Untersuchung von Raumladungsschichten an Zinkoxid-Oberflächen mit Hilfe der Elektroreflexion

Using the electroreflectance method space charge layers on crystals of different conductivities have been identified. The space charge layers were formed by adsorption of oxygen or atomic hydrogen. The limit of sensitivity required the irradiation with 5×10¹³ photons/cm²×sec of band gap energy. After exposure to atomic hydrogen all samples showed accumulation layers. With a partial pressure of oxygen above 350 mm Hg crystals of high conductivity (σ=47 ohm^?1 cm^?1) exhibit depletion layers, which change into accumulation layers, if the partial pressure is reduced below the limit. Crystals of a lower conductivity (σ=10^?3–10^?1 ohm^?1 cm^?1) show accumulation layers up to the highest applied oxygen pressure of 760 mm Hg. The phenomena are attributed to a dynamical equilibrium between adsorption and photo-desorption of oxygen. This equilibrium depends on oxygen pressure and free carrier concentration. By comparing a calculated curve with the experimental results the value of 3.31 ev is obtained for the energy gap, light polarized perpendicular to thec-acis.     

On the field penetration into semiconductors in the field ion microscope

The calculation of field penetration in semiconductors and consequent band bending during field ionization/evaporation is discussed. The shielding by surface states is also taken into account. The Si(111) face example demonstrates that neglection of surface states may give unrealistic high band bending values. Because of the lack of reliable data for the density of surface states, a possible maximal band bending has been calculated for GaAs. Its value in the case of an external applied field of $\text{1}\text{V}\text{A}\text{?}$ may be such smaller as formerly assumed in recent works.     

New experimental results for electron transport in weak accumulation layers of ZnO crystals

Previous Hall measurements on (0001) and (0001&#x0304;) faces of ZnO have shown a Hall mobility oscillating as a function of Hall surface electron density in the range between N_SH=10⁶ and 10¹¹ cm^?2. Here we report on new results obtained by a field effect arrangement for free surfaces in UHV. With donors from H exposure or by illumination weak accumulation layers (n_sh <10¹¹ cm^?2) are established. The field effect shows oscillations in surface conductivity as a function of gate voltage. Also the combination of a field effect with a Hall effect measurement reveals discrete values of Hall surface electron density n_sh. Various pretreatments do not change the periodicity of these oscillations. Necessary preconditions are a temperature below 130 K, a Hall surface electron density below 3 × 10¹² cm^?2 and a source-drain field of a few V/cm. A model regarding impurity levels in the space charge layer relates the results of the field effect measurements to the results of the Hall effect measurements.     

A parametric study of the surface potential and band bending in silicon

A unified approach to band bending is described, and the macroscopic electronic potential through the silicon surface is calculated as a function of temperature in the ranges 300–500°K and 100–1600°K, externally applied electric field, for zero field and for 10³ to 10⁵ V/cm, and donor and acceptor concentrations, from 10¹² to 10¹⁸ cm^?3. The results, presented in graphical and tabular forms, are intended to serve the convenience of researchers in a wide area of surface and high temperature silicon physics such as in thermionic, field, and photoelectric emission work and in high temperature, and field modulated transport studies. The calculations are based on an essentially classical approach to the solution of the electrostatic band bending problem, using the surface state density for silicon proposed by Allen and Gobeli on the basis of their photoelectric investigations. The cases considered were limited to nondegenerate, intrinsic, and n- and p-doped silicon in which all impurity states are fully ionized.     

Photovoltaic properties of a heterojunction based on copper phthalocyanine films on the surface of polycrystalline cadmium sulfide

The photovoltaic effect has been detected and studied in thin-film structures based on thermally deposited 200-nm-thick copper phthalocyanine (CuPc) films on the surface of polycrystalline CdS. The structures under study demonstrate the linear current-voltage characteristics at external electric fields to 3.5 × 10⁴ V/cm. Two components of the photovoltage of different signs have been revealed when the sample is illuminated in the wavelength range from 350 to 700 nm. The first component has the positive sign on the CuPc film side and is observed when using the radiation with a wavelength lesser than 500 nm, i.e., in conditions of predominant absorption of the radiation in the CdS layer. The second component has the negative sign on the CuPc film side and is observed when using the radiation with a wavelength in the range from 500 to 570 nm, corresponding to the spectral region of the absorption edge of the CuPc films. The dependences of the photovoltage on the radiation intensity studied in the range from 5 × 10¹² to 10¹⁴ photons cm^?2 s^?1 are different in the cases of the two detected components. Mechanisms of generation of the photovoltage components associated with a change in the band bending during photogeneration of charge carriers in the region of space charge in CdS and a change in conditions of the charge transfer in the interfacial CuPc/CdS region during the radiation absorption in the CuPc film have been proposed.     

