Showing papers on "Schottky barrier published in 1987"

Tin dioxide gas sensors. Part 1.—Aspects of the surface chemistry revealed by electrical conductance variations

Abstract: Detailed measurements are reported of the dependence of the conductance of porous, sintered pellets of tin dioxide, on temperature, moisture and oxygen partial pressure. Comparison is made with the behaviour of thin layers prepared by radio-frequency sputtering. The effect of introducing a non-equilibrium mixture of a combustible gas in air, on the conductance and conductance activation energy is described. The theoretical basis for current interpretations of the behaviour of this material is reviewed. The results are discussed using a model in which the conductance of the porous pellets is controlled by different sorts of intergrain contact, represented as necks, necks depleted of conduction electrons, and Schottky barriers. The results are best rationalised by postulating that the conductance is in fact controlled by Schottky barriers separating domains or agglomerates, each comprising a rather large number of crystallites. The effects of temperature, moisture, oxygen partial pressure and combustible gases are discussed in terms of their effect on the Schottky barrier height by way of altrations in the area density, charge and occupancy of surface states (chemisorbed oxygen species). The existence of a surface state level located ≳ 1.1 eV below the conduction band edge is deduced. Adsorbed water strongly affects both the conductance and the response to combustible gases. Loss of water from the surface over the temperature range 280–450 °C results in a sigmoidal conductance-temperature relationship in moist air. The effect is to lower the resistance at temperatures below this range to no more than one-tenth of the values observed for dried pellets. A surface transformation O2–↔ OH– is inferred. In dried air, with dried pellets, inflexions in the conductance-temperature behaviour at temperatures below 230 °C are tentatively attributed to the effect of O2–↔ O–↔ O–2 surface transformations. The onset of a conductance response to the presence of a combustible gas coincides with the onset of a surface-catalysed combustion. At higher temperatures an effect of combustible gases is to lower the conductance activation energy for the porous pellets, but not for the sputtered layers. The effect of moisture on the response to CO was to extend the response to lower temperature; on dried pellets in dried air the response disappeared abruptly when the temperature fell below 350 °C.

Role of virtual gap states and defects in metal-semiconductor contacts

Abstract: Chemical trends of barrier heights reported for metal- and silicide-silicon contacts are analyzed. The data are easily explained when both virtual gap states of the complex band structure of the semiconductor and electronic levels of defects created in the semiconductor close to the interface during its formation are considered. The virtual gap states determine the barrier heights when either the defect density is low or the defects are completely charged or all neutral.

High-temperature point-contact transistors and Schottky diodes formed on synthetic boron-doped diamond

Abstract: Point-contact transistors and Schottky diodes have been formed on synthetic boron-doped diamond. This is the first report of diamond transistors that have power gain. Further, the transistors exhibited power gain at 510°C and the Schottky diodes were operational at 700°C.

Schottky-barrier formation on a covalent semiconductor without Fermi-level pinning: The metal-MoS2(0001) interface.

Abstract: Chemical interaction and Schottky-barrier formation at the metal-${\mathrm{MoS}}_{2}$ interface were studied by evaporating metals (Ag, Al, Au, Co, Fe, In, Mn, Pd, Rh, Ti, and V) onto the (0001) basal-plane surface of cleaved molybdenite, and then analyzing the interface with x-ray photoelectron spectroscopy (XPS). Except for Mn, negligible changes were revealed in the Mo (3${d}_{5/2}$) and S (2${p}_{3/2}$) peak shapes, or widths, after deposition. The shifts in the binding energies did not correlate with the electron configuration of the metal but rather with the metal electronegativity, and are interpreted in terms of band bending at the metal-semiconductor interface, rather than chemical reaction. Plots of both Mo and S binding energies versus metal electronegativity yield approximately linear curves with nonzero (positive) slopes, which provide an average ``index of interface behavior'' of S'=1.28\ifmmode\pm\else\textpm\fi{}0.22. This value is considerably higher than for other covalent semiconductors, which exhibit S'0.3 due to Fermi-level pinning. The anomalous behavior of ${\mathrm{MoS}}_{2}$ results from the extreme inertness of the basal-plane surface and the stability of the layered crystal lattice of ${\mathrm{MoS}}_{2}$. The absence of chemical interaction at the interface causes the formation of a Schottky barrier exhibiting behavior that may approach the Schottky limit. This behavior for ${\mathrm{MoS}}_{2}$(0001) is compared with that of other semiconductors, and is discussed in terms of their ionicity, reactivity, and dielectric response. Metals (Ag, Au, Co, Cu, Fe, Pd, Ti, and V) were also deposited onto ${\mathrm{MoS}}_{2}$(0001) surfaces that were bombarded with 10-keV ${\mathrm{Ar}}^{+}$ ions. The Fermi level was not strongly pinned, even though defect densities as high as \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ were produced in the surface region.

