bluuebunny/crossref_metadata_2025 huggingface dataset:
Longitudinal carrier density profiling in semiconductor lasers via spectral analysis of side spontaneous emission
Authors: Sargent Edward H. , Pavlidis Dorothy , Anis Hanan , Golinescu Nicolae , Xu J. M. , Clayton Richard , Kim Hyung B.
Published: July 2002

A combined experimental and theoretical approach to measuring the variation in carrier density along the length of a semiconductor laser is developed. It is shown that by following the rate of increase of the principal spectral peak, rather than monitoring the optical power at a fixed energy, measurements can be made less susceptible to the effects of heating in the sample. Experimental results showing the development of the longitudinal carrier density profile with injected current are presented and, when compared with the results of self-consistent modeling, provide insights into the internal operating mechanisms of the laser.
Electrochromism in lithiated Sn oxide: Mössbauer spectroscopy data on valence state changes
Authors: Isidorsson J. , Granqvist C. G. , Häggström L. , Nordström E.
Published: July 2002

Lithiated Sn oxide films, denoted LixSnO2, were produced by reactive rf magnetron sputtering of Sn and electrochemical post-treatment. Optical transmittance and Mössbauer spectra were recorded for progressively increased lithiation. Increasing x from zero to ∼0.1 did not have any significant effect on the optical data or Mössbauer spectra, and it is likely that the lithium is located in internal double layers in the film. Further increasing x from ∼0.1 to ∼0.2 yielded significant transmittance drops and Mössbauer spectra unambiguously showing a conversion Sn4+→Sn2+. Hence the optical absorption can be reconciled with intervalency transitions as in other cathodically coloring electrochromic oxides. Electrocrystallization appeared to dominate the electrochemistry at x≳0.2.
Growth and characterization of mid-infrared InGaAs/InAlAs strained triple-quantum-well light-emitting diodes grown on lattice-mismatched GaAs substrates
Authors: Grietens B. , Van Hoof C. , Van Daele P. , Borghs G.
Published: July 2002

Strained triple-quantum-well In0.9Ga0.1As/In0.8Al0.2As light-emitting diodes were grown on lattice-mismatched GaAs substrates by molecular-beam epitaxy. The diodes exhibit room-temperature external electroluminescent light emission at 2.4 μm with efficiencies up to 6.4×10−5. The maximum output power at room temperature is 1.85 μW under pulsed operation at 150 mA, (1 kHz, 50% duty cycle). Although the diodes have excellent electrical properties, their optical performance is still found to be limited by the nonradiative Shockley–Read–Hall recombination.
Annealing behavior and atomic composition of substitutional carbon–oxygen complexes in silicon crystals
Authors: Shirakawa Yoshimi , Yamada-Kaneta Hiroshi
Published: July 2002

We investigated the annealing behavior of the infrared-absorption peaks at 1052 and 1099 cm−1 due to C–OB complex, and those at 1112 and 1026 cm−1 due to C–OC complex. Similar to previous observations [Y. Shirakawa, H. Yamada-Kaneta, and H. Mori, J. Appl. Phys. 77, 41 (1995)], the formation-dissociation reaction of C–OB and C–OC complexes rapidly reaches the quasithermal-equilibrium state at the onset of annealing. Based upon the mass action law, it is suggested that the formation of the C–OB and C–OC complexes involves one carbon atom and two and three oxygen atoms, respectively.
An integral equation approach to electromigration under arbitrary time-dependent stress
Authors: Dwyer V. M. , Kearney M. J. , Bressloff P. C.
Published: July 2002

We present an integral equation reformulation of the drift–diffusion equation for modeling void buildup in integrated circuit interconnects due to the electromigration of aluminum atoms. The method can deal with arbitrary time-dependent current stress and offers a number of advantages over the conventional finite-difference approach. In particular, the boundary conditions are included in an intuitive and straightforward fashion. By way of illustration we identify the conditions under which the time-to-failure scales as r−2 for unidirectional square-wave stress of duty cycle r.
Microstructural and electrical investigations of Pd/Ge/Ti/Au ohmic contact to n-type GaAs
Authors: Kwak J. S. , Kim H. N. , Baik H. K. , Lee J.-L. , Shin D. W. , Park C. G. , Kim H. , Pyun K.-E.
Published: July 2002

Interfacial microstructures of Pd/Ge/Ti/Au ohmic contact to n-type GaAs have been investigated using cross-sectional transmission electron microscopy, and the results are used to interpret the electrical properties. Annealing at 300 °C yields a contact resistance of 0.62 Ω mm and the layer structure is changed to GaAs/PdGe/Au4Ti/TiO. The ohmic contact is formed through a solid phase regrowth of GaAs heavily doped with Ge below the PdGe layer. At 380 °C, the lowest contact resistance of 0.43 Ω mm is obtained. The layer structure is changed to GaAs/(Ge–Ti)/PdGe/TiO. Spikes composed of Au and AuGa are found at the grain boundaries of the PdGe compound. The formation of AuGa at 380 °C reduces the contact resistance through the creation of more Ga vacancies at the interface of GaAs/PdGe, and the incorporation of elemental Ge.
Raman scattering by longitudinal acoustic phonons in metallic W/Ti Fibonacci superlattice
Authors: Pan F. M. , Jin G. J. , Wu X. L. , Feng J. W. , Hu A. , Jiang S. S.
Published: July 2002

