Au-TiB<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-<Emphasis Type="Italic">n</Emphasis>-6<Emphasis Type="Italic">H</Emphasis>-SiC Schottky barrier diodes: Specific features of charge transport in rectifying and nonrectifying contacts

Mechanism of charge transport in a diode of a silicon carbide’s Schottky barrier formed by a quasi-amorphous interstitial phase TiB _x on the surface of n-6H-SiC (0001) single crystals with an uncompensated donor (nitrogen) concentration of ～10¹⁸ cm^?3 and dislocation density of ～(10⁶–10⁸) cm^?2 has been studied. It is demonstrated that, at temperatures T ? 400 K, the charge transport is governed by the tunneling current along dislocations intersecting the space charge region. At T > 400 K, the mechanism of charge transport changes to a thermionic mechanism with a barrier height of ～0.64 eV and ideality factor close to 1.3.     

Scanning tunneling spectroscopy of a-C:H and a-C:(H, Cu) films prepared by magnetron sputtering

Scanning tunneling spectroscopy was used to study a-C:H and a-C:(H, Cu) films under atmospheric conditions; these films were formed on semiconductor (Si) and metallic (Cr/Si) substrates using dc magnetron sputtering of graphite or graphite/copper targets. The local density of electron states was determined from normalized differential tunneling conductance with the aim of probing the individual sp ²-phase clusters. The well-defined valence-band edge and the varying (i.e., dependent on the scanning coordinate) shape of the distribution of the density of electron states within the conduction band are characteristic of the a-C:H films; also, the largest experimental value of the band gap in these films is ～3 eV; finally, the tendency towards the stable position of the Fermi level at a level of ～1 eV above the valence-band top is observed in a-C:H films. The a-C:(H, Cu) films are homogeneous with respect to the local density of electron states, which is accounted for by the formation of a homogeneous surface layer in the course of growth.     

Direct tunneling of electrons in Al-n+-Si-SiO2-n-Si structures under conditions of nonstationary depletion of the semiconductor surface of the majority charge carriers

Specific features of direct tunneling of electrons through an ultrathin (∼40 ?) oxide in metal-SiO₂-Si structures under nonstationary conditions of depletion of the semiconductor surface, in which case the potential relief in the insulator is only slightly perturbed by external electric fields, have been experimentally studied. Penetrability of the tunneling barrier is appreciably limited by a classically forbidden region in n-Si (this region is brought about by fixed negative charge in SiO₂). As the voltage drop across oxide is increased, the electrons localized within this oxide transfer to the semiconductor, which is accompanied by a drastic increase in the tunneling current. The values of coefficients linear rise in the logarithm of tunneling current as the voltage at the isolator is increased are determined from the experiment. These values are not consistent with the data calculated on the basis of a model of a rectangular barrier with parameters typical of “thick” oxides. It is shown that actual values of the effective mass are bound to be larger than 0.5m ₀, while the height of the barrier is bound to be lower than 3.1 eV.     

Charge transport at the interface of <Emphasis Type="Italic">n</Emphasis>-GaAs (100) with an aqueous HCl solution: Electrochemical impedance spectroscopy study

Charge transport processes at the interface of n-GaAs (100) with an aqueous HCl solution are studied by electrochemical impedance spectroscopy. It is found that when open-circuit potential and anodic potentials are applied to the semiconductor the impedance spectra contain two capacitive semicircles corresponding to the capacitances of the space charge layer and surface states. In the case of open-circuit potential, semiconductor band bending at the interface with the solution is about 0.7 eV and the density of occupied surface states in the dark and under daylight conditions is 1.6 and 2.8 × 10¹² cm² eV⁻¹, respectively. When cathode potentials are applied to GaAs, hydrogen evolution begins at the semiconductor/electrolyte interface and an additional inductive loop appears in the impedance spectra. At the same time, the density of occupied surface states increases considerably both due to a straightening of the semiconductor bands and to the appearance of As-H bonds. Thus, charge transport through the n-GaAs (100)/aqueous HCl solution interface is always mediated by semiconductor surface states.     

Ohmic contacts to low-resistivity ZnS: Preparation,noise properties and nature of contact resistance

The results of simultaneous investigations of noise characteristics and contact resistance of different contacts to low-resistivity (ρ_b = 1–10 ohm cm) ZnS single crystals including so-called ohmic contacts with a contact resistance ρ_c = 0.1–0.5 ohm cm² are described and analyzed. A conclusion is drawn that the difficulties in obtaining ohmic contacts to low-resistivity ZnS are connected with the presence on the semiconductor contact surface of acceptor centers filled by electrons which are responsible for the increase of band bending at the semiconductor surface. In the case of ohmic contacts the contact barrier is lowered significantly owing to formation of the ionized donor centers in the contact region; electrons penetrate such barriers by tunneling.     

