bluuebunny/crossref_metadata_2025 huggingface dataset:
On the nanometer-scale solid-state reactions at thin-film Ni/amorphous SiC and Co/amorphous SiC interfaces
Authors: Nathan M. , Ahearn J. S.
Published: July 2002

Solid-state interfacial reactions between self-supporting thin (50, 100, and 200 Å) films of Ni and Co, and amorphous (a) films of SiC, Si and C were investigated in the 300–1000 °C temperature range. The metastable solubility of C (from SiC) in Ni and Co after short annealings was calculated from the metal lattice expansion after accounting for the effect of Si. A maximum of ∼7 at. % C dissolves in (Ni) at ∼300 °C, a value close to the maximum solubility of C in rapidly quenched Ni-C alloys. In contrast, a lack of change in the Co lattice parameter after reactions with either SiC, Si, or C indicates negligible dissolution of C prior to silicide nucleation. The first silicides in metal/a-SiC reactions (Ni31Si12, and Co2Si) are more metal-rich than in metal/a-Si reactions.
Theoretical explanations of magnetocrystalline anisotropy behaviors in RTiFe11Nx compounds
Authors: Yang Ying-chang , Pei Xiedi , Li Hailin , Zhang Xiaodong , Kong Linshu , Pan Qi , Zhang Minghou
Published: July 2002

The RTiFe11Nx compounds have higher Curie temperatures, larger saturation magnetization, and entirely different magnetocrystalline anisotropy behavior as compared with the RTiFe11 compounds. In contrast to RTiFe11, the c axis becomes the easy magnetization direction of RTiFe11Nx when R=Nd, Tb, Dy, and Ho, while SmTiFe11Nx has an easy plane and ErTiFe11Nx presents a spin reorientation at about 45 K. Theoretical calculations were made to explain those anisotropy behaviors. The calculation results show that the changes in anisotropy behaviors could be attributed to the large positive contribution of nitrogen atoms located at 2b interstitial sites to the second-order crystal-field coefficient A20.
Magnetoresistance in Fe80−xNixCr20 (21≤x≤30) alloys
Authors: Nath T. K. , Majumdar A. K.
Published: July 2002

Longitudinal and transverse magnetoresistances (MR) and dc magnetization (M) have been measured in Fe80−xNixCr20 (21≤x≤30) alloys where the magnetic phase diagram was already established. The motivation behind the present work is to study the behavior of magnetoresistance (Δρ/ρ) in a concentrated alloy system with continuously varying magnetic phases resulting from strongly competitive exchange interactions. We find that all the three samples show negative MR whose magnitude increases with field (as Hn with n<1) and decreases with temperature up to 50 K. The MR of the alloy with x=21 (spin glass), although isotropic like the canonical spin glasses (SG), does not show the usual low-field H2 dependence but exhibits a correlation with M like Δρ/ρ ∝ M2.5. The MR behavior of the x=23 (mixed phase) alloy is qualitatively similar to that of x=21 and is like neither a pure SG nor a ferromagnet (FM) showing ferromagnetic anisotropy of resistance (FAR). However, it is rather similar to that of the mixed phase AuFe alloy but with quantitative differences in Δρ/ρ vs H and M vs H behaviors. However, Arrott plots (M2 vs H/M) for x=23 confirm long-range FM order in the mixed phase regime. Similar plots for x=30 show FM order but the MR shows very small FAR. Large high-field dc susceptibility at 11 K indicates SG ordering for x=21 and 23. Thus the MR in this system is quite different from those expected of pure magnetic phases.
Bean–Livingston barriers and first field for flux penetration in high-T
c crystals
Authors: Burlachkov L. , Konczykowski M. , Yeshurun Y. , Holtzberg F.
Published: July 2002

We present evidence for the importance of Bean–Livingston (BL) barriers for field penetration into high-Tc crystals. The magnetization curves M(H) and the first field Hp for flux penetration were measured near the transition temperature Tc of untwinned Y-Ba-Cu-O crystal by using a miniature Hall probe. There are three observations that serve as evidence for the efficiency of BL barriers: (1) the magnetization was found to be almost zero on the descending branch of the magnetization loop; (2) The slope of Hp(T) exhibits a clear change close to Tc, being largest at Tc; (3) after introducing damage by irradiating the sample, both the field Hp and the width of the M(H) loops reduce significantly, showing almost reversible behavior for the sample. We explain these observations in terms of BL barriers which are shown to be especially important in high-Tc superconductors, and these could be responsible for the controversy of the Hc1 values reported previously in the literature.
Specific resistivity of delta-doped contacts in n-GaAs
Authors: Marcy D. L. , Maby E. W. , Newman P. G. , Khanna R.
Published: July 2002

Specific contact resistivities of nonalloyed Si delta-doped contacts to n-GaAs have been measured using six-terminal Kelvin test structures. The results indicate that multiple near-surface delta layers are needed to obtain ohmic contacts with resistivities of about 10−5 Ω cm2. Lower resistivity values may be possible if the Si donor incorporation limit can be increased above 3×1019 cm−3 through improved growth conditions. Delta-layer effectiveness is degraded by process conditions that feature a Si3N4 overlayer.
Impurity effects on the mechanical behavior of GaAs crystals
Authors: Yonenaga Ichiro , Sumino Koji
Published: July 2002

