bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional model for cw laser-induced mode-mismatched dual-beam thermal lens spectrometry and time-resolved measurements of thin-film samples
Authors: Shen Jun , Baesso Mauro L. , Snook Richard D.
Published: July 2002

Conventional thermal lens spectrometry theory assumes that the sample is an infinite medium, and only the radial temperature rise and heat flow in the sample are considered. This approach is not suitable for thin-film samples. Considering the axial boundary conditions of the sample, a three-dimensional model for cw laser-induced mode-mismatched thermal lens spectrometry is presented. However, its mathematical expression is not so convenient as that of the conventional theory to deal with the experimental data. The model shows that, besides axial boundary conditions of the sample, the ratio of excitation laser spot size to the sample thickness ωe/l is a crucial parameter to minimize the axial heat flow. By choosing small ωe/l, it is possible for the conventional theory to describe the development of the thermal lens of a thin sample for a short duration. Experimental proofs for the theoretical prediction are made, and good agreements are achieved.
Electroabsorption and light modulation with ZnSe/ZnSSe multiquantum wells grown by metalorganic vapor phase epitaxy
Authors: Marquardt E. , Opitz B. , Scholl M. , Heuken M.
Published: July 2002

ZnSe/ZnSSe multi-quantum wells were grown on GaAs by metalorganic vapor phase epitaxy and investigated by photocurrent and photoluminescence measurements. Photocurrent spectra show clear excitonic peaks. The observed peaks were identified as light and heavy hole excitonic transitions by comparing the data to theoretical ones. Field dependent photocurrent measurements show features useable for modulator applications in the blue spectral range at energies above 2.7 eV at 300 K.
Magnetic structure of the spin valve interface
Authors: Nicholson D. M. C. , Butler W. H. , Zhang X.-G. , MacLaren J. M. , Gurney B. A. , Speriosu V. S.
Published: July 2002

Nonferromagnetic atoms present at Ni/Cu and Permalloy/Cu interfaces in sputtered spin valve magnetoresistive layered structures have been shown to cause reduced magnetoresistance. Here we show that a model in which the moments on the Ni atoms in the interfacial region of Ni/Cu are reduced substantially by interdiffusion with Cu is consistent with the experimental results. In contrast, we believe that moments persist at the permalloy/Cu interface, which first principle total energy calculations suggest will be disordered at finite temperatures. These reduced or disordered moments are expected to significantly reduce the GMR.
Magnetic and Mössbauer studies of single-crystal Fe16N2 and Fe-N martensite films epitaxially grown by molecular beam epitaxy (invited)
Authors: Sugita Yutaka , Takahashi Hiromasa , Komuro Matahiro , Mitsuoka Katsuya , Sakuma Akimasa
Published: July 2002

Single-phase, single-crystal Fe16N2(001) films and Fe-11 at. %N martensite films of 200–900 Å thickness have been epitaxially grown on In0.2Ga0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N2 and NH3, followed by annealing. The saturation magnetizations 4πMs’s for Fe16N2 and Fe-N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πMs for Fe-N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe16N2. Mössbauer spectra have been measured for those films. The spectrum for Fe-N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe16N2. 4πMs of 29 kG for Fe16N2 (3.2 μB/Fe atom) and 4πMs of 24 kG for martensite (2.6 μB/Fe atom) has not been explained based on the conventional band theory of 3d metal magnetism. Behaviors of Mössbauer spectra could not be understood based on the conventional concept either. Thus a new physical concept is likely to be needed for clarification of giant magnetic moments and Mössbauer spectra for Fe16N2 and Fe-N martensites.
Limiting loss mechanisms in 23% efficient silicon solar cells
Authors: Aberle Armin G. , Altermatt Pietro P. , Heiser Gernot , Robinson Stephen J. , Wang Aihua , Zhao Jianhua , Krumbein Ulrich , Green Martin A.
Published: July 2002

The ‘‘passivated emitter and rear locally diffused’’ (PERL) silicon solar cell structure presently demonstrates the highest terrestrial performance of any silicon-based solar cell. This paper presents a detailed investigation of the limiting loss mechanisms in PERL cells exhibiting independently confirmed 1-sun efficiencies of up to 23.0%. Optical, resistive, and recombinative losses are all analyzed under the full range of solar cell operating conditions with the aid of two-dimensional (2D) device simulations. The analysis is based on measurements of the reflectance, quantum efficiency, dark and illuminated current–voltage (I–V) characteristics, and properties of the Si–SiO2 interfaces employed on these cells for surface passivation. Through the use of the 2D simulations, particular attention has been paid to the magnitudes of the spatially resolved recombination losses in these cells. It is shown that approximately 50% of the recombination losses at the 1-sun maximum power point occur in the base of the cells, followed by recombination losses at the rear and front oxidized surfaces (25% and <25%, respectively). The relatively low fill factors of PERL cells are principally a result of resistive losses; however, the recombination behavior in the base and at the rear surface also contributes. This work predicts that the efficiency of 23% PERL cells could be increased by about 0.7% absolute if ohmic losses were eliminated, a further 1.1% absolute if there were no reflection losses at the nonmetallized front surface regions, about 2.0% by introducing ideal light trapping and eliminating shading losses due to the front metallization, and by about 3.7% absolute if the device had no defect-related recombination losses. New design rules for future efficiency improvements, evident from this analysis, are also presented.
Nitrogen implantation in (100)-β-SiC layers grown on Si substrate
Authors: Hirano Yasuaki , Inada Taroh
Published: July 2002

