High Work Function Metal Research Articles

Carrier selective contacts:a selection of high efficiency silicon solar cells

Solar cell has two basic units:the photon absorption layer and the contact layer. The contact layer is a region between the highly recombination-active metal interface and the photon absorption layer. It is vital to reduce the recombination loss between the photon absorption layer and the contact layer in pursuit of the higher conversion efficiency of silicon solar cell. In recent years, carrier selective contact is arousing research interest in photovoltaic industry because it is deemed as one of the last remaining obstacles in approaching to the theoretical efficiency limit of silicon solar cell. In this paper, three different types of carrier selective contacts are analyzed, which includes:1) sandwiching a heavily doped thin layer between the photon absorption layer and the metal interface, which is the so-called emitter or back surface field; 2) aligning the conduction bands or the valence bands of two materials; 3) inducing the band bending through a high work function metal oxide contacting crystalline silicon. Based on one-dimensional solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulated, which includes:1) diffused homojunction silicon solar cell[(p+)c-Si/(n)c-Si/(n+)c-Si]; 2) silicon heterojunction solar cell with amorphous silicon thin films[(p+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n+)a-Si]; 3) silicon heterojunction solar cell with metal oxide thin films[(n)MoOx/(n)c-Si/(n)TiOx], then the energy band structures and the spatial distributions of carrier concentrations of solar cells in the dark are discussed. The simulation results show that the key factor of carrier selective contacts is the asymmetric spatial distribution of the carrier concentrations, i.e. the asymmetric conductivities of electrons and holes. This leads to the formation of high resistance to electrons and low resistance to holes, or high resistance to holes and low resistance to electrons, so the holes will go through the contact easily and the electrons will be blocked simultaneously, or the electrons will go through the contact easily and the holes will be blocked simultaneously. Therefore a hole selective contact or a electron selective contact is formed, respectively.

Review: Fullerene based acceptors for efficient bulk heterojunction organic solar cell applications

Bulk hetro-junction organic solar cells (BHJ-OSCs) which made by coating the blend of highly electron rich, poly-3-hexylthiophene (P3HT) as a donor and extremely electron deficient, soluble C60 derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an acceptor between the high work function positive electrode, ITO and low work function metal negative electrode Al, showed exciting results than the bilayer counterpart in the last two decades, thus the scientific circle and solar cell manufacturing groups are focusing on this treasured field. Fullerene derivative based BHJ-OSCs showed astonishing efficiencies due the high donor-acceptor (D-A) interface, high open-circuit voltage (Voc), easy processing, and thermal stability. Hence, the design of the fullerene derivative plays a vital role in the efficiency of the OSCs. Numerous papers were published on the PC61/71BM and various functionally modified fullerene acceptors in the past several years with higher efficiency, higher LUMO level, and improved solubility. The symmetrical structure of PC61BM showed high aggregation properties, insufficient absorption in the visible region, but unsymmetrical PC71BM had a good optical absorption in the visible region, hence it was the most desirable choice for best performing solar cells. From the comparison, it was clear that the modification in the PC61/71BM core with aryl group, alkyl chain length, and end group modification didn’t show any significant difference in the PCE with the P3HT polymer donor in BHJ-OSCs device structure. Fullerenes multi-adducts performed better than the PC61/71BM due to the development in the Voc. In multi-functionalization, mainly bis-functionalization showed better performance than mono-adduct. C60 and C70 based hetro bis-adducts namely, C60(CH2)(Ind), and OQMF70 showed the highest PCE of 5.90%, and 6.61% respectively, which was comparable with the indene based bis-adducts. Commonly, tris-adduct fullerenes showed the lowest PCE due to the isomer effect and electron trapping, but OQBMF was a hetro tris-adduct of C60-methanofullerenes, which showed a excellent PCE of 6.43% due to improved Voc. Similarly, indene substituted C60 and C70 fullerenes based bis-adducts showed supreme PCE, of 6.5% (IC60BA) and ~6.7% (IC70BA) respectively, peak PCE was attributed to the high electron donating indene group, which enhanced the LUMO level due to the shrinkage in the π-system. The dihydronapthyl based fullerene bis-adduct derivatives such as, NC60BA, and NC70BA showed the best performance of 5.37%, and 5.95% respectively, than the mono-adduct. Even though abundant fulleropyrrolidines were reported, the performance of the fulleropyrrolidines based device was very deprived than the PC61/71BM based devices. Though the OSCs are comparatively efficient for commercial application due the insufficient efficiency and lower environmental stability factors controls the early arrival to the market. Therefore, it is essential to go through the recent fullerene derivatives based literatures, including mono-, bis-, tris- and multi-adducts of various methanofullerene derivative, indene based fullerene derivatives, dihydronaptyl substituted fullerenes, fulleropyrolidines, 1,2 and 1,4-adducts and fullerene derivatives for OSCs applications. In this regard, we discussed the present development in fullerene derivative and summarized the efficient fullerene derivatives for the BHJ-OSCs application and suggest a possible scenario for the future development in the fullerene cage.

