bluuebunny/crossref_metadata_2025 huggingface dataset:
Interference effects in photoreflectance and contactless electroreflectance spectra of CdTe films grown on Si substrate
Authors: Ghosh S. , Arora B. M.
Published: July 2002

We report the observation of oscillatory features (OFs) in the photoreflectance (PR) and contactless electroreflectance (CER) spectra of CdTe films grown on Si substrate, at energies below the band gap of CdTe. The simultaneous observation of OF in the reflectance (R) spectrum having the same period as those in the PR and CER spectra (as also their dependence on film thickness) provides a direct proof for optical interference effects as being the source of these features. However, in the present case the amplitude of the OF gets damped towards shorter wavelengths while remaining nearly wavelength independent in the longer wavelength region indicating a modulation mechanism different from those reported earlier. A series of experiments and simulations performed by us seem to indicate that while in PR the principal mechanism is the pump beam induced periodic temperature changes which in turn modulates the optical path length of the CdTe film, in CER the mechanism is the electric field induced modulation of the subband gap refractive index of the film through the electro-optic effect. The damping of the OF has been explained on the basis of subband gap absorption by the CdTe film.
High conductivity and nonlinear Seebeck coefficient of Ag2O containing (Bi–Pb–Sr–Ca–Cu–O) glassy precursors for high Tc superconductors
Authors: Chatterjee S. , Banerjee S. , Chaudhuri B. K.
Published: July 2002

Electrical conductivities (σdc) of the as-quenched Bi3.5Pb0.5Sr3Ca3Cu4Ox+zAg2O (with z=1, 3, 5, and 10 wt %) glassy precursors for high Tc superconductors are found to be much higher (∼10−5−101 Ω−1cm−1) than those of the corresponding Ag2O free Bi3.5Pb0.5Sr3Ca3Cu4Ox (denoted by BPB) precursor glass (∼10−13−10−6 Ω−1cm−1). This unusually high conductivity is attributed to the increase of carrier concentrations caused by the addition of Ag2O (also observed from the Hall effect measurements). The experimentally observed high values of σdc do not follow Mott’s variable range hopping model which is in sharp contrast to the behavior of the corresponding pure BPB and many other conventional transition metal oxide glasses having high resistivities. Moreover, the Seebeck coefficients (S) of these glassy precursors show nonlinear variations (from negative at lower temperature to positive at higher temperature) which cannot be clearly explained by phonon drag or electron-phonon interaction. This behavior of S which is also supported from Hall effect measurement is considered to be due to the nonlinear thermal variations of carrier concentrations (both hole and electron) present in the glassy samples.
Analysis of subsurface damage in silicon by a combined photothermal and photoluminescence heterodyne measurement
Authors: Geiler H. D. , Karge H. , Wagner M. , Ehlert A. , Kerstan M. , Helmreich D.
Published: July 2002

A nondestructive evaluation technique based on the detection of the real part of the excess charge carrier wave by photoluminescence in a frequency range from 5 kHz up to 12 MHz is introduced for semiconductor defect analysis. Crystalline imperfections in silicon due to wafer manufacturing and processing are investigated and the results are correlated with those obtained from conventional photothermal measurements.
Influence of quasi-two-dimensional doping on the Raman spectrum of Si1−xGex/Si quantum wells
Authors: Bendayan M. , Beserman R. , Dettmer K.
Published: July 2002

We present a study of p-doping influence on the Raman spectrum of thin SiGe quantum wells. We observe a laser wavelength dependence of the phonon frequencies. We show that the quantum confinement induces special effects which can be described by the renormalization of the phonon energy by the free charge continuum transitions. It is shown that the polarization of the laser and of the phonon can probe the confinement and the density of states of the quasi-free holes.
Determination of defect density in ZnCdMgSe layers grown on InP using a chemical etch
Authors: Zeng L. , Yang B. X. , Shewareged B. , Tamargo M. C. , Wan J. Z. , Pollak Fred H. , Snoeks E. , Zhao L.
Published: July 2002

ZnCdMgSe epitaxial layers grown on InP (001) substrates have been characterized by etch pit density (EPD) measurements. A hydrobromic acid and acetic acid solution at a concentration of 1:7 by volume was found to work well for the EPD measurements. Atomic force microscopy (AFM) was used to characterize the morphology of the etched surface. The AFM results showed that the etch pit depth became constant after a depth corresponding to roughly the II-VI layer thickness. The accuracy of the etch pit density method for revealing stacking faults and dislocations was verified by plan-view transmission electron microscopy (TEM). Both stacking faults and threading dislocations originating from misfit dislocations were revealed by this etchant. The TEM and AFM results showed that the etching is very selective. We show that we have grown ZnCdMgSe layers with an EPD of ∼106 cm−2.
The effect of substrates on the transverse geometry photorefractive thin films
Authors: Wang Qingnan , Temple Doyle A.
Published: July 2002

Although photorefractive thin films in the transverse geometry retain most of the basic features of the bulk photorefractive materials, the abrupt changes in the dielectric constant across the boundary regions always accompany polarized charge gratings. These gratings are generated by the perpendicular component of the space charge field and they are absent in the bulk transport equations. Our systematic theoretical calculation indicates that these boundary charge gratings will strongly influence device performance. Adjustment of the dielectric properties of media sandwiching the thin film can serve as a method by which control device performance.
Regenerative pulsations in semiconductor etalon due to competition between carrier generation and heating effects on band filling
Authors: Tolstikhin Valery I. , Grigor’yants Alexander V. , van de Roer Theo G.
Published: July 2002

