bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorine implantation effect on boron diffusion in Si
Authors: Park Yong-Jik , Kim Jong-Jean
Published: July 2002

Boron diffusion was studied in the BF2+ implanted Si by secondary ion mass spectroscopy. Anomalous boron diffusion was observed in the excess Si region of the amorphized layer at low temperatures below 300 °C. This was explained by a diffusion model where boron was assumed to diffuse by way of an intermediate species BI formed of interstitial Si(SiI) and substitutional boron. It is also found that fluorine plays a significant role in enhancing boron diffusion even at an annealing temperature as low as 300 °C, although the enhanced boron diffusion was reported only to occur at annealing temperatures above 600 °C. Fluorine may hinder SiI from growing into large clusters which need a high temperature annealing to dissociate into freely migrating species. Exponential diffusion profile of fluorine could be explained by a model of SiI–F complex formation. This complex consumes SiI resulting from small Si clusters which could dissociate at low annealing temperatures. This result also supports that boron diffusion at high annealing temperatures may be suppressed by co-implantation of fluorine.
Optimization of double pulse pumping for Ni-like Sm x-ray lasers
Authors: Lin J. Y. , Tallents G. J. , Smith R. , MacPhee A. G. , Wolfrum E. , Zhang J. , Eker G. , Keenan R. , Lewis C. L. S. , Neely D. , O’Rourke R. M. N. , Pert G. J. , Pestehe S. J. , Wark J. S.
Published: July 2002

We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 Å, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model.
Electrical resistance and mechanical stress in NiCr/Cu/NiCr thin films
Authors: Brückner W. , Baunack S.
Published: July 2002

NiCr/Cu/NiCr film configurations are used for metallization of chip resistors. The influence of a heat treatment on the electrical resistance of these films has been investigated ex situ and in situ. This has been correlated with concentration depth profiles determined by Auger electron spectrometry. Up to 450 °C only weak interdiffusion has been found, so that distinct resistance changes occur only for films with small Cu-sublayer thicknesses. The distinction of NiCr/CuNi/NiCr resistor films with characteristic interdiffusion effects is explained on the basis of phase diagram. In comparison to copper films, the temperature dependence of the effective biaxial stress in the NiCr/Cu/NiCr film configuration shows a reduced stress relaxation due to the plastically undeformable NiCr films.
Structural and photoluminescence studies of Er implanted Be doped and undoped low-temperature grown GaAs
Authors: Maltez R. L. , Liliental-Weber Z. , Washburn J. , Behar M. , Klein P. B. , Specht P. , Weber E. R.
Published: July 2002

Characteristic 1.54 μm Er3+ emission has been observed from Er-implanted and annealed, low-temperature grown GaAs Be doped and undoped samples. Er plateau implantations (480, 155, and 40 keV successive implants) were performed at 300 °C covering calculated Er concentrations from 1018 up to 1020 Er/cm3. Cross-sectional transmission electron microscopy studies reveal very little structural damage for these elevated temperature implants up to an Er total fluence of 1.36×1014 Er/cm2. No Er emission was observed from any of the as-implanted samples but it was observed after postimplantation annealings at 650 and 750 °C temperatures. The Er emission was significantly more intense after 650 °C anneals, for Be doped samples, and after 750 °C anneals for undoped samples. It appears on top of a broad background luminescence associated with midgap states. The Er emission intensity was found to scale linearly with the total Er implantation fluence up to Er concentration of ∼1019 Er/cm3. Er precipitation was observed after 750 °C annealing for 1019 Er/cm3 samples, but could be observed for 1020 Er/cm3 Er-doped samples even after a 650 °C anneal. These precipitates are most likely ErAs.
Electrical properties of highly strained modulation-doped InAs/GaAs (110) quantum-well heterostructures
Authors: Jaszek R. , Harris J. J. , Roberts C.
Published: July 2002

In this article results of Hall measurements on highly strained, modulation-doped InAs quantum-wells (QWs), grown by molecular beam epitaxy on GaAs (110) substrates are presented. These structures exhibit noticeably anisotropic strain relaxation, with an interfacial misfit dislocation network consisting of 90° dislocations along the in-plane [001] direction, and 60° dislocations along the perpendicular [11̄0] direction. While the linear density of the former is essentially independent of strain, that of the 60° dislocations increases with the thickness of the InAs layer. Using samples patterned with a bridge “Hall-bar” geometry and the current channels aligned along the dislocation lines, we find very low anisotropic mobilities, with those in the [11̄0] direction consistently higher than those along the [001]. When compared to self-consistent calculations, we also measure higher than expected carrier densities in these samples. Both effects appear to be directly related to the degree of relaxation in the structures and, in particular, to the density of 60° misfit dislocations at the QW interfaces. In terms of the dislocation scattering of carriers, our results show that 60° dislocations are more effective at scattering than 90° dislocations. We suggest the 60° dislocations possess a significant line charge, whereas the 90° dislocations are electrically neutral. We have also found evidence of donor-like behavior at the InAs/GaAs interface, which we associate with the known pinning by defects in InAs of the Fermi level in the conduction band.
Ellipsometric examination of optical property of the Si–SiO2 interface using the s-wave antireflection
Authors: Cho Yong Jai , Lee Yun Woo , Cho Hyun Mo , Lee In Won , Kim Sang Youl
Published: July 2002