Reduction of Fermi level pinning at Au–MoS2 interfaces by atomic passivation on Au surface

Monolayer molybdenum disulfide (MoS2), which is a semiconducting material with direct band gap of ∼1.8 eV, has drawn much attention for application in field effect transistors (FETs). In this connection, it is very important to understand the Fermi level pinning (FLP) which occurs at metal–semiconductor interfaces. It is known that MoS2 has an n-type contact with Au, which is a high work function metal, representing the strong FLP at Au–MoS2 interfaces. However, such FLP can obstruct the attainment of high performance of field effect devices. In this study, we investigate the reduction of FLP at Au–MoS2 interfaces by atomic passivation on Au(111) using first-principles calculations. To reduce the FLP at Au–MoS2 interfaces, we consider sulfur, oxygen, nitrogen, fluorine, and hydrogen atoms that can passivate the surface of Au(111). Calculations show that passivating atoms prevent the direct contact between Au(111) and MoS2, and thus FLP at Au–MoS2 interfaces is reduced by weak interaction between atom-passivated Au(111) and MoS2. Especially, FLP is greatly reduced at sulfur-passivated Au–MoS2 interfaces with the smallest binding energy. Furthermore, fluorine-passivated Au(111) can form ohmic contact with MoS2, representing almost zero Schottky barrier height (SBH). We suggest that SBH can be controlled depending on the passivating atoms on Au(111).

High-gain complementary metal-oxide-semiconductor inverter based on multi-layer WSe2 field effect transistors without doping

A high-gain complementary metal-oxide-semiconductor (CMOS) logic inverter was implemented by fabricating p- and n-type field effect transistors (FETs) based on multi-layer WSe2 on the same wafer. Au as a high work-function metal is contacted to WSe2 for the source/drain of the p-type FET. The n-type FET has an Al electrode contacted to WSe2 for the source/drain. Both FETs were designed to have similar on-current densities (>10−7 A μm−1) and high on/off current ratios (>106). The inverter shows excellent switching characteristics including relatively high voltage gains (>25) and high noise margins (>0.9) in the range of supply voltage from 2 V to 8 V. This work has a great significance in the realization of a CMOS logic gate based on WSe2 without an additional doping scheme.

Effects of plate electrode materials on hydrogen production by pulsed discharge in ethanol solution

Hydrogen production from an ethanol solution by pulsed high voltage spark discharge was optimized by varying the material of plate electrode. It is the first time that metal work function has been used in hydrogen production by plasma reforming. With low work function metal plate electrode, both energy efficiency and hydrogen yield can be increased. The flow rate and the percentage concentration of hydrogen were achieved 1.3L/min and 75% respectively while discharging with zinc plate electrode for hydrogen production, which is much better than traditional stainless steel electrodes at the same conditions. The analysis of emission spectra was also accomplished in this work. All the intensity of existing spectral lines showed a decline with higher work function metal as plate electrode, and H may be the key to the process of hydrogen production. Additionally, electron temperature and density were also estimated. Both of which were increased with lower work function metal plate electrodes. The electron temperature and density can reach 24,000K, 7.5×1018cm−3 respectively.