Raman scattering from longitudinal acoustic phonons in W/Ti Fibonacci superlattice is studied. Doublet peaks which result from the folded longitudinal acoustic phonons appear in the Raman spectra as expected. Based on the Kronig–Penney model or the Rytov model, both the normal-mode frequencies and scattering intensities are numerically calculated by using a transfer-matrix method. Noticeable agreement is obtained between the results of quantitative calculations and experimental data.
Thermoacoustic analysis of solar cells by current modulation
Authors: Harms U. , Barkow U. , Metzdorf J. , Kessler F. R.
Published: July 2002

The photoacoustic effect and a similar thermoacoustic effect with an alternating current (electroacoustic effect) are used to investigate the spatial distribution of heat sources in crystalline silicon solar cells. The use of a reference signal produced inside the same sample to normalize the acoustic signal allows background and photoacoustic cell effects to be reduced. The frequency dependence of these normalized measurements can be calculated analytically. Covering frequencies up to 10 kHz, the electroacoustic effect allows the nondestructive determination of diffusion lengths and recombination velocities at the interfaces. Volume and surface effects can thus be separated, which is useful for the analysis of high-efficiency solar cells.
Barrier formation at graded HgTe/CdTe heterojunctions
Authors: Goren D. , Asa G. , Nemirovsky Y.
Published: July 2002

Numerical calculations of graded HgTe/CdTe heterojunction (HJ) band diagrams at equilibrium are presented and discussed. The calculations are performed in the entire compositional range (0<x<1), using a nonparabolic conduction band and Fermi-Dirac statistics. The dependence of barrier formation at graded HJs are examined as a function of the graded region width and the graded region doping profiles. The graded region width and doping profiles were found to be the two main factors that determine whether barriers are formed as well as their shape and magnitude. The calculated results indicate that epitaxial ohmic HgTe contacts to extrinsic CdTe are possible, provided that the graded region is wider than one micron, and that it has the same doping type as the doping of the substrate with equal or higher absolute value. Further numerical calculations take into consideration the possible existence of distributed interface charges in the graded region of the HJ. It is shown that by assuming a classical transport over the potential barrier, the effective graded interface charge can be determined from the zero bias differential resistance of the HJ. Experimental transport measurements of metalorganic chemical vapor deposition (MOCVD) grown HgTe/p-CdTe graded HJs show a varying degree of rectification, indicating variations in the graded interface charge distributions which result from different MOCVD growth conditions.
Effect of high oxygen partial pressure heat treatment on the superconducting properties of Bi2Sr2CaCu2Ox/Ag tapes
Authors: Kumakura H. , Kitaguchi H. , Togano K. , Sugiyama N.
Published: July 2002

We investigated the effect of oxygen partial pressure pO2 during melt-and-solidification heat treatment on the superconducting properties of grain oriented Bi-2212 thick films. Tc and c-axis parameter decreased with increasing pO2, indicating the samples were in the over-doped state. The heat treatment temperature which gave highest Jc increased with increasing pO2. This was explained by the increase of the solidification temperature of Bi-2212 with pO2. Both the resistive Jc and intergrain Jc estimated from the dc magnetization increased with increasing pO2 when the samples were melt-solidified under an optimum condition for each pO2. Amplitude dependence of ac susceptibility suggested that the coupling of Bi-2212 grains was improved with increasing pO2. Irreversibility field Birr below ∼60 K was also enhanced with increasing pO2. Low temperature annealing under high pO2 after the melt-solidification also increased the oxygen content in Bi-2212 and hence Birr. However, the annealing did not change the amplitude dependence of ac susceptibility, and transport Jc at 4.2 K was almost independent of the annealing. The results indicate that the increase of Jc at 4.2 K with increasing pO2 during melt-solidification is primarily attributed to the improvement of the grain couplings.
Thermoelectric power studies on Nd1.82−xSrxCe0.18CuOy:x≤0.18 superconductors
Authors: Singh Okram G. , Padalia B. D. , Prakash Om , Agarwal S. K. , Narlikar A. V.
Published: July 2002

Thermoelectric power (S) studies on a Nd1.82−xSrxCe0.18CuOy:x≤0.18 superconducting system in the temperature range 35–250 K are reported here. In the x=0.09 sample, synthesized in the reduced environment, the small magnitude of S is highly metalliclike and its sign is negative, a characteristic of electron conduction. The sign of S for the x=0.18 sample shows a crossover below 75 K from negative to positive, in apparent conflict with electronic conduction. Interestingly, after oxygenation this sample exhibits a broadened but positive phonon draglike peak. This oxygenated sample shows overcompensation of the carrier (electron) concentration. Critical analysis of the data suggests that Sr doping seemingly causes a competition between electron- and holelike conduction. The slope dS/dT is, in general, negative suggesting that the main contribution is coming from the diffusive part. The observed thermopower features seem to fall in line with the theoretical curves of Durczewski and Ausloos [Z. Phys. B 85, 59 (1991); Phys. Rev. B 53, 1762 (1996)] based on the inelastic scattering of quasifree electrons by phonons.
Crystallographic and magnetic properties of NdCo11−xSix compounds
Authors: Zhao Yanming , Liang Jingkui , Rao Guanghui , Guo Yongquan , Tang Weihua
Published: July 2002