Molecular N ions (N+2) have been implanted at an incident energy of 30 keV in a heteroepitaxial β-SiC layer grown on a (100)-Si substrate to form an n+ layer in the substrate. Furnace and rapid-thermal annealing are carried out on N-implanted substrates. Crystalline properties of the implanted layers have been characterized by Rutherford backscattering measurements and transmission electron microscope observations. An anodic oxide growth and removal technique has been developed, by which a very thin SiC layer can be reproducibly stripped from the substrate. Carrier-concentration and mobility profiles for n-type layers formed under various implant and annealing conditions have been investigated by differential Hall measurements combined with the layer stripping process. Rutherford backscattering data indicate that heating of substrate during implantation is important to reduce the amount of defects existing in the substrate after annealing. It has been revealed that rapid thermal annealing is useful for achieving high electrical activation of implanted N atoms. A very shallow (<100 nm) n+ layer with a maximum carrier concentration of around 1×1020/cm3 can be formed in the SiC layer by N implantation at 400 °C and by subsequent rapid thermal annealing at 1100 °C for 10 s. This highly doped layer is applicable to an emitter in a SiC/Si heterobipolar transistor. It has been also shown that implantation with a very high dose, e.g., 1.2×1016/cm2, gives rise to the formation of Si3N4 in the substrate after annealing, suppressing electrical activation of implanted atoms.
Polarization suppression in Pb(Zr,Ti)O3 thin films
Authors: Warren W. L. , Dimos D. , Tuttle B. A. , Pike G. E. , Schwartz R. W. , Clews P. J. , McIntyre D. C.
Published: July 2002

Switchable polarization can be suppressed in Pb(Zr,Ti)O3 thin films by optical, thermal, electrical, and reducing processes. The optical suppression effect occurs by biasing the ferroelectric near the switching threshold and illuminating the material with band gap light; the thermal suppression effect occurs by biasing the ferroelectric near the switching threshold and heating the material to ≊100 °C. The electrically induced suppression effect, known as electrical fatigue, occurs by subjecting the ferroelectric capacitor to repeated polarization reversals. We find that the suppressed polarization in these three cases can be restored to essentially its initial polarization value by creating electronic charge carriers in the ferroelectric. This strongly suggests that all three forms of degradation largely involve locking domains by electronic charge trapping at domain boundaries. The fourth form of polarization suppression, a reducing treatment, was obtained by annealing the crystallized PZT films at 400 °C in nitrogen. The suppressed polarization could not be restored by injecting electronic charge into the reduced films, indicating that the mechanism for polarization suppression is different. In this case, it appears as though ionic defects, such as oxygen vacancies, are responsible for locking the domains, and hence, suppressing the polarization.
Creation of a conductive surface layer on polypropylene samples by low-pressure plasma treatments
Authors: Coen M. Collaud , Groening P. , Dietler G. , Schlapbach L.
Published: July 2002

Polypropylene (PP) samples were treated in an electron cyclotron resonance plasma with several gases and under different treatment conditions (pressure, treatment time, rf potential). The surfaces were chemically characterized in situ by x-ray photoelectron spectroscopy (XPS). Changes in the electrical resistance of the surface layer were analyzed by four points measurements. Noble gas plasma treatments led to a great decrease (ten orders of magnitude) of the resistance, whereas reactive gas plasma treatments induced no modification of the resistance. Taking into account that the treated layer corresponds to the ion penetration depth, an estimation gives 0.1 Ω cm for the bulk resistivity of He plasma treated PP. For noble gases, this value depends on the plasma gas and on the ion kinetic energy. The resistance decrease is explained by a dehydrogenation and an increase of carbon–carbon bonds. During reactive gas plasma treatments, the incorporation of new chemical species prevents this graphitization. Correlations of the resistance with XPS measurements and comparisons with literature on high energetic ion treatments allow a better understanding of the effects of the plasma treatment.
Elastic and anelastic properties of chemical vapor deposited epitaxial 3C-SiC
Authors: Su C. M. , Wuttig Manfred , Fekade A. , Spencer M.
Published: July 2002

Chemical vapor deposited 3C-SiC films were micromachined into free standing cantilevers and their anelastic and elastic properties were determined by a vibrating reed technique. Despite a high density of defects, epitaxial 3C-SiC exhibits extremely high mechanical Q which is essential for resonator sensors and actuators. An anelastic relaxation peak was found with an associated activation energy of 0.94 eV. Doping caused splitting of this peak. The mechanism of the mechanical relaxation peak is discussed in relation to defect movement under stress. Young’s modulus of epitaxial undoped 3C-SiC was found to be 694 GPa, p-doping reduced it to 474 Gpa.
Diffusion of tin in germanium studied by secondary ion mass spectrometry
Authors: Friesel M. , Södervall U. , Gust W.
Published: July 2002