Using variable-angle spectroscopic ellipsometry and introducing two models (a three-phase and a two-film model), we examined the optical properties of thermally grown SiO2 layers on Si, with special focus on phase difference Δ and amplitude ratio tan ψ for the s and p waves. We found an abrupt flip of the cos Δ curve from which the s and p-wave antireflection conditions were determined and evaluated the interface sensitivities for cos Δ and tan ψ based on the three-phase (ambient-oxide substrate) and the two-film (ambient-oxide-interlayer substrate) model. The sensitivities for cos Δ and tan ψ were shown to have maximum values at the same angle of incidence and photon energy in the s-wave antireflection condition. By fitting the variable-angle spectroscopic ellipsometry data measured in the s-wave antireflection condition, the thickness of the Si–SiO2 interface was determined as 0.784±0.003 nm for a 52-nm thick oxide sample and 0.764±0.002 nm for a 150-nm-thick oxide one. We also found that the effective refractive index of the interface was 2.060 at 546 nm for the 52 nm sample and 1.981 for the 150 nm sample, respectively.
Femtosecond melting and ablation of semiconductors studied with time of flight mass spectroscopy
Authors: Cavalleri Andrea , Sokolowski-Tinten Klaus , Bialkowski Joerg , Schreiner Michaela , von der Linde Dietrich
Published: July 2002

Using time-of-flight mass spectroscopy, we have investigated melting and ablation of gallium arsenide and silicon irradiated by femtosecond pulses. Below the ablation threshold the maximum surface temperature is obtained from the collisionless time-of-flight distributions of evaporated or sublimated particles. At the melting threshold, we estimate a temperature for the silicon surface which is approximately 500 K higher than the equilibrium melting temperature. In the fluence regime where melting is known to be a nonthermal process, we measure maximum surface temperatures in excess of 2500 K for both silicon and gallium arsenide, indicating rapid thermalization after nonthermal melting. At the ablation threshold, we estimated for both materials surface temperatures between 3000 and 4000 K. We observed a clear threshold-like effect in the number of detected particles, indicating the occurrence of a bulk effect. The flow parameters above the ablation threshold are discussed and compared to the different models of collisional expansion. For Fabl<F<2Fabl, transition from the liquid state to the gas phase occurs through the two-phase regime. For F>2Fabl, we find evidence that expansion takes place at temperatures that are higher than the critical temperature. Plasma formation appears only at fluences above 1 J/cm2 (F>5Fabl).
Spin valves with a thin pinning layer of α-Fe2O3 or α-Fe2O3/NiO
Authors: Kawawake Yasuhiro , Sugita Yasunari , Satomi Mitsuo , Sakakima Hiroshi
Published: July 2002

Magnetoresistive (MR) properties of spin valves pinned by a thin α-Fe2O3 layer were investigated. In spin valves consisting of α-Fe2O3/Ni–Fe(2 nm)/Co(1 nm)/Cu(2 nm)/Co(1 nm)/Ni–Fe(5 nm), the MR ratio remained nearly constant at about 12%, when the α-Fe2O3 layer thickness was reduced from 50 to 10 nm, while the exchange anisotropy field Hex decreased from 22 to 6 kA/m. The spin valves with a thinner α-Fe2O3 layer showed higher sensitivity to magnetic field than the ones with a thicker α-Fe2O3 layer. The measurement of MR ratio after annealing at 573 K in the α-Fe2O3/Co/Cu/Co spin valves revealed that the spin valve with a 30-nm-thick α-Fe2O3 layer was more stable against heat treatment than the one with a 50-nm-thick α-Fe2O3 layer. Heat treatment increased Hex in spin valves with both thick and thin α-Fe2O3 layers. Spin valves with NiO(10 nm)/α-Fe2O3(10 nm) as a pinning layer showed larger Hex than spin valves with either α-Fe2O3(10 nm) or NiO(10 nm) pinning layers.
Switching dependence on fabrication accuracy of tapered ends of a single giant magnetoresistance memory cell in word disturb condition
Authors: Redjdal M. , Gross P. W. , Kazmi A. , Humphrey F. B.
Published: July 2002

Direct integration of the Landau–Lifshitz–Gilbert equation in a three-dimensional rectangular lattice shows that fabrication asymmetry of tapered ends affects the magnetic switching behavior of the magnetic layers of a pseudospin valve (PSV) memory cell in word disturb condition. When a 10 nm asymmetry is introduced with the longer sides of the tapers on the same side of the PSV memory cell, a “360°” wall in the storage layer is formed when a 40 Oe uniform word disturb field is applied opposite to the magnetization direction in the storage layer. The initial magnetization state is recovered when the field is turned off. When the longer sides of the tapers are on opposite sides of the cell, complete magnetization reversal occurs in the storage layer. The sense layer and the storage layer relax to an antiparallel configuration magnetization that is opposite to the initial configuration in the memory cell when the field is turned off. The information is lost. Successful operation of the memory depends upon a fabrication accuracy that is better than 10 nm.
Excited-state absorption of Er3+-doped LiNbO3
Authors: Burlot-Loison R. , Doualan J.-L. , Le Boulanger P. , Han T. P. J. , Gallagher H. G. , Moncorgé R. , Boulon G.
Published: July 2002

The excited state absorption spectra of an Er:LiNbO3 single crystal are reported and analyzed versus polarization in both visible and near infrared spectral regions between 500 and 1800 nm. These spectra are completed with excited state excitation (ESE) spectra of the bright green anti-Stokes fluorescence observed in this material. These ESE spectra are recorded more particularly around 800 and 980 nm, and the results are analyzed in view of up-conversion laser operation after diode laser pumping around these two wavelengths.
X-ray response of epitaxial GaAs
Authors: Owens A. , Bavdaz M. , Kraft S. , Peacock A. , Nenonen S. , Andersson H. , Gagliardi M. A. , Gagliardi T. , Scholze F. , Ulm G.
Published: July 2002

