWF values for various metal gate electrodes, validating both techniques for EWF extraction.

Influence of oxygen diffusion through capping layers of low work function metal gate electrodes

This letter evaluates Ru and W capping layers for MoTa metal gate electrodes in MOS capacitor applications. The authors report that the oxygen diffusion from the capping layer plays an important role in determining the MoTa alloy effective work function value on SiO/sub 2/. A MoTa alloy metal gate with Ru capping exhibits stable effective work function up to 900/spl deg/C annealing but is not stable with W capping. Auger electron spectroscopy and Rutherford backscattering spectroscopy analyses show minimal oxygen diffusion into MoTa gate stacks with Ru capping while severe oxygen diffusion into the gate is observed with W capping metal after 900/spl deg/C annealing. Current-voltage analysis also demonstrates different barrier heights of MoTa on SiO/sub 2/ with Ru or W capping layer after 900/spl deg/C annealing, confirming the effective work function value change.

Evaluation of tantalum silicon alloy systems as gate electrodes

The effect of Si concentration on the effective work function of tantalum silicon (Ta–Si) alloy systems as gate electrodes in direct contact with SiO2 and HfSiOx gate dielectrics has been studied extensively. It was found that the Si concentration in the Ta–Si electrodes (⩾60at.% Si) has a strong effect on the effective work function, and three discrete composition-dependent phases (tantalum metal, tantalum silicide, and silicon) coexist in the films. The film resistivity and density also change dramatically as a function of Si concentration. Physical analysis shows that these Si-rich Ta–Si electrodes are amorphous at room temperature and crystallize with a 1000°C, 5-s anneal. Finally, an effective work function value of 3.98eV has been achieved by arsenic implantation of a capacitor electrode layer in Ta–Si∕HfSiOx film systems, thereby producing a potential n-type metal gate electrode in conjunction with high-k gate dielectrics.

Growth mechanism of TiN film on dielectric films and the effects on the work function

We have investigated the growth mechanism of ALD-TiN film on different dielectrics and the resulting effective work function value. TiN nucleation rate and growth rate are found to be dependent on the dielectric films. TiN growth mechanism changed from 3-D type on SiO 2 to layer-by-layer type on HfO 2. The minimum TiN thickness required for a complete surface coverage varies according to the growth mechanism. Capacitor (MOSCAP) characterization revealed that the effective work function of TiN is dependent not only on dielectric films but also on the TiN thickness. The behavior of work function and fixed charge correlated with the growth mechanism of TiN on dielectric films.

Silicon tips with diamond particles on them: New field emitters?

A new approach to preparation of effective field emitters is proposed. The approach consists in deposition of diamond particles on high aspect ratio silicon tips. Field-emission measurements of such objects have given rather low effective work function values, from 0.3 to 1.2 eV.