Showing papers on "Doping published in 1997"

Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering

Abstract: Single-walled carbon nanotubes1 (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls2. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles1 to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements3, suggests that doping does increase the carrier concentration of the SWNT bundles.

Erbium implanted thin film photonic materials

Abstract: Erbium doped materials are of great interest in thin film integrated optoelectronic technology, due to their Er3+ intra-4f emission at 1.54 μm, a standard telecommunication wavelength. Er-doped dielectric thin films can be used to fabricate planar optical amplifiers or lasers that can be integrated with other devices on the same chip. Semiconductors, such as silicon, can also be doped with erbium. In this case the Er may be excited through optically or electrically generated charge carriers. Er-doped Si light-emitting diodes may find applications in Si-based optoelectronic circuits. In this article, the synthesis, characterization, and application of several different Er-doped thin film photonic materials is described. It focuses on oxide glasses (pure SiO2, phosphosilicate, borosilicate, and soda-lime glasses), ceramic thin films (Al2O3, Y2O3, LiNbO3), and amorphous and crystalline silicon, all doped with Er by ion implantation. MeV ion implantation is a technique that is ideally suited to dope these materials with Er as the ion range corresponds to the typical micron dimensions of these optical materials. The role of implantation defects, the effect of annealing, concentration dependent effects, and optical activation are discussed and compared for the various materials.

Electric-field and temperature dependence of the hole mobility in poly(p-phenylene vinylene)

Abstract: The current-voltage characteristics of poly(dialkoxy p-phenylene vinylene)-based hole-only devices are measured as a function of temperature. The hole current is space-charge limited, which provides a direct measurement of the hole mobility ${\mathrm{\ensuremath{\mu}}}_{\mathrm{p}}$ as a function of electric field E and temperature. The hole mobility exhibits a field dependence ln ${\mathrm{\ensuremath{\mu}}}_{\mathrm{p}}$\ensuremath{\propto}$\sqrt{E}$ as has also been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The combination of a field-dependent mobility and space-charge effects provides a consistent description of the hole conduction in conjugated polymer films as a function of voltage, temperature, and layer thickness.

Conductivity enhancement in single-walled carbon nanotube bundles doped with K and Br

Abstract: Single-walled carbon nanotubes (SWNTs), prepared by metal-catalysed laser ablation of graphite, form close-packed bundles or ‘ropes;1. These rope crystallites exhibit metallic behaviour above 50K (ref. 2), and individual tubes behave as molecular wires, exhibiting quantum effects at low temperatures3,4. They offer an all-carbon host lattice that, by analogy with graphite5 and solid C60 (ref. 6), might form intercalation compounds with interesting electronic properties, such as enhanced electrical conductivity and superconductivity. Multi-walled nanotube materials have been doped with alkali metals7 and FeCl3 (ref. 8). Here we report the doping of bulk samples of SWNTs by vapour-phase reactions with bromine and potassium—a prototypical electron acceptor and donor respectively. Doping decreases the resistivity at 300K by up to a factor of 30, and enlarges the region where the temperature coefficient of resistance is positive (the signature of metallic behaviour). These results suggest that doped SWNTs represent a new family of synthetic metals.

A silicon/iron-disilicide light-emitting diode operating at a wavelength of 1.5 μm

Abstract: Although silicon has long been the material of choice for most microelectronic applications, it is a poor emitter of light (a consequence of having an ‘indirect’ bandgap), so hampering the development of integrated silicon optoelectronic devices. This problem has motivated numerous attempts to develop silicon-based structures with good light-emission characteristics1, particularly at wavelengths (∼1.5 μm) relevant to optical fibre communication. For example, silicon–germanium superlattice structures2 can result in a material with a pseudo-direct bandgap that emits at ∼1.5 μm, and doping silicon with erbium3 introduces an internal optical transition having a similar emission wavelength, although neither approach has led to practical devices. In this context, β-iron disilicide has attracted recent interest4,5,6,7,8,9,10,11,12 as an optically active, direct-bandgap material th might be compatible with existing silicon processing technology. Here we report the realization of a light-emitting device operating at 1.5 μm that incorporates β-FeSi2 into a conventional silicon bipolar junction. We argue that this result demonstrates the potential of β-FeSi2 as an important candidate for a silicon-based optoelectronic technology.