A simple interfacial-layer model for the nonideal I-V and C-V characteristics of the Schottky-barrier diode

Abstract: An interfacial-layer theory considering the nonequilibrium generation-recombination mechanism of the real interface states for Schottky-barrier diodes is presented. It is shown in the theory that the soft behavior of the reverse I - V characteristics associated with the observed nearly ideal C −2 - V characteristics can be modeled in terms of the interfacial-layer capacitance in a consistent manner, and that the nonideality of the forward I - V as well as C −2 - V characteristics should be attributed to the transition from the metal—controlled to semiconductor (majority carrier)—controlled occupancy of the interface states. The experimental verification for the theory is done by comparing the density distribution of the interface states in the semiconductor bandgap extracted from the nonideal forward I – V characteristics of the fabricated AlpSi and AlnSi Schottky barrier diodes with those directly measured by the multifrequency admittance methods. Excellent agreement from these comparisons strongly supports the physical validity of our theory.

Mechanism of Schottky barrier formation: The role of amphoteric native defects

Abstract: A correlation between the Fermi level pinning deduced from Schottky barrier heights and from electrical properties of irradiated III–V semiconductors is found. The correlation indicates that similar defects are responsible for the Fermi level stabilization in both cases. It is proposed that amphoteric native defects, i.e., the defects which change their electrical characteristics depending on the Fermi level position, play a dominant role in the processes leading to a Schottky barrier formation. A detailed analysis of metal–GaAs contacts shows that in this case the amphoteric defects responsible for the barrier heights are V Ga (acceptor) and a donor complex AsGa+V As. It is shown that for thick metal coverages two barriers are formed. A surface barrier determined by the charge associated with a native defect and the bulk barrier controlled by the bulk doping. The sum of the two barrier heights satisfies the Schottky condition for the interface, but it is the bulk barrier that determines the macroscopic electrical properties of the contact. The model explains the evolution of the Fermi level position at the interface observed for metal coverages varying in a broad range of thicknesses. The relationship of the present proposal to previous models of Schottky barriers is discussed.

Initial stages of Schottky barrier formation: Temperature effects

Abstract: Recent experiments on the formation of Schottky barriers reveal a temperature-dependent pinning of the Fermi level (E F ) which cannot be explained in terms of the current published models. Al, Au, Ag, In, and Sn are evaporated on room temperature (RT) and low temperature (LT= 80 K) cleaved n- and p-GaAs(110). We find a substantial decrease in surface metal clustering at LT, leading to more homogeneous films, especially with Al, In, and Sn. Interface mixing and chemical reaction are also partly inhibited. Cancellation of the semiconductor surface relaxation is observed for Al and Sn. From the electronic point of view, low temperature has opposite effects on the pinning rate of E F on n- andp-GaAs. Pinning on n-GaAs as a function of coverage is dramatically retarded. Pinning on p-GaAs remains faster than on n-GaAs, as fast as at RT, faster for Al and In. These asymmetric rates make it impossible to explain the initial stages of Schottky barrier formation for all substrates with a single mechanism. They suggest that several independent but concomitant mechanisms might be at work in the pinning process.

Method of manufacturing a field effect transistor device having a multilayer gate electrode

Abstract: This Schottky barrier gate field effect transistor has N + -type source and drain regions formed in the surface area of a GaAs semi-insulation substrate, a channel region formed between the source and drain regions, and a gate electrode formed on this channel region. Particularly, in this Schottky barrier gate field effect transistor, the gate electrode has a first metal portion, which is preferably in Schottky contact with the channel region, and a second metal portion, which stably affixes to the first metal portion. The first and second metal portions are fixed to an insulative portion formed on the channel region.

