Showing papers on "Doping published in 2002"

Diamond-like amorphous carbon

Abstract: Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

Growth of nanowire superlattice structures for nanoscale photonics and electronics.

Abstract: The assembly of semiconductor nanowires and carbon nanotubes into nanoscale devices and circuits could enable diverse applications in nanoelectronics and photonics1. Individual semiconducting nanowires have already been configured as field-effect transistors2, photodetectors3 and bio/chemical sensors4. More sophisticated light-emitting diodes5 (LEDs) and complementary and diode logic6,7,8 devices have been realized using both n- and p-type semiconducting nanowires or nanotubes. The n- and p-type materials have been incorporated in these latter devices either by crossing p- and n-type nanowires2,5,6,9 or by lithographically defining distinct p- and n-type regions in nanotubes8,10, although both strategies limit device complexity. In the planar semiconductor industry, intricate n- and p-type and more generally compositionally modulated (that is, superlattice) structures are used to enable versatile electronic and photonic functions. Here we demonstrate the synthesis of semiconductor nanowire superlattices from group III–V and group IV materials. (The superlattices are created within the nanowires by repeated modulation of the vapour-phase semiconductor reactants during growth of the wires.) Compositionally modulated superlattices consisting of 2 to 21 layers of GaAs and GaP have been prepared. Furthermore, n-Si/p-Si and n-InP/p-InP modulation doped nanowires have been synthesized. Single-nanowire photoluminescence, electrical transport and electroluminescence measurements show the unique photonic and electronic properties of these nanowire superlattices, and suggest potential applications ranging from nano-barcodes to polarized nanoscale LEDs.

Epitaxial core–shell and core–multishell nanowire heterostructures

Abstract: Semiconductor heterostructures with modulated composition and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core-shell heterostructures formed by the growth of crystalline overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures have also been formed by a one-dimensional modulation of nanowire composition and doping. However, modulation of the radial composition and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here we synthesize silicon and germanium core-shell and multishell nanowire heterostructures using a chemical vapour deposition method applicable to a variety of nanoscale materials. Our investigations of the growth of boron-doped silicon shells on intrinsic silicon and silicon-silicon oxide core-shell nanowires indicate that homoepitaxy can be achieved at relatively low temperatures on clean silicon. We also demonstrate the possibility of heteroepitaxial growth of crystalline germanium-silicon and silicon-germanium core-shell structures, in which band-offsets drive hole injection into either germanium core or shell regions. Our synthesis of core-multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices.

Band gap narrowing of titanium dioxide by sulfur doping

Abstract: Titanium dioxide (TiO2) doped with sulfur (S) was synthesized by oxidation annealing of titanium disulfide (TiS2). According to the x-ray diffraction patterns, TiS2 turned into anatase TiO2 when annealed at 600 °C. The residual S atoms occupied O-atom sites in TiO2 to form Ti–S bonds. The S doping caused the absorption edge of TiO2 to be shifted into the lower-energy region. Based on the theoretical analyses using ab initio band calculations, mixing of the S 3p states with the valence band was found to contribute to the band gap narrowing.

Origin of p -type doping difficulty in ZnO: The impurity perspective

Abstract: We investigate the p-type doping difficulty in ZnO by first-principles total-energy calculations. The dopants being considered are group-I elements Li, Na, and K and group-V elements N, P, and As. We find that substitutional group-I elements are shallow acceptors, while substitutional group-V elements such as P and As are deep acceptors. The AX centers that convert acceptors into deep donors are found to be unstable except for P and As. Without compensation by intrinsic defects, the most likely cause for doping difficulty is the formation of interstitials for group-I elements and antisites for group-V elements. Among all the dopants studied here, N is a relatively better candidate for p-type ZnO.

