Showing papers on "Doping published in 2001"

Magnetic and electric properties of transition-metal-doped ZnO films

Abstract: 3d-transition-metal-doped ZnO films (n-type Zn1−xMxO (x=005–025): M=Co, Mn, Cr, Ni) are formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties are examined The Co-doped ZnO films showed the maximum solubility limit Some of the Co-doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than room temperature The magnetic properties of Co-doped ZnO films depend on the concentration of Co ions and carriers

Observation of ultraslow and stored light pulses in a solid.

Abstract: We report ultraslow group velocities of light in an optically dense crystal of Pr doped ${\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$. Light speeds as slow as 45 m/s were observed, corresponding to a group delay of 66 \ensuremath{\mu}s. Deceleration and ``stopping'' or trapping of the light pulse was also observed. These reductions of the group velocity are accomplished by using a sharp spectral feature in absorption and dispersion that is produced by resonance Raman excitation of a ground-state spin coherence.

Identification of electron donor states in N-doped carbon nanotubes

Abstract: Nitrogen-doped carbon nanotubes have been synthesized using pyrolysis and characterized by scanning tunneling spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular heterojunction devices.

High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties

Abstract: Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 297 eV and 371 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 325 eV Different magnetoresistance behavior was observed for the samples codoped with n-type carriers Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K

Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping

Abstract: TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Electrochemistry of silicon and its oxide

Abstract: 1: Basic Theories of Semiconductor Electrochemistry. 1.1. Introduction. 1.2. Energetics of Semiconductor/Electrolyte Interface. 1.3. Potential and Charge Distribution in Space Charge Layer. 1.4. Kinetics of Charge Transfer. 1.5. Photoeffects. 1.6. Open-Circuit Potential. 1.7. Experimental Techniques. 2: Silicon/Electrolyte Interface. 2.1. Basic Properties of Silicon. 2.2. Thermodynamic Stability in Aqueous Solutions. 2.3. Surface Adsorption. 2.4. Native Oxide. 2.5. Hydrophobic and Hydrophilic Surfaces. 2.6. Surface States. 2.7. Flatband Potentials. 2.8. Open-Circuit Potentials. 3: Anodic Oxide. 3.1. Introduction. 3.2. Types of Oxides. 3.3. Formation of Anodic Oxides. 3.4. Growth Mechanisms. 3.5. Properties. 4: Etching of Oxides. 4.1. Introduction. 4.2. General. 4.3. Thermal Oxide. 4.4. Quartz and Fused Silica. 4.5. Deposited Oxides. 4.6. Anodic Oxides. 4.7. Etching Mechanisms. 5: Anodic Behavior. 5.1. Introduction. 5.2. Current-Potential Relationship. 5.3. Photoeffect. 5.4. Effective Dissolution Valence. 5.5. Hydrogen Evolution. 5.6. Limiting Current. 5.7. Impedance of Interface Layers. 5.8. Tafel Slope and Distribution of Potential. 5.9. Passivation. 5.10. Current Oscillation. 5.11. Participation of Bands and Rate-Limiting Processes. 5.12. Reaction Mechanisms. 6: Cathodic Behavior and Redox Couples. 6.1. Introduction. 6.2. Hydrogen Evolution. 6.3. Metal Deposition. 6.4. Deposition of Silicon. 6.5. Redox Couples. 6.6. Open-Circuit Photovoltage. 6.7. Surface Modification. 7: Etching of Silicon. 7.1. Introduction. 7.2. General. 7.3. Fluoride Solutions. 7.4. Alkaline Solutions. 7.5. Etch Rate Reduction of Heavily Doped Materials. 7.6. Anisotropic Etching. 7.7. Surface Roughness. 7.8. Applications. 8: Porous Silicon. 8.1. Introduction. 8.2. Formation of Porous Silicon. 8.3. Morphology. 8.4. PS Formed at OCP. 8.5. PS Formed under Special Conditions. 8.6. Formation Mechanisms. 8.7. Properties and Applications. 9: Summaries and General Remarks. 9.1. Complexity. 9.2. Surface Condition. 9.3. Oxide Film. 9.4. Sensitivity to Curvature. 9.5. Sensitivity to Lattice Structure. 9.6. Relativity. 9.7. Future Research Interests.

Stabilization of Ferromagnetic States by Electron Doping in Fe-, Co- or Ni-Doped ZnO

Abstract: Detailed guidelines for controlling magnetic states in ZnO-based diluted magnetic semiconductors are given based on ab initio electronic structure calculations within the local spin density approximation using the Korringa-Kohn-Rostoker method. Effects of disorder were taken into account by the coherent potential approximation. It was found that the ferromagnetic state was stabilized by electron doping in the case of Fe-, Co- or Ni-doped ZnO. From the view point of practical applications, it is possible to realize a high-Curie-temperature ferromagnet, because n-type ZnO is easily available.

Insulating behavior for DNA molecules between nanoelectrodes at the 100 nm length scale.