The diffusion of tin in intrinsic germanium has been studied in the temperature range from 555 to 930 °C by secondary ion mass spectrometry. Tin has been indiffused under vacuum conditions both from the gas phase and from thin films. In both cases, pure metal was used as a source of tin. In the studied temperature range the diffusion coefficient of tin as a function of temperature can be expressed as D(cm2/s)=8.4×102 exp[(−3.26 eV)/kT]. Based on the similarities between tin diffusion and germanium self-diffusion, it is concluded that tin diffuses in germanium via the monovacancy mechanism.
Electrode contacts on ferroelectric Pb(ZrxTi1−x)O3 and SrBi2Ta2O9 thin films and their influence on fatigue properties
Authors: Lee J. J. , Thio C. L. , Desu S. B.
Published: July 2002

The degradation (fatigue) of dielectric properties of ferroelectric Pb(ZrxTi1−x)O3 (PZT) and SrBi2Ta2O9 thin films during cycling was investigated. PZT and SrBi2Ta2O9 thin films were fabricated by metalorganic decomposition and pulsed laser deposition, respectively. Samples with electrodes of platinum (Pt) and ruthenium oxide (RuO2) were studied. The interfacial capacitance (if any) at the Pt/PZT, RuO2/PZT, and Pt/SrBi2Ta2O9 interfaces was determined from the thickness dependence of low-field dielectric permittivity (εr) measurements. It was observed that a low εr layer existed at the Pt/PZT interface but not at the RuO2/PZT and Pt/SrBi2Ta2O9 interfaces. In the case of Pt/PZT, the capacitance of this interfacial layer decreases with increasing fatigue while the εr of the bulk PZT film remains constant. This indicates that fatigue increases the interfacial layer thickness and/or decreases interfacial layer permittivity, but does not change the bulk properties. For the capacitors with RuO2/PZT/RuO2 and Pt/SrBi2Ta2O9/Pt structures, however, the εr does not change with ferroelectric film thickness or fatigue cycling. This implies no interfacial layer exists at the interfaces and which can be correlated to the observed nonfatigue effect. Additionally, the equivalent energy-band diagrams of these different capacitor structures were proposed to complement the proposed fatigue mechanism.
Epitaxial growth of 3C–SiC films on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition
Authors: Zorman Christian A. , Fleischman Aaron J. , Dewa Andrew S. , Mehregany Mehran , Jacob Chacko , Nishino Shigehiro , Pirouz Pirouz
Published: July 2002

Silicon carbide (SiC) films have been grown on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition, using propane, silane, and hydrogen. X-ray photoelectron spectroscopy data confirm that the films are stoichiometric SiC, with no major impurities. X-ray diffraction and transmission electron microscopy (TEM) data indicate that the films are single-crystalline cubic polytype (3C) across the 4 in. substrates. With the exception of slip lines near the edge of the wafers, the films appear featureless when observed optically. The nitrogen concentration, as determined by secondary ion mass spectroscopy, is 4×1018 cm3. Cross-sectional TEM images show a fairly rough, void-free interface.
Stochastic scattering in charged particle projection systems: A nearest neighbor approach
Authors: Mkrtchyan M. M. , Liddle J. A. , Berger S. D. , Harriott L. R. , Gibson J. M. , Schwartz A. M.
Published: July 2002

Image blurring as a result of stochastic particle–particle interactions has been investigated for projection electron- and ion-beam lithography systems. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results and experimental data. Large variations in results and serious disagreements between the different theoretical approaches are found. We have formulated a new theory on the basis of a simple, analytical approach that overcomes most of the difficulties experienced by earlier theories with two key concepts: consideration of nearest-neighbor interactions only, and a randomization length, over which the interactions are correlated. Our model displays satisfactory functional and numerical agreement with Monte Carlo simulation results over a large range of beam currents, as well as with the only available experimental data. The physical basis of our model also enables us to understand the origins of the discrepancies arising from earlier theories.
Compositional disordering of InGaAs/GaAs heterostructures by low-temperature-grown GaAs layers
Authors: Tsang J. S. , Lee C. P. , Lee S. H. , Tsai K. L. , Tsai C. M. , Fan J. C.
Published: July 2002

Compositional disordering of InGaAs/GaAs superlattices using a low-temperature-grown GaAs cap layer (LT-GaAs) by molecular beam epitaxy has been studied. The disordering of the superlattice was verified by photoluminescence and double-crystal x-ray rocking curve measurements. The Ga-vacancy-enhanced interdiffusion due to the presence of LT-GaAs was found to be the disordering mechanism. Diffusion equations and the Schrödinger’s equation were solved numerically to obtain the composition profile and the transition energies in the disordered quantum well, respectively. The simulated energy shifts for samples under different annealing conditions agreed very well with the experimental results. The calculated effective diffusivity for the In–Ga interdiffusion has an activation energy of 1.63 eV, which is smaller than the activation energy 1.93 eV, for intrinsic interdiffusion. The diffusivity for the enhanced In–Ga interdiffusion due to the presence of LT-GaAs is about two orders of magnitude larger than the intrinsic In–Ga diffusivity.
The effect of deposition and annealing conditions on textured growth of sputter-deposited strontium ferrite films on different substrates
Authors: Acharya B. Ramamurthy , Prasad Shiva , Venkataramani N. , Shringi S. N. , Krishnan R.
Published: July 2002

