Showing papers in "Journal of Applied Physics in 1986"

Evaporated Sn‐doped In2O3 films: Basic optical properties and applications to energy‐efficient windows

Abstract: We review work on In2O3:Sn films prepared by reactive e‐beam evaporation of In2O3 with up to 9 mol % SnO2 onto heated glass. These films have excellent spectrally selective properties when the deposition rate is ∼0.2 nm/s, the substrate temperature is ≳150 °C, and the oxygen pressure is ∼5×10−4 Torr. Optimized coatings have crystallite dimensions ≳50 nm and a C‐type rare‐earth oxide structure. We cover electromagnetic properties as recorded by spectrophotometry in the 0.2–50‐μm range, by X‐band microwave reflectance, and by dc electrical measurements. Hall‐effect data are included. An increase of the Sn content is shown to have several important effects: the semiconductor band gap is shifted towards the ultraviolet, the luminous transmittance remains high, the infrared reflectance increases to a high value beyond a certain wavelength which shifts towards the visible, phonon‐induced infrared absorption bands vanish, the microwave reflectance goes up, and the dc resisitivity drops to ∼2×10−4 Ω cm. The corre...

Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals

Abstract: The temperature dependence of the saturation magnetization and the magnetocrystalline anisotropy field have been measured on single‐crystal samples of the R2Fe14B compounds for R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm from 4.2 K to the magnetic ordering temperatures. A spin reorientation transition of the Nd2Fe14B type has been found in Ho2Fe14B at 57.6 K in zero field. Another type of spin reorientation caused by anisotropy compensation between the Fe and the R sublattices exists in Er2Fe14B and Tm2Fe14B. The temperature dependence of the angle of the easy direction of magnetization from the c axis has been measured for R=Nd, Ho, Er, and Tm. The relation between the magnetocrystalline anisotropy and the sublattice magnetization is investigated by employing a simplified two‐sublattice molecular field model.

Near‐field optical‐scanning microscopy

Abstract: Near‐field optical‐scanning (NFOS) microscopy or ‘‘optical stethoscopy’’ provides images with resolution in the 20‐nm range, i.e., a very small fraction of an optical wavelength. Scan images of metal films with fine structures presented in this paper convincingly demonstrate this resolution capability. Design of an NFOS microscope with tunnel distance regulation, its theoretical background, application potential, and limitations are discussed.

Optical properties of AlxGa1−x As

Abstract: We report pseudodielectric function 〈e〉 data for AlxGa1−xAs alloys of target compositions x=0.00–0.80 in steps of 0.10 grown by liquid‐phase epitaxy and measured by spectroellipsometry. Cleaning procedures that produce abrupt interfaces between the technologically relevant alloys x≤0.5 and the ambient are described. The 〈e2〉 data are corrected near the fundamental direct absorption edge by a Kramers–Kronig analysis of the 〈e1〉 data to circumvent a limitation of the rotating‐analyzer ellipsometric technique. The results and the associated pseudooptical functions 〈n〉, 〈R〉, and 〈α〉 are listed in tabular form. Accurate values of the E0 and E1 threshold energies are determined from these spectra by Fourier methods. From these values, and from similar values for a GaAs‐capped AlAs sample grown by organometallic chemical vapor deposition, the dependencies of the E0 and E1 interband critical point energies on nominal composition are obtained. Cubic polynomial representations of these dependences are determined to...

Metal deposition from a supported metal film using an excimer laser

Abstract: A method for the direct writing of metal features from a metal film supported on an optically transparent substrate using a single pulse from a high‐energy excimer laser (193 nm) is presented. The technique eliminates the need for gas‐phase precursors in many cases and is an inherently clean process. Results of copper depositions onto silicon substrates are shown to exemplify the technique and a mechanism for the process is proposed.

Optical band gap of indium nitride

Abstract: Room‐temperature optical absorption data in the 1.5–2.5‐eV range are reported for indium nitride thin films prepared by reactive radio‐frequency sputtering. The fundamental absorption edge in high‐purity material is located at 1.89±0.01 eV and corresponds to a direct transition at k=0, in agreement with band‐structure calculations. A significant Moss‐Burstein shift is noted for carrier concentrations in excess of 1019 cm−3 and obeys the empirical relationship EG =1.89+2.1×10−8 n1/3 eV.