The effects of substitution of Si for Co on the crystal structural stability and magnetic properties of NdCo11−xSix compounds have been studied. X-ray diffraction and magnetic measurement show that the single phase region can be expressed as NdCo9.8–9.2Si1.2–1.8 other than NdCo9Si2. X-ray-diffraction study indicates that the compounds adopt the tetragonal BaCd11-type struc- ture. The magnetic properties of a series of NdCo11−xSix solid solutions have been studied by magnetic measurement. Our investigations show that the Curie temperature of NdCo11−xSix increases drastically from 528 K (x=1.8) to 833 K (x=1.2) as the Si content decreases. The Co magnetic moment reaches a maximum between NdCo9.8Si1.2 and NdCo9.6Si1.4, and decreases with the further increase in x. The substitution of Si for Co in NdCo11−xSix leads to a contraction of lattice constant a by 0.2%, c by 0.2%, and a consequent reduction of the unit-cell volume by about 0.6%. The Rietveld refinement results show that the silicon atoms preferentially occupy the 8(d) sites. The occupation of 8(d) sites by proper amount of Si will shift the Fermi level EF upward and to a valley with a lower local density of states in NdCo11−xSix compounds, which is favorable for the stability of the BaCd11-type structure.
Microstructural and magnetic characteristics of nanocrystalline Fe89Zr7B4 soft magnetic alloys
Authors: Zhou F. , He K. Y. , Bai K. C.
Published: July 2002

Nanocrystalline (NC) Fe89Zr7B4 soft magnetic alloys, consisting of an α-Fe crystalline phase and a residual amorphous phase, were synthesized by crystallizing the original amorphous samples at temperatures between 773 and 923 K. Microstructure of the alloys was characterized by x-ray diffraction and Mössbauer spectroscopy. Few of solute atoms was found in the α-Fe phase precipitated at various temperatures. The volume fractions of the residual amorphous phase in the alloys are in the range of 23%–36%. Moreover, the difference in values for the intensity of the specific saturation magnetization between the residual amorphous phase and the α-Fe phase, evaluated by the Mössbauer spectroscopy and the magnetic analysis technique, is considerably large, which might be mainly responsible for the relatively lower permeability of the NC Fe89Zr7B4 alloys than of the NC Fe73.5Cu1Nb3Si13.5B9 alloy.
Contribution of single InGaAs quantum wells to the guided mode dispersion of InGaAs/GaAs/AlGaAs waveguides: Model and experimental results
Authors: Pacradouni V. , Morin R. , Kanskar M. , Young Jeff F. , Johnson S. R. , Tiedje T.
Published: July 2002

High resolution Fabry–Perot fringe spacing measurements are used to determine the group index dispersion of TE and TM polarized modes in single quantum well InGaAs/AlGaAs/GaAs graded index separate confinement heterostructure waveguides. The TE mode data, over a ∼175 nm range below the quantum well band gap, is compared with model calculations of guided mode dispersion using existing empirical formulas for the index dispersion of AlxGa1−xAs and GaAs, and different phenomenological expressions for the TE index dispersion of the InGaAs quantum well. A satisfactory fit is obtained when the quantum well is modeled as a two-dimensional set of equal strength oscillators with a constant density of states, plus a single 1S excitonic level.
Theory of the generation and detection of carriers in ultrafast pump-and-probe spectroscopy of semiconductors
Authors: Kuznetsov A. V. , Kim Chang Sub , Stanton C. J.
Published: July 2002

We present an analysis of several of the factors that contribute to experimentally measured pump-probe spectra. We point out that for a correct description of the carrier generation, a full quantum mechanical description is essential. In addition, we discuss the regimes where the quantum mechanical description based on the Bloch equations can be replaced by a simplified semiclassical approach. We also demonstrate that the detection of nonequilibrium carrier distributions with the probe pulse is a nontrivial process due to quantum effects, and derive expressions for the detected signal for different experimental schemes.
The quenching and recovery of photoluminescence in porous silicon
Authors: Salonen J. , Laine E.
Published: July 2002

We have investigated the photoluminescence quenching of porous silicon in different atmospheres and in vacuum. We observe a much faster quenching in vacuum than reported before, which we propose is caused by the difference of structure in p- and n-type porous silicon, or a re-reaction with an electrolyte vapor. We have also investigated the recovery of the photoluminescence. We observe the luminescence intensity to partially recover under the influence of a hydrogen atmosphere, up to 0.1 of its initial value. We propose that this is associated with the stabilization of surface dangling bonds. When the hydrogen atmosphere was changed to the normal atmosphere of air, the photoluminescence intensity started to decrease again due to oxidation.
X-ray-diffraction study of quasipseudomorphic ErSi1.7 layers formed by channeled ion-beam synthesis
Authors: Wu M. F. , Vantomme A. , Pattyn H. , Langouche G. , Yang Qinqing , Wang Qiming
Published: July 2002

ErSi1.7 layers with high crystalline quality (χmin of Er is 1.5%) have been formed by 90 keV Er ion implantation to a dose of 1.6×1017/cm2 at 450 °C using channeled implantation. The perpendicular and parallel elastic strain e⊥=−0.94%±0.02% and e∥=1.24%±0.08% of the heteroepitaxial erbium silicide layers have been measured with symmetric and asymmetric x-ray reflections using a double-crystal x-ray diffractometer. The deduced tetragonal distortion eT(XRD)=e∥−e⊥ =2.18%±0.10%, which is consistent with the value eT(RBS)=2.14±0.17% deduced from the Rutherford backscattering and channeling measurements. The quasipseudomorphic growth of the epilayer and the stiffness along a and c axes of the epilayer deduced from the x-ray diffraction are discussed.
Effect of the substrate on the ac response of superconductors with strong pinning to an incident plane wave
Authors: Pérez-Rodríguez F. , Makarov N. M. , Yampol’skii V. A. , Lyubimova I. O. , Lyubimov O. I.
Published: July 2002