Contact Metal–MoS2 Interfacial Reactions and Potential Implications on MoS2-Based Device Performance

Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS2 using electron beam deposition under two different reactor base pressures to determine the contact metal–MoS2 interface chemistry and its dependence on the reactor ambient. The high work function metal Au does not react with MoS2 regardless of reactor ambient. In contrast, interfacial reactions between MoS2 and another high work function metal, Ir, are observed when it is deposited under both high vacuum (HV, ∼ 1 × 10–6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10–9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS2 when the contact metal is deposited under UHV conditions. In addition, Sc is rapidly oxidized on the MoS2 surface, whereas Cr is only partially oxidized when deposited under HV conditions. This indicates that deposition chamber ambient can affect the contact metal chemistry in addition to the chemistry present at the contact metal...

Surface State Engineering of Metal/MoS2Contacts Using Sulfur Treatment for Reduced Contact Resistance and Variability

Variability and lack of control in the nature of contacts between metal/MoS2 interface is a major bottleneck in the realisation of high-performance devices based on layered materials for several applications. In this letter, we report on the reduction in Schottky barrier height at metal/MoS2 interface by engineering the surface states through sulphur treatment. Electrical characteristics for back-gated MoS2 field effect transistor structures were investigated for two high work-function metal contacts Ni and Pd. Contacts on MoS2 treated with sulphur exhibited significant improvements in Ohmic nature with concomitant reduction in variability compared to those on untreated MoS2 films leading to a 2x increase in extracted mobility. X-ray Photoelectron Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold voltages indicated absence of additional doping or structural changes due to sulphur treatment. The Schottky barrier heights were extracted from temperature-dependent transfer characteristics based on the thermionic current model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2 and Pd/MoS2 contacts respectively is hence attributed to the increase in surface states (or stronger Fermi level pinning) due to sulphur treatment. The corresponding charge neutrality levels at metal/MoS2 interface, were extracted to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur treatment. This first report of surface states engineering in MoS2 leading to superior contacts is expected to significantly benefit the entire class of devices based on layered 2D materials.

Fabricating Ohmic contact on Nb-doped SrTiO3 surface in nanoscale

Fabricating reliable nano-Ohmic contact on wide gap semiconductors is an important yet difficult step in oxide nanoelectronics. We fabricated Ohmic contact on the n-type wide gap oxide Nb-doped SrTiO3 in nanoscale by mechanically scratching the surface using an atomic force microscopy tip. Although contacted to high work function metal, the scratched area exhibits nearly linear IV behavior with low contact resistance, which maintains for hours in vacuum. In contrast, the unscratched area shows Fowler–Nordheim tunneling dominated Schottky rectifying behavior with high contact resistance. It was found that the Ohmic conductivity in the scratched area was drastically suppressed by oxygen gas indicating the oxygen vacancy origin of the Ohmic behavior. The surface oxygen vacancy induced barrier width reduction was proposed to explain the phenomena. The nanoscale approach is also applicable to macroscopic devices and has potential application in all-oxide devices.

Fabrication of efficient polymer light-emitting diodes using water/alcohol soluble poly(vinyl alcohol) doped with alkali metal salts as electron-injection layer

