Showing papers in "Journal of Applied Physics in 1989"

Electroluminescence of doped organic thin films

Abstract: Electroluminescent (EL)devices are constructed using multilayer organic thin films. The basic structure consists of a hole‐transport layer and a luminescent layer. The hole‐transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8‐hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The ELquantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue‐green to orange‐red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron‐hole recombination and emission zones can be confined to about 50 A near the hole‐transport interface. In the undoped Alq, the EL emission zone is considerably larger due to excitondiffusion. The multilayerdopedEL structure offers a simple means for the direct determination of excitondiffusion length.

Impedance and modulus spectroscopy of semiconducting BaTiO3 showing positive temperature coefficient of resistance

Abstract: Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain‐boundary and bulk effects may be distinguished from capacitance data: grain‐boundary effects have temperature‐independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain‐boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Improved laser‐beam uniformity using the angular dispersion of frequency‐modulated light

Abstract: A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high‐intensity spikes within the laser chain, making the technique attractive for the high‐power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency‐modulated bandwidth on the laser beam using an electro‐optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction‐limited focal spot is the size of the target. The time‐averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction‐limited intensity pattern on target.

An atomic-resolution atomic-force microscope implemented using an optical lever

Abstract: We present the first atomic‐resolution image of a surface obtained with an optical implementation of the atomic‐force microscope (AFM). The native oxide on silicon was imaged with atomic resolution, and ≊5‐nm resolution images of aluminum, mechanically ground iron, and corroded stainless steel were obtained. The relative merits of an optical implementation of the AFM as opposed to a tunneling implementation are discussed.

Kim model for magnetization of type‐II superconductors

Abstract: We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Trap creation in silicon dioxide produced by hot electrons

Abstract: Trap creation in both the bulk of silicon dioxide films and at its interfaces with silicon and metallic contacting electrodes is shown to depend on the presence of hot electrons in the oxide. For thick oxides (≥100 A), little trap creation is observed in the near‐thermal transport regime at electric field magnitudes less than 1.5 MV/cm. At these low fields, electrons travel in a streaming fashion close to the bottom of the oxide conduction band at energies less than that of the dominant optical phonon mode at 0.153 eV. At higher electric fields, the rate of bulk trap creation is proportional to the average energy of the hot electrons, which move in a dispersive manner and can reach energies as large as 4 eV. For thin oxides (<100 A) where electrons can travel ballistically (i.e., without scattering), traps are not produced unless injected electrons acquire more than 2 eV of kinetic energy from the applied electric field, regardless of the magnitude of this field. All data on both thin and thick oxides are shown to give a threshold for trap creation of about 2.3 eV by the hot electrons in the oxide conduction band. Also, trap creation is shown to be suppressed by lowering the lattice temperature below ≊150 K. Our results are discussed in terms of a model involving hydrogen‐related‐species release from defect sites near the anode by the hot electrons and the subsequent motion of these molecules to regions near the cathode where they can interact with the lattice and form the trapping sites which are measured.

Theoretical models of the electrical discharge machining process. I. A simple cathode erosion model

Abstract: A simple cathode erosion model for the electrical discharge machining (EDM) process is presented. This point heat‐source model differs from previous conduction models in that it accepts power rather than temperature as the boundary condition at the plasma/cathode interface. Optimum pulse times are predicted to within an average of 16% over a two‐decade range after the model is tuned to a single experimental point. A constant fraction of the total power supplied to the gap is transferred to the cathode over a wide range of currents. A universal, dimensionless model is then presented which identifies the key parameters of optimum pulse time factor (g) and erodibility (j) in terms of the thermophysical properties of the cathode material. Compton’s original energy balance for gas discharges is amended for EDM conditions. Here it is believed that the high density of the liquid dielectric causes plasmas of higher energy intensity and pressure than those for gas discharges. These differences of macroscopic diele...