Effects of In, Zn, and Si impurities on the mechanical behavior of GaAs are investigated. Experimental results are interpreted in terms of the impurity effect on the dislocation velocity and of dislocation locking due to impurities which have been clarified by previous experiments. It is shown that in the temperature range lower than about 600 °C the impurity effect on the dislocation mobility in glide motion plays a dominant role in determining the mechanical strength, while in the higher temperature range dislocation locking by impurities controls the strength. Thus, Si impurity is the most effective in enhancing the strength in the low-temperature range while In impurity is so in the high-temperature range. The dislocation processes which take place during plastic deformation of any impurity-doped GaAs crystals in the low-temperature range are essentially the same as those taking place in highly pure crystals of other kinds of semiconductors such as Ge and Si.
Structural and magnetic studies of the alloy system GdAg1−xFex
Authors: Saleh A. S. , Saleh L. A. , Nayfeh N. M. , Mahmood S. H. , Abu-Aljarayesh I.
Published: July 2002

We report the results of structural and magnetic studies of the alloy series GdAg1−xFex for x=0.0, 0.1, 0.2, and 0.3. The structural study indicates that this alloy is not single phase for all values of x, but rather consists of three phases: the cubic disordered (A2) GdAg phase, the hexagonal α-Gd phase, and the cubic (MgCu2-type) GdFe2 phase. The magnetic measurements confirm the existence of these phases. The Néel temperature for the GdAg phase is TN≊140 K, the Curie temperature for the α-Gd phase is TC≊300 K, and the Curie temperature for the GdFe2 phase is TC≊820 K.
Discharge-excited free jet source of rare gas-halide and oxide molecules
Authors: Jones R. B. , Schloss J. H. , Eden J. G.
Published: July 2002

Intense rare gas-halide and rare gas-oxide excimer emission has been observed by coupling a pulsed electric discharge to a supersonic free jet expansion. A simple modification in the placement of the discharge anode drastically reduces the number of energetic electrons in the supersonic expansion region and permits large peak excimer populations to be produced by harpoon collisions involving rare gas metastables. The expansion velocity has been determined by spatially resolved spontaneous emission and laser-induced fluorescence measurements. Strong emission spectra from XeF, XeCl, KrF, KrCl, ArF, and the 2 3Π ion pair state of XeO have been recorded.
On the correspondence between poroelasticity and thermoelasticity
Authors: Norris Andrew
Published: July 2002

An interesting and useful analogy can be drawn between the equations of static poroelasticity and the equations of thermoelasticity including entropy. The correspondence is of practical use in determining the effective parameters in an inhomogeneous poroelastic medium using known results from the literature on the effective thermal expansion coefficient and the effective heat capacity of a disordered thermoelastic continuum.
Reflectivity and transmission of amorphous thin films at the melting temperature
Authors: Solis J. , Afonso C. N.
Published: July 2002

A method to evaluate the evolution of the optical properties (reflectivity and transmission) of thin solid films as a function of temperature has been developed. A pulsed Ar+ laser focused on the sample surface to a 1/e-beam radius of 4 μm is used as a local heat source while the optical properties of the irradiated surface are monitored in real time by means of another laser focused to 1.7 μm. The use of laser pulses combined with real time optical measurements provides a suitable means to determine accurately the optical properties of thin-film materials for temperatures up to the melting point. In addition, the irradiation pulse can be sufficiently shortened to avoid crystallization prior to melting in the case of amorphous films and therefore the optical properties of amorphous films up to the melting point can be also determined. The analysis of the optical transients with a simple thermal model allows either the determination of the thermal conductivity of a transparent substrate if the film melting temperature is known or the film melting temperature if the thermal properties of the substrate are well characterized. The results obtained in Ge and Si amorphous thin films prove the accuracy of the method and the melting temperature of amorphous Ge is determined to be 987 K.
Silicon-doping level dependent diffusion of Be in AlGaAs/GaAs quantum well lasers
Authors: Swaminathan V. , Chand N. , Geva M. , Anthony P. J. , Jordan A. S.
Published: July 2002

We have investigated Be diffusion during molecular beam epitaxial growth of GaAs/AlGaAs graded index separate confinement heterostructure laser structures using secondary ion mass spectrometry (SIMS). For growth at 700 °C we find that Be from the p-type AlGaAs cladding layer diffuses into the quantum well and beyond. As a result, the p-n junction is displaced from the heterojunction. The extent of Be diffusion is found to depend on the dopants in the graded index (GRIN) regions adjoining the GaAs active layer. When the GRIN segments are left intentionally undoped, Be diffuses through the entire p-side GRIN, the quantum well active and a significant portion of the n-side GRIN. However, when the GRIN regions are doped, respectively, with Be and Si on the p and n sides, the displacement of the p-n junction caused by Be diffusion is significantly reduced. Assuming that Be diffuses from a constant source at the surface into a n-type layer as a singly charged interstitial donor, our analysis predicts that increasing the doping of the n layer retards the diffusion of Be while that of the p layer enhances it. Further, including the electric field of the p-n junction in the model leads to peaks and inflections resembling those observed in the experimental SIMS profiles. In view of Be-related oxygen contamination and Be diffusion on the p-side GRIN region, Be should be dispensed with on the p side, however, Si addition on the n side is beneficial as it minimizes Be diffusion and p-n junction displacement.
Photovoltage and photocurrent spectroscopy of p+-i-n+ GaAs/AlGaAs quantum well heterostructures
Authors: Tarricone L. , Arena C. , Parisini A. , Genova F.
Published: July 2002

