Large Work Function Research Articles

Electronic Structures of S-Doped Capped C-SWNT from First Principles Study

The semiconducting single-walled carbon nanotube (C-SWNT) has been synthesized by S-doping, and they have extensive potential application in electronic devices. We investigated the electronic structures of S-doped capped (5, 5) C-SWNT with different doping position using first principles calculations. It is found that the electronic structures influence strongly on the workfunction without and with external electric field. It is considered that the extended wave functions at the sidewall of the tube favor for the emission properties. With the S-doping into the C-SWNT, the HOMO and LUMO charges distribution is mainly more localized at the sidewall of the tube and the presence of the unsaturated dangling bond, which are believed to enhance workfunction. When external electric field is applied, the coupled states with mixture of localized and extended states are presented at the cap, which provide the lower workfunction. In addition, the wave functions close to the cap have flowed to the cap as coupled states and to the sidewall of the tube mainly as extended states, which results in the larger workfunction. It is concluded that the S-doped C-SWNT is not incentive to be applied in field emitter fabrication. The results are also helpful to understand and interpret the application in other electronic devices.

A New Two-Dimensional Analytical Model for Nanoscale Symmetrical Tri-Material Gate Stack Double Gate Metal–Oxide–Semiconductor Field Effect Transistors

Based on the resultant solution of two-dimensional (2D) Poisson's equation in silicon region, a new, compact and analytical model for nanoscale fully depleted, symmetrical tri-material gate stack double-gate (STMGSDG) metal–oxide–semiconductor field-effect transistor (MOSFET) has been developed. The STMGSDG MOSFET exhibits significantly reduced short-channel effects (SCEs) when compared with the symmetrical single-material gate stack double-gate (SSMGSDG) MOSFET. It is found that the threshold voltage roll-off for the fully depleted STMGSDG MOSFET can be effectively reduced by using both thin Si film and thin gate oxide. Besides, the high ratio of large work function of metal gate 1 (L1) to the total gate can efficiently suppress the drain-induced barrier lowering (DIBL) and maintain the low threshold voltage degradation. This study not only presents a precise 2D analytical model of the surface potential and threshold voltage, but also discusses the electric field distribution in the channel region, subthreshold swing and subthreshold current for the STMGSDG MOSFET. The new model is verified to be in a good agreement with numerical simulation results over a wide range of the device parameters.

Narrowing SWNT diameter distribution using size-separated ferritin-based Fecatalysts

Sensors and devices made from single-walled carbon nanotubes (SWNTs) are most oftenelectrically probed through metal leads contacting the semiconducting SWNTs (s-SWNTs).Contact barriers in general and Schottky barriers (SBs) in particular are usuallyobtained at a metal–semiconductor interface. The unique one-dimensional structure(1D) of SWNTs allows tailoring of the SB heights through the contact metaltype and the size of the s-SWNT bandgap. A large workfunction reduces the SBheight (e.g. using Pd as the metal contact material). The bandgap of an SWNT isinversely proportional to its diameter. Ohmic contacts—the preferable choice—areachieved for s-SWNTs with diameters greater than 2 nm on Pd metal leads. SWNTdevice reproducibility, on the other hand, requires a narrow distribution of theSWNT diameters. Here, we present a method to fabricate SWNTs with a large andadjustable mean diameter (1.9–2.4 nm) and very narrow diameter distribution (± 0.27 nm at mean diameter 1.9 nm). The results are achieved through a size separation of theferritin catalyst particles by sedimentation velocity centrifugation prior to their use in thechemical vapor deposition (CVD) formation of SWNTs.

P‐161: Effectiveness of p‐Dopants in an Organic Hole Transporting Material

AbstractWe investigated the effectiveness of p‐dopants such as CuI, MoO3 and ReO3 having different work functions to generate holes in a hole transporting material of 4,4′,4″‐tris(N‐(2‐naphthyl)‐N‐phenyl‐amino)‐triphenylamine (2TNATA). Dopants with larger work function (ReO3>MoO3>CuI) are more effective in the formation of CT complexes and in the generation of the charges.