Anisotropic charge transport in flavonoids as organic semiconductors

A quantum mechanical approach has been used to investigate on the potential for using two naturally occurring flavonoids: quercetin and luteolin as candidates for organic semiconductor. Selection of flavonoids enables to evaluate the effects of hydroxyl group structural features. The relationship between molecular packing and charge transport in flavonoids is presented. The calculated results indicate that quercetin should be an ideal candidate as high-performance p-type organic semiconductor material, while luteolin is predicted as n-type organic semiconductor material. The predicted maximum electron mobility value of quercetin is 0.075 cm² V^?1 s^?1, which appears at the orientation angle near 91°/271° of conducting channel on the reference planes b–c. Theoretical investigation of natural semiconductors is helpful for designing higher performance electronic materials used in biochemical and industrial field to replace expensive and rare organic materials.     

Quantum properties of strong accumulation layers on ZnO surfaces

Extremely strong accumulation layers with surface electron densities ΔN approaching 10¹⁴ cm^?1 have been achieved on ZnO surfaces in contact with an electrolyte. Quantization effects, which are very pronounced in such narrow (?10 Å) layers, are studied by measurements of ΔN versus surface barrier height V_s. Comparison of the results with self-consistent calculations shows very good agreement up to ΔN = 2 × 10¹³ cm^?2. Deviations observed at higher ΔN are probably associated with the huge electric fields (～10⁷ V/cm) experienced by the surface electrons.     

Charge transfer over localized states in a TlS single crystal

It is revealed that TlS single crystals exhibit a variable range hopping conduction along a normal to their natural layers at temperatures T ≤ 230 K in a dc electric field and a nonactivated hopping conduction at low temperatures in strong electric fields. Estimates are made for the density of states near the Fermi level (N _F = 2.8 × 10²⁰ eV^?1 cm^?3 and their energy spread (ΔW = 0.02 eV), the localization radius (a = 33 Å), the average jump distance in the region of activated (R _av(T) = 40 Å) and nonactivated (R _av(F) = 78 Å) hopping conduction, and also the drop in the charge carrier potential energy along the jump distance in an electric field F: eFR = 0.006 and 0.009 eV at F = 7.50 × 10³ and 1.25 × 10⁴ V/cm, respectively.     

Electron spectrum and intrinsic absorption of heavily doped n-GaAs

The article presents results of a numerical calculation, made with a computer, of several parameters and functions characterizing the electron spectrum of heavily doped semiconductors; in addition, the article presents theoretically calculated and experimentally obtained dispersion curves of the absorption coefficient in the region of interband transitions in heavily doped n-GaAs. The numerical calculation method is based upon the work of V. L. Bonch-Bruevich, who used Green functions. Calculations and experimental measurements were made for n-GaAs with free electron concentrations of 4.8·10¹⁸ cm^?3 at temperatures of 10–610?K. Agreement between the theoretically calculated and the experimentally obtained values of the absorption coefficient is observed in the edge region at photon energies exceeding the width of the forbidden band of the pure semiconductor.     

Analysis of the charge transfer mechanisms responsible for the current-voltage characteristics of Ca<Subscript>4</Subscript>Ga<Subscript>2</Subscript>S<Subscript>7</Subscript>: Eu<Superscript>3+</Superscript> single crystals

The current-voltage characteristics of Ca₄Ga₂S₇: Eu³⁺ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca₄Ga₂S₇: Eu³⁺ single crystals are high-resistance semiconductors with a resistivity of ～10⁹ Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×10¹⁴ to 2.7×10¹⁵ cm^?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×10³ V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×10⁴ V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level.     