Competition between carrier concentration and effective temperature effects on the dielectric function of a degenerate semiconductor in a spectral range near its fundamental absorption edge is suggested as a mechanism for regenerative pulsations in a stationary pumped optical etalon. The origin of self-pulsations is similar to that in a bistable etalon with competing concentration and thermal optical nonlinearities, but, due to its purely electronic nature, the proposed mechanism provides a way for a much higher repetition frequency, probably in the GHz range. The model of the phenomenon and modeling examples are all related to a Fabry–Perot resonator filled with the bulk n+-In0.53Ga0.47As as an active medium.
Strain relaxation in high electron mobility Si1−xGex/Si structures
Authors: Li J. H. , Holy V. , Bauer G. , Schäffler F.
Published: July 2002

We have studied the strain relaxation in Si1−xGex/Si (001) structures with high electron mobility grown by molecular beam epitaxy. The structures contain a Si1−xGex layer with linearly graded composition, followed subsequently by a uniform composition buffer Si1−yGey, a thin Si layer serving as two-dimensional electron gas channel, and a modulation n-doped Si1−xGex layer. We found that a major part of the graded layer is basically completely strain relaxed, whereas a very thin layer close to the graded-uniform layer interface, as well as the uniform alloy buffer, are just partly relaxed. We performed also model calculations of the strain status of a graded-uniform two-layer system using an equilibrium approach. It is found that for our Si0.7Ge0.3 systems, the residual strains of the samples with different composition, grading rate, and a uniform buffer thickness of 0.6 μm is almost the same at equilibrium. However, experiments show a clear dependence of the residual strain on the grading rate of the graded buffer. The higher the grading rate, the higher is the residual strain in the constant composition alloy buffer. This indicates that with a lower grading rate, the structure is closer to equilibrium, and is thus, thermally more stable. Furthermore, lower grading rates produce also smoother surfaces.
Effect of preoxidation on deposition of thin gate-quality silicon oxide film at low temperature by using a sputter-type electron cyclotron resonance plasma
Authors: Gao D. W. , Kashiwazaki Y. , Muraoka K. , Nakashima H. , Furukawa K. , Liu Y. C. , Shibata K. , Tsurushima T.
Published: July 2002

We have studied a method of combining preoxidation and subsequent sputter deposition for fabricating Si oxide films with a thickness of less than 10 nm at low temperature by using a sputter-type electron cyclotron resonance plasma system. As a key process to achieving high quality composite oxide structures, plasma preoxidation was investigated under different gas flow rates at a substrate temperature of 130 °C. The optimum conditions for the preoxidation were clarified. The structural properties of Si oxide formed by this method with the preoxidation were characterized and compared with those of Si oxide which was directly sputtered without a preoxidation step. It was found that the method with the preoxidation provided a useful way of establishing an abrupt Si/SiO2 interface region and achieving films which have a lower network disorder degree. This procedure was then followed by a thermal annealing in Ar ambient at 450 °C. A thin Si oxide film was produced with a fixed charge density of less than 1.0×1011 cm−2. 98% of the detected samples had a breakdown field of greater than 8 MV/cm. These properties essentially match the electrical quality of thermally grown Si oxide. In contrast, the oxide film deposited without a preoxidation step showed much poorer electrical properties as only 28% of the samples had a breakdown field of greater than 8 MV/cm. Also, the film showed a higher fixed charge density of 1.0×1012 cm−2. The effects of the preoxide on structural and electrical properties of deposited oxide films are discussed in detail.
Characterization of erasable inorganic photochromic media for optical disk data storage
Authors: Huang Xu Guang , Wang Michael R. , Tsui Yuen , Wu Chuck
Published: July 2002

Optical recording characteristics of an ion-exchanged inorganic photochromic media is investigated. The experiments show that the ion-exchanged recording layer can support high data recording density with excellent optical readout contrast. Its recording speed at available red diode laser power can be comparable and even faster than most CD-erasable drives. A focused UV light or electron beams can erase the recorded data on the ion-exchanged layer and facilitate further data rewriting by focused laser beams. The recording medium has excellent thermal stability and supports nondestructive data readout for erasable optical data storage applications.
Reflectance anisotropy spectroscopy study of the surface reconstructions of decapped InP(001)
Authors: Johal T. K. , Barrett S. D. , Hopkinson M. , Weightman P. , Power J. R.
Published: July 2002

A reflectance anisotropy spectroscopy (RAS) and low-energy electron diffraction study of the InP(001) surface is presented. The surface was prepared by thermal desorption of an As-P capped epilayer grown by molecular beam epitaxy. RA spectra have been monitored over a spectral range of 1.5–5.5 eV at regular intervals during thermal decapping and annealing up to the point of decomposition (553–973 K). Each of the RA spectra of the surface reconstructions comprise positive (at 2.9 eV) and negative (at 1.8 eV) anisotropies which have been previously associated with P- and In-related bonding, respectively. Unlike other III-V (001) semiconductor surfaces, the evolution of different reconstructions cannot be explained in terms of a change in surface stoichiometry which involves loss of the anion species. In the case of InP(001) the P species contributes to the clean surface reconstruction from the early stages of decapping to the point of decomposition.
Efficient electromagnetic analysis of two-dimensional finite quasi-random gratings for quantum well infrared photodetectors
Authors: Jandhyala Vikram , Sengupta Deepak , Shanker Balasubramaniam , Michielssen Eric , Feng Milton , Stillman Greg
Published: July 2002