Manufacture of semiconductor device

Abstract: PURPOSE:To realize a quality MESFET provided with a source, a gate, and a drain with the gaps between them constant and smaller and provided with a gate electrode enhanced in thickness by a method wherein a trench is provided in the surface of a semiconductor layer and, in the trench, a Schottky junction is formed between a gate electrode buried in the trench and the semiconductor layer for the construction of a MESFET. CONSTITUTION:On the semi-insulating GaAs substrate 11, an N-type layer 12 is epitaxially grown. Onto the entire surface of such a wafer, an AuGe layer and then an ohmic metal layer 13 are attached. Heat treatment is accomplished, which is followed by patterning whereby a 2.5mum gap is provided between a source and a drain. The ohmic metal layer 13 and the N-type layer 12 are then subjected to ion beam etching. A plasma SiO2 film 14 is provided and subjected to etching which is accomplished vertical to the substrate surface. Next, three layers of Ti, Pt, and Au are laid on the entire surface, in that order. Ion beam etching is accomplished at 30 deg. to the direction vertical to the substrate surface, after which Au and Pt are retained only at a gate section 15. Etching follows, after which Ti is retained only under the gate section 15 for the completion of a three-layer Schottky gate electrode.

Particulate semiconductor devices and methods

Abstract: Particulate semiconductor devices and method of preparation by a low temperature process. A particulate layer is screen printed on a metallized substrate and a rear contact is formed by alloying the semiconductor particules to the substrate. The layer is fired and a front Schottky contact applied. The resulting device has sharp diode IV-characteristics, low leakage current, and significant reverse break-down voltages. In the manufacture of efficient and red-enhanced particulate silicon pn-junction solar cells, prediffused particles are used, offering major advantages compared to other techniques, such as where the junction is formed after completion of a particulate layer.

Transition-metal impurities in semiconductors-their connection with band lineups and Schottky barriers.

Abstract: The dependence of cation-substitutional transition-metal impurity levels upon the host semiconductor is calculated self-consistently in a defect-molecule approach. Heterojunction band lineups and Schottky-barrier heights can be calculated within the same tight-binding model. In all three systems, the characteristic behavior is determined by an approximate local charge-neutrality condition imposed by electrostatic self-consistency ("pinning"), with a dangling-bond level playing the role of the "neutrality level." In this way we explain the observed correlation between transition-metal impurity levels, heterojunction band lineups, and Schottky barriers.

Thin film transistor

Abstract: PURPOSE:To obtain a vertical TFT which is made of amorphous semiconductor and is operated with a high speed and has a high ON/OFF ratio by a method wherein a gate electrode forms a Schottky junction with an amorphous semiconductor in the boundary in a channel part and forms an MIS junction with the amorphous semiconductor in the boundary in the part other than the channel part with an insulating film between. CONSTITUTION:A 1st main electrode 20 formed on an insulating substrate 10, amorphous semiconductor layers 30 and 70 formed on the electrode 20, 2nd main electrodes 90 selectively formed on them, gate electrodes 50 formed in the amorphous semiconductor layers 30 and 70 and insulating films 40 and 60 which are formed on the gate electrodes 50 on at least one of the 1st electrode 20 side and the 2nd electrode 90 side are provided. The gate electrode 50 forms Schottky junctions with the amorphous semiconductor 70 in the boundaries in channel parts in which carriers travel and, moreover, forms MIS junctions with the amorphous semiconductor layers 30 and 70 in the boundaries in the parts other than the channel parts. With this constitution, a leakage current into the gate electrode can be minimized so that a TFT with a high speed operation and a high ON/OFF ratio can be realized.

Engineered Schottky barrier diodes for the modification and control of Schottky barrier heights

Abstract: A technique for fabricating controlled Schottky barrier heights to GaAs over the entire band gap is demonstrated. Thin, highly doped semiconductor layers at the metal‐semiconductor interface allowed the reproducible control of the effective barrier height on n‐type GaAs from near zero (i.e., ohmic behavior at 300 K) to 1.33 eV (the band gap equals 1.43 eV at 300 K) with diode ideality factors 1.02≤n≤1.21. Molecular‐beam epitaxy was used to grow GaAs epitaxial layers with in situ deposited Al metal layers, resulting in diodes with nearly ideal electrical and structural characteristics. Electrical characterization by current‐voltage (I‐V) and capacitance‐voltage (C‐V) techniques, models for these I‐V and C‐V characteristics, and structural characterization by high resolution transmission electron microscopy lattice images are presented. Implications of this work for models of Schottky barrier formation are discussed, as well as some applications for these ‘‘engineered Schottky barrier diodes.’’