The photoelectric properties of p+-i-n+ heterostructures that contain a GaAs/AlGaAs superlattice, grown by molecular beam epitaxy, have been investigated. The nominally undoped multi-quantum-well region was excited by photons whose energies were varied within the spectral region of valence-to-conduction subband transitions. The dark- and light-induced perpendicular transport was examined. The intense spectral features observed by photovoltage (PV) and photocurrent (PC) measurements were ascribed to excitonic transitions between the heavy and light hole electron states; their energetic location and line shape (half width at half peak and integrated area) were carefully studied in the temperature interval 10–300 K. The excitonic nature of the spectral peaks showing a conspicuous red shift under a reverse bias (Stark shift) were confirmed by electroreflectance measurements. The full analogy and the origin of the two (PV and PC) photoresponse signals are clearly proved. The spectral features can be explained by the absorption properties of the quantum wells, while the variation of the peak integrated area as a function of the temperature is dominated by the particular transport mechanism. The integrated area of the peaks shows a minimum at about 100 K which seems to be due to the occurrence of two different tunneling mechanisms.
In situ Raman spectroscopy of diamond during growth in a hot filament reactor
Authors: Bernardez L. , McCarty K. F. , Yang N.
Published: July 2002

We report a system capable of obtaining Raman spectra during growth of carbon films in a hot filament reactor. A gated, multichannel detection system was used to discriminate against the high levels of background radiation produced by the hot substrate and the hot filament. The ability to detect and distinguish between diamond and nondiamond carbon films during growth is shown. Diamond was grown on silicon substrates at 925 °C, with a filament temperature of 2100 °C and with CH4/H2 ratios between 0.002 and 0.008. A nondiamond carbon film was produced with CH4/H2 ratio of 0.016. In order to estimate the sensitivity of the system to detect diamond during growth, the average particle size and fractional coverage of the substrate were determined when a diamond Raman signature was first observed. Currently, the system is capable of detecting diamond particles about 0.5 μm in diameter covering about 3/4 of the surface.
The effect of the planar doping on the electrical transport properties at the Al:n-GaAs(100) interface: Ultrahigh effective doping
Authors: Geraldo J. M. , Rodrigues W. N. , Medeiros-Ribeiro G. , de Oliveira A. G.
Published: July 2002

Measurements of the current density as a function of the applied bias have been performed to study the electronic properties of the Al:n-GaAs(100) interface. The samples were grown by molecular beam epitaxy. A Si planar doping was placed in GaAs(100) at different depths underneath the metal-semiconductor interface and the charge transport across this interface has been investigated. An increasing tunneling contribution to the net current through the Schottky barrier has been observed by decreasing the planar doping depth. For sufficiently small depths, the tunneling current dominated. Planar doping near the metal-semiconductor interface was found to be equivalent, concerning charge transport properties, to a bulk semiconductor doped well beyond the currently achievable limit.
Temperature dependence of the circular magnetic x-ray dichroism in the Invar alloy Fe72Pt28
Authors: Stähler S. , Knülle M. , Schütz G. , Fischer P. , Welzel-Gerth S. , Buchholz B.
Published: July 2002

The method of spin-dependent absorption has been used to examine the temperature dependence of the circular magnetic x-ray dichroism (CMXD) of the (disordered) Invar alloy fcc Fe72Pt28. At the Fe K edge the difference of the transmission for the reversed magnetization direction was determined with an accuracy of 10−5. By comparing the dichroic effects at 110 and at 410 K evidence was found for a temperature-dependent change of the magnetic absorption profile different from the pure amplitude reduction due to the reduced magnetization. It indicates a change of the spin splitting in the low-lying unoccupied p states. The polarization-averaged absorption spectrum shows also a small variation between 4 and 11 eV above the threshold indicating that the p-like electron density drops when the sample is heated. The experimental CMXD profiles are compared with spin-polarized band-structure calculations for ordered Fe3Pt and discussed within the frame of the model of high-spin–low-spin transitions in the Invar regime.
Magnetism in RENiAl compounds
Authors: Tuan N. C. , Sechovský V. , Diviš M. , Svoboda P. , Nakotte H. , de Boer F. R. , Kim-Ngan N. H.
Published: July 2002

Results of magnetic, resistivity and specific heat measurements of RENiAl compounds are presented. Except for CeNiAl, all the compounds with the unfilled f-shell rare earths exhibit magnetic ordering at low temperatures. In the majority of cases, we observe one or two additional magnetic phase transitions below the ordering temperature. In RENiAl antiferromagnets, metamagnetic transitions are observed in moderate magnetic fields. Magnetization measurements performed on oriented powders reveal a strong magnetocrystalline anisotropy (except for GdNiAl and CeNiAl) which originates in crystal field interaction. The energy spectra of the RE 4f states were obtained from the analysis of the Schottky anomalies found in the specific heat data. This analysis also revealed indications of strong magnetic correlations in the paramagnetic state.
A proposal of a vacuum micro quantum interference transistor
Authors: Ugajin Ryuichi , Ishibashi Akira , Funato Kenji , Mori Yoshifumi
Published: July 2002

A quantum interference transistor that can be fabricated by available technology and can operate at room temperature is proposed. This device uses the phase interference effect of a vacuum electron that is not influenced by thermal fluctuations, in contrast to an electron in solid-state materials. The device consists of a field emitter, a collector, and segmented capacitors between the emitter and the collector. The capacitors control electron trajectories and the phase interference of the electron in vacuum by their electrical potential. This quantum interference effect is found not to be the same as the Aharonov–Bohm effect contrary to our expectations. We are convinced that the new transistor is capable of room-temperature operation because the large kinetic energy of the electron in vacuum suppresses energy fluctuations caused by the field emission itself and by thermal fluctuations in the emitter material to about 50 meV. The switching time of the transistor is limited in order to average the number of electrons at the collector to allow the emitter-collector current to be a deterministic variable against the quantum stochastic property. The threshold of the switching time is estimated to be of the order of magnitude of 10−11 s for the field emitter reported by C. A. Spindt, I. Brodie, L. Hummphrey, and E. R. Westerberg [J. Appl. Phys. 47, 5248 (1976)].
Deposition and device application of in situ boron-doped polycrystalline SiGe films grown at low temperatures
Authors: Lin Horng-Chih , Jung Tze-Guei , Lin Hsiao-Yi , Chang Chun-Yen , Lei Tan-Fu , Wang P. J. , Deng Ray-Chern , Lin Jandel , Chao Chih-Yeh
Published: July 2002

