Metal-semiconductor Schottky Barrier Research Articles

Improving organic transistor performance with Schottky contacts

Organic field-effect transistors (OFETs) with non-Ohmic contacts, e.g., pentacene with gold electrodes, exhibit a linearly growing threshold voltage with increased film thickness due to tunnel injection [R. Schroeder et al., Appl. Phys. Lett. 83, 3201 (2003)]. In this letter, we demonstrate gold/pentacene OFETs with a low threshold voltage independent of pentacene thickness. By doping the pentacene in the contact area with FeCl3 (iron-III-chloride), the metal-insulator-type tunneling barrier was changed to a metal-semiconductor Schottky barrier. Since the injection through a Schottky barrier depends on the potential and not on the electric field, the threshold voltage is no longer a function of the semiconductor thickness. Through selective doping of the area under the electrode, the channel remains undoped, and large on/off ratios are retained.

Room-temperature ballistic electron emission luminescence spectroscopy with a scanning tunneling microscope

We present a luminescence spectroscopy for semiconductor heterostructures based on local hot electron injection from a scanning tunneling microscope tip. In addition to a tip voltage bias exceeding the metal-semiconductor Schottky barrier height, this process requires a collector bias voltage to satisfy energy conservation. These results indicate that this method could be used to study local electron transport and simultaneous electroluminescence in buried luminescent layers at depths greater than the ballistic electron mean free path in the collector.

Calculation of band bending in ferroelectric semiconductor

In order to investigate the electronic characteristics of ferroelectric thin films that behave as semiconductors, we derive an equation to calculate the band bending in ferroelectrics, to replace the Poisson equation. We find that the space-charge density term, , in the Poisson equation should be replaced by - Pr, where Pr is the remanent polarization. In order to simplify the band bending calculation, only one dimension is considered here. For example, in the case of the 3 V band bending in a metal-semiconductor Schottky barrier junction with a 1 µm thick ferroelectric semiconductor, the depletion region extends over the whole semiconductor, even with a dielectric constant of 10 and a donor density of 1 × 1017 cm-3 due to dPr /dx. However, the depletion layer width in a semiconductor without remanent polarization is only about 0.18 µm under the same conditions.

Three-dimensional nanoscale W-GaAs heterostructures

Layers of semi-insulating GaAs have been realised by epitaxial overgrowth over matrices of nm-sized tungsten discs. By fabricating and characterising structures with different metal disc densities, the influence of metal-induced carrier depletion on the resistivity in GaAs is demonstrated. The vertical current transport in this material shows a distinct decrease as the metal discs are spaced closer than 400 nm, which indicates an onset of overlapping Schottky depletion regions. From the temperature dependence of the current transport at different bias conditions, we deduce the height of the internal barriers, which are built-up in the semiconductor material between the discs, and their variation with the applied voltage. Finally, the actual metal–semiconductor Schottky barrier height is obtained from the spectral distribution of a photoresponse, which we attribute to electron excitation in the buried discs in the otherwise semi-insulating GaAs layer.

Coulomb effects on charged, buried metal disks at room temperature

Capacitance transients caused by capture and emission of electrons from buried metal disks are investigated. A single layer of tungsten disks, arranged in a square lattice, is introduced into GaAs by epitaxial overgrowth and a depleted layer is formed around the disks due to the metal–semiconductor Schottky barrier. The number of captured electrons on each disk is measured by the capacitance associated with the width of the depletion layer, whereas the capacitance transients reflect the changes in the number of excess electrons on the disks. By investigating the emission time constants for varying numbers of electrons in excess on the disks, the Coulomb effect is studied. In combination with a temperature-dependent capture, a Coulomb charging energy of only 4 meV is shown to shift the measured activation energies erroneously by hundreds of meV.

Liquid Encapsulated Czochralski Growth of Large size Gallium Arsenide and Indium Phosphide Single Crystals and their Characterisation—A Review

GaAs and InP single crystals have been grown using Liquid Encapsulated Czochralski (LEC) technique. The grown GaAs ingots are of 2-2.5 inch in diameter and 1–1.5 kg in weight; InP ingots are of 2 inch diameter and 0.75 kg—1 kg in weight. Several n-type, p-type and doped as well as undoped semi-insulating crystals were grown using <111> and <100> oriented seed crystals. The important crystal characteristics such as resistivity, carrier concentration, mobility and etch-pit density have been studied using electrical and optical techniques. A test solar cell has been fabricated using Si doped GaAs crystal and the efficiency of the cell has been determined to be 14.8% as compared to the 17% of the commercial GaAs solar cells. For the realisation of metal-semiconductor contacts, Au, Al and Ag Schottky Barrier Diodes (SBDs) were fabricated and were characterised using Current-Voltage (I-V), Capacitance-Voltage(C-V) and Deep Level Transient Spectroscopy(DLTS). High energy and low energy ion implantation studies were carried out and are reported.