An effective electron-injection layer (EIL) is crucial to efficient polymer light-emitting diodes (PLEDs) with high work-function metal as cathode. This work presents the use of water/alcohol soluble poly(vinyl alcohol) (PVA), especially doped with alkali metal salts, as a highly effective EIL to fabricate efficient multilayer PLEDs, allowing the use of stable aluminum as the cathode. Using neat PVA as EIL, the maximum brightness and maximum current efficiency of the device [ITO/PEDOT:PSS/SY/PVA/Al(90 nm)] were significantly enhanced to 5518 cd/m2 and 2.64 cd/A (from 395 cd/m2 and 0.06 cd/A without the EIL) due to promoted electron-injection and hole-blocking. The device performance is further enhanced by doping the PVA with alkali metal salts (M2CO3 or CH3COOM; M: Na, K, Cs), and the enhancement is increased with increasing dopant concentration. Particularly, the PVA doped with 30 wt% alkali metal carbonates revealed the best performance (20214–25163 cd/m2, 5.83–6.83 cd/A). This has been attributed to improved electron-injection from aluminum cathode, which has been confirmed by the corresponding increase in the open-circuit voltages (V oc) obtained from photovoltaic measurements. Current results indicate that commercially available PVA are promising electron-injection layer for PLEDs when doped with appropriate alkali metal salts.

Organic semiconductor heterojunctions: electrode-independent charge injectors for high-performance organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are driven by injected charges from an anode and a cathode. The low and high work function metals are necessary for the effective injection of electrons and holes, respectively. Here, we introduce a fully novel design concept using organic semiconductor heterojunctions (OSHJs) as the charge injectors for achieving highly efficient OLEDs, regardless of the work functions of the electrodes. In contrast to traditional injected charges from the electrodes, the injected charges originate from the OSHJs. The device performance was shown to be significantly improved in efficiency and stability compared to conventional OLEDs. Attractively, the OLEDs based on OSHJs as charge injectors still exhibited an impressive performance when the low work function Al was replaced by air- and chemistry-stable high work function metals, such as Au, Ag, and Cu, as the cathode contact, which has been suggested to be difficult in conventional OLEDs. This concept challenges the conventional design approach for the injection of charges and allows for the realization of practical applications of OLEDs with respect to high efficiency, selectable electrodes, and a long lifetime.

Material selection for hybrid floating gate NAND memory applications

This article discusses the materials aspects involved in the successful integration of hybrid floating gate (HFG) devices for NAND applications. In HFG, a Si(n type)\metal(p type) stack replaces the standard poly‐Si FG. The high work function metal helps to enlarge the program window by limiting the leakage through the high‐k intergate dielectric (IGD); the use of high‐k IGD enables to scale the equivalent oxide thickness (EOT) of the HFG cell. Our recent progresses in the understanding of the materials characteristics (IGD crystallinity and HFG‐IGD thermal stability) that influence memory performance have allowed to demonstrate that HFG can be a solution to scale planar flash beyond the 20 nm node with the best preferable hybrid floating gate structure being a Ru‐based HFG and a 3‐layers IGD stack of HfAlO\Al2O3\HfAlO. Indeed, through the correct selection and combination of materials for process conditions that are relevant in flash memory fabrication flows, we will show that program windows can be suitably engineered. Finally, the materials challenges for keeping acceptable memory retention characteristics are also highlighted.

Electron energy distribution in Si/TiN and Si/Ru hybrid floating gates with hafnium oxide based insulators for charge trapping memory devices

Scaling the planar NAND flash cells to the 20 nm node and beyond mandates introduction of inter‐gate insulators with high dielectric constant (κ). However, because these insulators provide a smaller electron barrier at the interface with the poly‐Si floating gate, the program window and the retention properties of these scaled cells are jeopardized. To reduce the charge loss from the floating to the control gate, one may consider the introduction of a hybrid floating gate (HFG) structure comprised of poly‐Si and a high work function (WF) metal, e.g., TiNx (x ∼ 1; WF ∼ 4.7 eV) or Ru (WF ∼ 5.3 eV). However, the very HFG concept is based on the assumption that electron trapping occurs inside the HFG stack rather than on traps present in the high‐κ insulator. To examine this critical hypothesis, we analyzed the energy distribution of electrons trapped in flash cells with poly‐Si(2 nm)/TiN (6 nm)/Hf0.8Al0.2Ox(19 nm, κ ∼ 15–19)/TiNx (10 nm) and Si(2 nm)/Ru (1 nm)/Hf0.8Al0.2Ox(5 nm)/Al2O3 (5 nm)/Hf0.8Al0.2Ox (5 nm)/TiNx (10 nm) trapping gate stacks using the exhaustive photo‐depopulation spectroscopy. We found that trapped electron energy levels show a broad distribution (± 0.3 eV) centred at ∼3.2 eV below the oxide conduction band. The energy onset of electron de‐trapping at ∼2.8 eV matches the TiNx/HfO2 barrier height found from internal photoemission experiments, indicating that electrons are predominantly trapped inside the HFG.