In this work, a recently developed full-wave electromagnetic analysis technique is applied to the simulation of two-dimensional finite quasi-random gratings for quantum well infrared photodetectors. This steepest descent fast multipole method is a mathematically rigorous technique that permits the rapid and accurate solution of the electric field integral equation governing scattering from a quasi-planar structure. In the present application, it enables the efficient and accurate simulation of scattering by finite two-dimensional grating structures interfacing with GaAs. Grating absorption is predicted by evaluating the scattered optical electric field component at the device layer along the growth direction. Numerical examples illustrating the functional dependence of the absorption on grating parameters and wavelength are discussed. The simulation approach presented here should prove to be a useful tool for the a priori design of novel aperiodic, quasi-random and rough surface two-dimensional gratings for infrared imaging applications.
Differential reflection dynamics in InAsxP1−x/InP (x⩽0.35) strained-multiple-quantum wells
Authors: Zhao Y.-G. , Zou Y.-H. , Huang X.-L. , Wang J.-J. , Qin Y.-D. , Masut R. A. , Beaudoin M.
Published: July 2002

Using the pump-probe technique, we have observed time-resolved differential reflection in InAsxP1−x/InP (x⩽0.35) strained-multiple-quantum wells. The experimental results show that barrier height, interface roughness and well width influence strongly the differential reflection dynamics. For samples with the same interface quality and almost the same well width, the delay time of the differential reflection decreases with increasing barrier height, while for the sample with rough interface and narrower wells, the delay time of the differential reflection is much slower, although it has a larger barrier height. To understand the experimental results, we have performed a simulation study of temporal and spatial evolutions of photoexcited carriers in the samples, and the influence of various physics processes on the photoexcited carrier dynamics has been discussed. From the calculated and the measured results, we conclude that carrier diffusion in the cap layer and the barriers plays a dominant role in determining the differential reflection dynamics.
Spectroscopic ellipsometric study of the size evolution of Ge islands grown on Si (100)
Authors: Palange E. , Ragni L. , Di Gaspare L. , Capellini G. , Evangelisti F.
Published: July 2002

In this article we discuss the use of spectroscopic ellipsometry for an in situ and real time probe of three-dimensional self-organized Ge island growth on Si (100) surfaces. We will show that atomic force microscopy and x-ray photoemission spectroscopy can be combined with spectroscopic ellipsometry to give information on the size and shape evolution of the Ge islands as well as on the amount of Ge deposited on the Si surface.
Coercivity of sintered Nd(Fe0.92−xGaxB0.08)5.5 permanent magnets
Authors: Kou X. C. , de Boer F. R. , Kronmüller H.
Published: July 2002

The temperature dependence of the coercive field of Nd(Fe0.92−xGaxB0.08)5.5 compounds with x=0 and 0.01 has been measured from 10 K up to the Curie temperatures in fields applied parallel or perpendicular to the magnetic alignment direction and has been analyzed in terms of a micromagnetic model. Two temperature ranges in which different mechanisms control the coercivity can clearly be distinguished. At temperatures above 170 K, a nucleation process of reversed domains determines the coercivity mechanism. From the micromagnetism point of view, partial substitution of Fe by Ga into NdFeB magnets leads to a reduction of the local effective demagnetization field (−NeffMs) which makes the nucleation of reversed domains more difficult, and therefore enhances the coercivity.
Giant spontaneous Hall effect and magnetoresistance in La1−xCaxCoO3 (0.1⩽x⩽0.5)
Authors: Samoilov A. V. , Beach G. , Fu C. C. , Yeh N.-C. , Vasquez R. P.
Published: July 2002

Results of resistivity and Hall effect measurements in La1−xCaxCoO3 (0.1⩽x⩽0.5) epitaxial films and ceramics are presented. The spontaneous Hall effect in La1−xCaxCoO3 (LCCO) is observed for ferromagnetic samples with x⩾0.2. The Hall effect is largest near the magnetic percolation threshold x=0.2. For x=0.2, the low-field slope of the Hall resistivity, ρxy/(μ0H), attains a large value of 2×10−6 m3/C below the Curie temperature Tc, which may be applied to sensitive low-field detection. Except near the magnetic percolation threshold, the longitudinal resistivity of LCCO decreases with increasing field at all temperatures. Anomalous temperature-dependent magnetoresistance occurs in the sample with x=0.2, which may be associated with the spin-state transition in LCCO.
Magnetic transitions in Tb0.7Nd0.3Mn2Ge2 compound
Authors: Saha Shibaji , Ali Naushad , Labroo Sunil , Zych Dale
Published: July 2002

In this article we report a new compound, Tb0.7Nd0.3Mn2Ge2, which behaves like SmMn2Ge2, a compound with reentrant ferromagnetic order. Magnetization and thermal expansion measurements have been performed to study polycrystalline Tb0.7Nd0.3Mn2Ge2. Temperature dependent magnetization data shows that from 800 to 426 K the sample behaves like a paramagnet. A Néel transition is observed at 426 K (T4) and the antiferromagnetic phase persists until 334 K (T3). From 334 K (T3) to 277 K (T2) the sample exists in a ferromagnetic state. At 277 K (T2) the sample transforms into an antiferromagnetic phase. Interestingly, at 117 K (T1) the sample enters into a ferromagnetic-like phase. Thermal expansion study shows a sharp change in the sample dimension around T1 (125 K). This change in the dimension most likely triggers the reentrant magnetic transition (T1) observed in the magnetization measurement. The sharp change in thermal expansion suggests this transition at T1 to be of first order. The relatively small and shallow dimensional change around T2 (275 K) implies a weak first order transition. The dimensional changes found at the two transitions strongly suggest that Mn–Mn intralayer distance plays an important role in the multiple magnetic phases in Tb0.7Nd0.3Mn2Ge2.
Experimental evidence of macroscopic resonant tunneling of magnetization in antiferromagnetic ferritin
Authors: del Barco E. , Luis F. , Tejada J. , Zhang X. X. , Bartolomé J. , Hernandez J. M. , Chudnovsky E. M.
Published: July 2002