Deposition of in situ boron-doped polycrystalline silicon-germanium (poly-Si1−xGex) films at temperatures below 550 °C was investigated using an ultrahigh-vacuum chemical- vapor-deposition system. These films with a fine grain structure were obtained for boron concentrations higher than 1021 cm−3. It is attributed to the enhanced nonequilibrium doping effect due to the addition of GeH4 gas during film deposition. Poly-Si0.56Ge0.44 films with a carrier concentration of 8×1020 cm−3 were achieved at a growth temperature of 500 °C. Such a high activated carrier concentration resulted in a film resistivity less than 2 mΩ cm. Utilizing these characteristics, a novel approach was proposed and demonstrated to fabricate p-channel polycrystalline silicon thin-film transistors at process temperatures below 550 °C. These transistors with a maximum field effect mobility up to 28 cm2/V s and an on/off current ratio over 106 were achieved without employing any post-treatment step, indicating the feasibility of this approach on the fabrication of polycrystalline silicon thin-film transistors at low temperatures.
Kinetics of pulsed ultraviolet laser induced oxidation of c-Ge: The role of optical coupling and material losses
Authors: Vega F. , Afonso C. N. , Ortega C. , Siejka J.
Published: July 2002

Ultraviolet pulsed laser induced oxidation kinetics of crystalline germanium (c-Ge) is determined from real-time optical measurements in conjunction with absolute measurements of the oxygen incorporation performed by nuclear reaction analysis. Although the oxidation process can be triggered at laser fluences initially below the melting threshold of c-Ge, it is strongly activated when surface melting occurs and therefore the fast oxidation process observed is mainly a thermally activated process. Because an optical coupling between the oxide layer and the c-Ge underneath, the growth kinetics is complex and leads to nonconstant rates. The oxygen incorporation reaches a saturation value which depends both on the laser fluence and the oxygen pressure. The results show that the oxygen incorporation is limited by an overlapped laser-induced material removal process rather than by the diffusion length of oxygen species.
Transport of detrapped charges in thermally wet grown SiO2 electrets
Authors: Günther Peter , Xia Zhongfu
Published: July 2002

The transport mechanism of detrapped charges was investigated in thermally wet grown SiO2 electrets, which were first charged by the corona method or by electron-beam irradiation and then aged at different temperatures. The discharge and retrapping behavior was analyzed with a recently suggested method for determination of the charge centroid and charge density and by measuring open-circuit termally stimulated discharge current spectra. Thus, the thermal activation process could be compared with the local redistribution and the decay of the trapped charges. It was found that the fast retrapping effect plays the dominant role for the released negative charge carriers. For positive charge carriers, however, this could not be confirmed. Independent of the charging polarity, it could be shown that the aging procedure resulted in a controlled shift of the charge centroid from a near surface region up to several hundred nanometers deep into the bulk of SiO2. Unexpectedly, samples treated with hexamethyldisilazane exhibited a larger shift of the charge centroid accompanied by a stronger charge decay compared to untreated samples.
Depth-dependent porous silicon photoluminescence
Authors: Ookubo Norio
Published: July 2002

Porous silicon showing a two-layer structure is studied by stepwise peeling the surface layer to clarify the nonuniformity in photoluminescence (PL) emission as a function of depth. The upper layer is amorphous and luminesces at higher energy and efficiency. The deeper the depth or the lower the excitation energy Eex, the lower the PL peak Ep; but, at a low Eex, Ep is not sensitive to the depth. Both intrinsic PL emission and the variation of penetration depth with Eex contribute to the linear dependence of Ep on Eex, which is in contrast to the cases of a-Si:H and siloxene exhibiting thermalization gaps. The total PL excitation spectrum, the integrated PL intensities versus Eex, saturates rather than exhibiting a peak. Its leading edge profile is similar to that for the absorption spectrum, unchanged by the depth, and described by an Urbach tail with energy of 0.26 eV, which is 4–5 times larger than that of a-Si:H. The results can be understood based on silicon clusters embedded in amorphous silicon incorporating oxygen and/or hydrogen.
57Fe Mössbauer studies of Nd(Fe12−xMox) and Nd(Fe12−xMox)N0.9 (x=1.25 and 1.50)
Authors: Qi Qi-Nian , Hu Bo-Ping , Coey J. M. D.
Published: July 2002

Nd(Fe12−xMox)Ny (x=1.25 and 1.50, y=0.9) 57Fe Mössbauer spectra of parent compounds and nitrides have been measured at 15 and 293 K. Hyperfine fields are little changed by the presence of nitrogen at 15 K, where the average value is about 25 T but at 293 K there is a change in average hyperfine field from 19 T for parent compounds to 24 T for nitrides due to the increase in Curie temperature. Spin reorientations occur in the parent compounds in the temperature range between 160–190 K but not in the nitrides, which show c-axis anisotropy throughout this range.
Effect of surface layers on ferromagnetic resonance in thin Fe films: Ni, Co, Si, and YBa2Cu3O7−δ
Authors: Lubitz P. , Rubinstein M. , Chrisey D. B. , Horwitz J. S. , Broussard P. R.
Published: July 2002