Controlling doping and carrier injection in carbon nanotube transistors

Abstract: Carbon nanotube field-effect transistors (CNTFETs) fabricated out of as-grown nanotubes are unipolar p-type devices. Two methods for their conversion from p- to n-type devices are presented. The first method involves conventional doping with an electron donor, while the second consists of annealing the contacts in vacuum to remove adsorbed oxygen. A comparison of these methods shows fundamental differences in the mechanism of the transformation. The key finding is that the main effect of oxygen adsorption is not to dope the bulk of the tube, but to modify the barriers at the metal–semiconductor contacts. The oxygen concentration and the level of doping of the nanotube are therefore complementary in controlling the CNTFET characteristics. Finally, a method of controlling individually the contact barriers by local heating is demonstrated.

High efficiency single dopant white electrophosphorescent light emitting diodes

Abstract: Efficient white electrophosphorescence has been achieved with a single emissive dopant. The dopant in these white organic light emitting diodes (WOLEDs) emits simultaneously from monomer and aggregate states, leading to a broad spectrum and high quality white emission. The dopant molecules are based on a series of platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C2′] β-diketonates. All of the dopant complexes described herein have identical photophysics in dilute solution with structured blue monomer emission (λmax = 468, 500, 540 nm). A broad orange aggregate emission (λmax ≈ 580 nm) is also observed, when doped into OLED host materials. The intensity of the orange band increases relative to the blue monomer emission, as the doping level is increased. The ratio of monomer to aggregate emission can be controlled by the doping concentration, the degree of steric bulk on the dopant and by the choice of the host material. A doping concentration for which the monomer and excimer bands are approximately equal gives an emission spectrum closest to standard white illumination sources. WOLEDs have been fabricated with doped CBP and mCP luminescent layers (CBP = N,N′-dicarbazolyl-4,4′-biphenyl, mCP = N,N′-dicarbazolyl-3,5-benzene). The best efficiencies and color stabilities were achieved when an electron/exciton blocking layer (EBL) is inserted into the structure, between the hole transporting layer and doped CBP or mCP layer. The material used for an EBL in these devices was fac-tris(1-phenylpyrazolato-N,C2′)iridium(III). The EBL material effectively prevents electrons and excitons from passing through the emissive layer into the hole transporting NPD layer. CBP based devices gave a peak external quantum efficiency of 3.3 ± 0.3% (7.3 ± 0.7 lm W−1) at 1 cd m−2, and 2.3 ± 0.2% (5.2 ± 0.3 lm W−1) at 500 cd m−2. mCP based devices gave a peak external quantum efficiency of 6.4% (12.2 lm W−1, 17.0 cd A−1), CIE coordinates of 0.36, 0.44 and a CRI of 67 at 1 cd m−2 (CIE = Commission Internationale de l'Eclairage, CRI = color rendering index). The efficiency of the mCP based device drops to 4.3 ± 0.5% (8.1 ± 0.6 lm W−1, 11.3 cd A−1) at 500 cd m−2, however, the CIE coordinates and CRI remain unchanged.

Chemical trends of defect formation and doping limit in II-VI semiconductors: The case of CdTe

Abstract: Using first-principles band structure methods we studied the general chemical trends of defect formation in II-VI semiconductors. We systematically calculated the formation energies and transition energy levels of intrinsic and extrinsic defects and defect complexes in the prototype CdTe and investigated the limiting factors for p-type and n-type doping in this material. Possible approaches to significantly increase the doping limits are discussed. Our general understanding of the chemical trends of defect formation energies and transition energy levels in CdTe is expected to be applicable also to other II-VI semiconductors.

General parameterization of Auger recombination in crystalline silicon

Abstract: A parameterization for band-to-band Auger recombination in silicon at 300 K is proposed. This general parameterization accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants. We confirm that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. Further, the rate of enhancement is shown to be less for highly injected intrinsic silicon than for lowly injected doped silicon, consistent with the theory of Coulomb-enhanced Auger recombination. Variations on the parameterization are discussed.