Macroscopic resonant quantum tunneling of magnetization has been observed in antiferromagnetic ferritin particles. The dependence of magnetic viscosity on the applied magnetic field has been systematically investigated in the temperature range 1.8–10 K. The zero maximum in field dependence of viscosity and the maximum in the field dependent blocking temperature are interpreted to be due to the resonant spin tunneling between matching spin levels.
Doping effects on the high-frequency mobility of minority carriers in p-GaAs
Authors: Caetano E. W. S. , Wang H. , Freire V. N. , da Costa J. A. P.
Published: July 2002

The high-frequency complex mobility of minority carriers in p-GaAs subjected to a ac+dc strong electric field is calculated for two doping concentrations, 1.5×1017 and 1.5×1018 cm−3. The high-frequency minority carrier mobility in p-GaAs is shown to be dependent on the doping concentration. When the doping is high, the electron-hole interaction turns to be a scattering mechanism that has to be take into account to the description of the terahertz operation regime of devices where p-GaAs minority carriers are important.
Local vibrational mode bands due to a DX-like hydrogen-related center in silicon
Authors: Markevich V. P. , Suezawa M. , Murin L. I.
Published: July 2002

An absorption line at 1025.5 cm−1 has been observed in hydrogenated Czochralski-grown silicon crystals after irradiation with fast electrons and following annealing in the temperature range of 300–400 °C. The line was found to be related to a local vibrational mode (LVM) due to a DX-like hydrogen-related center, having a shallow donor and a deep acceptor electronic level. This LVM band was observed when the center was in the singly negatively charged state. Transformation of the defect into the neutral charge state due to photoionization resulted in disappearance of the LVM line and the appearance of several absorption lines in the range of 250–325 cm−1. These lines were interpreted earlier to be associated with ground-to-excited-state electronic transitions in an effective-mass-like shallow donor state of the center. Substitution of hydrogen by deuterium resulted in a shift of the LVM band to 1027.9 cm−1 that unambiguously indicates hydrogen incorporation into the defect. The structure of the center which gives rise to the observed absorption bands is discussed.
N
-type doping of lattice-matched ZnCdSe and ZnxCdyMg1−x−ySe epilayers on InP using ZnCl2
Authors: Lin W. , Cavus A. , Zeng L. , Tamargo M. C.
Published: July 2002

Chlorine on Se-site forms a shallow donor for ZnSe. In this article, we use Cl, obtained from ZnCl2, as the n-type dopant for ZnxCdyMg1−x−ySe lattice matched to InP, a new wide band gap II-VI material grown by molecular beam epitaxy. An 800 Å p-type doped InGaAs buffer layer was grown to improve the doping behavior, consistent with improved crystalline quality. The highest free-carrier concentrations measured by Hall effect are 7×1018 cm−3 with mobility of 240 cm2/V s and 3×1018 cm−3 with mobility of 230 cm2/V s for Zn0.5Cd0.5Se (77 K Eg=2.17 eV) and ZnCdMgSe (77 K Eg=2.74 eV), respectively. A small systematic reduction of maximum carrier concentration was observed as the quaternary layer band gap is increased. No deep level emission is introduced by the chlorine dopant. High n-type doping levels, consistent with semiconductor laser applications were achieved for quaternaries of band gaps as high as 2.9 eV.
Quantification of local remanence magnetization in perpendicular magnetic recording film by scanning interference electron microscopy
Authors: Takahashi Yoshio , Yajima Yusuke
Published: July 2002

The local remanence magnetization of perpendicular magnetic recording bit patterns has been quantified by scanning interference electron microscopy. When two coherent probes run across each bit, the interference image contrast changes periodically with a unit of h/e (h: Planck’s constant, e: electron charge). Our numerical analysis shows that the magnetic field effect on the image contrast is less than 1/1000 that of the magnetization under the experimental conditions. This allows the local remanence magnetization of the film to be determined accurately by averaging the periodic contrast change measured over repetitive bit patterns. From the interference image of bit patterns on a Co (0.4 nm)/Pt (1.2 nm)×18 layered film, a value of 1.4×10−8 T m was obtained as the product of the remanence and the film thickness.
Electron attachment to photofragments and Rydberg states in laser-irradiated CCl2F2
Authors: Pinnaduwage Lal A. , Datskos Panos G. , Tav Cumali
Published: July 2002

We report electron attachment measurements on ArF-excimer-laser irradiated CCl2F2, obtained using an improved experimental technique that allows simultaneous measurements on multiple electron attaching species. Compared to a maximum electron attachment rate constant of ∼2×10−9 cm3 s−1 for the ground electronic state of CCl2F2, we measure an order of magnitude larger rate constant for the CClF2 radical produced via laser photodissociation. However, the highly excited electronic states of CCl2F2 produced by the laser irradiation have an associated electron attachment rate constant that is at least four orders of magnitude larger compared to the ground electronic state value. Implications of these findings for plasma processing discharges using CCl2F2 are discussed.
Schottky barrier effects in the electronic conduction of sol–gel derived lead zirconate titanate thin film capacitors
Authors: Yang Y. S. , Lee S. J. , Kim S. H. , Chae B. G. , Jang M. S.
Published: July 2002