Theory of Semiconductor Superjunction Devices

Abstract: A new theory of semiconductor devices, called "semiconductor superjunction (SJ) theory", is presented. To overcome the trade-off relationship between breakdown voltage and on-resistance of conventional semiconductor devices, SJ devices utilize a number of alternately stacked, p- and n-type, heavily doped, thin semiconductor layers. By controlling the degree of doping and the thickness of these layers, according to the SJ theory, this structure operates as a pn junction with low on-resistance and high breakdown voltage. Analytical formulas for the ideal specific on-resistance and the ideal breakdown voltage of SJ devices are theoretically derived. Analysis based on the formulas and device simulations reveals that the on-resistance of SJ devices can be reduced to less than 10-2 that of conventional devices.

New Well-Defined Poly(2,7-fluorene) Derivatives: Photoluminescence and Base Doping

Abstract: Well-defined poly(2,7-fluorene) derivatives have been prepared through palladium-catalyzed couplings between various 9,9-disubstituted or 9-monosubstituted 2,7-dibromofluorenes and 2,7-bis(4,4,5,5- tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene. Using this versatile synthetic method, process- able polyfluorenes have been obtained in good yields. In solution, all these neutral yellow polymers exhibit blue emission (maximum of emission around 410 nm) with high quantum yields (up to 0.87). Moreover, novel acidic polyfluorene derivatives have been synthesized (i.e., poly(2,7'-(alkyl 9,9-dioctyl-7,2'-bifluorene- 9'-carboxylate))s) which show, upon base doping, electrical conductivities of 10-6-10-5 S/cm. This new doping method for conjugated polymers could open the way to the preparation of air-stable electron- injecting electrodes. Both photophysical and electrical properties of these polymers are quite promising for the fabrication of efficient blue-light-emitting devices.

Ferroelectric Field Effect Transistor Based on Epitaxial Perovskite Heterostructures

Abstract: Ferroelectric field effect devices offer the possibility of nonvolatile active memory elements. Doped rare-earth manganates, which are usually associated with colossal magnetoresistive properties, have been used as the semiconductor channel material of a prototypical epitaxial field effect device. The carrier concentration of the semiconductor channel can be "tuned" by varying the manganate stochiometry. A device with La0.7Ca0.3MnO3 as the semiconductor and PbZr0.2Ti0.8O3 as the ferroelectric gate exhibited a modulation in channel conductance of at least a factor of 3 and a retention loss of 3 percent after 45 minutes without power.

Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering

Abstract: Aluminum doped zinc oxide (AZO) films are prepared by rf magnetron sputtering on glass or Si substrates using specifically designed ZnO targets containing different amount of Al2O3 powder as the Al doping source. The structural, electrical, and optical properties of the AZO films are investigated in terms of the preparation conditions, such as the Al2O3 content in the target, rf power, substrate temperature and working pressure. The crystal structure of the AZO films is hexagonal wurtzite. The orientation, regardless of the Al content, is along the c axis perpendicular to the substrate. The doping concentration in the film is 1.9 at. % for 1 wt % Al2O3 target, 4.0 at. % for 3 wt % Al2O3 target, and 6.2 at. % for 5 wt % Al2O3 target. The resistivity of the AZO film prepared with the 3 wt % Al2O3 target is ∼4.7×10−4 Ω cm, and depends mainly on the carrier concentration. The optical transmittance of a 1500-A-thick film at 550 nm is ∼90%. The optical band gap depends on the Al doping level and on the microstr...

Growth and characterization of phosphorous doped {111} homoepitaxial diamond thin films

Abstract: An n-type semiconducting diamond thin film was obtained by microwave enhanced plasma chemical vapor deposition using phosphine (PH3) as a dopant source. A homoepitaxial diamond thin film with a thickness of about 300 nm was grown on the {111} surface of a type Ib diamond with a variety of dopant concentrations. Over a wide range of dopant concentrations (PH3/CH4: 1000–20 000 ppm), the n-type conduction was confirmed by Hall-effect measurements. The activation energy of carriers was 0.43 eV. The Hall mobility of about 23 cm2/V s has been obtained at around 500 K for the 1000 ppm sample. No significant increase of hydrogen has been observed by secondary-ion-mass-spectroscopy analysis for the phosphorous doped layers.