Aluminum or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass

Abstract: To utilize the excellent properties of silica (SiO2) glass for a glass laser host, neodymium‐aluminum (Nd–Al) and neodymium‐phosphorous (Nd–P) co‐doped SiO2 glasses were studied. They were prepared by plasma‐torch chemical vapor deposition (CVD). It was found that a doping level less than ten times the number of Nd for the Al co‐dopant and less than about fifteen times for the P co‐dopant was enough to remove undesirable fluorescence properties of Nd‐doped SiO2 glasses and make them suitable for laser application. The clustering Nd ions disperse well in a glass matrix and lasing fluorescence increases. The effects of the Al dopant on the density and Raman spectra were also studied to obtain structural information. On the basis of glass science and solution chemistry, the marked effects of both dopants were explained by the following model. Nd ions can be well incorporated into a SiO2 glass network through co‐dopant oxide forming a solvation shell around the Nd ions. This model leads to an expansible metho...

Silicon‐to‐silicon direct bonding method

Abstract: It was found that strong bonding takes place when a pair of clean, mirror‐polished silicon surfaces are contacted at room temperature after hydrophilic surface formation. Bonding strength reaches the fracture strength of silicon bulk after heating above 1000 °C. Electric resistivity at the interface is less than 10−6 Ω/cm2. Bonding p‐type silicon to n‐type silicon forms a diode. The reaction between silanol groups formed on the surface may cause the bonding force. Heating above 1000 °C was thought to diffuse oxygen to inside the silicon bulk, forming an epitaxial‐like lattice continuity at the interface.

Polymer dispersed nematic liquid crystal for large area displays and light valves

Abstract: A new electro‐optical material based on nematic liquid crystal dispersed in a polymer matrix has recently been introduced by Fergason. This technology (termed NCAP, for nematic curvilinear aligned phase) is suitable for making very large area (thousands of square centimeter) light valves and displays. The device consists of micron size droplets of liquid crystal dispersed in and surrounded by a polymer film. Light passing through the film in the absence of an applied field is strongly forward scattered, giving a milky, translucent film. Application of an electric field across the liquid crystal/polymer film places the film in a highly transparent state. Pleochroic dyes may be employed in the system in order to achieve controllable light absorption as well as scattering. Microscopically, it is shown that the liquid‐crystal director lies preferentially parallel to the polymer wall, leading to a bipolar‐like configuration of the liquid‐crystal directors within the droplet. The symmetry axes of the droplets are randomly oriented in the unpowered, scattering state, but align parallel to the field in the powered, transparent state. The electric field required to reorient a given droplet varies inversely with the diameter of that droplet, and it is shown that the macroscopic electro‐optical properties of the film can be modeled if the distribution of liquid‐crystal droplet sizes is known.

Transient thermoreflectance from thin metal films

Abstract: This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one‐dimensional heat flow equation using a two‐parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

Preparation of c‐axis oriented PbTiO3 thin films and their crystallographic, dielectric, and pyroelectric properties

Abstract: Highly c‐axis oriented and epitaxial PbTiO3 thin films were grown on MgO single crystals and epitaxial Pt film substrates by the rf‐magnetron sputtering method. The films with a 98% c‐axis orientation rate were obtained under low deposition rate (<20 A/min) and low gas pressure (∼1 Pa) using a PbO‐rich target. Phase transition of the PbTiO3 thin film was studied by high‐temperature x‐ray diffraction. It was found that the c‐axis of the tetragonal phase was parallel to the substrate just below Tc and the c‐axis becomes perpendicular to the substrate with lowering of temperature. These films show high resistivity (∼1010/Ω cm). It was found that significant pyroelectric currents are detected on all specimens without poling treatment. The polarization directions of the films are the same in all specimens. One of these films had a low relative dielectric constant of 97 and a high pyroelectric coefficient of 2.5×10−8 C/cm2 K without poling treatment. These films will be suitable for applications such as pyroelectric infrared detector and piezoelectric transducer.