Pure and Fe-doped Pb(Zr0.53Ti0.47)O3 (PZT) thin film capacitors were fabricated by the sol–gel method and the leakage current versus voltage characteristics of these films were investigated at several temperatures and initial polarization states. After the initial poling of ferroelectric thin films, we measured two kinds of leakage current: (i) the full-switching current measured against an initial polarization direction and (ii) the nonswitching current measured toward an initial polarization direction. In the case of the full-switching current measurement, the anomaly of leakage current due to the switching of space charges was observed, which we suggest to be accumulated at a Schottky barrier region near an electrode or a gain boundary. In the case of the nonswitching current measurement, the Schottky diode rectifier current and the field-enhanced Schottky (ES) emission current came out without the switching current. The Poole–Frenkel emission process was dominant over ES emission for a Fe-doped PZT thin film having a large amount of space charge and traps. The height of the Schottky barrier in a Au/PZT film/Pt capacitor was measured to be 0.64 eV.
Transport properties of Hg0.80Mg0.20Te grown by molecular beam epitaxy
Authors: Gui Yongsheng , Li Biao , Zheng Guozhen , Guo Shaoling , Chu Junhao , Oehling S. , Becker C. R. , Landwehr G.
Published: July 2002

Variable magnetic-field Hall measurement has been used to investigate the transport properties in Hg0.80Mg0.20Te molecular beam epitaxy films in the temperature range from 1.5 to 250 K. The experimental data have been analyzed by using a hybrid approach consisting of the mobility spectrum (MS) technique followed by a multicarrier fitting (MCF) procedure. Both the Shubnikov–de Haas measurements and the hybrid MS+MCF approach show three- and two-dimensional electronic behaviors. The two-dimensional electrons, with mobility in the range of 1–3×103 cm2/V s and a sheet density about 1012 cm−2, are found and come from an accumulation layer near the HgMgTe–CdTe interface or the HgMgTe–vacuum interface. The temperature-dependent evolution of the bulk electron mobility indicates that the scattering mechanism in HgMgTe is very similar to that in HgCdTe, that is, ionized impurity scattering dominates at low temperature while lattice scattering dominates above 100 K.
Oscillations of interlayer coupling and magneto-optical Kerr effect in NiCo/Cu multilayers
Authors: Tong Liuniu , He Longqing , He Xianmei , Du Jun , Wu Jing , Pan Minhu , Lu Mu , Feng D. , Zhai Hongru , Chen Liangyao , Jin Xiaofong , Xia Hui
Published: July 2002

A series of rf sputtered [NiCo(1.3 nm)/Cu(tCu)]25 multilayers with tCu=0.5–4.0 nm was prepared and the behavior of interlayer coupling, magnetoresistance, and magneto-optical Kerr effect was studied. A concurrent oscillation in the magnitude of the magnetoresistance and magneto-optical Kerr effect in visible wavelength range as a function of Cu layer thickness was observed. The changes of both the optical and magneto-optical transition due to interlayer coupling may be responsible for the magneto-optical activities.
Lattice parameter variation in doped GaAs substrates determined using high resolution photoluminescence spectroscopy
Authors: Hu J. , Harrison D. A. , Karasyuk V. A. , Watkins S. P. , Thewalt M. L. W. , Bassignana I. C. , Beckett D. J. S. , Hillier G. C. , SpringThorpe A. J.
Published: July 2002

Variations in the lattice parameters of commercially available undoped and Si-doped GaAs substrates have been studied using high resolution photoluminescence (PL) spectroscopy. The lattice parameter difference between high quality epitaxial layers of GaAs and the doped substrates upon which they are grown induces a biaxial strain in the epitaxial layer which can be detected with remarkable sensitivity using low temperature PL. The PL results show that a lattice mismatch as low as 0.0013% can cause significant and well-resolved changes in the features of donor (D°X) and acceptor (A°X) bound exciton spectra, which are consistent with results obtained from samples subjected to externally applied biaxial stress. In addition to studying a variety of low lattice mismatch GaAs/GaAs:Si heterostructures, the technique is also successfully employed in the case of more highly strained GaAs grown on an In0.004Ga0.996As substrate.
Pulsation of a liquid excited by a fast-moving crystallization front with segregation of surface active impurities
Authors: Balandin V. Yu. , Nink T. , Bostanjoglo O.
Published: July 2002

A new mechanism is described which produces short-wave patterns in a freezing metal melt that contains surface active impurities. The main feature of the process is a periodic accumulation of the impurity atoms in sharp crests of a rake wave, which forms due to segregation of the impurity atoms ahead of the crystallization front, and which moves with the same velocity. The local rise of the concentration of the impurity increases the chemocapillary surface forces, whereby “excitations” are emitted in opposite directions into the melt. These excitations interact with the crystallization front and produce ripples on a solidifying surface. The model was used to explain ripples in solidified melts produced by short laser pulses in Au and Al films.
Low temperature growth of GaInNAs/GaAs quantum wells by metalorganic chemical vapor deposition using tertiarybutylarsine
Authors: Pan Z. , Miyamoto T. , Schlenker D. , Sato S. , Koyama F. , Iga K.
Published: July 2002