Low-temperature thermionic emitters using metal-semiconductor composites

Small metal particles (10–100 nm in size) buried in semiconductor matrices provide a source of electrons which can be optically or thermally excited into the semiconductor across the metal-semiconductor Schottky barrier. These electrons which are now in the conduction band of the semiconductor have only to surmount the surface barrier of the semiconductor, i.e. the electron affinity EA, in order to escape into vacuum. Since Schottky barriers can be made considerably smaller than the bandgap Eg of most common semiconductors (1–3 eV), the threshold for electron emission into the vacuum is reduced, i.e. (ESchottky+EA)<(Eg+EA). Some preliminary results on Au-CuInSe2 composites provide strong support for these ideas.

Tuning Schottky Barriers by atomic layer control at metal—semiconductor interfaces

Metal‐semiconductor contacts are essentially of two kinds : ohmic contacts and Schottky barriers. The physics governing the formation of metalsemiconductor Schottky barriers–which are the basis of most electronic devices–and ways of tuning their height are reviewed, concentrating on the theory. It is shown that tuning can be achieved by modifying the interface geometry, by modifying the surface dangling bonds, or, in the case of heterojunction interfaces, by the addition of an interlayer. Directions for (experimental) future research are suggested.

Proton irradiation effects on strained Si1−xGex/Si heterostructures

Proton irradiation effects on strained Si1−xGex/Si heterostructures have been studied. For the experiment, p+-Si1−xGex/p−-Si heterojunction diodes were fabricated by molecular beam epitaxy (MBE) growth of strained p+-boron doped SiGe layers on p−-Si(100) substrates. Due to the valence band discontinuity between SiGe and Si layers, and degenerate doping in the SiGe layer, the characteristics of these heterojunction diodes are similar to those of metal-semiconductor Schottky barrier diodes. The SiGe/Si heterojunction diodes are irradiated by 1 Mrad of protons at 1 and 8.5 MeV energies. The current-voltage (I-V) characteristics are measured as a function of temperature before and after irradiation. I-V characteristics show a decrease of the reverse bias leakage current after irradiation. The effective heterojunction barrier heights (Φb) and Richardson constants (A**) are measured before and after irradiation using activation energy measurements. The measurements show an increase of Φb and A** after irradiation. The increase of the effective barrier height is attributed to reduction of free-holes in the SiGe layers due to proton induced displacement defects. The increase of effective barrier height suggests that the strain in the SiGe layers is conserved after 1 Mrad of proton irradiation.

Activating technology of SnO 2 layers by metal particles from ultrathin metal films

The selectivity and sensitivity of gas sensitive semiconductor materials can be improved by doping the resistor material with catalytically active dopants. Recent work deals with the activating technology of SnO 2 sensors by metal particles originating from a sputtered ultrathin metal layer. Ultrathin metal films are sputtered on top of the SnO 2 layer (0.5 W/cm 2 r.f. power density in argon at 0.1 Pa pressure, 0–60 s sputtering time, resulting in 0–8 nm layer thickness). The sputtered samples were heated up to 600 K in the atmosphere until the end of agglomeration of the ultrathin metal film. The sheet resistivity of the Pd-SnO 2 layer is 10–100 Ω before agglomeration, and 10 6–10 8 Ω after the heat treatment. Before the heat treatment the sheet resistivity is determined by the continuous Pd layer only, after the agglomeration process the resistivity is controlled by the SnO 2 layer and the metal-semiconductor Schottky barriers. SIMS and XPS results clearly show the decrease in surface coverage of the activator after agglomeration, and the presence of small amounts of oxidized metal phase on the surface. The agglomerated Pd layer is the best activator for H 2 sensors: the sensitivity increases by three to four orders of magnitude after this procedure, compared to the sensitivity of the simple SnO 2 layer. Ultrathin metal films from other materials (Ag, Pt, Au) behave similarly to the Pd layers. An agglomerated silver layer is a very good activator for H 2S. Gold and platinum are also effective for CO and H 2, respectively.

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.

Electronic structure of epitaxial interfaces

Metal-semiconductor (Schottky barrier) and semiconductor-semiconductor (heterojunction) interfaces show rectifying barrier heights and band offsets, which are two key quantities required to optimize the performance of a device. A large number of models and empirical theories have been put forward by various workers in the field during the last 50 years. But a proper understanding of the microscopic origin of these quantities is still missing. In this article, our focus is mainly to present a unified framework for first principles investigation of the electronic structure of epitaxial interfaces, in which one of the constituents is a semiconductor. LMTO method is now a well established tool for self-consistent electronic structure calculations of solids within LDA. Such calculations, when performed on supercell geometries, are quite successful in predicting a wide range of interface specific electronic properties accurately and efficiently. We describe here the basic formalism of this LMTO-supercell approach in its various levels of sophistication and apply it to investigate the electronic structure of A- and B-type NiSi2/Si(111) interface as a prototype metal-semiconductor system, and CaF2/Si(111) interface as a prototype insulator-semiconductor system. These are a few of the most ideal lattice matched epitaxial interfaces whose atomic and electronic structures have been extensively studied using a wide range of experimental probes. We give here a glimpse of these experimental results and discuss the success as well as limitations of LDA calculations to achieve accuracies useful for the device physicists.