Nonlinear optical properties of thin films of polysilane

Abstract: Optical third harmonic generation measurements by transmission have been performed on thin films of polysilane deposited by a spinning technique with a thickness varying between 0.15 and 0.45 μm. The films have a good optical quality and support large laser power (∼200 MW/cm2 at 1.064 μm). They are transparent in visible and near infrared. The measured average value of cubic susceptibility 〈χ(3)xxxx (−3ω; ω,ω,ω)〉 is equal to (1.5±0.1)×10−12 esu at 1.064 μm. The technique used for the third harmonic generation measurements allows determination of both modulus and phase of χ(3). It is argued that the large value of χ(3) is due to a three‐photon resonance at 1.064 μm.

Laser generation of acoustic waves in liquids and gases

Abstract: The laser generation of sound in liquids and gases is reviewed. The sound‐generating mechanisms of laser interaction with matter are discussed with emphasis on the thermoelastic process. The studies on strongly absorbing liquids include detailed theoretical considerations of the thermoelastic sound generation with pulsed lasers. Acoustic waveforms for H2O and D2O are calculated analytically on the basis of a model laser‐pulse shape. Both free and rigid boundaries on the surface of the liquid are considered. Good agreement between theory and experiments with respect to waveforms and amplitudes is obtained. The experiments are performed with a hybrid CO2 laser and piezoelectric or optical detection of the acoustic transients. In view of a present controversy, special emphasis is put on the temperature dependence of the acoustic amplitudes in H2O, D2O, and in aqueous MgSO4 solutions. Good agreement is found between experimental data and a new, pure thermal model which takes heat conduction into account. The distortion of the acoustic waveform during the propagation through the liquid is treated in terms of sound absorption, diffraction, and nonlinear acoustics. A simple experimental method for the determination of Beyer’s nonlinearity parameter B/A is presented. In the last section some characteristics of photoacoustic spectroscopy (PAS) in gaseous media are reviewed. This method has been demonstrated to be highly sensitive. The measurement of absorption coefficients as low as 10− 8 cm− 1 is possible. PA studies on H2O vapor are discussed with new results on line and continuum absorption in the 9–11‐μm wavelength range. Finally, the impact of PAS on trace gas analysis is demonstrated. With PAS the detection of gas concentrations in the ppb range is feasible. The operational characteristics of a stationary CO laser and a mobile CO2 laser‐PAS system are presented, including first results on continuous i n s i t u air pollution monitoring.

Dynamics of UV laser ablation of organic polymer surfaces

Abstract: A model based on a time‐dependent treatment of the ablation of organic polymer surfaces by UV laser radiation is proposed. It relates the dynamics of the etching process to the experimental parameters such as the fluence, wavelength, and the width of the laser pulse. The model is applied to poly(methyl methacrylate) and to polyimide in order to predict the etch characteristics quantitatively and to compare them systematically to the experimental results. This model accounts for a wide variety of observations such as fluence thresholds, wavelength‐dependent etch rates, ‘‘incubation’’ pulses, filtering by photofragments, fast intrapulse etching, thermal contributions, and even the influence of pulse compression so that a consistent picture of the mechanisms underlying the ablation process becomes apparent.

The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. II.

Abstract: The study of the excess conductivity induced in a material by pulsed optical excitation yields information on the optoelectronic properties of the material and is receiving increasing attention. As conventional conductivity techniques are hampered by the need to apply electrical contacts, we have investigated the reliability and the possibilities of microwave conductivity measurements. This paper first presents the general background for excess conductivity measurements in the microwave range, and then derives the quantitative relationship between the reflected microwave signal and the change in conductivity for a wafer of single‐crystalline Si. For this sample, the theory of excess charge carrier kinetics is also developed. After a short description of our apparatus, kinetic measurements on a nano‐ and microsecond timescale are compared to theory.