A simple technique for measuring the interface‐state density of the Schottky barrier diodes using the current‐voltage characteristics

Abstract: A simple technique for measuring the interface‐state density of a Schottky barrier diode fabricated on the moderately doped semiconductor is presented. It requires the I‐V characteristics and substrate doping concentration as inputs which can be easily measured by existing techniques. Checked by the Schottky capacitance spectroscopy method, the proposed technique is proved to be highly accurate for various metal‐Si and metal silicide‐Si Schottky barrier diodes.

Silicon waveguide with monolithically integrated Schottky barrier photodetector

Abstract: The invention comprises a Schottky barrier type infrared photodetector which is monolithically integrated on a silicon waveguide. A Schottky barrier contact is positioned directly on a silicon waveguide to absorb grazing incidence optical signals passing through the waveguide. The Schottky contact is operated in the avalanche or reverse bias mode to generate a useable electrical signal.

Hydrogenation of GaAs on Si: Effects on diode reverse leakage current

Abstract: Plasma hydrogenation for 3 h at 250 °C of GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition, followed by a 5‐min, 400 °C anneal to restore the passivated shallow donor electrical activity, increases the reverse breakdown voltage of Schottky diode structures from 2.5 to 6.5 V. This improvement appears to be a result of the passivation by atomic hydrogen of defects such as threading dislocations caused by the large (4%) lattice mismatch between GaAs and Si. A reduced Schottky barrier height is exhibited by hydrogenated samples, consistent with As depletion of the surface occurring during the long duration plasma processing.

Structure and optical properties of Ge‐Si ordered superlattices

Abstract: We report the synthesis, structural characterization, and optical studies of ultrathin Ge‐Si superlattices, grown by molecular beam epitaxy, on (001) silicon substrates. Structures consist of alternating layers of pure Ge and Si, with layer thicknesses of 1, 2, 4, and 6 monolayers. Using high‐resolution transmission electron microscopy, we provide direct observation of order in these pseudomorphic layered films. Systematic study of optical transitions by means of Schottky barrier electroreflectance reveals that each of the ordered structures displays a unique set of optical transitions. Of particular interest is the 4×4 structure which shows new, well defined optical transitions at 0.76, 1.25, and 2.31 eV. These transitions constitute the first observation of structurally induced optical transitions in Ge‐Si and may make the 4×4 structure suitable for optoelectronic devices.

Sputter-induced damage in Al/n-GaAs and Al/p-GaAs Schottky barriers

Abstract: Schottky barrier diodes were made by DC magnetron deposition and by thermal evaporation of Al on both n- and p-type GaAs substrates. Information about the electrical behaviour of these diodes was obtained from I-V, C-V and I-V-T measurements. It was found that for the diodes with a sputter-deposited metal contact a large density of donor-like defects is present in the near-surface region, together with a region of decreasing density deeper into the semiconductor. An out-annealing of these donor-type defects is observed at temperatures lower than 250 degrees C. The interaction between Al and GaAs, leading to the formation of AlxGa1-xAs, may be responsible for the observed changes in barrier heights and doping densities for the diodes annealed at temperatures higher than 250 degrees C.

Electron emitting element

Abstract: An electron emitting device is provided with an N type semiconductor disposed in contact with a first electrode. A P type semiconductor contacts the N type semiconductor to define a PN junction. A low work function metal electrode contacts the P type semiconductor thus defining a Schottky barrier. First and second means are provided to forward bias the PN junction and to reversed bias the Schottky barrier, respectively.