M-type strontium ferrite films were prepared by radio-frequency (rf) sputtering on fused quartz substrates using different deposition conditions and were subjected to two different types of annealing treatments. The study showed that in addition to the deposition conditions such as rf power, oxygen to argon ratio in the sputtering gas, and target to substrate distance, the postdeposition annealing conditions also play an important role in determining the texture and properties of the films. The films with random orientation or with preferred c-axis orientation either normal to the film plane or in the film plane could be deposited depending on the process parameters chosen. The study carried out by depositing these films on different substrates such as Si(100), Si(111), sapphire(110), and Gd3Ga5O12(111) showed that though the nature of the substrates plays a role in determining the texture and properties of the films, such effects are less dominant in comparison to the effect of deposition and annealing conditions in the case of strontium ferrite films.
Diffraction of ultrasonic waves from elastic Galois gratings
Authors: Costa Bernadette , Folacci Antoine , Gabrielli Paul
Published: July 2002

A significant attenuation of the specular echo in a wide frequency domain occurs in scattering of waves by Galois gratings. Diffraction of ultrasonic waves by immersed one-dimensional elastic gratings constructed from the finite Galois fields GF(pm) is investigated. We develop a theoretical model taking into account the elasticity of the structure, its finite thickness, the boundary conditions at the fluid–solid and solid–fluid interfaces, and the periodicity of the gratings. In addition to the typical attenuation of the specular echo predicted by discrete Fourier transform, such a model predicts minima in the spectrum of the reflection coefficient corresponding to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) by mode conversion and to the excitation of Lamb-type modes. The attenuation as well as the minima are experimentally observed and these results are in good agreement with theoretically predicted ones. Galois gratings could constitute an advance in the search for new anechoic devices.
Magnetic properties of epitaxial ferrite multilayer films
Authors: Suzuki Y. , van Dover R. B. , Gyorgy E. M. , Phillips Julia M. , Korenivski V. , Werder D. J. , Chen C. H. , Felder R. J. , Cava R. J. , Krajewski J. J. , Peck W. F.
Published: July 2002

We have fabricated and studied the structure and magnetic properties of high quality single crystalline (Mn,Zn)Fe2O4, NiFe2O4, and CoFe2O4 films. Although (Mn,Zn)Fe2O4 and NiFe2O4 films grown directly on SrTiO3 and MgAl2O4 show mediocre structural and magnetic properties, these same films grown on SrTiO3 and MgAl2O4 buffered with CoCr2O4 or NiMn2O4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the underlying layer. X ray, Rutherford backscattering spectroscopy, atomic force microscopy, and transmission electron microscopy analysis provide a consistent picture of the structural properties of these ferrite films.
Anisotropy and angular variation of the giant magnetoresistance in magnetic multilayers (invited)
Authors: Dieny B. , Cowache C. , Nossov A. , Dauguet P. , Chaussy J. , Gandit P.
Published: July 2002

The giant magnetoresistance (GMR) of magnetic multilayers is usually considered as isotropic, i.e., independent of the direction of the sensing current with respect to the applied field. In spin-valve samples of the form NiFe/Cu/NiFe/FeMn it is possible to accurately determine the amplitude of the GMR (without any contribution from the usual anisotropic magnetoresistance) for various direction of the current with respect to the direction of the magnetization of the two ferromagnetic layers, both in the parallel and antiparallel magnetic configurations. In three series of spin-valve samples of the composition F tF/Cu tCu/NiFe/FeMn, we have observed that the GMR amplitude is larger when the current is perpendicular to the magnetizations than when it is parallel to it. This intrinsic anisotropy in the GMR shows a pronounced maximum (relative amplitude of the anisotropy of the order of 10% at the maximum) for a thickness of the ferromagnetic layer of the order of 150 Å. In contrast, this anisotropy depends very weakly on the nonmagnetic spacer layer thickness. The results are compared with semiclassical calculations of Rijks et al. [Phys. Rev. B 51, 283 (1995)]. On another respect, we have measured the in-plane (CIP) and perpendicular to the plane (CPP) giant magnetoresistance of antiferromagnetically coupled (NiFe/Ag) multilayers. Particular attention has been paid on the variation of resistivity with the angle Δθ between the magnetization in the successive magnetic layers. While the CIP GMR varies almost linearly with cos(Δθ), the CPP GMR shows strong deviations from linearity especially at large NiFe thicknesses. The results are discussed in terms of relative role of s-like and d-like electrons in CIP and CPP transport.
Electrical properties of vapor-deposited yttria-stabilized zirconia thin films
Authors: Yeh W. , Patuwathavithane C. , Zee R. H.
Published: July 2002