The low temperature growth of highly strained GaInNAs/GaAs quantum wells was investigated by low-pressure metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine (TBAs) and dimethylhydrazine. We found that the incorporation behavior of indium in the strained GaInAs layers at low growth temperature was very different from that at high growth temperature. The N content dropped rapidly with increasing In content in the strained GaInNAs layer. It is pointed out that the V/III ratio is an important growth parameter for TBAs based MOCVD. The V/III ratio strongly affected both the photoluminescence intensity and the alloy composition of the GaInNAs.
Terahertz response of metal-semiconductor-metal photodetectors
Authors: Ryzhii M. , Willander M. , Khmyrova I. , Ryzhii V.
Published: July 2002

Calculations by ensemble Monte Carlo particle method on ultra-high-frequency response of metal-semiconductor-metal (MSM) photodetectors are reported. It is shown that the MSM photodetectors with relatively large contact spacing (0.2–1.0 μm) with lateral light input into a thin absorbing layer reveal rather high-frequency response in the terahertz range of signals due to the velocity overshoot effect manifested by the photoexcited electrons. The response-signal frequency product exhibits a pronounced maximum at terahertz frequencies being significantly higher than that evaluated without the inclusion of the velocity overshoot effect. The obtained numerical results are interpreted invoking the developed analytical device model.
Magnetic response of Cd0.63Mn0.37Te under illumination: Dynamic behavior of spin glass with step-like heating and cooling
Authors: Kawai H. , Sato T.
Published: July 2002

The magnetic behavior of a typical diluted magnetic semiconductor Cd0.63Mn0.37Te was investigated under the unpolarized light illumination in a magnetic field. Both the zero field cooled (ZFC) and field cooled (FC) values of magnetization in Cd0.63Mn0.37Te decrease under illumination below the spin freezing temperature. This magnetic response was explained on the basis of a change in temperature of the sample under illumination, since the intrinsic photo-induced magnetic change was less than the resolution of superconducting quantum interference device magnetometer. The transient behavior observed in the FC and ZFC values of magnetization in Cd0.63Mn0.37Te was analyzed in terms of the step-like heating and cooling induced by the optical method and it was observed that the FC magnetization showed hardly any relaxation in spite of the change in temperature below the spin freezing temperature. In addition, the analysis of ZFC magnetization indicated that two types of spins coexist in the low temperature state of Cd0.63Mn0.37Te: one belongs to the freezing spin cluster and the other shows paramagnetic behavior.
Steady-state, planar growth parameters in the reactive deposition of thin films
Authors: Carter G.
Published: July 2002

A generalized model is developed which predicts the composition and growth rate of thin films when multiple atomic fluxes impinge onto a substrate and react chemically to form a composite film. The effects of simultaneous energetic ion irradiation which can mediate reaction rates and cause differential sputter removal of different film components are included and are shown to modify film composition and growth rate. A comparison of the model predictions with available experimental data is shown to give satisfactory agreement and it is demonstrated that two distinct cases of gas–solid and solid-state reactions can dominate and be distinguished, with theoretical analysis being more straightforward in the former case.
Raman scattering in hexagonal GaN epitaxial layer grown on MgAl2O4 substrate
Authors: Li G. H. , Zhang W. , Han H. X. , Wang Z. P. , Duan S. K.
Published: July 2002

The room temperature Raman spectra of the hexagonal GaN epilayer grown on [111]- oriented MgAl2O4 substrate were measured in various backscattering and right angle scattering geometries. All of the symmetry—allowed optical phonon modes were observed except the E2 (low frequency) mode. The quasitransverse and quasilongitudinal modes were also observed in the x(zx)z and x(yy)z configurations, which are the mixed modes of pure transverse and longitudinal modes with A1 and E1 symmetry, respectively.
Photoreflectance spectroscopy of CdTe(001) around E1 and E1+Δ1: linear electro-optic spectrum
Authors: Lastras-Martı́nez A. , Balderas-Navarro R. E. , Cantú-Alejandro P. , Lastras-Martı́nez L. F.
Published: July 2002

We report on the measurement of the linear electro-optic (LEO) reflectance spectrum of CdTe (001) in an energy range around the E1 and E1+Δ1 interband transitions. This spectrum shows a sharp peak localized in energy around E1 and a second shorter peak around E1+Δ1. We show that the theoretical model developed in an earlier article for the LEO line shape of GaAs (001) gives an accurate description of the experimental LEO CdTe spectrum as well. This model includes two contributions to the LEO line shape, a first one proportional to the normalized energy derivative of the reflectance spectrum and a second one associated to the sample reflectance. The large spin-orbit splitting energy of CdTe, (Δ1≈0.6 eV) allows for a neat separation of the contributions to the LEO spectrum of the E1 and E1+Δ1 critical points, providing a critical test for the LEO line shape model. From the fitting we obtain d′/d=1.5 for the conduction band to valence band deformation potential ratio and E2=9.1 eV for the interband deformation potential in the Brooks notation.
Resonance enhancement of electronic Raman scattering from nitrogen defect levels in silicon carbide
Authors: Burton J. C. , Long F. H. , Ferguson I. T.
Published: July 2002

Electronic Raman scattering from nitrogen defect levels in SiC is seen to be significantly enhanced with excitation by red (633 nm, 1.98 eV) or near-IR (785 nm, 1.58 eV) laser light at room temperature. Four nitrogen peaks are observed in 6H–SiC (380, 430, 510, and 638 cm−1) and three peaks in 4H–SiC (about 400, 530, and 570 cm−1). The peaks in the 4H–SiC spectrum are seen to shift to lower frequency with increasing nominal doping concentration. Raman spectra taken at low temperature in 6H–SiC reveal differences between wafers and Lely grown platelets by the appearance of several additional peaks. The origin of the resonant enhancement is the near-IR absorption band associated with the green color characteristic of n-type SiC. These results demonstrate that the laser wavelength is a key parameter in the characterization of SiC by Raman scattering.
Epitaxy barium ferrite thin films on LiTaO3 substrate
Authors: Fang H. C. , Ong C. K. , Xu S. Y. , Tan K. L. , Lim S. L. , Li Y. , Liu J. M.
Published: July 2002