Evaluation of exciton diffusion lengths and apparent barrier widths for metal/porphyrin Schottky barrier cells by use of the optical filtering effect

Abstract: The optical filtering effect due to the porphyrin phase in the Al/porphyrin/Au (Schottky-type) photovoltaic cell is investigated in an attempt to estimate the exciton diffusion length L/sub exc/ and the apparent width of the Schottky barrier W/sub ap/. The observed filtering effects are interpreted in terms of a theoretical model in which the photogenerated charge carriers are attributed to the dissociation of excitons diffusing toward the Schottky barrier. The results show that the quantum efficiency of carrier generation from excitons is not constant over the porphyrin layer but exponential at the Schottky barrier. In other words, either ionized impurities (acceptors or donors) in the porphyrin film are distributed not uniformly but exponentially or the excitons dissociate into free carriers owing to interaction with exponentially distributed impurities in the barrier. The values of L/sub exc/ = 15 +/- 10 and 3 +/- 2 nm estimated for 5,10,15,20-tetraphenylporphyrin (H/sub 2/TPP) and its metalloporphyrin ZnTPP from the proposed model seem reasonable in comparison with the values of 24 and 13 nm, respectively, estimated by fluorescence spectroscopic measurements. In addition, the W/sub ap/ values of 39 +/- 2 and 14 +/- 1 nm obtained for H/sub 2/TPP and ZnTPP correspond fairly well to themore » depletion layer thickness determined by low-frequency capacitance measurements at zero bias.« less

Barrier heights and electrical properties of intimate metal‐AlGaAs junctions

Abstract: The dependence of the Schottky barrier height of Mo‐n:AlGaAs junctions, fabricated in situ by molecular beam epitaxy, on the Al mole fraction (x) was determined by internal photoemission measurements and by activation energy plots of the current versus voltage dependence on temperature. Both techniques yielded similar values. The difference in barrier height of Mo‐AlGaAs as a function of x, compared to that of Mo‐GaAs, was found to be equal to the conduction band discontinuity in AlGaAs‐GaAs heterojunctions for Al concentrations in the range 0≤x≤0.4. For x>0.4, values of the barrier heights were somewhat lower than values of the band discontinuity; however, both dependencies on x were quite similar. The temperature dependence of the current‐voltage characteristics showed that thermionic emission was the dominant transport mechanism at forward bias for temperatures higher than 250 K. At lower temperatures, current transport was governed by thermionic field emission.

Metal oxide semiconductor gated turn off thyristor including a schottky contact

Abstract: An MOS gate turn-off thyristor structure includes non-regenerative (three-semiconductor-layer) portions interspersed with four-semiconductor-layer regenerative (thyristor) portions, gate electrode segments disposed adjacent to relatively narrow portions of the base region within the regenerative portion, and either ohmic contacts or Schottky barrier contacts to the non-regenerative portions. Upon application of an appropriate turn-off gate bias to the gate electrode segments, the base region of the regenerative portion in which they are disposed is pinched off and the current flowing therethrough is derived to flow through the non-regenerative portion of the structure. This interrupts regeneration in the regenerative structure and the device turns off. Upon application of a high voltage of one polarity across a device of this type which includes the Schottky barrier contacts, the Schottky barrier maintains the same high stand-off voltage in the non-regenerative portion as the regenerative portion can hold off, thereby enabling the device to hold off high voltages of either polarity.

Method for fabrication of monolithic integrated circuits

Abstract: During fabrication of monolithic microwave integrated circuits, active devices having sources, gates, drains, and/or Schottky barrier junctions are first provided for an epitaxial layers. Then many layers of metals and oxides are produced thereover in situ without removing the circuit from its environmental chamber. Circuit elements are then defined by processing of the many layers sequentially by photolithography and other processes from the top of the chip downward. Certain combinations of metals, oxides, and processes are selected to enable fabrication of circuits from the top down in this way. This reduces inclusion of contaminating chemical films and particles between the desired layers. Lumped and distributed capacitors, resistors, inductors, transmission lines, contacts, and complete active devices are monolithically defined, with a reduced number of process steps. An all-refractory MESFET is described, having a Schottky barrier gate and nonalloyed ohmic contacts for source and drain producible at room temperatures. Source, gate, and drain can be defined with a single mask. A thinner gold layer is formed for FET contacts than for other circuit conductors and elements by means of a configured tantalum layer buried in a thick gold layer.