Piezoelectricity and related properties of vinylidene fluoride and trifluoroethylene copolymers

Abstract: The piezoelectric properties and ferroelectric transition behaviors of vinylidene fluoride and trifluoroethylene copolymers, P(VDFx−TrFE1−x), were studied as a function of x in the range of x=0.37–0.94. The electromechanical coupling factor kt and piezoelectric constant d31 are strongly dependent on both x and annealing temperature. In the copolymers of 0.65≲x≲0.82, a large kt value of 0.27–0.3 has been found when annealed above ferroelectric‐to‐paraelectric transition temperature Tc. The large kt is attributable to their high remanent polarization (∼110 mC/m2) which arises from high degrees of crystallinity and preferred orientation of well‐grown crystallites. Structural and morphological changes induced by annealing and poling were characterized by SEM, DSC, and x‐ray diffraction studies. Phase diagram of the copolymers were obtained by DSC measurements. Tc increases with increase of x and coincides with melting temperture Tm at x≂0.82. In the range of x≳0.82, Tc=Tm.

Zinc-oxide thin-film ammonia gas sensors with high sensitivity and excellent selectivity

Abstract: A sensor with a high sensitivity and an excellent selectivity for ammonia gas was prepared by using sputtered ZnO thin films. The sensor exhibited an increase of resistance for exposure to ammonia gas whereas it exhibited a decrease of resistance for exposure to many other gases such as inflammable and organic gases. The resistance change and the selectivity of the sensor were enhanced by doping group III metal impurities such as Al, In, and Ga. The lower limit of the detection for ammonia gas was about 1 ppm at a working temperature of 350 °C.

Ferroelectric (Pb,La)(Zr,Ti)O3 epitaxial thin films on sapphire grown by rf-planar magnetron sputtering

Abstract: Ferroelectric (Pb,La)(Zr,Ti)O3 (PLZT) thin films have been epitaxially grown on the c plane of sapphire by rf‐planar magnetron sputtering. The sputtering conditions were investigated to obtain epitaxial and transparent films. Dielectric, piezoelectric, and electro‐optic properties of the films were measured. Piezoelectricity of the PLZT(28/0/100) film was confirmed and was as strong as that of BaTiO3. Excellent quadratic electro‐optic effects for PLZT(28/0/100) and PLZT(9/65/35) films and a linear electro‐optic effect for PLZT(21/0/100) film were observed at 0.633‐μm wavelength. Epitaxial PLZT thin film on sapphire is presently the most promising material for new functional devices.

Pulsed photothermal modeling of layered materials

Abstract: Exact analytical solutions of the heat‐diffusion problem encountered in the pulsed photothermal evaluation of layered materials are presented. The analysis relates to the long‐time decay of the front face temperature following the absorption of a surface heat pulse. The configuration of a coating on a substrate is analyzed, and the parameters governing the temperature evolution are identified with a particular emphasis on the thermal contact resistance term. This model provides a new method of measuring the quality of the interface. Previously published data and new experiments with laminated materials and living tissues are analyzed using the proposed analytical solutions.

Some considerations on the electric field induced in insulators by electron bombardment

Abstract: Starting from a simple model of the distributions of charge created in an insulator by bombardment with electrons, the components of the electric field are evaluated by using Maxwell’s equations and image effects. The results are applied to the most common experimental situations: a semi‐infinite sample (i) bounded by a vacuum or (ii) covered by a conducting film, and a sample in the form of a film (iii) unsupported or (iv) covering a conducting substrate. The results are compared to some experimental data concerning, for instance, electromigration and electron‐stimulated desorption. In surface analysis the decay of the Auger signal from ions of opposite charges and the opposite behavior of ions of the same charge are explained. Similar effects observed in electron‐probe microanalysis of glasses are also elucidated. The results concern scanning electron microscopy, transmission electron microscopy, and electron‐beam lithography applied to biological objects, polymers, ceramics, minerals, glasses, and electronic devices. With slight modifications, the same model can be applied to cases of irradiation with ions or x rays. The evolution of the trapped charges with time is suggested, and the need to indicate the electric parameters (e and γ) of the investigated samples is outlined.