Deep Defect Centers in Silicon Carbide Monitored with Deep Level Transient Spectroscopy

Abstract: Electrical data obtained from deep level transient spectroscopy investigations on deep defect centers in the 3C, 4H, and 6H SiC polytypes are reviewed Emphasis is put on intrinsic defect centers observed in as-grown material and subsequent to ion implantation or electron irradiation as well as on defect centers caused by doping with or implantation of transition metals (vanadium, titanium, chromium, and scandium)

Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 µm

Abstract: Erbium-doped multicomponent phosphate glass waveguides were deposited by rf sputtering techniques. The Er concentration was 5.3×1020 cm−3. By pumping the waveguide at 980 nm with a power of ∼21 mW, a net optical gain of 4.1 dB at 1.535 μm was achieved. This high gain per unit length at low pump power could be achieved because the Er–Er cooperative upconversion interactions in this heavily Er-doped phosphate glass are very weak [the upconversion coefficient is (2.0±0.5)×10−18 cm3/s], presumably due to the homogeneous distribution of Er in the glass and due to the high optical mode confinement in the waveguide which leads to high pump power density at low pump power.

Red Luminescence in Pr3+‐Doped Calcium Titanates

Abstract: A strong single red emission band (λ max = 613 nm, FWMH = 450 cm -1 ) was observed at room temperature in Pr 3+ -doped calcium titanates. The evolution of the intensity of this luminescence was analysed in compensated and uncompensated samples (using Na + , Ag + or TI + as charge compensators) and also as a function of Pr concentration. Emission, excitation and reflectivity spectral repartitions were investigated at various temperatures, as well as the luminescence decays. The origin of the red luminescence is discussed.

Micromechanical and tribological characterization of doped single-crystal silicon and polysilicon films for microelectromechanical systems devices

Abstract: Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films, and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Micromechanical and tribological characterization of p-type (lightly boron-doped) single-crystal silicon (referred to as “undoped”), p+-type (boron doped) single-crystal silicon, polysilicon bulk, and n+-type (phosphorous doped) LPCVD polysilicon films have been carried out. Hardness, elastic modulus, and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Friction and wear properties were measured using an accelerated ball-on-flat tribometer. It is found that the undoped silicon and polysilicon bulk as well as n+-type polysilicon film exhibit higher hardness and elastic modulus than the p+-type silicon. The polysilicon bulk and n+-type polysilicon film exhibit the lowest friction and highest resistance to scratch and wear followed by the undoped silicon and with the poorest behavior of the p+-type silicon. During scratching, the p+-type silicon deforms like a ductile metal.

Acoustically Driven Storage of Light in a Quantum Well

Abstract: The dynamics of photogenerated carriers in semiconductor structures with reduced dimensionality has been the subject of intensive investigations in recent years [1,2]. State-of-the-art band-gap engineering technologies enable us to tailor low-dimensional semiconductor systems with desirable optoelectronic properties and study the fundamental aspects of carrier dynamics. This has increased tremendously our fundamental understanding of the dynamic properties of artificial semiconductor structures and has also resulted in a wide range of novel devices such as quantum well lasers, modulators, and detectors, as well as all-optical switches. Nevertheless, the bulk band structure of semiconductors seems to dominate optoelectronic properties since the strength of interband transitions is largely governed by the atomiclike Bloch parts of the wave function [3]. Thus it appears at first glance unavoidable that strong interband optical transitions are linked to direct band-gap semiconductors with short radiative lifetimes such as GaAs, whereas long radiative lifetimes of photogenerated carriers imply utilization of semiconductors with indirect band gaps such as Si and correspondingly reduced interband absorption. Initial attempts to employ band-gap engineering in order to combine strong interband absorption with long radiative lifetimes have focused on so-called doping superlattices [4]. There, alternate n and p doping along the growth direction is utilized to combine a direct gap in momentum space with an indirect gap in real space which causes a spatial separation of photogenerated electron-hole se-hd pairs and hence considerably prolonged lifetimes. Here, we introduce a new way of band-gap engineering in which we expose a semiconductor quantum well of a direct gap material to a moving potential superlattice modulated in the plane of the well. We show that the confinement of photogenerated e-h pairs to two dimensions, together with the moving lateral superlattice, allows reversible charge separation [5]. We demonstrate that the combination of both the advantages of strong interband absorption and extremely long lifetimes of the optical excitations is achieved without affecting the superior optical quality of the quantum well material. The spatial separation of the electron-hole pairs is achieved via the piezoelectric potential of acoustic waves propagating along the surface of a semiconductor quantum well system. On a piezoelectric substrate, the elliptically polarized surface acoustic waves (SAWs) are accompanied by both lateral and vertical piezoelectric fields which propagate at the speed of sound. Those fields can be strong enough to field ionize optically generated excitons and to confine the resulting electrons and holes in the moving lateral potential wells separated by one-half wavelength of the SAW. The spatial separation dramatically reduces the recombination probability and increases the radiative lifetime by several orders of magnitude as compared to the unperturbed case. We further demonstrate that the dynamically trapped electron-hole pairs can be transported over macroscopic distances at the speed of sound and that deliberate screening of the lateral piezoelectric fields of the SAW leads to an induced radiative recombination after long storage times at a location remote from the one of e-h generation. This conversion of photons into a long lived e-h polarization which is efficiently reconverted into photons can serve as an optical delay line operating at sound velocities. The undoped quantum well samples used in our experiments are grown by molecular beam epitaxy on a (100)GaAs substrate. The quantum well consists of 10 nm pseudomorphic In0.15Ga0.85As grown on a 1 mm thick GaAs buffer and is covered by a 20 nm thick GaAs cap layer. The active area of the sample is etched into a 2.5 mm long and 0.3 mm wide mesa (see inset of Fig. 1) with two interdigital transducers (IDTs) at its ends. The IDTs are designed to operate at a center frequency fSAW › 840 MHz. They are partially impedance matched to the 50 V radio frequency (rf) circuitry using an on-chip matching network, thus reducing the insertion