Operation of Schottky‐barrier field‐effect transistors of 3C‐SiC up to 400 °C

Abstract: Schottky‐barrier field‐effect transistors have been fabricated from 3C‐SiC and transistor operation has been studied at temperatures up to 400 °C. B‐doped high‐resistivity and undoped n‐type 3C‐SiC epilayers were successively grown on p‐type Si substrates by chemical vapor deposition. Au and Al electrodes were used for Schottky‐barrier gate contacts and source and drain contacts for n‐type 3C‐SiC, respectively. Transconductances of 1.7 and 0.15 mS/mm were obtained at room temperature and 400 °C, respectively.

Schottky barrier formation of various metals on n‐GaAs (100) by electrochemical deposition

Abstract: Electrochemistry is used to deposit various metals (Pt, Pd, Ni, Co) on n‐GaAs (100) surfaces to form Schottky barriers. For the first time very good rectifying characteristics, with an ideality factor n=1.04–1.08, are obtained, while the barrier heights match quite well those made under ultrahigh vacuum conditions. It is also shown that the possible disadvantages of this wet technique (oxidation, contamination,...), can be overcome provided that the conditions of deposition are carefully looked after: strong correlations between the electrical characteristics (studied by I–V and C–V measurements) and the interfacial morphology and composition (studied by Rutherford backscattering, nuclear reaction observation, and transmission electron microscopy) are evidenced when the electrodeposition conditions are varied for a given metal. The advantages of the method are also discussed within the framework of our results.

Electrical study of Schottky-barrier heights on atomically clean p-type InP(110) surfaces

Abstract: We report here a systematic study of the electrical properties of a large number of metal (Ag, Cr, Cu, Au, Pd, Mn, Al, Ni) on p-type InP diodes. Schottky-barrier diodes were fabricated by in situ metal deposition on atomically clean InP(110) surfaces in ultrahigh vacuum. Schottky-barrier heights were determined from current-voltage (I-V) and capacitance-voltage (C-V) measurements. A small, but finite, range in barrier heights (0.76--0.98 eV) is found for the metal--p-type InP systems investigated. When a comparison is made to our earlier work on n-type surfaces, we find that the interface Fermi level of n-type and p-type samples pins at the same position within the band gap for each of the metal InP systems studied. Our experimental results indicate that models that use metal-independent surface states (energy and density) and potential normalization conditions (i.e., natural band lineups) can predict the general trends in the interface Fermi-level pinning behavior. They cannot, however, successfully predict the details of this behavior to within measurement error. A theoretical method to determine the natural band lineups at the interface (using a scheme developed by Anderson) is presented within this context. Also investigated was the effect of air exposure on the electrical characteristics of diodes. For in situ I-V measurements, the metal semiconductor systems were characterized by a near-unity (1.03--1.10) ideality factor n. Upon exposure to air, a large increase in the current and ideality factor n was found for several (Cu, Au, Pd, Mn, Ni) metal--p-type InP systems at all measured biases. A detailed investigation of the Pd--p-type InP system indicated that the ``excess'' current pathway which results from exposure to air is at the periphery and can be eliminated by mesa etching.

Thermal stability and barrier height enhancement for refractory metal nitride contacts on GaAs

Abstract: Self‐aligned GaAs metal‐semiconductor field‐effect transistor process requires a very thermally stable gate material which must maintain good Schottky contact with GaAs after high‐temperature annealing. The electrical characteristics of rf‐sputtered ZrN, TiN, and NbN contacts on n‐GaAs substrate have been investigated as a function of annealing temperature. We show that all these refractory metal nitride contacts on GaAs have ideality factors very close to unity after annealing at temperatures as high as 850 °C. The barrier height for these contacts increases with annealing temperature and very low reverse leakage current is obtained. We also observe similar behavior from previous work on WN/GaAs contacts. Such barrier height enhancement at elevated temperatures has been attributed to the incorporation of nitrogen into GaAs near the metal/GaAs interface.

A subthreshold current model for GaAs MESFET's

Abstract: A GaAs MESFET model capable of accurately describing currents in the subthreshold region is described. The model is based on the concept of drain-induced barrier lowering (DIBL) together with the reverse-bias Schottky diode conduction. Agreement between measured and calculated data based on this model was excellent.