The electrical properties of rf sputter-deposited thin films of ZrO2 doped with 4 mol % of Y2O3 were examined from 296 to 773 K. The films were deposited in an Ar/O2 (80/20) atmosphere at a pressure of 2 Pa under various substrate dc bias conditions (from −30 to +20 V). A Rutherford backscattering spectroscopy technique was used to determine the thickness of the films. Results showed that the deposition rate increased with applied negative dc bias ranging from 0.0486 nm/s for films produced with +20 V to 0.0972 nm/s for films produced with −30 V. Current–time measurements as a function of temperature were made in an electric field of 103 V/mm. Distinct conduction mechanisms of the ZrO2–Y2O3 films were observed from the slope change in Arrhenius plots of dc resistivity at 523 K. At low temperatures, the conduction was due to the migration of free oxygen vacancies. At temperatures above 500 K, the conduction was governed by the dissociation of oxygen vacancies trapped by the yttrium cations resulting in a higher activation energy for conduction. The effects of thermal cycling on the electrical behavior of the thin films were also investigated.
Cl2/Ar and CH4/H2/Ar dry etching of III–V nitrides
Authors: Vartuli C. B. , MacKenzie J. D. , Lee J. W. , Abernathy C. R. , Pearton S. J. , Shul R. J.
Published: July 2002

Electron-cyclotron-resonance (ECR) and reactive ion etching (RIE) rates for GaN, AlN, InN, and InGaN were measured using the same reactor and plasma parameters in Cl2/Ar or CH4/H2/Ar plasmas. The etch rates of all four materials were found to be significantly faster for ECR relative to RIE conditions in both chemistries, indicating that a high ion density is an important factor in the etch. The ion density under ECR conditions is ∼3×1011 cm−3 as measured by microwave interferometry, compared to ∼2×109 cm−3 for RIE conditions, and optical emission intensities are at least an order of magnitude higher in the ECR discharges. It appears that the nitride etch rates are largely determined by the initial bond breaking that must precede etch product formation, since the etch products are as volatile as those of conventional III–V materials such as GaAs, but the etch rates are typically a factor of about 5 lower for the nitrides. Cl2/Ar plasmas were found to etch GaN, InN, and InGaN faster than CH4/H2/Ar under ECR conditions, while AlN was etched slightly faster in CH4/H2/Ar plasmas. The surface morphology of InN was found to be the most sensitive to changes in plasma parameters and was a strong function of both rf power and etch chemistry for ECR etching.
Dynamics of laser-produced carbon plasma in an inert atmosphere
Authors: Kerdja T. , Abdelli S. , Ghobrini D. , Malek S.
Published: July 2002

Time- and space-resolved emission spectroscopy measurements were performed to investigate plasma dynamics during laser evaporation of a graphite target. The evolution of the velocity and the emission intensity of the excited species versus the distance and the pressure of the foreign gas are studied. The time-resolved emission intensity of C II* transition at 426.7 nm shows a double-peak structure, resulting from the stratification of the plasma into fast and slow components. Intense molecular emission is found to occur behind a front separating the plasma from the foreign gas. Two stages of expansion are found in the generated plume, the first one starting just after the laser irradiation, the second beginning some time later depending on the nature and the pressure of the foreign gas. The two stages are well described, using a viscous drag force model for the first one and a delayed ideal blast wave model for the second.
Effects of rapid thermal anneal on refractive index and hydrogen content of plasma-enhanced chemical vapor deposited silicon nitride films
Authors: Cai L. , Rohatgi A. , Yang D. , El-Sayed M. A.
Published: July 2002

The objective of this paper is to understand and quantify the effects of rapid thermal anneal (RTA) on refractive index, thickness, and hydrogen content of plasma-enhanced, chemical vapor-deposited (PECVD) silicon nitride films. It is shown that RTA is more effective than identical furnace anneal. A threshold in as-deposited refractive index value is found, above which the index of a silicon nitride film increases, while the thickness and bonded hydrogen content decreases as result of the RTA. In addition, the magnitude of increase in the index is proportional to the as-deposited index value. The threshold index value increases with the increase in silicon nitride deposition temperature. A direct correlation is found between the annealing-induced increase in refractive index and the corresponding decrease in total bonded hydrogen concentration in the PECVD silicon nitride films. Finally, it is shown that the release of bonded hydrogen from the film can passivate defects in the underlying silicon substrate and increase the performance of silicon devices such as solar cells.
Optical spectra and analysis for Pr3+ in SrAl12O19
Authors: Zandi Bahram , Merkle Larry D. , Gruber John B. , Wortman Donald E. , Morrison Clyde A.
Published: July 2002

Energy levels of the 4f2 electronic configuration of Pr3+ in SrAl12O19 (SAM) have been determined from polarized absorption and fluorescence spectra using crystals, charge-compensated with Mg2+, at temperatures between about 20 and 300 K. Energy level assignments were made initially by comparing the crystal spectra with energy levels calculated by using crystal-field parameters, Bkm, compatible with those previously reported for Nd3+ in SAM. The Bkm were varied to obtain a best fit between experimental and theoretical energies, and the final values: B20=−94, B40=1047, B60=−1397, and B66=−1245 cm−1 give an rms fit of 14.5 cm−1. The Bkm for Pr3+ in SAM were also used along with a result relating the Bkm to crystal-field components, Anm, derived in a lattice sum calculation to obtain Bkm values for all the triply ionized rare-earth ions in SAM. Odd-fold Akm are also given that are required in the intensity and lifetime calculations and yield results in good agreement with the measurements. In addition, the results of our energy level fitting were useful in estimating absorption and emission cross sections, as well as lifetimes and branching ratios, for the various J multiplets. The evidence given here that the room temperature fluorescence quantum efficiency of P03 in lightly doped Pr in SAM is near unity strengthens the case that this is a potentially useful laser material. The theoretical model and method of analysis used is useful for the calculation of energy levels, wave functions, transition probabilities, etc., for any rare-earth RE3+ ion in any host of known crystal structure.
Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?
Authors: Sandoghdar V. , Wegscheider S. , Krausch G. , Mlynek J.
Published: July 2002