The influence of the electromagnetic properties of the substrate upon the size effect in hard superconductors is theoretically studied. Within the frame of the critical state model we calculate the distribution of the magnetic induction and the electric field inside a superconducting plate subject to unilateral electromagnetic excitation. This calculation allows to obtain the dependences of surface impedance and absorptivity on the amplitude of the external signal. We find that the manifestation of the size effect in the ac response of the superconducting plate depends strongly on the value of the dielectric constant of the substrate.
Oxidation state and superconducting properties of Y1−xPrxSr2Cu2.85Re0.15O7+δ
Authors: Murugesan M. , Selvam P. , Sharon M. , Gupta L. C. , Pinto R. , Rao M. S. R. , Gopinath C. S. , Subramanian S.
Published: July 2002

Superconductivity exists in orthorhombic Y1−xPrxSr2Cu2.85Re0.15O7+δ up to a critical concentration (xcr) of 0.65. A progressive decrease in Tc occurs as x increases from 0 to 0.65. A further increase in x leads to a tetragonal transformation and as a consequence the Tc vanishes; however, the orthorhombicity of these Sr-based compounds is much lower than that observed for the Ba analog, Y1−xPrxBa2Cu3O7−δ and, hence, the Tc. On the one hand, crystal chemistry correlations indicate that the Pr ion is in trivalent state while on the other hand, the stabilizing cation, viz., Re, is in hexavalent state which accounts for the excess oxygen (>7.0) in the system. The high xcr value of the Sr series compared to the Ba series (xcr=0.55) is attributed to the much reduced orbital overlap of the trivalent Pr(4f ) state with the Cu(3dx2−y2)–O(2p) conduction band, via hole localization and/or pair breaking, and is not due to the much discussed hole filling by tetravalent Pr.
The bonding of CAs acceptors in InxGa1−xAs grown by chemical beam epitaxy using carbon tetrabromide as the source of carbon
Authors: Ashwin M. J. , Pritchard R. E. , Newman R. C. , Joyce T. B. , Bullough T. J. , Wagner J. , Jeynes C. , Breuer S. J. , Jones R. , Briddon P. R. , Öberg S.
Published: July 2002

InxGa1−xAs layers (0≤x≤0.37) doped with carbon (>1020 cm−3) were grown on semi-insulating GaAs substrates by chemical beam epitaxy using carbon tetrabromide (CBr4) as the dopant source. Hall measurements imply that all of the carbon was present as CAs for values of x up to 0.15. The C acceptors were passivated by exposing samples to a radio frequency hydrogen plasma for periods of up to 6 h. The nearest-neighbor bonding configurations of CAs were investigated by studying the nondegenerate antisymmetric hydrogen stretch mode (A−1 symmetry) and the symmetric XH mode (A+1 symmetry) of the H–CAs pairs using IR absorption and Raman scattering, respectively. Observed modes at 2635 and 450 cm−1 had been assigned to passivated Ga4CAs clusters. New modes at 2550 and 430 cm−1 increased in strength with increasing values of x and are assigned to passivated InGa3CAs clusters. These results were compared with ab initio local density functional theory. Modes due to AlInGaCAs clusters were detected in samples containing grown in Al and In. These results demonstrate that for InGaAs, CBr4 is an efficient C doping source since both In–CAs bonds as well as Ga–CAs bonds are formed, whereas there is no evidence for the formation of In–CAs bonds in samples doped with C derived from trimethylgallium or solid sources.
Predicting the crystallization temperature variation with composition for amorphous silicon-based binary alloy thin films
Authors: Carlsson J. R. A. , Sundgren J.-E. , Li X.-H. , Madsen L. D. , Hentzell H. T. G.
Published: July 2002

A semi-empirical model for predicting the crystallization temperature variation with composition for amorphous Si-based binary alloys is presented. Based on the calculated heat of formation, ΔHfor, Si-based amorphous alloys are proposed to crystallize according to one of three classes: Si-nonmetal, Si-nonreactive metal (ΔHfor⩾0) or Si-reactive metal (ΔHfor<0) alloy systems. For all three classes, the variation in ΔHfor with composition is also used to qualitatively predict the concentration dependence of the crystallization temperature, Tc. The validity of the model was investigated by comparing the predicted concentration dependence of Tc with our own experimental results as well as with available literature data. It was found that the model explains the qualitative crystallization temperature variation with composition well for the hitherto studied amorphous Si-based binary alloy systems.
Magneto-transport properties of semiconductors from flatband magnetocapacitance spectroscopy
Authors: Liu Kun , Chu J. H. , Zheng G. Z. , Guo S. L. , Tang D. Y.
Published: July 2002

We introduce the flatband magnetocapacitance measurement to study magnetotransport properties in bulk semiconductors. This method is better, in some cases, than the Hall measurement because it can avoid some problems inherent to the latter method, such as the influence of the surface conducting layer, the carrier mobility, and the sample geometry correction factor. By using this experimental method some interesting phenomena concerning magnetotransport properties in an n-type InSb sample were observed, including Shubnikov–de Haas oscillations, resonant defect states, conduction process transitions, and the metal–insulator phase transition. The experimentally determined magnetotransport properties for the InSb sample are compared with those reported in literature.
On the interaction between anhysteretic magnetization and demagnetizing fields in iron strips
Authors: de Wit H. J.
Published: July 2002