Performance analysis of different top metal electrodes in inverted polymer solar cells

Abstract In recent years, since the introduction of bulk heterojunction concept in an organic solar cell, the efficiency is increased to ∼6%. The benefits of bulk heterojunction (BHJ) organic solar cells are drawing interest for applications in next-generation solar cells. In this study, we analyze the optimal top electrode for practical polymer solar cells (PSC) fabrication by utilizing the optical properties of the electrode material and study their performance of PSC devices. The high work function metal electrodes having enhanced optical properties such as aluminum (Al), silver (Ag) and gold (Au) are considered in fabricating inverted PSC devices. It comprises of a blended polymer of poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM). Among the key photovoltaic parameters, the open circuit voltages ( V oc ) were strongly affected by the optical properties of the top electrode . From the experimental evidence, the increase in V oc of the Al and Ag electrode devices were found to be approximately 11.32% and 26.42% respectively. These values were significantly higher when compared to the Au electrode, as the parasitic absorption of incident photons were below 600 nm. Ag electrodes have excellent stability to ambient exposure which is comparable to Au. These investigations directed to conclude that, Ag is the optimal top electrode material for use in inverted devices. Also, in this study we investigated the incorporation of monodispersed ZnO as an n-type buffer layer to effectively transport electrons from active layer to the cathode electrode.

Static and Dynamic Performance of Complementary Inverters Based on Nanosheet α-MoTe2 p-Channel and MoS2 n-Channel Transistors.

Molybdenum ditelluride (α-MoTe2) is an emerging transition-metal dichalcogenide (TMD) semiconductor that has been attracting attention due to its favorable optical and electronic properties. Field-effect transistors (FETs) based on few-layer α-MoTe2 nanosheets have previously shown ambipolar behavior with strong p-type and weak n-type conduction. We have employed a direct imprinting technique following mechanical nanosheet exfoliation to fabricate high-performance complementary inverters using α-MoTe2 as the semiconductor for the p-channel FETs and MoS2 as the semiconductor for the n-channel FETs. To avoid ambipolar behavior and produce α-MoTe2 FETs with clean p-channel characteristics, we have employed the high-workfunction metal platinum for the source and drain contacts. As a result, our α-MoTe2 nanosheet p-channel FETs show hole mobilities up to 20 cm(2)/(V s), on/off ratios up to 10(5), and a subthreshold slope of 255 mV/decade. For our complementary inverters composed of few-layer α-MoTe2 p-channel FETs and MoS2 n-channel FETs we have obtained voltage gains as high as 33, noise margins as high as 0.38 VDD, a switching delay of 25 μs, and a static power consumption of a few nanowatts.

Electron transfer during metal-assisted and stain etching of silicon

The etching of silicon in fluoride solutions is limited by the kinetics of charge transfer not thermodynamics. This characteristic is what gives fluoride etching its great versatility in making different types of nanostructures as the result of self-limiting chemistry. This review approaches the kinetics of electron transfer from silicon and metal-coated silicon to a solution phase species from a fundamental point of view in order to establish a better understanding of the mechanisms of nanostructure formation during metal-assisted and stain etching of silicon. Band bending calculations demonstrate that diffusion of holes away from low work function metals such as Ag is not possible. Similarly diffusion of holes outside of the space charge layer is not possible for high work function metals such as Au, Pd and Pt. While direct hole injection may be important for etch track pore formation in the immediate vicinity of the metal, the charge imbalance on or near the metal causes the metal to act like a nanopower supply that polarizes the surrounding Si. This second mechanism is implicated in nonlocal etching of Si during metal-assisted etching.