Temperature-dependent thermal conductivity of porous silicon

Abstract: The thermal conductivity of electrochemically etched porous silicon (PS) layers was determined over a wide temperature range (T = 35 - 320 K) using the dynamic technique. Both the doping level of the silicon wafers (p and ) and the porosity P of the porous layers (P = 64 - 89%) were varied. The measured thermal conductivities were three to five orders of magnitude smaller than the values for bulk silicon. Furthermore, they increase with increasing the wafer doping level and with decreasing the porosity P of the layers. For all investigated PS layers the thermal conductivity increases with temperature. The results are discussed in terms of a simple model for heat conduction in PS based on the phonon diffusion model.

Investigation of Structure and Electrical Conductivity in Doped Polyaniline

Abstract: Polyaniline (PAn) was synthesized chemically and doped with various dopants, such as HCI, HCOOH, I 2 and methylene blue (C 16 H 18 ClN 3 S), by an immersion method. The structure of these samples was investigated by infrared (IR) spectroscopy and wide-angle X-ray diffraction (WAXD) analysis. Remarkable changes have been observed in the IR spectra of doped PAn, indicating that doping is affecting the chemical structure. The percentage crystallinity was also found to increase after doping. The electrical conductivity (σ) of these samples was measured at various temperatures (T = 308K to 423K). Plots of log σ versus T y , where y = - 1/2, - 1/3, - 1/4, were obtained and used to identity the conduction mechanism. Undoped PAn shows semiconducting behaviour, while doped samples show a variable range hopping mechanism. A primary cell was constructed with HCl-doped PAn as one of the electrodes and a copper plate as the other electrode. It gave an open circuit voltage of 0.38 V and a short circuit current of about 5.4 mA.

Nature of Conduction in Doped Silicon

Abstract: Via ultrafast optoelectronic THz techniques, we are able to test alternative theories of conduction by precisely measuring the complex conductivity of doped silicon from low frequencies to frequencies higher than the plasma frequency and the carrier damping rate. These results, obtained for both $n$ and $p$-type samples, spanning a range of more than 2 orders of magnitude in the carrier density, do not fit any standard theory. We only find agreement over the full frequency range with the complex conductivity given by a Cole-Davidson type distribution applied here for the first time to a crystalline semiconductor, and thereby demonstrate that fractal conductivity is not just found in disordered material.

Investigation of carrier lifetime instabilities in Cz-grown silicon

Abstract: In the literature it is well known that the low-injection bulk carrier lifetime of boron-doped Cz-grown silicon is not a constant material property but, depending on previous thermal treatments and light exposure, varies between two states corresponding to a high and a low lifetime value. The upper state is obtained by means of low-temperature annealing, while illumination degrades the lifetime towards the value of the lower state. In order to improve the understanding of this phenomenon, we performed comprehensive carrier lifetime measurements on solar- and electronic-grade boron, gallium, and phosphorus doped Cz wafers obtained from different manufacturers. Based on the experimental results, a new model is introduced which attributes the disappointingly low stable lifetimes of illuminated boron-doped Cz silicon with resistivity around 1 /spl Omega/cm to boron-oxygen pairs. From this model, simple recipes are derived which might lead to an improvement of the efficiency of commercial Cz silicon solar cells.

