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We performed room-temperature far-infrared ($40--650\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra show enhanced transmittance after bromine doping and all spectra were fit to a Drude-Lorentz (DL) model. A decreased metallic conductivity along with a redshift of the lowest semiconducting gap was found with doped samples. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of nondispersive reflectance and subsequently compared with DL-model-derived spectra. The free-carrier density of the $n$-type powder and the electronic mean free path were estimated and compared with reported values for single-walled nanotubes and pyrolytic graphite.

We report on the use of bromine intercalation of graphite to perform in situ tuning of the Schottky barrier height (SBH) formed at many-layer-graphene (MLG) semiconductor interfaces. The intercalation of Br into MLG simultaneously increases interlayer separation between the graphene planes, while at the same time giving rise to an increase (decrease) in the free hole carrier density (Fermi energy) because of the transfer of electrons from carbon to bromine. The associated increase in the graphite work function results in an increase of the SBH, as manifested by lower forward/reverse current densities and higher depletion capacitances. These results are quantitatively understood within the context of the Schottky–Mott model and thermionic emission theory. The presented results have important implications for sensing and high power applications as well as the integration of carbon into semiconductors and carbon/graphene electronics.

Recent efforts on doping ZnO films for charge and spin functionality are reviewed, focusing on chemical doping for charge and spin device formation. Discussion includes the behavior of phosphorus as an acceptor and magnetism in transition metal-doped ZnO. Evidence for p-type behavior in phosphorus-doped (Zn,Mg)O grown by pulsed laser deposition is presented. The magnetic properties of ZnO co-doped with Mn and Sn are also discussed.

Implantation of Co or Mn into single-crystal BaTiO3(K), SrTiO3 or KTaO3(Ca), followed by annealing at 700 � C, produced ferromagnetic behavior over a broad range of transition metal concentrations. For BaTiO3, both Co and Mn implantation produced magnetic ordering temperatures near 300 K with coercivities 670 Oe. The M–T plots showed either a near-linear decrease of magnetization with increasing temperature for Co and a non-Brillouin shaped curve for Mn. No secondary phases were detected by high-resolution X-ray diffraction. The same basic trends were observed for both SrTiO3 and KTaO3, with the exception that at high Mn concentrations (� 5 at.%) the SrTiO3 was no longer ferromagnetic. Our results are consistent with recent reports of room temperature ferromagnetism in other perovskite systems (e.g. LaBaMnO3) and theoretical predictions for transition metal doping of BaTiO3 [Nakayama et al., Jap. J. Appl. Phys. 40 (2001) L1355].

Both ion implantation and epitaxial crystal growth provide convenient methods of introducing transition metals such as Mn,Cr,Fe,Ni and Co into GaN, GaP, SiC and ZnO for creating dilute magnetic semiconductors exhibiting room temperature ferromagnetism. In this paper we review progress in wide band gap ferromagnetic semiconductors and the role of defects and doping on the resulting magnetic properties.

Results on the synthesis of ferromagnetic GaMnN and GaMnP by both Molecular Beam Epitaxy or implantation of Mn directly into p-GaN or GaP(C) at elevated temperatures to avoid amorphization will be described. There is a relatively broad range of growth conditions under which single-crystal, single-phase material may be obtained with significant Mn concentrations. The effects of background doping level are discussed, along with a comparison of results on direct implantation of Fe and Ni instead of Mn. Essential requirements for utilizing the spin of the electron in device structures include the ability to achieve efficient electrical spin injection and transport of spin-polarized carriers, along with effective detection of these carriers.

Structural and magnetic characteristics of Ni-implanted GaP were measured for doses in the range 3/5/10 16 cm 2 . After subsequent annealing at 700 8C, transmission electron microscopy (TEM) showed residual lattice damage which was more significant in highly carbon-doped epi layers as compared to bulk GaP substrates. The magnetization measurements showed two different contributions, one present at 5/75 K and the other below � /225 K. No secondary phase formation was detected in either type of GaP. # 2002 Elsevier Science B.V. All rights reserved.

High concentrations (0.1-5 at.%) of Mn or Fe were introduced into the near-surface region (≤2000 A) of 6H-SiC substrates by direct implantation at ˜300°C. After annealing at temperatures up to 1000°C, the structural properties were examined by transmission electron microscopy (TEM) and selected-area diffraction pattern (SADP) analysis. The magnetic properties were examined by SQUID magnetometry. While the Mn-implanted samples were paramagnetic over the entire dose range investigated, the Fe-implanted material displayed a ferromagnetic contribution present at >175 K for the highest dose conditions. No secondary phases were detected, at least not to the sensitivity of TEM or SADP.