We have investigated the optical resolution of a scanning near-field optical microscope in reflection collection mode using an uncoated fiber tip. We demonstrate that the apparent resolution in the optical signal (better than 70 nm) is a topography-induced effect. We believe that the purely optical resolution is only of the order of λ/2 and diffraction limited.
Dislocation motion in GaN light-emitting devices and its effect on device lifetime
Authors: Sugiura Lisa
Published: July 2002

In this study, dislocation motions in GaN-based materials and devices were quantitatively estimated in order to determine why GaN-based light-emitting diodes have remarkable reliability and longevity in spite of extremely high dislocation density. The dislocation velocity of GaN-based materials was calculated by estimating the activation energy of dislocation, and comparing it with that of GaAs, which are typically used for light-emitting devices. It was estimated that the dislocation mobility of GaN-related materials was lower than that of GaAs by a factor of approximately 10−10–10−16, at room temperature. Furthermore, dislocation velocity under current injection became about 10−20 times lower than that of GaAs, under the assumption that the dislocations in GaN-related materials do not act as nonradiative recombination centers. The possibility of degradation under high current densities and high temperature, as would be found in GaN-based laser diodes, is also discussed.
Dynamic micromagnetics of nanocomposite NdFeB magnets
Authors: Schrefl T. , Roitner H. , Fidler J.
Published: July 2002

Magnetization processes in nanocomposite permanent magnets are investigated using a dynamic micromagnetic model. Numerical examples show the effects of the dispersion of the intrinsic magnetic properties at grain boundaries and the influence of gyromagnetic precession on magnetization reversal. Reduced exchange interactions within the intergranular phase between soft magnetic and hard magnetic grains suppress the onset of irreversible switching. A paramagnetic intergranular phase gives rise to flux closure and leads to rectangular hysteresis loops. The gyromagnetic precession taken into account in dynamic micromagnetic calculations reduces the coercive field as compared to static micromagnetic calculations by about 10%–20%.
Higher-order contribution and temperature dependence of the magnetic anisotropy in ultrathin films (invited)
Authors: Baberschke K. , Farle M.
Published: July 2002

The pseudomorphic growth of ultrathin films produces artificial structures, and with it anomalously large magnetic anisotropy energies (MAEs). Second- and fourth-order MAEs K2,K4 can be measured and analyzed. To understand the continuous (steplike) rotation of M in a spin reorientation transition (SRT), K4<0(K4>0) is necessary. By means of ferromagnetic resonance measurements we show that for fct Ni/Cu(001) the SRT is of second order. It is important to measure the “surface” and “volume” contribution for all MAE contributions K2,K4,… as a function of temperature. These results are compared with T=0 K first principle electronic structure calculations.
Epitaxial and island growth of Ag/Si(001) by rf magnetron sputtering
Authors: Je J. H. , Kang T. S. , Noh D. Y.
Published: July 2002

In this study, we examined the microstructure of Ag films grown on Si(001) substrates by radio frequency (rf) magnetron sputtering in a synchrotron x-ray scattering experiment. At a low rf power of 0.22 W/cm2, the film was initially nucleated in the form of fine-grained epitaxial film with the crystalline axes parallel to the substrate crystalline axes. As the growth proceeded further, it changed to nonepitaxial three dimensional island growth. The Ag islands were not epitaxial, but grew preferentially along the 〈111〉 direction. At a higher rf power of 0.44 W/cm2, the Ag film developed a nonepitaxial island growth from the early stage. Annealing the films at 500 °C increased the island size and enhanced the crystalline quality. The thin epitaxial film grown at the low rf power was recrystallized into islands during the annealing. This study suggests that it is feasible to grow heteroepitaxial Ag films on silicon substrates even by a sputtering process when the energy of the sputtered particles is minimized.
Magnetic properties and Mössbauer study of Fe nanocrystals prepared by the gas-deposition method
Authors: Sasaki Y. , Hyakkai M. , Kita Eiji , Tasaki A. , Tanimoto H. , Iwamoto Y.
Published: July 2002

Fe nanocrystalline films were prepared by the gas-deposition method (GDM). The films were composed of small particles and their aggregates. With an increase of pressure difference between an evaporation chamber and a deposition chamber, the grain size is slightly reduced and lower coercivity is realized. The Mössbauer spectra consisted of a crystalline component (α-Fe) and a boundary component with a hyperfine field of 310 kOe; no oxide subspectrum was observed. Magnetizations of the samples are around 95% of that of bulk α-Fe. Fe nanocrystals prepared by GDM are found to show good oxidization resistance.
Growth and characterization of phosphorus doped diamond films using trimethyl phosphite as the doping source
Authors: Roychoudhury Rajat , Charlson E. J. , Stacy T. , Hajsaid M. , Charlson E. M. , Meese J. M.
Published: July 2002