We summarize the results of a number of x-ray experiments on an epitaxial GaAs device carried out in both our laboratory and the Physikalisch-Technishe Bundesanstalt radiometry Laboratory at the Berliner Elektronenspeicherring-Gesellschaft fur Synchrotronstrahlung Synchrotron Radiation Source. The detector has a diameter of 1.5 mm and is fully depleted to a depth of 40 μm. It has been characterized as a function of energy, bias, and temperature. At −35 °C we determine the charge collection efficiency to be 97% and find that energy resolutions ranging from 730 to 930 eV full width at half maximum (FWHM) can be readily achieved using conventional pre-amplifiers over the energy range 6–60 keV. By considering the various contributions to the FWHM, we show that leakage current and charge trapping noise dominate the resolution function. From detector modeling we determine the effective electron and hole density/cross section products to be 7 and 3 cm−1, respectively.
Response to “Comment on ‘Mixed electron emission from lead zirconate–titanate ceramics’ ” [J. Appl. Phys. 85, 8495 (1999)]
Authors: Zhang Weiming , Huebner Wayne , Sampayan Stephen E. , Krogh Mike L.
Published: July 2002

Understanding the mechanism of electron emission from ferroelectrics is in deed crucial for the application of ferroelectric cathodes. We appreciated the comments of Dorfman et al. on our recent article [J. Appl. Phys. 85, 6055 (1998)]. We believe the original proposed model is consistent and reasonable, i.e., the coexistence of ferroelectric and plasma emission from the same sample.
Macro- and microstress analysis in sol-gel derived Pb(ZrxTi1−x)O3 thin films
Authors: Schäfer J. D. , Näfe H. , Aldinger F.
Published: July 2002

Thin films of Pb[(ZrxTi1−x)1−yMey]O3 (Me=Al, Nb) (abbreviated PZT) have been prepared by spincoating of material synthesized by a sol-gel method. In accordance with the widely used arrangement for ferroelectric random access memories, the following layer sequence served as substrate: Si(100)/SiO2/Ti/Pt. Surface profilometry has been used to characterize the macrostresses in the films. Microstresses have been determined on the basis of grazing incidence x-ray diffraction profiles evaluated by the Warren–Averbach algorithm. The macrostress behavior of the PZT thin films is independent of the doping species, whereas the determined microstress behavior strongly depends on doping species aluminum or niobium and the dopant concentration. Furthermore, the general applicability of grazing incidence x-ray diffraction measurements in combination with microstress determination following the Warren–Averbach method has been proven.
Correlation of diffuse scattering with nanocrystallite size in porous silicon using transmission microscopy
Authors: Yamani Zain , Gurdal Osman , Alaql A. , Nayfeh Munir H.
Published: July 2002

We use high resolution cross sectional transmission electron microscopy to image the nanostructure of (100) p-type porous Si. A network of pore tracks subdivide the material into nano-islands and nanocrystallites are resolved throughout the material. With distance from the substrate, electron diffraction develops noncrystalline-like diffuse patterns that dominate the coherent scattering in the topmost luminescent layer. Also, with distance from the substrate, crystalline islands evolve such that their size drops to as small as 1 nm in the topmost luminescence material. Although the topmost luminescent layer is very rich in nanocrystallites, it has the strongest diffuse scattering of all regions. This confirms that diffuse scattering is due to size reduction effects rather than to an amorphous state.
An interface state mediated junction leakage mechanism induced by a single polyhedral oxide precipitate in silicon diode
Authors: Tsuchiaki M. , Fujimori H. , Iinuma T. , Kawasaki A.
Published: July 2002

The impact of an oxide precipitate on the junction leakage of a silicon p/n diode was investigated. A 70–100 nm diam single polyhedral oxide precipitate was placed in the depletion region of a p+/n junction by hydrogen annealing and polishing a Czochralski grown silicon wafer. In the reverse-bias current–voltage (I–V) curve, an anomalous hump structure was observed. This excess leakage component exhibited a flicker-type low frequency noise. Functional decomposition of the I–V curves demonstrated that a single gap state regulates the leakage current and that two distinct physical processes are involved in the leakage mechanism. The temperature and electric field dependence of the processes revealed that the gap state was a shallow attractive Coulomb center and that electric carriers were supplied to the Coulomb center by phonon-assisted tunneling and emitted from the center by the Poole–Frenkel mechanism. The presence of excessive low frequency noise indicated spatial proximity between the Coulomb center and the oxide precipitate. Computer simulations of the electric field around the oxide precipitate suggested that the center is doubly charged. Based on these findings, a leakage model was proposed. In the vicinity of the oxide precipitate, a few interstitial oxygen atoms are thought to be clustered to form a shallow double donor. The electric field facilitates the hopping of electrons from the valence band into the oxygen donor via interface states on the exterior of the oxide precipitate. Electrons are then emitted onto the conduction band by the Poole–Frenkel mechanism. Charge state transitions of traps inside the oxide precipitate generate a low frequency electric field fluctuation. The field fluctuation is then amplified by the Poole–Frenkel mechanism into the excessive low frequency noise in the leakage current. Thus a clear case of the detrimental impact of a shallow Coulomb center associated with an extended defect is established.
Demonstration of Li-based alloy coatings as low-voltage stable electron-emission surfaces for field-emission devices
Authors: Auciello O. , Krauss A. R. , Gruen D. M. , Shah P. , Corrigan T. , Kordesch M. E. , Chang R. P. H. , Barr T. L.
Published: July 2002