In p-GaN implanted with Mn (3 × 1016cm-2 at 250 keV), the material after annealing shows ferromagnetic properties below 250 K. Cross-sectional transmission electron microscopy (TEM) revealed the presence of platelet structures with hexagonal symmetry. These regions are most likely GaxMn1-xN, which produce the ferromagnetic contribution to the magnetization. In p-GaN implanted with Fe, the material after annealing showed ferromagnetic properties at temperatures that were dependent on the Fe dose, but were below 200 K in all cases. In these samples, TEM and diffraction analysis did not reveal any secondary phase formation. The results for the Fe implantation are similar to those reported for Fe doping during epitaxial growth of GaN.

ZnO films grown by pulsed-laser deposition on c-plane Al2O3 substrates were annealed at temperatures up to 600 °C to produce n-type carrier concentrations in the range 7.5×1015–1.5×1020 cm−3. After high-dose (3×1016 cm−2) Mn implantation and subsequent annealing at 600 °C, all the films show n-type carrier concentrations in the range 2–5×1020 cm−3 and room temperature hysteresis in magnetization loops. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn, and that factors such as crystalline quality and residual defects play a role.

In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si− plus C− and Au+ ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even las...

Single-phase (Ga,Mn)N layers grown by molecular beam epitaxy with ∼3 at. % Mn were exposed to 2H or 1H plasmas under conditions designed to permeate layers with hydrogen. No evidence of the formation of Mn–H complexes could be detected from either infrared spectroscopy or the changes in magnetic properties. However, the residual magnetization increased ∼20% after hydrogenation, consistent with passivation of crystal defects in (Ga,Mn)N. Hydrogenation may be an attractive method for improving the magnetic properties of these heteroepitaxial materials.

We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segregated secondary phases, and strongly suggests that a near-room-temperature dilute magnetic semiconducting oxide has been realized. Based on these results, ZnO doped with Mn and Sn may prove promising as a ferromagnetic semiconductor for spintronics.

We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native(undoped) device performance by a factor of 4.5 and the best previously reported PCE in similar devices by a factor of nearly 6. Current-voltage, capacitance-voltage and external quantum efficiency measurements show the enhancement to be due to the doping induced shift in the graphene chemical potential which increases the graphene carrier density (decreasing the cell series resistance) and increases the built-in potential., 14 pages, 5 figures

The unpretentious origins in pure carbon soot of the highly symmetric C 60 molecule and the ease in which pure C 60 solids can be intercalated with alkali metals provide the essential motivation behind much of the current research activity on superconductivity in the doped fullerenes. In this paper the salient experimental phenomenology of these new superconductors is briefly reviewed and placed into context with respect to what is known about other classes of superconductors. Although the ‘soot to superconductivity’ story has had an auspicious beginning, a conclusion to the story is not yet in sight and additional experimental results and theoretical analyses are needed.

We report on the p doping of graphene with the polymer TFSA ((CF(3)SO(2))(2)NH). Modification of graphene with TFSA decreases the graphene sheet resistance by 70%. Through such modification, we report sheet resistance values as low as 129 Ω, thus attaining values comparable to those of indium-tin oxide (ITO), while displaying superior environmental stability and preserving electrical properties over extended time scales. Electrical transport measurements reveal that, after doping, the carrier density of holes increases, consistent with the acceptor nature of TFSA, and the mobility decreases due to enhanced short-range scattering. The Drude formula predicts that competition between these two effects yields an overall increase in conductivity. We confirm changes in the carrier density and Fermi level of graphene through changes in the Raman G and 2D peak positions. Doped graphene samples display high transmittance in the visible and near-infrared spectrum, preserving graphene's optical properties without any significant reduction in transparency, and are therefore superior to ITO films in the near infrared. The presented results allow integration of doped graphene sheets into optoelectronics, solar cells, and thermoelectric solar cells as well as engineering of the electrical characteristics of various devices by tuning the Fermi level of graphene.