The results of ferromagnetic resonance experiments on 10 nm polycrystalline films of Fe with very thin surface layers of Ni and Co, using Si and a high temperature superconductor (YBCO) as substrates is reported herein. An earlier fmr investigation of Fe films on YBCO showed strongly temperature dependent linewidths and resonance fields but only small effects in Fe on Si. In this work, the resonance of Fe on Si is found to be significantly narrower when the Fe film has thin (0.5 to 1.0 nm) Ni surface layers, compared to uncoated Fe; linewidths are as narrow as 15 Oe at 9 GHz. However, when YBCO substrates are used, the iron resonance properties are nearly the same as in the case where no Ni was used. This indicates that the effects seen in earlier work were not the result of an oxide layer on the Fe but may indicate a magnetically active surface layer in the YBCO. Co surface layers have the effect of producing strongly temperature dependent anisotropies and linewidths on all substrates used, with the strongest temperature dependence near 160 K, possibly caused by the ordering of an antiferromagnetic surface layer.
Observation and simulation of space-charge effects and hysteresis in ZnS:Mn ac thin-film electroluminescent devices
Authors: Neyts K. A. , Corlatan D. , De Visschere P. , Van den Bossche J.
Published: July 2002

We have shown from basic considerations that the negative resistance effect measured in thin-film electroluminescent devices must involve some variation of space charge in the phosphor layer. Time-delayed voltage pulse experiments indicate that positive space charge decreases spontaneously. By introducing the presence of deep donor levels which can be ionized by accelerated electrons and, when positively charged, have some typical decay time for the release of a hole, we have created a numerical model which is able to simulate negative resistance, time-delayed pulse experiments and the influence of temperature and frequency on the hysteretic behavior.
Phase change of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole for information storage applications
Authors: Mizuguchi Jin , Giller Gérald , Baeriswyl Erwin
Published: July 2002

Three crystal modifications I, II, and III are known to exist for 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole (DTPP; blue pigment). Among these, only modification III exhibits an intense near-IR absorption due to interplanar π-π interactions of the alternating nitrogen and thiocarbonyl carbon atoms along the stacking axis. An optical information storage system of the ‘‘write once’’ type has been developed in which the near-IR absorption (‘‘on’’ state) is switched-off (‘‘off’’ state) on irradiation with GaAs laser diodes in the presence of a hydrazone compound. The near-IR absorption ceases to exist due to a molecular rearrangement caused by laser-generated vapors of hydrazone molecules. X-ray-diffraction analysis revealed that the present phenomenon is due to the phase change from modification quasi-III to modification II. The optical disk (structure: substrate/hydrazone/DTPP/Al) exhibits a reflectivity change from about 30% to 45% on writing with a power of about 9 mW at 780 nm.
Fe and Ti doping of InP grown by metalorganic chemical-vapor deposition for the fabrication of thermally stable high-resistivity layers
Authors: Wolf T. , Zinke T. , Krost A. , Scheffler H. , Ullrich H. , Bimberg D. , Harde P.
Published: July 2002

The optimum conditions for the fabrication of semi-insulating InP epitaxial layers grown by metalorganic chemical-vapor deposition are investigated in a comparative study of the structural, electrical, and diffusive properties of Fe- and Ti-doped material. Thermally stable InP:Fe layers with resistivities approaching the intrinsic limit can be prepared in an environment of n-type material if the Fe concentration does not exceed but is close to its solubility limit of 8×1016 cm−3 at 640 °C. In contact with p-type layers, however, semi-insulating characteristics of InP:Fe turn out to be difficult to reproduce because of a pronounced interdiffusion of Fe and p-type dopants. Here, Ti doping of InP is shown to be a useful scheme for the fabrication of high-resistivity layers. New processes for the deposition of InP:Ti using (C5H5)2Ti(CO)2 and Ti[N(CH3)2]4 as metalorganic precursors are described in detail. Ti is found to compensate up to 2×1016 cm−3 of shallow acceptors in metalorganic chemical-vapor-phase-deposition-grown InP. Ti-doped InP layers containing more electrically active deep Ti donors than net shallow acceptors exhibit semi-insulating characteristics with a resistivity of 5×106 Ω cm. Codoping of InP:Fe with Ti turns out to be a universal process for the preparation of thermally stable high-resistivity layers. If the material is appropriately grown, Fe+Ti doping compensates both excess shallow donors and excess shallow acceptors up to concentrations of 8×1016 and 2×1016 cm−3, respectively. In contrast to InP:Fe, resistivities in excess of 107 Ω cm are obtained in contact with both symmetric n- and p-type current injecting contacts. Moreover, codoping of semi-insulating InP:Fe with Ti is found to suppress the interdiffusion of Fe and p-type dopants. Therefore, the outdiffusion and accumulation of Fe in other regions of complex device structures can be significantly reduced. The interdiffusion of Fe and p-type dopants as well as its suppression by additional doping with Ti, finally, is studied in detail, which enables a comprehensive model accounting for this phenomenon to be developed.
Broadband near-infrared pulse generation in KTiOPO4
Authors: Takeuchi S. , Kobayashi T.
Published: July 2002