Perpendicular magnetic anisotropy and magneto-optical Kerr effect of vapor-deposited Co/Pt-layered structures

Abstract: We prepared by vapor deposition at room temperature thin (500 A) Co/Pt multilayers or layered structures directly onto glass or Si substrates. They show a preferential magnetization perpendicular to the film plane for Co thicknesses below 12 A and a 100% perpendicular remanence for Co thicknesses below 4.5 A. The magnetic anisotropy can be explained by an interface contribution to the anisotropy. We also investigated the magneto‐optical (MO) polar Kerr effect of these multilayers. Because of their excellent magnetic properties and their potentially high oxidation and corrosion resistance, these Co/Pt‐layered structures are very promising candidates for MO recording. The Kerr rotation θk at λ=820 nm for a 35×(4.0 A Co+12.7 A Pt)‐layered structure, which has 100% magnetic remanence, is modest (−0.12°), but the reflectivity R is high (70%), which results in a respectable figure of merit Rθ2k. Furthermore, the Kerr effect increases towards shorter wavelengths and thus favors future higher‐density recording.

Anharmonicity and the equation of state for gold

Abstract: The temperature dependence of the thermodynamic and the elastic properties of elemental gold are found from published data. It is shown that measurements for (∂KT/∂P)T near 5.5 are more thermodynamically consistent than are higher values of this parameter which have been reported earlier. Using 5.5 for (∂KT/∂P)T, we find that (∂KT/∂T)V is not zero but −11.5×10−3 GPa K−1 for high temperatures (T>θD, where θD is the Debye temperature). One consequence of this is that above θD the thermal pressure, PTH, along the room‐pressure isobar can be expressed as PTH(T)−PTH(300)=[7.14×10−3 +(∂K T/∂T)v ln(Va/V)] ×(T−300) GPa for T at absolute temperature and Va being the volume at ambient conditions. These results give thermal pressure values near those previously reported at small compressions, but give lower thermal pressures at large compressions. This study suggests that in order to ensure thermodynamic consistency, the value of d ln γ/d ln V is near 2.5–3.0 which is higher than values of 1.0 and 1.7 reported previ...

Optical dispersion relations for GaP, GaAs, GaSb, InP, InAs, InSb, AlxGa1−xAs, and In1−xGaxAsyP1−y

Abstract: A method is described for calculation of the optical constants (the refractive index, extinction coefficient, and absorption coefficient) of some III‐V binaries (GaP, GaAs, GaSb, InP, InAs, and InSb), ternaries (AlxGa1−xAs), and quaternaries (In1−xGaxAsyP1−y) in the entire range of photon energies (0–6.0 eV). The imaginary part of the dielectric function [e2(ω)] is derived first from the joint density‐of‐states functions at energies of various critical points (CPs) in the Brillouin zone; then its real part [e1(ω)] is obtained analytically using the Kramers–Kronig relation. The indirect band‐gap transitions are also assumed to provide a gradually increasing e2 spectrum expressed by a power law of (ℏω−EIDg)2, where ℏω is the photon energy and EIDg is the indirect band‐gap energy. The optical dispersion relations are expressed in terms of these model dielectric functions. The present model reveals distinct structures in the optical constants at energies of the E0, E0+Δ0 [three‐dimensional (3‐D) M0 CP], E1, E...

Theoretical determination of net radiation force and torque for a spherical particle illuminated by a focused laser beam

Abstract: Series expressions for the net radiation force and torque for a spherical particle illuminated by an arbitrarily defined monochromatic beam are derived utilizing the spherical‐particle/arbitrary‐beam interaction theory developed in an earlier paper. Calculations of net force and torque are presented for a 5‐μm‐diam water droplet in air optically levitated by a tightly focused (2 μm beam waist diameter) TEM00‐mode argon‐ion (λ=0.5145 μm) laser beam for on and off propagation axis, and on and off structural resonance conditions. Several features of these theoretical results are related to corresponding experimental observations.