Efficient organic electroluminescent devices using single-layer doped polymer thin films with bipolar carrier transport abilities

Abstract: Detailed studies of electroluminescent devices made from single-layer doped polymer blend thin films having bipolar carrier transport abilities are presented. The active organic layer consists of the hole-transport polymer poly(N-vinylcarbazole) (PVK) containing dispersed electron-transport molecules, as well as different fluorescent small molecules or polymers as emitting centers to vary the emission color. Both the photoluminescence and electroluminescence (EL) properties are extensively studied. In photoluminescence, very efficient transfer of energy can occur from the host to very dilute (/spl sim/1 wt.%) amounts of emitting materials. When covered with a metal layer, the intensity of photoluminescence from blend thin films was found to be dependent on the type of metal coverage. The optical and electrical properties of materials and devices were systematically studied to understand the operating mechanisms and to optimize the devices. In EL, excitons appear to be formed at doped emitting centers, rather than in the host. We show that in an optimized device, a relatively high external quantum efficiency (>1%, backside emission only) and a low operating voltage (<10 V for over 100 cd/m/sup 2/) can be easily achieved by this class of devices. It was also found air-stable Ag is as good as reactive Mg-Ag alloy for the cathode contact in devices using PVK containing dispersed electron-transport oxadiazole molecules.

Optical properties of Si-doped GaN

Abstract: The optical properties of n-type GaN are investigated for Si doping concentrations ranging from 5×1016 to 7×1018 cm−3. The photoluminescence linewidth of the near-band gap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled in terms of potential fluctuations caused by the random distribution of donor impurities. Good agreement is found between experimental and theoretical results. The intensity of the near-band-gap transition increases monotonically as the doping concentration is increased indicating that nonradiative transitions dominate at a low doping density. The comparison of absorption, luminescence, reflectance, and photoreflectance measurements reveals the absence of a Stokes shift at room temperature demonstrating the intrinsic nature of the near-band edge transition.

Noncontact semiconductor wafer characterization with the terahertz Hall effect

Abstract: We demonstrate noncontact measurements of the Hall mobility of doped semiconductor wafers with roughly 250 μm spatial resolution, using polarization rotation of focused beams of terahertz (THz) radiation in the presence of a static magnetic field. Quantitative and independent images of both carrier density and mobility of a doped semiconductor wafer have been obtained.

Excimer laser crystallization and doping of silicon films on plastic substrates

Abstract: We report the pulsed laser recrystallization and doping of thin film amorphous silicon deposited on oxide-coated polyester substrates. Although our heat-flow simulation of the laser recrystallization process indicates that the plastic is briefly subjected to temperatures above its softening point, we see no evidence of damage to the plastic or film delamination from the substrate. Film grain size is found to vary up to ∼0.1 μm. Electrical characteristics obtained from simple strip line resistors and thin film transistors indicate that device-quality silicon films have been produced on an inexpensive flexible plastic substrate.

Conductive Blends of Thermally Dodecylbenzene Sulfonic Acid-Doped Polyaniline with Thermoplastic Polymers

Abstract: In the present study, a conductive polyaniline-dodecyl benzene sulfonic acid (PANI-DBSA) complex, prepared by a thermal doping process, and its blends with thermoplastic polymers, prepared by melt processing, were investigated. PANI- DBSA characterization included conductivity measurements, morphology, crystallogra- phy, and thermal behavior. The blends' investigation focused on the morphology and the interaction between the components and on the resulting electrical conductivity. The level of interaction between the PANI and the matrix polymer determines the blend morphology and, thus, its electrical conductivity. Similar solubility parameters of the two polymeric components are necessary for a high level of PANI dispersion within the matrix polymer and, thus, enable the formation of conducting paths at low PANI content. The morphology of these blends is described by a two-structure hier- archy: (a) a primary structure, composed of small dispersed polyaniline particles, and (b) a short-range fine fibrillar structure, interconnecting the dispersed particles. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 243-253, 1997