Crystalline films of C60 with ~1 μm grain size and a preferred [111] texture have been doped with K and Rb to form superconducting compositions that have sharp zero-resistance transitions. The temperature dependence of the resistance above Tc for both K3C60 and Rb3C60 films reveals metallic behavior up to temperatures as high as 520 K without any evidence of saturation. For Rb3C60, electronic mean free paths significantly shorter than nearest-neighbor C60 distances and anomalously high values of the electron-phonon coupling strength are inferred. These results suggest that there is a strong interaction between the conduction electrons and the intramolecular vibrational modes and also bring into question the assumption that all of the donated charge (three electrons per C60) is itinerant.

We report the observation and assignment of the doping-induced features in the UV-visible and near-IR optical spectra of ${\mathrm{C}}_{60}$ thin films and their direct correlations with electrical resistivity. A pronounced bleaching and several induced absorptions are observed upon doping with the alkali metals, potassium and rubidium. The absence of a strong dependence of the induced spectra upon intercalant suggests that the induced features observed are intrinsic to the host fullerene film. The evolution of these transitions, as doping proceeds from pristine ${\mathrm{C}}_{60}$ to ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$, is consistent with a two-domain picture for the formation of the metallic state. As the intercalation proceeds beyond the resistance minimum, an overall change in spectral shape begins to occur and energy shifts of many of the features become apparent. These observations indicate changes in the electronic bands accompanying the structural phase transformations from ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ to ${\mathrm{K}}_{6}$${\mathrm{C}}_{60}$. There is no spectral evidence for the presence of ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$ during doping.

Carbon 60 is a fascinating and arrestingly beautiful molecule. With 12 pentagonal and 20 hexagonal faces symmetrically arrayed in a soccer‐ball‐like structure that belongs to the icosahedral point group Ih, its high symmetry alone invites special attention. The publication in September 1990 of a simple technique for manufacturing and concentrating macroscopic amounts of this new form of carbon (see Donald R. Huffman's article in PHYSICS TODAY, November 1991, page 22) announced to the scientific community that enabling technology had arrived. Macroscopic amounts of C60 (and the higher fullerenes, such as C70 and C84) can now be made with anapparatus as simple as an arc furnace powered with an arc welding supply. Accordingly, chemists, physicists and materials scientists have joined forces in an explosion of effort to explore the properties of this unusual molecular building block.

A brief review is presented discussing the role of charge transfer and the appearance of superconductivity in the three-dimensional (3D) alkali-metal-doped fullerene compounds (A3C60). An argument is made that similar phenomenology may occur in two dimensions (2D) when a C60 monolayer is doped by charge transfer from an adjacent planar metal layer. Illustrative data are presented showing that charge transfer gives rise to a decrease in resistance when C60 is deposited onto a thin smooth nickel film. Accordingly, the physics of charge transfer and electron delocalization, which plays such an important role in the superconductivity of the 3D A3C60 compounds, could well play a similar role in these 2D analogues where a C60 monolayer is doped to a conducting state by charge transfer from an underlying metal.

ZnO is an attractive candidate for spintronics studies because of its potential for exhibiting high Curie temperatures and the relative lack of ferromagnetic second phases in the material. In this paper, we review experimental results on transition-metal (TM) doping of ZnO and the current state of theories for ferromagnetism. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field that has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analog. We have been unable to detect optical spin polarization in ZnO.

ZnO is a very promising material for spintronics applications, with many groups reporting room temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during PLD, we find an inverse correlation between magnetization and electron density as controlled by Sn doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for the ferromagnetism include the bound magnetic polaron model or exchange is mediated by carriers in a spin-spilt impurity band derived from extended donor orbitals. Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors.

We report the femtosecond impulsive excitation of coherent phonons in alkali metal-doped fullerenes. These excitations appear as small oscillations of the reflectivity (ΔR/R∼10−7) produced by the pump in an optical pump-probe experiment. The observed phonons include previously unobserved highly damped modes near 150 cm−1 as well as long-lived Ag(1) modes near 490 cm−1.

In recent years, significant interest has emerged in the synthesis of magnetically-doped semiconductors. For computing, these materials provide a venue to exploit spin for computation. Direct bandgap spin-doped semiconductors are potential sources for polarized optical emission. In addition, quantum-entangled spins have been proposed as mediators in photon-based quantum communication schemes. In this paper, we discuss the properties of spin-doped ZnO. Mn-doped ZnO demonstrates ferromagnetism at relatively high temperatures as evidenced in hysteretic behavior in the magnetic flux versus field behavior. Transition metal doping was explored via ion implantation. Investigations of group V doping for p-type conduction are also discussed.