The equation of state of platinum to 660 GPa (6. 6 Mbar)

Abstract: Platinum metal was shock compressed to 660 GPa using a two‐stage light‐gas gun to qualify this material as an ultrahigh‐pressure standard for both dynamic and static experiments. The shock velocity data are consistent with most of the previously measured low‐pressure data, and an overall linear us−up relationship is found over the range 32–660 GPa. As a part of this work, we have also extended the Hugoniot of the tantalum standard we use to 560 GPa; we have included these data into a new linear fit of the tantalum Hugoniot between 55–560 GPa. We also present the results of a first‐principles theoretical treatment of compressed platinum. The fcc phase is predicted to remain stable to beyond 550 GPa. In addition, we have calculated the 300‐K pressure‐volume isotherm and the Hugoniot. The latter is in excellent agreement with experimental results and qualifies the former to at least 10% accuracy.

Structural and thermodynamic properties of heavily mechanically deformed Ru and AlRu

Abstract: We report on high‐energy ball milling of Ru and AlRu. The deformation results in a drastic decrease of the crystal size to a nanometer scale and in an increase of atomic‐level strain. This is accompanied by a disordering of the crystal lattice as is shown by means of the long‐range‐order parameter in AlRu. The specific heat increases by more than 15%–20%, indicating large changes in the vibrational and configurational part of the entropy. The stored energy of cold work is up to 6 kJ/mol for AlRu and 10 kJ/mol for Ru. This is almost 40% of the heat of fusion of Ru and exceeds by far the energies stored by other deformation processes.

Using a magnetometer to image a two‐dimensional current distribution

Abstract: We describe a mathematical algorithm to obtain an image of a two‐dimensional current distribution from measurements of its magnetic field. The spatial resolution of this image is determined by the signal‐to‐noise ratio of the magnetometer data and the distance between the magnetometer and the plane of the current distribution. In many cases, the quality of the image can be improved more by decreasing the current‐to‐magnetometer distance than by decreasing the noise in the magnetometer.

Theoretical models of the electrical discharge machining process. II. The anode erosion model

Abstract: As a second in a series of theoretical models for the electrical discharge machining (EDM) process, an erosion model for the anode material is presented As with our point heat‐source model in the previous article, the present model also accepts power rather than temperature as the boundary condition at the plasma/anode interface A constant fraction of the total power supplied to the gap is transferred to the anode The power supplied is assumed to produce a Gaussian‐distributed heat flux on the surface of the anode material Furthermore, the area upon which the flux is incident is assumed to grow with time The model is capable of showing, via the determined migrating melt fronts, the rapid melting of the anodic material as well as the subsequent resolidification of the material foation from plasma dynamics modeling could improve substantially our results

Fifth-order corrected electromagnetic field components for a fundamental Gaussian beam

Abstract: Fifth‐order corrected expressions for the electromagnetic field components of a monochromatic fundamental Gaussian beam (i.e., a focused TEM00 mode laser beam) propagating within a homogeneous dielectric media are derived and presented. Calculations of relative error indicate that the fifth‐order Gaussian beam description provides a significantly improved solution to Maxwell’s equations in comparison with commonly used paraxial (zeroth‐order) and first‐order Gaussian beam descriptions.

Magnetic and magneto‐optical properties of rare‐earth transition‐metal alloys containing Dy, Ho, Fe, Co

Abstract: Amorphous rare‐earth transition‐metal alloys of composition REl−xTMx with RE=Dy, Ho; TM=Fe,Co and 0

Rate equation analysis of microcavity lasers

Abstract: We describe the light output properties of single mode lasers having cavity dimensions on the order of the emitted wavelength. A simple rate equation formula is derived for a four‐level laser assuming enhanced spontaneous emission into the cavity. These rate equation analyses show that increasing the coupling of spontaneous emission into the cavity mode causes the lasing properties to become quite different from those of usual lasers having cavity dimensions much larger than a wavelength. We find that the lasing threshold disappears, the light emission efficiency increases, relaxation oscillations do not occur, and the dynamic response speed is improved. It is shown that the spontaneous emission rate alteration caused by the cavity plays an essentially important role for these characteristics.

Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area

Abstract: To investigate the effect of growth area on interface dislocation density in strained‐layer epitaxy, we have fabricated 2‐μm‐high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500‐, 7000‐, and 8250‐A‐thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large‐area samples, were then deposited by molecular‐beam epitaxy. For the 3500‐A layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface‐dislocation density was found to vary linearly with mesa...