Alkali metals have extremely low work functions and are, therefore, expected to result in significant enhancement of the electron emission if they are used as coatings on Mo or Si microtip field-emission arrays (FEAs). However, the alkali metals are physically and chemically unstable in layers exceeding a few Å in thickness. Maximum enhancement of electron emission occurs for alkali–metal layers 0.5–1 ML thick, but it is extremely difficult to fabricate and maintain such a thin alkali–metal coating. We present here an alternative means of producing chemically and thermally stable, self-replenishing lithium coatings approximately 1 ML thick, which results in a 13-fold reduction in the threshold voltage for electron emission compared with uncoated Si FEAs.
Direct deposition of a blanket tungsten layer on SiO2 by preexposure of helium plasma
Authors: Noma Takashi , Seol Kwang Soo , Fujimaki Makoto , Ohki Yoshimichi
Published: July 2002

By utilizing preexposure of helium plasma, a method of depositing a blanket tungsten layer directly on SiO2 has been developed. When SiO2 films are exposed to a helium plasma, oxygen atoms are knocked on by active species in the plasma and the surface becomes covered with reduced silicon. It is assumed that this silicon reacts with WF6 to form tungsten nuclei, and that tungsten layers grow through the reaction of H2 and WF6 at these nuclei. The deposited tungsten layer is stable with a low resistivity in the form of the α-W crystal and shows strong adhesion to the SiO2 film due to the anchoring effect of tungsten.
Effectiveness of multiple-pair buffer layer to improve the GaN layers grown by metalorganic chemical vapor deposition
Authors: Yang Chien-Cheng , Wu Meng-Chyi , Chang Chin-An , Chi Gou-Chung
Published: July 2002

High-quality GaN epitaxial layers with a multiple-pair buffer layer have been grown on sapphire substrates in a separate-flow reactor by metalorganic chemical vapor deposition. Each pair of buffer layer consists of a 300 Å thick GaN nucleation layer grown at a low temperature of 525 °C and a 1–4 μm thick GaN epitaxial layer grown at a high temperature of 1000 °C. The GaN samples with a multiple-pair buffer layer are characterized by double-crystal x-ray diffraction (DC-XRD), Hall method and photoluminescence (PL) at 300 K, and etch-pit density measurements. The optimized condition to obtain the best quality of GaN epitaxial layers is to grow the four-pair buffer layer with a pair thickness of 4 μm. The GaN samples with the optimized buffer layer exhibit a narrow full width at half maximum (FWHM) of 150 arcsec and a strong intensity in DC-XRD, a high electron mobility of 450 cm2/V s, a low background concentration of 3×1017 cm−3, a low etch-pit density of mid-105 cm−2, and a narrow FWHM of 56 meV in PL spectrum.
Multiple phase gratings in pure, Yb- and P-doped Pb5Ge3O11 after different thermal treatments
Authors: Yue Xuefeng , Mendricks S. , Nikolajsen T. , Hesse H. , Kip D. , Krätzig E.
Published: July 2002

Multiple phase gratings are written in pure, Yb- and P-doped Pb5Ge3O11 crystals: a fast grating and a slow grating with substantially different response times compensate each other. Doping and thermal treatments have strong influences on the behavior of both gratings. Reduction treatments of Yb-doped samples lead to a significant decrease of the response time of the slow grating, while that of the fast grating is diminished by more than two orders of magnitude. P doping significantly increases the response time of the fast grating. Possible origins of both gratings are discussed.
Spectroscopic measurement of packaging-induced strains in quantum-well laser diodes
Authors: Tomm J. W. , Müller R. , Bärwolff A. , Elsaesser T. , Gerhardt A. , Donecker J. , Lorenzen D. , Daiminger F. X. , Weiß S. , Hutter M. , Kaulfersch E. , Reichl H.
Published: July 2002

Packaging-induced strain is studied in high-power semiconductor lasers by a noninvasive optical technique. Fourier-transform photocurrent measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of optical transitions within the active region. These shifts by up to 10 meV serve as a measure for the strain status within the active layer of the devices and are compared with model calculations. For different packaging architectures we quantify the strain portion which is transmitted to the optically active region of the semiconductor device.
Structural changes in InP/Si solar cells following irradiation with protons to very high fluences
Authors: Messenger S. R. , Jackson E. M. , Burke E. A. , Walters R. J. , Xapsos M. A. , Summers G. P.
Published: July 2002

Precisely how the short circuit current (JSC) is produced in a proton irradiated n+p InP/Si solar cell at very high fluence levels has been determined from combined measurements of the cell structure using electrochemical capacitance–voltage profiling and detailed analysis of the spectral quantum efficiency. Type conversion in the base region of the cell is shown to occur before an anomalous peak in the degradation curve for JSC is reached at high damage levels. The short circuit current, and hence the cell efficiency, ultimately collapse because the high absorption coefficient of InP eventually prevents the generation of electron–hole pairs close enough to the effective cell junction from being collected.
Optical constants of amorphous Se
Authors: Innami Takeshi , Miyazaki Takayuki , Adachi Sadao
Published: July 2002