High doses (10(exp 15) - 5 x 10(exp 16)/sq cm) of Mn(+) ions were implanted into p-GaN at ^ 350 deg C and annealed at 700 - 1000 deg C. At the high end of this dose range, platelet structures of Ga(x)Mn(1-x)N were formed. The presence of these regions correlated with ferromagnetic behavior in the samples up to ^ 250 K. At low doses, the implantation led to a buried band of defects at the end of the ion range. There is tremendous interest in structures involving the manipulation of the spin of the electron in addition to its charge for switching and memory devices with new functionality. Applications for these spintronic devices are envisioned in communications technology, data processing and storage and in photonics. It has been demonstrated in a number of semiconductors (including GaMnAs, InMnAs and ZnMnSe) that quantum spin states are quite robust and can be transported over very large distances (> 100 micrometer in some cases). The ferromagnetism in dilute magnetic III-V semiconductors is carrier-induced and is still far from completely understood. The Curie temperature (T(sub c)) in these materials is believed to be governed by the interaction between localized ferromagnetic clusters (bound magnetic polarons). Recent calculations suggest that wide bandgap semiconductors may have T(sub c) values well above those for GaMnAs (110 K) and InMnAs (< 35 K). In particular GaMnN with ^ 5 at.% Mn, and high hole concentrations is predicted to have a T(sub c) exceeding 300 K. At present, little is known about doping GaN with Mn, although some initial results have been published on n-type Ga(x)Mn(1-x)N single crystallites with paramagnetic behavior. In this paper, we report on the magnetic properties of p-GaN implanted with high doses (3-5 at.%) of Mn.

Photoemission spectra of vacuum deposited layers of C(60), before and after exposure to K vapor, show that the K donates its conduction electron into the band derived from the lowest unoccupied molecular orbital. A compound with composition of K(3)C(60), corresponding to the maximum conductivity, has been prepared. In it the potassium atoms presumably occupy both the octahedral and the two tetrahedral interstitial sites of the face-centered-cubic (fcc) C(60) structure.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

A facile synthesis of 3–6 nm, water dispersible, near-infrared (NIR) emitting, quantum dots (QDs) magnetically doped with Fe is presented. Doping of alloyed CdTeS nanocrystals with Fe was achieved in situ using a simple hydrothermal method. The magnetic quantum dots (MQDs) were capped with N-acetyl-cysteine (NAC) ligands, containing thiol and carboxylic acid functional groups to provide stable aqueous dispersion. The optical and magnetic properties of the Fe doped MQDs were characterized using several techniques. The synthesized MQDs are tuned to emit in the vis-NIR (530–738 nm) wavelength regime and have high quantum yields (67.5–10%). NIR emitting (738 nm) MQDs having 5.6 atomic% Fe content exhibited saturation magnetization of 85 emu per g [Fe] at room temperature. Proton transverse relaxivity of the Fe doped MQDs (738 nm) at 4.7 T was determined to be 3.6 mM−1 s−1. The functional evaluation of NIR MQDs has been demonstrated using phantom and in vitro studies. These water dispersible, NIR emitting and MR contrast producing Fe doped CdTeS MQDs, in an unagglomerated form, have the potential to act as multimodal contrast agents for tracking live cells.

As developed by Senturia and co-workers [1], microdielectrometry is a technique for measuring complex permittivity utilizing microfabrication technology to incorporate both the interdigital sensing electrodes and associated circuitry on the same microchip. By covering a microdielectrometer chip with a thin layer of C60, it is possible to dynamically monitor the film's frequency response and dielectric properties as a function of doping with selected gases and other species.It is known that solid C60 has a substantial amount of interstitial volume. The presence of mobile ions in these spaces compromises the breakdown voltage and makes pure C60 unsuitable for applications requiring high-quality dielectric films. However, various immobile ions or neutral species (e.g. oxygen) can be made to fill the interstitial volume, changing the characteristics of the C60 films and, in some cases, improving the dielectric properties. In-situ microdielectric measurements of pristine and modified C60 films were performed for frequencies ranging from 0.005 Hz to 100 kHz. Based on the low-frequency behavior of the dielectric constant, a model is proposed for the mechanism of oxygen diffusion into the interstitial spaces of the fullerene material.