A broadband near-infrared pulse covering 700 nm around 2.4 μm has been generated by a dispersion-free parametric mixing in a KTiOPO4 crystal between the fundamental pulse (0.8 μm) from a Ti:sapphire regenerative amplifier and the white continuum pulse. It is tunable within the wavelength region of 1.7–2.9 μm. The comparison with a theoretical bandwidth indicates that the noncollinear phase matching plays a part for the spectral broadening.
Optical behavior of textured silicon
Authors: Nussbaumer H. , Willeke G. , Bucher E.
Published: July 2002

The optical behavior of polycrystalline SILSO silicon wafers with a mechanically V-grooved surface has been studied between 300 and 1500 nm. The texturization was carried out by a conventional dicing saw using beveled blades. For a 35° V-grooved surface and a nonmetallized backside the optical path length in the weakly absorbing part of the spectrum (1100–1200 nm) was found to be enhanced by a factor of 33 as compared to a nongrooved wafer. The enhanced reflectance in the nonabsorbing spectral region for the former is analyzed and explained. The different loss contributions due to a nonideal grooved structure are discussed.
Cathodoluminescence study of domains, defects, and interdiffusion in ZnSe/GaAs(100)
Authors: Lin H. T. , Rich D. H. , Wittry D. B.
Published: July 2002

The distribution of dislocations and domains found in thin ZnSe films grown by molecular-beam epitaxy on GaAs(100) has been examined with low-temperature cathodoluminescence (CL) imaging and spectroscopy. Dark-line and bright-line defects in the low-temperature CL imaging of the free-exciton (FE) and Y-band emissions, respectively, are found to correlate with the presence of [11̄0]-oriented misfit dislocations for 1-μm-thick films found to grow nearly two dimensionally. For a sample exhibiting mixed two- and three-dimensional growth characters, large domains (∼1–5 μm widths) in the CL imaging of the ZnSe FE emission were found to correlate with a cellular pattern found in the imaging of the GaAs exciton and band-edge–to–acceptor emissions. These results show that the optical properties of the ZnSe film and GaAs substrate are coupled and influenced by Zn diffusion into the substrate during growth.
Anisotropic glasslike characteristics of strontium barium niobate relaxors
Authors: Huang W. H. , Viehland Dwight , Neurgaonkar R. R.
Published: July 2002

The complex dielectric response of strontium barium niobate relaxor ferroelectrics has been studied as a function of orientation and composition. The dielectric response was found to be strongly dependent on orientation, in distinct comparison to the nearly orientation-independent response found for Pb-based perovskite relaxors. Investigations of the temperature dependence of the high-field polarization behavior found that the square-to-slim loop hysteresis transition occurred rapidly with increasing temperature. In addition, no deviations from Curie–Weiss behavior were found, however two distinct linear regions were observed. An anisotropic dielectric glasslike model involving the freezing of heterophase fluctuations is then discussed in reference to the data.
Light scattering in a thin film of a mixture of dielectric and magnetic fluids under an electromagnetic field
Authors: Fujita T. , Yamaguchi K.
Published: July 2002

Light scattering in a thin film of a mixture of dielectric and magnetic fluids has been studied under both electric and magnetic fields. The film can transmit visible light in no electric field. When an electric field is applied across the film, the transmittance decreases rapidly. The phenomenon is explained by Rayleigh scattering due to the dielectric clusters formed under the field. The decay time to return to the transmittance at no electric field after switching off the field is much larger than the response time after switching on the field; however, the usage of a magnetic as well as an electric field allows the decay time to be shortened.
YBa2Cu3O7 Josephson junctions and dc SQUIDs by mechanically induced growth disorder
Authors: Schmitt M. , Freisem S. , Becherer T. , Hadish A. , Adrian H.
Published: July 2002

A simple and reproducible method for the fabrication of high-Tc Josephson junctions and SQUIDs is presented. A very weak disturbance is produced by pressing a diamond needle onto the substrate surface before film deposition. The junction parameters such as the critical current Ic, the IcRn product, or β=2IcL/Φ0 (with L the self-inductance of the SQUID loop) are accessible by controlling the film thickness and can be varied by several orders of magnitude. The junctions usually have critical current densities jc in the range of 5×104–1×105 A/cm2 at 77 K and show Shapiro steps when exposed to rf power. The current-voltage characteristics are resistively shunted junctionlike and nonhysteretic. In dc SQUIDs a maximum peak-to-peak voltage response to dc flux of up to 12 μV at 77 K is observed.
Interface formation of Bi on ceramic ZnO: A simple model varistor grain boundary
Authors: Magnusson K. O. , Wiklund S.
Published: July 2002

The Schottky barrier formation between Bi and polycrystalline, ceramic ZnO has been studied with photoelectron spectroscopy (PES) under ultrahigh-vacuum conditions. This system is a simple model of a varistor compound. Evaporation of Bi on highly n-doped (Al) sintered ZnO surfaces, fractured in situ and held at room temperature, results in considerable upward band bending. After evaporation of 10 Å of Bi (approximately 3 monolayer coverage), the Bi-induced band bending amounts to 0.43 eV, as evident from the energy shift of the Bi 5d emission in PES. From valence-band and band-gap studies using ultraviolet PES, the states responsible for the observed band bending could be identified: Bi induces states in the ZnO band gap at 0.9 eV above the valence-band maximum. The filling of this high density of band-gap states results in a pinning of the surface Fermi level which makes the band bending proportional to the Bi coverage, with a rapid increase during the formation of the first monolayer and markedly slower thereafter. These results show the importance of Bi in the formation of the varistor Schottky barriers.
Sputter synthesis of TbCu7 type Sm(CoFeCuZr) films with controlled easy axis orientation
Authors: Hegde H. , Samarasekara P. , Rani R. , Navarathna A. , Tracy K. , Cadieu F. J.
Published: July 2002