The pseudodielectric-function spectra, ε(E)=ε1(E)+iε2(E), of amorphous (a-) selenium (Se) in the 1.2–5.2 eV photon-energy range at room temperature were measured by spectroscopic ellipsometry. The a-Se films investigated were deposited by vacuum evaporation in a base pressure of 2×10−6 Torr on Si(100) substrates at room temperature. The ε2(E) spectrum showed only a broad peak at ∼4 eV. Such a structureless feature is known to be typically observed in amorphous semiconductors. An ex situ atomic-force-microscopy image confirmed a microscopically flat surface (root-mean-square roughness of ∼0.4 nm) with relatively sharp cones in spots. The Bruggeman effective-medium-approximation analysis suggested that the correction for this microroughness is very small (Δε1,2<0.2). The present ε(E) data and previously published data were used for the modeling of the optical constants of a-Se over the 0–15 eV photon-energy range. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of a-Se were also presented.
Visible photoluminescence in hydrogenated amorphous silicon grown in microwave plasma from SiH4 strongly diluted with He
Authors: Dian J. , Valenta J. , Hála J. , Poruba A. , Horváth P. , Luterová K. , Gregora I. , Pelant I.
Published: July 2002

Room temperature visible photoluminescence (PL) of wide-band gap hydrogenated amorphous silicon (a-Si:H) thin films prepared in SiH4 microwave plasma strongly diluted with He is reported. Films were characterized by means of optical and infrared absorption, hydrogen thermal desorption, and Raman scattering. The band gap of a-Si:H films varies within the interval 2.0–2.2 eV, corresponding PL maxima are located at 1.4–1.6 eV. The highest PL intensity was observed in samples with a position of H–Si–H symmetric stretching vibration of the –(SiH2)n– units near the frequency of 2100 cm−1. The strong evidence for two distinct types of PL processes is presented: one being linked with oligosilanes and the second one attributed to electron-hole recombination in tail states.
Electronic structure of the layered compound Ti3GeC2
Authors: Zhou Yanchun , Sun Zhimei
Published: July 2002

The electronic structure and properties of layered ceramic Ti3GeC2 have been examined by means of ab initio linear combination of atomic orbital calculations. The calculated band structure shows Ti3GeC2 to be strongly metallic with high densities of states at the Fermi level. The electrical conductivity is dominated by a Ti (2) 3d state with less contribution from Ti (1) 3d, Ge 4p, and C 2p states. The major factors governing the electronic properties are pd hybridization from Ti 3d, Ge 4p, and C 2p states, and p-d bonding stabilizes the structure.
A comparison of photoluminescence properties of InGaAs/GaAs quantum dots with a single quantum well
Authors: Kong Mei-Ying , Wang Xiao-Liang , Pan Dong , Zeng Yi-Ping , Wang J. , Ge Weikun
Published: July 2002

Variable temperature photoluminescence (PL) measurements for In0.3Ga0.7As(6 nm)/GaAs(34 nm) quantum dot superlattices with a period of 20 and an In0.3Ga0.7As(6 nm)/GaAs(34 nm) reference single quantum well have been conducted. It is found that the temperature dependence is different between the quantum dots and the reference single quantum well. The PL peak energy of the single quantum well decreases faster than that of the quantum dots with increasing temperature. The PL peak energy for the InGaAs/GaAs quantum dots closely follows the InAs band gap in the temperature range from 11 to 170 K, while the PL peak energy for the InGaAs/GaAs quantum well closely follows the GaAs band gap. In comparison with InAs/GaAs quantum dots, the InGaAs/GaAs quantum dots are more typical as a zero-dimensional system since the unusual PL results, which appear in the former, are not obvious for the latter.
The role of S passivation on magnetic properties of Fe overlayers grown on GaAs(100)
Authors: Zhang F. P. , Xu P. S. , Lu E. D. , Guo H. Z. , Xu F. Q. , Zhang X. Y.
Published: July 2002

We have produced epitaxial Fe overlayers on sulfur-passivated GaAs(100) surfaces by CH3CSNH2 treatment, and investigated the correlation between magnetic properties of the overlayers and surface chemical structure of GaAs(100) surfaces by ferromagnetic resonance and synchrotron radiation photoemission. The surface chemical properties were modified by changing the annealing temperature of the surfaces prior to the growth. The results show that the magnetization of Fe overlayers is crucially determined by the presence of Ga–S chemical bonds and excess As after the anneals. A comparative investigation of the magnetization has been made on both S passivated and clean GaAs(100). It is confirmed that S passivation on the GaAs surface can effectively eliminate the magnetization deficiency previously attributed to interdiffusion of As into the Fe overlayer.
Analytic reconstruction of Compton scattering tomography
Authors: Wang Jiajun , Chi Zheru , Wang Yuanmei
Published: July 2002

Compton scattering can be used to determine the electron densities of tissues for medical applications and those of materials for industrial applications. Much work has been devoted in solving the reconstruction problem. Norton proposed an analytic transform method for the reconstruction of Compton scattering tomography [J. Appl. Phys. 76, 2007 (1994)]. However, it is difficult to relate the response function presented by Norton to the measurement quantity. The aim of this article is to present an improved form of the detector response function which corresponds to the actual measurement and to verify the validation of the transform method for this problem.
Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8−xCax)Mn2O7 manganites
Authors: Shen C. H. , Liu R. S. , Hu S. F. , Lin J. G. , Huang C. Y. , Sheu H. S.
Published: July 2002