Electrical resistances of polycrystalline fullerene (C(60)) films were monitored while the films were being doped in ultrahigh vacuum with potassium from a molecular-beam effusion source. Temperature- and concentration-dependent resistivities of K(chi) C(60) films in equilibrium near room temperature were measured. The resistance changes smoothly from metallic at chi approximately 3 to activated as chi --= 0 or chi --6. The minimum resistivity for K(3)C(60) films is 2.2 microohm-centimeters, near the Mott limit. The resistivities are interpreted in terms of a granular microstructure where K(3)C(60) regions form nonpercolating grains, except perhaps at chi approximately 3. Stoichiometries at the resistivity extrema were determined by ex situ Rutherford backscattering spectrometry to be chi = 3 +/- 0.05 at the resistance minimum and chi = 6 +/- 0.05 at the fully doped resistance maximum.

The room temperature Raman spectra of the intramolecular modes between 100 cm(-1) and 2000 cm(-1) are reported for alkali-metal doped AxC(60) films. For A = K, Rb, and Cs, phase separation is observed with the spectra of C(60), K(3)C(60), K(6)C(60), Rb(3)C(60), Rb(6)C(60), and Cs(6)C(60) phases reported. The x = 3 phases show only three Raman active modes: two of Ag symmetry and only the lowest frequency Hg mode. The other Hg modes regain intensity in the x = 6 films, with several mode splittings observed. For A = Na, such phase separation is not clearly observed, and reduced mode shifts are interpreted as due to incomplete charge transfer in these films.

THE synthesis of macroscopic amounts of C60 and C70 (fullerenes)1 has stimulated a variety of studies on their chemical and physical properties2,3. We recently demonstrated that C60 and C70 become conductive when doped with alkali metals4. Here we describe low-temperature studies of potassium-doped C60 both as films and bulk samples, and demonstrate that this material becomes superconducting. Superconductivity is demonstrated by microwave, resistivity and Meissner-effect measurements. Both polycrystalline powders and thin-film samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor.

The magnetic and transport properties of Mn-doped ZnO thin films co-doped with P are examined. Superconducting quantum interference device magnetometry measurements indicate that the films are ferromagnetic with an inverse correlation between magnetization and electron density as controlled by P doping. In particular, under conditions where the acceptor dopants are activated leading to a decrease in free-electron density, magnetization is enhanced. The result is consistent with hole-mediated ferromagnetism in Mn-doped ZnO, in which bound acceptors mediate the ferromagnetic ordering. Increasing the electron density decreases the acceptor concentration, thus quenching the ferromagnetic exchange. This result is important in understanding ferromagnetism in transition metal doped semiconductors for spintronic devices.

Electrical and magnetic properties, room temperature optical absorption bands, and 300 and 90K microcathodo luminescence (MCL) bands were studied in heavily Mn and Co doped (1–5at.%) bulk ZnO crystals. Optical absorption bands near 1.9eV (Co) and 2eV (Mn) and MCL bands near 1.84eV (Co) and 1.89eV (Mn) are found to be associated with transition metal (TM) ions. These bands are assigned to internal transitions between the levels of the substitutional TM ions. The temperature dependence of the resistivity of the ZnO showed activation energies of ∼35meV in all cases and the electron mobilities were decreased relative to the undoped material.

The effects of high dose (3 3 1016 cm22) implantation of Ni, Fe, Co, or Mn ions into bulk, single-crystal SnO2 substrates carried out at substrate temperature of ;350 C to avoid amorphization of the implanted region on the magnetic properties of the material are reported. X-ray diffraction showed no evidence of secondary phase formation in the SnO2 . The Mn-implanted samples remained paramagnetic, as also reported for samples doped during thin film growth, but the Fe, Co-, and Ni-implanted SnO2 showed evidence of hysteresis with approximate Curie temperatures of ;120 K ~Co and Cr! or 300 K ~Fe!. The carrier density in the implanted region appears to be too low to support carrier-mediated origin of the ferromagnetism and formation of bound magnetic polarons may be one explanation for the observed magnetic properties. The much reduced Curie temperature seen in Co-implanted SnO2 compared to material doped during pulsed laser deposition suggests the residual implant damage degrades the magnetic properties.

(Ga, Mn)N layers were grown by radio-frequency plasma-assisted molecular beam epitaxy with Mn concentrations of ~5 or 50 atom %. In the material doped with 5 atom % Mn, the growth conditions were adjusted to produce second phases in one case, detectable by X-ray diffraction. Under our conditions, the dominant second phases are -related and not All three types of material (single-phase or GaMn second phases at different concentrations) exhibit 300 K hysteresis in magnetic moment vs. field (M-H) plots, but the temperature dependence of the field-cooled (FC) and zero-field cooled (ZFC) magnetization is quite different in each case. A clear difference in FC-ZFC magnetization at 300 K in the single-phase material is observed. © 2003 The Electrochemical Society. All rights reserved.