Single phase TbCu7 type films of Sm(CoFeCuZr) have been sputter synthesized for a range of sputter gas pressure, deposition temperature, and two different gas species: Ar and Ar50%Xe. The magnetic and crystallographic properties of these films as a function of sputter deposition parameters have been studied. Films synthesized at temperatures near their crystallization temperatures at sputter gas pressures exceeding 60 mTorr of Ar50%Xe, had a strong c-axis in-plane texture. The remanent magnetization ratio for measurement perpendicular to the film plane versus in plane was close to zero for such films. X-ray diffraction patterns of these films showed only (hk0) type reflections. At gas pressures around 30 mTorr predominant reflection was (111) type. The perpendicular to the in-plane BR ratio for such films was around 0.55. For films with in-plane c-axis texture, in-plane Br of 9.0 kGauss and coercivities in the range 3–10 kOe were possible. Pressures of Ar and Ar50%Xe correlated roughly in the ratio 1:2 for the synthesis of films, with comparable crystallographic and magnetic properties.
Enhancement of the formation of Fe16N2 on Fe films by Co additions (invited)
Authors: Inoue Yoshiharu , Takebayashi Shigeto , Mukai Toshio
Published: July 2002

Effects of Co additions on the formation of Fe16N2 have been investigated by observing nitrides formed on thin-film surfaces. Thin films of Fe-Co alloys with a (100) surface sputter-deposited on MgO(100) substrates are exposed to a mixed gas of NH3 and H2. The 16:2 nitride was observed to form on a surface of Fe-10 at. % Co film at a nitriding temperature of 500 °C. The formation temperature is 50 °C higher than for pure iron. The amount of the formed 16:2 nitride has been found to be 3× larger for Fe-10 at. % Co than for pure iron.
Synthesis and magnetic properties of PrFe12−xMox and PrFe12−xMoxNy (0.5≤x≤1.0, y≊1)
Authors: Kalogirou O. , Psycharis V. , Saettas L. , Niarchos D.
Published: July 2002

A series of iron-rich ternary intermetallics PrFe12−xMox (x=0.5, 0.75, 1.0) and their nitrides have been prepared by arc melting, annealing at 1273–1393 K and water quenching. A single-phase structure of the ThMn12 type has been obtained for x=1.0 when annealed at 1345 K (with α-Fe<5%). The lattice constants are a=8.5978(2) Å, c=4.7795(3) Å. The alloys exhibit planar anisotropy, a Curie temperature of 498 K, and a room-temperature magnetization of 120.4 A m2/kg prior to the nitrogenation. After nitrogenation the anisotropy changes to uniaxial, the Curie temperature increases to 681 K, and the room-temperature magnetization becomes 136 A m2/kg. A lattice expansion corresponding to 2.4% of the cell volume is observed. In the x=0.5 and x=0.75 alloys, a secondary 2:17 phase is also present, besides the 1:12-type structure. The peritectic decomposition temperature is 1393 K for all samples.
Hysteresis of binary clusters
Authors: Klik Ivo , Yang Jyh-Shinn , Chang Ching-Ray
Published: July 2002

A pair of parallel uniaxial particles with dipole-dipole coupling has, for bond angles β=0 and π/2, up to four locally stable configurations: ↑↑, ↑↓, ↓↑, and ↓↓. Assuming thermal relaxation via coherent rotation and a periodic driving field we solve a master equation for the occupation probabilities of these states and find the coercivity and switching field distribution of the ensemble of coupled particles. For either value of the bond angle we compare numerical solutions of the master equation with approximate expressions based on extremal analysis and find good agreement between the two.
Low-temperature phase diagram of YbBiPt
Authors: Movshovich R. , Lacerda A. , Canfield P. C. , Thompson J. D. , Fisk Z.
Published: July 2002

Resistivity measurements are reported on the cubic heavy-fermion compound YbBiPt at ambient and hydrostatic pressures to ≊19 kbar and in magnetic fields to 1 T. The phase transition at Tc=0.4 K is identified by a sharp rise in resistivity. That feature is used to build low-temperature H-T and P-T phase diagrams. The phase boundary in the H-T plane follows the weak-coupling BCS expression remarkably well from Tc to Tc/4, while small hydrostatic pressure of ≊1 kbar suppresses the low-temperature phase entirely. These effects of hydrostatic pressure and magnetic field on the phase transition are consistent with an spin-density-wave (SDW) formation in a very heavy electron band at T=0.4 K. Outside of the SDW phase at low temperature, hydrostatic pressure increases the T2 coefficient of resistivity, signaling an increase in heavy-fermion correlations with hydrostatic pressure. The residual resistivity decreases with pressure, contrary to trends in other Yb heavy-fermion compounds.
Phase formation in melt-spun Nd-Fe-Mo-Ti alloys
Authors: Pinkerton F. E. , Fuerst C. D. , Herbst J. F.
Published: July 2002