Investigation of electron properties of semiconductor surface by modulation spectroscopy of electroreflection

The relation of the Franz-Keldysh oscillations to electronic parameters of semiconductor materials is analyzed using the high-field measurement mode. The potential of using modulation spectroscopy of electroreflection for investigating electronic properties of a semiconductor surface is shown by the example of electroreflection spectra of n-GaAs (100) homoepitaxial films with the electron concentration of 10¹⁷–10¹⁸ cm^?3. The spectra are measured by the Schottky-barrier method at a room temperature using unpolarized light in the spectral range of 1.3–1.65 eV in the vicinity of the transition E ₀ (Γ_8V -Γ_6C ). From the quantitative analysis of electroreflection spectra, electronic parameters of films are obtained: the electron-transition energy E ₀, the electro-optical energy ??, the surface electric field F _s, the energy-relaxation time τ of charge carriers, the oscillation length λ_KF of the wave function of a quantum-mechanical particle with a reduced effective mass μ for a given surface electric field F _s, and the electron mobility μ_e.     

New mechanism of semiconductor polarization at the interface with an organic insulator

A semiconductor—organic-insulator system with spatially distributed charge is created with a uniquely low density of fast surface states (N _ss ) at the interface. A system with N _ss ≈ 5 × 10¹⁰ cm^–2 is obtained for the example of n-Ge and the physical characteristics of the interface are measured for this system with liquid and metal field electrodes. For a system with an organic insulator, the range of variation of the surface potential from enrichment of the space-charge region of the semiconductor to the inversion state is first obtained without changing the mechanism of interaction between the adsorbed layer and the semiconductor surface. The effect of enhanced polarization of the space-charge region of the semiconductor occurs due to a change in the spatial structure of mobile charge in the organic dielectric layer. The system developed in the study opens up technological opportunities for the formation of a new generation of electronic devices based on organic film structures and for experimental modeling of the electronic properties of biological membranes.     

<Emphasis Type="Italic">C-V</Emphasis> and <Emphasis Type="Italic">I-V</Emphasis> characteristics of ultrathin-oxide MOS structures: Identification and analysis

For an NMOS structure with 3.7-nm-thick oxide, dynamic I-V characteristics are digitally measured by applying an upward and a downward gate-voltage ramp. An averaging procedure is employed to deduce the tunneling (active) current component and the quasi-static C-V characteristic (CVC). Analyzing the depletion segment of the CVC provides reliable values of the semiconductor doping level, the oxide capacitance and thickness, and the sign and density of oxide-fixed charge, as well as estimates of the dopant concentration in the poly-Si region. These data are used to identify the Ψ_s(V _g), V _i(V _g), and I _t(V _i) characteristics, where Ψ_s is the n-Si surface potential, V _i is the voltage drop across the oxide, V _g is the gate voltage, and I _t is the tunneling current; the gate-voltage range explored extends to prebreakdown fields (～13 MV cm^?1). The results are obtained without recourse to fitting parameters and without making any assumptions as to the energy spectrum of electrons tunneling from the n-Si deep-accumulation region through the oxide. It is believed that experimental I _t-V _i and Ψ_s-V _g characteristics will provide a basis for developing a theory of tunneling covering not only the degeneracy and size quantization of the electron gas in the semiconductor but also the nonclassical profile of the potential barrier to electron tunneling associated with the oxide-fixed charge.     

The effect of damaging radiation (<Emphasis Type="Italic">p,e, γ</Emphasis>) on photovoltaic and tunneling GaAs and GaSb <Emphasis Type="Italic">p-n</Emphasis> junctions

The properties of multiple-junction solar cells depend on the properties of the constituent photovoltaic and tunneling p-n junctions. In this study, the properties of the space-charge region for photovoltaic and tunneling p-n junctions were examined using the dark current-voltage characteristics for two semiconductors: GaSb (a narrow-gap semiconductor) and GaAs (a wide-gap semiconductor). The effects of irradiation with protons (the energy of 6.78 MeV and the maximum fluence of 3 × 10¹² cm^?2), electrons (the energy of 1 MeV and the maximum fluence of 3 × 10¹⁶ cm^?2), and γ-ray photons (the energy of 1.17–1.33 MeV and the maximum dose of 17 Mrad) on the lifetime of charge carriers in the space-charge region of photovoltaic p-n junctions and on the peak current of connecting tunneling p-n junctions were studied. The coefficients of the damage for the inverse lifetime are determined for photovoltaic p-n junctions. The coefficients of equivalence between the used types of radiation are determined; these coefficients are found to be almost independent, on the order of magnitude, of the type and material of the p-n-junction (and nearly equal for photovoltaic GaAs p-n junctions and tunneling GaAs and GaSb p-n junctions).     