Phosphorus doped polycrystalline diamond films were grown by hot-filament chemical vapor deposition using trimethyl phosphite as the doping source. Phosphorus incorporation into the diamond films was established using secondary ion mass spectroscopy. Current–voltage characteristics were measured and the resistivities of the films were found to be of the order of 1012 Ω cm at room temperature. The diamond films gave indications of n-type behavior when electron beam induced current studies were performed.
Amorphouslike chemical vapor deposited tungsten diffusion barrier for copper metallization and effects of nitrogen addition
Authors: Chang Kow-Ming , Yeh Ta-Hsun , Deng I-Chung , Shih Chieh-Wen
Published: July 2002

In this article, we propose an amorphouslike chemical vapor deposited tungsten (CVD-W) thin film as a diffusion barrier for copper metallization. Experimental results gave no evidence of interdiffusion and structural change for Cu/amorphouslike CVD-W/Si samples annealed up to 675 °C for 30 min in N2. At higher temperatures (700 °C), Cu penetration results in the formation of η′′-Cu3Si precipitates at the CVD-W/Si interface. This is due to the crystallization of the amorphouslike CVD-W film above 650 °C, rendering the grain-boundary structure and, hence, fast pathways for Cu diffusion. The Cu/amorphouslike CVD-W/p+n diodes, thus, sustain large increases in reverse leakage current. In addition, the effects of nitrogen addition by using an in situ nitridation on the amorphouslike CVD-W film are also discussed. The effectiveness of the nitrided barrier is attributed to the blocking of the grain boundaries in the tungsten film by nitrogen atoms. This slows down Cu diffusion significantly. Physical and chemical analyses indicate that interfaces in the Cu/WNx/W/Si multilayer maintain their integrity while the annealing is carried out at 750 °C. Moreover, the reverse leakage current densities of Cu/WNx/W/p+n diodes remain at 10−7 A/cm2 after 725 °C annealing.
High intensity illumination effects in LiNbO3 and KTiOPO4 waveguides
Authors: Eger D. , Arbore M. A. , Fejer M. M. , Bortz M. L.
Published: July 2002

Quasi-phase-matched waveguides are known to degrade when generating high intensity short wavelength light. Photoinduced changes in the refractive indices can lead to reduced efficiency by broadening phase-matching curves or inducing power dependent losses. In this work a pump-probe technique was used to investigate photorefractive effects in nonlinear optical waveguides. A strong two photon photorefractive effect in single domain and Ti-poled LiNbO3 was found that is considerably reduced in electric field periodically poled LiNbO3 and is absent in Rb-exchanged KTiOPO4.
Correlation of stress behavior with hydrogen-related impurities in plasma-enhanced chemical vapor deposited silicon dioxide films
Authors: Haque M. S. , Naseem H. A. , Brown W. D.
Published: July 2002

The presence of undesirable hydrogen-related impurities and the resulting stress instability in chemical vapor deposited silicon dioxide films are important issues. In this work, the bonding nature and stress behavior of relatively low-temperature deposited silicon dioxide films deposited at high rates were investigated. Films were deposited at 1000 Å/min and at a substrate temperature in the 250–350 °C range. A considerable change in stress was observed in these films upon annealing in the 250–400 °C temperature range. Both as-deposited and annealed films were then stored in a cleanroom environment for long periods of time, and their stress was monitored intermittently. In parallel, Fourier transform infrared studies were performed on an identical set of as-deposited and annealed films to investigate changes in the bonding nature of the films during aging. Thus, film stress and their bonding nature were studied concurrently over an extended period of time. Si–H and silanol (Si–OH) were identified as impurities responsible for the observed stress instability of the deposited films. Initial concentrations of these impurities have been observed to vary depending on the deposition conditions. Also, depending on the concentrations of these impurities, both reversible and irreversible bond reconstruction were observed in the films upon annealing. Concomittantly, reversible and irreversible changes in stress were observed in annealed films, the amount of change depending on the impurity type and content. Impurities responsible for reversible and irreversible bond reconstruction were identified. Good correlation between film stress and bonding was observed.
Optical constants of epitaxial AlGaN films and their temperature dependence
Authors: Brunner D. , Angerer H. , Bustarret E. , Freudenberg F. , Höpler R. , Dimitrov R. , Ambacher O. , Stutzmann M.
Published: July 2002

We have studied the dependence of the absorption edge and the refractive index of wurtzite AlxGa1−xN films on temperature and composition using transmission and photothermal deflection spectroscopy. The Al molar fraction of the AlxGa1−xN films grown by plasma induced molecular beam epitaxy was varied through the entire range of composition (0⩽x⩽1). We determined the absorption edges of AlxGa1−xN films and a bowing parameter of 1.3±0.2 eV. The refractive index in the photon energy range between 1 and 5.5 eV and temperatures between 7 and 295 K was deduced from the interference fringes. The static refractive index n(0) changed from 2.29 for GaN to 1.96 for AlN at room temperature. A variation of temperature from 295 to 7 K resulted in a decrease of refractive index (at photon energies close to the band gap) by 0.05±0.01 and in an energy shift of the absorption edge of about 64±5 meV independent of the Al content of the films. Using the Kramers–Kronig dispersion relation and an approximation for the dispersion coefficient for photon energies near the band gap, the refractive index could be described as a function of photon energy, Al content, and temperature.
Effects of the growth temperature and As/Ga flux ratio on the incorporation of excess As into low temperature grown GaAs
Authors: Luysberg M. , Sohn H. , Prasad A. , Specht P. , Liliental-Weber Z. , Weber E. R. , Gebauer J. , Krause-Rehberg R.
Published: July 2002