Capillary instabilities in thin films. I. Energetics

Abstract: A stability theory is presented which describes the conditions under which thin films rupture. It is found that holes in the film will either grow or shrink, depending on whether their initial radius is larger or smaller than a critical value. If the holes grow large enough, they impinge to form islands; the size of which are determined by the surface energies. The formation of grooves where the grain boundary meets the free surface is a potential source of holes which can lead to film rupture. Equilibrium grain boundary groove depths are calculated for finite grain sizes. Comparison of groove depth and film thickness yields microstructural conditions for film rupture. In addition, pits which form at grain boundary vertices, where three grains meet, are another source of film instability.

Low‐temperature deposition of high‐quality silicon dioxide by plasma‐enhanced chemical vapor deposition

Abstract: Thin films of high‐quality silicon dioxide have been deposited at low temperatures by plasma‐enhanced chemical vapor deposition. A deposition rate much lower than that used in conventional plasma‐enhanced processes is found to be crucial in obtaining material with reproducible, good properties. Controlled, slow deposition is achieved by using very low flow rates of reactive gases, together with a much higher flow of inert carrier gas to ensure uniformity. Films deposited at usual high deposition rates (∼500 A/min) exhibit irreproducible and poor electrical properties and are porous. Those deposited slowly (∼60 A/min) have very reproducible properties, are relatively dense and exhibit very good electrical integrity. Oxides deposited using a substrate temperature of 350 °C compare favorably with those deposited at 700°C using atmospheric‐pressure chemical vapor deposition and can be deposited routinely over a wide range of oxide thickness. Deposition at 275 °C results in similar properties but with increase...

Magnetic viscosity and thermal activation energy

Abstract: Magnetic viscosity arises when the activation energy E required to produce a change in mangetization is provided thermally. Activation energy functions for single domain particles and materials with ‘‘strong’’ and ‘‘weak’’ domain‐wall pinning are discussed. The magnetic viscosity parameter Sv, given by kT(∂E/∂H)−1T, where kT is the Boltzmann energy and H is the magnetic field, is shown to be proportional to kT/(v Ms), where Ms is the spontaneous magnetization and v is the activation volume swept out as the energy barrier is overcome. For single domain particles and strong domain‐wall pinning the constant of proportionality is one while for weak pinning it is 1/2. The generality of this simple relationship is shown to be independent of the details of the activation model.

A model for conductor failure considering diffusion concurrently with electromigration resulting in a current exponent of 2

Abstract: A model was constructed for electromigration failure where both Fickian diffusion and mass transport due to the electromigration driving force are considered concurrently. The solution to the resulting diffusion equation yields a current exponent of 2 and an activation energy consistent with grain‐boundary self‐diffusion. A modification of the standard median time to failure equation first proposed by Black is suggested.

Optical constants and associated functions of metastable diamondlike amorphous carbon films in the energy range 0.5–7.3 eV

Abstract: The complex refractive index N(ω)=n+ik and the complex dielectric constant e(ω)=e1+ie2 are presented for diamondlike amorphous carbon (a‐C) films in the photon energy range 0.5–7.3 eV. The effective number of valence electrons neff per carbon atom, the static dielectric constant e0,eff, and the energy loss function Im[−1/e(ω)] are deduced via the use of sum rules and are used to interpret the optical data. The a‐C films were deposited using an unbalanced magnetron gun to sputter a graphite target (effective sputtering area of 20 cm2) in ultrapure argon gas. The magnetron is characterized by a high deposition flux of condensing atoms (1.5×1014–1.2×1016 cm−2 s−1) and a concomitant high ion flux (6×1014–2.5×1016 cm−2 s−1). A series of films were prepared by sputtering at different power levels in the range 5–500 W. Insulating substrates were used which allowed the films to self‐bias negatively with respect to the plasma, so that the films were bombarded during their growth by Ar+ ions of energy 16–13 eV at a...