Effect of annealing temperature on optical and electrochemical properties of chitosan–ZnO nanostructure

Chitosan–ZnO nanostructures were prepared by chemical precipitation method using different concentration of zinc chloride and sodium hydroxide solutions. Nanorod-shaped grains with hexagonal structure for samples annealed at 300 °C and porous structure with amorphous morphology for samples annealed at 600 °C were revealed in SEM analysis. X-ray diffraction patterns confirmed the hexagonal phase ZnO with crystallite size found to be in the range of ~24.15–34.83 nm. Blue shift of UV–Vis absorption shows formation of nanocrystals/nanorods of ZnO with marginal increase in band gap. Photoluminescence spectra show that blue–green emission band at 380–580 nm. The chitosan–ZnO nanostructures used on surface of a glassy carbon electrode gives the oxidation peak potential at ~0.6 V. The electrical conductivity of chitosan–ZnO composites were observed at 2.1?×?10^?5 to 2.85?×?10^?5?S/m. The nanorods with high surface area and nontoxicity nature of chitosan–ZnO nanostructures observed in samples annealed at 300 °C were suitable as a potential material for biosensing.     

Band alignment in SiC-based one-dimensional van der Waals homojunctions

The density functional theory method is utilized to verify the electronic structures of SiC nanotubes (SiCNTs) and SiC nanoribbons (SiCNRs) one-dimensional (1D) van der Waals homojunctions (vdWh) under an applied axial strain and an external electric field. According to the calculated results, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-II band alignment and robust electronic structures with different diameters or widths. Furthermore, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-I band alignment, respectively, in a range of[-0.3, -0.1] V/Å and[0.1, 0.3] V/Å and change into metal when the electric field intensity is equal to or higher than 0.4 V/Å. Interestingly, the SiCNTs/SiCNRs 1D vdWhs have robust electronic structures under axial strain. These findings demonstrate theoretically that the SiCNTs/SiCNRs 1D vdWhs can be employed in nanoelectronics devices.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Evaluation of NaFeTiO<Subscript>4</Subscript> as an electrode for energy storage application

We report a polycrystalline NaFeTiO₄ prepared via conventional solid-state reaction route. X-ray diffraction (XRD) results and Rietveld refinement confirmed single-phase NaFeTiO₄ having an orthorhombic unit cell with lattice parameters a = 9.17051 Å, b = 2.96310 Å, and c = 10.73676 Å and Pnma space group (No. 62). Energy dispersive spectrum (EDS) yielded sample stoichiometry that agrees well with its molecular formula. The surface morphology indicated a cylindrical rod-like microstructure comprising well-defined grains having variable dimension, i.e., diameter ~?250 to 350 nm and length ~?1 to 5 μm. Vibrational spectroscopy (FTIR/Raman) results indicated presence of FeO₆ and TiO₆ octahedra in good agreement with crystallographic study. Brunner-Emmet-Teller (BET) surface area measurement yielded a specific surface area as high as ~?4.28 m² g^?1. Electrical impedance spectrum indicated presence of grains separated by well-defined grain boundaries in agreement with microstructural analysis. Electrical conductivity of the material was estimated to be ~?6.05 × 10^?6 S cm^?1. The structural model obtained using XRD and vibrational spectrum results suggest layered tunnel/cage structure of cage dimension ~?4.65 Å, along [010] direction in the xz plane, which is larger than the size of Na⁺ ion (0.98 Å). So, easier Na⁺ migration feasibility exists in NaFeTiO₄ crystal lattice making it a good candidate for electrode applications.     

Structure and electrophysical properties of ferrocobaltites <Emphasis Type="Italic">Ln</Emphasis>BaFeCoO<Subscript>5 + δ</Subscript> (<Emphasis Type="Italic">Ln</Emphasis> = Tb,Dy, Ho,Y)