We report for the first time the range of ThMn12-type crystal structure formation in melt-spun Nd-Fe-Mo-Ti ribbons. These materials, which combine the melt-spun microstructure with the favorable intrinsic magnetic properties of the NdFe12−x(Mo,Ti)x phase, are important as precursors to the formation of hard magnetic materials by nitrogen absorption. Alloys with compositions Nd1.15Fe10+xMo2−2xTix (0≤x≤1), which grade the composition smoothly between the two end point alloys Nd1.15Fe10Mo2 and Nd1.15Fe11Ti, were melt spun at quench wheel velocities 5 m/s≤vs≤30 m/s. From x-ray diffraction patterns we construct a phase formation diagram as a function of Ti content x and wheel speed vs. We find that the desired ThMn12 structure is obtained only in ribbons quenched at low vs. At higher speeds, the ribbons quench instead into the disordered TbCu7-type crystal structure. Further, the ThMn12 structure is considerably less stable at the Ti-rich end of the composition range than at the Mo-rich end: for Nd1.15Fe10Mo2, the ThMn12 structure is obtained for vs≤17.5 m/s, whereas in Nd1.15Fe11Ti only ribbons quenched at vs=5 m/s are of the ThMn12 type. Heat treatments of Nd1.15Fe11Ti ribbons melt spun at 30 m/s confirm the relatively difficult formation of the ThMn12 structure type; annealing temperatures in excess of 1000 °C are required to form the ThMn12 crystal structure.
Stress-temperature behavior of electron cyclotron resonance oxides and their correlation to hydrogenous species concentration
Authors: Cramer J. K. , Murarka S. P.
Published: July 2002

Deposited oxides of silicon are used for a wide variety of purposes in integrated circuit technology; two of the most important being as interlevel dielectrics and for passivation purposes. It is important to characterize oxides deposited by newer techniques like electron cyclotron resonance (ECR), plasma enhanced (PE) chemical vapor deposition (CVD), and sputtering of quartz for these purposes. In this article, we studied ECR oxides and sputtered quartz oxides to correlate their mechanical behavior to the hydrogenous species concentration. Sputtered quartz oxides were chosen as they are not known to have hydrogenous species concentration, and hence would serve as a comparison tool for ECR oxides which are known to have significant H content. Si-rich ECR oxides were found to have significant hydrogenous species content while near-stoichiometric ECR oxides had low bulk H content (as seen from nuclear reaction analyses). The hydrogenous species was identified from Fourier transform infrared spectroscopy to be Si:H. In these oxides, the stress-temperature behavior is directly related to the H content, with oxides high in hydrogenous species concentration ending up more tensile after thermal cycling. Realizing that these oxides are Si-rich, the above observation indicates that these oxides have oxygen vacancies (as opposed to Si interstitials). In order to achieve stable stress-temperature behavior (i.e., linear, and without hysteresis during thermal cycling), these oxides have to be annealed at temperatures greater than 700 °C. Sputtered quartz oxides exhibit stable stress-temperature behavior up to 600 °C.
Evidence of point defect supersaturation during Zn diffusion in InP single crystals
Authors: Wittorf D. , Rucki A. , Jäger W. , Dixon R. H. , Urban K. , Hettwer H.-G. , Stolwijk N. A. , Mehrer H.
Published: July 2002

Formation of defects during Zn diffusion into undoped and semi-insulating Fe-doped InP single crystals at 700 °C was observed by transmission electron microscopy for various diffusion conditions. Agglomerates of predominantly perfect interstitial-type dislocation loops, dislocations, and small indium precipitates inside voids are observed in the Zn-diffused crystal region. In addition, large planar arrays of precipitates are formed by climbing dislocations. From these observations it is concluded that the incorporation of Zn on In sublattice sites creates a supersaturation of In self-interstitials which is relieved by dislocation loop formation leading to a supersaturation of P vacancies and void formation.
Polar optical-phonon scattering in three- and two-dimensional electron gases
Authors: Gelmont B. L. , Shur M. , Stroscio M.
Published: July 2002

The concept of the polar optical momentum relaxation time for phonon energies larger than the thermal energy is applied to the analytical calculations of the electron mobility in bulk GaAs and in a two-dimensional (2D) electron gas. The theory also accounts for nonparabolicity. For the bulk material, the analytical formula is in good agreement with experimental data even at elevated temperatures where it can be used as an extrapolation formula. The comparison with numerical simulations for the 2D gas shows that the analytical formula for the 2D case applies when intersubband scattering is unimportant. When many subbands are involved in scattering processes, the polar optical mobility can be estimated using the analytical formula for the bulk case. Hence the results provide convenient analytical equations for polar optical mobility which can be used in device simulators.
Velocity distributions of ions in the ablation plume of a Y1Ba2Cu3Ox target
Authors: Fukushima Kensuke , Kanke Yukio , Badaye Massoud , Morishita Tadataka
Published: July 2002

The velocity distribution of ionic species in the plume was monitored by a time-of-flight mass spectrometer during laser ablation of Y1Ba2Cu3Ox. As oxygen pressure was raised, the velocity of Cu+ ions shifted to the slower side, and the width of the velocity distribution narrowed down to a minimum width at around 3×10−2 Torr. Beyond this pressure the width rapidly broadened. This change is qualitatively interpreted considering the dynamics in the flow of the supersonic expansion and thermal equilibrium, and is also discussed in connection with the surface morphology of ablated films.
Electrical and optical properties of thermally evaporated LiBO2–LiF composite films
Authors: Can Nguyen , Truong Vo-Van
Published: July 2002

Optical and electrical properties of the LiBO2–LiF composite thin films prepared by thermal evaporation have been studied for their use as ion conductors in solid-state electrochromic (EC) systems. All the LiBO2–LiF films exhibited a high degree of optical transmission (over 90%) and the best lithium ion conductivity attainable was about 8×10−7 S cm−1, a value considerably higher than that for the ionic conductivity of LiBO2. A typical activation energy of 0.545 eV was found for the composite film. The films obtained thus have a good potential for use as ion conductors in EC devices.