Thermal conduction in doped single-crystal silicon films

Abstract: This work measures the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1×1017 to 3×1019 cm−3 at temperatures between 15 and 300 K. The impurity concentrations are measured using secondary ion mass spectroscopy (SIMS) and the thermal conductivity data are interpreted using phonon transport theory accounting for scattering on impurities, free electrons, and the layer boundaries. Phonon-boundary scattering in the 3-μm-thick layers reduces the thermal conductivity of the layers at low temperatures regardless of the level of impurity concentration. The present data suggest that unintentional impurities may have strongly reduced the conductivities reported previously for bulk samples, for which impurity concentrations were determined from the electrical resistivity rather than from SIMS data. This work illustrates the combined effects of phonon interactions with impurities, free electrons, and material interfaces, which can be particularly...

Growth of Fe doped semi-insulating GaN by metalorganic chemical vapor deposition

Abstract: Iron doped GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) using ferrocene as the Fe precursor. Specular films with concentrations up to 1.7×1019 cm−3, as determined by secondary ion mass spectrometry, were grown. The Fe concentration in the film showed a linear dependence on the precursor partial pressure, and was insensitive to growth temperature, pressure, and ammonia partial pressure. Memory effects were observed, similar to Mg doping of GaN by MOCVD. The deep acceptor nature of Fe was used for growth of semi-insulating GaN films on sapphire substrates. A 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that Fe doping did not affect the structural properties of the film. Fe doping allows for growth of semi-insulating GaN on sapphire without the high threading dislocation densities and/or high carbon leve...

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Abstract: Self-assembly pyrolytic routes to large arrays (<2.5 cm2) of aligned CNx nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CNx nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs.

Atomic-scale imaging of individual dopant atoms and clusters in highly n -type bulk Si

Abstract: As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters1. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously2,3,4, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.

Rare-earth doped polymers for planar optical amplifiers

Abstract: Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.

Nanomaterials‐Based Electrochromics for Paper‐Quality Displays

Abstract: Electrochromic displays based on nanostructured films modified with electrochromophores are capable of becoming high quality paper-like displays due to their excellent ink-on-paper optical qualities, fast response times, and low power consuming features. The nanostractured films are composed of nanoparticles of a semiconductor, e.g., TiO 3 and other doped metal oxides. The high coloration efficiencies of these devices is due to the use of organic chromophores and the umplification of the color change due to the extremely high surface area of the nanostructured film they are bound to.

Group III nitride based light emitting diode structures with a quantum well and superlattice, group III nitride based quantum well structures and group III nitride based superlattice structures

Abstract: A Group III nitride based semiconductor device is disclosed, comprising: a doped Group III nitride layer; and a gallium nitride based superlattice directly on the doped Group III nitride layer, the gallium nitride superlattice being doped with an n-type impurity and having at least two periods of alternating layers of InXGa1-XN and InYGa1-YN, where 0 ≤ X < 1 and X is not equal to Y and wherein a thickness of a first of the alternating layers is less than a thickness of a second of the alternating layers.

Thermal conductivity of GaN films: Effects of impurities and dislocations

Abstract: We report details of the calculation of the lattice thermal conductivity κ in wurtzite GaN. Numerical simulations are performed for n-type wurtzite GaN with different density of silicon dopants, point defects and threading dislocations. Using the material specific model we verified the experimentally observed linear decrease of the room-temperature thermal conductivity with the logarithm of the carrier density n. The decrease was attributed mostly to the increased phonon relaxation on dopants. Our calculations show that the increase in the doping density from 1017 to 1018 cm−3 leads to about a factor of 2 decrease in thermal conductivity from 1.77 W/cm K to 0.86 W/cm K. We have also established that the room-temperature thermal conductivity in GaN can be limited by dislocations when their density is high, e.g., ND>1010 cm−2. The obtained results are in good agreement with experimental data. The developed calculation procedure can be used for accurate simulation of self-heating effects in GaN-based devices.

Bipolar doping and band-gap anomalies in delafossite transparent conductive oxides.