Impact of metal electrode on charge transport behavior of metal-Gd2O3 systems

In this paper, we have grown an 80 nm thick Gd2O3 thin film by electron beam evaporation on glass substrate and fabricated different metal (Al, Cu, Cr and Au) electrodes on grown sample under same condition. To investigate the charge transport mechanism in these metal-semiconductor systems, the electrical conductivities and current–voltage (I–V) measurements have been measured over temperature range of 250–400 K. We have found that Mott variable range hopping (VRH) is responsible for conduction behavior in all systems for entire temperature range. A strong correlation between transport properties and metal work function has been observed. A space charge model successfully explained the decreasing trend of conductivity with increasing the metal work function. The conductivity decreased from 2.9 × 10−5 to 1.8 × 10−11 S/cm as the metal work function increased from 4 to 5.1 eV for Al to Au metals respectively. The ideality factor also increased from 1.67 to 2.2 with metal work function from Al to Au metal. The observed result can be explained as; high work function metal forms higher depletion layer as compared to metal having low work function, which compensate the empty sites available for hopping and consequently decreased the hopping conductivity.

(Invited) Simulation and Modeling of the Switching Dynamics in Resistive Switching Devices

Resistive switching devices based on the valence change mechanism (VCM) or electrochemical mechanism (ECM) have attracted great attention due to their potential use in future non-volatile memories. Both types of cells exhibit a bipolar operation principle: The cells can be set from a high resistive state (HRS) to a low resistive state (LRS) with one voltage polarity and reset back to the HRS by applying the opposite voltage polarity. ECM cells consist of an ion-conducting layer sandwiched between a Ag/Cu active electrode and an inert counter electrode. The switching mechanism is based on the growth and dissolution of Ag/Cu filament within the ion-conducting host matrix involving the physical and electrochemical processes illustrated in Fig. 1.[1] The VCM stack typically consists of a mixed ionic-electronic conducting layer, e.g. a metal oxide, a high work function metal electrode and an oxidizable counter electrode. The switching process involves the movement of oxygen vacancies within the MIEC and subsequent local redox reactions in the cation sublattice.[2] Fig.1: Illustration of the electrochemical processes occurring during SET switching in an ECM cell: charge-transfer reactions, nucleation, ion migration, filamentary growth. From [1] – Reproduced by permission of the PCCP Owner Societies In order to fulfill the requirements of a nonvolatile memory a resistive switching device has to be switchable with a voltage amplitude of a few volts in the ns-regime and to offer a read-disturb immunity of up to ten years. This is equivalent to the driving physical processes exhibiting a nonlinearity of more than 15 orders of magnitude.[3]To understand the underlying electrochemical and physical processes is hence of fundamental importance. In the present paper, simulation models for ECM and VCM cells will be presented with special emphasis on the underlying physical and electrochemical processes. Based on simulation results the SET and RESET switching kinetics will be discussed in detail. ECM compact models,[1] continuum models[4] and kinetic Monte Carlo (kMC) models[5, 6] will be presented. All ECM models include the electrochemical and physical processes illustrated in Fig. 1: nucleation, electron-transfer reactions at the boundaries, ion migration and filamentary growth. The models are capable of capturing the observed experimental phenomena: nonlinear switching kinetics, multilevel switching, current-voltage characteristics and switching variability.[5]The multilevel switching is attributed to different tunneling gaps between the growing filament and the counter electrode at low current compliance and a filament widening if a galvanic contact is achieved. By comparison of the simulation results with experimental data the processes limiting the switching speed are discussed. Furthermore, the influence of mechanical stress on the filamentary growth will be addressed using kMC simulations. The developed VCM simulation model is based on the solution of the drift-diffusion equation for oxygen vacancies and electrons coupled to Poisson’s equation and the heat transfer equation. Electron transport across the metal/oxide interface via thermionic emission and tunneling is also included within the model. Using this simulation model the nature of the gradual RESET and abrupt SET transition will be discussed. The gradual RESET can be explained as a result of a drift-diffusion balance. Moreover, the simulation results reveal that the switching kinetics in VCM devices are dominated by temperature-accelerated ion movement. Furthermore, the transition between bipolar and complementary switching can be explained by the variation of the metal/oxide interface properties.[7] This work was supported in part by the Deutsche Forschungsgemeinschaft (SFB 917). [1] S. Menzel, S. Tappertzhofen, R. Waser, I. Valov, PCCP 2013, 15, 6945. [2] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 2009, 21, 2632. [3] S. Menzel, M. Salinga, U. Böttger, M. Wimmer, Adv. Func. Mater. 2015 (submitted) [4] S. Menzel, N. Adler, J. van den Hurk, S. Tappertzhofen, I. Valov, R. Waser, 2013 5th IEEE International Memory Workshop (IMW) 2013, 92. [5] S. Menzel, B. Wolf, S. Tappertzhofen, I. Valov, U. Böttger, R. Waser, 2014 6th IEEE International Memory Workshop (IMW), 2014 [6] S. Menzel, U. Böttger, R. Waser, J. Appl. Phys. 2012, 111, 014501/1. [7] A. Schönhals, D. J. Wouters, A. Marchewka, T. Breuer, K. Skaja, V. Rana, S. Menzel, R. Waser, 2015 7th IEEE International Memory Workshop (IMW), 2015 (accepted) Figure 1