Abstract: Electrical transport measurements are reported for double-stranded DNA molecules located between nanofabricated electrodes. We observe the absence of any electrical conduction through these DNA-based devices, both at the single-molecule level as well as for small bundles of DNA. We obtain a lower bound of 10 TΩ for the resistance of a DNA molecule at length scales larger than 40 nm. It is concluded that DNA is insulating. This conclusion is based on an extensive set of experiments in which we varied key parameters such as the base-pair sequence [mixed sequence and homogeneous poly(dG)⋅poly(dC)], length between contacts (40–500 nm), substrate (SiO2 or mica), electrode material (gold or platinum), and electrostatic doping fields. Discrepancies with other reports in the literature are discussed.

p-type conductivity in CuCr1−xMgxO2 films and powders

Abstract: CuCr1−xMgxO2, a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV.

Ion implantation into GaN

Abstract: The current status of ion beam processing of GaN is reviewed. In particular, we discuss the following aspects of ion implantation into GaN: (i) damage build-up and amorphization, (ii) preferential surface disordering and loss of nitrogen during ion bombardment, (iii) ion-beam-induced porosity of amorphous GaN due to material dissociation, (iv) anomalous surface erosion during ion bombardment at elevated temperatures, (v) the effect of implantation disorder on mechanical properties, (vi) current progress on annealing of implantation disorder, (vii) electrical and optical doping, and (viii) electrical isolation. Emphasis is given to current problems which may hinder a successful application of ion implantation in the fabrication of GaN-based devices.

Large remanent polarization of vanadium-doped Bi4Ti3O12

Abstract: Effects of vanadium doping on the ferroelectric properties of Bi4Ti3O12 were investigated using dense ceramics. The incorporation of vanadium resulted in a large remanent polarization (2Pr) of over 40 μC/cm2 without sacrificing other physical properties, and the polarization characteristics were shown to be superior to SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9. In addition, dense ceramics of vanadium-doped Bi4Ti3O12 could be obtained by sintering at temperatures 100–200 °C lower than those for the SrBi2Ta2O9 system.

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

Abstract: A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities For this purpose polybenzimidazole (PBI) membranes have been prepared and H3PO4-doped in a doping range from 300 to 1600 mol % Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level At 160 °C a conductivity as high as 013 S cm−1 is obtained for membranes of high doping levels Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide

Energetics of native defects in ZnO

Abstract: We have investigated the formation energies and electronic structure of native defects in ZnO by a first-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type conductivity of undoped ZnO.

Highly conductive epitaxial CdO thin films prepared by pulsed laser deposition

Abstract: Epitaxial growth of both pure and doped CdO thin films has been achieved on MgO (111) substrates using pulsed laser deposition. A maximum conductivity of 42 000 S/cm with mobility of 609 cm2/V s is achieved when the CdO epitaxial film is doped with 2.5% Sn. The pure CdO epitaxial film has a band gap of 2.4 eV. The band gap increases with doping and reaches a maximum of 2.87 eV when the doping level is 6.2%. Both grain boundary scattering and ionized impurity scattering are found to contribute to the mobility of CdO films.

Intrinsic limitations to the doping of wide-gap semiconductors

Abstract: Doping limits in semiconductors are discussed in terms of the amphoteric defect model (ADM). It is shown that the maximum free electron or hole concentration that can be achieved by doping is an intrinsic property of a given semiconductor and is fully determined by the location of the semiconductor band edges with respect to a common energy reference, the Fermi level stabilization energy. The ADM provides a simple phenomenological rule that explains experimentally observed trends in free carrier saturation in a variety of semiconductor materials and their alloys. The predictions of a large enhancement of the maximum electron concentration in III–N–V alloys have been recently confirmed by experiment.

Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure

Abstract: A transparent oxide semiconductor with delafossite structure, CuInO2, was found to exhibit both p-type and n-type conduction by doping of an appropriate impurity and tuning of proper film-deposition conditions. Thin films of Ca-doped or Sn-doped CuInO2 (optical band gap=∼3.9 eV) were prepared on α-Al2O3(001) single crystal substrates by pulsed laser deposition method. The films were deposited at 723 K in O2 atmosphere of 1.0 Pa for the Ca-doped films or 1.5 Pa for the Sn-doped films. The positive sign of the Seebeck coefficient demonstrated p-type conduction in the Ca-doped films, while the Seebeck coefficient of the Sn-doped films was negative indicating n-type conductivity. The electrical conductivities of Ca-doped and Sn-doped CuInO2 thin films were 2.8×10−3 S cm−1 and 3.8×10−3 S cm−1, respectively, at 300 K.

Electronic band structure of indium tin oxide and criteria for transparent conducting behavior

Abstract: Indium-based transparent conductors, notably indium tin oxide (ITO), have a wide range of applications due to a unique combination of visible light transparency and modest conductivity. A fundamental understanding of such an unusual combination of properties is strongly motivated by the great demand for materials with improved transparent conducting properties. Here we formulate conditions for transparent conducting behavior on the basis of the local density full-potential linear muffin-tin orbital electronic band structure calculations for Sn-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and available experimental data. We conclude that the position, dispersion, and character of the lowest conduction band are the key characteristics of the band structure responsible for its electro-optical properties. Further, we find that this lowest band is split with Sn doping due to the strong hybridization with dopant s-type states and this splitting contributes to both the decrease of the plasma frequency and the mobility of the carriers.