Theory for a quantum modulated transistor

Abstract: We present a theoretical study of semiconductor T‐structures which may exhibit transistor action based on quantum interference. The electron transmission through a semiconductor quantum wire can be controlled by an external gate voltage that modifies the penetration of the electron wavefunction in a lateral stub, affecting in this way its interference pattern. The structures are modeled as ideal two‐dimensional electron waveguides and a tight‐binding Green’s function technique is used to compute the electron transmission and reflection coefficients. The calculations show that relatively small changes in the stub length can induce strong variations in the electron transmission across the structure. Operation in the fundamental transverse mode appears to be important for applications. We also show that a bound state of purely geometrical origin nucleates at the intersection between waveguide and stub. The performance of the device can be improved by inserting additional stubs of slightly different lengths. ...

Oxidation studies of SiGe

Abstract: We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

Time domain reflection methods for dielectric measurements to 10 GHz

Abstract: Total reflection methods and instrumentation for their use are described for measurements of dielectric permittivity and loss at frequencies to 10 GHz or more. Several cell designs are shown, together with analyses of their performance. Procedures are given for correcting effects of wave propagation in the cells and residual reflections in the cells by bilinear analysis with calibrations using dielectrics of known permittivity. Representative results are presented for highly polar liquids, dilute solutions of polar molecules in nonpolar solvents, electrolyte solutions, and ionic glasses with appreciable ohmic conduction.

Laser performance of LiSrAlF6:Cr3+

Abstract: We have lased the new material, LiSrAlF6:Cr3+ (Cr3+:LiSAF). The single crystals were grown by the horizontal zone melting technique. The spectroscopic properties of Cr3+:LiSAF are similar to those of other low‐field Cr3+‐doped systems, although the emission cross section is strongly π polarized and is also somewhat larger than has been measured for other fluoride hosts. The free‐running lasing wavelength of Cr3+:LiSAF is 825 nm, and the tuning range extends from at least 780 to 920 nm. Using Kr laser pumping, we obtained slope efficiencies of 36% and 14% by utilizing output couplings of 4.8% and 0.8%, respectively. On the basis of these results, the extrapolated maximum efficiency of 53% is determined, to be compared to the quantum defect‐limited value of 78%. It is concluded that a moderate level of excited state absorption (ESA) loss is responsible for the reduced efficiency of the Cr3+:LiSAF system. This contrasts with the related results previously obtained for LiCaAlF6:Cr3+, where it was concluded th...

Model of plasma immersion ion implantation

Abstract: In plasma immersion ion implantation, a target is immersed in a plasma and a series of negative high‐voltage pulses are applied to implant plasma ions into the target. We develop an approximate analytical model to determine the time‐varying implantation current, the total dose, and the energy distribution of the implanted ions.

Large displacement transducers based on electric field forced phase transitions in the tetragonal (Pb0.97La0.02) (Ti,Zr,Sn)O3 family of ceramics

Abstract: Several properties associated with the field‐induced phase change between the antiferroelectric and ferroelectric states in tetragonal (Pb0.97 La0.02 )(Sn,Ti,Zr)O3 antiferroelectric ceramic family were investigated for high‐strain displacement transducer applications. The longitudinal field‐induced strain accompanying the phase change is in the range of 0.2%–0.9%. The single‐shot switching time between the two states is on the order of 1–2 μs. Under continuous ac field driving, the hysteretic heating effect introduces a temperature rise, changing the original room‐temperature switching behavior. The ceramics degrade under ac field excitation, the average life cycles is in the range of 106 –107 cycles which can be greatly improved by carefully polishing the sample surfaces.

