Deterministic addressing of nanoscale devices assembled at sublithographic pitches

Multiple techniques have now been proposed using random addressing to build demultiplexers which interface between the large pitch of lithographically patterned features and the smaller pitch of self-assembled sublithographic nanowires. At the same time, the relatively high defect rates expected for molecular-sized devices and wires dictate that we design architectures with spare components so we can map around defective elements. To accommodate and mask both of these effects, we introduce a programmable addressing scheme which can be used to provide deterministic addresses for decoders built with random nanoscale addressing and potentially defective wires. We describe how this programmable addressing scheme can be implemented with emerging, nanoscale building blocks and show how to build deterministically addressable memory banks. We characterize the area required for this programmable addressing scheme. For 2048/spl times/2048 memory banks, the area overhead for address correction is less than 33%, delivering net memory densities around 10/sup 11/ b/cm/sup 2/.     

Demagnetizing factors for square bars

Using a finite-element method, we have calculated fluxmetric and magnetometric demagnetizing factors N/sub f/ and N/sub m/ for square cross-section bars as functions of susceptibility /spl chi/ and length-to-width ratio c/a over the range /spl chi/=-1 to /spl infin/ and c/a=0.01 to 100. We present the results in figures and tables and compare them with those for cylinders. We give an approximate conjugate relation for N/sub m/(/spl chi/) of a cube.     

Recent developments in high curie temperature perovskite single crystals

The temperature behavior of various relaxor-PT piezoelectric single crystals was investigated. Owing to a strongly-curved morphotropic phase boundary, the usage temperature of these perovskite single crystals is limited by T/sub R-T/- the rhombohedral to tetragonal phase transformation temperature - which occurs at the significantly lower temperatures than the Curie temperature T/sub c/. Attempts to modify the temperature usage range of Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZNT) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMNT) rhombohedral crystals (T/sub c/ /spl sim/ 150-170/spl deg/C, T/sub R-T/ /spl sim/ 60-120/spl deg/C) using minor dopant modifications were limited, with little success. Of significant potential are crystals near the morphotropic phase boundary in the Pb(Yb/sub 1/2/Nb/sub 1/2/)O/sub 3/-PbTiO/sub 3/ (PYNT) system, with a T/sub c/ > 330/spl deg/C, even though T/sub R-T/ was found to be only half the value at /spl sim/160/spl deg/C. Single crystals in the novel BiScO/sub 3/-PbTiO/sub 3/ system offer significantly higher T/sub c/s > 400/spl deg/C, while exhibiting electromechanical coupling coefficients k/sub 33/ > 90% being nearly constant till the T/sub R-T/ temperature around 350/spl deg/C, which greatly increases the temperature range for transducer applications.     

Preparation of ferroelectric Ba(Ti/sub 0.85/Sn/sub 0.15/)O/sub 3/ thin films by metal-organic decomposition

Ferroelectric Ba(Ti/sub 0.85/Sn/sub 0.15/)O/sub 3/ (BTS/sub 15/) thin film is newly prepared on the Pt/Ti/SiO/sub 2//Si substrate by metal-organic decomposition. The firing condition is determined by thermogravimetric and differential thermal analysis. The BTS/sub 15/ thin film with a flat surface and uniform thickness is obtained by spin coating in N/sub 2/ atmosphere that avoids moisture. The BTS/sub 15/ film has a perovskite phase and a preferential [110] texture. It is also found that the crystalline structure is cubic at 24/spl deg/C with a lattice constant of 4.01 /spl Aring/, and a grain size of about 30 nm was estimated by Scherrer equation and SEM image. From P-E hysteresis loop at 20/spl deg/C, the polarization at E=0 and the electric field at P=0 are found to be 1.07 /spl mu/C/cm/sup 2/ and 24.0 kV/cm, respectively. It is observed that the dielectric constant decreases monotonously from about 830 to 630 with increasing temperature ranging from 20/spl deg/C to 50/spl deg/C. Finally, it is found that the BTS/sub 15/ thin film shows a sufficient ferroelectricity and is an attractive material for functional ferroelectric devices, such as thermal-type infrared sensors.     

Mechanical and electrical characterization of /spl beta/-Ga/sub 2/O/sub 3/ nanostructures for sensing applications

Single crystalline /spl beta/-Ga/sub 2/O/sub 3/ nanowire and nanoribbon materials were synthesized, and electrical and mechanical properties were studied for sensing applications. The structural analysis showed that the Ga/sub 2/O/sub 3/ nanomaterials were stoichiometric and had the same crystal lattice structure as the /spl beta/ phase Ga/sub 2/O/sub 3/ crystal. The mechanical study on individual Ga/sub 2/O/sub 3/ nanowires and nanoribbons showed that they had a bending modulus of around 300 GPa, are flexible (in bending and twisting), and are easy to be cleaved along their crystal lattice. The current-voltage electrical characterization through the thickness of nanoribbon and along the length of nanowire confirmed their semiconducting characteristic. A two-terminal device fabricated with an individual Ga/sub 2/O/sub 3/ nanowire showed good sensing response to ethanol gas at low-operating temperature, which revealed the potential of using such nanostructures for effective sensing applications.     

Magnetic and structural properties of Co ion-implanted GaN

A GaN epilayer was grown on Al/sub 2/O/sub 3/ substrate by metal-organic chemical vapor deposition, and Co/sup -/ ions with a dose of 3/spl times/10/sup 16/ cm/sup -2/ were implanted into GaN at 350/spl deg/C. The implanted samples were postannealed at 700/spl deg/C-900/spl deg/C to recrystallize the samples and to remove implantation damage. We have investigated the magnetic and structural properties of Co ion-implanted GaN by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, and X-ray photoelectron spectroscopy (XPS). XRD results did not show any peaks associated with the second phase formation, and only the diffraction from the GaN layer and substrate structure were observed. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions showed the features of superparamagnetic system in films annealed at 700/spl deg/C-900/spl deg/C. The magnetization curves at 5 K for samples annealed at 700/spl deg/C-900/spl deg/C exhibits ferromagnetic hysteresis loops, and the highest residual magnetization (M/sub R/) and coercivity (H/sub c/) of M/sub R/=1.5/spl times/10/sup -4/ emu/g and H/sub c/=107 Oe were found in the 800/spl deg/C annealed sample. XPS measurement showed the metallic Co 2p core levels and the metallic valence band spectra for as-implanted and 700/spl deg/C-900/spl deg/C annealed samples. From these, it could be explained that the magnetic property of our films originated from Co and CoGa magnetic clusters.     

Prediction of ultra-high aspect ratio nanowires from self-assembly

We employ a combination of ab initio total energy calculations and classical molecular dynamics (MD) simulations to investigate the possible self-assembly of nanoscale objects into ultrahigh aspect ratio chains and wires. The ab initio calculations provide key information regarding selective chemical functionalization for end-to-end attraction and the subtle interplay of the energy landscape, which is then used to fit classical potentials. MD simulations are carried out to predict short-time dynamical properties of assembly as a function of synthesis conditions, including solvent, chemical functionalization, temperature, and concentration. Our results suggest an efficient technique for bringing nanoscale objects together to form ultrahigh aspect ratio nanowires with high-quality alignment. We show that the electronic structure of the resulting nanowires depends strongly on the end functionalization.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Field-emission triode of low-temperature synthesized ZnO nanowires

A field-emission triode based on the low-temperature (75/spl deg/C) and hydrothermally synthesized single-crystalline zinc-oxide nanowires (ZnO NWs) grown on Si substrate with a silicon dioxide (SiO/sub 2/) insulator was fabricated for the controllable field-emission device application. Field-emission measurement reveals that the ZnO NWs fabricated on the Si substrate exhibit a good emission property with the turn-on electric field and threshold electric field (current density of 1 mA/cm/sup 2/) of 1.6 and 2.1 V//spl mu/m, respectively, with a field enhancement factor /spl beta/ of 3340. The field-emission properties of the ZnO NW-based triode exhibit the controllable characteristics. The well-controlled field-emission characteristics can be divided into three parts: gate leakage region, linear region, and saturation region. Therefore, this study provides a low-temperature field-emission triode fabrication process that is compatible with the Si-based microelectronic integration, and the field-emission measurements also reveal that the emission behavior can be well controlled by adopting the triode structure.     

Site-Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin–Orbit Coupling

Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain-relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin–orbit coupling, with a spin–orbit length similar to that of III–V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.     

An atomic force microscope study of surface roughness of thin silicon films deposited on SiO/sub 2/

Atomic force microscope analysis, with a resolution of /spl lsim/1.1 nm, shows that peak-to-peak surface roughness (/spl Delta/h/sub p-p/) of amorphous silicon films thinner than /spl ap/50 nm on silicon dioxide can be controlled to better than 5 nm. Low-pressure, chemically-vapor-deposited silicon films on silicon dioxide initially show an approximately linear increase in the surface roughness due to growing nuclei as the deposition progresses, followed by a decrease in the surface roughness as growth nuclei coalesce. A simple model based on random nucleation and nuclei growth displays similar trends. Films deposited on rougher substrates show more surface roughness. Surface treatment during the predeposition cleaning process does not significantly affect /spl Delta/h/sub p-p/. As a means of producing smooth surfaces, films thinner than about 20 nm are first deposited more thickly than needed, and then etched back to the desired dimension; the use of a binary HNO/sub 3/ and HF etching process improves roughness control. Boron-ion implanted and subsequently crystallized 45-nm-thick Si films show significant smoothing with /spl Delta/h/sub p-p//spl ap/2.2 nm. Thin amorphous silicon films deposited by source evaporation are attractive because they can be deposited at room temperature, and have smoother surfaces (/spl Delta/h/sub p-p//spl ap/2.5 nm) than comparable films produced by chemical vapor deposition.     

Influence of spinel substrate and over-layer for enhanced SAW and AO properties with KNbO3 thin film

Surface acoustic wave (SAW) propagation characteristics have been studied using modeling calculations for a potassium niobate (KNbO/sub 3/) thin film-layered structure with [001] and [110] orientation on a single crystal spinel (MgAl/sub 2/O/sub 4/) substrate, and a spinel buffer layer on silicon. Variation in the electromechanical coupling and acoustic attenuation has been compared. A significantly high value of coupling factor (k/sub max//sup 2/=23%) is obtained for the [001]KNbO/sub 3//spinel structure by introducing an optimum thickness of spinel over-layer for potential wide bandwidth SAW device applications. The dispersion characteristics with the [110] KNbO/sub 3/ orientation indicate an initial peak in the coupling coefficient value (k/sub max//sup 2/=8.8%) at a relatively low KNbO/sub 3/ film thickness that appears attractive for fabricating devices with thinner films. The KNbO/sub 3/ film with [001] orientation is found attractive for efficient acousto-optic (AO) device application with the formation of a symmetric waveguide structure (spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel). A high value of k/sup 2/=23.5% with 50% diffraction efficiency has been obtained for the spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel structure at 1 GHz SAW frequency and 633 nm optical wavelength at a very low input drive power of 15.4 mW.     

CO-sensing properties of In/sub 2/O/sub 3/-doped SnO/sub 2/ thick-film sensors: effect of doping concentration and grain size

In/sub 2/O/sub 3/-doped SnO/sub 2/ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl/sub 4/ 5H/sub 2/O) and indium nitrate. The doping concentration was varied from 7.718/spl times/10/sup -5/ to 3.859/spl times/10/sup -4/ moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300/spl deg/C-900/spl deg/C. Thick-film samples with a thickness of /spl sim/15 /spl mu/m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220/spl deg/C-240/spl deg/C. A blue shift in the sensing temperature and increase in sensitivity factor (S/sub f/) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of /spl sim/5 is found for the highest doping concentration (3.859/spl times/10/sup -4/ moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.     

Design and fabrication of a mode rectangular X-cut quartz resonator with zero temperature coefficient

Rectangular X-cut quartz crystal resonators with cut angles /spl theta/ > 5.0/spl deg/ and aspect ratios R/sub zy/ (= width 2z/sub 0//length 2y/sub 0/) from 0.3 to 0.5 are investigated. The resonators oscillate mode is a length-extensional mode. A semiempirical frequency equation was derived from the stress expressed in terms of the trigonometric and the hyperbolic transcendental functions with constants estimated by the regression curve fit to the stress simulated by the finite-element method (FEM). Contours on which a point satisfies a zero first order temperature coefficient condition are shown in a cut angle /spl theta/ and R/sub zy/ diagram. We proved that a fabricated resonator with R/sub zy/ = 0.400 and /spl theta/ = 16.0/spl deg/, whose design parameter is located in the area of the contour, had a zero temperature coefficient.     

Dedicated temperature sensing with c-axis oriented single-crystal ruby (Cr/sup 3+/:Al/sub 2/O/sub 3/) fibers: temperature and strain dependences of R-line fluorescence

Single-crystal fibers of ruby (Cr/sup 3+/:Al/sub 2/O/sub 3/) with approximately 0.1 wt.% Cr/sup 3+/ have been produced by the laser heated pedestal growth (LHPG) technique. The fluorescence emissions of the R/sub 1/ and R/sub 2/ lines were studied as functions of temperature and strain. Fluorescence decay lifetime measurements indicate that these fibers may be suitable for thermometric applications up to 973 K while strain measurements show only a very weak dependence. Similarly, characterization of the R-line shifts also show a weak strain sensitivity and an appreciably larger change with temperature. One fiber sample has been tested to destruction to demonstrate this weak strain dependence. Single-crystal ruby fibers are, thus, found to be potential candidates for dedicated temperature sensing from room temperature to /spl sim/923 K.     

Structure, properties, and thermal stability of nanocrystallite Fe-Ti-N soft magnetic films

We deposited Fe-Ti-N magnetic films with a high sputtering power of 7 W/cm/sup 2/. When the composition of the films was in the range of Fe-Ti(3.9 at.%)-N(8.8 at.%) to Fe-Ti(3.3 at.%)-N(13.5 at.%), the films were composed of /spl alpha/' and Ti/sub 2/N precipitates. With the addition of nitrogen, 4/spl pi/M/sub s/ became higher than that of pure iron, reaching a maximum of 23.8 kG. At the same time, H/sub c/ was reduced to a minimum of 1.12 Oe. The best films can meet the needs of the recording head in dual-element giant magnetoresistive/inductive heads, yielding high storage density (10 Gb/in/sup 2/). The incorporation of N in /spl alpha/-Fe brought about the /spl alpha/' phase with its higher saturation magnetization. Ti additions inhibited the equilibrium decomposition /spl alpha/'/spl rarr//spl alpha/+/spl gamma/'. Because H/sub C//sup D//spl prop/D/sup 6/, where D is average grain diameter, grain size control is very important. The nitrogen induces severe distortion of the /spl alpha/' lattice, which can cause the grains to break into pieces and reduce the grain size. High sputtering power also led to the formation of fine grains, with diameter in the order of 14 nm. Probably Ti/sub 2/N is preferentially precipitated on the grain boundary, pinning the grain boundary and stabilizing the grain size during high-temperature heat treatment. The temperature limit for stability of the structure and its associated low coercivity was not less than 520/spl deg/C.     

CMOS chip as luminescent sensor for biochemical reactions

We describe a novel biochemical sensing method and its potential new biosensing applications. A light-sensitive complementary metal oxide semiconductor (CMOS) chip prepared through a standard 0.5-/spl mu/m CMOS process was developed for measuring biochemical reactions. A light producing enzymatic reaction catalyzed by horseradish peroxidase (HRP) was designed as a platform reaction to determine the concentration of hydrogen peroxide (H/sub 2/O/sub 2/) by the CMOS chip with a standard semiconductor parameter analyzer (HP4145). The kinetics of enzymatic reaction were determined and compared with a standard and sophisticated fluorometer (Hitachi F-4500) in a biochemical laboratory. Similar results were obtained by both instruments. Using glucose oxidase as an example, we further demonstrated that the HRP platform can be used to determine other H/sub 2/O/sub 2/ producing reactions with the CMOS system. The result points to an important application of the CMOS chip in biological measurements and in diagnosis of various health factors.     

Chemically synthesized FePt nanoparticle material for ultrahigh-density recording

We have examined the magnetic anisotropy of the "heat-treated FePt nanoparticles" annealed in a magnetic field. The magnetic easy axis of the "heat-treated FePt nanoparticles" is found to be three-dimensional (3-D) random and a partial ordering fct structure is observed before annealing in the presence of a magnetic field. The value of M/sub r//M/sub s/ obtained is 0.5. After annealing in the presence of a magnetic field, the M-H loop indicates that the easy axis is oriented preferably in the perpendicular direction than along the in-plane direction. The value of H/sub c/(//)/H/sub c/(/spl perp/) at 10 K is 0.62 (1410 Oe/2250 Oe). The value of M/sub r//M/sub s/(/spl perp/) is 0.58 at 10 K larger than the value of M/sub r//M/sub s/(//). Therefore, a weak magnetic easy axis orientation is fundamentally possible on the chemically synthesized FePt nanoparticles. We have studied the recording characteristics of a 3-D random nanoparticle medium using a GUZIK spinstand and observed the recorded patterns for the medium by imaging with a magnetic force microscopy.     

Inspection of intrinsic critical currents for spin-transfer magnetization switching

We examined the relationships between critical current, I/sub c/, and switching time, /spl tau//sub p/, for spin-transfer switching in two regions: (region I) /spl tau//sub p//spl Gt//spl tau//sub 0/, where thermal switching is accompanied and (region II) /spl tau//sub p/< several tens times /spl tau//sub 0/, where /spl tau//sub 0/ is the attempt time for thermal switching (/spl ap/1 ns). We estimated I/sub c0/, defined as the intrinsic I/sub c/ at 0 K, for both regions and confirmed experimentally that those I/sub c0/ coincided with each other at room temperature (RT). The value of I/sub c/ at /spl tau//sub p/=1 ns, measured with microwaves, was approximately 1.6 times the I/sub c0/. This suggested that we use at least two times I/sub c0/ as the writing currents of magnetic memory devices for nsec spin-transfer switching at RT. Although I/sub c0/ for both regions were defined as I/sub c/ at 0 K (I/sub c//sup 0K/) in theory, they showed temperature dependence at low temperatures; |I/sub c0/| for region I increased with decreasing temperature, and the estimated I/sub c//sup 0K/ was approximately three times I/sub c0/ for RT. This temperature dependence was quite different from that for region II.