Electrical analysis of organic dye-based MIS Schottky contacts

In this work, we prepared metal/interlayer/semiconductor (MIS) diodes by coating of an organic film on p-Si substrate. Metal(Al)/interlayer(Orange GOG)/semiconductor(p-Si) MIS structure had a good rectifying behavior. By using the forward-bias I-V characteristics, the values of ideality factor (n) and barrier height (BH) for the Al/OG/p-Si MIS diode were obtained as 1.73 and 0.77 eV, respectively. It was seen that the BH value of 0.77 eV calculated for the Al/OG/p-Si MIS diode was significantly larger than the value of 0.50 eV of conventional Al/p-Si Schottky diodes. Modification of the potential barrier of Al/p-Si diode was achieved by using thin interlayer of the OG organic material. This was attributed to the fact that the OG organic interlayer increased the effective barrier height by influencing the space charge region of Si. The interface-state density of the MIS diode was found to vary from 2.79 × 10¹³ to 5.80 × 10¹² eV⁻¹ cm⁻².     

Capacitance of MOS diodes on substrates doped non-uniformly near the surface

The usual assumption of uniform doping in the space charge region no longer holds if the doping near the semiconductor surface is changed appreciably by thermal oxidation and/or ion implantation. Poisson's equation becomes the non-linear, but can be solved using suitable approximations for the dependence on the voltage. Thus the exact form of the doping profile is taken into account.The C?V curves of MOS diodes differ considerably from those for uniform doping due to the altered relation between the surface potential and the total charge of the semiconductors as well as due to the altered width of the space charge region. These two contributions are reduced to additive terms if the space charge region covers completely the region with altered doping.For weak depletion the slope of the C?V curves is larger for pile-down of acceptors and pile-up of donors in p-type silicon. The slope becomes smaller for increasing depletion, because the space charge region spreads now to more heavily doped regions of the semiconductor. For pile-up of donors and pile-down of acceptors in n-type silicon, the curves are initially less declined and become steeper according to the smaller doping in the bulk of the semiconductor.     

Mechanism of dislocation-governed charge transport in schottky diodes based on gallium nitride

A mechanism of charge transport in Au-TiB _x -n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 10⁷ cm^?2, which is close in magnitude to the dislocation density measured by X-ray diffractometry.     

Generation of surface electron states with a silicon–ultrathin-oxide interface under the field-induced damage of metal–oxide–semiconductor structures

The high-frequency capacitance–voltage characteristics of metal–oxide–semiconductor structures on n-Si substrates with an oxide thickness of 39 Å are studied upon being subjected to damage by field stress. It is shown that the action of a high, but pre-breakdown electric field on an ultrathin insulating layer brings about the formation of a large number of additional localized interface electron states with an energy level arranged at 0.14 eV below the conduction band of silicon. It is found that, as the field stress is increased, the recharging of newly formed centers provides the accumulation of excess charge up to 8 × 10¹² cm^–2 at the silicon–oxide interface. The lifetime of localized centers created under field stress is two days, after which the dependences of the charge localized at the semiconductor–insulator interface on the voltage at the gate after and before field stress are practically the same.     

Recombination effects in p-type silicon S.B.S.C'S

A model of a p-type Schottky Barrier Solar Cell with an insulating interfacial layer is described which takes into account a constant distribution of interfacial surface states in the energy gap of the semiconductor. The electron quasi-Fermi level is able to rise above the Fermi level in the metal when the device is illuminated. This allows for the imperfect communication between the metal and the semiconductors conduction band. Recombination in the semiconductor bulk and space charge regions are shown to be of great importance (for interfacial layer thicknesses 25 Å these recombination currents can dominate). The recombination through the surface states has been coupled with the tunneling of carriers between these interfacial surface states and the metal.The effects mentioned above play an important role for interfacial surface state densities 10¹¹ cm⁻² (eV)⁻¹. The results of the calculations made show that the I-V characteristics of the p-type Schottky Barrier Solar Cell are strongly controlled by the parameters of the interfacial layer, which in effect determine the extent of the recombination within the solar cell. The reduction V_i in the potential developed across the interfacial layer when the device is illuminated, is found to be negative when short-circuit conditions apply, and positive when the cell is operating at its maximum power point. This is a new effect which has not appeared previously in the literature.     