Platinum Germanosilicide Contacts Formed on Strained and Relaxed $\hbox{Si}_{1 - x}\hbox{Ge}_{x}$ Layers

Contact resistivity is a key contributor to the parasitic series resistance of nanoscale MOSFETs. Since the contact resistivity is an exponential function of the Schottky barrier height, new contact materials that can provide smaller barrier heights to source-drain junctions are needed. Platinum germanosilicide (PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) is of interest as a contact material to the recessed Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> junctions of p-channel MOSFETs due to the large work function of platinum silicide (PtSi). In this paper, we explore the impact of in-plane biaxial compressive strain in Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layers on PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> formation and the impact of the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> on the strain in Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . The parameters considered in this paper include the Ge content, the thickness of the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> epitaxial layer, and the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thickness. The results show that the resistance, surface morphology, and the crystalline structure of the PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films are independent of the strain in the original Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer. The results also indicate that PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> does not influence the strain in the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer. The barrier-height measurements suggest the presence of Fermi-level pinning, and the pinning position is independent of the strain in the alloy, and it is primarily determined by the Ge concentration. As a result of Fermi-level pinning, hole Schottky barrier height of PtSi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> contacts is 0.1-0.2 eV higher than that of the PtSi-Si contacts.

Electrical contacts for II–VI semiconducting devices

High resistivity II–VI semiconductors in general and CdTe and its associated materials like CdZnTe and CdMnTe in particular are suffering from ohmic contacting problem due to their high electron affinity and consequently large work function. Ni, Au, Pt and Pd have large work function and have possibility to match with the above materials. However, except Ni other materials have problems in large-scale commercial applications. In order to overcome the ohmic contacting problem to these semiconductors the following studies has been conducted in the text of the present paper. These are: (i) Work function engineering to modulate the work function through combination materials like, Cu, Au, Mo, W and Co. (ii) Introduction of a charge diluting intermediate semiconducting layer media in between the metal and CdTe to neutralize the bound polarized charges. These two aspects were applied in case of thin film CdTe–CdS solar cells to evaluate their contacting performances and their influences in solar cell parameters. Both cell performances and the contact characteristics of these contacting technologies were studied at depth and indicated their applicability in semiconducting devices.

Interface electronic structures of organic light-emitting diodes with WO 3 interlayer: A study by photoelectron spectroscopy

The energy level alignment and chemical reaction at the interface between the hole injection and transport layers in an organic light-emitting diode (OLED) structure has been studied using in-situ X-ray and ultraviolet photoelectron spectroscopy. The hole injection barrier measured by the positions of the highest occupied molecular orbital (HOMO) for N,N′-bis(1-naphthyl)- N,N′-diphenyl-1,1 ′-biphenyl-4,4 ′-diamine (NPB)/indium tin oxide (ITO) was estimated 1.32 eV, while that with a thin WO 3 layer inserted between the NPB and ITO was significantly lowered to 0.46 eV. This barrier height reduction is followed by a large work function change which is likely due to the formation of new interface dipole. Upon annealing the WO 3 interlayer at 350 °C, the reduction of hole injection barrier height largely disappears. This is attributed to a chemical modification occurring in the WO 3 such as oxygen vacancy formation.

A strategy towards p-type doping of organic materials with HOMO levels beyond 6 eV using tungsten oxide

The authors present a concept to p-dope organic hole transport materials with highest occupied molecular orbital (HOMO) levels on the order of 6 eV (e.g.4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) or 4,4′-bis(carbazol-9-yl)biphenyl (CBP)) by using WO3 as acceptor. Owing to its large work function, WO3 allows for efficient electron transfer from organic semiconductors with HOMO levels in this region. The effect of p-type doping is evidenced by optical absorption spectroscopy, electrical transport characteristics and Kelvin probe analysis. Moreover, OLED devices have been realized which exhibit 30% enhanced power efficiency due to the p-type doping. The results are expected to have a significant impact on the development of organic (opto-)electronic devices.