Barium hexaferrite (BaM) thin films were deposited on (0001) LiTaO3 substrates by pulsed laser deposition. Effects of the substrate temperature and oxygen gas pressure on the formation and quality of these films were studied. Films deposited at a substrate temperature of 800 °C and an oxygen pressure around 0.23 mbar showed the best c axis normal to the film plane with locked in-plane orientation. The saturation magnetization Ms and anisotropy field Ha measured by vibrating sample magnetometer were almost the same as those reported on bulk barium ferrite. Decreasing oxygen pressure hinders the formation of the Ba layer in BaM magnetoplumbite structure and gives rise to the spinel phase, which greatly decreases coercivity Hc of the films and finally destroys the whole BaM structure. Effects of the lattice mismatch and substrate-induced strains on the film structure were also studied. It was found that barium ferrite thin films grown on LiTaO3 substrates tend to choose a matching mode with compressional strains rather than shear strains.
Asymptotic solutions for a symmetric plasma under ionization nonequilibrium
Authors: Thornhill Lindsey D. , Desai Prateen V.
Published: July 2002

Leading-order solutions for electron and ion density, electron and ion velocity, and electric potential have been obtained in a planar, symmetric geometry for the case of a dense, low-temperature plasma bounded by perfectly absorbing walls. The solutions are obtained in terms of the Jacobi elliptic functions. These solutions are shown to reduce to those previously obtained for the case of a semi-infinite plasma when the appropriate transformations are applied. The solutions are also shown to reduce to the well-known solutions from linear diffusion theory, in terms of circular functions, when the linear source approximation is made. The solutions are evaluated and discussed for a representative thermal plasma where the influence of explicitly accounting for recombination is illustrated.
X-ray photoelectron spectroscopy of molybdenum-containing carbon films
Authors: Yoon S. F. , Huang Q. F. , Rusli  , Yang H. , Ahn J. , Zhang Q. , Blomfield C. , Tielsch B. , Tan Lawrence Y. C.
Published: July 2002

Results from x-ray photoelectron spectroscopy (XPS) measurements of molybdenum-containing carbon films (Mo–C:H) deposited using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system are reported in this article. The Mo–C:H films were deposited using a technique with two Mo screen grids incorporated inside the ECR-CVD chamber. The versatility of this technique arises from the ability to control the degree of plasma ionization, sputtering rate of the metal grids, and energy of the impinging ions. Variation of the (CH4/Ar) gas flow ratio results in a change of the Mo fraction within the range of 0.32–15.11 at. %. For large amounts of Mo, the C 1s peak was split into four components with binding energies of 283.05, 284.67, 286.22, and 288.17 eV. These were identified as carbidic (metallic), polymeric, and oxidic (single- and double-bond) carbon, respectively. The presence of oxygen was detected in the films, due possibly to free-radical absorption at the film surface during deposition, or oxidation of the metallic Mo at the surface upon exposure to atmosphere. The results showed that the ECR-CVD technique is useful and effective for the deposition of Mo–C:H films with low- and high-Mo content.
Luminescence of In+ in Ce3+ and Tb3+-doped elpasolite-type fluoroindates
Authors: Buñuel M. A. , Moine B. , Jacquier B. , Garcia A. , Chaminade J. P.
Published: July 2002

This work deals with the identification of In+ ions in octahedral symmetry and the study of their optical properties in crystalline Rb2KInF6:Ce3+ and Rb2KInF6:Tb3+. The presence of In+ ions in the sites of In3+ has been confirmed exciting the Ce3+-doped sample at 41 600 and 32 360 cm−1 and exciting the Tb3+-doped sample at 41 600 cm−1. An hypothesis based on the In3+–Ce3+ and In3+–Tb3+ redox couples for the Ce3+- and Tb3+-doped elpasolite-type fluoroindates has been proposed in order to explain the presence of In+ at the mentioned excitation energies. This redox process is more efficient for the In3+–Ce3+ couple than for the In3+–Tb3+ one. Emission spectra and luminescence decays of In+ have been investigated at 5–300 K in both samples under the above mentioned excitations. Our experimental results have been compared to those obtained for In+-type ions as In+, Ga+, and Tl+ in alkali halides in literature. The transitions between the 5s5p excited configuration and the 5s2 configuration of In+ ions modified by the elpasolite matrix in Oh symmetry explain the optical data, if we take into account the spin-orbit coupling and Jahn–Teller effect (JTE). Finally, an additional fluorescence has been found in the Tb3+-doped sample; it has been assigned to In+ ions in potassium sites.
Induced electric fields in anisotropic thermoelectric materials
Authors: Bies W. E. , Radtke R. J. , Ehrenreich H.
Published: July 2002