Abstract: Doping wide-gap materials p type is highly desirable but often difficult. This makes the recent discovery of p-type delafossite oxides, CuM(III)O2, very attractive. The CuM(III)O2 also show unique and unexplained physical properties: Increasing band gap from M(III) = Al,Ga, to In, not seen in conventional semiconductors. The largest gap CuInO2 can be mysteriously doped both n and p type but not the smaller gaps CuAlO2 and CuGaO2. Here, we show that both properties are results of a large disparity between the fundamental gap and the apparent optical gap, a finding that could lead to a breakthrough in the study of bipolarly dopable wide-gap semiconductor oxides.

Infrared spectra of zinc doped lead borate glasses

Abstract: The infrared spectra of zinc-doped lead borate glasses (10–30 mol% ZnO) were measured over a continuous spectral range (400–4000 cm−1) in an attempt to study their structure systematically. No boroxol ring formation was observed in the structure of these glasses. The formation of Zn in tetrahedral coordination was not observed. The conversion of three-fold to four-fold coordinated boron took place.

Crystal structure and spiral magnetic ordering of BiFeO 3 doped with manganese

Abstract: Neutron powder diffraction has been performed on polycrystalline BiMnxFe1-xO3 (x=0, 0.1 and 0.2). The structures of the doped compounds are described by the space group R3c of ferroelectric BiFeO3. Refined structure parameters are presented. Mn doping generates microstructural changes manifested by broadening in the diffraction patterns. The lattice parameters show a non-linear behaviour from 4 K to 630 K. Mn doping results in a transformation of the long-range spiral spin modulation of BiFeO3 to a collinear antiferromagnetic structure with spins along c. The average magnetic moments and the ordering temperatures decrease with increasing Mn concentration.

n-type doping of oxides by hydrogen

Abstract: First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity<3.0±0.4 eV) will remain nonconductive since hydrogen forms a deep impurity.

Very low bulk and surface recombination in oxidized silicon wafers

Abstract: Bulk and surface processes determine the recombination rate in crystalline silicon wafers. In this paper we report effective lifetime measurements for a variety of commercially available float-zone silicon wafers that have been carefully passivated using alnealed silicon oxide. Different substrate resistivities have been explored, including both p-type (boron) and n-type (phosphorus) dopants. Record high effective lifetimes of 29 and 32 ms have been measured for 90 Ω cm n-type and 150 Ω cm p-type silicon wafers, respectively. The dependence of the effective lifetime has been measured for excess carrier densities in the range of 1012–1017 cm−3. These results demonstrate that very low bulk and surface recombination rates can be maintained during high-temperature oxidation (1050 °C) by carefully optimizing the processing conditions.

1,8-Naphthalimides in phosphorescent organic LEDs: the interplay between dopant, exciplex, and host emission.

Abstract: Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.

Dopant precursors and processes

Abstract: Silicon alloys and doped silicon films are prepared by chemical vapor deposition and ion implantation processes using Si-containing chemical precursors as sources for Group III and Group V atoms. Preferred dopant precursors include (H3Si)3-xMRx, (H3Si)3N, and (H3Si)4N2, wherein R is H or D, x = 0, 1 or 2, and M is selected from the group consisting of B, P, As, and Sb. Preferred deposition methods produce non-hydrogenated silicon alloy and doped Si-containing films, including crystalline films.

Strained silicon MOSFET technology

Abstract: Mobility and current drive improvements associated with biaxial tensile stress in Si n- and p-MOSFETs are briefly reviewed Electron mobility enhancements at high channel doping (up to 6 /spl times/ 10/sup 18/ cm/sup -3/) are characterized in strained Si n-MOSFETs For low inversion layer carrier concentrations, channel-dopant ionized impurity scattering does reduce the strain-induced mobility enhancement, but the enhancement is recovered at higher inversion charge concentrations, where screening is efficient Mobility enhancement in strained Si p-MOSFETs is also discussed There are process integration challenges and opportunities associated with this technology Dopant diffusion, and its impact on profile engineering in strained Si CMOS structures, is one example While the slower diffusion of B in Si/sub 1-x/Ge/sub x/ enables improved doping profile control, the diffusivity of the n-type dopants is dramatically enhanced in Si/sub 08/Ge/sub 02/