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in La1.2(Sr1.8−xCax)Mn2O7 (x=0–0.8) are investigated. The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H=1.5 T) at a temperature of 102 K was observed for the x=0.4 sample. The Curie temperatures decreased from 135 to 102 K for x=0–0.4, respectively. Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration. The antiferromagnetic behavior with Néel temperature around 30 K was found in the x=0.8 sample. The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Néel temperatures.
Effects of elastic stress introduced by a silicon nitride cap on solid-phase crystallization of amorphous silicon
Authors: Kimura Yasuo , Kishi Masato , Katoda Takashi
Published: July 2002

Effects of stress on solid-phase crystallization of amorphous silicon (a-Si) were studied by laser Raman spectroscopy. Compressive stress was introduced in a-Si with a Si3N4 cap. The speed of crystallization decreased with the increase of the stress while it increased again with an additional cap of SiO2 on a Si3N4 cap. A SiO2 cap introduced tensile stress in an a-Si film and relaxed compressive stress by a Si3N4 cap. The reason why crystallization of a-Si is suppressed is that the stress is elastic and that it does not relax with crystallization.
Defects in p+-gate metal–oxide–semiconductor structures probed by monoenergetic positron beams
Authors: Uedono Akira , Hiketa Masako , Tanigawa Shoichiro , Kitano Tomohisa , Kubota Taishi , Makabe Mariko , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa
Published: July 2002

Defects in p+-gate metal–oxide–semiconductor structures were probed using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured for BF2+- or B+-implanted polycrystalline-Si(300 nm)/SiO2(4 nm)/Si specimens. The line-shape parameter, S, corresponding to the annihilation of positrons near SiO2 films was decreased by annealing treatments performed after ion implantation. This fact was attributed to the trapping of positrons by vacancy-oxygen complexes introduced by the boron diffusion in the SiO2 film. The introduction of such complexes was enhanced by incorporation of fluorine, but suppressed by doping of phosphorus into polycrystalline films.
Room-temperature lasing of strain-compensated CdSe/ZnSSe quantum island laser structures
Authors: Engelhardt R. , Pohl U. W. , Bimberg D. , Litvinov D. , Rosenauer A. , Gerthsen D.
Published: July 2002

Efficient resonant excitonic waveguiding is achieved in laser structures, grown by metallorganic chemical vapor deposition, with stacked CdSe quantum islands which were separated by ternary ZnSSe barriers. Plastic relaxation within the stack is shown to be suppressed by adjusting the sulfur content in the barriers to compensate the strain. Excitonic lasing with low threshold intensities is demonstrated well above room temperature with Ith77 K=0.8 kW/cm2 and Ith300 K=55 kW/cm2.
Dendritic domain configurations in Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Yu Hanxing , Randall Clive A.
Published: July 2002

Domain structures arising as a result of pulsed electric fields are investigated in Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) 〈001〉 single crystals by means of etching and environmental scanning electron microscope. It is shown that randomly orientated dendritic domain patterns were created under a pulsed electric field condition. The dendritic patterns are believed to be the result of local electric field and elastic deformation. Dendritic structures are also found during a back-switching process in 0.92PZN-0.08PT [001] single crystals. It is hypothesized that these unusual nonequilibrium domain structures are the result of local defects and space charge in the relaxor PZN-PT crystals.
Electromagnetic missiles and radial dependence of pulses transmitted by transient sources
Authors: Xu B. Y. , Lai H. M.
Published: July 2002

The retarded solution to the inhomogeneous wave equation in electrodynamics is studied with a purpose to understand Wu’s missile and subsequently the radial dependence of energy transmitted by transient sources. The finite rate of change of the source current is shown to imply the inverse-square law at a sufficiently large distance, the region of which is determined by the minimum scale time tm of the current and may be called the (generalized) Fraunhofer region. With a uniform disk source, the electric field on the symmetry axis for a wide range is shown equal to the difference of the current at two times. Hence, examples with analytic results are given to demonstrate clearly how the pulse energy decreases with distance at a much slower rate in the (generalized) Fresnel region and how the inverse-square dependence is recovered in the farther region. Moreover, Wu’s current profile, though with infinite slopes, is a Lighthill’s generalized function and missiles refer to the transmitted pulses in the limit of zero tm which gives an infinite Fresnel region. A square temporal profile and other profiles with a sudden rise (or fall), including Samaddar’s single-cycled cosine, which also generate missiles with an inverse-linear dependence, are studied.
Atomic hydrogen-assisted molecular beam epitaxy for the fabrication of multi-quantum-well solar cells
Authors: Suzuki Yoshiyuki , Kikuchi Takatoshi , Kawabe Mitsuo , Okada Yoshitaka
Published: July 2002

The atomic hydrogen-assisted molecular beam epitaxy (H-MBE) technique has been applied to fabricate multi-quantum-well (MQW) based compound semiconductor solar cells. A number of different InGaAs/GaAs MQW structures were inserted within the photon absorption region of a p–i–n GaAs junction diode. The spectral response characteristics of each device with varying number, thickness, and In composition of MQWs were measured and analyzed to investigate the effects of atomic H on the device performance. In the photon wavelength region of 800–1000 nm, the spectral response of H-MBE-grown cells was increased by >25% for the GaAs homojunction control cell, and by as much as ∼35% for a 10-period MQW cell (In0.07Ga0.93As: GaAs=50 nm:50 nm) as compared to the conventionally MBE-grown cells with identical layer structures.
Measurement of the diffuse double-layer forces between zirconia and α-alumina
Authors: El Ghzaoui A.
Published: July 2002