Field emission from a metal covered with a semiconducting layer: A model calculation

The electron field emission from a metal covered with a thin layer of a semiconductor with electron affinity χ and dielectric constant ɛ is considered. The model takes into account the metal-semiconductor (Schottky) barrier of height ϕb, the conduction current inside the semiconductor, and the band bending at the semiconductor-vacuum interface due to the external field penetration. For thick films under moderately high electric fields, the metal-semiconductor interface does not influence the emission behaviour whereas for thin films, the interface plays an important role, depending on the barrier heights. In particular, for χ/ϕb<e2/3 the I–V characteristics will, for strong fields, be dominated by the field emission process at the interface. In such cases important deviations from Fowler-Nordheim behaviour are found.

Effect of nitridation on the density of interface states in W–Ti/n-GaAs Schottky diodes

The effective density of interface states that exist at the metal–semiconductor Schottky barrier formed between W–Ti and silicon implanted (n‐type) GaAs has been characterized using forward‐biased capacitance–voltage (C–V) measurements. The Schottky diodes were fabricated by sputtering from a tungsten titanium target under different nitrogen gas ambients of 0%, 6%, and 12%. The nitrogen incorporation and interface stability were verified by secondary ion mass spectroscopy (SIMS) and Rutherford backscattering (RBS). The results of this study clearly show that the lowest effective density of interface states was obtained for the highest nitrogen ambient of 12% at a deposition rate of 10 nm/min.

Theory of a metal-semiconductor photodiode with grating coupling of the incident light to surface plasma waves

A theoretical model has been developed to design a metal-semiconductor (Schottky barrier) narrow-band photodiode with grating coupling of the incident light to surface plasma waves. With optimal choice of grating parameters and the right combination of materials, the quantum efficiency can be increased over the flat surface geometry. The results of this model are in good agreement with the experimental results found in the literature.

The effect of carrier concentration in the semiconductor on the voltage-current characteristics of Schottky barrier diodes at low temperatures

We report the results of a study of voltage-current characteristics of gallium arsenide metal-semiconductor Schottky barrier structures, in which palladium and W-Ni alloy were electrochemically deposited in SiO2 windows to form the barrier. It is demonstrated that the voltage-current characteristics are significantly distorted at low temperatures. The observed behavior of the voltage-current characteristics at low temperatures —the appearance of excess current and the increase in the ideality factor with carrier concentration in the excess current regime —is explained by a model in which peripheral mechanical stresses in the contacts lower the potential barrier height on the periphery of the contacts.

Metal-semiconductor Schottky barrier junctions and their applications

Metal-semiconductor Schottky barrier junctions and their applications

A simple model for computer simulation of Schottky-barrier diodes

To simulate the electrical characteristics of metal-semiconductor Schottky barrier diodes, a numerical analysis program based on the Shockley's semiconductor equations has been established. The thermionic emissions of electrons and holes from semiconductor to metal as well as the electric field in the interfacial layer are taken as the derivative boundary conditions of the nonlinear equations. The forward and reverse current-voltage characteristics of various metal-silicon and metal-silicide-silicon Schottky barrier diodes can be simulated by properly choosing the zero-field barrier height and the interfacial-layer capacitance. The barrier height variation as a function of applied voltage is related to the space-charge density and the interfacial-layer capacitance. The nonideality of forward characteristics is attributed to the bending of majority carrier imref and the raising of barrier height. The soft behavior of reverse characteristics can be modeled in terms of the interfacial-layer capacitance.

Diffusion length measurements in CdS and CdSe Schottky barrier junctions

The minority carrier diffusion length has been measured on CdS and CdSe single crystals by means of the surface photovoltage method. By illuminating the samples through semi-transparent metal semiconductor Schottky barriers and for a given photovoltage signal, a linear relation was found between the intensity of light and the absorption length. By extrapolating to zero light intensity such a straight line, the diffusion length was obtained as the intercept with thex-axis. For good accuracy, a least-square fit method was employed to calculate the diffusion length values. The agreement of measurements performed on some Schottky diodes prepared in different ways and displaying different spectral responses confirmed a near independence of the SPV technique on some material parameters and experimental conditions.

Ionicity and the theory of Schottky barriers

We have investigated the role of ionicity in metal-semiconductor Schottky barriers by examining interfaces of increasing semiconductor ionicity. The electronic structure of four separate interfaces consisting of jellium (of A1 density) in contact with the (111) surface of Si and the (110) surfaces of GaAs, ZnSe, and ZnS is investigated through the use of a self-consistent pseudopotential method. The barrier height and the surface density of states in the semiconductor band gap are determined. The phenomenological index of interface behavior $S$ (studied by Kurtin, McGill, and Mead for semiconductors of different ionicity) is discussed in terms of a simple model involving metal-induced states in the semiconductor gap.