Anhysteretic magnetization is the magnetization that results if a constant bias field and an alternating field with decreasing amplitude are applied simultaneously to a ferromagnetic sample. Both anhysteretic magnetization and magnetization curves were measured on strips of iron shielding material in two cases where the product of the demagnetization factor and magnetization is much smaller than or comparable with the external field. The influence of demagnetizing fields on the measured effective anhysteretic permeability can be described neither by “normal’’ shearing of the magnetization curve nor by a dependence predicted by Néel for anhysteretic permeability. A crucial role in Néel’s theory is played by so-called “frozen magnetization.’’ During anhysteretic magnetization an increasing fraction of the magnetization ceases to follow the applied alternating field and remains fixed. Comparison with Preisach models suggests that deviation from normal behavior is connected with a gradual rather than an instantaneous buildup of frozen magnetization. It is concluded that Néel’s theory gives a qualitative but not a quantitative explanation of the dependence of anhysteretic permeability on demagnetizing fields in the material investigated.
Optical characterization of tin diffusion and defect generation in strained GaAs
Authors: Rashid A. B. M. Harun-ur , Katoda Takashi
Published: July 2002

The effects of strain on tin diffusion in GaAs were studied by using laser Raman and photoluminescence spectroscopies. It was found that an increase of compressive strain on GaAs causes an increase of the carrier concentration while a decrease of compressive strain or an increase of tensile strain causes a decrease of the carrier concentration at the surface of GaAs. The results are due to the decrease of the diffusion coefficient of tin in GaAs with compressive strain and the increase of it with tensile strain. Photoluminescence data showed that the peak due to Ga antisite defects increased with increase of compressive stress. This indicates a decrease of Ga vacancy concentration from the equilibrium concentration in an unstressed sample. On the other hand, photoluminescence data from the tensile stressed sample showed an increase of Ga vacancy concentration from the equilibrium concentration in an unstressed sample. Thus, the change of the diffusion coefficient with strain seems to be related to Ga vacancy. It was also found that the diffusion coefficient decreased exponentially with compressive strain and increased exponentially with tensile strain with the result that the activation energy of tin diffusion in GaAs varied linearly with strain.
Radiative recombination lifetime of excitons in thin quantum boxes
Authors: Gotoh Hideki , Ando Hiroaki , Takagahara Toshihide
Published: July 2002

Exciton radiative recombination lifetime in a thin quantum box in the intermediate spatial dimension between the two-dimension and the zero-dimension is investigated by a theoretical analysis which rigorously treats the electron-hole Coulomb interaction. The higher exciton states as well as the ground exciton state are explicitly taken into account to estimate the temperature dependence of exciton recombination lifetime. We clarify how the temperature dependence of the recombination lifetime varies with a change in the quantum confinement dimension which can be controlled by the lateral width of a thin quantum box. We also discuss the effect of the exciton localization due to structural imperfection on the radiative recombination lifetime.
Transport properties of sputtered W/C multilayers in high fields
Authors: Broto J. M. , Ousset J. C. , Rakoto H. , Vidal B. , Jiang Z. , Sdaq A. , Khmou A.
Published: July 2002

In this article, we present the transport properties of W/C multilayers which are generally studied for their optical properties (X-ray mirrors). For the thinnest tungsten layers the magnetoresistance is clearly governed by weak localization. We point out the importance of electron-electron interactions.
Effects of microwave fields on recombination processes in 4H and 6H SiC
Authors: Son N. T. , Sörman E. , Chen W. M. , Bergman J. P. , Hallin C. , Kordina O. , Konstantinov A. O. , Monemar B. , Janzén E. , Hofmann D. M. , Volm D. , Meyer B. K.
Published: July 2002

The effects of microwave fields on recombination processes, which are responsible for the optical detection of cyclotron resonance (ODCR) in 4H and 6H SiC epitaxial layers, have been investigated. We present experimental evidence indicating that the dominant mechanism of ODCR in SiC, at low temperatures and in a common range of microwave power (<200 mW), is microwave-induced lattice heating under the cyclotron resonance conditions. The results also show that at low temperatures and low microwave power the dominant scattering mechanism is impurity scattering, while carrier scattering by lattice phonons dominates under high microwave power conditions.
Raman spectra and lattice dynamics of intermixed AlAs/GaAs superlattices
Authors: Halsall M. P. , Dawson P.
Published: July 2002

We present Raman scattering measurements on a series of [100] short-period AlAsn/GaAsn superlattices. The Raman spectra show evidence for the effects of AlxGa1−xAs layer formation at the heterointerfaces and the experimental spectra are modeled using a random element isodisplacement type model. The model described is based on a rigid-ion lattice dynamic model and involves the construction of a superlattice cell from individual alloy unit cells. The resulting eigenvectors are used to calculate Raman spectra for the samples studied via the bond polarizability model. Comparison between experiment and theory shows near perfect agreement after intermixing effects are included. The effects of interface alloying on selection rules and interface modes are discussed. We also deduce dispersion in the AlAs optic phonon along the [100] direction.
Deposition of diamond films on SiO2 surfaces using a high power microwave enhanced chemical vapor deposition process
Authors: Lee Jau-Sung , Liu Kuo-Shung , Lin I-Nan
Published: July 2002

Diamonds were successfully nucleated on SiO2-coated silicon substrates using a high power microwave plasma enhanced chemical vapor deposition process. Nucleation rates on SiO2 surfaces (i.e., 0.5×1010 cm−2) were, however, still smaller than those on Si surfaces (i.e., 1.0×1010 cm−2). The major advantage in using high power microwaves was revealed by optical emission spectroscopy to be that the atomic C and H species produced are more abundant and energetic. Therefore, the negative bias effect is enabled and the formation of sp3 bonds is enhanced. The nucleation of diamonds on SiO2 surface is thus made possible. The growth of diamonds behaved similarly on the prenucleated surface, regardless of the nature of the substrates. Diamonds were of single grain columnar structure with random orientation when deposited without bias and were of multi-grain columnar structure with [111] or [001] preferred orientation when deposited under −100 V dc bias. Multi-grain columnar structure was ascribed to the induction of secondary nucleation at the presence of bias voltage.
Numerical study of an optically pumped multimode D2O laser
Authors: Sasaki K. , Tsukishima T.
Published: July 2002