Atomic force microscopy measurements were carried out between zirconia and α-alumina colloids over a wide pH range and in 1×10−4 M sodium chloridel aqueous solution. The electrophoretic mobility and potentiometric titrations were performed on the zirconia and α-alumina colloid samples, respectively. The electrical double-layer forces measured by atomic force microscopy were related to the zeta potentials and the isoelectric point of the both mineral dioxides, determined from the electrophoretic mobility measurements.
Enhanced electrical activation of indium coimplanted with carbon in a silicon substrate
Authors: Boudinov H. , de Souza J. P. , Saul C. K.
Published: July 2002

Electrical activation of In of 18%–52% of the implanted dose (5×1014 cm−2) was obtained in Si samples having a C+ coimplantation after rapid thermal annealing (RTA) at 800–1000 °C for 15 s. This electrical activation yield markedly contrasts with that in samples singly implanted with In in which only ≅0.5% of the dose was activated. The following features were observed in the coimplanted samples: (i) a reverse annealing of the electrical activation in the temperature range of 800–900 °C; (ii) significant reduction of the In profile redistribution during RTA; and (iii) the electrically activated In concentration is substantially higher than the substitutional In concentration. These findings are discussed in terms of the interaction between C atoms and Si self-interstitials (SiI), strain compensation between C and In atoms in the Si lattice, and formation of stable In substitutional–C substitutional acceptors centers.
Optical properties of Si-doped GaN films
Authors: Yang H. C. , Lin T. Y. , Huang M. Y. , Chen Y. F.
Published: July 2002

Results of the front-side and back-side photoluminescence (PL) measurements in a set of Si-doped GaN epifilms are presented. From the back-side PL spectrum, the enhancement of the yellow emission implies that most of the intrinsic defects responsible for the yellow band exist mainly near the interface between the buffer layer and the epilayer. We also found that the intensity of the yellow luminescence decreases with increasing Si dopants, which is consistent with the fact that the microscopic origin of the yellow emission can be attributed to gallium vacancies VGa. In additions, our investigations reveal that the potential fluctuations, that give rise to the effect of band-gap narrowing and linewidth broadening, are mainly caused by randomly distributed doping impurities instead of other defects.
Dielectric response in pulsed laser ablated (Ba,Sr)TiO3 thin films
Authors: Saha S. , Krupanidhi S. B.
Published: July 2002

The dielectric response of pulsed laser ablated barium strontium titanate thin films were studied as a function of frequency and ambient temperature (from room temperature to 320 °C) by employing impedance spectroscopy. Combined modulus and impedance spectroscopic plots were used to study the response of the film, which in general may contain the grain, grain boundary, and the electrode/film interface as capacitive elements. The spectroscopic plots revealed that the major response was due to the grains, while contributions from the grain boundary or the electrode/film interface was negligible. Further observation from the complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Conductivity plots against frequency at different temperatures suggested a response obeying the ‘universal power law’. The value of the activation energies computed from the Arrhenius plots of both ac and dc conductivities with 1000/T were 0.97 and 1.04 eV, respectively. This was found to be in excellent agreement with published literature, and was attributed to the motion of oxygen vacancies within the bulk.
On a shock-induced martensitic phase transition
Authors: Abeyaratne Rohan , Knowles James K.
Published: July 2002

A recently developed continuum-mechanical model for stress-induced phase transitions in solids is applied to a transition generated by impact. The role of transition kinetics in determining the macroscopic response to impact is discussed; in addition, the special way that “overdriven” phase boundaries emerge in this model is described. The predictions of the model are compared with experiments involving shock-induced graphite-to-diamond phase transitions.
Properties of nanoporous silica thin films determined by high-resolution x-ray reflectivity and small-angle neutron scattering
Authors: Wu Wen-li , Wallace William E. , Lin Eric K. , Lynn Gary W. , Glinka Charles J. , Ryan E. Todd , Ho Huei-Min
Published: July 2002

A new methodology based on a novel combination of a high-resolution specular x-ray reflectivity and small-angle neutron scattering has been developed to evaluate the structural properties of low-dielectric-constant porous silica thin films about one micrometer thick supported on silicon wafer substrates. To complement these results, film composition was determined by high-energy ion scattering techniques. For the example thin film presented here, the overall film density was found to be (0.55±0.01) g/cm3 with a pore wall density of (1.16±0.05) g/cm3 and a porosity of (53±1)%. The characteristic average dimension for the pores was found to be (65±1) Å. It was determined that (22.1±0.5)% of the pores had connective paths to the free surface. The mass fraction of water absorption was (3.0±0.5)% and the coefficient of thermal expansion was (60±20)×10−6/°C from room temperature to 175 °C. Lastly, model fitting of the specular x-ray reflectivity data indicated the presence of a thin surface layer with an increased electron density compared to the bulk of the film as well as an interfacial layer with a reduced electron density.
Structural and electrical characterization of epitaxial, large area ferroelectric films of Ba2Bi4Ti5O18 grown by pulsed excimer laser ablation
Authors: James A. R. , Pignolet A. , Hesse D. , Gösele U.
Published: July 2002