Surface voltage and surface photovoltage: history, theory and applications

Abstract: Surface voltage and surface photovoltage measurements have become important semiconductor characterization tools, largely because of the availability of commercial equipment and the contactless nature of the measurements. The range of the basic technique has been expanded through the addition of corona charge. The combination of surface charge and illumination allows surface voltage, surface barrier height, flatband voltage, oxide thickness, oxide charge density, interface trap density, mobile charge density, oxide integrity, minority carrier diffusion length, generation lifetime, recombination lifetime and doping density to be determined. In this review I shall briefly review the history of surface voltage, then discuss the principles of the technique and give some examples and applications.

Material Design of GaN-Based Ferromagnetic Diluted Magnetic Semiconductors

Abstract: Material design of GaN-based ferromagnetic diluted magnetic semiconductors is given based on ab initio calculations within the local spin density approximation. The electronic structure of 3d-transition-metal-atom-doped GaN was calculated by the Korringa-Kohn-Rostoker method combined with the coherent potential approximation. It was found that the ferromagnetic ground states were readily achievable in V-, Cr- or Mn-doped GaN without any additional carrier doping treatments. A simple explanation on the systematic behavior of the magnetic states in GaN-based diluted magnetic semiconductors is also given.

Organic-Capped ZnO Nanocrystals: Synthesis and n-Type Character

Abstract: Wurtzite ZnO nanocrystals capped with trioctylphosphine oxide or alkylamines are synthesized and characterized. These ZnO nanocrystals can be made n-type either by electron transfer doping from reducing species in solution or by above band gap photoexcitation with a UV lamp. The n-type nanocrystals exhibit a strong intraband infrared absorption, an extensive bleach of the interband band-edge absorption, and a complete quenching of the photoluminescence.

Ferromagnetism in a transition metal atom doped ZnO

Abstract: Ferromagnetism in a 3d transition metal atom doped ZnO was investigated by ab initio electronic structure calculations based on the local density approximation. It was shown that the anti-ferromagnetic state was stable in Mn atom doped ZnO and the ferromagnetic state was stable in the other transition metal, i.e., V, Cr, Fe, Co or Ni, doped ZnO, if no additional carrier dopant was introduced. Carrier induced ferromagnetism in the Mn atom doped ZnO was also investigated. The results showed that the ferromagnetism was induced by hole doping in the Mn atom doped ZnO. The present calculations will provide us with guidelines to produce ferromagnetic magnetic semiconductors and to control their magnetic state.

High critical current density of MgB2 bulk superconductor doped with Ti and sintered at ambient pressure

Abstract: Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and Jc of the samples change significantly with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2×106 A/cm2 in the self-field and 5×104 A/cm2 in 5 T. At 20 K, the Jc is still as high as 1.3×106 A/cm2 in the self-field and 9.4×104 A/cm2 in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB2 matrix, and this may play an important role in flux pinning enhancement.

Magnetic and magneto-transport properties of ZnO:Ni films

Abstract: Magnetic semiconductor (MS), Ni doped ZnO film was fabricated by pulsed laser deposition method on sapphire (0 0 0 1) substrates. Ni dissolves until 25 at % into ZnO by low temperature growth technique. Lattice constant c once increases with increasing Ni content and has maximum point at the Ni content of 5 at % , and then it suddenly decreases. In all the Ni content range, the films exhibits n-type conduction. With increasing the Ni content, the carrier density and mobility decrease. ZnO films with the Ni content range from 3 to 25 at % exhibit ferromagnetic behavior at 2 K . At 30 K , the magnetization against applied magnetic field shows superparamagnetic behavior and it maintains at least up to 300 K . To study the effect of carrier density, Al was additionally doped. The effect of carrier density and Ni content on the structure, magnetic and magneto-transport behaviors are described.

Ferromagnetism in magnetically doped III-V semiconductors.

Abstract: The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band can explain ferromagnetism in GaAs(Mn) in both degenerate and nondegenerate samples. Formation of ferromagnetic clusters and the percolation picture of phase transition describes well all available experimental data and allows us to predict the Mn-composition dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.

Enhanced Hole Injection into Amorphous Hole-Transport Layers of Organic Light-Emitting Diodes Using Controlled p-Type Doping

Abstract: We demonstrate enhanced hole injection and lowered driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a hole-transport layer using the starburst amine 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) p-doped with a very strong acceptor, tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) by controlled coevaporation. The doping leads to high conductivity of doped TDATA layers and a high density of equilibrium charge carriers, which facilitates hole injection and transport. Moreover, multilayer OLEDs consisting of double hole-transport layers of thick p-doped TDATA and a thin triphenyl-diamine (TPD) interlayer exhibit very low operating voltages.