Fermi Level Engineering of Single-Walled Carbon Nanotubes by AuCl3 Doping

We investigated the modulation of optical properties of single-walled carbon nanotubes (SWCNTs) by AuCl 3 doping. The van Hove singularity transitions (E 11 (S), E 22 (S), E 11 (M)) in absorption spectroscopy disappeared gradually with an increasing doping concentration and a new peak appeared at a high doping concentration. The work function was downshifted up to 0.42 eV by a strong charge transfer from the SWCNTs to AuCl 3 by a high level of p-doping. We propose that this large work function shift forces the Fermi level of the SWCNTs to be located deep in the valence band, i.e., highly degenerate, creating empty van Hove singularity states, and hence the work function shift invokes a new asymmetric transition in the absorption spectroscopy from a deeper level to newly generated empty states.

Using different work function nanocrystal materials to improve the retention characteristics of nonvolatile memory devices

Recently, nanocrystal nonvolatile memory (NVM) devices have attracted great research interest. Taking into account the effect of work function to account for the better retention characteristics for nanocrystals with larger work function, utilizing different work functions Au, W and Si as floating gates is proposed and comparatively studied in this paper. It was found that Au nanocrystals have better retention characteristic than W and Si. The good retention characteristic of the Au nanocrystal device is due to the larger work function and it is difficult for electrons captured by Au nanocrystal to escape from them. So, the retention characteristic of the device can be improved by using larger work function nanocrystal materials.

Electrical and optical properties of copper‐based chalcogenide thin films deposited by pulsed laser deposition at room temperature: Toward p‐channel thin film transistor fabricable at room temperature

AbstractCopper‐based chalcogenide thin films deposited at room temperature were examined for exploring an active layer material for a p‐channel thin film transistor (TFT) fabricable at room temperature. The electrical conductivities of chalcopyrite and simple copper chalcogenide thin films were controlled by introducing hydrogen sulfide and oxygen gases, respectively, during the film deposition. However, no current modulation by gate bias was observed in fabricated TFTs. Optical absorption spectra showed intense subgap absorptions, and these subgap states are thought to pin the Fermi levels. Although these copper‐based chalcogenides did not work as TFT channels, we found that simple copper chalcogenides deposited under a high vacuum showed p‐type degenerate conduction with a high conductivity up to 7.3 × 103 S/cm and a reasonably large work function of ∼4.7 eV. These properties are rather favorable for a hole injection layer of flexible electronic devices and suggest a promising application as an anode layer in organic light emitting devices. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Analysis of ITO/Mg:GaN interfaces by synchrotron radiation hard X-ray photoemission spectroscopy and their electrical characteristics

Interfacial chemical states and Schottky barrier heights (SBHs) for indium tin oxide (ITO) electrodes on Mg:GaN films have been investigated by high-resolution hard X-ray photoemission spectroscopy (HX-PES), and have been correlated with electrical properties. HX-PES has revealed that the large downward band bending of 2.6 eV was drastically reduced by ITO deposition and annealing, resulting in low SBH of 0.2 eV which is well correlated with good ohmic contact. Improvements of electrical properties can be attributed to the combination of (1) the interfacial layer with large work function, (2) the ordered interfacial dipole layer and (3) activation by ITO catalytic effect.

Achieving low-VT Ni-FUSI CMOS via lanthanide incorporation in the gate stack

This work reports that introducing lanthanide in the gate dielectric or in the gate electrode results, in both cases, in large effective work function (eWF) modulation towards n-type band-edge for Ni-FUSI devices. This is done by: (a) deposition of a Dy2O3 capping layer on the host dielectric (SiON or HfSiON), or (b) simple Yb implantation of nMOS poly gates prior to FUSI. We show that: (1) both cases result in dielectric modification with gate leakage (JG) reduction; (2) adding a cap has no significant impact on Tinv(<1Å), while up to ∼5 and 2Å reduction occurs for SiON and HfSiON Yb-implanted devices, respectively, (3) the largest JG reduction (150×) is obtained for capped SiON devices due to dielectric intermixing and formation of a new high-k dielectric (DySiON), comparable to HfSiON in JG and mobility but with 500mV smaller VT; (4) on the other hand, being less invasive to the host dielectric, the optimized Yb I/I option gives 18% improved mobility compared to capped SiON devices; (5) excellent process control and reliability behavior (VT instability by a.c. pulsed IV, PBTI and TDDB) is reported for both eWF tuning methods. They allow ΔeWF(n−p) values up to ∼800meV when combined with Ni–silicide FUSI phase engineering, promising for low-VT CMOS.