Excimer laser ablation was used to deposit epitaxial thin films of Ba2Bi4Ti5O18 having mostly c-axis oriented grains. The ferroelectric layer was deposited on an epitaxial conducting oxide electrode layer of LaNiO3, the electrode layer in turn being grown on an epitaxial buffer layer stack on single crystalline (100)-oriented silicon wafers of 3 in. diameter. X-ray diffraction measurements indicate a strong c-axis orientation of the ferroelectric layer. Scanning electron microscopy and atomic force microscopy were used to observe the microstructure of the films and these revealed the homogenous quality of the film with good uniformity over the wafers. Well-defined, saturated ferroelectric hysteresis loops were observed in the compound with values of 1.3 and 0.6 μC/cm2 as saturation and remnant polarization values, respectively, and a coercive field of 47 kV/cm. The results of I–V measurements and complex impedance spectroscopy measurements are presented and a defect formula is proposed for the compound.
Microstructural and magnetic properties of exchange-coupled Co/Fe multilayers
Authors: Asti G. , Carbucicchio M. , Ghidini M. , Rateo M. , Ruggiero G. , Solzi M. , D’Orazio F. , Lucari F.
Published: July 2002

Cobalt/iron multilayers with different layer thickness were electron beam evaporated in ultrahigh vacuum, keeping constant both the number of layers and the Co/Fe thickness ratio. Structural and magnetic properties are thoroughly investigated with different techniques. The multilayers have clean and sharp interfaces. All samples show single-phase magnetic behavior in the temperature range of 5–300 K, due to the strong exchange coupling between the layers. The room temperature hysteresis loops present a sharp switching at a field of ∼8 kA/m, followed by a residual hysteresis extending up to saturation, at fields of several tens of kA/m.
Effects of an intermediate layer on the magnetic and recording characteristics of CoCrPtTa media
Authors: Tanahashi K. , Tamai I. , Matsunuma S. , Kanbe T. , Ishikawa A. , Matsuda Y. , Yahisa Y. , Hosoe Y.
Published: July 2002

Effects of bcc and hcp intermediate layers between a bcc–Cr80Ti20 underlayer and a hcp–CoCrPtTa magnetic layer on magnetic and recording characteristics were investigated. The medium without an intermediate layer has a low coercivity Hc of 0.27 kOe due to the perpendicular orientation of the c axis of the CoCrPtTa layer. By introducing a 5-nm-thick intermediate layer of bcc–Cr60Mo40 or hcp–Co70Cr30, the c axis of the CoCrPtTa layer can be effectively in-plane oriented, resulting in a high Hc of 2.71–2.87 kOe and a coercive squareness S* of 0.72–0.81. The medium with the Cr60Mo40 layer has lower media noise than the medium with the Co70Cr30 layer. This lower noise is considered to be caused by the small grain size of 10.4 nm. The addition of the thin Cr60Mo40 intermediate layer is therefore effective to obtain both high Hc and low noise in the CoCrPtTa media with the CrTi underlayer.
Spin valve structures with artificial antiferromagnets
Authors: de Morais A. , Petford-Long A. K.
Published: July 2002

An artificial antiferromagnetic (AAF) structure as substitute for the ferromagnetic layer may overcome the limitations of sensitivity and dynamic range of a standard spin valve (SV). The AAF consists of two ferromagnetic films (CoFe or NiFe in this article) antiferromagnetically coupled across a thin nonmagnetic layer (Ru). Improved sensitivities combined with high magnetoresistance (MR) ratios and low coercivities and interlayer coupling fields are obtained in NiFe based AAF SVs. The differences between the observed reversal mechanism of SVs with parallel and crossed easy axes configurations have been studied by Lorentz microscopy and correlated with their different MR and sensitivity characteristics.
Induced and cooperative order of Nd ions in NdNiO3
Authors: Bartolomé F. , Bartolomé J. , Eccleston R. S.
Published: July 2002

Low-temperature specific heat measurements on NdNiO3 evidence the onset of Nd cooperative ordering at TN2=0.77 K. Because of the particular arrangement of the Ni magnetic moments, half of the Nd ions have an antiferromagnetically compensated environment while the other half have a noncompensated one. We show that both types of Nd ions are affected by a different but non-negligible Nd–Ni exchange field, in contrast with the current model used in literature.
Domain wall pinning, bulging, and displacement in circumferential domains in CoFeBSi amorphous wires
Authors: Garciá K. L. , Valenzuela R.
Published: July 2002

A detailed study of the complex inductance response of as-cast CoFeBSi wires as a function of frequency (100 Hz–13 MHz range) and circular field amplitude [0.1–17 A/m room mean square (rms) on the surface of the wire] in the giant magnetoimpedance arrangement is presented. The analysis of experimental results show evidence of magnetization processes associated with circumferential domain walls, such as domain wall pinning, bulging, and displacement. The unpinning field was measured as 0.2 A/m (rms) at 5 kHz. It is shown that the analysis of spectroscopic plots of real and imaginary inductance leads to characterization of the magnetization process involved.
Controlled n-type doping of a molecular organic semiconductor: Naphthalenetetracarboxylic dianhydride (NTCDA) doped with bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)
Authors: Nollau A. , Pfeiffer M. , Fritz T. , Leo K.
Published: July 2002

We present a study of controlled n-type doping in molecular organic semiconductors: Naphthalenetetracarboxylic dianhydride is doped by cosublimation with the donor molecule bis(ethylenedithio)-tetrathiafulvalene. Electrical parameters are deduced from temperaturedependent measurements of the conductivity and the thermopower for various dopant concentrations. The results are compared to the predictions of a standard model commonly used for crystalline semiconductors. The Fermi level shifts towards the transport level, the conductivity is increased, and the mobility decreases with the doping ratio.