A numerical simulation of an optically pumped, multimode D2O laser was carried out based on Lamb’s semiclassical laser theory. The numerical code includes the dispersion effect of the laser medium (D2O gas in the cavity) in order to simulate the strength of mode coupling accurately. As a result, multiple-longitudinal-mode oscillations around the Raman resonance frequency were obtained that are roughly in agreement with experimental observations. Energy spectra of the D2O laser emission obtained by the simulation were compared with those of the experimental results for various D2O gas pressures. By including the dispersion effect, temporal variations of mode frequencies were calculated. In addition, resonant pumping was examined to obtain spectrally narrow outputs by tuning a frequency of a pump CO2 laser pulse to the line center of the absorption band of D2O molecules.
Effects of deviatoric stress and radial strain on the shock-induced diffusionless transformation in boron nitride
Authors: Sekine T. , Kobayashi T. , Nameki H.
Published: July 2002

The phase transformation of graphitelike BN (h-BN) to wurtzite-type high-pressure BN (w-BN) was investigated through shock-recovery techniques under quasihydrodynamic and nonhydrodynamic shock compressions and under various strain conditions. The experimental results support a diffusionless mechanism, by which the hydrodynamic c-axis compression of h-BN is preferred. This mechanism is topologically considered based on the relationship of crystal structures between h-BN and w-BN. The presence of deviatoric stress and strain depresses the yield of w-BN and the development of w-BN (100) relative to (002).
Calculated electronic structure of GaAs/Ge2 (001) superlattices
Authors: Rufinus J. , Crook G. E.
Published: July 2002

We use the semiempirical tight binding method with an sp3s* basis and second-nearest-neighbor interactions to investigate the electronic structure of (GaAs)m/(Ge2)n (001) superlattices (SLs) with 1⩽(m,n)⩽20. We have found no correspondence between the calculated band structures of these (GaAs)/(Ge2) SLs and those for (GaAs)1−x(Ge2)x random alloys. The inclusion of second-nearest-neighbor, compared with the nearest-neighbor interactions, raised the SL band gap for (m,n)>1, while lowering that for m=n=1. For the (GaAs)20/(Ge2)20 (001) SLs our calculation gives a band gap energy of 0.79 eV, while for (GaAs)1/(Ge2)1 the band gap energy is 0.11 eV. For small values of m, the (GaAs)m/(Ge2)n SLs are predicted to have indirect band gaps regardless of the valence band offset used in the calculation. For larger values of m, however, the predicted SL band gaps become direct for large values of valence band offset. For ΔEv=0.85 eV we find a direct to indirect band gap transition for m between 14 and 16. In the indirect-gap SLs, the electrons and holes are confined in the Ge layers, while the direct-gap SLs have holes confined in the Ge layers and electrons in the GaAs layers.
Silicidation process of Ti/TiNx/Si structures
Authors: Pérez-Rigueiro J. , Jiménez C. , Pérez-Casero R. , Martı́nez-Duart J. M.
Published: July 2002

One of the most promising metallization schemes on silicon is the TiN/TiSi2/Si structure, since it takes advantage from both the good electrical contact between Si and TiSi2 and the property as a diffusion barrier of TiN. A related system that shows some interesting features is the Ti/TiNx/Si system. The results of two Ti/TiNx/Si structures with different nitride compositions (TiN0.7 and TiN0.8) have been studied. These structures show good electrical characteristics after thermal treatments (>900 °C) and allow one to obtain a smooth variation of the Schottky barrier height, which could lead to tunable contacts in the 0.57–0.53 eV range. Rutherford backscattering spectrometry and grazing incidence x-ray diffraction have been used to establish that the phase sequence observed during the silicidation process is very dependent on the diffusion rate of silicon through the TiNx layer. Finally, the slowest silicidation rate in the Ti/TiNx/Si system permits a higher incorporation of nitrogen from the atmosphere during the thermal treatment, as determined by nuclear reaction analysis. All these characteristics show that these structures can lead to the preparation of the TiN/TiSi2 C54/TiNx/Si in a single thermal treatment.
Study of deposition dependent characteristics of gold on n-GaAs by photoreflectance spectroscopy
Authors: Badakhshan Ali , England Jeff L. , Thompson P. , Cheung P. , Yang C. H. , Alavi Kambiz
Published: July 2002

We used the noncontact electric field modulation technique of photoreflectance to study the effect of gold on GaAs grown by two different metalization methods. A semitransparent 7 nm gold overlayer was grown on low doped n-GaAs (2×1016 cm−3) using metalization by evaporation and by sputtering. Our experimental results indicate that the photoreflectance lineshape depends on the metalization method in a characteristic way. We present evidence of a previously unreported modulation mechanism in photoreflectance. We employed simulation of photoreflectance lineshape based on a multilayer model to reproduce characteristic features of experimental lineshapes. For sputtered Au/GaAs samples the best simulated lineshape was obtained through an unusual modulation, which is based on a strongly pined surface electric field. This produced a lineshape somewhat similar to a third derivative functional form with severely suppressed Franz–Keldysh oscillations (FKO). Simulation with the same set of parameters and a parallel field modulation restored FKO and reproduced the experimental lineshape of the evaporated Au/GaAs. The parallel field modulation spontaneously reproduced the below–band-gap feature, which is often observed in electromodulation spectroscopy. We believe the change in FKO is correlated with the density of interface states as they influence the extent of Fermi-level pinning.
Energy and orientation dependence of neutron depolarization in a large single crystal of ferromagnetic holmium
Authors: Alfimenkov V. P. , Chernikov A. N. , Lason L. , Mareev Yu. D. , Novitsky V. V. , Pikelner L. B. , Skoy V. R. , Tsulaya M. I. , Gould C. R. , Haase D. G. , Roberson N. R.
Published: July 2002