The characteristics and interfacial electronic structures of organic thin film transistor devices with ultrathin (HfO2)x(SiO2)1−x gate dielectrics

Pentacene-based thin film transistors with ultrathin (6nm) (HfO2)x(SiO2)1−x gate dielectric layers (x=0.25 and 0.75) were fabricated for low-voltage operation. The devices with ultrathin (HfO2)x(SiO2)1−x as the gate dielectric layer were operated at a gate voltage lower than −4.0eV. However, the threshold voltage and drain current have different values depending on the composition of the (HfO2)x(SiO2)1−x gate dielectric layer. The device with (HfO2)0.75(SiO2)0.25 gate dielectrics, having larger capacitance, shows a higher drain current than that with (HfO2)0.25(SiO2)0.75 gate dielectrics. On the other hand, the device with (HfO2)0.25(SiO2)0.75 gate dielectrics, which has a larger work function, shows a lower threshold voltage. The in situ ultraviolet photoelectron spectroscopy shows that this is caused by the difference in electronic structures and by changes in band alignment of the interface between the pentacene and dielectric layers.

Low and small resistance hole-injection barrier for NPB realized by wide-gap p-type degenerate semiconductor, LaCuOSe:Mg

LaCuOSe:Mg is a wide-gap p-type semiconductor with a high conductivity and a large work function. Potential of LaCuOSe:Mg as a transparent hole-injection electrode of organic light-emitting diodes (OLEDs) was examined by employing N, N′-diphenyl- N, N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB) for a hole transport layer. Photoemission spectroscopy revealed that an oxygen plasma treated surface of LaCuOSe:Mg formed a hole-injection barrier as low as 0.3 eV, which is approximately a half of a conventional ITO/NPB interface. Hole-only devices composed of a LaCuOSe:Mg/NPB/Al structure showed a low threshold voltage ∼0.2 V and high-density current drivability of 250 mA cm −2 at 2 V, which is larger by two orders of magnitude than that of ITO/NPB/Al devices. These results demonstrate that LaCuOSe:Mg has great potential as an efficient transparent anode for OLEDs and other organic electronic devices.

Electrode-dependent electrical properties of metal/Nb-doped SrTiO3 junctions

In this study, we discuss the electrical properties of junctions consisting of metal electrodes and Nb-doped SrTiO3(001) single crystals. The junctions formed with large work function metals (Ni, Au, Pd, and Pt) resulted in rectifying transport. A hysteretic feature was observed in the current (I)-voltage (V) and capacitance (C)-V characteristics of these junctions upon polarity reversal. The ideal Schottky–Mott rule could not explain the barrier height obtained from the I-V data, indicating the existence of interface states. Analyses of the C-V data revealed that a low dielectric constant layer existed at the interface. The interface states and layers affected the transport and the related resistance switching characteristics of the junctions.

Work function engineering and its applications in ohmic contact fabrication to II–VI semiconductors

CdTe and the associated materials are suffering from ohmic contacting problem due to their high electron affinity and consequently large function. Ni, Au, Pt and Pd have large work function and have possibility to match with CdTe. However, except Ni other materials have problems in large-scale applications. In the present paper possibility of Ni has been explored through work function engineering. Work function and bulk resistivity of Ni has been modulated with other materials like, Cu, Au, Mo, W and Co. A theoretical model has been developed to calculate the effective work function and bulk resistivity after modulation. Modulated materials have been deposited over thin film CdTe using electroless technique to evaluate the validity between the theoretical and experimental results. Results indicated about good matching between them. To the best of knowledge this is the first time applications on such type of methodology has presented in contact applications to CdTe.