The ferrocobaltites LnBaFeCoO^{5 + δ} (Ln = Tb, Dy, Ho, Y) have been synthesized, and the parameters of their crystal structure have been determined. The thermal expansion, electrical resistivity ρ, and thermopower S of the synthesized compounds have been investigated in air at temperatures in the range from 300 to 1100 K. The compounds have a tetragonal structure (symmetry space group P4/mmm) with the unit cell parameters a = 3.9000 Å and c = 7.5922 Å (Ln = Tb, δ = 0.31), a = 3.8973 Å and c = 7.5679 Å (Ln = Dy, δ = 0.34), a = 3.8970 Å and c = 7.5507 Å (Ln = Ho, δ = 0.28), and a = 3.9029 Å and c = 7.5538 Å (Ln = Y, δ = 0.25). The ferrocobaltites under investigation are p-type semiconductors, and their electrical resistivity ρ and thermopower S decrease in the sequence Tb → Ho → Y → Dy (at room temperature). The linear thermal expansion coefficient of the LnBaFeCoO^{5 + δ} phases in the vicinity of the temperatures ranging from 465 to 535 K increases from (1.15?1.23) × 10^?5 to (1.73?1.93) × 10^?5 K^?1. The parameters of charge transfer in these ferrocobaltites have been determined. It has been found that an increase in the temperature leads to an increase in the excitation energy of charge carriers and a decrease in the activation energy of charge carrier transfer.     

Sur la contribution des differents porteurs à la zone de charge d'espace d'un semiconducteur multivallée

We study, in the case of the parabolic approximation, the space charge of a two-valley conduction band semiconductor; the electrons associated to these valleys are supposed to have different effective masses. By using a non-degenerate distribution we analyse the contribution of the different carriers to the space charge and the effect of surface band bending, temperature, effective masses, dope and type of material. We point out that the space charge can be governed by either type of carrier or both. By comparing the results with those given by a single valley model, we show the incidence of the initial hypothesis on the value and evolution of the space charge with temperature and surface band bending. As an exemple, we study briefly n-type GaSb.     

Density of states of a two-dimensional electron gas measured by high-resolution photoelectron spectroscopy

We present high energy-resolution photoemission measurements of the spectral density at the discrete quantized electronic levels of a two-dimensional (2D) electron gas. The dynamical 2D electron gas has been obtained by generating a strong accumulation layer at the (110) surface of narrow-gap III–V semiconductors. Exploitation of a number of cases generating band bending (metallic chains or clusters, atomic structure, defects) demonstrates the generality of 2D electron gas formation at charge-accumulated semiconductor surfaces. A self-consistent solution of the Poisson and Schrödinger equations gives the potential well shape, the sub-band energy level position and the accumulated charge density, in excellent agreement with the present experimental data.     

III–V semiconductor interface properties as a knowledge basis for modern heterostructure devices

Some examples of interface studies are reported which show their close link with progress in III–V modern semiconductor device physics and technology. The surface electronic properties investigated in-situ by reflectance anisotropy spectroscopy during InGaP/InP growth (metal-organic vapor-phase epitaxy) are essential for the control of ordering phenomena in these layers, which is relevant for high-performance optoelectronic devices. Studies of electronic interface states at metal/narrow-gap III–V semiconductors are presented, which enabled the successful preparation of semiconductor/superconductor hybrid devices. For group-III nitrides with wurtzite structure the presence of fixed polarization interface charges yields new challenges in order to understand and control Schottky-barrier heights, band offsets and 2D confinement in heterostructure field-effect transistors. Received: 26 April 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Li+ transportation kinetics of FeF3 · 0.33H2O/C nanocomposite synthesized by one-step solid state method

A new cathode material for lithium ion battery FeF₃?·?0.33H₂O/C was synthesized successfully by a simple one-step chemico-mechanical method. It showed a noticeable initial discharge capacity of 233.9 mAh g^?1 and corresponding charge capacity of 186.4 mAh g^?1. A reversible capacity of ca.157.4 mAh g^?1 at 20 mA g^?1 can be obtained after 50 charge/discharge cycles. To elucidate the lithium ion transportation in the cathode material, the methods of electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were applied to obtain the lithium diffusion coefficients of the material. Within the voltage level of 2.05–3.18 V, the method of EIS showed that \( {D}_{{\mathrm{Li}}^{+}} \) varied in the range of 1.2?×?10^?13?~?3.6?×?10^?14 cm² s^?1 with a maximum of 1.2?×?10^?13 cm² s^?1 at 2.5 V. The method of GITT gave a result of 8.1?×?10^?14?~?1.2?×?10^?15 cm² s^?1. The way and the range of the variation for lithium ion diffusion coefficients measured by the GITT method show close similarity with those obtained by the EIS method. Besides, they both reached their maximum at a voltage level of 2.5 V.