We report measurements of the depolarization of epithermal neutrons (1.7–59 eV) in magnetic domains of a 2.0-cm-diam cylindrical single crystal of ferromagnetic holmium. The neutrons were polarized by a dynamically polarized proton target and polarization analyzed using a statically polarized rare-earth spin filter. Based on the dependence of the depolarization on neutron energy and crystal orientation, we determined the domains to be laminar, or needlelike, with the long axis slightly deflected from the c crystalline axis and having an average width of 59 μm.
Diamond ultraviolet photovoltaic cell obtained by lithium and boron doping
Authors: Popovici G. , Melnikov A. , Varichenko V. V. , Sung T. , Prelas M. A. , Wilson R. G. , Loyalka S. K.
Published: July 2002

Polycrystalline high quality freestanding 300-μm-thick diamond films were doped by diffusion of B and Li under electric bias in order to fabricate vertical p-n junctions. Circular contacts were obtained by high dose ion implantation of B and Li. The I–V characteristics were rectifying. When illuminated by deuterium lamp, an open circuit voltage was 2.6 eV. The shape of the I–V characteristic under illumination points to the existence of shunt and series resistances. The obtained structure is most probably a p-n junction with bad contacts.
Soft x-ray emission produced by a sub-picosecond laser in a single- and double-pulse scheme
Authors: Pelletier J. F. , Chaker M. , Kieffer J. C.
Published: July 2002

The interaction of a high-contrast, 400 fs, 5×1017 W/cm2 laser pulse with a preformed tantalum plasma produced in a double-pulse configuration is investigated. The x-ray emission characteristics are studied for different time delays between the two laser pulses. A time-resolved transmission grating spectrometer is used to measure the x-ray conversion efficiency and emission duration in the 0.2–1.2 keV energy range. Our results show that it is possible to increase the x-ray source conversion efficiency by an order of magnitude simply by optimizing the delay between the laser pulses. To give an overview of the performances available from such laser-plasma x-ray sources, these results are compared those obtained with a single laser pulse in different interaction regimes.
Failure of the charge-transfer hypothesis for superconductivity in YBa2Cu3Ox and in La2−βSrβCuO4
Authors: Blackstead Howard A. , Dow John D.
Published: July 2002

By extracting the layer-charges in YBa2Cu3Ox as functions of dopant oxygen content x from neutron diffraction data (using the bond-valence-sum method), we show that the charge-transfer hypothesis of high-temperature cuprate-plane superconductivity fails to describe the data. A similar failure occurs for La2−βSrβCuO4 as a function of Sr dopant concentration, and was implicitly predicted (before the charge-transfer hypothesis was enunciated) by self-consistent local-density-approximation calculations of R. V. Kasowski, M.-H. Tsai, J. D. Dow, and M. T. Czyzyk [Physica C 162, 1349 (1989)]. Both failures can be traced to difficulties the hypothesis has with (i) the rules of chemistry which require electronegative dopants such as oxygen to attract electrons, not holes, and (ii) the law of conservation of (neutral) charge in each unit cell.
The effect of underlayer for Ba-ferrite sputtered films on c-axis orientation
Authors: Morisako Akimitsu , Liu Xiaoxi , Matsumoto Mitsunori , Naoe Masahiko
Published: July 2002

A thin amorphous Ba–Fe–O film was deposited as an underlayer for the growth of c-axis oriented hexagonal barium ferrite (BaM) films using a facing targets sputtering system. This underlayer facilitates the crystallization and improves the crystallinity for hexagonal M phase. BaM films with c-axis orientation can be prepared at Ts of 525 °C on the underlayer. BaM films prepared on this amorphous underlayer exhibit a good c-axis orientation with c-axis dispersion angle Δθ50 as small as 3°. It is shown by the x-ray photoemission spectroscopy profile that this underlayer prevents deep diffusion of Ba and Fe into the substrate surface.
Magnetic evidence for structural-phase transformations in Fe-Co alloy nanocrystals produced by a carbon arc
Authors: Turgut Z. , Huang M.-Q. , Gallagher K. , McHenry M. E. , Majetich S. A.
Published: July 2002

In previously reported work FexCo1−x[C] (x=0.0, 0.2, 0.4, 0.5, 0.6, and 0.8 nominally) nanoparticles were prepared by a Kratschmer–Huffman carbon-arc method. Fe0.5Co0.5[C] exhibited the largest magnetizations heretofore observed in similarly produced nanoparticles. Here we present a more detailed study of the magnetic properties of Fe0.5Co0.5[C] nanocrystals. Magnetic hysteresis loops have been measured to temperatures exceeding 1050 K. This is attributed to rotational processes in monodomain particles and is shown to be sensitive to ordering of the particles. Low-field thermomagnetic data clearly show features which we attribute to the α→α′ disorder–order and α→γ phase transformations, respectively.
Reduced lateral carrier diffusion for improved miniature semiconductor lasers
Authors: Strand T. A. , Thibeault B. J. , Coldren L. A.
Published: July 2002