The controlled incorporation of excess As into GaAs grown by molecular beam epitaxy at low growth temperatures (LT-GaAs) is explored. The substrate temperature and the As/Ga flux ratio were systematically varied to investigate the influence of growth parameters on the formation of native defects and structural properties. Near infrared absorption, magnetic circular dichroism of absorption, and slow positron annihilation were applied to determine point defect concentrations of As antisites (AsGa) and Ga vacancies (VGa). Structural properties of as-grown and annealed LT-GaAs layers were investigated by x-ray diffraction and transmission electron microscopy. In a well defined parameter range the lattice expansion of the LT-GaAs layers correlates with the amount of AsGa. The VGa acceptor concentration can quantitatively account for the ionization of the AsGa donors.
InGaAs island shapes and adatom migration behavior on (100), (110), (111), and (311) GaAs surfaces
Authors: Lobo C. , Leon R.
Published: July 2002

The evolution of InGaAs island formation on (100), (110), (111), and (311) GaAs substrates was studied by atomic force microscopy. In addition to determining the growth mode, shape, average size and distribution of InGaAs islands on each orientation, measurement of the saturation island densities enabled an estimation of effective group III adatom surface diffusion lengths. Small lens-shaped islands in addition to larger faceted islands were formed on (100) and (311) surfaces, while trapezoidal and triangular islands were obtained on (110) and (111)B orientations, respectively. Adatom diffusion lengths on these surfaces were found to range from 0.06 μm on (311)B to 3 μm on (111)B.
Analysis of shear modes in a piezoelectric vibrator
Authors: Cao Wenwu , Zhu Shining , Jiang Bei
Published: July 2002

A shear piezoelectric vibrator has been analyzed in detail. There are two types of shear motion associated with the same shear stain deformation. It is found experimentally that the lower frequency mode among the two has lower strength and its vibration amplitude continuously decreases as the long dimension of the vibrator increases. A nearly pure shear mode may be obtained with the aspect ratio greater than 20, but the influence of the other low frequency mode could not be totally eliminated even at such a large aspect ratio. We found that such phenomena can be understood by the conservation of angular momentum and the shared origin of these two types of shear motion. The ratio of the driving forces for these two types of shear motion is related directly to the aspect ratio of the vibrator. Formulas for calculating k15 from the resonant and antiresonant frequencies using both type of shear modes are given.
The origin of visible photoluminescence from silicon oxide thin films prepared by dual-plasma chemical vapor deposition
Authors: Zhu M. , Han Y. , Wehrspohn R. B. , Godet C. , Etemadi R. , Ballutaud D.
Published: July 2002

In order to understand the radiative recombination mechanisms in silicon oxides, photoluminescence properties (PL) of H-rich amorphous silicon oxide thin films grown in a dual-plasma chemical vapor deposition reactor have been related to a number of stoichiometry and structure characterizations (x-ray photoelectron spectroscopy, vibrational spectroscopy, and gas evolution studies). The visible photoluminescence at room temperature from a-SiOx:H matrixes with different compositions, including different bonding environments for H atoms, has been studied in the as-deposited and annealed states up to 900 °C. Three commonly reported PL bands centered around 1.7, 2.1, and 2.9 eV have been detected from the same type of a-SiOx:H material, only by varying the oxygen content (x = 1.35, 1.65, and 2). Temperature quenching experiments are crucial to distinguish the 1.7 eV band, fully consistent with bandtail-to-bandtail recombination, from the radiative defect luminescence mechanisms attributed either to defects related to Si–OH groups (2.9 eV) or to oxygen-vacancy defects (2.1 eV). In the latter case, a red-shift of the PL peak energy as a function of annealing temperature is probably attributed to some matrix-induced strain effect.
Phase transition and phase diagram of the J1−J2 Heisenberg model on a simple cubic lattice
Authors: Pinettes C. , Diep H. T.
Published: July 2002

By extensive standard and histogram Monte Carlo simulations, we show that the magnetic phase transition is of first order in the frustrated J1−J2 Heisenberg model on a simple-cubic lattice. Despite the infinite ground-state degeneracy, the system is shown to retain only the collinear spin configurations at low temperatures.
Fine grained Mn–Zn ferrite for high frequency driving
Authors: Takadate K. , Yamamoto Y. , Makino A. , Yamaguchi T. , Sasada I.
Published: July 2002

A Mn–Zn ferrite with a grain size of about 2 μm has been developed for the transformer driven at around 1 MHz. The developed ferrite exhibits considerably lower core losses than a conventional Mn–Zn ferrite with a grain size of about 10 μm at a frequency of 0.5 to 2 MHz. The thin-type transformer using the developed ferrite driven at 1 MHz shows a high efficiency of more than 95% at an output power of 10 to 17 W and a much lower surface temperature rise than the transformer using the conventional ferrite.