Modeling the charge transport mechanism in amorphous Al2O3 with multiphonon trap ionization effect

The charge transport mechanism in amorphous alumina, Al₂O₃, is investigated both theoretically and experimentally. We found that the experimental current-field-temperature dependencies can hardly be understood based on the commonly used Frenkel effect or the thermally-assisted tunneling model. Instead, we suggest that the charge transport in Al₂O₃ is related to the ionization of the deep trap by multiphonon tunneling. Excellent agreements between the predicted, the measured data were obtained by using the proposed multiphoton model with the following values of trapping parameters: thermal ionization energy of 1.5 eV, optical ionization energy of 3.0 eV, phonon energy of 0.05 eV, electron effective mass of 0.4m_e. The density of electron trap and electron capture cross-section of neutral traps are 2 × 10²⁰ cm⁻³ and 5 × 10⁻¹⁵ cm², respectively.     

Multiphonon capture of charge carriers by deep-level centers in a depletion region of a semiconductor

Multiphonon field-assisted thermal capture of thermally equilibrium charge carriers by deep-level centers located in a depletion region of a semiconductor is analyzed. It is shown that, in the case of strong electron-phonon coupling (SEPC), the multiphonon capture with preliminary tunneling of an electron through a potential barrier in the depletion region occurs with a lower rate as compared to the direct multiphonon capture in the electrically neutral bulk of the semiconductor, whereas, in the case of weak electron-phonon coupling (WEPC), the capture rate in the depletion region of a semiconductor may exceed that in the electrically neutral bulk by several orders of magnitude. The results of experimental study of capture processes in AlGaAs doped with silicon indicate that electron-phonon coupling is strong in DX centers.     

Model for Charge Accumulation in <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-MOS Transistors during Tunneling Electron Injection from a Gate

A quantitative model for charge accumulation in an undergate dielectric during tunneling electron injection from a gate according to the Fowler–Nordheim mechanism is developed. The model takes into account electron and hole capture at hydrogen-free and hydrogen-related traps as well as the generation of surface states during the interaction of holes with hydrogen-related centers. The experimental dependences of the threshold voltage shift and gate voltage shift of n- and p-channel MOS (metal–oxide–semiconductor) transistors on the injected charge in the constant current mode are analyzed based on the model.     

Potential barrier formation at a metal-semiconductor contact using selective removal of atoms

The possibility of forming a potential profile in a semiconductor by forming a metal film on its surface via selective removal of oxygen atoms from a deposited metal oxide layer was studied. Selective removal of atoms (SRA) was performed using a beam of accelerated protons with an energy of about 1 keV. Epitaxially grown GaAs films with a thickness of ～100 nm and an electron concentration of 2×10¹⁷ cm^?3 were chosen as the semiconductor material, and W obtained from WO₃ was used as the metal. The potential profile appeared due to the formation of a Schottky barrier at the metal-semiconductor interface. It was found that the Schottky barrier formed at W/GaAs contacts made by the SRA method is noticeably higher (～1 eV) than the barrier formed at the contacts made by conventional metal deposition (0.8 eV for W/GaAs). The data presented indicate that there is no damaged layer in the gate region of the structures, which is most strongly affected by the proton irradiation. Specifically, it was shown that the electron mobility in this region equals the mobility in bulk GaAs with the same doping level.     

Controlling of electronic parameters of GaAs Schottky diode by poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol) organic interlayer

The electronic properties of metal-organic semiconductor-inorganic semiconductor structure between GaAs and poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol) organic film have been investigated via current-voltage and capacitance-voltage methods. The Au/PEDOT/n-GaAs contact exhibits a rectification behavior with the barrier height of 0.69 eV and ideality factor value of 3.94. The barrier height of the studied diode (0.67 eV) is lower than that of Ni/n-GaAs/In (0.85 eV) and Au/n-GaAs/In Schottky diodes. The decrease in barrier height of Au/n-GaAs/In Schottky diode is likely to be due to the variation in the space charge region in the GaAs. The obtained results indicate that control of the interfacial potential barrier for metal/n-GaAs diode was achieved using thin interlayer of the poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol).