Showing papers in "Journal of Applied Physics in 1991"

Frequency modulation detection using high‐Q cantilevers for enhanced force microscope sensitivity

Abstract: A new frequency modulation (FM) technique has been demonstrated which enhances the sensitivity of attractive mode force microscopy by an order of magnitude or more. Increased sensitivity is made possible by operating in a moderate vacuum (<10−3 Torr), which increases the Q of the vibrating cantilever. In the FM technique, the cantilever serves as the frequency determining element of an oscillator. Force gradients acting on the cantilever cause instantaneous frequency modulation of the oscillator output, which is demodulated with a FM detector. Unlike conventional ‘‘slope detection,’’ the FM technique offers increased sensitivity through increased Q without restricting system bandwidth. Experimental comparisons of FM detection in vacuum (Q∼50 000) versus slope detection in air (Q∼100) demonstrated an improvement of more than 10 times in sensitivity for a fixed bandwidth. This improvement is evident in images of magnetic transitions on a thin‐film CoPtCr magnetic disk. In the future, the increased sensitivi...

Barrier inhomogeneities at Schottky contacts

Abstract: We present a new analytical potential fluctuations model for the interpretation of current/voltage and capacitance/voltage measurements on spatially inhomogeneous Schottky contacts. A new evaluation schema of current and capacitance barriers permits a quantitative analysis of spatially distributed Schottky barriers. In addition, our analysis shows also that the ideality coefficient n of abrupt Schottky contacts reflects the deformation of the barrier distribution under applied bias; a general temperature dependence for the ideality n is predicted. Our model offers a solution for the so‐called T0 problem. Not only our own measurements on PtSi/Si diodes, but also previously published ideality data for Schottky diodes on Si, GaAs, and InP agree with our theory.

Rapid‐phase transitions of GeTe‐Sb2Te3 pseudobinary amorphous thin films for an optical disk memory

Abstract: Amorphous films having a component of the stoichiometric GeTe‐Sb2Te3 pseudobinary alloy system, GeSb2Te4 or Ge2Sb2Te5 representatively, were found to have featuring characteristics for optical memory material presenting a large optical change and enabling high‐speed one‐beam data rewriting. The material films being sandwiched by heat‐conductive ZnS layers can be crystallized (low power) or reamorphized (high power) by laser irradiation of very short duration, less than 50 ns. The cooling speed of the sandwiched film is extremely high: more than 1010 deg/s, which permits the molten material to convert to the amorphous state spontaneously; whereas, a low‐power pulse irradiation of the same duration changed the exposed portion into the crystalline state. The optical constant changes between the amorphous state and the crystalline state of them were measured to be large: from 4.7+i1.3 to 6.9+i2.6 and from 5.0+i1.3 to 6.5+i3.5, respectively. The crystallized portion was known to have a GeTe‐like fcc structure ...

Dynamic tortuosity and bulk modulus in air‐saturated porous media

Abstract: In this paper, the concept of characteristic length introduced in the definition of the dynamic tortuosity by Johnson, Koplik, and Dashen [J. Fluid Mech. 176, 379 (1987)] is extended to express the frequency dependence of the bulk modulus of the saturating fluid at high frequencies. A general phenomenological frequency dependence for this dynamic bulk modulus is obtained using the expression for the dynamic tortuosity. The theoretical predictions for dynamic tortuosity and bulk modulus are compared with experimental results obtained from acoustic measurements on a rigid‐frame porous material saturated with air.

Electron transport of inhomogeneous Schottky barriers: A numerical study

Abstract: Numerical simulations are presented of the potential distribution and current transport associated with metal‐semiconductor (MS) contacts in which the Schottky barrier height (SBH) varies spatially. It is shown that the current across the MS contact may be greatly influenced by the existence of SBH inhomogeneity. Numerical simulations indicate that regions of low SBH are often pinched‐off when the size of these regions is less than the average depletion width. Saddle points in the potential contours in close proximity to the low‐SBH regions, which are shown to vary with the dimension and magnitude of the inhomogeneity as well as with bias, essentially determine the electron transport across the low‐SBH regions. It is these dependences of the saddle point which give rise to various abnormal behaviors frequently observed from SBH experiments, such as ideality factors greater than unity, various temperature dependences of the ideality factor, including the T 0 anomaly, and reverse characteristics which are strongly bias‐dependent. The results of these numerical simulations are shown to support the predictions of a recently developed analytic theory of SBH inhomogeneity.

Magnetotransport properties of magnetically soft spin‐valve structures (invited)

Abstract: The magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 A/NiFe 62 A/Cu 22 A/NiFe 40 A/FeMn 70 A/Ta 50 A and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 A for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin‐dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness,...

An ecological optimization criterion for finite‐time heat engines

Abstract: An endoreversible Carnot‐type heat engine is studied under the usual restrictions: no friction, working substance in internal equilibrium (endoreversibility), no mechanical inertial effects, and under Newton’s cooling law for heat transfer between working fluid and heat reservoirs. A monoparametric family of straight lines which is isoefficient is found; i.e., all points (engine configurations) that belong to same line have the same efficiency. Along each line the power output divided by entropy production is a constant. From these properties and by using some dissipated quantities, relationships are obtained between reversible work and finite‐time work and between reversible efficiency and finite‐time efficiency. An ‘‘ecological’’ criterion is proposed for the best mode of operation of this heat engine. It consists in maximizing a function representing the best compromise between power and the product of entropy production and the cold reservoir temperature. The corresponding efficiency results almost equal to the average of the Carnot and the Curzon and Ahlborn [Am. J. Phys. 43, 22 (1975)] efficiencies.

Stress‐related problems in silicon technology

Abstract: The silicon integrated‐circuits chip is built by contiguously embedding, butting, and overlaying structural elements of a large variety of materials of different elastic and thermal properties. Stress develops in the thermal cycling of the chip. Furthermore, many structural elements such as CVD (chemical vapor deposition) silicon nitride, silicon dioxide, polycrystalline silicon, etc., by virtue of their formation processes, exhibit intrinsic stresses. Large localized stresses are induced in the silicon substrate near the edges and corners of such structural elements. Oxidation of nonplanar silicon surfaces produces another kind of stress that can be very damaging, especially at low oxidation temperatures. Mismatch of atomic sizes between dopants and the silicon, and heteroepitaxy produce another class of strain that can lead to the formation of misfit dislocations. Here we review the achievements to date in understanding and modeling these diverse stress problems.

Quantitative measurement of space-charge effects in lead zirconate-titanate memories

Abstract: By combining Auger data on the width of an oxygen depletion layer near the Pt electrodes with a modified Langmuir–Child law for the leakage current: I(V) = aV + bV2, we deduce parameters related to the space‐charge density and field in 210‐nm‐thick PbZr1−xTixO3 memories. The results are compared with the space charge fields inferred by Okazaki (∼10 kV/cm for PZT), which involve measuring the switching speeds ts(E) for positive and negative voltages. Differences in the voltage dependencies of the leakage current are found after fatigue and are related to specific electrochemical processes involving oxygen deposition on electrode surfaces.

Deposition of device quality, low H content amorphous silicon

Abstract: Device‐quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device‐quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.

Soft magnetic properties of nanocrystalline bcc Fe‐Zr‐B and Fe‐M‐B‐Cu (M=transition metal) alloys with high saturation magnetization (invited)

Abstract: A mostly single bcc phase with nanoscale grain sizes of 10 to 20 nm was found to form by annealing amorphous Fe‐Zr‐B, Fe‐Hf‐B, and Fe‐M‐B‐Cu(M=Ti, Zr, Hf, Nb, and Ta) alloys for 36 ks in the range of 723 to 923 K The high permeability (μe) above 10 000 at 1 kHz combined with high saturation magnetization (Bs) above 15 T was obtained for the bcc alloys The highest μe and Bs values reach 14 000 and 17 T for Fe91Zr7B2, 20 000 and 155 T for Fe87Zr7B5Cu1, and 48 000 and 152 T for Fe86Zr7B6Cu1 Magnetostriction (λs) decreases significantly by the phase transition from amorphous to bcc phase and is measured to be 1 × 10−6 for the bcc Fe86Zr7B6Cu1 alloy The small λs as well as the small grain size is concluded to be the reason for the good soft magnetic properties The lattice parameter of this bcc phase is 02870 nm being larger than that of pure α‐Fe The small λs seems to be achieved by the dissolution of solute elements above an equilibrium solubility limit The bcc Fe86Zr7B6Cu1 alloy also shows the l

Impurity enhancement of the 1.54‐μm Er3+ luminescence in silicon

Abstract: The effect of impurity coimplantation in MeV erbium‐implanted silicon is studied. A significant increase in the intensity of the 1.54‐μm Er3+ emission was observed for different coimplants. This study shows that the Er3+ emission is observed if erbium can form an impurity complex in silicon. The influence of these impurities on the Er3+ photoluminescence spectrum is demonstrated. Furthermore we show the first room‐temperature photoluminescence spectrum of erbium in crystalline silicon.

CuInSe2 for photovoltaic applications

Abstract: The properties and most successful methods for producing CuInSe2 films for solar‐cell applications are reviewed and the production, analysis, and performance of photovoltaic devices based on CuInSe2 are discussed. The most successful methods for depositing thin CuInSe2 films for high‐efficiency solar cells are three‐source elemental evaporation and selenization of Cu/In layers in H2Se atmospheres. Devices based on CuInSe2 have achieved the highest conversion efficiencies for any nonepitaxial thin‐film solar cell, 14.1% for a small cell and 10.4% (aperture efficiency) for a 3916‐cm2 (4 sq. ft) device. Furthermore, high‐efficiency devices have been produced by several groups and have shown no evidence of degradation of performance with time. The internal quantum efficiency is remarkably close to 100%, although various losses prevent making use of all of the generated carriers. The high performance results, in part, from the very‐high‐absorption coefficient of CuInSe2, which is of the order of 105 cm−1 for photons with energies slightly above 1 eV. Models of the operation of CuInSe2/CdS heterojunctions have begun to explain the processes limiting the device performance. The success of the models is based, in part, on the large amount of data which has accumulated on CuInSe2 in spite of the relatively short time it has been extensively studied.

Strain relaxation kinetics in Si1−xGex/Si heterostructures

Abstract: A semiempirical kinetic model is presented which maps out the thermal budget for processing of strained layer devices through epitaxial growth and postgrowth anneals. Misfit strain relaxation in Si1−xGex/Si heterostructures by the injection and propagation of a/2 〈110〉 60°‐type misfit dislocations has been studied for a range of geometries and dimensions. Strained layer superlattices, Si1−xGex alloy layers, 0

Magnetic properties of Fe deposited into anodic aluminum oxide pores as a function of particle size

Abstract: A combination of magnetic and electron microscopic measurements on iron deposited in chemically widened anodic aluminum oxide films has allowed us to relate the coercivity of these materials to the dimensions of the metal particles. The coercivity is found to be highly anisotropic and to depend on the aspect ratio of the metallic particles. The functional dependence of Hc on the aspect ratio fits the expression reported by Jacobs and Bean [Phys. Rev. 100, 1060 (1955)] for magnetization reversal by symmetric fanning. This is likely due to the fact that in our unannealed samples the metal deposit consists of a cylindrical assembly of fused single‐domain particles. Pore widening by chemical dissolution of the anodic oxide is found to depend quadratically on the time that the sample is soaked in the acid bath.

The maximum possible conversion efficiency of silicon‐germanium thermoelectric generators

Abstract: The thermoelectric properties of N‐type and P‐type Si‐Ge alloys have been reviewed and detailed calculations for the efficiency of a thermoelectric generator made from a 70% Si‐30% Ge alloy have been made over the temperature range from 300 to 1300 K. A model employing one valence band and two conduction bands has been used. A generator of standard material, optimally doped, and infinitely segmented will have an efficiency of 12.1% operating over this range. If the lattice thermal conductivity can be reduced to its minimum value without upsetting the electrical properties, then the efficiency can be raised to an ultimate maximum of 23.3%. A more modest increase in efficiency to 14.7% could be obtained by a 2.4 volume percent of finely dispersed second‐phase precipitates which would act as phonon scatterers. The utility/futility of GaP additions and grain‐boundary scattering as methods to increase the efficiency is discussed.

Atmospheric impregnation of porous silicon at room temperature

Abstract: Microporous and mesoporous Si layers contain a very large surface area that affects both their optical and electrical properties. Secondary ion mass spectroscopy (SIMS) analysis is used for the first time to simultaneously monitor all the major impurities on that surface. SIMS data on a microporous layer demonstrate that its chemical composition changes dramatically with time during ambient air exposure. Similar trends are observed for mesoporous layers. Extended storage in air at room temperature converts the hydride surface of freshly anodized layers to that of a contaminated native oxide. Characterization techniques need to take the metastability of the hydride surface into account since the structural, optical, and electrical properties of porous Si can consequently change with time upon exposure to ambient air. Low‐temperature photoluminescence and spectroscopic ellipsometry data on freshly anodized and ‘‘aged’’ microporous and mesoporous layers are chosen to illustrate typical changes in optical pro...

The extrinsic nature of nonlinear behavior observed in lead zirconate titanate ferroelectric ceramic

Abstract: The nonlinear electric and electromechanical responses of lead zirconate titanate Pb(ZrxTi1−x)O3 ceramics to an external ac electric field have been measured under different driving levels. The onset of measurable nonlinearity is observed to be accompanied by the appearance of hysteresis loops. This lossy nature suggests that the nonlinearities in a ferroelectric ceramic are generated by the domain‐wall motion. In addition, aging experiments and the bias field dependence of the threshold field (onset of nonlinearity) all indicate the extrinsic nature of the nonlinear behavior of ferroelectric ceramics. A phenomenological theory of Arlt, Dederichs, and Herbiet [Ferroelectrics 74, 34 (1987)] has been extended to include the nonlinear contributions. With only 90° wall vibration being considered, the theory leads to some basic understanding of the experimental results.

Stress and plastic flow in silicon during amorphization by ion bombardment

Abstract: In situ wafer curvature measurements were performed during amorphization of silicon by MeV ion implantation. These measurements provide information about density changes and plastic phenomena in the implanted region. Experiments were performed for a variety of ions, a range of fluxes, and for temperatures between −175 and 200 °C. In all cases, the implanted region expanded due to the creation of damaged crystal, creating compressive stress in the implanted region on the order of 108 N/m2. Once heavily damaged or amorphous regions were formed, radiation‐enhanced plastic flow of material out of the plane of the wafer occurred in order to relieve the stress created by the expansion. The value of the shear viscosity responsible for this phenomena could be measured by comparing samples with the same history but different stresses. For 2‐MeV Xe implantation at room temperature and 1011 ions/cm2 s, the radiation‐enhanced shear viscosity is ∼ 1013 Ns/m2, which is at least four orders of magnitude smaller than the thermally activated shear viscosity. Possible contributions to flow from a homogeneous distribution of broken bonds and from fluid‐like collision cascade regions are discussed.

Granular magnetic solids (invited)

Abstract: Granular metals consist of nanometer size metal granules embedded in an immiscible medium. They display a rich variety of physical properties as a result of their unique nanostructure and extra degrees of freedom. They are also suitable for the exploration of finite‐size effects, enhanced and tailored properties of fundamental interest, and for technological applications. Granular magnetic solids of elemental metals and alloys will be discussed. Single‐domain characteristics, superparamagnetic relaxation, enhanced ferromagnetic properties, granular alloys, and spin glasses are some of the topics covered.

A model for the high‐temperature transport properties of heavily doped n‐type silicon‐germanium alloys

Abstract: A model is presented for the high-temperature transport properties of large-grain-size, heavily doped n-type silicon-germanium alloys. Electron and phonon transport coefficients are calculated using standard Boltzmann equation expressions in the relaxation time approximation. Good agreement with experiment is found by considering acoustic phonon and ionized impurity scattering for electrons, and phonon-phonon, point defect, and electron-phonon scattering for phonons. The parameters describing electron transport in heavily doped and lightly doped materials are significantly different and suggest that most carriers in heavily doped materials are in a band formed largely from impurity states. The maximum dimensionless thermoelectric figure of merit for single-crystal, n-type Si(0.8)Ge(0.2) at 1300 K is estimated at ZT about 1.13 with an optimum carrier concentration of n about 2.9 x 10 to the 20th/cu cm.

Particle generation and behavior in a silane‐argon low‐pressure discharge under continuous or pulsed radio‐frequency excitation

Abstract: The generation and behavior of particles in a low‐pressure silane‐argon discharge have been analyzed under continuous and pulsed radio‐frequency (rf) excitation conditions. In the continuous rf excitation regime, the influence of parameters such as gas temperature and silane partial pressure are determined. By using rf pulsed excitation, it is shown that gas‐flow effects play a predominant role for particle dynamics when the excitation is stopped. Radio‐frequency regimes with short and adjustable rf off sequences are used to study both the inhibition of particle formation and the elimination of particles from the dusty plasmas. The electrical properties of the discharge are shown to be sensitive to the presence of the particles. Simple models for particle trapping in the plasma edge and for particle dynamics when the discharge is turned off are presented.

Thermoelectric properties of a composite medium

Abstract: We study the thermoelectric properties of a composite medium. Various approximations are developed for calculating the matrix Qe of the bulk effective transport coefficients of the medium, including exact upper and lower bounds for Qe under various conditions. Results are especially detailed for two‐component composites, where a field decoupling transformation is used to reduce the thermoelectric problem to two uncoupled quasi‐conductivity problems. Exact bounds are then obtained for the absolute thermopower αe and the thermoelectric figure of merit Ze in two‐component composites. We prove that Ze of the composite can never exceed the largest value of Z in any component. Some of these results are extended to certain classes of multicomponent composites.

Charging of pattern features during plasma etching

Abstract: The localized charging of a rectangular trench during the plasma etching of a perfectly insulating surface was modeled assuming an isotropic electron flux and monodirectional ion bombardment. The field set up by the localized charging acts to deflect arriving ions, modifying the ion flux densities within the feature, and thus, etching rates. Preliminary simulations indicate that this may be important in the shaping of etching profiles.

Force constant and effective mass of 90° domain walls in ferroelectric ceramics

Abstract: Domain wall contributions to the dielectric, piezoelectric, and elastic properties of tetragonal ferroelectric ceramics, as discussed extensively in the past, are calculated. A simple model shows that the motion of 90° domain walls causes a shear deformation and an approximately homogeneous electric field in the grain. The elastic and electric field energies involved allow the calculation of the force constant for the domain wall displacement by external fields. The displacements agree with experimental results. In a moderate electric field the displacement is a small fraction of the lattice cell only. By averaging over the orientational distributions of all grains the contributions of the 90° domain walls to the material properties are calculated for unpolarized and for polarized ceramics and agree with experimental results. The effective mass, which has to be attributed to the domain walls is the mass of the whole grain reduced by the factor S0 (spontaneous deformation), is independent of the domain wid...

Effect of oxygen pressure on the synthesis of YBa2Cu3O7-x thin films by post-deposition annealing

Abstract: The effect of ambient oxygen pressure on the synthesis of epitaxial YBa2Cu3O7−x films on (100) SrTiO3 substrates by post‐deposition annealing of amorphous precursor films was studied for oxygen partial pressures pO2 between 1.0 and 8.0×10−5 atm and annealing temperatures between 890 and 650 °C. A pO2−1/T diagram containing recent literature data regarding YBa2Cu3O7−x oxygen stoichiometry, phase stability, and liquid‐phase formation was compiled to provide guidance for the selection and interpretation of annealing conditions. The results evidence a strong dependency of growth properties on the oxygen pressure with enhanced c‐oriented epitaxy at lower pO2 values. A particularly interesting result is the formation of predominantly c‐oriented films at 740 °C and pO2=2.6×10−4 atm (0.2 Torr). Similar to YBa2Cu3O7−x films produced by in situ laser ablation at the same temperature and oxygen pressure, the films exhibited low ion channeling yields (χmin<0.1) and a dense (smooth) surface morphology, while critical ...

An analytical model for organic‐based thin‐film transistors

Abstract: A model describing the current‐voltage characteristics of organic thin‐film transistors (TFTs) is presented. The model is based on the trap distribution deduced from temperature‐dependent current‐voltage measurements on Au/alpha‐sexithienyl (α6T)/Au symmetrical structures, which comprises a dominant single shallow trap level located near the valence‐band edge. Numerical and approximate analytical derivations of the saturation current density as a function of the gate voltage have been made. From these calculations, the dependence of the threshold voltage on the parameters of the trap level (density and energy) is deduced. It appears that the threshold voltage corresponds to the filling of traps, and is a surface equivalent of the trap‐filled limit voltage in bulk space‐charge‐limited current. The model is in good agreement with experimental data on α6T TFTs. The energy of the trap level compares well with that obtained from the temperature‐dependent conductivity. However, the mobility is much lower in the...

Prediction of phase formation sequence and phase stability in binary metal-aluminum thin-film systems using the effective heat of formation rule

Abstract: The effective heat of formation (ΔH’) concept allows heats of formation to be calculated as a function of concentration. In this work the effective heat of formation rule is used to predict first phase formation in metal‐aluminum thin‐film systems and to predict subsequent phase sequence for thin metal films on thick aluminum or thin aluminum on thick metal substrates. The effective concentration at the growth interface is taken to be that of the lowest temperature eutectic (liquidus) for the binary system. Although the effective heat of formation rule may predict that formation of a certain phase would lead to the largest free energy change, this phase does not necessarily form at the moving reaction interface if it has difficulty to nucleate. By excluding phases with a large number of atoms per unit cell and which thus have difficulty to nucleate, the effective heat of formation rule successfully predicts first phase aluminide formation for all 15 metal‐aluminum binary systems for which experimental data could be found. It is also shown how the effective heat of formation rule can be used to predict formation and decomposition of aluminide phases in contact with each other or in contact with their component metals.

Ultrafine particles of Fe, Co, and Ni ferromagnetic metals

Abstract: Fe, Co, and Ni ultrafine particles (UFP) were prepared by a gas evaporation method in an inert atmosphere. The different preparing conditions of the Fe, Co, and Ni UFP included inert gas pressures and kinds were investigated by means of x‐ray diffraction, transmission electron microscopy (TEM), and magnetic measurements with low or high temperatures. The experimental results show that coercivities Hc of the Fe, Co, and Ni UFP, depending on particle sizes, obey a single‐domain theory. The maximum Hc was 450, 1000, and 1500 Oe, respectively, for Ni (310 A), Fe (210 A), and Co (200 A) at room temperature. The coercivities came up to high in the low temperatures. The saturation magnetizations Ms of the Fe, Co, and Ni UFP decreased with small particle sizes. A face‐centered structure of Co UFP (200–300 A) was confirmed by x‐ray diffraction. Some Fe UFP in the shape of balls (200–400 A) observed in the TEM may be an amorphous structure.

Exchange resonance modes in a ferromagnetic sphere

Abstract: Magnetostatic resonance modes are usually calculated by assuming that the exchange energy is negligible compared to the magnetostatic energy. It is shown that in the other extreme, when the magnetostatic energy is small compared to the exchange energy, the resonance modes are a quadratic function of the reciprocal size.