The desire to fabricate very small semiconductor lasers requires that we address problems associated with surface recombination. We have proposed and demonstrated a segmented quantum well active region in which lateral diffusion is intentionally reduced. Such a structure should prevent the transfer of electrons and holes from the interior of a laser device to the edges. The supply of carriers to surface-related carrier traps can thus be eliminated, and we need not concern ourselves with the electrical quality of the surfaces. In this work we present calculations which predict laser performance for various lateral diffusion conditions.
Simulation and analysis of 1.55 μm double-fused vertical-cavity lasers
Authors: Piprek Joachim , Babić Dubravko I. , Bowers John E.
Published: July 2002

Using a comprehensive numerical model, we analyze the first long-wavelength (1.55 μm) vertical-cavity surface-emitting lasers operating continuous-wave at room temperature (up to 33 °C). These double-fused lasers employ strain-compensated InGaAsP multi-quantum wells sandwiched between GaAs/AlGaAs distributed Bragg reflectors that are fused on both sides of the InP spacer. The two-dimensional model includes drift and diffusion of electrons and holes, finite-element thermal analysis, calculation of the internal optical field at threshold, and k⋅p band structure computations. The simulation shows excellent agreement with a large variety of experimental characteristics. Internal laser parameters like optical losses and injection efficiency are obtained. The thermal conductivity of the multilayer mirror is found to be only one third of the value expected. Temperature dependent absorption and Auger recombination within the active region as well as lateral leakage currents are identified as dominating loss mechanisms. The analysis shows great potential for high-temperature operation of double-fused vertical-cavity lasers.
Spectroscopic characterization of laser-induced plasma created during welding with a pulsed Nd:YAG laser
Authors: Lacroix D. , Jeandel G. , Boudot C.
Published: July 2002

A spectroscopic study of a laser-induced plume created during the welding of stainless steel and other materials (iron and chromium) has been carried out. A pulsed Nd:YAG laser of 1000 W average power is used. The evolutions of the electron temperature and electron density have been studied for several welding parameters. We use working powers from 300 to 900 W and pulse durations between 1.5 and 5 ms. The influence of shielding gases like nitrogen and argon has been taken into account. Temperature and density calculations are based on the observation of the relative intensities and shapes of the emission peaks. We assume that the plasma is in local thermal equilibrium. The temperature is calculated with the Boltzmann plot method and the density with the Stark broadening of an iron line. The electron temperatures vary in the range of 4500–7100 K, electron density between 3×1022 and 6.5×1022 m−3. The absorption of the laser beam in the plasma is calculated using the Inverse Bremsstrahlung theory.
Compensation effect in the rate of solid-phase epitaxial growth of Si1−xGex alloys
Authors: Suh K. Y. , Lee Hong H.
Published: July 2002

The compensation effect, well known in catalytic reactions, is shown to apply equally well to the solid-phase epitaxial (SPE) growth of Si1−xGex alloys. A linear relationship exists between the logarithm of the pre-exponential factor and the activation energy of the SPE growth rate. This linear relationship, together with the activation energy obtained earlier, enables one to completely describe the growth rate of Si1−xGex alloys. The effect holds for both strained and unstrained SPE. The model is applicable to other binary alloys.
A structural and topographical study of low-pressure chemical-vapor-deposited polysilicon by scanning probe microscopy
Authors: Pleschinger A. , Lutz J. , Kuchar F. , Noll H. , Pippan M.
Published: July 2002

The surface topography and structure of low-pressure chemical vapor deposited silicon films on thermal oxide grown on (100) silicon substrates have been investigated after different processing steps. Atomic force microscopy topographic measurements are performed on undoped as-grown samples and after ex situ phosphorous doping from a POCl3 source. In addition, topographies of the doped films are obtained using constant current scanning tunneling microscopy on hydrofluoric acid passivated surfaces under high vacuum. As a result we have found that surface topography and roughness are mainly determined by the deposition process. Roughness of films deposited at 620 °C is related to the grain structure represented by hillocks with typical lateral dimensions between 50 and 150 nm in the images. Doping by high temperature diffusion and subsequent annealing causes a complete recrystallisation of the film, leading to typical lateral grain sizes between 200 and 600 nm. However, the surface topography of the doped films still remains determined by the hillocks formed by the deposition process. Values of the surface roughness are between 7 and 14 nm depending on the process step.
The calculation and the photoluminescence characterization of energy levels in a strained GaAs/In0.2Ga0.8As/GaAs quantum well heterostructure for the application of light-emitting real-space transfer devices
Authors: Yeh Yung-Hui , Lee Joseph Ya-min
Published: July 2002

Real-space transfer (RST) light-emitting devices are implemented with a strained GaAs/InGaAs/GaAs quantum well heterostructure fabricated on GaAs substrates. The device energy band diagrams are simulated by using the MEDICI program. The effects of critical thickness, quantum size, and strain-induced band-gap shift on the optical transition energies at 77 and 300 K are calculated. Photoluminescence (PL) measurement is carried out to characterize the RST light-emitting devices. The measured results agree well with calculated values. The narrowest full widths at half-maximum of PL spectra are measured to be 17 meV at 300 K and 9 meV at 77 K for an undoped GaAs/In0.2Ga0.8As/GaAs quantum well heterostructure. The PL spectra confirm that the emission is dominated by emission from the strained GaAs/InGaAs/GaAs quantum well.