Perpendicular magnetic anisotropy and magnetostriction of sputtered Co/Pd and Co/Pt multilayered films

Abstract: Magnetic properties of Co/Pd and Co/Pt multilayers were studied as a function of sputtering gas pressure. It was found that the magnetic properties of the films depended upon the sputtering gas pressure, and large coercivity (several kOe) and a perfect squareness of the perpendicular hysteresis loop were attained by the deposition at the high gas pressure. These multilayers are suitable for perpendicular magnetic recording media. In the Co/Pd, the origin of the perpendicular magnetic anisotropy is mainly an interfacial anisotropy, although there exists a stress‐induced anisotropy for the films with the relatively short periodicity. The perpendicular magnetic anisotropy of the Co/Pt can be concerned with the Co‐Pt mixture formed at the interface of the multilayer structure. The magnetostriction of the Co/Pd films with the relatively short periodicity is extraordinarily large with a negative sign, since Co atoms adjacent to Pd atoms at the interface could induce larger magnetostriction than that of pure Co....

Spin coating: One‐dimensional model

Abstract: The model of the spin‐coating process presented here accounts for variations of concentration, viscosity, and diffusivity across the thickness of the spin‐coated film. The flow of the liquid is governed by a balance between centrifugal driving force and viscous resisting force. Radial variations in film thickness and concentration are neglected. The Galerkin/finite‐element method is employed to solve the equation set. Film thinning slows initially due to decreasing film thickness and ceases finally due to dramatically increasing viscosity of the coating liquid as solvents evaporate. The formation of a region of extremely low solvent concentration and correspondingly high viscosity and low binary diffusivity at the free surface, i.e., a solid ‘‘skin,’’ is predicted. Coating defects can occur if convective flow has not completely ceased when this skin forms. Skin formation can be eliminated or delayed by partially saturating the overlying gas with solvent or by using mixed solvents (having both high and low...

Silicon surface passivation by hydrogen termination: A comparative study of preparation methods

Abstract: Clean silicon surfaces having low carbon and oxygen contamination are necessary for good epitaxial overgrowth. Methods for low‐temperature preparation of clean surfaces are needed to fully enable low‐temperature fabrication processes on silicon. In this paper silicon surfaces are compared for residue and chemical passivation after (i) hydrogen termination by various low‐temperature, wet‐chemical techniques, (ii) cleaving in ultrahigh vacuum (UHV), and (iii) ion sputtering. Surface residue was characterized with x‐ray photoelectron spectroscopy (XPS) and small‐spot Auger electron spectroscopy (AES). Low‐energy electron diffraction (LEED) was also used. Evaluations of passivation were done by exposing the cleaned surfaces to various environments, e.g., UHV, N2 gas, and room air. We have obtained especially promising results with a technique whereby the wafer surface oxide is etched using an HF‐alcohol reagent in a flowing nitrogen atmosphere at room temperature while the wafer is spinning, i.e., a spin etch...

A constitutive model for strain rates from 10−4 to 106 s−1

Abstract: We have developed an addition to the Steinberg–Guinan high strain‐rate constitutive model that extends its validity to strain rates as low as 10−4 s−1. With this new model, we have successfully reproduced a number of rate‐dependent, shock‐induced phenomena in tantalum, such as precursor on reshock, precursor decay, and shock smearing. We have also successfully calculated a plate‐impact experiment at a loading stress of 230 GPa as well as extensive data for yield strength versus strain rate at room temperature and yield strength versus temperature at a strain rate of 10−4 s−1.

Gettering in silicon

Abstract: A series of gettering experiments have been carried out for a better understanding of gettering mechanism(s) in silicon. We find that oxidation and oxynitridation, which are known to inject silicon interstitials, do not getter metallic impurities such as Au, Cu, Fe, and Ni while phosphorus (P) diffusion does produce effective gettering of these metals. We also find from P diffusion, Ar ion implantation, and Ni film gettering performed as a function of temperature, there exists an optimum gettering temperature. From a comprehensive discussion of the existing models, we conclude that neither the enhanced metal solubility nor the silicon interstitial model explains our experimental results. Furthermore, it is shown that generation of dislocations is not a prerequisite for effective gettering. A model, based on the segregation of impurities at high temperatures and on the release/diffusion of metallic impurities at lower temperatures, is proposed to explain all of our results. A general form of the segregatio...