Transport in an anisotropic material can create potentially large induced transverse fields which reduce the measured electric and thermal conductivities relative to those computed without the induced fields. These affect the thermoelectric figure of merit ZT modestly. The induced electric field in n-type Bi2Te3 is predicted to be as much as 76% of the external one and can lower the measured electrical conductivity by up to 60%. In Hg1−xCdxTe superlattices, the anisotropy may be increased by varying the composition and width of the barrier and well to give induced fields much larger than the applied one. These effects should be easily observable. The present work utilizes general results applied to a microscopic model relevant for multivalleyed materials within the effective-mass and relaxation-time approximations.
Conductivity of inhomogeneous nanowires
Authors: Ruda H. , Shik A.
Published: July 2002

Conductivity of wires with thickness less than the screening length is shown to be very sensitive to any kind of inhomogeneity causing a long-range carrier redistribution along the wire. This deviation from quasineutrality depends on the applied voltage and results in a nonlinear current–voltage characteristic and a noticeable frequency dispersion of the wire conductivity. These effects were analyzed in detail for the two different models: (i) wires with nonuniform doping, (ii) uniform wires separated from a contact by a short high-resistive (e.g., tunnel) region. The second model describes also the Coulomb blockade effects in wires. The effects can be large in spite of the large static capacity of wires, since at the high frequencies responsible for a Coulomb blockade, the capacity drops to the values comparable with those of quantum dots.
Metal-insulator transition in La0.7Sr0.3Mn1−xFexO3
Authors: Tiwari Ashutosh , Rajeev K. P.
Published: July 2002

We report the effect of Fe doping at the Mn site in La0.7Sr0.3MnO3 oxides. We find that the doping of Fe does not cause any structural change, but the electrical transport in the system is strongly affected. The parent compound La0.7Sr0.3MnO3 shows a resistivity peak at T=Tp (365 K) and behaves as a ferromagnetic metal at low temperature. Double exchange (DE) interaction between Mn3+ and Mn4+ is understood to be the cause of ferromagnetism and metallicity of the material. As Fe is doped into the system a depletion in the number of hopping electrons occurs and consequently the DE interaction becomes weak, however, as long as x⩽0.2 the system shows metallic behavior at low temperatures. For higher Fe doping the system becomes insulating throughout the whole temperature range. Electron tunneling conductance measurements show that the density of states of metallic samples exhibits a dip at EF. As the value of x increases the dip deepens and finally at x=0.25 a gap opens up at the Fermi level.
Piezoreflectance study of an Fe-containing silicon carbon nitride crystalline film
Authors: Hsieh C. H. , Huang Y. S. , Tiong K. K. , Fan C. W. , Chen Y. F. , Chen L. C. , Wu J. J. , Chen K. H.
Published: July 2002

A detailed piezoreflectance (PzR) study of an Fe-containing silicon carbon nitride crystalline film in the temperature range between 15 and 580 K was performed. From the line shape fit of the PzR spectra, the impurity to band and the direct band-to-band transition energies which are denoted as Ei and Egd, respectively, at various temperatures were accurately determined. The parameters that describe the temperature dependence of Ei and Egd are evaluated and discussed.
Fabrication and magnetotransport properties of nanoscaled MnSb dots
Authors: Mizuguchi M. , Akinaga H. , Ono K. , Oshima M.
Published: July 2002

We report the fabrication and magnetotransport properties of nanoscaled clusters of manganese antimonide (MnSb) grown on sulfur-passivated GaA(001) substrates by molecular-beam-epitaxy (MBE). The typical diameter and height of dots were about 22 nm and 4 nm, respectively. Uniform dots in size were formed in a self-assembled mode. Magnetotransport properties of MnSb granular films were investigated, and the anisotropic magnetoresistance were observed at room temperature. Large jumps appeared at 9 K in magnetoresistance curves, and the jump structure depends on the sweep rate of the magnetic field. These magnetotransport properties describe well the magnetic behavior of MnSb dots.
Ab initio local-spin-density study of oscillatory exchange coupling and spin density waves in Fe/Cr multilayers
Authors: Hafner R. , Spišák D. , Lorenz R. , Hafner J.
Published: July 2002

The interlayer exchange coupling and the magnetic structure of bcc (001) Fe/Cr multilayers have been studied using a first-principles self-consistent tight-binding linear-muffin-tin-orbital method in the atomic sphere approximation. A generalized gradient approximation to the exchange correlation potential was used. Antiferromagnetic coupling between neighboring Fe layers is found to be energetically favorable for an even number of monolayers in the Cr spacer, while ferromagnetic coupling is preferred for an odd number of monolayers. Also it is found that for certain Cr thicknesses incommensurate spin density waves with different wavelengths coexist.
Magnetoelastic dependence of switching field in TbFe–FeCo giant magnetostrictive spring-magnet multilayers
Authors: Chopra Harsh Deep , Yang David X. , Wilson P.
Published: July 2002

The role of magneto-elastic interactions in controlling the magnitude of switching field and coercivity is investigated using sputter-deposited TbFe/FeCo multilayers. In the absence of long-range anisotropy in amorphous TbFe layers, along with negligible magneto-crystalline anisotropy in FeCo layers, reversal behavior in these multilayers is largely determined by stress-induced anisotropy. Results show that magneto-elastic constraints at the TbFe–FeCo interfaces arising due to different values of saturation magnetostriction in adjacent layers lead to biaxial stresses. This manifests itself as highly square and similar M–H loops measured in different directions in the plane of the films. The measured values of coercivity in these films (between 4.5 and 7.0 mT) are found to be in close agreement with theoretical coercivity values predicted on the basis of biaxial stress-induced anisotropy. Biaxial stresses resulting in corresponding stress-induced anisotropy were found to have interesting consequences vis-à-vis magnetization reversal. In particular, self-organized domain patterns are formed during reversal which are mediated by local stress state in the films.