Laser‐intensity‐modulation method: A technique for determination of spatial distributions of polarization and space charge in polymer electrets

Abstract: A new method for measuring the spatial distribution of combined polarization and space charges in poled polymer electrets has been developed. The technique, called the laser‐intensity‐modulation method (LIMM), is based upon measurement of the pyroelectric current generated by the electret when it is heated with a laser beam which is intensity‐modulated at frequencies varying from 200 to 2500 Hz. A special analytical technique is used to transform the current‐frequency measurements to spatial distributions. The method gives high resolution and is very stable with respect to measurement errors. The technique can be easily implemented in a laboratory with conventional equipment. The advantages and limitations of LIMM are illustrated by means of calculations based upon simulated data. Experimental results on a number of laboratory and commercially poled samples are presented.

The statistical self‐similarity hypothesis in grain growth and particle coarsening

Abstract: The time dependence of particle size (e.g., mean volume) in normal grain growth, bubble growth, and late‐stage coarsening is deduced from a statistical self‐similarity (SSS) hypothesis, according to which consecutive configurations of the system in the self‐similar mode are geometrically similar in a statistical sense, and from the scaling characteristic of v, the rate of change of the volume of a given particle. It is shown that if v scales as vα under uniform magnification, where v is the mean particle volume and α a constant depending on the controlling kinetics and geometry of the system, then v1−α is a linear function of the time. Values of α are obtained for a number of cases. The treatment is free of many of the approximations and geometrical simplifications used in most theories. Evidence for the SSS hypothesis is discussed. Special new results are obtained for two‐dimensional bubble and idealized grain growth that should be testable by computer simulation or by observation of a well‐behaved soap‐bubble array.

Use of digital image analysis to estimate fluid permeability of porous materials: Application of two-point correlation functions

Abstract: Scanning electron microscope images of cross sections of several porous specimens have been digitized and analyzed using image processing techniques. The porosity and specific surface area may be estimated directly from measured two‐point spatial correlation functions. The measured values of porosity and image specific surface were combined with known values of electrical formation factors to estimate fluid permeability using one version of the Kozeny‐Carman empirical relation. For glass bead samples with measured permeability values in the range of a few darcies, our estimates agree well (±10–20%) with the measurements. For samples of Ironton‐Galesville sandstone with a permeability in the range of hundreds of millidarcies, our best results agree with the laboratory measurements again within about 20%. For Berea sandstone with still lower permeability (tens of millidarcies), our predictions from the images agree within 10–30%. Best results for the sandstones were obtained by using the porosities obtained at magnifications of about 100× (since less resolution and better statistics are required) and the image specific surface obtained at magnifications of about 500× (since greater resolution is required).

Atomic layer epitaxy

Abstract: Atomic layer epitaxy (ALE) is not so much a new technique for the preparation of thin films as a novel modification to existing methods of vapor‐phase epitaxy, whether physical [e.g., evaporation, at one limit molecular‐beam epitaxy (MBE)] or chemical [e.g., chloride epitaxy or metalorganic chemical vapor deposition (MOCVD)]. It is a self‐regulatory process which, in its simplest form, produces one complete molecular layer of a compound per operational cycle, with a greater thickness being obtained by repeated cycling. There is no growth rate in ALE as in other crystal growth processes. So far ALE has been applied to rather few materials, but, in principle, it could have a quite general application. It has been used to prepare single‐crystal overlayers of CdTe, (Cd,Mn)Te, GaAs and AlAs, a number of polycrystalline films and highly efficient electroluminescent thin‐film displays based on ZnS:Mn. It could also offer particular advantages for the preparation of ultrathin films of precisely controlled thickne...

A fractal model of dielectric breakdown and prebreakdown in solid dielectrics

Abstract: We introduce and discuss a fractal model for dielectric breakdown which exhibits a breakdown voltage and a region of stable prebreakdown structures. The model provides a unifying picture covering homogeneous space charge injection, treelike structures, and filamentary breakdown. A simple qualitative relation between the global form of the pattern and two simple physical parameters is found. The model illustrates the intricate relationship between local stochastic and global deterministic aspects of dielectric instablilities.