Molybdenum and tungsten oxide: High work function wide band gap contact materials for hole selective contacts of silicon solar cells

The high work function metal oxides, tungsten oxide (WOx) and molybdenum oxide (MoOx), were investigated regarding their ability to form a hole-selective contact for a crystalline silicon absorber. We show that in principle both materials have the potential to (i) either replace the p-type amorphous silicon thin films typically used as the high work function contact material in silicon based heterojunction solar cells or (ii) to assist the hole extraction if used as an additional contact layer placed between the p-type amorphous silicon and the TCO electrode. For an integral evaluation of the actual loss mechanisms limiting the contact characteristics both the ability of the contact scheme to passivate the absorber and to selectively extract the excess holes from the absorber are analyzed.

Modification of metal–InGaAs Schottky barrier behaviour by atomic layer deposition of ultra-thin Al2O3 interlayers

The effect of inserting ultra-thin atomic layer deposited Al2O3 dielectric layers (1nm and 2nm thick) on the Schottky barrier behaviour for high (Pt) and low (Al) work function metals on n- and p-doped InGaAs substrates has been investigated. Rectifying behaviour was observed for the p-type substrates (both native oxide and sulphur passivated) for both the Al/p-InGaAs and Al/Al2O3/p-InGaAs contacts. The Pt contacts directly deposited on p-InGaAs displayed evidence of limited rectification which increased with Al2O3 interlayer thickness. Ohmic contacts were formed for both metals on n-InGaAs in the absence of an Al2O3 interlayer, regardless of surface passivation. However, limited rectifying behaviour was observed for both metals on the 2nm Al2O3/n-InGaAs samples for the sulphur passivated InGaAs surface, indicating the importance of both surface passivation and the presence of an ultra-thin dielectric interlayer on the current–voltage characteristics displayed by these devices.

Layer dependence and gas molecule absorption property in MoS2 Schottky diode with asymmetric metal contacts.

Surface potential measurement on atomically thin MoS2 flakes revealed the thickness dependence in Schottky barriers formed between high work function metal electrodes and MoS2 thin flakes. Schottky diode devices using mono- and multi- layer MoS2 channels were demonstrated by employing Ti and Pt contacts to form ohmic and Schottky junctions respectively. Characterization results indicated n-type behavior of the MoS2 thin flakes and the devices showed clear rectifying performance. We also observed the layer dependence in device characteristics and asymmetrically enhanced responses to NH3 and NO2 gases based on the metal work function and the Schottky barrier height change.