Mn ion implantation into single-crystal BaTiO 3 (K). SrTiO 3 , or KTaO 3 (Ca) was investigated for its effects on the magnetic properties of these materials. Following annealing at 700°C, the Mn implantation was found to induce ferromagnetic behavior for most of the concentration range investigated. For BaTiO 3 , Mn implantation produced magnetic ordering temperatures near 300 K with coercivities ≤70 Oe. The magnetization/temperature plots showed a non-Brillouin-shaped curve. No secondary phases were detected by high resolution X-ray diffraction. The results are consistent with theoretical predictions for transition metal doping of BaTiO 3 . The same basic trends were observed for both SrTiO 3 and KTaO 3 , with the exception that at high Mn concentrations (∼5 atom %) the SrTiO3 was no longer ferromagnetic.

n- and p-type AlxGa1−xN (x=0.38 for n-type, x=0.13 for p-type) layers grown on Al2O3 substrates were ion implanted with the transition metals Mn, Cr, and Co at high concentrations (peak doping levels ∼3 at. %). After implantation and annealing at 1000 °C, only impurity transitions at ∼2.9 and 3.9 eV and no band-edge photoluminescence could be observed in all the samples. X-ray diffraction did not detect any peaks associated with second phase formation. Room-temperature hysteresis loops were obtained for Co-implanted n-type AlGaN, while there was no convincing evidence for ferromagnetism in the Mn- or Cr-implanted n-AlGaN. By sharp contrast, Mn implantation in p-AlGaN did produce ferromagnetic behavior and 300 K hysteresis. Both carrier type and crystalline quality can influence the resulting magnetic properties.

The discovery of epitaxially grown ferromagnetic, type III–V semiconductors (Ga,Mn)As (Tc=110 K) and (In,Mn)As (Tc=35 K) holds promise for developing semiconductor electronics that utilize the electron’s spin degree of freedom in addition to its charge. It has been theoretically predicted that some semiconducting systems could be ferromagnetic above room temperature, when optimally doped (p-GaN with 5% Mn). We report here on the use of ion implantation to incorporate magnetic ions into a variety of semiconducting substrates, thereby facilitating investigation of the nature of ferromagnetism in semiconducting systems that are difficult to grow with other methods. The magnetic ions, Mn, Fe, and Ni, were implanted into each of the epitaxially grown semiconductors GaN, GaP, and SiC to achieve volume concentrations between 1 and 5 at. %. The implanted samples were subsequently annealed at 700–1000 °C to recrystallize the samples and remove implant damage. The implanted samples were examined with both x-ray dif...

ZnO is a very promising material for spintronics applications, with many groups reporting room-temperature ferromagnetism in films doped with transition metals during growth or by ion implantation. In films doped with Mn during pulsed laser deposition (PLD), we find an inverse correlation between magnetization and electron density as controlled by Sn-doping. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn and factors such as crystalline quality and residual defects play a role. Plausible mechanisms for ferromagnetism include the bound magnetic polaron model or exchange that is mediated by carriers in a spin-split impurity band derived from extended donor orbitals. The progress in ZnO nanowires is also reviewed. The large surface area of nanorods makes them attractive for gas and chemical sensing, and the ability to control their nucleation sites makes them candidates for microlasers or memory arrays. Single ZnO nanowire depletion-mode metal-oxide semiconductor field effect transistors exhibit good saturation behavior, threshold voltage of ∼−3 V, and a maximum transconductance of 0.3 mS/mm. Under ultraviolet (UV) illumination, the drain-source current increased by approximately a factor of 5 and the maximum transconductance was ∼5 mS/mm. The channel mobility is estimated to be ∼3 cm2/Vss, comparable to that for thin film ZnO enhancement mode metal-oxide semiconductor field effect transistors (MOSFETs), and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination. The Pt Schottky diodes exhibit excellent ideality factors of 1.1 at 25°C, very low reverse currents, and a strong photoresponse, with only a minor component with long decay times thought to originate from surface states. In the temperature range from 25°C to 150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089 eV and was insensitive to ambient used. By contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. Sensitive pH sensors using single ZnO nanowires have also been fabricated.