Small Molecular Organic Nonvolatile Memory Fabricated with Ni Nanocrystals Embedded in Alq3

AbstractRecently, organic nonvolatile memory has attracted much interest as a candidate device for next generation nonvolatile memory because of its simple process, small device area, and high speed. To investigate electrical characteristics of small molecular organic nonvolatile memory with Ni as a middle metal layer, we developed a small molecular organic nonvolatile memory with the device structure of Aluminum tris (8-hydroxyquinolate) (Al/Alq3), Ni nanocrystals, and Alq3/Al. A high vacuum thermal deposition method was used for the device fabrication. It is critical that the fabrication process condition for Ni nanocrystals be optimized, including ∼100 Å thickness, 0.1 Å/sec-evaporation rate, and in-situ plasma oxidation for effective oxidation. The reasons we chose Ni for the middle metal layer are that Ni has a smaller grain boundary, which is beneficial for scaling down and has a larger work function (∼5.15 eV) that can make a deep quantum well in an energy band diagram, compared with that of Al. Our device showed an electrical nonvolatile memory behavior including Vth of ∼2 V, Vw (write) of ∼3.5 V, negative differential region (NDR) of 3.5∼7 V, Ve (erase) of 8 V, and symmetrical electrical behavior at reverse bias. In addition, an interesting behavior of electrical properties was that, although retention and endurance characteristics were similar to the Al device, the Ion/Ioff ratio was greater than 104 at Vr (read) of 1 V. This value of the Ni device was higher than 102 compared to that of the Al device. Also, small molecular organic nonvolatile memory with a Ni middle layer with α-NPD at same fabrication condition showed more unstable characteristics than Alq3. We can speculate that there is a relationship in fabrication condition between the middle metal material and the organic material. Finally, we conclude that our device with a Ni nanocrystals middle layer is more reliable and useful for small molecular organic nonvolatile memory.

Dependence on Organic Thickness of Electrical Characteristics Behavior in Low Molecular Organic Novolatile memory

AbstractRecently, organic nonvolatile memory of nonvolatile memories have attracted attention because application as next generation memory devices. In effort for realizing low-molecular organic nonvolatile memory, the dependence of organic thickness on electrical characteristic behavior in low-molecular organic nonvolatile memory was investigated. We developed an low-molecular organic nonvolatile memory fabricated with the device structure of Al/Alq3 (Aluminum tris(8-hydroxyquinoline))/Ni nanocrystals/Alq3/Al. Four different organic thicknesses, i.e., 30, 40, 50, and 100 nm, with fixing middle layer thickness were deposited by using a high vacuum thermal deposition method. The reason we chose Ni for middle metal layer is that Ni has smaller grain boundary which is benefit for scaling down and larger work function (∼5.15 eV) that can make a deep quantum well in energy band diagram, compared with those of Al. We confirmed that, as an organic thickness increases, a current level linearly decreases by an order of magnitude in a log-scale except for the 100 nm sample case. That is, Ion of 30-nm-thick sample was about 1 mA and 50-nm-thick sample was about 10 ¥ìA. The reason why the decrease in the current with increasing an organic thickness can be attributed to that electron transfer occurs less frequently because of the decrease in the hopping frequency. In addition, a 100-nm-thick sample was not shown the electrical characteristic of low-molecular organic nonvolatile memory. Meanwhile, the switching characteristics of our device showed that Vth of 2V, Vp(program) of 4V, Ve(erase) of 7V, and Ion (after programming)/Ioff (after erasing) of ~6x103. In addition, the interesting behavior of those characteristics is that the switching voltages (e.g; Vth, Vp, Ve Ion/Ioff) were not much changed with a varying an organic thickness. Therefore, we can conclude that an organic thickness does not affect significantly to the switching characteristics but current level. In addition, it was confirmed that a 30-nm-thick organic thickness was the best process condition in realizing low-molecular organic nonvolatile memory fabrication process.

Transport Mechanism of SiGe Dot MOS Tunneling Diodes

The blockage of hole transport due to excess holes In SiGe dots was observed in the MOS tunneling diodes for the first time. The five layers of self-assembled SiGe dots are separated by 74-nm Si spacers and capped with a 130-nm Si. The hole tunneling current from Pt gate electrode to p-type Si dominates the inversion current at positive gate bias and is seven orders of magnitude higher than the Al gate/oxide/p-Si device. The large work function of Pt is responsible for the hole transport current from Pt to p-Si. The incorporation of SiGe dots confines the excess holes in the valence band and forms a repulsive barrier to reduce the hole transport current from Pt to SiGe dots by 2-3 orders of magnitude in comparison with the Pt/oxide/p-Si device. This repulsive barrier also reduces the hole tunneling current from SiGe dots to Pt at negative gate bias.